Numerical investigation of coherent structures in plane and curved wall jets

Turbulent wall jets over convex surfaces (Coanda flows) and turbulent plane wall jets are investigated using 3-D Navier-Stokes simulations. By comparing the two flows, the effect of wall-curvature on the development of coherent structures that are present in the flow field can be isolated. These coherent structures, namely large spanwise "rollers" and longitudinal G̈ortler-type vortices, are likely to influence the development of the Coanda wall jet along the surface of a cylinder and its eventual separation from the cylinder surface. The simulations show that both types of vortical structures are naturally amplified in the Coanda flow. When forced, the longitudinal structures reach amplitudes large enough to suppress the formation of spanwise rollers. In contrast, for the plane wall jet, longitudinal vortices form only in response to forcing and they exhibit a weaker growth in downstream direction. As a consequence, they are unable to prevent the spanwise rollers from developing. This indicates that the G̈ortler-type centrifugal instability does contribute to the growth of the longitudinal structures in the Coanda flow. The flow fields are analyzed using Proper Orthogonal Decomposition (POD) to extract time-dependent coherent structures from the flow. Progress in the development of the Flow Simulation Methodology (FSM) for computing the entire Coanda flow geometry is also discussed.