Boundary-layer transition investigations are earned out on swept wings in supersonic flows (M = 2.4) by employing IRT (infrared thermography). Transition is clearly visible only for sweep angles below 45. In all other cases the flow over the wing is almost completely turbulent, as proved by the use of isolated roughness elements which produced effects typical of turbulent flows. The transition front appears on the thermographic image as a strong gradient (discontinuity) from hot to cold regions or vice-versa, depending upon the initial temperature of the model (respectively warmer or colder than the oncoming flow). This behavior is a direct consequence of the difference in the convective heat-transfer coefficient between laminar and turbulent boundary layers, A laminar boundary layer experiences low heat exchange and acts as an insulator, whereas in a turbulent boundary layer the heat exchange is accelerated. When the model is cooled with liquid nitrogen, i.e. the surface temperature before the run is lower than the external incoming flow, laminar regions are cold and turbulent regions are warm. Transition detection is enhanced when the model surface is colder than the freestream flow. Results demonstrate that IRT is a very appropriate tool for studying laminar-to-turbulent boundary-layer transition. Its straightforward use and the absence of additional operations required by the other techniques available for transition detection (e.g. modification of the model surface, spreading of substances or extra light sources) make it very portable and suited for both wind-tunnel and in-flight testing.
Infrared thermography investigations in transitional supersonic boundary layers
Infrarot-thermographische Untersuchungen in Überschall-Übergangsgrenzschichten
Experiments in Fluids ; 44 , 1 ; 145-157
2008
13 Seiten, 15 Bilder, 36 Quellen
Article (Journal)
English
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