Abraham N. Gissen

Controlled Streamwise Vorticity in Diffuser Boundary Layer using Hybrid Synthetic Jet Actuation

The formation of streamwise vorticity concentration by exploiting the interaction of surface-mounted passive and active flow control elements with the cross flow is investigated experimentally in a small-scale wind tunnel at high subsonic speeds (up to M = 0.5). Controlled formation of streamwise vortices can be a key element in the mitigation of the adverse effects of secondary flows in embedded propulsion system with complex inlet geometries that can affect pressure recovery and distortion at the engine inlet face. The evolution of these vortices is investigated on a converging-diverging insert along one of the test section walls that is designed to provide an adverse pressure gradient that mimics the pressure gradient within a typical offset diffuser. Counterrotating vortex pairs and single-sense vortices are formed and characterized using conventional passive micro-ramps and micro-vanes, respectively. It is demonstrated that similar streamwise vortices can also be realized using synthetic jet actuators having rectangular orifices that are slanted or skewed to produce single-sense vortices, or streamwise aligned to produce vortex pairs. Hybrid actuation is demonstrated by combining the passive and active actuation approaches to yield a “fail-safe” device with significant degree of controllability.

Distortion Management in a Boundary Layer Ingestion Inlet Diffuser Using Hybrid Flow Control

Flow distortion in a 5% scale model of a blended-wing-body, boundary-layer-ingesting offset diffuser is mitigated at high subsonic speeds (up to M=0.55) using integrated, hybrid actuators comprised of tandem configurations of passive (vanes) and active (synthetic jets) flow control elements. The inlet flow of the model diffuser is conditioned to simulate the inlet flow of an airborne blended-wing-body offset diffuser by implementation of a novel boundary layer fence. It is shown that flow distortion at the engine face is reduced by each of the passive and active elements, and the reduction is even more significant when both actuation approaches are implemented, concurrently, over the entire range of test Mach numbers. Hybrid actuation yields an overall distortion reduction of 35% at the design flow condition (M=0.55) using a control system that is fail safe and does not require an external air supply.

Fluidic Control of Transonic Shock-Induced Separation

†The feasibility of active flow control approaches in suppression of ‘large scale’ separated flow unsteadiness resulting from the transonic flow separation over rounded geometry is investigated experimentally. The subsonic upstream flow (M ≈ 0.59) accelerates over the rounded ramp and terminates at the normal shock that induces the flow separation. Two active flow control approaches are tested, focusing on the shock and the shear layer, respectively. Both control approaches significantly suppress sharp velocity/density gradients in the shear layer proportional to the jets’ mass flow rate coefficient, but the underlying mechanisms that lead to such results are different. The former utilizes the control jets upstream from the shock, which virtually shape an ‘apparent flow boundary’ in their interaction with the outer flow. As a consequence, the outer flow becomes locally slowed down just upstream from the shock formation, which contributes to its weakening. The latter flow control approach utilizes the control jets downstream from the shock, which directly target the flow separation, and only indirectly target the normal shock. Overall, it is argued that the main effect of the second flow control approach is in the enhanced mixing and spreading of the shear layer for a low jets’ mass flow rate coefficient, and a combination of the flow separation delay and mixing for the higher jets’ mass flow rate coefficient.

Distortion Management in a BLI Inlet Diffuser using Synthetic-Jet Hybrid Flow Control

Flow distortion in a 5% scale model of a blended-wing-body (BWB) offset diffuser is mitigated at high subsonic speeds (up to M = 0.55) using integrated hybrid actuators comprised of tandem configurations of passive (vanes) and active (synthetic jets) flow control elements. The inlet flow of the model diffuser is conditioned to mimic the inlet flow of an airborne BWB offset diffuser by implementation of a novel boundary layer ‘fence’. It is shown that flow distortion at the engine face is reduced by each of the passive and active elements, and the reduction is even more significant when both actuation approaches are implemented concurrently over the entire range of test Mach numbers. Hybrid actuation yields overall distortion reduction of 35% at the design flow condition (M = 0.55) using a control system that is both fail safe and does not require an external air supply.

Dynamics of Hybrid Flow Control in a Boundary-Layer-Ingesting Offset Diffuser

** † * * The effectiveness of hybrid flow control, comprised of tandem arrays of passive (vanes) and active (synthetic jets) control elements for suppression of engine face total-pressure distortion is investigated experimentally. The experiments are conducted in a wind tunnel inlet model with an offset diffuser at speeds up to M = 0.55 using inlet flow conditioning that mimics boundary layer ingestion on a Blended-Wing-Body platform. Time-dependent distortion of the dynamic total pressure field at the AIP is measured using an array of forty total-pressure probes, and the control-induced distortion changes are analyzed using triple decomposition and proper orthogonal decomposition (POD). These data indicate that an array of small-scale synthetic jet vortices merge into two large-scale, counter-rotating streamwise vortices that exert significant changes in the flow distortion. The two most energetic POD modes appear to govern the distortion dynamics in either active or hybrid flow control approaches. Finally, it is shown that the present control approach is sufficiently robust to reduce distortion with different inlet conditions of the baseline flow.

Manipulation of Streamwise Vorticity in an Emulated Diffuser Boundary Layer Using Hybrid Flow Control

The formation and interaction of streamwise vorticity generated by surface-mounted passive (micro-vanes) and active (synthetic jets) flow control elements is investigated experimentally in a small-scale wind tunnel at high subsonic speeds (M < 0.6). Streamwise vortices are generated within the boundary layer of a converging-diverging wall insert that is designed to provide an adverse pressure gradient similar to the pressure gradient within a typical offset diffuser. Hybrid actuation is demonstrated by combining the micro-vanes with synthetic jets in tandem or by integrating the jets into vanes. Tandem actuation is accomplished using synthetic jet actuators that are yawed to match the vanes’ yaw angle and skewed to generate streamwise vortices of matching sense. Both approaches lead to augmentation of the primary vanes’ vortices and enhance their pairing. These approaches provide “fail-safe” control devices having robust nominal control effectiveness by the vane elements coupled with enhancement by the jets.

Flow Control of a Cascade Thrust Reverser

The present experimental investigation explores utilization of active flow control for mitigation of the flow separation over a shortened turning section of a cascade thrust reverser, also known as a bullnose. Such a reduction in the nominal thrust reverser length, with minimal or no penalty in increased drag and reduced reversed thrust, would reduce both the wetted area and weight of the nacelle. To minimize the performance penalty, the shortened thrust reverser sector model is equipped with an array of small-scale fluidic oscillating jets that are used as the flow control elements for the oncoming Mach numbers of M≈0.24–0.53. The flow control effectiveness is demonstrated over four bullnose geometries, down to 47% of the nominal bullnose length. The established scaling laws indicate that the mass flow rate recovery is directly proportional to the actuation flow rate for a given flow configuration and condition, and it is strongly nonlinear with the flow pressure ratio for a given flow control rate. Furt...