Experimental Investigation of Rarefaction Effects on Aerodynamic Coefficients of Slender and Blunt Re-entry Vehicles

Kurzfassung

During the design process of hypersonic flight vehicles operating in the rarefied flow regime, i.e. the transition region between continuum and free molecular flow, tools for flight stability predictions must be applied which correctly include the rarefaction effects on the aerodynamic coefficients. The focus of the present experimental investigation is to quantify these effects for two different classes of re-entry vehicles and to provide a data base which can be utilized to validate numerical design tools. A slender high lift/drag configuration is compared with a classical blunt-shaped lifting body. The ground based testing is conducted in the Hypersonic Vacuum Wind Tunnels Göttingen (VxG) of the German Aerospace Center (DLR). In the 2nd test section (V2G) of this continuously operating hypersonic test facility, near continuum rarefied flow conditions with Knudsen numbers based on the model length between 7.2e-4 to 8.8e-3 are generated. An optimized three-component strain gauge force balance, capable of simultaneously measuring lift, drag and pitching moment, is applied. The zero point drift due to increasing balance temperature, most critical for the accuracy of the measurements, is improved by one order of magnitude compared to earlier measurement campaigns. This is mainly achieved by an alternative test article set up and balance integration resulting in an improved decoupling of heat conduction from the model into the balance. Lift, drag and pitching moment are measured at angles of attack between 0° and 34°. Postulating the applicability of the Mach number independence principle, the basic approach to set up the test matrix is to adjust the Knudsen number by fixing the Reynolds number and varying the Mach number. Further, since the considered flow conditions are near continuum, the variation of the aerodynamic coefficients is analysed as function of the rarefaction parameter. Basically, similar trends are observed for both parameters. Increasing the Knudsen number, the lift coefficient of both configurations is reduced over the analysed Knudsen number regime. For angles of attack 6° < alpha < 30, a reduction between 20 - 30% and 10 - 20% is obtained for the blunt body configuration and the slender configuration, respectively. For the slender configuration, the drag coefficient increases by 100% at low angles of attack (Alpha ca. 4°), the maximum drag coefficient increase for the blunt configuration amounts to 50% (Alpha ca.0°). At higher angles of attack, the drag coefficient increase with increasing Knudsen number is reduced, but still above 20% at angles of attack below 30°. Consequently, at low angles of attack, the aerodynamic efficiency, i.e. lift/drag ratio is decreased for the slender configuration by 60%. At 30° angle of attack, a reduction of 30% is obtained. For the blunt configuration the lift/drag ratio reduction is approximately 40 - 50% over the considered angle of attack range. The pitching moment shows a tendency to increase with increasing Knudsen number, however, the observed dependence on, e.g., streamwise flow gradients do not allow a non-ambiguous interpretation and quantitative statements.