2 research outputs found
Etude expérimentale d'un décollement supersonique ouvert
Une interaction entre une onde de choc et une couche limite turbulente supersonique a Ă©tĂ© expĂ©rimentalement Ă©tudiĂ©e. Quand le gradient de pression adverse que subit la couche limite est assez intense, il entraine le dĂ©collement de celle-ci. Ce phĂ©nomĂšne est le siĂšge dâinstationnaritĂ©s Ă basses frĂ©quences. On sâintĂ©resse au dĂ©collement dit "ouvert" oĂč la couche limite se dĂ©tache de la paroi sans recollement aval. Cette configuration peut ĂȘtre rapprochĂ©e de celle observĂ©e en tuyĂšres dites surdĂ©tendues dans lesquelles lâorigine des instationnaritĂ©s Ă basses frĂ©quences est mal identifiĂ©e. Cette Ă©tude a pour but de dĂ©terminer les mĂ©canismes physiques responsables des instationnaritĂ©s observĂ©es dans ces interactions. Pour cela, une configuration expĂ©rimentale originale a Ă©tĂ© mise en place. La vĂ©locimĂ©trie par imagerie de particules, les mesures de pression statique et totale, ainsi que lâanĂ©momĂ©trie Ă fil chaud ont Ă©tĂ© utilisĂ©es afin de dĂ©crire spatio-temporellement le dĂ©collement ouvert dans une configuration bidimensionnelle. DiffĂ©rents modĂšles utilisĂ©s dans lâĂ©tude des dĂ©collements fermĂ©s ont Ă©tĂ© discutĂ©s afin de tester leur validitĂ© dans le cas ouvert, notamment lâeffet des diffĂ©rentes rĂ©gions de lâĂ©coulement sur les instationnaritĂ©s du choc en fonction de lâintensitĂ© de lâinteraction. LâinterprĂ©tation proposĂ©e est que les basses frĂ©quences du mouvement du choc sont dues aux fluctuations de pression extĂ©rieures imposĂ©es en aval de lâinteraction et quâil nâexiste pas dâĂ©vidence de crĂ©ation de basses frĂ©quences propres Ă lâinteraction. De plus, le systĂšme semble jouer le rĂŽle dâun filtre passe bas et d'un "amplificateur de bruit" vis Ă vis de lâexcitation avalAn interaction between a shock wave and a turbulent supersonic boundary layer isexperimentally investigated. When the adverse pressure gradient that the boundary layer incurs is strong enough, it leads to the flow separation. This project deals with an interaction, which compares with free shock separation regime (FSS), and occurs when the boundary layer does not reattach to the wall. This configuration is similar to that observed in over expanded nozzles. We note inside this interaction a development of unsteadiness of which the origin is not well identified. The aim of this study is to give an understanding of the physical mechanisms inducing these unsteady features. Here we present the results on an original experimental set-up of a 2D FSS. Pitot probe, static pressure probe, hot wire anemometry and particle image velocimetry are used to describe the spatial and temporal organization of the flow. We discuss different models which were documented in the literature and used to study the restricted shock separation regime in order to check their validity in FSS case, in particular the contribution of different regions of the flow (upstream boundary layer, flow downstream the interaction) in the interaction unsteadiness. No evidence of intrinsic low time scales of the shock has been observed and the dominant frequencies of the shock spectrum are found similar to that of the subsonic external flow downstream the interaction. Further more, shock reaction in frequency and amplitude to downstream perturbations has been enlightened : we suggest that the system plays the role of a low pass filter towards the downstream fluctuations and it behaves as a "noise amplifier
Unsteadiness in supersonic Free Shock Separation in overexpanded flows
International audienceWhile operating in an overexpanded condition, nozzles exhibit side loads. These loads are due to the presence of unsteadiness in the shock wave boundary layer interactions which are developing inside the nozzle. This paper deals with an interaction, which compares with Free Shock Separation regime, and occurs when the boundary layer doesn't reattach to the floor. We present the first results on an original experimental setup of a 2D Free Shock Separation, at Mach number of 2.1. Pitot probe, static pressure probe, hot wire anemometry and Particle Image Velocimetry are used to describe the spatial organization of the flow, as well as the frequency scales obtained in various configurations. No evidence of intrinsic low time scales of the shock has been observed and the dominant frequencies of the shock spectrum are found similar to that of the subsonic external flow. Nevertheless, the amplitude of the low frequency motions are found to increase when the flow separates