Fabrizio Vergine

Vortex dynamics studies in supersonic flow: Merging of co-rotating streamwise vortices

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 superso...

Supersonic Combustion of Pylon-Injected Hydrogen in High-Enthalpy Flow with Imposed Vortex Dynamics

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. Stagn...

Evolution of large-scale structures generated by a strut injector in a mach 2.5 flow

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 (TiO2, 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.

Supersonic Combustion and Flame-Holding Characteristics of Pylon Injected Hydrogen in a Mach 2.4 High Enthalpy Flow

This work investigates the structure of the reaction zones downstream of a pylon injecting hydrogen fuel into a Mach 2.4 high enthalpy flow, with the aim of characterizing the behavior and evolution of the expected vortical structures. The pylon injector incorporates expansion ramps that serve as vortex generators. Fuel is injected through a thin slit along the entire length of the base of the ramps. Tests were conducted in the Stanford University Expansion Tube Facility at the High Temperature Gasdynamics Laboratory. All the experiments were conducted at a stagnation enthalpy of 2.8MJ/kg, a static temperature of 1400K, a static pressure of 40kPa, and Mach number of 2.4. The total temperature of the injected hydrogen was 300K. The ignition and flame holding characteristics were investigated using schlieren, time-integrated OH* chemiluminescence, and instantaneous OH planar laser-induced fluorescence (OH PLIF) imaging. OH PLIF was applied to capture the evolution of the reactive vortical system in planes normal to the freestream flow direction at a distance of 1.8cm, 4.3cm, 7.6cm and 10.7cm from the fuel exit plane. Resulting images show the distribution of OH radicals in the plume; these images also show a peculiar geometric pattern in the plume shape, suggesting that the vortices generated by the pylon configuration, and the consequent vortex dynamics, played a dominant role in the mixing and combustion processes.

Study of two supersonic streamwise vortex interactions in a Mach 2.5 flow: Merging and no merging configurations

This work presents the detailed analysis of the flow fields resulting from two supersonic vortex interaction modes in a Mach 2.5 cold flow. The vortex interactions were selected beforehand by means of a reduced order modeling tool to obtain merging of the co-rotating structures in two counter-rotating vortex pairs in one case and to prevent their merging in the second. To experimentally target the flow physics of interest, expansion ramps of the same height but different width and mutual distance were placed on the surface of a strut injector. The resulting flow fields have been characterized using stereoscopic particle image velocimetry surveys conducted at selected streamwise planes. Accurate measurements of the velocity and vorticity fields, strain rates, circulation, and the rate of change of the area of each vorticity patch in the surveyed flow highlight the differences between merging and non-merging scenarios as well as the role of viscosity and turbulent diffusion in the resulting vortex dynamics....

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...

Experimental study of strut injectors for scramjet combustors (invited)

Despite the recognized importance of streamwise vortices in the enhancement of fuel/air mixing processes in scramjet combustors, the effects of their interactions on mixing and associated total pressure losses are still largely unexplored. This work represents the first effort to find answers to fundamental questions such as: can selected vortex interactions be identified and effectively used in an injection system for scramjets? For this reason, experimental investigations of the flowfields resulting from selected vortex interaction scenarios in cold flow are presented. The experiments were conducted in the supersonic wind tunnel of the Aerodynamics Research Center at the University of Texas at Arlington. The mean velocity flowfields of two selected vortex dynamics scenarios, chosen based on the outcomes of the simulations of an inviscid reduced order model, are probed by means of stereoscopic particle image velocimetry. The same streamwise vortices were used to investigate the dynamics of a vortex merging scenario and those in which the merging is voluntarily avoided. The results from the mean flow highlight major differences between the two cases and corroborate the use of an inviscid model to predict the main flow physics. Also other different vortex interaction cases have been successfully targeted and investigated, showing the possibility of a new approach in the design of injection strategies suitable for adoption in scramjet combustors based on the ability to predict, with basic vortex dynamics concepts and a highly reduced computational cost, the main features of flows of technological interest.

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.

Filtered rayleigh scattering measurements in helium-air mixing experiments of selected streamwise vortex interactions in a mach 2.5 flow

An experimental study of the mixing capabilities of a pre-selected streamwise vortex interaction mode in a Mach 2.5 flow is presented. The vortex dynamics of interest were targeted by placing expansion ramps of the same height with a mutual overlap distance on the surface of a strut injector specifically designed for the study of multiple vortex interactions in supersonic flow. The experiments were performed in the supersonic wind tunnel facility of the Aerodynamics Research Center at the University of Texas at Arlington. This investigation complements and expands on previous studies that have primarily focused on the dynamics and merging characteristics of large-scale streamwise, turbulent, vortical structures, in order to augment the understanding of the effects of the resulting flowfield on the mixing processes. Non-intrusive measurements based on the laser diagnostic technique of Filtered Rayleigh Scattering have been used to measure the concentration of an injected passive scalar (helium) at two planes downstream of the injection point. The injection scheme characterized by an imposed vortex dynamics scenario is investigated and timeaveraged plots of helium concentration are calculated. The data related to helium concentration are then compared with the flowfields provided by Stereoscopic Particle Image Velocimetry to identify key controlling parameters of the injection scheme under investigation to assess the influence of the resulting streamwise vortex interactions on the mixing process. An estimate of the peak helium concentration decay rate is found using the data obtained from the two streamwise stations.

Vortex dynamics in supersonic mixing flows

The effectiveness of supersonic combustion is crucial for the development of air-breathing hypersoni…