Afterbody separation control using passive porosity

A computational investigation was conducted to examine passive control of a nozzle afterbody flow through the use of a porous surface. A threedimensional Navier-Stokes solver was used to obtain two-dimensional computational solutions. Baseline solutions (no porosity) were found to compare well with experimental data. Passive control at subsonic, off-design conditions had a slight loss in pressure recovery compared to the baseline case. Nomenclature C permeability factor Cd discharge coefficient, wp/w,Cp coefficient of pressure, (p-poo)/qoo FA axial thrust, N Fi ideal isentropic thrust, N H ratio of hole depth to plate thickness MOO freest ream Mach number NPR nozzle pressure ratio, p«j/Poo P ratio of hole area to total area d diameter of porous holes, m p local static pressure, Pa pp plenum pressure, Pa Pt,j jet total pressure, Pa pw surface pressure, Pa Poo freestream static pressure, Pa goo freestream dynamic pressure, Pa t thickness of porous plate, m vw transpiration velocity, m/s Wf ideal mass flow rate, kg/s wp mass flow rate, kg/s Xf finish location of the porous region, m 'Graduate Research Assistant,Department of Mechanical and Aerospace Engineering, Student Member AIAA. 'Associate Professor, Department of Mechanical and Aerospace Engineering, Senior Member AIAA. Copyright © 1997 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. xs start location of the porous region, m fi molecular viscosity, kg/m-s p density, kg/m 111 fine grid sequence 122 medium grid sequence 144 coarse grid sequence Introduction One of the main characteristics of afterbody flow is separation. In subsonic flow, separation occurs when the attached flow is unable to negotiate an adverse pressure gradient imposed by the afterbody geometry. In transonic flows, the afterbody flow expansion often terminates in a shockwave, which then induces boundary layer separation. In either case, separation causes viscous dissipation and increased afterbody drag. Hence, boundary layer control on an afterbody can provide beneficial decreases in drag, which, in turn, can improve the aeroprepulsive efficiency. One such means of boundary layer control is active control through suction. Suction decreases the boundary layer thickness, thus, delaying or preventing flow separation. However, active control methods are complicated, and often, the additional weight of the control system can be significant. An alternative to active control is passive control. Passive control makes use of naturally occurring phenomena to obtain desirable flow characteristics. Advantages of passive control include simplicity of the system, no additional weight, and reduced cost. In this study, the passive porosity control concept was examined. As shown in Figure 1, this control approach uses a porous surface placed above a plenum in a region with large pressure gradients; in this example, beneath a shockwave. The high pressure region behind the shock is allowed to communicate with the low pressure region ahead of the shock. Copyright© 1997, American Institute of Aeronautics and Astronautics, Inc. This communication reduces the shock strength. The reduction in the strength of the shockwave reduces its adverse effects on the boundary layer and flow separation. Previous research has shown that the method of passive porosity can enhance wing performance at subsonic, transonic, and supersonic speeds by reducing or eliminating shock-induced and trailingedge separation". Testing on ogive forebodies has demonstrated the capability of passive porosity to reduce side force at subsonic and supersonic speeds. There is also potential in reducing aircraft afterbody drag through the application of passive porosity to relieve afterbody separation. The objective of this study is to computationally examine the effectiveness of afterbody flow control using passive porosity.