Effects of periodic excitation on the flow around A D-shaped cylinder at low reynolds numbers

The flow around a bluff cylinder with half-elliptical D-Shape cross section was investigated using 2D numerical simulations of the Navier-Stokes equations at low Reynolds numbers. The Strouhal-Reynolds numbers correlation, the mean drag, the lift and drag oscillations dependence on the Reynolds number were investigated and compared to circular cylinder data. It was found, that the D-Shaped body causes smaller resistance and disturbances than circular cylinder of the same diameter. Wake studies, at a Reynolds number of 150, examined the influence of zero-mass flux slot excitations, located at the separation points, for various actuation amplitudes/frequencies were performed. The slot excitation influence on the drag and lift forces was studied. The evolution of a single vortex, that is part of the von-Karman wake, and excitation influence on its evolution were also investigated. Good agreement with theoretical viscous vortex models was found. The formation and convection regions of the vortex evolution were documented. Proper Orthogonal Decomposition (POD) analysis of the near wake flow field was performed. The influence of the bluff body shape (circular versus "D" shaped) on the POD modes in the wake was found to be small. The effect of periodic excitation on the shape of the velocity and vorticity modes and on the corresponding time coefficients was also studied. It was found that the Eigenfunctions (mode shapes) of the POD velocity modes are less sensitive to slot excitations than the vorticity modes. As a result of the excitation, two types of mode shape changes were observed. The mode shape can be exchanged with lower energy mode or shifted to low energy level. The influence of one slot excitation operating in otherwise still fluid was studied and compared to published data and to "actuator" modes with external flow present. Based on these findings it is hypothesized that the vorticity field is the more suitable parameter for closed-loop flow control using POD modes for cases without external flow, i.e., for controlling "synthetic jets". On the other hand, the vertical velocity POD modes are more suitable for feedback control of wake flow using periodic excitation.