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.
The ignition and combustion characteristics of the hydrogen plume issued from two pylon-type injectors in a Mach 2.4, high-enthalpy airflow are presented. Specifically, the focus of the study is on the effects of the imposed interaction and subsequent dynamics of a system of selected supersonic streamwise vortices on the reacting plume morphology and its evolution. The design phase of the experimental campaign was carried out with a reduced-order model, with the goal of identifying peculiar interactions among streamwise vortical structures introduced in the flow of interest. Two vortex interaction modes have been selected and later implemented using ramp-type vortex generators positioned symmetrically and asymmetrically on the pylon injectors, as prescribed by the results of the simulations reported here. Hydrogen/air combustion experiments, aimed at investigating the selected cases, were conducted in the expansion tube facility of the High-Temperature Gas Dynamics Laboratory at Stanford University. Stagnation enthalpy of 2.8 MJ∕kg, static temperature of 1400 K, and static pressure of 40 kPa were the chosen test conditions. Hydrogen at 300 K was delivered with two different total injection pressures. The supersonic combustion, ignition, and flameholding characteristics were documented with schlieren photography, OH chemiluminescence, and instantaneous OH radicals planar laser-induced fluorescence. The evolution of the reactive vortical system was probed in cross-sectional planes at a distance of 1.8, 4.3, 7.6, and 10.7 cm from the fuel exit plane. The results are thoroughly analyzed and the marked difference in the plume morphology between the two cases is explained. The similarities between the injectant's predicted plumes and the measured distribution of OH radicals across the surveyed planes allow for an interpretation of the results based on supersonic vortex dynamics considerations.
Only a few fundamental studies on the dynamics and interactions of supersonic streamwise vortices have been conducted so far despite the recognized potential of these structures to enhance supersonic mixing. In an effort to shed light on this largely unexplored field, multiple experimental campaigns were conducted in a Mach 2.5 flow to probe the dynamics of turbulence decay in complex flows originating from selected modes of supersonic streamwise vortex interaction. The first part of the manuscript presents the detailed study of two vortex interaction scenarios: one selected to obtain merging of co-rotating vortices and the other to prevent vorticity amalgamation. In the second part, data from three additional vortex merging cases are used to substantiate the findings of the first part of the study and characterize the decay of turbulence. Stereoscopic particle image velocimetry was employed to probe the resulting flow fields at different downstream stations. It was found that these complex vortex interactions measurably affect both the morphology and the magnitude of the streamwise vorticity and turbulent kinetic energy as well as the associated decays. Particularly, while the turbulent kinetic energy across each vorticity patch undergoes an initial production before decreasing monotonically in both scenarios, its content in the coalesced structure is roughly double that of the isolated vortices. The manuscript also presents the analysis of the turbulence data from 27 supersonic vortical structures differing in shape, strength and modes of interaction, acquired within a range of vortex Reynolds numbers of almost one order of magnitude. Dimensional analysis was then used to correlate the spatial decay of turbulent kinetic energy with the vortex Reynolds number. For all the cases considered here, where the fluctuating Mach number was found to be subsonic, the form of the resulting law was similar to that reported in previous scholarly publications, despite the complexity of the vortex dynamics considered in this work.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.