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

Kurzfassung

The prediction of the flight stability and of the aerodynamic coefficients in the transition region between the rarefied and the continuum flow regime is important for the design of hypersonic vehicles. The aim of this investigation is to analyse experimentally how aerodynamics – lift, drag and pitching moment – of re-entry and hypersonic transport vehicles is affected by rarefaction effects for classical “blunt bodies” and high lift / drag flight configurations. The analysis is conducted by experiments in the DLR Hypersonic Vacuum Wind Tunnel Göttingen (VxG), a continuously operating facility which can simulate hypersonic flows around vehicle configurations in this gas kinetic transition regime. For the analysis of aerodynamic forces a measurement technique is established for the 2nd test section of the Hypersonic Vacuum Wind Tunnel Göttingen (V2G). The 3-component strain gauge force balance is based on the design of former V2G force balances, and is optimised to obtain a better resolution and improved zero-point stability which results in precision improvement of the output signal during the experiment. Based on the relation of Knudsen number to Mach number and Reynolds number, the experimental test matrix is setup. For a non-ambiguous result it is important to vary only one parameter at a time. Applying the Mach number independence principle, it is straight forward to vary the Mach number since the resulting effect can be interpreted as a pure effect of rarefaction. Although the experimental setup allows a factor two of Mach number and, hence, an equal variation of Knudsen number, the variation is little considering the overall rarefaction regime covers about three magnitudes of Knudsen numbers. By performing several test series, each at a constant Reynolds number, it is possible to extend the Knudsen number variation to one order of magnitude. This assumption requires that the Reynolds number influence is of secondary importance. This and the applicability of the Mach number independence principle is addressed in the discussion of the results. Further the effect of the flow inhomogeneity is evaluated. This analysis contains an investigation of one blunt test configuration (COLIBRI) and one slender test configuration (SHEFEX III) both investigated at angles of attack between 0° and 34° in 2° steps at 26 test conditions in the rarefied flow. The rarefaction effects are analysed and assessed individually for lift-, drag- and pitching moment coefficient and lift / drag ratio. Within the analysed Knudsen number range between 7 ∗ 10−4 and 9 ∗ 10−3, significant rarefaction effects could be observed in all considered aerodynamic coefficients. The effects are strong enough to cause a reduction in lift / drag ratio of up to more than 50%. In parallel to the evaluation of aerodynamic coefficients versus Knudsen number, the evaluation is conducted as well versus the rarefaction parameter. The rarefaction parameter is an appropriate parameter to distinguish flow regimes in the transition region between rarefied flow and continuum, where rarefaction effects start to arise but boundary layers are still defining the overall flow field. Based on the obtained aerodynamic data, solvers for rarefied flow can be compared and simplified engineering methods, as e.g. bridging methods can be developed. These tools are especially important for the pre-design and optimisation phase where fast and simple tools are necessary.