* Collaborators: Matthew Berry, Andrew Magstadt, Cory Stack, Mark Glauser, Datta Gaitonde.* A rectangular supersonic multi-stream jet with aft deck is analyzed using large eddy simulation (LES) and experimental data. Proper orthogonal decomposition (POD) and Dynamic mode decomposition (DMD) are applied to time-resolved schlieren, stereo particle image velocimetry (PIV), and LES data. The spatial eigenfunctions of the two datasets reveal similar trends in the flow field, where structures with high mean squared value propagate from the nozzle lip and mixing layer. In the schlieren, acoustic waves can also be seen radiating from the nozzle lip and edge of the aft deck. By combining the spatial eigenfunctions from the schlieren experiment and the time-dependent snapshots of the LES, low-dimensional time characteristics for the flow are reconstructed. Additionally, we compare the spatial modes from the PIV and LES to determine how well the coherent structures are being captured in the simulation.

- Berry, M. G., Ali, M. Y., Magstadt, A. S., Glauser, M. N., “DMD and POD of Time-resolved Schlieren on a Multi-stream Single Expansion Ramp Nozzle,”
*In print – International Journal of Heat and Fluid Flow*, 2017 - Berry, M. G., Stack, C. M., Ali, M. Y., Magstadt, A. S., Gaitonde, D. V., Glauser, M. N., “Analysis of a Rectangular Supersonic Multi-stream Jet by LES and Experiments,”
*10th International Symposium on Turbulence and Shear Flow Phenomena (TSFP10)*, Chicago, USA, July, 2017. - Berry, M. G., Magstadt, A. S., Ali, M. Y., Glauser, M. N., Ruscher, C. J., Gogineni, S., “Time- resolved schlieren POD and aft deck pressure correlations on complex supersonic jet nozzles,”
*AIAA 2017-0553*, January 2017. - Berry, M. G., Magstadt, A. S., Stack, C. M., Ali, M. Y., Gaitonde, D. V., Glauser, M. N.,“Time- resolved schlieren POD and aft deck pressure correlations on a rectangular supersonic nozzle and sonic wall jet,”
*69th Annual APS Division of Fluid Dynamics Meeting*, Portland, OR, November 2016.

Fast-response pressure sensitive paint (PSP) is used to measure and analyze the acoustic pressure field in a rectangular cavity. The high spatial resolution and fast frequency response of PSP effectively captures the spatial and temporal detail of surface pressure resulting in the acoustic pressure field. A high-speed camera is used to generate a continuous time record of the acoustic pressure fluctuations with PSP. Since the level of the acoustic pressure is near the resolution limit of the sensor system, advanced analysis techniques are used to extract the spatial modes of the pressure field. Both dynamic mode decomposition (DMD) and proper orthogonal decomposition (POD) are compared with phase averaging for data analysis. While all three techniques effectively extract the pressure field and reduce the impact of sensor noise, DMD and POD are more robust techniques that can be applied to aperiodic or multi-frequency signals. Furthermore, DMD is better than POD at suppressing noise in particular regions of the spectrum and at effectively separating spectral energy when multiple acoustic excitation frequencies are present.

- Ali, M. Y., Pandey, A., and Gregory, J. W., “Dynamic Mode Decomposition of Fast Pressure Sensitive Paint Data,” Sensors, Vol. 16, No. 6, 2016.

The flow physics of liquid spray generated by different nozzle geometric configurations and inlet pressures have been characterized theoretically, computationally, and experimentally for integration into next-generation emission control systems. This research and development has focused on swirl or angular momentum enhanced micro-droplet breakup of the liquid jet to facilitate control of mixing urea (or water used as a model) with engine exhaust. The research has combined new nozzle design concepts, computational fluid dynamics, reduced order modeling, and experimental characterization. The results illustrate advanced performance in a simple, compact nozzle body relative to the commercial nozzle design.

Nozzle Designs to Enhance Spray Formation using Tailored Swirl and Shear – Pending Patent, 2015.

An array of high-momentum microjets are used upstream of a compression corner to control the shock wave/boundary layer
interaction on a 24° unswept compression ramp in a Mach 2 flow. Measurements include schlieren flow visualization and
unsteady pressure measurements using fast-response pressure sensors of the interaction region. Results show that the array
of microjets issuing in the supersonic crossflow create oblique shocks, which effectively reduce the incoming Mach number
at the compression corner. This leads to a modified separation shock of significantly reduced strength. The location of
the modified shock is moved upstream by as much as 4δ_{o} from its mean undisturbed location. The mean
pressure distribution on the surface is altered with microjet control leading to a more gradual compression of the
incoming flow relative to the separation shock without control. The wall-pressure fluctuations in the interaction region
are reduced by approximately 50%, and the flow near the compression corner appears to be energized with control, based
on the unsteady surface-pressure measurements. The pressure spectra show that microjet control results in a redistribution
of energy on the wall and the ramp surfaces.

Ali, M. Y., Alvi, F. S., Kumar, R., Manisankar, C., Verma, S. B., and Venkatakrishnan, L., "Studies on the Influence of Steady Microactuators on Shock-Wave/Boundary-Layer Interaction," AIAA Journal, Vol. 51, No. 12, 2013, pp. 2753 - 2762.

Jets in supersonic crossflow are known to produce a three-dimensional bow-shock structure due to the blockage of the flow. 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 (AIAA Journal, Vol. 49, No. 12, 2011, pp. 2751 – 2759). Multiple jets in a linear array interact with both one another and the incoming supersonic flow. Jet injection in supersonic crossflow is known to create a pair of counter-rotating vortex pairs (CVPs) and generate streamwise vorticity. The streamwise CVPs generated by the each microjet in an array remain coherent until about 20 diameters from the point of injection for arrays with larger spanwise separation distances between the micro-orifices (AIAA Paper 2013-3117). However, as the center-to-center spacing is reduced the CVPs interact with one another and dissipate rapidly leading to reduced vorticity generation. The flow structure for the array with reduced center-to-center spacing approaches to that of a two-dimensional shock oblique shock.

- Ali, M. Y. and Alvi, F. S., “Jet Arrays in Supersonic Crossflow - An Experimental Study,” Physics of Fluids, Vol. 27, 126102, 2015.
- Ali, M. Y., Alvi, F. S., Kumar, R., and Ahmed, K. A.,“Flowfield Characteristics of Oblique Shocks Generated using Microjet Arrays,” International Journal of Flow Control, Vol. 6, No. 3, 2014.
- Emerick, T. M., Ali, M. Y., Foster, C. H., Alvi, F. S., Popkin, S. J., and Cybyk, B. Z., "SparkJet Actuator Characterization in Supersonic Crossflow," Experiments in Fluids, Vol. 55, 2014.
- Ali, M. Y. and Alvi, F. S., “Three – dimensional Flowfield of Microjets in Supersonic Crossflow”, AIAA Paper 2013-3117, June 2013.
- Emerick, T. M., Ali, M. Y., Foster, C. H., Alvi, F. S., Popkin, S. J., and Cybyk, B. Z., "SparkJet Actuator Characterization in Supersonic Crossflow," AIAA Paper 2012-2814, June 2012.
- Topolski, M., Arora, N., Ali, M. Y., Solomon, J. T., and Alvi, F. S., "Experiments on Resonance Enhanced Pulsed Microjet Actuators in Supersonic Cross flow," AIAA Paper 2012-2813, June 2012.
- Kumar, R., Ali, M. Y., Alvi, F. S., and Venkatakrishnan, L., "Generation and Control of Oblique Shocks Using Microjets," AIAA Journal, Vol. 49, No. 12, 2011.
- Ali, M. Y., Ahmed, K. A., Kumar, R., and Alvi, F. S., "Flowfield Characteristics of Oblique Shocks Generated using Microjet Arrays," AIAA Paper 2011-0485, January 2011.

The Resonance-Enhanced Microjet (REM) actuator developed at our laboratory produces pulsed, supersonic microjets by utilizing a number of microscale, flow-acoustic resonance phenomena. The microactuator used in this study consists of an underexpanded source jet flowing into a cylindrical cavity with a single orifice through which an unsteady, supersonic jet issues at a resonant frequency of 7 kHz. The flowfields of a 1 mm underexpanded free jet and the microactuator are studied in detail using high-magnification, phase-locked flow visualizations (microschlieren) and 2-component particle image velocimetry. The challenges of these measurements at such small scales and supersonic velocities are discussed. The results clearly show that the microactuator produces supersonic pulsed jets with velocities exceeding 400 m/s. This is the first direct measurement of the velocity field and its temporal evolution produced by such actuators. Comparisons are made between the flow visualizations, velocity field measurements, and simulations using Implicit LES for a similar microactuator. With high, unsteady momentum output, this type of microactuator has potential in a range of flow control applications.

- Kreth, P., Ali, M. Y., Fernandez, E. J., and Alvi, F. S.,“Velocity field measurements on high-frequency, supersonic microactuators,” Experiments in Fluids , Vol. 57, No. 5, 2016.
- Kreth, P., Fernandez, E., Ali, M. Y. and Alvi, F. S.,“High-magnification velocity field measurements on high-frequency, supersonic microactuators,” 67th Annual APS Division of Fluid Dynamics Meeting, San Francisco, CA, November 2014.

- Ahmed, K. A., Ali, M. Y., and Alvi, F. S.,“Mixing Characteristics of Active Microjet-Based Actuators in a Supersonic Backward-Facing-Step Flow,” AIAA Journal, Vol. 52, No. 12, 2014.
- Ahmed, K. A., Ali, M. Y., and Alvi, F. S., "Mixing Characteristics of Active Microjet-Based Actuators in a Supersonic Backward-Facing Step Flow," AIAA Paper 2011-0309, January 2011.
- Ali, M. Y., Solomon, J. T., Gustavsson, J., Kumar, R., and Alvi, F. S., "Control of Resonant Flow Inside a Supersonic Cavity Using High Bandwidth Pulsed Micro-actuators," AIAA Paper 2010-1198, January 2010.