A series of low-speed swept-wing stability and transition studies beginning in the 1980s have been conducted by Saric, Reed and coworkers (Saric et al. 2003). This provided a near-complete picture of the crossflow instability including the importance of leading-edge micron-sized roughness elements, the early importance of nonlinearities, amplitude saturation at high amplitudes, and other factors as described above.
It is proposed to advance the UAC methodology using combined theoretical developments, numerical simulations and experiments to optimize porous structures. These studies will address a number of factors concerning the UAC approach. (1) The UAC coatings must be demonstrated to be “aerodynamically smooth” so that micro-cavities do not trip the boundary layer. (2) In deriving the boundary conditions on the UAC surface, each pore is assumed not to interact with others and to be infinitely deep. These assumptions must be examined and, if necessary, corrected. (3) The UAC structure needs to be optimized within modern TPS constraints. (4) UAC effects on the nonlinear phases of transition need to be investigated theoretically and experimentally. Bountin et al. (2004) have shown that the harmonic resonance, which is quite pronounced in the latter stages of the disturbance evolution, can be completely suppressed by the porous coating so the porous coating may play multiple favorable roles. Finally, (5) the theoretical and CFD methodologies developed during these investigations must be integrated into a toolbox suitable for design of UAC TPS articles. Prof. Zhong’s group has already performed preliminary DNS studies that will form the basis of the simulation efforts.
To examine the stability of particular spanwise vortex wavelengths, experiments often feature spanwise arrays of discrete 3-D roughness elements to trigger stationary crossflow vortices. (This approach is planned for the proposed hypersonic experiments.) Saric discovered that the nonlinear behavior of the stationary crossflow vortices in distorting the mean flow profile could be exploited to control the crossflow instability. By designing a favorable pressure gradient to be subcritical to T-S growth and promote early growth of crossflow wavelengths shorter that the predicted most unstable (critical) crossflow, we discovered that passive nonlinear biasing of stationary crossflow wave growth near the attachment line can maintain a laminar boundary layer. Placing spanwise-periodic discrete roughness elements (DREs) at wavelengths shorter than the (predicted) critical wavelength at the leading edge, causes these shorter waves to grow just enough upstream to mildly distort the basic-state boundary layer without causing transition and then they decay. The (predicted) critical crossflow disturbance does not appear and the flow remains laminar. Over the past decade, Saric and co-workers have conducted various wind tunnel and flight experiments, and demonstrated that the basic mechanism is valid in subsonic and supersonic flow but that increasing care is needed in placing and sizing roughness elements as the Reynolds number increases (Saric et al. 2004, 2008).
It is proposed to study DRE effectiveness for hypersonic crossflow control on our slender cone modes at various angles of attack. This will require careful coordination between the various theoretical approaches, DNS, and receptivity experiments, leading to the eventual control experiments in the M6QT and possibly the M3.5QT. As with the UAC approach, a thorough understanding of the instability physics is required to implement a robust control technique.