Abstract
The influence of localized nitrogen transpiration on second mode instabilities in a hypersonic boundary layer is experimentally investigated. The study is conducted using a 7^circ half-angle cone with a length of 1100 mm and small nose bluntness at 0^circ angle-of-attack. Transpiration is realized through a porous Carbon/Carbon patch of 44 times 82 mm located near the expected boundary layer transition onset location. Transpiration mass flow rates in the range of 0.05–1% of the equivalent boundary layer edge mass flow rate were used. Experiments were conducted in the High Enthalpy Shock Tunnel Göttingen (HEG) at total enthalpies around 3 MJ/kg and unit Reynolds numbers in the range of 1.4 cdot 10^6 , to 6.4 cdot 10^6 , {text {m}}^{-1}. Measurements were conducted by means of coaxial thermocouples, Atomic Layer Thermopiles (ALTP), pressure transducers and high-speed schlieren. The present study shows that the most amplified second mode frequencies were shifted to lower values as nitrogen is transpired into the boundary layer. In some cases the instability amplitudes were found to be significantly reduced. The observed frequency reduction was verified to correlate with the change of the relative sonic line height in the boundary layer. The amplitude damping was observed to occur only until the most amplified frequencies were reduced to around 50% of their undisturbed values. When transpiration within this limit was performed shortly upstream of the natural boundary layer transition onset, a transition delay of approximately 17% could be observed.Graphic abstract
Highlights
The understanding of the boundary layer transition process in supersonic and hypersonic boundary layers is crucial for the optimized design of vehicles operating at such speeds
Mass addition into a boundary layer through porous surfaces in hypersonic flow has been used in the past in experimental investigations in the 1960s and 1970s to simulate the ablation effect of thermal protection systems
The experimental results presented in this paper were obtained in the High Enthalpy Shock Tunnel Göttingen (HEG) which is a free-piston driven reflected shock tunnel
Summary
The understanding of the boundary layer transition process in supersonic and hypersonic boundary layers is crucial for the optimized design of vehicles operating at such speeds. Mass addition into a boundary layer through porous surfaces in hypersonic flow has been used in the past in experimental investigations in the 1960s and 1970s to simulate the ablation effect of thermal protection systems For these investigations, conical shapes were frequently used, and many different gases were transpired at various flow conditions and locations. The steep rise of the N-factor following further downstream was found to be due to the overlap of multiple unstable frequencies These results suggest an explanation why early investigations consistently observed a negative effect of transpiration on the boundary layer stability. As a possible explanation for the disagreement between the numerical results and the consolidated experimental trends at similar, large-surface transpiration conditions in the literature, it was suggested that an absolutely unstable, wall bounded mixing layer could be the key mechanism of transition in this case, instead of second modes.
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