Report No, 232,
LABORATORIUM VOOR
SCHEEPSBOUWKUNDE
TECHNISCHE HOGESCHOOL DEIFT
E
SOME NOTES ON THE DISCREPANCIES BETWEEN THE LATERAL
MOTIONS OF OSCILLATION TESTS AND FULL SCALE MANOEUVRES.
by
G. van Leeuwen.
L
Prepared for the Planar Motion Mechanism post graduate course,
A.EW., Haslar.
Contents.
Summary,
1.The motiyation of horizontal oscillation tests. 2
2.Horizontal oscillations due to rudder forces. 2
3.Consequences of small path amplitudes.
).Some remarks on the range of frequencies II
5.An example of a large amplitude pmzn, 5
6.Ashort review of the motives for a large amplitude pmm! 5
T.Final remark. 6
Summary.
Some aspects of the present horizontal oscillation testing technique are considered.
The discrepancies between full-scale manoeuvres and the motions of the shipmodel during such tests are discussed.
On this connection the advantages of a large amplitude planar motion mechanism are considered while the general arrangements of such an oscillator in development for the Netherlands Ship Model Basin are
shown.
-2-I
The mtivation ¿f hrizontals1lt
iontests.
In the past ten years horizontal oscillation tests have proved to be useful with respect to the research on manoeuvrability qualities of ships. Especially the determination of the coefficients of non.-linear terms in the equations of motion appeared a means to get a better understanding of the non..1inear phenomenae which play an important part in manoeuvring.
Fro&one point of view we might say that the horizontal oscillation
tests are descended from the vertical oscillation tests, the origin of which is found in the oscillating motions of a ship in waves. With respecj to the horizontal motions of a ship the relationship is not clear. Though a ship on a straight course will have a slight oscillating motion due to the small disturbances of wind and waves
or to the action of the helmsman, this motion can hardly be the motivation for our horizontal oscillation tests. Simulation of such motions on model scale would result in unmeasurably small forces
and
besides, nonoticeable non-linear effects would occur.
Oscillating horizontal motions with somewhat larger course- and path deviations are found in a heavy sea, but also these motions are not the motivation for horizontally oscillating. Simulated on model scale these rough-wheather motions would only provide information about the ship among these specified circumstances.
So, the application of horizontal oscillation tests for the pu.rpose of determining the coefficients of a set qf equations to describe the various kinds of manoeuvres is not self evidente it rather seems
to be an imitation of th research on pitch and heave motions.
.,' ' -.
2.Horizontal oscillations dueo rudder forces.
Considering the oscillatory motions which a ship may have due to rudder forces only, a very rough estimation of these motions is sufficient to show that they do not resemble in any sense the forced motIons we give
our
models during horizontal oscillation testsThis will be considered in greater detail.
Provided the frequencies used in oscillation tests cover a Bo]newhat realistic range, then two typical phenomenae must be corisided,
-3-1.The path-amplitud.es, which we choose are in general based upon the consideration that for the highest frequency we can just attain
the non-dimensional yawrate the ship has in a turning circle
test with maximum rudder deflection.
2.Tests with smaller yaw-rate amplitudes are performed with lower frequencies and nearly the same path amplitude.
Figure 1 shows a typical force plot of yawing tests.
Performing sinus-response tests we see just the reverse:
1,The large yaw-rates are found for low frequencies while the path amplitudes amount to s&veral ship lengths.
2.For faster rudder movements, which means higher frequencies the amplitudes of the path and the yaw-rate decrease to very small values,
In figure 2 we see three typical paths of a ship during mus-response
tests with the same low frequency; in the upper diagram with small and in the lower with large rudder deflection. For the purpose of comparison figure 3 shows the path of a ship with a high frequency and large rudder angle.
As is generally known, for oscillation tests we use path amplitudes which do not exceed some tenths of the model length and though the relation between frequency, rudder angle and width of the path is different for
each ship, we are sure that in the requency range in which for sinus-response tests the yaw-rate is the greatest, the width of the path will be many ship lengths.Figures 1 and 5 illustrate these facts.
Up to now it is an unanswered question to what extent the difference between realistic path amplitudes and those used in oscillation tests disturbs the prediction of full scale manoeuvres - for instance frequency response manoeuvres-. It is not very likely however that it will have no
influence, Indeed tests have been carried out with variable path-amplitudes for
instance
from five percent to ten percent of the model length, but ifthe right amplitude should be ten ship lengths it is rather risky to decide that there is no significant amplitude influence if it is not found. in such a small region,
13
r.5
-3. Consequences' of srnal1
tnpìtudes,
I
The consequence of taking too small a path amplitude is that the accele-rations are some hundreds of percents too large. In general we neglect this and for the determination of the inertia coefficients we only use about ten percent of the total acceleration range. A rough estimation of the maximum values o the non dimensional angular acceleration a ship
can have due to the rudder, gives a value of
.5
to i while for oscillation tests this quantity is about ten times larger:With respect to this phenomenae we may also formulate the problem of
the path amplitude in the following way To what extent are turning-and. lateral resistance influenced, by the instantaneous accelerations?
N N
,'
'
-1L.Some remarks on the rane of frequencies.
We just supposedthatve had a somewhat realistic range of freqencies To find out what we have to understand bya "realistic range" may aleo
follow from the frequency-response tests.
The highest frequency a ship can have, due to the rudder, follows from
the rate of turn of the rudder. Provided this is
2.5
degrees per secondthe maximum nondimensional frequency will depend on the length and the
forward speed of the ship.
In figure 6 the rudder movement is approximated. by triangles. Using this
approximation table I was made in which the non-dimensional maximum
frequencies are given for three ships and three speeds each, The quantity 2ir/w' denotes the number of shiplenghts sailed during the time of one oscillation.
It needs no argument that these frequencies must be considered very high, nor that the motions of the ship differ very much from a steady
turn.
Independently from the rudder rate to be reached we may fix a limit on the frequency, considering the quantity 2ir/w', We can propose that then
this quantity is more than four, the frequency is too high to may
expect that the results of such tests will contain employable information for the prediction of "normal" manoeuvres, taking into account that
each mathematical model has its limits of application.
5
In figure 7 a sketóh of an extreme manoeuvre is given, illustrating the case 2ir/' = 13.s
5
5.An
e'ample of a large amlitude PJVfi4, in develomeit for theNè1vffi at Wageningen.It is true that simulated freq.uency response tests with a PMM make high demands upon this facility, particularly when we want to apply it to determine non-linear effects. Anyhow if we have the occasion to build a PMM which allows amplitudes, much larger then some tenths of a model length, for instance ten or twenty feet, this may help to split up the concept "scale effect" in a wide sense- into details, which can more easily be understood.
Some time ago the management of the NSMB asked to make a proposal for a new horizontal oscillator because the facilities of the present one were
too few, taking into account the kind of tests which had to beperformed.
Though in a first stage the NSMB-management was thinking of a usual -whiéh means: small amplitude- oscillator, the instruction was given to design the main lines of a large amplitude P1M the range of which could be
3.75
meters.In figure 8 a sketch of the mechanical part is given. Herein is:
a:the centre wheel which makes the model turn around its centre of gravity b:electromotor, electronically governed, to adjust the yaw rate wanted c:sub-carriage, movable in longitudinal direction to provide an
adjustable speed along the path
d:carriage movable ma direction perpendicular to the axis of the towing tank
e:oscillator frame, which might be an independent carriage, but also attached to the present towing carriage.
All movable parts are to be governed electronically. The employable width
has been estimated to be one half of the tank width, being 15 meters. This was done naturally to avoid tank wall effects.
6.A
short review of the motives for a large aitude.rn.m.The main thoughts behind the large amplitude p.m.m. may be summarized as follows:
the motions of the model during oscillation tests should resemble as much as possible the full scale motions.
this means that the non-.dimensional velocities and accelerations are equal for ship and model.
the frequencies used should cover a realistic range so as to avoid typical frequency effects.
6
d.The forces to be neasured should be generated by introducing relatively small deviations with respect to the estimated full scale manoeuvres, This means that uncoupling of the sway- and yaw motion is avoided.
e.An eCectronicaliy governed large amplitude p.m,nl. provides the
possibility of performing quasi-free running model tests. The forces on the measuring devices will be zero or at least minimum.
7.Final remark.
The results of the I,T.T,C..-Mariner model tests showed that the various institutions found considerable differences even in the linear
hydrodynamic coefficients. On the one side this will be caused by the
differences in model size but also the choice of different ranges of noidimensional frequencies and path. amplitudes will play an important role. It would. be interesting to repeat these tests in such a way that the testing method and the ranges of nondimensional variables were bindingly prescribed.
FORCE
I
i,2iZ
2ruoL
INCREASING FREQUENCY
(NEARLY CONSTANT WIDTH OF PATH)
.rr0
\ U0dt
FIGURE 1
FORCE MEASUREMENTS OF
YAWING-OSCILLATIÖN TESTS
I
y
ONE SHIPLENGTH
X
X
FIGURE 2
LOW FREQUENCY PATH-CHARACTERISTICS
X
y
FIGURE 3
WIDTH OF
PATH
AMPLItUDE
OF
YAW-RAT E
FREQUENCY RESPONSE TESTS
OSCILLATION
TESTS
FIGURE 4
FREQUENCY RESPONSE TESTS
OSCILLATION
TESTS
w
FIGURE 5
DISCREPANCIES BETWEEN REALISTIC SHIP
s
(w&s 7112)
37
m
3.75 ni.path amplitude
15.00 meters
width of the towing tank.
O
SHIP LENGTH
FIGURE 7
EXAMPLE OFAN OSCILLATING SHIP
RUDDER-ANGLE
Q
