Cody Ground

Strut Injectors for Scramjets: Total Pressure Losses in Two Streamwise Vortex Interactions

Pressure losses and mixing effectiveness are key parameters in the performance of an injection system suitable for scramjet engines. This work thus focuses on the characterization of the total pressure losses associated with two selected vortex interaction processes in a Mach 2.5 flow in order to better understand the role of vortex dynamics in the operation of scramjet combustors. Specifically, the interaction modes are targeted to obtain merging of two corotating vortices in one case while preventing it in the second. The streamwise vortices are shed from ramps placed on the surface of a strut mounted in a cold-flow supersonic wind tunnel. In-stream pitot and total temperature measurements are conducted at two downstream streamwise stations located within a shock-free region. Stereoscopic particle image velocimetry is used in conjunction with the intrusive measurement techniques to retrieve the local Mach numbers and the total pressure in the flows of interest. The losses are quantified through the pres...

Computational analysis and characterization of the UTA 1.6 MW arc-heated wind tunnel facility

The present work focuses on a Computational Fluid Dynamics (CFD) investigation of the high-enthalpy flow produced by the 1.6 MW arc-heated wind tunnel facility of the University of Texas at Arlington. The numerical analysis is complementing a parallel experimental campaign dedicated to the aerothermodynamic characterization of the archeated plume originated by a nominal M=1.8 conical nozzle that has been used for the screening of candidate reusable C-C/SiC based materials for thermal protection system (TPS) applications. Due to dramatic changes in the thermo-mechanical response of the TPS when specific conditions of temperature, species composition, and pressure are reached, the knowledge of the flow conditions at the nozzle’s exit is an essential element for the identification of appropriate test conditions during the design of the experiment first, and the interpretation of the results later. Vibrational and chemical non-equilibrium conditions must be considered in the high-temperature nozzle flow when the characteristic times for the chemical reactions and vibrational energy are comparable to the characteristic flow time. The Navier-Stokes equations are solved with the implicit finite volume density-based solver of the commercially available CFD code Fluent. To solve the species equations two separate chemical kinetics models, the Park and Gupta models, are implemented in order to compare their results and performance in the current facility characterization. From the numerical characterization of the nozzle flow, new facility performance envelopes have been generated providing analytical correlations between the relevant aerothermodynamic parameters at the nozzle exit plane and the input facility operation conditions.

Computational study and error analysis of an integrated sampling-probe and gas-analyzer for mixing measurements in supersonic flow

Concentration probes are employed in supersonic flow mixing measurements. Because the typical design of such probes is essentially based on an inviscid, adiabatic, quasi-1D analysis, the scope of this work is to understand better and quantify the severe impact of viscous effects on the probes internal gasdynamics and the associated uncertainties in the measured quantities via a computational fluid dynamics analysis. Specifically, the focus is on the augmented errors due to the aforementioned viscous effects when coupled with various cases of probe-flow misalignment, which is a typical scenario encountered in mixing measurements of binary gas compositions (air and helium in the present work) in vortex-dominated flows. Results show phenomena such as shock induced boundary layer separation and the formation of an oblique shock train. These flow features are found to noticeably affect the accuracy of the composition measurement. The errors associated with the inviscid, adiabatic, quasi-1D analysis of the probes are quantified in this study.

Design and Application of Filtered Rayleigh Scattering Experiments for Mixing Studies of New Strut Injectors for Scramjets

The description of the setup and experimental methodology used to quantify the local mixing of a helium-air mixture with the non-intrusive laser based Filtered Rayleigh Scattering (FRS) technique in a Mach 2.5 vortical flow is presented. Of particular interest is the effect of the imposed vortex dynamics on the degree of mixedness of the two constituent mixture components in the region of the mutually interacting co-rotating vortices generated by a strut injector specifically designed for the study of multiple vortex interactions in supersonic flow. The application of the FRS technique in such highly complex vortical flows, however, is not straightforward. Thus, detailed preliminary experimental characterization of all subsystems of the measurement is required along with a methodical experimental design procedure in order to achieve consistently successful measurements that are meaningful and relevant. All aspects of the experimental design and methodology will be presented, beginning with the necessary theoretical background of the FRS procedure and concluding with the preliminary results and supporting evidence showing the proper design of the mixing experiments.

Visualization of supersonic turbulent vortical flows with filtered Rayleigh scattering

The interactions of multiple streamwise vortices have the potential to enhance the fuel/air mixing process in the supersonic flowfields intrinsic of scramjet combustors. As part of an ongoing experimental study on the mixing enhancement capabilities of imposed streamwise vortex interactions, high quality visualizations of the resulting turbulent vortical flowfields have been obtained with the filtered Rayleigh scattering technique. By visualizing the structure and organization of the studied flowfields, which are generated by a ramped strut injector in a Mach 2.5 flow, insight into the relevant flow physics governing the vortex interactions is obtained. Filtered Rayleigh scattering is a non-intrusive laser-based diagnostic technique that images the light scattered from the constituent gas molecules of a flow, thus providing a distinct advantage over other flow visualization techniques, in that it is not required to seed the flow with gaseous or particulate flow tracers. However, because the Rayleigh scattered signal is relatively weak, it is easily obscured by light scattered from other sources, necessitating the implementation of a molecular notch filter. This fact, combined with the complex supersonic turbulent vortical flows that are of interest, makes the application of the filtered Rayleigh scattering technique challenging and non-trivial. This work will highlight the experimental considerations necessary to achieve successful visualization of the flows of interest by presenting the methods used to image a single downstream station of a non-merging vortex interaction mode. The resulting flow visualizations illustrate the highly turbulent nature of the probed flowfield while confirming the behavior observed in previous experiments.Graphical abstract

Design of a model scramjet combustor for vortex-enhanced mixing and combustion studies

The present work details the preliminary analysis and design of a model scramjet combustor intended to serve as a research platform for mixing and combustion studies in support of current non-reactive research. The shock tunnel located at the Aerodynamics Research Center of The University of Texas at Arlington, capable of producing test conditions simulating scramjet flight at Mach numbers ranging from ~5 to ~16 at altitudes of ~24 km to ~85 km, will serve as the test facility for the model combustor. Specifically, the modular design of the model scramjet combustor will allow for the study of supersonic combustion and flameholding characteristics of specific fuel injection strategies based on enhanced fuel-air mixing via pre-imposed selected modes of streamwise vortex interactions and consequent dynamics. In this paper, the fundamental requirements for the combustor design and the performance of the UTA hypersonic tunnel are discussed. A parametric sizing of the model combustor is utilized to define a suitable design that meets the specified requirements, and the inevitable constraints imposed by the limitation of ground testing facilities, discussed in the manuscript.

Experimental and numerical investigation of the flow characteristics of a strut injector for scramjets

A peculiar dynamical interaction of two supersonic counter-rotating vortex pairs (CVPs) has been generated by a pair of overlapping expansion ramps mounted on a strut injector in a Mach 2.5 flow. Similar CVP configurations generated by overlapping ramps on the same modular strut injector have previously been studied by the authors and experimentally corroborated the predictions based on the in-house developed reduced order model, VorTx. Two co-rotating vortices at the center of the generated structure have either merged or begun to orbit each other. However, for the case discussed in this work, experiments have revealed an additional structure; in fact, instead of the expected four streamwise vortices in the flowfield consisting of two inner co-rotating vortices and two oppositely rotating external vortices, the formation of a fifth structure, a central vorticity patch, was detected. The manuscript presents a detailed experimental characterization of the flow of interest via the use of stereoscopic particle image velocimetry and a parallel computational effort based on the solution of the Reynolds Averaged Navier-Stokes equations with the ANSYS Fluent CFD Package. The results of the CFD simulations highlight the near injector flowfield, which is unable to be characterized experimentally, in order to focus upon the mechanism that result in the formation and the effects of the central vorticity patch.

Teflon probing for the flow characterization of the 1.6MW arc-heated wind tunnel of the University of Texas at Arlington

From the beginning of space exploration, protecting the vehicle’s structural integrity from the high-enthalpy flow encountered during atmospheric reentry, has been a primary concern. Particularly in the last decade, the necessity to extend the flight duration in the hypersonic regime has prompted a continuous development of new heat-shields. In this scenario, the testing of Thermal Protection Systems (TPS) plays an essential role in understanding the thermal response of candidate TPS materials when prolonged exposures to highenthalpy flows are considered. In order to reproduce the aero-thermodynamic environment present in real flight conditions the Mach number, flight duration, altitude, surface temperature, gas-surface interaction effects are some of the parameters to be replicated. Existing ground testing facilities are not able to simultaneously reproduce all the aforementioned parameters. Furthermore, testing facilities introduce flow disturbances and contaminations (such as non-uniformities deriving from the arc stabilization techniques and debris due to the erosion of the material parts exposed to the hot flow within the test facility) with respect to the free-stream flow in real flight conditions. The flow characterization of the arc-heated wind tunnel of the University of Texas at Arlington is investigated in this work. The Teflon material has been selected for this analysis because of its well-documented ablation properties. The stagnation point heat flux retrieved by the analysis of the ablated surface is coupled with Pitot pressure measurements to calculate the total enthalpy. Several locations downstream of the nozzle exit have been surveyed to determine the values of the stagnation point heat flux. The analysis of the ablated surface of Teflon is used to identify possible flow non-uniformities.