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Ground Testing Investigation of Hypersonic Transition Phenomena for a Re-entry Vehicle

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VKI PHDT 2013-11, Davide Masutti, Ground Testing Investigation of Hypersonic Transition Phenomena for a Re-entry Vehicle, IISBN 978-2-87516-052-2, 170 pgs

Ground Testing Investigation of Hypersonic Transition Phenomena for a Re-entry Vehicle

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Ground Testing Investigation of Hypersonic Transition Phenomena for a Re-entry Vehicle
By Davide Masutti

PhD Thesis from the von Karman Institute / TU Delft, June 2013, ISBN 978-2-87516-052-2, 170 pgs


Abstract

Unpredicted boundary layer transition can impact dramatically the stability of a high speed vehicle, its aerodynamic coefficients and reduce the efficiency of the thermal protection system. In particular, the pressure fluctuations caused by growing instabilities in the boundary layer are considered the primary cause which leads to transition on a space vehicle during atmospheric re-entry. The knowledge of these unsteady fluctuations could help to identify and model the mechanisms which take part into the transition process in order to better control them. When performing an experimental investigation on hypersonic boundary layer transition, the disturbances in the freestream flow generated by the wind tunnel have a dominant role in the transition process itself. The knowledge of the freestream flow quality is therefore a fundamental element for the interpretation of the transition experiments in hypersonic ground test facilities. In this framework, an investigation of the free-stream disturbance level of the VKI H3 Mach 6 Wind Tunnel as been performed at different Reynolds number. High frequency response data are obtained by means of classical intrusive measurement techniques such as hot-wire anemometry and Pitot pressure probes with Kulite pressure transducers. Statistical and spectral analyses are compared to the data available in literature for other hypersonic wind tunnels, working at similar conditions for Mach and Reynolds number. A data reduction technique that allows the computation of all the fluctuations of the flow quantities from those directly measurable through the above-mentioned instrumentation is also developed: by means of this technique, the validity of some physical hypotheses on the flow, such as the Strong Reynolds Analogy, can be verified. Moreover, a Kovasznay modal analysis of the turbulent fluctuations can be carried out, in order to better understand the physical meaning and the origin of the disturbances.

Natural transition mechanisms are studied on a smooth 7deg half-angle cone with exchangeable nose tip by means of a stream-wise array of high frequency pressure transducers (PCB 132A31). Instabilities are investigated with a wall temperature ratio Tw /Taw =0.69 (Tw /T0 =0.59) for a freestream unit Reynolds number ranging from 6 × 106 /m to 28 × 106 /m in the VKI H3 Hypersonic Wind Tunnel at Mach 6. Experimental data shows the streamwise evolution of high frequency second mode waves (up to 650 kHz) in the boundary layer. In particular, these instabilities grow in amplitude and decrease in frequency in the streamwise direction before breaking down into a turbulent boundary layer. The results obtained from the unsteady pressure measurements at the wall are well supported by a steady state infrared thermography validating the observations. Moreover, the normalized power spectral densities of the pressure fluctuations are then compared with stability computations from the VKI VESTA and the NASA LASTRAC 3D code showing a very good agreement. From the comparison between the experimental results and the stability theory, the N factor of the wind tunnel facility is found to be equal to 4.8 for linear stability equations and 5.6 for parabolized stability equations.

To provide and perform in-flight experiments, ESA is about to launch the EXPERT (European eXPErimental Reentry Testbed) vehicle in order to obtain aerothermodynamic data for the validation of numerical models and of ground-to-flight extrapolation methodologies. The EXPERT vehicle is equipped with a specific payload dedicated to the assessment of the induced boundary layer transition by an isolated roughness element. In support to the forthcoming post flight analysis, the induced transition by the same isolated roughness element has been studied over the EXPERT wind tunnel model and over a 7deg half-cone with a combination of steady and unsteady measurement techniques. Results show the importance of both the wind tunnel noise as well as the surface quality upstream of the roughness element on the induced transition mechanisms.

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Manufacturer von Karman Institute for Fluid Dynamics

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