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Experimental investigation of induced supersonic boundary layer transition

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VKI PHDT 2014-07, Henny Bottini, Experimental investigation of induced supersonic boundary layer transition, ISBN 978-2-87516-069-0, 184 pgs

wind tunnel qualification, heat-flux measurement, adiabatic-wall temperature measurement, unsteady temperature, unsteady pressure, supersonic boundary layer, roughnessinduced transition, shock-wave/boundary-layer interaction

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  • wind tunnel qualification, heat-flux measurement, adiabatic-wall temperature measurement, unsteady temperature, unsteady pressure, supersonic boundary layer, roughnessinduced transition, shock-wave/boundary-layer interaction

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Experimental investigation of induced supersonic boundary layer transition
By Henny Bottini

PhD Thesis from the von Karman Institute / Universidad Politecnica de Catalunya (UPC), March 2014, ISBN 978-2-87516-069-0, 184 pgs


Abstract

Systems of oblique shock waves, generated by ramps upstream of the inlet entrance, are the preferred solution to compress the flow prior to the combustor of supersonic/hypersonic engines. To ensure an efficient compression, boundary layer separation along these ramps must be avoided. However, an oblique shock impinging onto a laminar boundary layer is recognized as one of the most dangerous trip to separation. To cope with this issue, turbulence triggers, in the form of single roughness element, can be located on the ramps as to change the boundary layer from the separation-prone laminar state into the more robust turbulent state. These turbulence triggers may need a finite distance to set a fully turbulent boundary layer. Within this distance the boundary layer in the wake of the roughness is neither laminar, nor turbulent, but transitional, i.e., in the process of becoming turbulent.

The measurements in the present doctoral thesis relate the height of the roughness to the appearance of turbulence at given freestream Mach numbers, and for different initial wall temperatures. To perform such measurements a supersonic multiple-Mach-number wind tunnel for transition experiments was qualified. The qualification test- campaign consisted in assessing the breadth of the achievable Mach range, and in measuring the freestream fluctuations intensity.

In the following experimental test campaign a flat plate model was equipped with a square-based roughness element of two different heights (0.1 and 1 mm), and tested at two different Mach numbers (1.6 and 2.3). Thin-film measurements were performed downstream of the roughness element, along the streamwise and spanwise directions, to track the spatial evolution of heat-transfer and adiabatic wall temperature, and to detect the spectral characteristics of the temperature fluctuations in the wake of the roughness. Fast response piezoelectric pressure sensors complemented these measurements by capturing the spectral characteristics of the pressure fluctuations.

A third test campaign was carried out in a Mach-2 facility to investi- gate the effects of an impinging oblique shock wave on the wall-temperature fluctuations within the wake of a single roughness element.

The goals of the measurements were the study of the steady ad unsteady characteristics of the roughness wake to assess the effectiveness of a single roughness element of given shape and dimensions in triggering transition in supersonic-air-intake-like flow conditions. Thus, the measurement results can be used to decide whether this roughness element placed on an air intake in a supersonic flow is capable of mitigating, or eliminating altogether, the danger of boundary layer separation.

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