Stabilization of hypersonic boundary layer waves using 2-D surface roughness

Abstract

Recent research has shown that 2-D roughness of finite height can damp the second mode instability under certain conditions. For instance, it has been shown that the relative location of 2-D roughness element and the synchronization point is important in determining the 2-D roughness effect on modal growth. It was found that if a roughness element is placed downstream of a disturbance's synchronization point, the disturbance is damped. In this paper, an extensive DNS parametric study on the effect of second mode instability from roughness height, width and spacing between roughness elements are conducted. The results reveal that a taller roughness within one boundary layer thickness height results a stronger damping effect on the second mode. On the other hand, the roughness width effect is relatively insignificant compared with the height effect. Lastly, the spacing between roughness elements has been studied. The results show the increasing in spacing of roughness elements has both advantages and disadvantages. For the second mode control using 2-D roughness elements, roughness elements spacing of 10 times the roughness width has been shown to have the best overall damping effect in a broad frequency bandwidth. Overall, our extensive study has shown that roughness height is an important factor for the amplification/damping effect. For a second mode control design using 2-D roughness, the roughness height of 50% boundary layer thickness, width of 2 boundary layer thicknesss and spacing of 10 have been chosen to optimize the damping mechanism. Ideally the laminar-turbulent transition process can be divided into four stages. The first involves small disturbance fields which are initialized via a process termed "receptivity" by the viscous flow. The initial disturbance fields can involve both freestream and vehicle self-induced fluctuations such as acoustics, dynamic vortices, entropy spottiness, etc. The next stage is the linear growth stage, where small disturbances are amplified until they reach certain amplitude where nonlinear effects become important. The amplification can be in the form of exponential growth of eigenmodes (Tollmien-Schlichting waves or Mack waves) and non-modal growth of optimal disturbances (Transient growth). Once a disturbance has reached a finite amplitude, it often saturates and transforms the flow into a new, possibly unsteady state, which is termed as the secondary instability stage. The last

Second Mode Suppression in Hypersonic Boundary Layer by Roughness: Design and Experiments

“…The roughness height in the current design is 50% of the local boundary-layer thickness, and the width is two times the local boundary-layer thickness. This particular roughness size is chosen because, in the previous study by Fong et al [19], this roughness size has shown effective damping effects on the second mode. On the other hand, based on the fact that the local boundarylayer thickness is constant, the size of each roughness element is the same.…”

“…Zhong's group argues that 2-D roughness elements can damp 2-D disturbances if the roughness element, with a height less than the local boundary-layer thickness, is downstream of the synchronization point [14]. This finding has motivated a series of parametric studies on roughness effects including roughness locations, heights, and widths by Fong et al [17][18][19]. All results are consistent with the initial finding that 2-D roughness elements can damp 2-D disturbances and have shown the importance of roughness locations and synchronization locations.…”

Assessment of the Impact of Two-Dimensional Wall Deformation Shape on High-Speed Boundary-Layer Disturbances

“…It was found that a wavy surface with the wavelength equal to twice the boundary layer thickness delayed the transition onset. Duan et al [19] and Fong et al [15,16,17,18] in a series of studies investigated the effect of two-dimensional roughness on the instability of the second mode (or mode S) by direct numerical simulations (DNS). Their numerical results proved that the roughness located at the downstream of the synchronization point is able to stabilize this mode.…”

Characterization of Structural Response to Hypersonic Boundary Layer Transition

“…[9][10][11][12][13][14] These tools have enabled broad study on hypersonic boundary layer transition, including geometry changes, 10,11,15,16 operating conditions, 10,11,15,17 mass injection, [16][17][18][19] surface temperature perturbations, 16,18 surface roughness, [20][21][22][23][24][25][26][27] and structural deformations. 28,29 Regarding the latter, it was found that deformed surfaces have a strong influence on flow stability.…”