Sensitivity of separated flow in a three-dimensional, asymmetric diffuser to inlet condition perturbations [electronic resource]
Keywords: Aeronautics and Astronautics.

The sensitivity of separated flow in an asymmetric, three-dimensional diffuser to inlet condition perturbations is investigated using particle image velocimetry. The diffuser has a simple design, expanding from a rectangular cross-section at its inlet to a square cross-section at its outlet, with an area expansion ratio of 4.8. The bottom wall and one sidewall are angled outwards while the other two walls remain flat. In the baseline flow, a large and stable separation bubble forms in the corner between the two diverging walls and spreads across the more aggressively expanding bottom wall. The diffuser is fed by a long, rectangular duct which develops secondary flows in the corners. The diffuser flow has proven to be highly sensitive to the modification of these secondary flows. Dielectric barrier discharge plasma actuators and passive vortex generators are used to purposefully and systematically manipulate the secondary flows in the inlet channel. Changes to the flow are documented using static pressure measurements, magnetic resonance velocimetry, and particle image velocimetry. In pilot-scale experiments, spanwise-acting dielectric barrier discharge plasma actuators installed on the floor of the inlet channel are used to generate a wide range of perturbations. The effects of these perturbations are reflected in changes as large as +/-25% to the baseline pressure recovery of the diffuser. Two sets of passive vortex generators were fabricated as analogs of two continuously operated plasma actuator cases. Magnetic resonance velocimetry measurements of the vortex generator cases reveal significant changes to the separation bubble geometry. An improved pressure recovery is associated with a delayed onset of separation and an overall smaller separation bubble. The case which degrades the pressure recovery develops a completely different separation geometry as flow separates from the expanding sidewall. The separation bubble grows rapidly, preventing the main flow from decelerating efficiently. In order to study the plasma actuator cases in more detail, particle image velocity measurements were acquired in the High Reynolds Number Facility. Following actuator scaling efforts and operating parameter optimization, a pulsed forcing case which improves the diffuser's performance, and a continuous forcing case which degrades it were chosen for further study. PIV measurements were obtained in five streamwise-wall normal planes and two spanwise-wall normal planes for all three cases. The measurements focus on the first half of the diffuser, where the majority of the losses in pressure recovery occur. Spanwise-wall normal data show that both pulsed and continuous forcing cases generate four distinct longitudinal vortices which redistribute streamwise momentum in the diffuser's inlet. The larger and stronger vortices formed by continuous forcing sweep low momentum fluid from the bottom wall and deposit it along the sidewalls. The separation bubble for this case forms along the expanding sidewall instead of along the bottom wall as in the other two cases. The protrusion of the separation bubble into the diffuser from the sidewall creates a blockage of forward-moving flow, distorting velocity contours and keeping velocities high in the core of the diffuser. Elevated streamwise and vertical normal stresses distributed throughout the planes suggest that this separation bubble is unsteady. Pulsed forcing delays separation along the bottom wall by increasing turbulent activity in the attached boundary layer upstream of separation. A large peak in shear stress is observed in one of the planes close to the expanding sidewall. Elevated levels of normal stresses are observed for this case as well, but the distribution of these stresses and the overall flow structure closely resemble the baseline flow.