56 



must "be taken that the Reynolds number is high enough so that similarity in 

 the flow patterns is appr mately obtained . This is especially of importance 

 in tests of models on which separation may occur at the lower Reynolds numbers. 

 In such cases, cavitation in separated zones on the model may be misleading 

 since this zone may be eliminated at the higher Reynolds numbers. Usually, it 

 is possible to determine whether this will be the case by running the model 

 test over a range of Reynolds numbers to determine the minimum value for which 

 changes in the resistance, lift, or pressure distribution disappear. 



In cavities in which the gravitational acceleration plays an impor- 

 tant role as in air-water entry of missiles, the Froude number is an addition- 

 al parameter of scaling. For this type of cavitation, it can be shown that for 

 dynamical similarity, the ratio of the mass of gas to liquid must be modelled 

 as well. Thus, for similarity in the test of an air-to-water entry, the tests 

 must be performed under reduced pressure but with a gas heavier than air. A 

 comprehensive discussion of such modelling is given in Reference 56. 



SOME REMARKS ON DESIGNING FOR CAVITATION PREVENTION 



METHODS OF REDUCING ADVERSE EFFECTS OF CAVITATION 



Although it is generally agreed that attempts should be made to elim- 

 inate the possibility of cavitation in hydraulic systems, this is not always 

 possible without excessive costs and difficulties. In such cases, methods 

 have been developed for reducing the adverse effects of cavitation and much 

 work has been done on selection of materials which are resistant to cavitation 

 damage (see, for example, References ~[6 , 87, and 88). To prevent excessive 

 damage in hydraulic systems, the use of air injection into the cavitating re- 

 gion to "cushion" the impacts at collapse has been adopted in many cases. ' 

 A striking illustration of the effect of small air contents on cavitation 

 damage to concrete is shown in tests made by the Bureau of Reclamation, Fig- 

 ure 33- It is seen that the damage is considerably reduced with increasing 

 air content. Another method proposed for amelioration of cavitation damage 

 is the use of "water cushions" 7 on the assumption that if a layer of water is 

 maintained between the cavitation and the solid, damage will be minimized. 



PREDICTION OF MINIMUM PRESSURE POINTS 



In designing hydraulic systems, the most important information 

 needed in investigating the possibility of cavitation is a knowledge of the 

 distribution of pressures. Primarily, it is required to know at what points 

 on the boundary minimum pressures occur and, thus, at what points cavitation 

 may be expected. For most engineering applications, the minimum pressure 



