Smooth leading edge transition in hypersonic flow

The boundary layer transition along the attachment line of a smooth swept circular cylinder in hypersonic flow is investigated in a blowdown wind tunnel. A wide range of spanwise Mach numbers Me (3.28 to 6.78) is covered with the help of different models at several sweep angles (60°?Λ?80°). The transition is indirectly detected by means of heat flux measurements. The influence of the wall to stagnation temperature ratio is investigated by cooling the model with liquid nitrogen.     

Slope seeking for autonomous lift improvement by plasma surface discharge

The present paper describes an experimental investigation of closed-loop separation control using plasma actuators. The post-stall-separated flow over a NACA 0015 airfoil is controlled using a single dielectric barrier discharge actuator located at the leading edge. Open-loop measurements are first performed to highlight the effects of the voltage amplitude on the control authority for freestream velocities of 10–30 m/s (chord Re = 1.3 × 10⁵ to 4 × 10⁵). The results indicate that partial or full reattachment can be achieved and motivate the choice of the slope seeking approach as the control algorithm. A single-input/single-output algorithm is used to autonomously seek the optimal voltage required to achieve the control objective (full flow reattachment associated with maximum lift). The paper briefly introduces the concept of slope seeking, and a detailed parameterization of the controller is considered. Static (fixed speed) closed-loop experiments are then discussed, which demonstrate the capability of the algorithm. In each case, the flow can be reattached in an autonomous fashion. The last part of the paper demonstrates the robustness of the gradient-based, model-free scheme for dynamic freestream conditions. This paper highlights the capability of slope seeking to autonomously achieve high lift when used to drive the voltage of a plasma actuator. It also describes the advantages and drawbacks of such a closed-loop approach.     

Hole injection and surface state effects at gallium arsenide electrodes

We report capacitance and luminescence measurements at gallium arsenide electrodes in 1 M H₂SO₄ electrolyte that indicate the injection of holes on cathodic polarization. The steady-state potential-current relations and photoeffects are reviewed critically. We show that the assumption of hole injection is equally plausible to that of electron extraction provided the surface recombination velocity is fast compared to the bulk diffusion and recombination process. In addition to the space charge there is a substantial surface charge of the type commonly observed at III–V semiconductor surfaces.