Particle image velocimetry measurements were made along the center plane of a scramjet cavity flameholder to analyze simulated inlet flow distortion in the direct-connect test environment. Mach 3 nonreacting tests examined an oblique shock impinging upon locations in-and upstream of the cavity, including cases with wall-normal air injection upstream of the cavity to simulate fuel injection. Addition of flow distortion altered the size and shape of the primary recirculation region within the cavity by deflecting the bounding shear layer: the recirculation region was compressed by shock impingement upstream of the cavity, and shock impingement on the cavity itself expanded it. Air injection upstream of the cavity thickened the shear layer and produced a stronger effect on velocity direction than magnitude, preventing the formation of a large-scale recirculation region in two of the three shock locations studied. Flow distortion and upstream air injection both increased flow unsteadiness, with the greatest increases occurring in the shear layer and above the cavity closeout ramp. Additionally, results suggest the formation of spanwise secondary flow patterns that may account for flow nonuniformities observed in prior studies. This work presents the first velocimetry characterization of a scramjet cavity flameholder under distorted-flow conditions.
The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. Direct-connect simulations of scram jet combustors typically use facility nozzles designed to produce uniform flow entering the test article. Conversely, in free-jet and flight experiments, where air is ducted to the supersonic combustor through an inlet, flow entering the test article will be inherently distorted. These distortion effects can include non-uniform boundary layer thicknesses on the walls and relatively strong oblique shock waves. In this work, a special piece of hardware (called a distortion generator) was designed to mimic the effects of inlet distortion in a direct-connect test environment. The design methodology for this distortion generator will be described along with details of its fabrication and installation into the experimental research facility. Finally, the results of computational and experimental calibrations will be presented. These results confirm that distortion characteristics anticipated in freejet and flight experiments can be effectively simulated in the direct-connect test environment. This new hardware will enable future experimental investigations aimed at understanding the effects of inlet-induced distortion on combustor operability and performance.
Simulation results are presented for non-reacting flow within a supersonic cavity flameholder. The freestream is air at Mach 2. A case is simulated with no fuel injection, and two cases are simulated with different rates of ethylene fuel injected through holes located on the back face of the cavity. The simulations correspond to a series of experiments for which particle image velocimetry measurements of two velocity components were made within the cavity. Reynolds-averaged Navier-Stokes simulations are used to examine the influence of the finite-width, low-angled slot used to seed the flow for the particle image velocimetry. The simulations indicate that the seeder has little influence on the flow within the cavity, allowing for the seeder to be neglected in the remainder of the simulations. The flow within the cavity is simulated using steady-state Reynolds-averaged Navier-Stokes simulations, as well as unsteady hybrid Reynolds-averaged Navier-Stokes and large-eddy simulations. A thorough grid resolution study is presented in which the resolution required to resolve the flow within the cavity is determined. The results of the simulations on the final grids are compared to the velocity measurements from the experiments and the hybrid Reynolds-averaged Navier-Stokes and large-eddy simulation results are found to provide improved agreement with certain aspects of the flow. The simulation results are then used to investigate the mixing within the cavity, which was not measured in the experiments. The mixing information from the simulations provides further insight into the physics of the cavity flameholder flowfield.
Computational fluid dynamics simulations were performed on four generic isolator configurations with aspect ratios of 1, 3, 6, and 9. Inflow conditions were supersonic flow at Mach 2 and Mach 3.2; outflow conditions represented 80% of the normal-shock pressure rise for the flow. The analysis focuses on the variation in shock train length and shape with isolator aspect ratio.
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