Mohd Y. Ali

Shock-Boundary Layer Interaction due to a Sharp Unswept Fin in a Mach 2 Flow

An experimental study on the interaction of a swept shock wave, generated by a sharp fin, with a turbulent boundary layer is presented. An un-swept fin is placed in a Mach 2 flow where the interaction strength is varied by changing the angle of attack (α) from 10◦ to 15◦. This corresponds to flowfields that are weakly separated (α = 10◦) to fully separated (α = 15◦). Surface oil-flow is used to study the topography of the flowfield. Detailed velocity field measurements using two component Particle Image Velocimetry (PIV) are used to examine the structure of the interaction in a conical reference frame. Two slices of the velocity field at a distance of 1.5 in and 2.0 in from the fin apex (moving along the shock) are examined herein. Typical flow features, including the lambda-shock structure, the slip line and the separation bubble with reverse flow are clearly seen. The velocity field obtained confirm the quasi-conical behavior as well as the physical models developed earlier for this flowfield, which had mainly relied on detailed flow visualizations. To our knowledge, this is the first study that provides direct quantitative measurements of this flow in the appropriate quasi-conical frame of reference.

Generation and control of oblique shocks using microjets

Jets in a supersonic crossflow are known to produce a three-dimensional bow-shock structure due to the blockage of the flow. In the present study, streamwise linear arrays of high-momentum microjets are used to generate either single or multiple oblique shocks in a supersonic crossflow. The shocks generated using microjets can be tailored in terms of their strength and be made either parallel or coalescing, depending on the application. Flow visualization using shadowgraph and density field measurements using background-oriented schlieren (BOS) technique were carried out for a range of microjet operating conditions. The results obtained using the two methods are consistent and complementary and show a linear variation of oblique shock angles with a microjet pressure ratio over the range of conditions tested. The density field obtained using BOS clearly shows the oblique shocks generated using these microjet arrays, the jump in density across the shock, the extent of the high-density field, the expansion fan, and the associated decrease in density. The results suggest that microjet arrays can be successfully used to develop techniques for sonic boom mitigation and high-performance supersonic inlets.

SparkJet Actuator Characterization in Supersonic Crossflow

There are a number of adverse flow phenomena in the high-speed flight regime that can potentially be addressed using flow control techniques. However, requirements for active flow control in high-speed flows have proven technologically challenging. An innovative zero-net-mass-flux device called the SparkJet, has been developed for use in supersonic flow applications. With no moving parts or external mass injection, it utilizes an electrical discharge for the thermal heating of air inside of a cavity to produce pulsed jets. Benchtop results from previous studies have demonstrated the actuator’s capability to produce pulsed high-speed jets. In this study, schlieren imaging of the device was performed in single-shot mode at atmospheric and sub-atmospheric conditions. Jet front velocities from the schlieren images were measured as 240 m/s and 310 m/s for the conditions tested respectively, indicating the potential for higher control authority at lower ambient pressures. An array of SparkJet microjets was then installed in a supersonic tunnel, where the actuator was tested in a Mach 1.5 crossflow in single shot mode and at a pulsing frequency of 700 Hz. Phaseconditioned shadowgraph imaging was used to characterize the influence of the SparkJet array on the supersonic crossflow. The results show the generation of an unsteady oblique shock as a result of the SparkJet issuing into the supersonic crossflow where the shock strengthens and then decays in a periodic manner directly correlated to the SparkJet dynamics. These results demonstrate this actuator’s high-frequency response and control authority for supersonic flow applications.

Flowfield Characteristics of Oblique Shocks Generated using Microjet Arrays

High momentum microjet arrays are used to generate single and/or multiple oblique shocks in a supersonic crossflow. The properties of these microjet generated shocks can be tailored to be parallel or coalescing, at variable supply pressures, depending on the application. Flowfield measurements using Particle Image Velocimetry were obtained for a range of test conditions. The velocity field obtained clearly shows the global effect of the microjet generated oblique shocks on the flowfield. The jump in the velocity field across the oblique shock and the extent of the influence of the shock and expansion fan are also clearly seen from the results. The microjet generated shocks were compared with a ramp shock and it was found that the properties of the two share some similarity. The strength of the microjet shocks was found to be constant along the length of the shock. The results obtained were compared to previous results obtained using Background Oriented Schlieren and shadowgraph techniques and were found to be consistent. The parallel and coalescing properties of the multiple microjet shocks were also investigated.

Jet arrays in supersonic crossflow — An experimental study

Jet injection into a supersonic crossflow is a classical fluid dynamics problem with many engineering applications. Several experimental and numerical studies have been taken up to analyze the interaction of a single jet with the incoming crossflow. However, there is a dearth of the literature on the interaction of multiple jets with one another and with the crossflow. Jets in a supersonic crossflow are known to produce a three-dimensional bow-shock structure due to the blockage of the flow. Multiple jets in a streamwise linear array interact with both one another and the incoming supersonic flow. In this paper, a parametric study is carried out to analyze the effect of microjet (sub-mm diameter) injection in a Mach 1.5 supersonic crossflow using flow visualization and velocity field measurements. The variation of the microjet orifice diameter and spacing within an array is used to study the three-dimensional nature of the flow field around the jets. The strength of the microjet-generated shock, scaling of the shock wave angle with the momentum coefficient, averaged streamwise, spanwise, and cross-stream velocity fields, and microjet array trajectories are detailed in the paper. It was found that shock angles of the microjet-generated shocks scale with the momentum coefficient for the three actuator configurations tested. As the microjets issue in the crossflow, a pair of longitudinal counter-rotating vortices (CVPs) are formed. The vortex pairs remain coherent for arrays with larger spanwise spacing between the micro-orifices and exhibit significant three-dimensionality similar to that of a single jet in crossflow. As the spacing between the jets is reduced, the CVPs merge resulting in a more two-dimensional flow field. The bow shock resulting from microjet injection also becomes nearly two-dimensional as the spacing between the micro-orifices is reduced. Trajectory estimations yield that microjets in an array have similar penetration as single jets. A notional schematic depicting the interaction of the microjet arrays with the supersonic crossflow is presented summarizing the principal features of the interaction.

Experiments on Resonance Enhanced Pulsed Microjet Actuators in Supersonic Cross flow

An array of high-bandwidth, high-momentum pulsed supersonic microjets, called Resonance Enhanced Microjets (REM), were implemented to study the effect of pulsed actuation in a M=1.5 supersonic cross flow over a flat plate. Cross correlated, phase locked flow imaging and unsteady pressure measurements were used to characterize the shock wave-boundary layer interactions resulting from the pulsed actuation. Pulsed microjet actuators in supersonic cross flow have generated oblique shocks whose strength and shock angle are observed to have an invariable correlation with the pulsing phase of the microjets. Unsteady pressure measured downstream also shows a strong correlation to the pulsing phase of the actuator and the resulting oblique shock properties. These experiments clearly point to the potential capabilities of the resonance enhanced microjet actuator to manipulate the unsteady properties of the boundary layer of a supersonic flow.