Numerical Investigation of Fluidic Oscillator Flow Control in an S-Duct Diffuser

Control of flow distortion in offset diffusers using trapped vorticity

In investigating the potential of a new actuator for use in an active flow control system, several objectives had to be accomplished, the largest of which was the experimental setup. The work was conducted at the NASA Langley 20x28 Shear Flow Control Tunnel. The actuator named "Thunder", is a high deflection piezo device recently developed at Langley Research Center. This research involved setting up the instrumentation, the lighting, the smoke, and the recording devices. The instrumentation was automated by means of a Power Macintosh running LabV1EW, a graphical instrumentation package developed by National Instruments. Routines were written to allow the tunnel conditions to be determined at a given instant at the push of a button. This included determination of tunnel pressures, speed, density, temperature, and viscosity. Other aspects of the experimental equipment included the set up of a CCD video camera with a video frame grabber, monitor, and VCR to capture the motion. A strobe light was used to highlight the smoke that was used to visualize the flow. Additional effort was put into creating a scale drawing of another tunnel on site and a limited literature search in the area of active flow control.

Active Flow Control

This paper implements Co-flow Jet (CFJ) active flow control (AFC) on the M2129 serpentine duct for mitigating flow separation with low energy consumption. Various locations and slot widths of CFJ suction duct are studied and the energy expenditure of each case is analyzed. The 3D Reynolds Averaged Navier-Stokes (RANS) equations with one-equation Spalart-Allmaras turbulence model is used. The experimental data of AGARD test case 3.1, which has throat Mach number of 0.79, is used for validation. The predicted total pressure recovery agrees very well with the experiment with the discrepancy of less than 1%. The distortion coefficient (DC60) is also in an acceptable agreement with the experiment. The simulation result is further validated using the wall static pressure distribution, which also achieves a good agreement with the experiment. For the CFJ S-duct, a horn shaped slot geometry is adopted for the CFJ injection and suction slots to minimize the separation caused by CFJ sides wall. For the optimum configuration of the CFJ S-duct, a small distortion coefficient of 1.7% is achieved with the averaged total pressure recovery increased by 1.9%. The optimum CFJ configuration is able to substantially reduce the required CFJ power consumption due to placing the suction at the geometric inflection point. Overall, the CFJ active flow control is demonstrated numerically to be very effective to mitigate the S-duct flow distortion and improve total pressure recovery.

Mitigation of serpentine duct flow distortion using co-flow jet active flow control

Toward Improved Turbulence-Modeling Techniques for Internal-Flow Applications

Trapped vorticity concentration engendered by deliberate modification of the internal surface of an offset diffuser is coupled with a spanwise array of surface-integrated fluidic-oscillating jets for hybrid flow control of the secondary flow that gives rise to flow distortions. The trapped vorticity mimics the effects of local separation within aggressive diffusers and is used as a testbed for investigations of the effectiveness of such hybrid flow control for alleviation the flow distortions at the engine inlet (AIP). The local and global characteristics of the diffuser flow in the absence and presence of the actuation are investigated at Mach numbers up to M = 0.7, using a forty-probe AIP total pressure rake, surface oil-flow visualization, hot-wire anemometry, and particle image velocimetry. The present investigations demonstrate that the actuation affects the strength and scale of the trapped vorticity and its interaction with the cross flow, and consequently alters the flow evolution within the diffuser and leads to significant suppression of pressure distortion at the AIP (by about 80%). These findings indicate that hybrid actuation based on trapped vorticity concentrations can be utilized for mitigation of flow distortion in diffuser flows by strong coupling to the natural formation of vorticity concentrations due to the local flow separation.

Investigation of Trapped Vorticity Concentrations Effected by Hybrid Actuation in an Offset Diffuser

Losses and flow distortions in a supersonic-inlet aggressive offset diffuser (Mthroat < 0.64) that are induced by closed, separated flow domains on the diffuser’s concave surfaces and by pairs of dynamically coupled counter-rotating streamwise vortices are mitigated using fluidic actuation. While pressure recovery is primarily limited by the separation in each of the diffuser’s turns, the distortion is governed by counter-rotating streamwise vortices that advect low-momentum fluid from the wall region into the core flow. The present investigations have shown that the secondary vortices are engendered by concentrations of streamwise vorticity that form in the outboard segments of the separated flow domains. Therefore, fluidic control of the scale and topology of the trapped vorticity within the internally-separated flow can be leveraged to control the structure and strength of the ensuing secondary vortices and thereby significantly reduce flow distortion and losses. In the present investigations, fluidic control is effected by a spanwise array of oscillating jets that are placed just upstream of the separation domain. The actuation alters the spacing and diminishes the strength of the base flow streamwise vortices by forming and adjoining streamwise vorticity concentrations of opposite sense. The spanwise distribution of the actuator array can be optimized to reduce the average circumferential distortion by as much as 60% at actuation to diffuser mass flow rate ratio of 0.4%.

Fluidic Control of an Aggressive Offset Diffuser for a Supersonic Inlet

Total-pressure losses and distortion in an aggressive offset diffuser, induced by secondary-flow vortices and boundary-layer separation, are mitigated using fluidic actuation for the aerodynamic in...

Experimental and Numerical Investigation of Active Flow Control of a Serpentine Diffuser

The total pressure losses and distortion in a serpentine diffuser over a range of flow rates that result in the formation of a transonic shock at the diffuser’s first convex turn are alleviated using fluidic-based flow control. The present investigations show that the shock strength is highest at the diffuser’s spanwise corners and has a local minimum at midspan. Flow distortions above the diffuser’s convex bottom surface are induced by two counterrotating streamwise vortices that originate at each corner and strengthen as a result of interaction of the corner flow with the shock. These vortices are controlled indirectly by manipulation of the shock using a spanwise array of fluidic oscillating jets that are integrated into the diffuser moldline upstream of the shock. It is shown that along with changing the shock footprint topology, flow control displaces the streamwise vortex pair farther apart and thereby diminishes the cooperative advection of a low momentum fluid from the wall region into the core flow. Consequently, the average circumferential distortion parameter is reduced by 35%, while the total pressure recovery increases by about 1%. These findings indicate that diffuser flow rates higher than the nominal operating condition, which are typically limited by shock losses, can be enabled by active flow control.

Travis J. Burrows

Papers

Investigation of Trapped Vorticity Concentrations Effected by Hybrid Actuation in an Offset Diffuser

Control of flow distortion in offset diffusers using trapped vorticity

Fluidic Control of an Aggressive Offset Diffuser for a Supersonic Inlet

Experimental and Numerical Investigation of Active Flow Control of a Serpentine Diffuser

Control of a Transonic Shock in a Serpentine Diffuser using Surface Fluidic Actuation