Reusable thermal protection systems are one of the key technologies that have to be improved to enable long-duration hypersonic flights. Transpiration cooling has been demonstrated to be one of the most promising active cooling techniques in terms of coolant mass requirements and disturbance of the external flow. Previous numerical studies, conducted by the authors on the conjugate boundary-layer/material-response analysis, along with the current manufacturing capability of manipulating the natural properties of porous materials (e.g., porosity, permeability, and thermal conductivity) have demonstrated the cooling potential when variable transpiration is considered. In this work, a methodology for the nonintrusive characterization of the local effective permeability of a complex carbon–carbon porous structure is proposed. The concept of effective permeability, conceived as the local blowing capability of a porous structure with respect to a selected coolant fluid, is also discussed. Specifically, the coolant (air) mass flux blown from a conical porous surface has been measured by a hot-film probe at a distance specified by an appropriate reference elementary area and the Reynolds number based on the channels’ diameter. These measurements have then been related to the pressure gradient across the local material’s thickness by using Darcy’s law. Measurements have revealed a higher effective permeability near the nose of the cone where two longitudinal delaminations have been identified. The asymmetric blowing capability of the cone highlights the importance of characterizing the entire thermal protection system instead of defining the overall properties of the material, which can be drastically different at the full-scale level due to the geometry, the system integration (i.e. structural constraints), and the intrinsic defectology coming from the manufacturing process. Additionally, the mass fluxes measured on the external porous surface will support the numerical aerothermal rebuilding of the wind-tunnel experiment on the transpiration cooling.
This work aimed to investigate the evolution of a counter-rotating vortex pair (CVP) in a flow-mixing configuration designed to explore the interaction of selected vortical structures. A CVP is shed from an expansion ramp mounted on a strut injector in a Mach 2.5 flow. Shear flow at the plume's edges is due to sonic, parallel injection throughout the thin port located at the base of the ramp. The investigation was conducted in the supersonic wind tunnel at the Aerodynamics Research Center of the University of Texas at Arlington. Stereoscopic particle image velocimetry was the technique adopted to probe the resulting flowfields. A mixture of air and tracer particles (TiO 2 , nominal diameter 20nm) was injected at a nominal jet-to-freestream momentum flux ratio of 0.27, corresponding to a total pressure of 1atm in the injector's plenum. Mean flowfield data were obtained at three different cross-planes (10, 16 and 32 ramp's heights downstream of the injection point) and, at two selected streamwise planes. The dominant effect of the generated CVP (one order of magnitude stronger than the spanwise rollers) profoundly impacted the plume morphology.
For air-breathing propulsion systems intended for flight at very high Mach numbers, combustion is carried out at supersonic velocities and the process is mixing limited. Substantial increase in mixing rates can be obtained by fuel injection strategies centered on generating selected modes of supersonic, streamwise vortex interactions. Despite the recognized importance, and potential of the role of streamwise vortices for supersonic mixing enhancement, only few fundamental studies on their dynamics and interactions have been conducted, leaving the field largely unexplored. A reduced order model that allows the dynamics of complex, interacting, supersonic vortical structures to be investigated, is presented in this work. The prediction of the evolution of mutually interacting streamwise vortices represents an enabling element for the initiation of an effective, systematic experimental study of selected cases of interest, and is an important step toward the design of new fuel injection strategies for supersonic combustors. The case presented in this work is centered on a merging process of co-rotating vortices, and the subsequent evolution of a system composed of two counter-rotating vortex pairs. This interaction was studied, initially, with the proposed model, and was chosen for the peculiarity of the resulting morphology of the vorticity field. These results were used to design an experimental investigation with the intent to target the same specific complex flow physics. The experiment revealed the same peculiar features encountered in the simulation.
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