The term plasma actuator has now been a part of the fluid dynamics flow-control vernacular for more than a decade. A particular type of plasma actuator that has gained wide use is based on a single–dielectric barrier discharge (SDBD) mechanism that has desirable features for use in air at atmospheric pressures. For these actuators, the mechanism of flow control is through a generated body-force vector field that couples with the momentum in the external flow. The body force can be derived from first principles, and the effect of plasma actuators can be easily incorporated into flow solvers so that their placement and operation can be optimized. They have been used in a wide range of internal and external flow applications. Although initially considered useful only at low speeds, plasma actuators are effective in a number of applications at high subsonic, transonic, and supersonic Mach numbers, owing largely to more optimized actuator designs that were developed through better understanding and modeling of...

Dielectric Barrier Discharge Plasma Actuators for Flow Control

This paper describes ongoing research efforts in the scramjet community on cavity e ame holders, a concept for e ame holding and stabilization in supersonic combustors. During the last few years, cavities have gained the attention of the scramjet community as a promising e ame-holding device, owing to results obtained in e ight tests and to feasibility demonstrations in laboratory-scale supersonic combustors. However, comprehensive studies are needed to determine the optimal cone guration that will yield the most effective e ame-holding capability with minimum losses. The e owe eld characteristics of cavities and research efforts related to cavities employed in lowand high-speed e ows are summarized. Open questions impacting the effectiveness of the cavities as e ame holders in supersonic combustors are discussed.

https://research.engr.utexas.edu/benyakar/images/stories/pdfs/Ben-Yakar_lab_Ben-Yakar_et_al_JPP_2001.pdf

Cavity Flame-Holders for Ignition and Flame Stabilization in Scramjets: An Overview

Experimentalandcomputationalinvestigationsofthee owe eldassociatedwithseveralcavity-basede ameholders in a nonreacting supersonic e ow are described. All cavity e ows were of the open type, that is, length-to-depth ratio L/D<10. Two values of L/D were studied with several offset ratios (OR) and aft ramp angles µ. Results indicate that the aft ramp angle plays an important role in determining the character of the shear layer that spans the cavity. For a rectangular cavity with OR=1 and µ=90 deg, a compression wave forms as the e ow separates from the cavity’ s upstream corner. A strong recompression occurs at the aft wall, and the e ow is visibly unsteady. The pressure on the cavity fore wall decreases steadily and the recompression process occurs more gradually with decreasing aftrampangle.Higherdrag coefe cientsandshorterresidencetimesarefoundin cavitieswithshallower ramp angles.

Fundamental Studies of Cavity-Based Flameholder Concepts for Supersonic Combustors

We review recent advances in understanding, modeling, and controlling oscillations in the flow past a cavity. The fundamental mechanisms underlying cavity flow oscillations have been known for at least 40 years, but suppressing these oscillations in a reliable and robust way is still a challenge today. Interest in controlling the flow past a cavity is motivated by aerospace applications, but in addition, cavity flows provide an attractive canonical problem for exploring general flow control techniques. The focus is on recent advances in modeling these flows, and in controlling them, using both open-loop and closed-loop techniques. A relatively new perspective is that cavity oscillations may not always be self-sustained, but under some flow conditions may be lightly damped resonances, sustained by external disturbances such as boundary layer turbulence. Areas in which our understanding is incomplete, and which deserve further study, are discussed, in particular the effects of high-frequency open-loop forcing, fundamental limitations of feedback control for a given configuration of sensors and actuators, and the development of a feedback design methodology that respects the limited range of validity of the available dynamical models.

Dynamics and control of high-reynolds-number flow over open cavities

SDBD plasma enhanced aerodynamics: concepts, optimization and applications

Experimental methods for suppressing flow-induced pressure oscillations in a shallow cavity resulting from tangential flows over the cavity are described. The effects of manipulating the shear layer over the cavity leading edge are examined. Static and oscillating fences and steady and pulsating flow injection at the leading edge are studied for their effect on cavity sound pressure levels. Both subsonic and supersonic flow conditions are considered. Of the methods tested, static fences at the leading edge were found to provide the most suppression. Suppression was dependent on the frequency mode and the flow Mach number. Nomenclature a = speed of sound D = cavity depth / = frequency k = vortex convection velocity to freestream velocity ratio L = cavity length M = Mach number m = frequency mode number P = pressure P0 = stagnation pressure Re = Reynolds number, Uxlv S = Strouhal number S * = modified Strouhal number U = freestream velocity x = distance from nozzle exit Z = cavity span, lateral dimension a = phase delay parameter y = ratio of specific heats 8 = boundary-layer thickness v = kinematic viscosity

Suppression of flow-induced pressure oscillations in cavities

The drag of a half-scale aircraft laser turret is investigated in a low-speed wind tunnel. Forces, moments, and pressures are recorded fortheturretand aft-mounted fairings and splitter plates. Oil traces and tufts indicatethat the e ow is characterized by dominant vortices shedding from the top of the turret and a large trailing wake of vorticity. Splitter plates are ineffective in reducing drag as a result of the strong e ow over the top of the turret. A small fairing reduces the baseline drag coefe cient by 49% but is unable to produce attached e ow near the turret. A large fairing eliminates nearly all separation regions and reduces the baseline drag coefe cient by 55%.

Wind-Tunnel Tests of an Aircraft Turret Model

Wing-in-Ground effects from a wind-tunnel study of a NACA 4415-profile wing model with an aspect ratio of 2.33 are described. The wing model contains a 20%-chord, full-span, adjustable flap and removable end and enter plates. Ground boards are used in the wind tunnel to simulate the ground. In this study, the ground effects are expressed as variations to the aerodynamic coefficients (lift and drag) and lift-to-drag ratio. The ground effects are described in terms of angle of attack, flap angle, wing height above ground and use and size of end and center plates.

Wind-tunnel investigation of wing-in-ground effects

A dielectric barrier discharge is used to alter the near wall velocity of a two dimensional separated boundary layer flow. The electrodes of the dielectric barrier discharge are oriented at sixty degrees to primary flow direction and skewed fifteen degrees downstream to the primary flow direction. A separation bubble was created on a flat test surface by creating an adverse pressure gradient on the opposite wall, typical of the suction side of a Pak B low-pressure turbine blade. Attachment of the flow to the top wall and creation of a separation bubble on the flat wall was accomplished by applying suction to the contoured surface. The electrodes mid points were at the expected location of separation. The induced separation at a chord Reynolds number of 50,000 is significantly more severe than that of the Pak B profile. The skewed orientation was chosen to see if stable longitudinal vorticity could be induced in the wall layer. The discharge was driven by a 2000 to 7000 hertz source, and plasma power levels of 20, 30, and 40 watts were investigated with PIV boundary layer profiles at simulated chord Reynolds numbers of 50,000, 75,000, and 100,000. Turbulence levels of 3.3% and 5.4% were considered which resulted in both laminar and turbulent approaching boundary layers. Laminar-separated, near-wall-layer velocities were increased by as much as 40% by the discharge for the Reynolds number equal 100k case. Only marginal increases were observed for the turbulent cases. In the comparisons presented the frequency was fixed at 3,000 hertz. With only the center of the plasma excitation located at the separation no significant longitudinal vorticity was observed.

Effects of plasma induced velocity on boundary layer flow

: The predictive capability of the two-dimensional compressible mass- averaged Navier-Stokes equations was investigated for a typical circulation control air-foil. The governing equations were solved using the implicit approximate factorization algorithm of Beam-Warming with the turbulence model of Baldwin-Lomax. To account for the unique characteristics of circulation control airfoils, an empirical turbulence model correction due to Bradshaw was used. This thesis is unique in that the predictive capability of the computational method is explored by examining the importance of the empirical Bradshaw curvature correction constant on the computed results. Using a generic value of the curvature constant at various blowing coefficient levels, the computational method was able to accurately predict airfoil pitching moment and lift curve slope due to blowing. Predicted levels of airfoil lift coefficient, although reasonable, were found to be consistently low compared with experiment due to the generic curvature constant providing premature jet detachment from the Coanda surface. Computed and measured airfoil drag results followed the same trends, but lack of overall drag coefficient agreement was disappointing. Lift coefficient was found to be quite sensitive, pitching moment not sensitive, and drag coefficient moderately sensitive to the value of the curvature constant used. For the highest blowing coefficient case considered, the value of curvature constant required for the computational lift coefficient to match the experimental lift coefficient was also determined.

Milton Franke

Papers

Suppression of flow-induced pressure oscillations in cavities

Wind-Tunnel Tests of an Aircraft Turret Model

Wind-tunnel investigation of wing-in-ground effects

Effects of plasma induced velocity on boundary layer flow

Navier-Stokes methods to predict circulation control airfoil performance