Oscillations of a turbulent jet incident upon an edge

For the case of a jet originating from a fully turbulent channel flow and i mpinging upon a sharp edge, the possible onset and nature of coherent oscil lations has remained unexplored. In this investigation, high-image-density particle image velocimetry and surface pressure measurements are employed t o determine the instantaneous, whole-field characteristics of the turbulent jet-edge interaction in relation to the loading of the edge. It is demonst rated that even in the absence of acoustic resonant or fluid-elastic effect s, highly coherent, self-sustained oscillations rapidly emerge above the tu rbulent background. Two clearly identifiable modes of instability are evide nt. These modes involve large-scale vortices that are phase-locked to the g ross undulations of the jet and its interaction with the edge, and small-sc ale vortices, which are not phase-locked. Time-resolved imaging of instanta neous vorticity and velocity reveals the form, orientation, and strength of the large-scale concentrations of vorticity approaching the edge in relati on to rapid agglomeration of small-scale vorticity concentrations. Such vor ticity field-edge interactions exhibit rich complexity, relative to the sim plified pattern of vortex-edge interaction traditionally employed for the q uasi-laminar edgetone. Furthermore, these interactions yield highly nonline ar surface pressure signatures. The origin of this nonlinearity, involving the coexistence of multiple frequency components, is interpreted in terms o f large- and small-scale vortices embedded in distributed vorticity layers at the edge. Eruption of the surface boundary layer on the edge due to pass age of the large-scale vortex does not occur; rather apparent secondary vor ticity concentrations are simply due to distension of the oppositely signed vorticity layer at the tip of the edge. The ensemble-averaged turbulent st atistics of the jet quickly take on an identity that is distinct from the s tatistics of the turbulent boundary layer in the channel. Large increases i n Reynolds stress occur due to onset of the small-scale concentrations of v orticity immediately downstream of separation; substantial increases at loc ations further downstream arise from development of the large-scale vortici ty concentrations. (C) 2001 Academic Press.