Rogier H. M. Giepman

Flow control of an oblique shock wave reflection with micro-ramp vortex generators: Effects of location and size

This study investigates the influences of micro-ramp size and location on its effectiveness as a flow control device for oblique shock wave reflections. The effectiveness is measured in terms of the size of the shock-induced separation bubble and the reflected shock unsteadiness. Particle image velocimetry measurements were carried out on the interaction region and the mixing region between micro-ramp and interaction. The separation bubble is shown to be most sensitive to the momentum flux contained in the lower 43% of the incoming boundary layer. The momentum flux added to this region scales linearly with micro-ramp height and larger micro-ramps are shown to be more effective in stabilizing the interaction. Full boundary layer mixing is attained 5.7δ downstream of the micro-ramp and this forms a lower limit on the required distance between micro-ramp and the start of the interaction region. Typical reductions in the average separated area and the shock unsteadiness of 87% and 51%, respectively, were reco...

Experimental Study into the Effects of Forced Transition on a Shock-Wave/Boundary-Layer Interaction

This study investigates the effects of boundary-layer tripping on an oblique shock-wave reflection (M=1.7, Re∞=35×106 m−1) by means of particle image velocimetry. Laminar boundary layers are sensitive to adverse pressure gradients, and the interaction with an oblique shock wave (θ=3  deg) results in the formation of a large separation bubble (∼45δ95). The bubble can be removed by tripping the boundary layer a short distance upstream of the incident shock wave. Three types of tripping devices were investigated in this study: a stepwise trip, a patch of distributed roughness, and a zigzag strip. The step was found to be least effective at tripping the boundary layer, yielding a turbulent boundary layer ∼55δ95 downstream of the trip; whereas the other two trips required a distance of only ∼30δ95. Consequently, at the same trip–to-shock-wave distance, the step was found to be less effective at removing separation than the other two trips. Although tripping the boundary layer allows for the removal of the sepa...

Mach and Reynolds Number Effects on the Wake Properties of Microramps

A parametric study has been conducted into the effects of device height (h=6−10  mm), Mach number (M∞=1.5−2.5), and Reynolds number (Re∞=28×106 to 63×106  m−1) on the effectiveness of microramp vortex generators as flow control devices. The control authority of the microramp comes from the two counter-rotating vortices it introduces into the boundary layer. The vortices transport high-momentum fluid toward the wall, thus creating a fuller velocity profile that is less prone to separation. This momentum transport, however, comes at the price of a low-momentum wake that is formed downstream of the device. The induced flowfield has been studied by means of two-dimensional particle image velocimetry, oil-flow, and schlieren visualizations. Most of the geometric flow features were found to scale linearly with microramp height h: the wake height, wake strength, vortex core height, and the momentum flux added to the near-wall region of the flow all increase linearly with h. The control effectiveness of the micro...

Numerical Experiments on Aerodynamic Resonance in Transonic Airfoil Flow

We present the results of 2-d URANS simulations of unsteady shock/-boundary layer interaction on a supercritical airfoil in transonic flow. At constant Mach and Reynolds number the angle of attack is gradually increased until self-sustained periodic shock buffet oscillations set in. Subsequently, we focus on the subcritical flow field dynamics below the identified shock buffet onset, where already damped flow oscillations can be observed. Therefore, various fixed-point stable flows are perturbed with small time-periodic deflections of the airfoil geometry or random impulses, after which the particular flow response is analyzed in the frequency domain to identify the dominant aerodynamic eigenvalue. Furthermore, we demonstrate an effective stabilization of sub- and supercritical shock buffet flows by means of a closed-loop controller.

The effects of Mach and Reynolds number on the flow mixing properties of micro-ramp vortex generators in a supersonic boundary layer

A parametric study has been conducted into the effects of device height (h = 6 − 10 mm), Mach number (M∞ = 1.5−2.5) and Reynolds number (Re∞ = 28−63×106 m−1) on the effectiveness of micro-ramp vortex generators as flow control devices. The control authority of the micro-ramp comes from the two counter-rotating vortices it introduces into the boundary layer. The vortices transport high-momentum fluid towards the wall, thus creating a fuller velocity profile which is less prone to separation. This momentum transport, however, comes at the price of a low-momentum wake that is formed downstream of the ramp. The induced flow field has been studied by means of particle image velocimetry, oil-flow and Schlieren visualizations. Most of the geometric flow features were found to scale linearly with micro-ramp height h: the wake height, wake strength, vortex core height and the momentum flux added to the near-wall region of the flow all increase linearly with h. The control effectiveness of the micro-ramp is reduced for higher Mach numbers, as less momentum is added to the near-wall region of the flow and a stronger wake is recorded. Only a weak Reynolds number effect is observed in the results, with the micro-ramp becoming slightly more effective at higher Reynolds numbers.