Type-4 Interactions: Post Wake-Surface Impingement

A fourth type of pressure signature is shown in Fig. 11.13(d). This type of signature generally occurs just downstream of the impingement of a tip vortex on the body and thus is called a post wake-surface impingement or Type-4 interaction. This signature contains certain features common to both Type-2 and Type-3 interactions; the basic saw-tooth pattern is present along with a transient loading, but with several additional pressure fluctuations. The amplitude of the dominant peak is also attenuated relative to the Type-3 interactions, a point made by Lorber & Egolf (1990). This is a very complicated sighature to predict because the flow hear the body surface under these conditions is more viscous dominated and turbulent as it interacts with the body surface boundary layer. This problem requires a more thorough understanding of the distortions and stretching of the vortex filaments during and after the wake interaction with the body.

Leishman & Bi (1994b) suggest that Type-4 pressure signatures can be further classified into two groups. One can be called a near-field, post wake-surface impingement interaction and the other can be called a far-field post wake-surface impingement interaction. The former occur immediately downstream of the surface location where the tip vortex impacts or at least comes very close to the body surface. The latter type occur some further distance downstream and contain high frequency fluctuations that are associated with turbulence in the boundary layer or separated flow. Unsteadiness also arises because part of the tip vortex that is very close to the body becomes unsteady (aperiodic with respect to rotor frequency) as it undergoes a complicated stretching and turbulent diffusion process (Fig. 11.18).