Circular cylinder in subsonic cross-flow at moderate Reynolds numbers: free and near-wall effects
Hanson, Jack Adrian
The flow around a cylinder is a crucial engineering approximation and is well studied for a range of Reynolds numbers. However, the research into the effects of compressibility has lagged behind as a recent paper notes (Nagata et al., 2020) that the low sub-critical Reynolds numbers are unexplored. Furthermore, the investigation of the interactions between a cylinder and wall has been gaining further attention in the last 50 years, however, compressible research in this area is extremely limited. With the advent of thin-atmosphere flight where Reynolds numbers are low and flight speed is appreciable, there is a growing need for this fundamental engineering approximation to be properly resolved. This thesis tackles the range of 400 < ReD < 800 in a subsonic flow, 0.20 ≤ Ma ≤ 0.45, for both the free and near-wall cases to fill the gaps identified in the literature for this critical range of Reynolds numbers. This is achieved using a high-order, quasi-spectral finite difference method to solve the compressible Navier-Stokes equations. Firstly, the case of a 3D cylinder in a free flow is investigated with good agreement with the incompressible literature at Ma = 0.20. At ReD = 400 it was found that compressibility has a limited impact on the flow and the drag followed the 2D model. At ReD = 800 however, increasing Mach number caused 2D flow features to strengthen and resulted in a 3 fold increase in shedding intensity as well as a significant increase in drag over what the model predicts. This is followed by a larger parametric study of the near-wall case for gap heights of 3 to 1 exposed to a boundary layer with thicknesses greater than 1 diameter. For ReD < 700, the effects due to Mach number were limited and these effects were removed entirely upon immersion in the boundary layer. However, at G/D = 1.5, ReD = 800, increasing the Mach number led to an 8.5% increase in cylinder drag, a strong negative lift and an 8% increase in the shedding frequency. It was shown that in this case, the wall had the combined effect with the Mach number of inducing strong circulation due to accelerated flow in the gap, further decreasing the base pressure and shortening the formation region. These effects were significant enough to cause the excitation of vortex shedding as well as the increased drag. Finally, this unique case was studied in 3D and it was found that there was a drag increase, a drop in base pressure and a shortening of the formation region due to wall proximity. However, these changes were smaller in comparison to the 2D results. Furthermore, there was no reversal in the lift, rather a stronger positive lift was generated. Although shedding intensity increased due to wall proximity, there was no discernible trend in the shedding frequency. It was demonstrated that the longer formation region in 3D was preventing larger pressure decreases. The longer shear layers move high speed fluid further downstream, keeping the circulation lower and preventing the low pressure in wake from interacting strongly with the rear of the cylinder.