 Main
A Numerical Study of 2D Surface Roughness Effects on the Growth of Wave Modes in Hypersonic Boundary Layers
 Fong, Kahei Danny
 Advisor(s): Zhong, Xiaolin
Abstract
The current understanding and research efforts on surface roughness effects in hypersonic boundarylayer flows focus, almost exclusively, on how roughness elements trip a hypersonic boundary layer to turbulence. However, there were a few reports in the literature suggesting that roughness elements in hypersonic boundarylayer flows could sometimes suppress the transition process and delay the formation of turbulent flow. These reports were not common and had not attracted much attention from the research community. Furthermore, the mechanisms of how the delay and stabilization happened were unknown. A recent study by Duan et al. showed that when 2D roughness elements were placed downstream of the socalled synchronization point, the unstable secondmode wave in a hypersonic boundary layer was damped. Since the secondmode wave is typically the most dangerous and dominant unstable mode in a hypersonic boundary layer for sharp geometries at a zero angle of attack, this result has pointed to an explanation on how roughness elements delay transition in a hypersonic boundary layer. Such an understanding can potentially have significant practical applications for the development of passive flow control techniques to suppress hypersonic boundarylayer transition, for the purpose of aeroheating reduction. Nevertheless, the previous study was preliminary because only one particular flow condition with one fixed roughness parameter was considered. The study also lacked an examination on the mechanism of the damping effect of the second mode by roughness. Hence, the objective of the current research is to conduct an extensive investigation of the effects of 2D roughness elements on the growth of instability waves in a hypersonic boundary layer. The goal is to provide a full physical picture of how and when 2D roughness elements stabilize a hypersonic boundary layer. Rigorous parametric studies using numerical simulation, linear stability theory (LST), and parabolized stability equation (PSE) are performed to ensure the fidelity of the data and to study the relevant flow physics. All results unanimously confirm the conclusion that the relative location of the synchronization point with respect to the roughness element determines the roughness effect on the second mode. Namely, a roughness placed upstream of the synchronization point amplifies the unstable waves while placing a roughness downstream of the synchronization point damps the secondmode waves. The parametric study also shows that a tall roughness element within the local boundarylayer thickness results in a stronger damping effect, while the effect of the roughness width is relatively insignificant compared with the other roughness parameters. On the other hand, the fact that both LST and PSE successfully predict the damping effect only by analyzing the meanflow suggests the mechanism of the damping is by the meanflow alteration due to the existence of roughness elements, rather than new mode generation. In addition to studying the unstable waves, the drag force and heating with and without roughness have been investigated by comparing the numerical simulation data with experimental correlations. It is shown that the increase in drag force generated by the Mach wave around a roughness element in a hypersonic boundary layer is insignificant compared to the reduction of drag force by suppressing turbulent flow. The study also shows that, for a cold wall flow which is the case for practical flight applications, the Stanton number decreases as roughness elements smooth out the temperature gradient in the wallnormal direction. Based on the knowledge of roughness elements damping the second mode gained from the current study, a novel passive transition control method using judiciously placed roughness elements has been developed, and patented, during the course of this research. The main idea of the control method is that, with a given geometry and flow condition, it is possible to find the most unstable secondmode frequency that can lead to transition. And by doing a theoretical analysis such as LST, the synchronization location for the most unstable frequency can be found. Roughness elements are then strategically placed downstream of the synchronization point to damp out this dangerous secondmode wave, thus stabilizing the boundary layer and suppressing the transition process. This method is later experimentally validated in Purdue’s Mach 6 quiet wind tunnel. Overall, this research has not only provided details of when and how 2D roughness stabilizes a hypersonic boundary layer, it also has led to a successful application of numerical simulation data to the development of a new roughnessbased transition delay method, which could potentially have significant contributions to the design of future generation hypersonic vehicles.
Main Content
Enter the password to open this PDF file:













