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Large-eddy simulations were carried out in order to numerically describe plasma-based control of turbulent boundary-layer separation. The configuration consisted of a flat-plate section, over which the boundary layer developed, followed by a curved convex rearward-facing ramp, corresponding to an experimental arrangement. A single dielectric-barrier-discharge plasma actuator was then employed to reduce the extent of the separated flow region. Solutions were obtained to the Navier--Stokes equations, which were augmented by source terms used to represent plasma-induced body forces imparted by the actuator on the fluid. A simple phenomenological model provided the electric field generated by the plasma, resulting in the body forces. The numerical method used a high- fidelity time-implicit scheme, employing domain decomposition in order to perform calculations on a parallel computing platform. Simulations were first conducted for an isolated actuator, which were used to optimize parameters inherent in the plasma model. Subsequent calculations were then carried out for the plate development section and for the plate/ramp combination, using both continuous and pulsed control. All simulations were compared with experimental measurements, which indicated that, although some deficiencies were apparent in the plasma model, they appeared to be adequate for use with large-eddy simulation in the exploration of plasma-based control for turbulent wall-bounded flows. DOI: 10.2514/1.J050014

Magnetic fields significantly affect the flow of ionized gases and may be exploited for the control of separation, transition, and turbulence. The wave structure of ionized gas high-speed flows under the influence of magnetic fields is more involved than the waves of ordinary gas dynamics and high-resolution is required to accurately compute the complex wave interactions of such flows with discontinuities. Numerical solutions of the nonlinear ideal and viscous magnetohydrodynamic equations are obtained with a high-order-accurate shock-capturing scheme. The numerical method is tested for flows containing strong magnetohydrodynamic discontinuities and smooth flow features to ensure that it maintains high-order accuracy for the smooth parts of the flow, it preserves numerical stability, and eliminates nonphysical features that result from the violation of the divergence-free condition for the magnetic field. Problems with exact solutions, such as the convection of smooth waves caused by magnetic fields, are also solved numerically to validate the method. The numerical method is applied to simulate separation control at the interaction region of an oblique shock with a laminar boundary layer under the influence of magnetic fields. DOI: 10.2514/1.45526

An experimental study of flow control using all array of three synthetic jets has been undertaken in a separated laminar flow over an inclined flat plate in a water channel. Particle image velocimetry was employed to obtain the information about the extent of flow separation delay at different synthetic jet operating conditions. A laser-induced florescence flow visualization technique was used to reveal the characteristics of the vortical structures produced by the synthetic jets, which result in a delay of separation. It was observed that the flow separation delay is typically associated with the presence of two or three streaks of high streamwise velocity in the separated flow. Based on the results from the present experiment, a parameter map indicating the flow patterns observed at different synthetic jet operating conditions is produced. In addition, a contour map showing the separation control effectiveness at the corresponding conditions is also obtained. It was found that the two-streak flow pattern is mostly produced at a jet- to-freestream velocity ratio between 0.3 and 0.5, whereas the three-streak pattern occurs at a velocity ratio between 0.5 and 1.5. The laser-induced florescence images confirm that the two-streak flow pattern is caused by the hairpin type vortical structures produced by synthetic jets, whereas the three-streak pattern is caused by the tilted vortex ring type structures. For the range of actuator operating conditions tested in this study, operating the synthetic jets at a dimensionless stroke length of around 2, a velocity ratio of around 0.5, and a Strouhal number of around 1.6 would deliver the best flow control effect with the least energy consumption. Under these conditions, hairpinlike vortical structures are observed with a streamwise spacing of 44 % of the local boundary-layer thickness at the orifices of the synthetic jets. In this experiment, the spacing between the jets and the distance between the jet array and the baseline separation line are fixed. As the level of interaction between neighboring synthetic jets and their flow control effectiveness are expected to alter upon changes in these two parameters, the generality of this finding remains to be established in future work. DOI: 10.2514/1.43673

Effects of spanwise-periodic disturbances generated by steady active ventilation on the surface of a cylinder are investigated. The wake flow is studied experimentally using flow visualization and hot-wire measurements and numerically using FLUENT[TM] Periodic porosity induces the formation of a series of steady counter-rotating vortex structures behind the porous patches, with or without suction being applied. A considerable closing of the wake occurs when porosity is placed on both the top and bottom surfaces, leading to a reduction in drag. The formation of spanwise vortices is suppressed, resulting in a reduction of the velocity fluctuation level and the elimination of oscillation in lift. The formation and spatial evolution of streamwise vortices is discussed. DOI: 10.2514/1.43321