Date of Award
Doctor of Philosophy (PhD)
Mechanical and Aerospace Engineering
Active Control;Experimental;Flow Control;Machine Learning;Schlieren;SPOD
Flow control techniques are experimentally applied to a multi-stream rectangular supersonic nozzle with the goal of mitigating undesirable flow phenomena and maxi?mizing understanding of the underlying complex flow physics. The nozzle of interest is characterized by a core (M = 1.23) and bypass (M = 1.0) streams that coalesce behind a splitter and expand to achieve M = 1.6 flow before exiting onto an aft-deck plate meant to simulate air frame integration. The region where the two streams first contact, the splitter plate trailing edge (SPTE), acts a genesis for an instability that produces a dominant high frequency resonant tone ubiquitous in the flow field. The SPTE region, as well as the aft-deck plate are identified as receptive locations for the application of flow control techniques seeking to: manipulate the resonant tone, reduce unsteady loading on the deck surface, and manipulate the shock system. Control is applied by way of geometric modification to the SPTE (passive), length of the aft-deck plate (passive), and blowing through an array of jets in the near?nozzle region (active). A schlieren imaging system is constructed within an anechoic chamber to obtain near-field time-resolved density measurements of the baseline and controlled flows as well as far-field acoustic measurements. Spectral Proper Orthogonal Decomposition (SPOD) algorithms separate the space-time scales within the schlieren sets, allowing for the tracking of energy at different scales. Momentum Potential Theory (MPT) filtering of the data sets uses the schlieren pixel intensities as a surrogate model for density gradients, allowing for the solving of a spatial Poisson equation, this in-turn isolates acoustic (irrotational) wavepackets and allows for the identification of noise sources. A span-wise perturbation added to the SPTE is found to induce stream-wise vorticity and mix-out the formation of the tone, as evident by the far-field acoustic measurements. The schlieren-SPOD analysis reveals remaining structures at high frequencies have a broadband energy distribution and move along the SPTE shear layer in direct downstream paths. This is in contrast to the baseline flow, who’s high?frequency structures are more narrow-band and radiate throughout the flow field. With the perturbed SPTE present, a neural network is constructed to analyze large data sets obtained from the deck length studies revealing a link between the shock structures relative to the deck’s end and the noise output of the system. It is observed that when the end of the aft-deck coincides with existing structures impacting the deck, the shock that forms at the deck’s end merges with the reflected shock from the shock-train leading to minimal plume deflection and a low-noise bucket. Deck lengths that do not line up with the pattern of the shock-train do not permit the merging of these structures and in-turn create additional shedding formations that lead to an increased noise output of the system. Active control via steady blowing in the region between the SPTE and nozzle exit reveals the ability to move the shock structure’s impingement locations and amplify the magnitude of the resonant tone while reducing its frequency for near?SPTE blowing. The closer the actuation array to the SPTE the more pronounced the amplification of the tone. This amplification is further compounded when increasing the mass-flow rate of the jet array. Orienting the jet array at different downstream angles is shown in some cases to reduce the magnitude of the resonant tone while displaying similar control authority over shock locations. A hybrid control case is implemented utilizing both the passive and active control methods together yielding the benefits of each - eradication of the tone via the passive control and control of the shock train via the active control. The novel MPT filtering approach is applied to the baseline, passive, active, and hybrid control cases to reveal the prominence of the shock-shear interaction along the lip shear layer as a source for noise generation.
Kelly, Seth, "On the Control of a Supersonic Multi-Stream Rectangular Nozzle" (2024). Dissertations - ALL. 1852.