On the hydrodynamic performance of an improved motion control device for fast ships

On the hydrodynamic performance of an improved motion control device for fast ships

A. A. K . Rijkens, Ship Hydromechanics and Structures, Delft University o f Technology, Delft, The Netherlands H . M . A . Cleijsen, Ship Hydromechanics and Structures, D e l f t University o f Technology, Delft, The Netherlands J . A . Keuning, Ship Hydromechanics and Structures, D e l f t University o f Technology, Delft, The Netherlands

S U M M A R Y

This article focuses on the hydrodynamic performance o f a new transom-interceptor configuration that can be used to control the motion behaviour o f a fast ship in waves. The present control device is based on a conventional interceptor design, however its configuration is modified i n such a way that it is able to produce a l i f t force i n both the positive and negative direction. It is assumed that the negative l i f t component can be generated by introducing a relatively large round transition between the hull bottom and the transom o f the ship. A systematic series o f model experiments have been carried out i n the towing tank o f the D e l f t University o f Technology to validate this assumption. Three different transom-interceptor configurations are tested on a planing type hull form. The first design has a conventional shaip transom edge, whereas the other two designs have a round transom edge with different radii o f curvature. A l l tests were carried out i n combination with a controllable interceptor. The hydrodynamic characteristics o f the different configurations are implemented into a ship motion prediction progi-am. The simulations illustrate that this new transom-interceptor configuration results i n a more favourable sea keeping behaviour o f the ship in comparison w i t h the conventional interceptor design.

N O M E N C L A T U R E R O M A N b W i d t h interceptor [m] Co Drag coefficient [-] Cl L i f t coefficient [-] Cv Speed coefficient [-] D Drag [N] f Oscillation frequency [Hz] F Force [N] 9 Gravitational acceleration [m/s^] h Deflection interceptor [mm]

Hs Significant wave height [m]

I<0 Pitch velocity gain interceptor [mm/(deg/s)]

L L i f t [N]

Lpp Length between perpendiculars [m]

Tv Peak period [s]

V Forward speed [m/s]

r Radius transom edge [m]

To Wave amplitude [m]

GREEK

9 Pitch velocity [deg/s]

Pitch amplitude [deg]

X Wave length [m]

P Water density [kg/m']

T T r i m angle [deg]

1. I N T R O D U C T I O N

The application o f motion control devices on high speed ships can significantly improve their sea keeping performance [1]. One o f the most efficient and yet simple trim control mechanisms is the interceptor design. A n interceptor is a vertical blade that can be mounted onto the transom o f the ship. The shape o f the blade generally follows the contours o f the transom edge and it can be vertically lowered into the flow stream. The blade creates a discontinuity i n the flow field when it extends below the transom edge, which causes a stagnation region underneath the hull. This stagnation region is characterised by a high pressure which eventually induces a l i f t force on the stern o f the ship at high forward speeds [ 2 ] .

The straightforward and low-cost construction makes the interceptor a rather suitable device for motion control applications o f high speed ships. A n important additional advantage, f o r m an actuation point o f view, is the relatively limited amount o f power necessary to control the position o f the vertical blade. I n contrast to many other control devices the l i f t is entirely generated by a pressure modification underneath the hull. This allows the interceptor to change its position without too much effort, provided that it is sufficiently suspended by a vertical guidance system.

The conventional interceptor design can produce a l i f t in the positive direction, resulting in a bow-down trimming moment o f t h e ship. For a wide range o f high speed ships this w i l l also be the most important direction, whereas the vast majority has a natural tendency to significantly t r i m the bow i n the upward direction as the forward speed o f the ship increases. This behaviour typically continues up to the "hump" speed after which most often a reduction in the trim angle can be observed. The

existence o f a pronounced "iiump" is actually the reason why many o f these high speed ships have been equipped some type o f passive control mechanism, usually i n the f o r m o f a pair o f adjustable transom flaps or interceptors. The position o f these passive devices is often predetermined to a setting that gives the "most optimal" behaviour throughout the entire speed range o f the ship.

I n the field o f motion control applications a large variation i n the added l i f t force o f the active mechanism is desirable, since this gives the most controllability over the motions o f the ship. W i t h this i n mind, it was believed that an extension o f the control force i n the opposite or negative direction would contribute to a better controllability o f the ship motions. This especially holds f o r a class o f fast ships that do not naturally have large running t r i m angles at high forward speeds.

A first step i n this research was to develop a new transom-interceptor configuration. The main objective o f the new design was that it should be able to generate a l i f t force i n both the positive and negative direction. For this purpose a modification was made to the transom edge o f the ship. It was assumed that w i t h the introduction o f a relatively large round transition between the hull bottom and the transom, a negative l i f t could be generated once the interceptor is raised above the virtual ttansom-bottom intersection line. This presumption is investigated by conducting a series o f model experiments. A f t e r the analysis o f the experimental results, simulations w i l l be used to illustrate the principle effect o f this improved motion control device on the sea keeping performance o f a fast ship.

2. E X P E R I M E N T S

A n experimental research was initiated to determine the hydrodynamic performance o f this new transom-interceptor configuration. These tests should be considered as an exploratory investigation to examine i f this new design is able to generate a l i f t force o f any significance i n the downward direction. A t the same time, it is verified whether these modifications would not compromise the favourable hydrodynamic characteristics o f a conventional interceptor configuration.

A systematic series o f model experiments has been carried out i n the towing tank o f the Ship Hydromechanics Department o f the D e l f t University o f Technology. I n these tests three different transom-interceptor configurations have been investigated. One conventional design with a sharp transom edge was used as a benchmark and two "new" configurations were constructed w i t h a round transom edge having a different radius o f curvature. The hydrodynamic characteristics o f these three designs w i l l be compared i n order to quantify their relative performance.

2.1 M O D E L SETUP

The main objective in the design o f the model was that it could be used to measure the force variation at the stern w i t h a high degree o f accuracy. I t was expected that forces generated by the interceptors were relatively small compared to the total hydrodynamic force on the complete hull at high forward speed. For this reason it was decided to divide the hull into two individual longitudinal segments. W i t h this segmented model more sensitive force transducers could be installed i n the aft part which increases the precision during the measurements.

The model is manufactured from laser cut plywood and it has an overall length o f 1.8 meters and a beam o f 0.4 meters. The stem segment has a constant rectangular cross-section and its length measures 0.8 meters. The purpose o f the b o w segment was to smoothly divert the fiow stream to the stem segment and it was designed i n such a way that it could be assembled using a number o f developable surfaces. Figure 1 shows the lines plan o f this simplified planing hull shape.

Figure 1: Lines plan o f the model

Both segments are connected to a steel girder extending over the length o f the model. This steel girder serves as the "backbone" o f the model and is used to keep both segments i n its predetermined position. The front section was rigidly connected to the backbone. The aft section is connected to the backbone v i a three force transducers; two are placed in the normal direction to measure the total l i f t force and t r i m m i n g moment and one transducer is placed in the longitudinal direction to measure the additional drag due to an interceptor deflection. The separation between the two segments is kept to a minimum and has a value o f approximately 2 m m . This spacing is sealed w i t h a thin mbber strip to prevent pressure losses and flow disturbances between the two segments. The setup is presented in Figure 2.

1 1 1

r-

-r

₄

The steel backbone is mounted via a six degrees o f freedom oscillator to the towing carriage, see Figure 3. The oscillator restrains the model i n all six degrees o f freedom. When the carriage accelerates, the oscillator pushes the model into the water at the desired draft and trim angle. When the towing carriage is decelerating at the end o f the towing tanl-c the model is raised out o f the water in order to avoid that the measurement equipment becomes wet due to the wake o f the model.

Figure 3;

oscillator

Photograph o f the model connected to the

The three interceptor configurations have been tested on the same hull f o r m i n order to determine performance and make a justifiable comparison. Modular transoms sections have been used to prevent removing and re-aligning o f the complete model underneath the carriage at each time a different configuration is being tested. Figure 4 shows the three different transom sections, one with a conventional sharp transom edge and two with a round transom edge having a different radius o f 25 m m and 50 m m .

Figure 4: Modular transom sections; conventional

interceptor and the two new transom-interceptor configurations

The interceptor blade covers the f u l l beam o f the model. Four linear guiding rails hold the interceptor in position

and only allow a vertical translation. The deflection o f the interceptor was governed by a linear motor that uses an electromagnetic driving mechanism.

2.2 M E A S U R E M E N T SCHEME

The measurement scheme is subdivided into two parts. I n the first part the stationary characterises o f the different configurations w i l l be examined. The second part covers the dynamic response o f the interceptor during harmonic oscillations.

A brief overview o f the stationary test parameters for the different interceptor configurations are listed in Table 1. In this table the speed coefficient is defined by:

(1)

Table 1: Stationary test parameters for the various

transom-interceptor configurations at a trim angle o f 3 degrees

Symbol Parameters Unit

Cv ho hso 1 . 5 / 2 . 0 / 2 . 5 [-] 0 / 2 . 5 / 5 / 1 0 / 1 5 / 2 0 [mm] -25 /-12.5 / - 1 0 / - 5 / 0 / 5 / 1 0 / 2 0 [mm] - 5 0 / - 3 0 / - 2 0 /-15 / - 1 0 / - 5 / 0 [mm]

The speed coefficients correspond to model speeds o f 3.0, 4.0 and 5.0 m/s, w h i c h cover most o f the planing region. The deflection heights o f the interceptors are represented by hg, hzs and h^o- These subscripts refer to the radii o f the different transom sections. A positive deflection notation indicates that the interceptor extends below the (virtual) bottom-transom intersection line and a negative sign is used f o r positions in which the blade is raised above this reference line, see Figure 5.

AT

1

AT

Figure 5: Definition o f positive and negative interceptor

deflections

The largest upward deflection o f the interceptor is equal to the radius o f the particular transom section and the maximum downward deflection is limited to 20 mm. For

the transom configuration w i t h a 50 m m radius, however, only the upward deflections could be considered due to mechanical constrains i n the range o f the measurement setup.

The harmonic oscillations are only performed f o r the conventional interceptor and f o r the transom section w i t h an edge radius o f 25 mm. The particular test conditions for these configurations are listed in Table 2.

Table 2: Harmonic oscillation test parameters for two

transom-interceptor configurations at a trim angle o f 3 degrees

Symbol Parameters Unit

Cv K f 2.0 [-] 0 to 20 [mm] -20 to 20 [mm] 0 . 5 / 1 . 0 / 2 . 0 / 4 . 0 [Hz]

The interceptor changes the pressure distribution underneath the hull over a certain distance forward o f the transom. A pressure increase due to an interceptor deflection has been schemaflcally presented in Figure 6. To isolate the added forces generated by the interceptors an additional series o f bare hull tests was performed. The difference i n the measured force w i t h one o f the interceptor configurations and the corresponding bare hull tests yields the total added force generated by the particular interceptor design.

Figure 6: Schematic pressure distribution o f the bare

hull (black) and pressure increase due to the interceptor (red)

The added forces are measured i n the body fixed reference frame. These measured forces are transferred into l i f t and drag forces according to:

L = cos T -I- sin T

D = Fy sin T — Fv cos r

(2)

The dimensionless l i f t and drag are obtained via the expressions:

(3)

2.3 S T A T I O N A R Y RESULTS

The l i f t and drag coefficients o f the three configurations are presented as a function o f the defiection height i n Figure 7 and Figure 8. The results o f all three forward speeds are indicated i n these figures. The l i f t and drag coefficients show a very close correlation at various forward speeds, on which basis one can conclude that the added forces due to the interceptor are proportional to the velocity squared.

The conventional interceptor configuration is only effective f o r positive deflections. Raising the interceptor blade above the bottom-transom intersection line w i l l not result in change o f the hydrodynamic forces since the transom type stem is f u l l y ventilated at these high forward speeds. However, this is not the case f o r the two other transom designs. I n these configurations the fiow adheres, up to a certain pomt, to the curved surface o f the round transom edge. This induces a downward or negative l i f t on the aft ship as can be seen fiom Figure 7. I t can be noted that the configuration w i t h the larger transom edge radius also generates a larger l i f t i n the downward direction.

-10 0 Dellection [mm]

Figure 7: Comparison o f the l i f t coefficients

The position o f the interceptor can be used to control the magnitude o f the added forces. The interceptor starts to have an effect on the l i f t when it is positioned at roughly 10 m m or 20 mm above the reference line, f o r the configuration with the 25 mm or 50 m m radius respectively. The deflection i n this position corresponds to approximately 40 % o f the particular transom edge radius. A further lowering o f the interceptor blade w i l l gradually increase the l i f t on the aft ship.

When the interceptor is exactly positioned at the reference line, three different l i f t coefficients for the

various configurations are obtained. For tlie conventional transom design no l i f t is being generated at this interceptor position, which may also be expected. I n case o f the two other transom sections, however, some additional upward l i f t is induced on the aft segment. Apparently, there is an increase in pressure caused by presence o f the recess between the interceptor blade and the rounded transom edge. The l i f t curves between the configurations at larger positive deflections are more or less similar in magnitude and shape.

From Figure 8 it can be seen that there is hardly any difference i n drag between the three configurations. For the larger negative defiections a minor decrease in drag is observed which may be caused by a small forward directed normal component on the transom, since there is no clean separation o f the flow.

-10 0 Dc-lleclion [mm]

Figure 8: Comparison o f the drag coefficients

These experimental results indicate that the new configurations are able to extend the range o f the l i f t force i n the downward direction by a significant amount. For the configuration w i t h a 25 m m radius a total range increase o f 30 % i n the l i f t has been achieved. In addition, the hydrodynamic characteristics at positive interceptor deflections remained largely unchanged with respect to the benchmark design. The range o f the 50 mm transom edge radius w i l l presumably be even bigger; however this could not be conclusively demonstrated due to a limitation o f the interceptor deflection i n the measurement setup.

2.4 H A R M O N I C O S C I L L A T I O N RESULTS

The results o f the harmonic oscillation tests w i t h the conventional interceptor are presented i n Figure 9. This figure shows the harmonic motion o f the interceptor and the measured l i f t expressed i n its dimensionless form for four different oscillation fi-equencies. Moreover, also a quasi-stationary approximation o f the l i f t is presented. The horizontal time scale is modified f o r each frequency to display four f u l l oscillation cycles.

0.3 0.2 0.1 0 -0.1 • / 1 / • / 1 / •^ \ / ' > ' * N \ / \ _{J /} / ^ ss \ \ • • f= 0.5Hz s \ • 40 £ 20 g 0.2 0.1 0 -0.1 0.2 0.1 0 • f=2Hz 40 E E f=4Hz , ) 0.2 0.4 0.6 Time [s] 0.8 1 40 e E

• measurement • • quasi stationary

Figure 9: Harmonic oscillation results o f the

conventional interceptor f o r various frequencies at 0.5, 1.0, 2.0 and 4.0 H z

For the lower frequencies a good correlation is achieved w i t h a quasi-stationary approximation. For the higher frequencies a small discrepancy is noticed. The magnitude o f the peak force is larger and it has a small phase lead with respect to the motion o f the interceptor.

Figure 10 presents a similar overview but this time f o r the new transom-interceptor configuration. Again, a good similarity between the quasi-stationary approximation and the measured results is obtained for lower oscillation frequencies and the same dynamic response tendency is observed for the higher frequencies. Here, the main interest was to determine whether the negative l i f t would also be developed to its f u l l extent during oscillatory motions o f the interceptor.

0.3 0.2 0.1 O

//

. :

i /

/ A

/

^

\\

• measurement quasi stationary

Figure 10: Harmonie oscillation results o f the new

transom-interceptor configuration for various frequencies at 0.5, 1.0, 2.0 and 4.0 H z

It may be concluded, based on the resuhs presented above, that a quasi-stationary approach gives a remarkably good approximation f o r the l i f t o f both configurations. Only f o r the rather high frequencies a deviation w i t h respect to the quasi-stationary method was seen. However, it is expected that this phenomenon is o f minor importance since the high frequency peaks contain a relatively small amount o f energy, through which i t only has a limited effect on the motion behaviour o f the ship.

The current experiments were set up to illustrate the principle idea o f this new configuration. More detailed investigations are necessary i n order to achieve a better optimized design. Follow-up experiments could, f o r example, be used to refine the curvature o f the transom edge to further improve the downward l i f t generating capacity.

3. S I M U L A T I O N S

A number o f sea keeping simulations has been performed to verity whether this new interceptor configuration w i l l improve the operability o f a fast ship i n waves. The hydrodynamic characteristics obtained during the experiments have been integrated into a dedicated ship

motion prediction program termed Fastship. This program can be used to predict the sea keeping behaviour o f fast ships i n irregular head waves and it has been extensively validated over the years as described in references [3] and [ 4 ] .

3.1 C O N T R O L S Y S T E M

The dimensionless l i f t curves are expressed by mathematical formulations and these have been implemented i n Fasthip. I t is assumed that the added forces can be scaled to fiill scale values using the dependent variables i n the equation (3). • The extrapolation o f the model test resuhs should however be subjected to further investigations. The current simulations are only carried out to illustrate the principle effect o f various control mechanisms on the motion behaviour o f the ship. It should not be considered as an attempt to exactly determine the appropriate f u l l scale dimensions o f the particular devices.

A quasi-stationary approach is adopted to approximate the l i f t o f the actively controlled interceptors, which seems to be a justifiable method based on the oscillation experiments. The defiection o f the interceptors is governed by a pitch velocity control feedback. Rijkens et al. [1] and Wang [5] found that this control scheme produces the most satisfactory resuhs among all feedback types. The pitch velocity motion o f the vessel is amplified by a control parameter Kg. The relation between deflection height o f the interceptors h and pitch velocity 6 is accordingly given by:

h^Kn (4)

3.2 M O D E L

I n the context o f this scientific application a standardized planing hull shape is used in the simulations. The selected model is the 25 degrees parent hull f o r m o f the D e l f t Systematic Deadrise Series p S D S ) . The ship length is scaled to 15 meters and its displacement is equal to 23.4 tons. Figure 11 shows the geometry o f the particular parent hull form. I n the simulations two interceptors have been modelled at either side o f the centreline having a width o f 0.78 m each.

3.3 I N F L U E N C E T R I M A N G L E

A proper calm water trim can greatly improve the operability o f a fast ship in waves. The comprehensive work o f Fridsma [6] indicated that a change i n trim angle f r o m 4 to 6 degrees can produce up to 100 % higher values for the vertical peak accelerations at the bow o f the ship. It is thought that the new transom-interceptor configuration w i l l be the most beneficial for designs that already underwent an optimisation w i t h respect to sea keeping behaviour and do not have an excessive trim angle at high forward speeds. To quantity the effect o f the t r i m angle on this particular hull design, several simulations i n regular head waves have been performed. The forward speed o f the model is 25 loiots and the wave height is 0.5 m . The longitudinal position o f the centre o f gravity is varied in order to change the calm water trim angle. From Figure 12 it can be seen that an increase i n t r i m angle from 3 to 6 degrees results i n a pitch response increase at resonance o f approximately 50%.

Figure 12: Pitch response for different t r i m angles

The vertical accelerations at the bow show an even stronger dependence on the reference t r i m angle o f the ship as is shown i n Figure 13. The difference between 3 and 6 degrees trim, gives roughly a 100 % increase o f the peak acceleration near the resonance region.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 L / J . l - l

pp ' '

Figure 13: Vertical acceleration response at the bow for

different t r i m angles

Based on the foregoing analysis a trim angle o f 4 degrees has been selected for the simulations i n irregular head waves. This position provides a good operational performance, while f o r the simulations at a t r i m angle o f 3 degrees some small instabilities o f the model are being noticed in the results.

3.4 S I M U L A T I O N S W A V E S

I N I R R E G U L A R H E A D

The sea keeping behaviour o f the ship is evaluated in irregular head waves f o r thi-ee different configurations. First a benchmark simulation without any control mechanism is performed which is followed by two actively controlled simulations, one with a conventional interceptor and the other with the new transom-interceptor configuration. The forward speed o f the vessel is 25 knots. The deflection o f the interceptor is governed by a pitch velocity gain coefficient oi Kg = 3 . The maximum deflection is limited to 30 m m and f o r the new configuration a minimum deflection o f minus 20 mm is defined. The waves are represented by a JONSWAP spectrum w i t h a significant wave height o f

Hs = 1.25 meters and a peak period o f Tp = 6.5 seconds.

Total simulation time equals 30 minutes. Figure 14 shows time traces o f the first 20 seconds o f these simulations. 10 Time [s] • Hcnclimark • Convcnlional inlcrccplor • N c « iransiim-intcRxplur connyuratlun

Figure 14: Comparison o f the time traces i n irregular

The resuhs w i t h the conventional interceptor clearly show a reduction o f the positive (bow-up) peak values i n the pitch motion signal compared to the benchmark. This could also be expected, while this configuration can only generate a positive l i f t on the aft ship as has been mentioned before. L o o k i n g at the results with the new transom-interceptor configuration both the positive as w e l l as the negative peaks i n the pftch motion are being reduced. A t around 16 seconds a typical wave impact occurs, that results i n a pronounced peak acceleration w h i c h is clearly distinguishable i n the acceleration time trace. The reduction o f these large peak acceleration values is o f foremost importance when it comes to improving the sea keeping behaviour o f fast ships [4]. A noticeable reduction o f the particular peak acceleration can be observed i n the actively controlled simulations.

Next, the statistical properties o f the simulations are analysed i n order to allow a more generalized assessment o f the sea keeping performance. The distribution o f vertical peak accelerations at the bow are visualised i n a Rayleigh plot presented i n Figure 15. I n this figure a comparison is made o f the probability o f exceedance o f a particular peak acceleration level between the three different configurations. The uncontrolled benchmark simulation clearly has the highest acceleration level, especially for the more extreme acceleration values. I f , for example, a comparison is made at the 0.5 % mark, a reduction o f 20% i n accelerations level is obtained using the conventional interceptor. A n even higher reduction o f almost 4 0 % is achieved when the new transom-interceptor configuration is used.

60 50 40 •S 30 S 20 10 Crests - Benchmark

Crests - Conventional imcrccplor

Crests - New transom-interceptor configuration

100 50 20 10 5 2 1 0.5 0.1

Probability orcwecdance | % |

Figure 15: Comparison o f Rayleigh distribution o f

vertical accelerations at the b o w w i t h and without active interceptors

C O N C L U S I O N S

This article describes an initial investigation to a new transom-interceptor configuration that was designed to generate l i f t i n both the positive and negative direction. The new configuration contains a round transom edge o f which it was believed that it was able to induce a

negative l i f t force on the aft ship, once the interceptor is raised some distance above the virtual transom-bottom intersection line. A systematic series o f model experiments was carried out and the results confirmed this presumption. Comparison o f the hydrodynamic characteristics between the new configuration and a conventional interceptor design clearly demonstrates the ability to produce a l i f t force i n the negative direction. Moreover, it was shown that the l i f t generating capacity o f the interceptor i n the positive direction was virtually unaffected by the transom modifications.

The hydrodynamic characteristics o f the control mechanisms have been implemented into a ship motion prediction program to investigate their possible merits on the motion behavior o f a fast ship i n waves. The simulations show that this new transom-interceptor configuration leads to a lower level o f the vertical peak accelerations w h i c h contributes to a more favourable sea keeping performance o f the ship.

A C K N O W L E D G E M E N T S

The authors w o u l d like to express their appreciation to Damen Shipyards for their kind permission to allow publication o f the experimental resuhs.

R E F E R E N C E S

[1] A . A . K . Rijkens, J. A . Keuning, and R. H . M . Huijsmans, " A computational tool for the design o f ride conttol systems for fast planing vessels,"

International Shipbuilding Progress, v o l . 58, no. 4,

pp. 165-190, A p r i l 2011.

[2] S. Brizzolara, "Hydrodynamic analysis o f interceptors w i t h CFD methods," i n Proceedings oj

the 7th International Conference on Fast Sea Transportation, Ischia, 2003.

[3] E. E. Zamick, " A nonlinear mathematical model o f motions o f a planing boat i n regular waves," David W . Taylor Naval Ship Research and Development Center, Techical report March 1978.

[4] J. A . Keuning, "The nonlinear behaviour o f fast monohulls i n head waves," Ship Hydromechanics Laboratory, D e l f t University o f Technology, PhD thesis 1994.

[5] L . W . Wang, " A study on motions o f high speed planing boats w i t h controllable flaps i n regular waves," International Shipbuilding Progi'ess, v o l . 32, no. 365, pp. 6-23, January 1985.

[6] G. Fridsma, " A systematic study o f the rough-water performance o f planing boats," Stevens Institute o f Technology, Davidson Laboratory, Technical report November 1969.