flow structure.* Of these, the long chain 'polyox' polymer has received 

 much attention in the hydrodynamic community for its astounding effect on 

 turbulent drag and heat transfer reduction. It would be surprising, in a 

 way, if there were not an equivalent effect for cavitation. We now take up 

 some of the features briefly, but with primary reference to cavitation. 



Transition and Cavitation 



Here we need to have some idea of the location of "natural" transition 

 of attached boundary layers to determine the "local" pressure needed for a 

 cavitation- inception measurement. This is a basic "reference" problem in 

 fluid mechanics that does not appear yet to be in a satisfactory state 

 (Mack 1978) . At best, there appear to be semi- empirical methods based upon 

 the use of linear stability theory. In its present form, the concept, as 

 explained in the comprehensive report by Wazzan, Okamura, and Smith (1968), 

 is to calculate the factor by which certain disturbances in the laminar 

 boundary layer may be amplified. As an empirical observation, transition 

 may be said to occur when this factor reaches a certain level. The 

 difficulties and objections to this approach are reviewed at length by 

 Reshotko (1976) and later by Mack (ibid) and Van Ingen (ibid). These 

 center on the fact that the ambient unsteady disturbances in the flow are 

 largely unknown, and that the "receptiveness" of the boundary layer to 

 these disturbances is not known either, a concept due to Morkovin (see 

 Reshotko 1976) . It does not necessarily follow that transition is due only 

 to growing, unstable two-dimensional waves; this process may be circumvent- 

 ed by a roughness element, for example. Nevertheless, this method, the e 

 method, as it is called, has value in comparing different flows in a fixed 

 environment, and is in any case the most quantitative method now available. 

 For example, simplified prediction schemes based on these stability methods 

 can now readily account for transition of heated water boundary layers 

 (Wazzan and Gazley 1978) . 



With the linear stability method of Smith, Wazzan, and coworkers, it 

 is readily possible to calculate the transition location (or the start of 



^Suspended 'dust' particles may have some similar effects in gas flows. 



32 



