A new technique for use in ship model tanks

A NEW TECHNIQUE FOR USE IN SHIP MODEL TANKS BY

T.S. RAGHURAM AND S.D. NIGAM.

Contribution to the discussion by A.J.W. Lap.

The Authors have drawn attention to a. new possibility to carry out model tests at low Reynolds numbers. They have

presented a number of results obtained according to this new method, from which a number of conclusions are drawn which the

present writer would like to comment.

In the first place it must be mentioned that the new technique is much more complicated than the usual one of simply accelerating a model to a wanted speed and then measuring the resistance at that

speed,. The new method will therefore be subject to a number of experimental errors not occurring during normal tests,which are partly mentioned in the Authors' conclusion nr. 3. It is possible, however, that not all disturbing effects are known. In this respect

one might consider the primary wave system which consists of

relatively long and very low waves. Are these waves, generated during the AD-period, damped out sufficiently to evade all disturbing

influences ? At shallow water it has been found that this primary wave system is still present in the tank long after all the shorter waves of the secondary wave system have disappeared.

There is no doubt that, when applying the AD-method,turbulent flow is stimulated during the deceleration period. The advantage

of the method is, that in principle the stimulation takes place over the whole body. A disadvantage is, however, that after the deceleration there is no stimulation at all, whereas the, current turbulence stimulators are effective during the whole run. The question must be put, therefore, whether the stimulation of the AD-method is still effective sufficiently long after the stable

condition during which the measurements have to be taken is

established. Could the Authors supply therefore some data on the time elapsed between the end of the deceleration and the beginning

of the measurements as well as data on the effect of the length of this period on the results of the measurements ?

In the writer's opinion the results of the paper do not prove that the AO-method leads to better results than can be obtained with ordinary stimulators. From unpublished experiments in the

Delft towing-tank it has appeared that turbulent flow _{can in most} cases be attained at speeds higher than 9,50 m/sec, independent

-20 -

--of model size-This result is in agreethent with the experience --of the N.S.M.B. as well as with _{a publication of the N.P.L. [1]}

where a minimum speed of 1i ft/sec is given for obtaining turbulent flow. At a water temperature of 150 C a speed of 0,6 m/sec leads to the following Reynolds numbers for which turbulent flow can be obtained with models of different size:

Model length _{Log Rn}

30 ft 6,68 24 ft _6,58 20 ft _6,51 16 ft _6,41 12 ft _6,28 9 ft _6,16 6 ft V 5,98 4i ft V 5,86 4 ft 5,81 3 ft _5,68

That these values can actually be reached _{can be seen in the} original B.S.R.A. paper [2) _{as well as in a paper by Troost and} Zakay [3] where results are given of tests with 6 ft and 4 ft models of the Lucy Ashton. These results are in very good

agreement with the results from the bigger B.S.R.A. _models.

The values given in the table are also in very good agreement with a publication by Nordstrom and _{Edstrand [4] .}

If therefore the Authors claim that the AD-Method is applicable for Reynolds numbers of from 106 onwards _{this does not mean an}

improvement which justifies the complicated _{experimental technique.} On the other hand the writer doubts whether the experimental data presented in this paper are already sufficiently reliable. The results of the A3)-iuns _{are not only}Vjfl _{complete disagreement} with the results of the 9 and 12 ft B.S.R.A. models, but also with the results of the 12 ft, 16 ft and even the 20 ft B.S.R,A. models. The writer can therefore not accept conclusion 2 of the Authors since this would mean that practically all the B.S.R.A. tests, carried out extremely carefully with and without stimulation would be erroneous as well as the results of Troost and Zakay.

From the above it will therefore be clear that also the

x) ( i.e. constant Reynolds number for the

writer does:not:agree

with -Conclusion

5.:-In his -opinion,

necessary . to .eicaniine, the possibilities of.._.tileyneW'' teChnique':

-more cloaelyHO*,., for instance, do the

_{Aut-hore'.:COfficludettiiat.::}

in certain ;Cases. the 'flow is turbulent during..-:AP',-runs

turbulent during the corresponding

_{.-normal. runs}

_{. OA that the}

_ . . .

..,_ ._ ...:

. . . ,

existance., Of '6,- difference:.

_{in resistance is not}

_sance

_{,'-'n. simply}

l

_caused

. . . , . ,

-.

. _{_:.}

by,;.ekperitental.-eir'or .?:'71100-e and other:

_{questions ,Will.:,.htive.'t_}

be:answered .before thel....A.b.,teC)4214:4!le'-',.:- _Can

_6:6Cepted

method for turbulence stimulation.

_{: POnciUding the Writer #64)66}

that this criticism will not discourage

_{the Authors and that}

they will 'go on with their

_{investigations into this difficult}

problein.

.. .

Troosti:.-L.. and Zakay:i

:.:

_{".A new evaluation of the "Luc}

:Ashton"7--,02i,p:':VICiael and lull ::.Scale

-eStd,"

Int'. ' Shitb. Progr.;.,

' 1954.

Nordstrom-; -H.P.. and Edstrand., H. "yodel -Tests, with Turbulence

and

_{"Turbulence Stimulation on Ship}

-Models", Trans. S.14,..A

1951.-Iiackenby, H. and'W.P..:

.

Resistance Experiments on the

-"LuC-3t: Itsiitoti"

Part

.

:producing _Devi:c6s",

rof the

Swedish Sti.t.te'''Siiipbuilding:.

'Experimental: :Tank,