ORGANIZED NATURE OF FLOW IMPINGEMENT UPON A CORNER

Oscillations of impinging flows, which date back to the jet-edge phenomenon (Sondhaus 1854), have been observed for a wide variety of impingement configurations. However, alteration of the structure of the shear layer due to insertion of an impingement edge (or surface) and the mechanics of impingement of vortical structures upon an edge have remained largely uninvestigated. In this study, the impingement of a shear layer upon a cavity edge (or corner) is examined in detail. Water is used as a working fluid and laser anemometry and hydrogen bubble flow visualization are used to characterize the flow dynamics. Reynolds numbers (based on momentum thickness at separation) of 106 and 324 are employed. Without the edge, the shear layer produces the same sort of non-stationary (variable) velocity autocorrelations observed by Dimotakis & Brown (1976). When the edge is inserted, the organization of the flow is dramatically enhanced as evidenced by a decrease in variability of autocorrelations and appearance of well-defined peaks in the corresponding spectra. This enhanced organization is not locally confined to the region of the edge but extends along the entire length of the shear layer, thereby reinforcing the concept of disturbance feedback. Comparison of spectra with and without insertion of the edge reveals a remarkable similarity to those of a non-impinging shear layer with and without application of sound at a discrete frequency (Browand 1966; Miksad 1972); with enhanced organization at the fundamental frequency, simultaneous enhancement occurs also at the sub- and higher-harmonics. Visualization of the vortical structures in the vicinity of the impingement edge shows that an impinging structure may experience one of three possible events: complete clipping, whereby the structure is swept down into the cavity; partial clipping, which results in severing of the vortex; or escape, involving deformation of the vortex while it is swept (intact) downstream past the edge. In general, no one of these events persisted continuously over a long period, but tended to occur alternately, meaning that “jitter” of an impinging structure occurs. Plots of paths of these structures versus time showed that the convective speed of the vortex was locally influenced a distance of about four momentum thicknesses upstream of impingement, which is less than the estimated diameter of an impinging vortical structure. Furthermore, this upstream influence of the edge is also evident in the distributions of transverse velocity. Laser measurements indicate that the presence of the edge substantially increases the local value of transverse velocity fluctuation in the region immediately upstream of the edge. © 1979, Cambridge University Press