Scheeps bydromechani ca
Archief
Mekelweg 2, 2628
Deift
door Tom J.C. van Terwisga
Samenvatting
Doelstelling van dit werk is het ontwikkelen en valideren van gereedschappen voor de analyse van waterjet-romp interactie.
Hoewel er reeds een aanzienlijke
hoeveelheid kennis bestaat omtrent de
afzonderlijke componenten in het waterjet-romp systeem, bestaat er een hiaat in
onze kennis met betrekking tot de wederzijdse interactie. Als gevoig daarvan worden verschillen in het vermogen-snelheidsverband tussen prototype en voorspelling vaak aan interactie toegeschreven.
Een van de belangrijkste oorzaken voor misverstanden op het gebied van waterjet voortstuwing lijkt de afwezigheid van duidelijke definities te zijn. Hoofdstuk i laat zien dat er in de literatuur veel verwarring bestaat over definities en beschrijving van waterjet-romp interactie. Dit werk begint derhalve met een theoretisch model waarmee een complete beschrijving van waterjet-romp interactie mogelijk is. Het interactie effect op de romp wordt uitgedrukt in een 'resistance increment' factor.
Het interactie effect van de waterjet wordt uitgedrukt in een 'thrust deduction' factor en een impuls interactie- en een energie interactie-efficiency. De beide laatste rendementen verdisconteren de verandering in ingenomen impuls- en
energieflux ten gevolge van de verstoring door de romp.
Hoewel een ruwe procedure voor voortstuwingsproeven met waterjets reeds voorgesteld is door de ITTC in 1987, leidt deze aanpak gemakkelijk tot grote
systematische fouten, waardoor het nut van de proeven dubieus wordt. Bovendien
was de
voorgestelde verwerkingsproceduregebaseerd op een incompleet
theoretisch model. In dit werk wordt een verbeterde experimentele procedure
beschreven die gebaseerd is op ijking van de opnemers tijdens een paaltrek proef, tezamen met een verwerkingsprocedure die bepaling van de interactie factoren mogelijk maakt.
Gedetailleerde stromingsberekeningen en LDV metingen zijn gemaakt aan de
stroming in en rondom de intake. De resultaten geven inzicht in de geldigheid van
de aannames die gemaakt zijn in de verwerkingsprocedure van
voort-stuwingsproeven. Zij tonen aan dat een rechthoekige dwarsdoorsnede van de
aan dat de 'thrust deduction' factor tijdens het droogvaren van de spiegel niet
verwaarloosbaar is.
Berekeningen met een potentiaalprogramma en de methode van Savitsky zijn gedaan voor een berekening van de interactie effecten. De resultaten hiervan komen echter niet bevredigend overeen met de experimentele resultaten. Een
empirisch model wordt aangeraden voor een bepaling van interactie effecten in voorlopige vermogensberekeningen.
Dit werk biedt een consistente verzameling van definities en relaties, waarmee
zowel de voortstuwingseigenschappen van de romp en de waterjet, als ook hun interactie termen volledig beschreven worden. Een experimentele methode met een groter betrouwbaarheidsniveau dan tot nu toe beschreven in de openbare literatuur
wordt eveneens voorgesteld. Deze resultaten kunnen bijdragen tot een bredere
acceptatie van het waterjet systeem en tot een betere afwikkeling van contractuele onderhandelingen. Immers, de te verwachten voortstuwingseigenschappen van het schip zijn hierdoor beter voorspelbaar.
De interactie tussen de romp en het waterjet systeem kan het gevraagde
motorvermogen tot meer dan 20% beïnvloeden.
Voor een juiste selectie van voortstuwer systeem voor een bepaalde
toepassing, dienen de effecten van romp-voortstuwer interactie meegewogen te worden.
De interactie term 'thrust deduction' suggereert dat de stuwkracht verminderd wordt als gevoig van de aanwezige romp. Bij een waterjet-romp systeem is dit deels het geval. Bij een schroef-romp systeem is dit echter per definitie
onj uist.
De beschrijving van waterjet-romp interactie met dezelfde interactie termen zoals algemeen aanvaard voor de beschrijving van propeller-romp interactie, is principieel onjuist.
Waterjet-romp interactie effecten worden bet meest nauwkeurig bepaald door middel van modeiproeven.
In een potentiaalstroming is er in het algemeen meer nul dan je denkt. Door
bet toenemend gebruik van numerieke analyse methoden wordt dit steeds
vaker over bet hoofd gezien.
Interactie-verschijnselen zijn effecten die bij de gratie van door de mens geschapen (te) simpele denkmodellen in het technisch jargon bestaan. De
natuur zelf kent ze niet.
AIs we het vanzelfsprekend vinden dat met name jongeren fouten maken
waarvan ze kunnen leren, dan moeten we het ook accepteren dat een
complexe en jonge organisatie als de VN fouten maakt. Een opvoeder die dit proces bewaakt is echter onontbeerlijk. De afwezigheid hiervan vormt daarom het grootste probleem bij bet voiwassen worden van de VN.
Het zou het welzijn van de individuen van beide seksen als ook die van organisaties ten goede komen, wanneer we de psychologische verschillen
tussen mannen en vrouwen niet krampachtig ontkennen, doch er dankbaar gebruik van maken.
een té eenvoudig denkmodel van de werkelijkheid. Als deze fout tijdens een borrel gemaakt wordt is er een excuus, zijn de consequenties gering en kan het de gezelligheid stimuleren. Bij alle andere gelegenheden kan het echter verstrekkende gevolgen voor ons welzijn hebben.
De volgende redeneer mechanismen kunnen naar afnemende mate van betrouwbaarheid van het resultaat genoemd worden: wiskunde, statistiek,
fuzzy reasoning, en 'no reasoning at all'. Het redeneren van de mens bevindt zich in het algemeen tussen de twee laatstgenoemde mechanismen.
Door de complexiteit en de veelheid van processen in het menselijk lichaam,
zijn er talrijke mogelijkheden voor ongewenste interacties tussen deze processen en medicijnen. Door gebruik te maken van de lichaamseigen
processen, geeft de horneopathie inherent een kleinere kans op bijwerkingen dan de allopathie.
Het succes van het in de logistiek gehanteerde JIT principe (Just In Time) leidt tot een groter aantal vrachtwagenkilometers per ton produkt. Dit leidt tot een grotere stroperigheid en verstoppingskans van bet verkeer, waardoor het uT principe uiteindelijk via het JTL (Just Too Late) principe za] overgaan in het NNA (Not Needed Anymore) principe.
Het veelgehoorde argument '1k heb geen tijd', heeft pas enige
overtuigingskracht nadat de spreker ervan is overleden. En zeifs dan is het discutabel.
Stellingen behorend bij het proefschrift van T.J.C. van Terwisga: "Waterjet-Hull Interaction'. 25 april 1996
Waterjet-Hull Interaction
Torn J.C. van Terwisga
t.aboratorium voor Scheepshydromechanica
Archief
Mekeiweg 2, 2628 CD Deft
Waterjet-Hull Interaction
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Deift,
op gezag van de Rector Magnificus Prof.ir. K.F. Wakker, in het openbaar te verdedigen ten overstaan van een commissie,
door het College van Dekanen aangewezen, op donderdag 25 aprii te 13.30 uur
door
Thomas Jan Cornelis VAN TERWISGA
scheepsbouwkundig ingenieur geboren te Sneek
Prof.dr.ir. G. Kuiper
Samenstelling promotiecommissie: Rector Magnificus, voorzitter
Prof.dr.ir. G. Kuiper, TU Deift, promotor Prof.dr.ir. J. Pinkster, TU Deift
Prof.Dr.Dipl.-Ing. C. Gallin, TU Deift Prof.dr.ir. A. Hermans, TU Deift
Prof.dr.ir. L. van Wijngaarden, U-Twente Prof.Dr.-Ing. C. Kruppa, TU Berlin
Dr. A.J. Bowen, Canterbury University, New Zealand
2
Introduction
1 .1 Aim and motivation
1 .2 Historic setting
1.3 Outline of work
1.4 Description of waterjet system
1.5 Relations with other propulsors
1.5.1 Comparison with propeller
1.5.2 Comparison with gasturbine jet system
1.6 Review of previous work
1.6.1 Parametric models
1.6.2 Experimental procedures 1 .6.3 Computational procedures
1 .7 Summary of present work
1.7.1 Theoretical model
1.7.2 Experimental procedure
1.7.3 Computational analysis
Theoretical model
2.1 Systems decomposition
2.1.1 Definition of jet system control volume 2. 1.2 Analysis of overall powering characteristics 2.2 Basic equations
2.2.1 Thrust
2.2.2 Power
2.2.3 Free stream conditions 2.2.4 Lift
2.3 Interaction
2.3.1 Momentum interaction efficiency 2.3.2 Energy interaction efficiency
2.3.3 Quantitative assessment and comparison with previous work
2.3.4 Hull resistance increment
2.4 Conclusions i 2 4 8 13 13 17 22 23 34 43 48 48 49 50 53 53 54 58 64 65 68 71 78 80 8 1 88 90 94 96
3.1 Propulsion test procedure 99
3.2 Flow rate measurement 105
3.2.1 Flowmeter selection 106
3.2.2 Calibration procedure 119
3.2.3 Bollard pull verification tests 139
3.3 Uncertainty analysis 144
3.4 Propulsion test results 154
3.4.1 Thrust deduction 155
3.4.2 Momentum and energy interaction efficiencies 164
3.5 Extrapolation method 166
3.6 Conclusions 168
4 Computational analysis 171
4. 1 Free stream intake analysis 171
4.1.1 Intake flow analysis 172
4.1.2 Intake induced drag and lift 189
4.2 Computational prediction of interaction 196
4.2.1 Resistance increment for hump speed 197
4.2.2 Resistance increment for design speed 210
4.3 Analysis of propulsion test procedure 215
4.3.1 Effect of intake geometry on interaction efficiency . . . . 216
4.3.2 Effect of hull and free surface effects on t1 assumption 219
5 Conclusions and recommendations 229
5.1 Methods and tools 230
Al Derivation of relations for ideal efficiency 235
A2 Expressions for the computation ofcm and Ce 239
A3 Outline of uncertainty analysis 243
A4 Description of facilities and models used for experiments 253
AS Description of potential flow panel codes HESM' and DAWSON 259
A6 Description of performance prediction code PLANE' 263
A7 Description of LDV experiments in the MARIN large cavitation tunnel 265
References 271
Nomenclature
23
Summary 291
Acknowledgement 293
Chapter 1
1
Introduction
1.1
Aim and motivation
Aim of this work is 'the development and validation of tools to analyze
inter-action effects in powering characteristics of waterjet-hull systems'.
Interaction may be regarded as the phenomenon that is responsible for
devi-ations in the actual characteristics of two or more integrated subsystems, from a synthesis of properties of the merely matched subsystems. Interaction
phenom-ena are only revealed after one has a theoretical model to derive interaction terms, or after one has the means to observe interaction effects from measure-ments. And interaction is only explained and used effectively in design, after one has the tools to analyze the responsible mechanisms in detail. The search
for the aforementioned instruments forms the incentive of this work.
What's new? Propeller-hull interaction is a well developed field of research in
propulsion hydrodynamics where many researchers have made useful contribu-tions, see e.g. Morgan [1992]. What makes waterjet-hull interaction different is
the degree of integration of hull and propulsor system, rendering the definition
of interaction less straightforward, causing complications in experimental
tech-niques, and finally, rendering the interaction mechanisms different from those
During the first propulsion tests with waterjet propelled hulls, it appeared that waterjet-hull interaction could affect the overall efficiency by well over 20%. This finding in itself would probably be sufficient to justify research on this
topic. Moreover, the interaction effect on thrust sometimes appears to create an unfortunate combination of working regions for both the pump of the jet system and the hull. Such a combination may prohibit the jet-hull combination to reach its design working point and may result in a maximum speed of the craft that is
several knots lower than could be expected from pure matching of the two single systems.
Apart from this technical motivation, an economic motivation may be extracted
from present industrial developments. A growing number of waterjet applica-tions and waterjet manufacturers can be observed from the literature. Apart from the increase of these numbers, there is also an increase in waterjet size perceptible. And with this increase in scale, one can simply see the growing
financial risks and the consequent request for performance warrants.
1.2
Historic setting
An obvious question one may ask when being confronted with such an effort into waterjet-hull interaction, is the one on 'Why it is only now that interaction
raises this interest'. The following section seeks for a plausible explanation.
In the development of a new concept, technicians are initially only concerned
with the performance and the relations governing the isolated system.
Interac-tion between this new system and other systems is consequently neglected. As
such, the development of the waterjet-hull system can be regarded to have
already started in the ancient times during the development of the first hull
forms. The first achievement was the actual building and operation of the
corresponding vessel. Building and operation were based on experience, necess-arily gained after having learnt how certain ideas failed to work. Improvements were gradually implemented after an empirical evaluation of the new idea.
Archimedes (287-212 BC), presumably puzzled by the appearance of floating vessels, found a rational theory to describe this phenomenon. With his theory, new ideas and improvements could more easily, and with greater certainty be
evaluated. His contribution undoubtedly speeded up the development in
naviga-tion and shipbuilding. In the course of times, a wealth of knowledge became
Another historical event of probably similar importance, was the development of mechanical ship propulsion. This could be achieved after the availability of mechanical power, which came within reach with the introduction of steam
engines. Talented engineers subsequently concentrated on the propulsor system to convert the available mechanical power from the engine to a propelling force for the ship.
Since the beginning of the 17th Century, written reference is made to waterjet propulsion. In 1631, a Scot called David Ramsey acquired English Patent No.
50, which included an invention to make Boates, Shippes and Barges goe against Stronge Winde and Tyde". This was at a time when there was great interest in using steam to raise water and to operate fountains, so there is good
reason to suppose Ramsey had a type of waterjet in mind' (Dickinson [19381)...
A more explicit reference to the waterjet is made in a patent granted to
Toogood and Hayes [1661] for their invention of "Forceing Water by Bellowes
together with a particular way of Forceing water through the Bottome or Sides of Shipps belowe the Surface or Toppe of the Water, which may be of
singuler Use and Ease in Navagacon".
'The theory of waterjet propulsion was subsequently investigated and discussed by the Frenchman Daniel Bernoulli in 1753. He suggested (Flexner [1944]) that
if a stream of water was driven out of the stern of a boat below the waterline, its reaction on the body of water in which the boat floated would drive the vessel forward. By pouring water into an L-shaped pipe stretching to the aft end, Bernoulli's simple model experiment confirmed the principle of waterjet
propulsion, yet left others to determine how to force the water from the vessel' (Roy [1994]).
Up until the mid-nineteenth century, there has been little or no development on
waterjet propulsors. 'Because of the limitations of technology and lack of
understanding of the principles of propulsion , waterjet propulsors were
unable to compete with paddle wheels and, later, propellers' (Allison [1993]). From the early days of waterjet development, attempts to improve the propulsor concentrated on a better understanding and a consequent improved design of the
propulsor system itself. Interaction effects may not have been noted due to a limited accuracy of measurements or due to the absence of a physical model which showed that the effect searched for, could only be explained by interac-tiOfl.
As with the first propeller propelled vessels, waterjet systems have long been used successfully before technical interest focused on a proper description and
prediction of interaction effects between propulsor and hull. This interest could also have been slumbering for quite a period, because the first waterjet projects
often allowed for extensive trials or initial model testing. Problems due to
interaction effects could thus timely be solved experimentally.
Nowadays, the design procedure of a waterjet propelled vessel is different. Because of the availability of a large number of well developed waterjet
sys-tems, the designer is confronted with a selection of existing waterjet units, more
than with the integrated design of the waterjet itself. Due to the success of
waterjet propulsion development, times and budgets available for the vessel
development have been reduced significantly.
Having less time available, it is the responsibility of the ship designer to select the most appropriate waterjet system for the vessel under development. For an optimal selection of the propulsor installation, the evaluation of candidates should not only be made on an evaluation of separate systems, but should also
take into account possible interaction phenomena that interfere with the vessels overall performance.
1.3
Outline of work
Philosophy in approach
The basic philosophy underlying this study is taken from what is called Systems Theory.
'For centuries, science has sought for insight by analysing, by breaking down
more complex matters into series of simpler problems. The solution of the total
problem would then be equal to the sum of the solutions of the partial
prob-lems. Partial problems are thereby studied independently. More and more, also
basic, elements being identified. At the same time, the field of science is
widened, causing a growth in the amount of phenomena that need to be
explained. The analysis getting keener and deeper. Caused by this increasing
amount of knowledge, spread over numerous disciplines, synthesis has become increasingly difficult.
These days, the subsystems or elements are largely known and their properties have been well investigated. Various combinations of elements or systems are
elements is limited. This is partly caused by the classical scientific dogma:
'Only change one variable at a time'.' (translated from [In't Veld, 1981]). Description of concepts
Systems may be described by an enumeration of their constituting elements,or
by a set of properties or 'attributes' of the system. This latter way is appropriate when interaction between systems is to be described.
According to the concepts adopted in Systems Theory, 'interaction' between two (sub)systems occurs when the attributes of one system are affected by the attributes of the other system. The distortion causing the interaction, is passed through by the environment of the subject system. Analogous to the way in which interaction occurs between two systems, it may occur directly between
the system and its environment (see e.g. In 't Veld [1981]).
The attributes are quantified by the so-called 'state variables'. A set of state variable values consequently defines the 'state' of the system. The values of
these state variables are constrained by the set of relations governing the system in its environment.
To be able to quantify interaction, the 'isolated' system's condition is intro-duced (Fig. 1.1). This condition is defined by the set of attributes and relations
describing the system in a predefined undisturbed environment. The undisturbed
environment will in this work be referred to as the
'free stream condition'. Examples of free stream conditions are the later defined free stream conditionfor the waterjet system and the 'open water' condition for propellers.
Before looking at a complete integration of two systems with mutual interac-tion, an intermediate system's condition is introduced. In this so-called matched condition', there is no interaction between the two systems yet. The systems do however limit the range of values of the state variables of each
system. The state itself is not affected, only the number of states that can occur is limited. The possible states are governed by the 'matching relations'.
The situation in which interaction occurs is referred to as the 'united condition' of the combined system. In addition to matching relations, 'interaction relations'
determine the possible states of the systems involved. It can be inferred from
Fig. 1.1 that interaction between two systems is caused by a change in the envi-ronment through the action of the other system.
Isolated Condition
Matched Condition
United Condition
Fig. I I Relation between the degree of integration and interaction between two systems
Waterjet-hull interaction as meant throughout this work, refers to the interaction
between the waterjet system and the hull system only. That is, to a change in
values of the relevant attributes due to the presence of the other system.
The following gives an example on the use of the three conditions described above. The isolated systems condition or free stream condition is used for the quantification of the bare hull resistance as a function of speed. Similarly, the jet system's thrust production can be determined in its isolated condition for varying values of flow velocity and flow rate. In the matched condition, the thrust of the waterjet should balance the resistance of the hull. However, no
interaction effects on thrust or resistance are accounted for yet. We speak of the
situ-ation, the resistance of the hull is affected by the distorted flow due to the
waterjet action, and vice versa, the thrust delivered by the waterjet for a certain impeller rotation rate is affected by the hull distortion in ingested flow.
It should be noted that the matched condition is a conceptual condition which
only occurs in reality when it coincides with the united condition. This is when
interaction between both systems does not occur. The reason for considering
this condition is that a rough indication of the attributes of the combined system
can easily be obtained during synthesis activities. A second reason is that for
analysis activities the concept of interaction is defined more precisely. Scope
This thesis deals with interaction effects in the hydrodynamic relations that may
occur in the relations of either the jet or the hull system. Interaction effects in
the geometrical or constructional relations of either system are consequently not dealt with.
We speak of hydrodynamic interaction when for instance the equilibrium
posi-tion of the jet-hull system is different from that of the bare hull resistance test.
This change in balance is caused by the jet action, causing a change in the
hull's environment. Interaction effects can be described in terms of integrated
variables, such as in the above example. They can however also be expressed in
terms of state variables of the environment, such as fluid pressures and
veloc-ities.
The present work is concerned only with steady operation of the jet-hull sys-tem. Unsteady operation occurs for instance during acceleration of the vessel and during operation in
a seaway. Unsteady operations can
initially be approached by a quasi-steady analysis for each time step considered. 1f such an approach does not yield the requested correspondence with experimental obser-vations. the next step would be to also take into account time derivatives of the state variables. However, as a first step, we will concentrate here on interaction during steady operation.Although the above limitations are considered necessary from a practical point
of view, it should at the same time be realized that the quasi-steady approach should be considered with care. Doctors et aI. [19721 for example, show that
both the height and the corresponding Froude number of the hump in the
wavemaking drag of a Surface Effect Ship depend on the acceleration of the vessel. This change in resistance characteristics may consequently lead to a
Up to now, the unsteady type of operation during acceleration (and deceleration) has not emerged to the author as a field of industrial interest.
This is different for the unsteady operation of a vessel in a seaway. Engine
damage problems have been reported on High Speed Marine Vehicles by Meek-Hansen [1991]. Possible reasons for these damage problems are believed to be
caused by either air ingestion by the intake, by separation of the flow in the
intake or by a combination of both (ITTC [1993]).
Waterjet systems occur with various versions of the intake geometry. The three
basic configurations are shown in Fig. 1.3, viz, the flush type intake, the ram
type and the scoop type intake. This work is restricted to waterjet systems with a flush type intake. The theoretical model describing waterjet-hull interaction, as
well as the experimental procedures can however be used for ram and scoop
type intakes as well, with only minor modifications.
1.4
Description of waterjet system
General principle
The general principle underlying the thrust production of the jet propulsor is tersely summarized in the conservation law of momentum. A consequence of
this physical law is, that an action force is required to accelerate a certain
amount of fluid. This action force is exerted on the fluid by an actuator. For a
waterjet system, the actuator normally consists of a mechanical pump. In steady
conditions, the action force has to be counterbalanced by a reaction force
exerted by the fluid on the actuator. This reaction force can be identified as a
thrust vector Tg:
Tg
4mnmi
where = momentum flux vector
subscripts n and i denote the nozzle and intake area respectively.
The minus sign for the gross thrust has been added because the thrust is defined as the reaction force to the force associated with the increase in momentum. The momentum flux for a uniform flow can be written as:
Fig. 1.2 GeneraI scheme of waterjet propulsor
Water from the jet system's environment is ingested by the intake, accelerated
by the actuator and discharged through the nozzle. The actuator usually consists
of a mechanical pump, which may have a range of characteristics. The first waterjet systems were primarily equipped with centrifugal pumps, offering a relatively low pump efficiency. Modern waterjet systems mostly apply axial flow or mixed flow type pumps at improved pump efficiencies. An alternative
where p= specific mass of fluid
volume flow rate through control volume
u,n = momentum velocity vector.
Although various propulsor concepts acting in a fluid or a mixture of fluids are
available, they all share this common principle. Examples of such propulsors are for instance the classical propeller and all its derivatives, the gasturbine jet
used in
airplanes and the
waterjet for marine applications. Analogies in propulsor-hull interaction with propellers and gasturbines will be discussed inthe next section, to ensure that research results on propulsor-hull interaction for similar propulsors are used whenever possible.
To obtain a better understanding of the way in which interaction occurs and how it can be treated, a description of the waterjet system is provided in the following. The elements and fundamental processes are shortly addressed, without pursuing completeness.
Description of waterjet system
A general scheme of the waterjet propulsor is presented in Fig. 1 .2. A suitable
control volume that is required for e.g. the thrust computation from eq. (1.1), is discussed in detail in Chapter 2.
actuator that has recently raised some interest is the Magneto Hydrodynamic Pump [Doss and Geyer, 1993] and [Tasaki et al., 1991]. Due to its current low
efficiency, it has only been applied in experimental craft.
A further distinction of waterjet systems can be made after the type of intake applied. Two basic types of intake exist. One being an intake with an opening
that is situated flush in the vessel's hull and consequently approximately
paral-lel to the local flow. The other being an intake with an opening that is situated at approximate right angles to the local flow. Based on these two basic
con-cepts, various hybrid forms may be conceived. The basic concepts are sketched in Fig. 1.3.
flush intake
ram intake
planing scoop intake scoop intake
Both types of intake have their specific area of application. An advantage of the
flush intake is that it does not suffer from any drag due to protruding parts, as in the case of a ram type intake. Due to its position in the hull's bottom how-ever, it may become subject to air ingestion, either caused by large relative motions and a shallow submergence, or by entrained air in the flow under the hull. These disadvantages are avoided with a ram type intake, which can be
situated at a more favourable position in the flow.
The flush type intake is presently the most frequently applied intake. Ram
intakes are mounted in Hydrofoil craft or in other craft where the flush intake would be situated too close to the water surface. Hybrid forms are not often applied any more. They may be used wherever the disadvantages of flush type intakes are considered too serious, but the drastic ram intake solution is
con-sidered overdone.
Process per element
The most important part of the jet system governing both the size of the unit
and its overall efficiency, is the nozzle. This part of the jet system converts the potential (pressure) energy in the flow, added by the pump, into kinetic energy, used for thrust production.
For a given thrust and speed requirement, the nozzle area determines the thrust loading coefficient CT,,:
CT,, =
Tg
(1.3)
-p UA
where = gross thrust as defined by eq. (1.1)
o = ship speed or free stream velocity
= nozzle exit area.
lt will be demonstrated in Chapter 2 that the ideal efficiency of the jet system
only depends on the magnitude of CT,,. The ideal efficiency largely determines
the overall efficiency. Thus, in analogy with propellers, the lower the thrust loading, the higher the efficiency of the system. This implies an increase in
efficiency with increasing nozzle area.
The nozzle is usually shaped such as to have the vena contracta of the
dis-charged jet coinciding with the nozzle exit. The vena contracta of the jet
the ambient pressure of the medium in which it is discharged. Some jets are equipped with a clearly converging or Pelton type nozzle, causing the vena
contracta of the jet to be situated outside the physical boundaries of the jet
system (Fig. 1.4).
jet boundary jet boundary
Vena contracta Fig. 1.4 Distinct nozzle geometries
The function of the pump in the waterjet system is to add potential energy to the flow, resulting in a pressure rise over the pump. The most suitable type of
pump depends on the jet's thrust loading coefficient CTfl. For high thrust
load-ing coefficients, the ratio between nozzle velocity and free stream velocity is large, causing a relatively high head at low flow rate requirement. In extreme cases, the radial flow pump would offer the best pump efficiency. Nowadays, jet systems are usually selected at a lower thrust loading coefficient to obtain
the highest efficiency. This design condition demands a high pump efficiency at
a relatively low head but large flow rate. For these requirements, the mixed flow and the axial flow pump offer a better performance.
The function of the intake and consecutive ducting to the pump inlet area is to
provide sufficient flow to the pump. Intake design requirements relate to a
maximum energy recovery from the flow about the hull and minimum energy
losses within the intake. Another important requirement for the intake is that it
should present, as good as possible, a homogeneous flow to the pump. The
intake should further be as small as possible to minimize the weight of its
construction and of the ingested water.
The intake and ducting mostly feature bends and a diffusing or contracting cross sectional area along the intake/ducting. It should be noted that for most
operational conditions, diffusion or contraction of the ingested flow already
starts ahead of the intake.
1.5
Relations with other propulsors
The waterjet, the marine propeller and the aeronautical jet engine share the
same principle for thrust production. The latter two propulsors have furthermore
been subject to a longer time of scientific interest than the marine waterjet. In this period, well established theories of and procedures for the assessment of
interaction effects have become available. When investigating interaction effects
in and about waterjets, due attention has therefore to be paid to these related propulsors.
Before discussing interaction aspects of the propeller and the gasturbine jet, the most salient similarities and differences between the propulsors are discussed.
1.5.1
Comparison with propeller
The major hydrodynamic difference between a propeller and a waterjet system
occurs in the state of the flow passing the actuator, and therefore the risk of cavitation. The state of flow can be characterized by the state variables static
pressure and velocity as a function of their position.
Restricting ourselves to the steady process, we are not concerned with vari-ations of these parameters in time. Averaging these parameters over the disk
area just in front of the actuator, yields the two parameters that govern the
cavitation number G:
where = vapour pressure.
i-pv
±p2
2
(1.4)
The higher this cavitation number, the better the resistance in the actuator disk
against cavitation.
Using a required thrust production at a given speed as a starting point for our comparison, the cavitation number of the propeller can only be increased by
decreasing the velocity through its disk area. This conclusion is simply verified through the use of Bernoulli's theorem. Lowering the velocity through the disk area can only be accomplished by increasing the disk area.
The cavitation number just in front of the impeller disk of the waterjet is
simi-larly controlled by the average velocity through this disk. This average velocity
is however not only controlled by the impeller diameter itself, but also by the
nozzle diameter. This is because the flow rate through the impeller disk is
controlled by the nozzle area. For this resulting flow rate, the average velocity through the actuator disk is subsequently controlled by the impeller diameter.
The smaller the nozzle area, the smaller the flow rate. And the bigger the
impeller diameter, the lower the average velocity.
As we have seen in the preceding section, decreasing the nozzle area results in a decrease of ideal efficiency. This process of increasing the resistance against
cavitation by decreasing the flow rate through the system is consequently at the
cost of efficiency. This implies that when the jet is designed such as to have a better resistance against cavitation than the propeller, the propeller shows the
better efficiency.
This lower efficiency of the waterjet is partly compensated for by the absence
of appendage drag, which does occur for submerged propellers. This appendage
drag can reach values at high hull speeds of over 20% of the bare hull drag,
causing a major effect in power requirements.
Fig. 1.5 shows the trends in thrust loading coefficient CTfl and non-dimensional appendage drag as a function of the non-dimensional speed FnL.
t
L)
Io
Fig. I .5 Thrust loading coefficient and appendage drag contribution as a function of
non-dimensional speed 0 6 o 0.2 0.4 0.6 0.8 1 1.2 1.4 FnL I-t 12 0.25
As discussed before, the decreasing thrust coefficient results in an increase in
waterjet efficiency. At the same time, the increasing importance of the
append-age drag to the overall efficiency benefits the waterjet. If one furthermore
considers the deteriorating effects of cavitation at the higher speeds on overall powering performance, it becomes clear that the waterjet system is especially
suitable for the higher speeds, when purely evaluated in terms of cavitation and efficiency.
Propeller-hull interaction
The most frequently used model for the description of propeller-hull powering performance is based on the overall performance of the combined system, and
the performance of the isolated systems in predefined conditions. This theoreti-cal model provides a parametric description of the interaction. For this reason, it is a suitable model for application in preliminary ship design. A short descrip-tion of the model is given below.
The powering characteristics of the isolated
hull areeither computed or
measured during a resistance test. The powering characteristics of the isolated propeller are measured or computed for a uniform inflow. The performance characteristics of the propeller are then represented in a so-called hopen water
diagram'. This diagram provides the relations between non-dimensional propel-ler speed of advance J, with non-dimensional thrust KT and torque KQ.
Considering the combined propeller-hull system, the propeller normally operates
in the wake of the hull. The effective propeller inflow velocity thereby differs from the ship speed. By definition, the relation between thrust coefficient and propeller advance speed is
set equal to that in free stream conditions. The
advance ratio subsequently derived from the thrust coefficient as obtained from
self propulsion tests, can now be compared with that based on ship speed (see
Fig. 1.6, Newman [19891). The difference with the corresponding advance ratio
as obtained from the open water characteristics is accounted for in the 'Taylor wake fraction' WT according to:
U = UO(l-wT) (1.5)
where U
= effective propeller speed of advanceI I
i difference in US-* 1 W = UEI/UMODEL
»1 I I I I difference in KQ S * (KQ )O I (KQ)SP KT = thrust coefficient KQ = torque coefficient Subscripts
O = open water condition
SP = self propelled condition
Fig. 1.6 Relationship between self propelled and open water propeller characteristics (from
Newman [1989])
Because the propeller open water characteristics may differ from the propeller characteristics in the 'behind hull' condition, and because we have assumed
identical thrust coefficients KT in both conditions for a given advance ratio J, a generally small discrepancy occurs in the torque coefficient KQ. This discrep-ancy is accounted for by the relative rotative efficiency iR (see Fig. 1.6):
iR
K20 KQ
SQSP
s.where K0 =
propeller torque coefficientsubscript O indicates open water conditions.
Usually, a discrepancy exists between the thrust T delivered by the propeller
and the resistance R that was measured during the resistance test. This discrep-ancy is accounted for by the thrust deduction fraction t:
(1.6) U
- nD
(T-R)
(1.7)
T
The overall efficiency fluA can now be obtained from the open water efficiency and the interaction contribution
flOA 1Oib1NT
where ib = open water efficiency
fl/NT = interaction efficiency,
and the interaction efficiency can subsequently be written as:
(l-t)
flINT - fiR
(1 WT)
The fraction occurring on the right-hand side of this equation is often referred
to as the hull efficiency flH
As can be observed from the above model, the interaction between the propeller and the supporting hull is defined on a basis of the propeller open water
charac-teristics. Characteristics that were obtained from thrust and torque
measure-ments or computations. The idea of relating interaction to the free stream
char-acteristics of the systems involved can also be used in a model for the
descrip-tion of waterjet-hull interacdescrip-tion. But because the waterjet free stream
character-istics are obtained in a different way than practised for propeller propulsion, it
is not obvious to use the same terminology and relations.
1.5.2
Comparison with gasturbine jet system
First, a short description of the basic elements and processes of the gasturbine
jet will be given. Having reviewed the working principle, a comparison is made between the elementary processes of the waterjet and the gasturbine jet. Finally, the applicability of the analogy with waterjet-hull interaction is addressed.
The description of aeronautical or gasturbine jet systems will be concentrated
on subsonic jet engines. The physics involved in the corresponding processes is better comparable to those of waterjet systems because of the absence of shock waves.
(1.8)
Although the jet-fuselage configuration is not directly comparable to jet-hull systems, similarities occur. Hush intake type waterjets may be compared with
similar intakes of jet engines embedded in the fuselage. Ram intake type
waterjets as occur on hydrofoil craft may be compared with podded jet engines underneath the wings or mounted on the fuselage.
Description of gasturbine jet
A general scheme of the gasturbine jet is presented in Fig. 1.7.
a 1 2
34
intake compressor combustion camber turbine nozzle
p(N/m2) volume a o S(J/(kgK)) entropy a-1 ram.effect 2 adiabatic compression 2 . 3 : continuous burning at even
pressure
3 .4 : adiabatic expension 4 . 5 adiabatic expansion in a
convergent or convergent-divergent nozzle to excition
of jet
Fig. 1.7 GeneraI scheme of gasturbine jet system (Bodegom, W. Van [19981)
In the actuator of a gasturbine jet (viz, the combustion chamber), chemical energy is directly converted into kinetic and internal energy in an essentially isobar process (path 2-3 in Fig. 1.7). By expanding the exhaust gases in the aft
part of the jet system, most of the internal energy is converted into kinetic
energy that can be used for thrust production (path 5-6 in Fig. 1.7).
This direct conversion of chemical energy within the jet can only occur in a
compressible fluid, governed by thermodynamic principles. Burning fuel in the
actuator of the waterjet would initially only result in a rise of the water
Because of the compressibility of air, the distinct elements constituting the gasturbine jet system are characterized by a greater variety of physical pro-cesses than in the case of waterjet propulsion. These propro-cesses are
schemati-cally presented in the ideal pV and TS-diagrams shown in Fig. 1.7.
The function of the ram intake of the gasturbine jet is to ingest the required mass flow from the external flow at a maximum energy recovery. Apart from maximum energy recovery at
a minimal external drag, another condition
imposed on inlet design relates to an even velocity distribution in front of the
compressor mouth, so that an even force distribution on the compressor is
obtained. At subsonic airspeeds, the process of air ingestion and compression in
the intake is isentropic (path a-I in Fig. 1.7). Analogous to the waterjet intake,
diffusion or contraction of the ingested flow already starts upstream of the
physical intake opening.
Before entering the combustion chamber, the ingested air is further compressed by a compressor, performing an adiabatic compression (path 1-2 in Fig. 1.7). A high pressure of the ingested air is favourable for the thermodynamic efficiency of the combustion process.
The pressure increase in inlet and compressor is of great importance to the total
efficiency. The pressure rise in the intake is about 1 .6 times the free stream
pressure. The pressure increase over the compressor is subsequently 12-15 times the pressure just in front of the compressor. For supersonic speeds, the pressure
rise over the intake alone can even reach values in excess of 20 times the free
stream pressure.
After the combustion chamber, the flow enters the turbine, where part of the
energy is converted into mechanical power (path 3-4 in Fig. 1.7). This power is
used to drive the upstream compressor. In the turbine, the exhaust gas is
expanded, resulting in a decrease of gas temperature and pressure and an
increase of flow velocity.
The final stage of the jet system consists of the nozzle, in which the potential energy of the flow is completely converted into kinetic energy (path 4-5 in Fig. 1.7). The discharged exhaust gases adopt the ambient pressure at a higher
temperature (or internal energy) than ambient. Comparison with waterjet
A remarkable difference with the waterjet propulsor is the conversion process of
itself. As a consequence, the ingested medium is not equal to the discharged medium. The ingested flows consist of air and fuel, the discharged flow of exhaust gases.
The ingestion of a uniform velocity distribution by the intake is more important than it is for waterjets. The origin for this difference is that the specific mass of
the ingested flow rate increases with speed, due to the compressibility of air
and the increasing ram pressure with speed. This effect results in a thrust-speed
relation that
is markedly distinct from the same relation for waterjets (see
Fig. 1.8). Increasing the mass flow through the system for a given speed and
thrust requirement, decreases the nozzle velocity ratio NVR. And a decrease of
this parameter implies a higher ideal efficiency, as will be discussed further in Chapter 2.
thrust - speed relation for thrust - speed relation for incompressible fluid compressible fluid
(waterjet) (gasturbine jet)
E
E-ingested watervelocity
Resultant of A & B
Fig. 1.8 Thrust-speed relations for waterjet and gasturbine jet system ingested airvelocity
The compressor, mainly responsible for the pressure rise, requires a uniform inflow for an efficient and balanced performance. As a consequence of this
uniform flow requirement, the inlets are either podded or fuselage integrated in such a way that no appreciable amount of low energy or vortex flow is likely to
be ingested (Antonatos et al. [1972]). This is contrary to what is practised in
marine jets (see Photo 1.1).
Despite the aforementioned differences, a remarkable correspondence occurs in operational conditions, expressed in Nozzle Velocity Ratio values NVR.
Oper-ational values for this ratio occur between 1.5 and 5 for both jet systems (Borg
Courtesy Royal Dutch Air Force
Courtesy Lips Jets BV
It is concluded that the compressibility of air causes major differences in the processes of the gasturbine and the waterjet. Additional complications for the
marine jet are furthermore the possibility of cavitation and consequent
perform-ance degradation, and the presence of an interface between the two media
(water and air) in which the vessel operates.
Analogy with aeronautical jet-fuselage interaction
Fuselage interference effects on jet performance are of prime importance for aircraft designed for supersonic speeds. Due to the close interrelation between fuselage and jet engine in the case of flush intakes, the jet system design is
closely integrated in the whole aircraft design process (Ferri [1972]).
To the knowledge of the author, there are no parametric models available to quantify jet-fuselage interaction effects for airplanes. Taking into account the
broad attention that is paid to the jet-fuselage integration in the design process, and the requirement for uniform flow ingestion, the need for such a description may not be so evident.
Due to the marked differences in processes taking place in the marine jet and
the aeronautical jet, as discussed in this section, aerodynamic publications have a limited significance as far as the marine jet is concerned. Nevertheless, sorne useful information can be obtained from this discipline. The work of Mossman
et al.
[1948] on flush type intakes may in this respect serve as a classical
example.
1.6
Review of previous work
A large number of publications dealing with the hydrodynamics or
aerodynam-ics of jet propulsion has been published since jet propulsion started to raise interest. Because the interest was first excited in aeronautics, the first tions have an aeronautical origin. As concluded in Section 1.5, these
publica-tions have only a limited significance for the present work.
Waterjet technology has in the past particularly been pushed in Germany, Italy,
New Zealand, the United States and Sweden. These countries have provided
also most of the available literature.
This review of previous work is split up into a review of parametric models,
1.6.1
Parametric models
For a meaningful set up of procedures and interpretation of the experimental
and numerical information, parametric models are indispensable. Description of parametric models
A parametric model is a special form of a more generic theoretical model. A
theoretical model is a set of symbols and relations, describing both the elements
and the processes occurring in the systems considered. The Navier-Stokes
equations are generally considered as a basic theoretical model governing all flows that may be described as a continuum. These equations can be written in
a differential form, describing the equations of motion for an infinitesimal
volume, or in its integral form, describing the equations of motion for a certain control volume (Fig. 1.9).
Newton's second law as applied to fluid mechanics Differential equation (Flow field) analysis Integral analysis Non-viscous flow Viscous flow
Navier-Stokes equations
Differential form Integral form
parametric model
1 CFD applications
Fig. 1.9 Place and relevance of a parametric model in fluid dynamics
A parametric model of these equations can be extracted from the integral form for a simplified description of specific processes. This is done by substituting parametric expressions for integral expressions. Such a model is useful to gain insight in the relevant physics for a given process. It furthermore allows for an efficient storage of data on the process, consequently allowing for empirical
Insight in relevant physics Storage of data for empirical use Analysis of model tests
approaches. A third application of such a model is the analysis of data obtained from model tests.
Organization of review
As can be inferred from Fig. 1. 1, a complete description of interaction phenom-ena is only obtained after the subsystems involved are completely defined. This includes their relations with the undisturbed environment.
This condition is considered to be satisfied for the powering characteristics of the hull system. The hull's resistance has been studied for many decades now. An example of a detailed analysis of the hull's resistance is given by Paffett
[1972]. Model test procedures for measurement of the hull's resistance are well defined (see e.g. HSMV report of ITTC [1987]).
The definition of the powering characteristics of the waterjet system in an
undisturbed environment is more complicated however. This is illustrated by the
confusion that exists in the literature about the control volume defining the waterjet system and the corresponding intake drag of flush waterjet systems. This consequently results in confusion about the defined gross thrust (eq. 1.1)
and the actual net thrust that is available to propel the hull.
Interaction is originated at the boundaries of the system and appears as a
change of powering characteristics. A change in environment results
in a
change in stress distribution on, and changes in mass, momentum and energy fluxes through its boundaries. The attention related to interaction on waterjetperformance will therefore be discussed first in terms of definitions of control
volume and free stream characteristics, and subsequently in terms of changes in stresses and fluxes due to the hull action.
Although there seems to be little specific interest in jet-hull interaction in the literature, the subject is addressed several times as part of an overall
perform-ance description. Within the themes mentioned in the preceding paragraph, the literature is reviewed in temporal sequence.
In reviewing the work that has been done on parametric models, we will con-centrate on jet-hull interaction for flush intakes, in line with the rest of this work. Subsequently, due attention will be paid to jet-hull interaction for ram
intake systems. We will close the review on parametric models with a
consider-ation of an original contribution by Schmiechen to the field of propulsor-hull interaction.
Flush waterjet system
Definition of jet system and free stream powering characteristics
The generally quoted waterjet gross thrust T is usually defined as the force that
results from the change in momentum fiu through a certain control volume (eq. 1.1). The selected control volume also determines the difference between defined gross thrust and actual net thrust exerted by the jet upon the hull. A logical first step in defining jet system characteristics therefore seems to be an
explicit definition of this control volume. Such a definition has long been
missing in literature however.
Early parametric models for the description of waterjet performance are given by e.g. Brandau [1967], Kruppa et al. [1968] and Gao et al. [1969]. Emphasis
in these theoretical models is put on the description of powering characteristics
of the jet system in free stream conditions. Explicit definitions of control vol-umes are omitted, but implicit reference is made to a control volume with an
intake area infinitely far upstream (control volume A in Fig. 1.10).
To the knowledge of the author, Bowen [1971] was the first author in the
mar-ine field, to bring about a discussion on the control volume that should be
considered as a model of the jet system with flush intake. He discusses the difference between a generally applied definition of gross thrust and the actual thrust acting upon the hull. Analogous to the practice in the aeronautical field
(Jakobsson [1951]), he designates the difference between these definitions as a
'pre-entry thrust', and derives an estimate of this thrust contribution for ram
inlets. Bowen thereby also applies Control Volume A (Fig. 1.10) for the
description of the jet system.
An early parametric model including an explicit definition of the waterjet
control volume is presented by Etter et al. [1980]. The definition of their
con-trol volume corresponds to volume C (Fig. 1.10). The free stream characteristics
of the waterjet system are not elaborated. The emphasis is placed on a
separ-ation of jet system net thrust and hull resistance.
The discrepancy between jet system gross thrust and the bare hull resistance is expressed in an inlet system drag. This intake drag is derived from model tests with a self propelled model, and therefore also incorporates a change in hull
resistance due to the jet action.
in a recent publication by Allison [1993], a complete review of existing rela-tions to describe waterjet performance is given, including a review of
interac-tion effects. Implicit use is made of control volume A (Fig.
1.10). Allison includes an inlet drag allowance in the jet efficiency, to allow for the difference between gross and net thrust, but omits a definition. He mentions that this term tends to zero for truly flush intakes.pump
X
4
fixed (material) boundaries
A = intake leading edge (imaginary)
= ramp tangency point
I'BC = lower dividing streamline
C = stagnation point
D = intake trailing edge or outer lip tangency point EE intake throat area
Possible waterjet control volumes: CV A : II'CFF'I
CV B :
A'FF'A
CVC : A'B'CFF'A'
Fig. 1.10 Definitions of jet system's control volume used in the literature (see also fold-out at the back)
Intake drag
Many authors refer to the difference between defined gross thrust and a net
thrust acting upon the hull as an intake drag. Although the intake drag is
yz
/y
/
AA
//
r
/
/
/
B' Baddressed several times (e.g. Mossman et al. [1948], Arcand et al. [1968], Hoshino et al. [19841), little attention is paid to its definition. An exception to this rule is the contribution by Etter et al. [1980], as discussed in the previous section.
Mossman and Randall [1948] determine the intake drag for a number of flush
type intakes in a wind tunnel. They implicitly use control volume A (Fig. 1.10).
It will be shown in Section 4.1.2 however, that their intake drag consequently includes a significant frictional drag contribution of the tunnel wall in front of the intake.
Arcand and Comolli [1968] use the same definition in their data reduction.
They state however that they believe that a better definition of the 'real external
drag' of the intake is possible. Although their idea is not elaborated in detail, it can be inferred that their improved definition gets rid of the major part of the
tunnel wall contribution.
Hoshino and Baba [19841 define the intake drag as the difference between gross
thrust required to propel the ship and its bare hull resistance. This intake drag consequently not only accounts for a discrepancy between gross and net thrust for the jet system, but also for a change in hull drag due to the jet action. The authors implicitly use Control Volume A (Fig. 1.10) for the definition of gross thrust.
Confusion about external intake drag and internal jet system forces is illustrated
by statements in Okamoto et al. [1993] and Kim et al. [1994]. Okamoto et al. state that 'the intake duct shows a thrust generation mechanism' for certain operational conditions. Whereas Kim et al. state that 'the pressure distribution
along the intake lip is responsible for additional appendage drag'.
Interaction effects
Effects on thrust and power
Using an equal flow rate through the system as a basis for comparison, both
distortions in the local velocity distribution and distortions in the local pressures affect the ingested and the discharged momentum and energy fluxes through the
waterjet system. These fluxes govern the associated thrust production and the
power requirement respectively.
Only a few publications address the problem of waterjet-hull interaction in
the ingestion of boundary layer flow. This flow shows a decelerated velocity
relative to the free stream velocity.
Although the effects of pressure distortions on jet performance are acknowl-edged in an early stage (see e.g. Kruppa [19681), much confusion arises as to how it should be accounted for in a parametric model, as noticed by Kruppa
[1992].
Kruppa et al. [1968] account in their parametric model for the effect of the
boundary layer velocity distribution on the ingested momentum and energy flux. The authors introduce momentum and energy wake fractions. The authors note
that, whenever a pressure gradient occurs between the undisturbed free stream
and the intake area of the waterjet, these wake fractions should be corrected for
pressure terms. The correction for the momentum wake fraction is not elabor-ated however. The difference between gross thrust and bare hull resistance is referred to as a parasitic drag. This parasitic drag consequently includes a drag contribution of the intake and a change in hull resistance due to the waterjet action.
Wilson [1977] gives a detailed account of the state of the art knowledge on waterjet-hull interaction in that time. The parametric model used by Wilson is based on schemes presented by Johnson et al. [1972] and Barr [1974]. The
essential relationships are outlined by Miller [1977].
Miller describes waterjet-hull interaction in terms of ingested momentum and
energy velocities, accounting for differences in the respective fluxes relative to
the corresponding free stream fluxes. The effects considered are solely caused
by the boundary layer velocity profile. The other interaction parameter is
referred to as an intake drag, being equal to the difference in net thrust and calculated gross thrust. The intake drag is to be determined experimentally, implying that this drag component not only accounts for a net force acting on the intake part of the jet, but that it also accounts for a change in hull drag due
to the jet action.
A more complete description of the parametric model by Miller is presented by
Etter et al. [1980]. This model is adopted by the High Speed Marine Vehicle Committee of the 18th ITTC [1987]. The model describes the powering
formance of the combined jet-hull system. without breaking it down in the
per-formances of the isolated systems and their mutual interaction. The relation
Haglund et al. [1982] express a waterjet-hull interaction efficiency as a hull efficiency, analogous to the same efficiency used in propeller-hull theory. The
same control volume for the waterjet system is used as described by Etter et al. [19801 (CV C in Fig. 1.10). The effect of the boundary layer velocity profile on momentum and energy flux is only taken into account by the use of an average volumetric intake velocity.
Later, Svensson [1989] includes a pressure term in the jet efficiency, accounting for the static pressure contribution in the ingested energy flux.
Hoshino et al. [19841 only account for interaction by using a thrust
effective-ness (1-t). The authors do not account for the ingested boundary layer flow.
As a further development to the model proposed by Kruppa et al.
[1968], Masilge [1991] breaks the distortions in ingested momentum and energy fluxes down into a pressure contribution and a velocity contribution. The requirement for a pressure term in the ingested fluxes was also noted and a model suggested by Van Terwisga [1991].As a sequel to their 1968 paper, Kruppa [19921 presents a relation for overall efficiency based on thrust (thrust power efficiency), in which he incorporates the potential flow and viscous flow interaction effects separately, as proposed earlier by Van Terwisga [1991]. He notes the lack of a pressure term in the
momentum equation for thrust and consequently observes that the equations for thrust power efficiency for ram type inlets on the one hand and flush type inlets on the other hand are not compatible. Based on this observation, Kruppa queries a conclusion by Svensson [19891, where the positive effect of a retarded poten-tial flow wake on thrust power efficiency is noted.
Allison [1993] gives a review of interaction effects. Similar to the hull-effi-ciency in propeller-hull interaction, an interaction effihull-effi-ciency
is used in the
overall efficiency equation, according to:
(l-t)
11= (1-w) where t = thrust deduction fraction
w = volumetric wake fraction.
lt is emphasized by Allison that this efficiency is not directly comparable to the
hull efficiency used in propeller-hull theory. Contrary to the model for propel-(1.10)
1cr-hull interaction, where no interaction terms occur in the propeller efficiency, interaction terms remain present in the efficiency of the jet system.
Effects on lift
Svensson [1989] mentions a net lifting force on the stern of the vessel with active waterjet. This conclusion was obtained from pressure measurements on
the hull in the vicinity of the waterjet system (see Fig. 1.11). He states that 'the total lifting force generated by the inlets can be in excess of 5% of the
displace-ment for a high speed craft. The total lifting force can thus exceed the weight
of the units. This will be recognized as a negative thrust deduction caused by a reduction of the vessels resistance'.
BOTTOM PLATING B = I .5D
BOTTOM PLATING
B= D
INLET INLET VELOCITY RATIO (IV R) D = NOMINAL INLET DIAMETER B = BREADTH OF BOTTOM PLATING
Fig. I Il Lifting force due to pressure on the bottom plating and inlet for a KaMeWa waterjet installation (from Svensson [1989])
The issue of lift production caused by waterjet-hull interaction will be treated in
Chapter 2. It will be shown that the jet system itself does not generate a lift
Determination of ingested momentum and energy fluxes
An important problem in determining interaction effects on the waterjet
per-formance, is the assessment of ingested amount of momentum and energy for a flush intake operating in a non-uniform velocity field. For this particular
prob-lem, a strong analogy occurs between the flow in and about a flush intake of a
waterjet system and the intake of a condenser scoop system.
Spannhake [1951] presents a detailed theoretical analysis of the flow through a
condenser scoop system. He defines clear control volumes for both the inlet and the outlet scoop. He also introduces an expression for the imaginary intake area of the protruding streamtube (area AB in Fig. 1.10). This expression is based on
data obtained from flow visualisation studies about the intake as published by
Hewins et al. [1940].
Quantitative experimental information on the shape of this imaginary upstream
intake area for a waterjet is provided by Alexander et al. [1993]. These data
will be reviewed in more detail in Section 4.3.1 of this work.
Interaction effects ou hull performance
Interaction effects on the hull are caused by a change in the flow about the hull due to the jet action. It can be expressed as a change in the hull resistance. This
change in resistance is in the literature expressed in a thrust deduction fraction or an intake drag, as discussed in the foregoing. A possible change in lift
pro-duction on the hull as mentioned by Svensson [19891 could, if present, also be ascribed to interaction.
Several authors, e.g. Hothersall [19921, Allison [1993], Van Terwisga [1992], mention the effect of waterjet weight on the hull performance. The weight of the jet system does not affect the hydrodynamic environment for a given hull
loading condition, and does therefore not cause a true hydrodynamic interaction
effect. As it does affect the vessel's weight breakdown, one should account for
it in the jet selection process.
Interaction in ram type intake jets
Sherman and Lincoln [1969] discuss the optimization of the ram type intake for
waterjet systems in detail. They include clear definitions of the net and gross thrust and the external drag of the jet system. They thereby implicitly consider
nozzle F
strut
The net thrust, available to propel the hull is obtained from the vector
summa-tion of gross thrust and external drag:
Tnet Tg+Dext
where
Tnet = net thrust
Tg= gross thrust
Dext = external drag.Their gross thrust is defined as:
Tg mnniO+(PnPO)An (1.12)
hull
A
F -- .
where = momentum flux
p
= pressure
A
= nozzle area
subscript n = nozzle
This definition grossly corresponds to the basic definition given in eq. (1.1), but
is extended with a pressure force term over the nozzle area. It will be demon-strated in Chapter 2 that this term is also present for flush waterjet intakes, but
is included in a thrust deduction fraction t there.
The external drag of the system consists of the sum of the external force FEXT,
a so-called pre-entry drag DPE and a possible interference drag DINT:
DEXT FEXT+DPE+DINT (1.13)
The external force FEXT acts on the material external part of the waterjet system
(AHH'A' in Fig. 1.12). It is obtained from the summation of all pressure and
frictional forces in x-direction, associated with the ram inlet system from the
stagnation line around the external surface to the termination of the system.
The pre-entry drag DPE acts on the protruding part of the streamtube ahead of the material intake. This is the same component that was introduced in the discussion on the net thrust of flush type waterjets by Bowen [1971] and dis-cussed in the foregoing. It is caused by external diffusion or contraction of the ingested streamtube. As a consequence thereof, a discrepancy occurs between the actual net thrust and the defined gross thrust after the external force FEXT
and a possible interference drag DINT have been accounted for.
The interference drag DINT can be interpreted as a drag component accounting for a change in hull drag due to the presence of the waterjet.
A discussion on the subject of thrust definition and actual net thrust was held
during the 20th ITTC in and about the Report of the High Speed Marine
Vehicle Committee {ITTC, 1993].
Because the flow is ingested at a certain distance from the hull, interaction between hull and intake is usually small. The hull drag may be affected by the
pressure distortion by the intake or by an interference drag due to the strut
piercing through the hull.
Parametric model due to Scizmniechen
An original approach to
the propulsor-hull interaction problem isdue to
Schmiechen [1968, 1970]. He presents a generic system of criteria to evaluate
the powering performance of propulsor-hull systems. He separates the propulsor-hull system, thereby only providing a definition of the propulsor
