Spencer Stahl

Air Force Research Laboratory | WPAFBRL

The impingement of supersonic jets on a flat surface, such as those found in rocket launch applications or vertical short take-off and landing (VSTOL) aircraft, induce detrimental ground effects, chiefly driven by an aeroacoustic resonance. The acoustic feedback resonance of a single impinging jet (SIJ) has been studied extensively and successfully...

A double-bend, serpentine inlet diffuser is simulated to investigate the unsteady dynamics that contribute to reduced jet engine performance. The aggressive turning of the transonic flow, in conjunction with a varying non-axisymmetric cross-sectional area, induce separation along the upper and lower surfaces. The separated flow results in pressure...

View Video Presentation: https://doi.org/10.2514/6.2022-1153.vid The acoustic feedback mechanisms in an unheated Mach 1.27 under-expanded impinging jet, are investigated using conditional proper orthogonal decomposition (CPOD) of validated Large Eddy Simulation data. The feedback loop consists of downstream convecting structures in the shear-layer...

A data-driven "cause and effect" analysis is developed to generate insights into critical events within a resonating jet. The two-step method adapts a Conditional Proper Orthogonal Decomposition (CPOD) to extract space-time modes that optimally capture the variance across an ensemble of large fluctuation events. The CPOD modes are then expressed in...

A theoretical model is constructed to understand and predict the aeroacoustic feedback coupling of dual impinging jet (DIJ) configurations of the type encountered in supersonic vertical take-off and landing aircraft. The proposed model extends the single impinging jet (SIJ) framework of Powell, which derives impinging tone frequencies from the spee...

The self-excited oscillations of shock/pseudo-shock systems created in a back-pressured transonic diffuser flow path are studied experimentally by way of a direct-connect inlet flow path rig. A facility converging/diverging nozzle, with modular hardware designed for either Mach 1.4 or 1.7 flow, supplies supersonic air to a rectangular throat/isolat...

A data-driven framework using snapshots of an uncontrolled flow is proposed to identify, and subsequently demonstrate, effective control strategies for different objectives in supersonic impinging jets. The open-loop, feed-forward control approach, based on a dynamic mode decomposition reduced-order model (DMD-ROM), computes forcing receptivity in...

A data-driven framework using snapshots of an uncontrolled flow is proposed to identify, and subsequently demonstrate, effective control strategies for different objectives in supersonic impinging jets. The approach, based on a dynamic mode decomposition reduced order model (DMD-ROM), determines forcing receptivity in an economical manner by projec...

The correlation and extraction of coherent structures from a turbulent flow is a principal objective of data-driven modal decomposition techniques. The Conditional space-time Proper Orthogonal Decomposition (CST-POD) offers insight into transient dynamics, revealing the causes of specific flow phenomena, or events, in a customizable manner. This wo...

The correlation and extraction of coherent structures from a turbulent flow is a principle objective of data-driven modal decomposition techniques. The Conditional space-time Proper Orthogonal Decomposition (CPOD) offers insight into transient dynamics, revealing the causation of specific flow phenomenon - or events, in a customizable manner. This...

View Video Presentation: https://doi.org/10.2514/6.2021-2923.vid The hydrodynamic coupling between two closely spaced, identical, sonic, impinging jets is modeled using a Large Eddy Simulation. The underexpanded round jets separated 4.3 nozzle diameters apart impinge on a flat surface 4 diameters away. The outboard sides of each jet of the twin con...

View Video Presentation: https://doi.org/10.2514/6.2021-2118.vid The flow of a supersonic jet impinging on a flat plate results in a very complex flow-field which is dominated by a self-reinforcing aeroacoustic feedback process resulting in the generation of discrete frequency tones. Although previous experimental observations have shown conclusive...

Impinging jets are characterized by an acoustic feedback resonance capable of generating intense tones. This investigation examines changes in single impinging jet (SIJ) dynamics when another jet is added alongside to form a dual impinging jet (DIJ) arrangement of interest in vertical takeoff and landing applications. The emphasis is on the hydrody...

Two closely spaced impinging jets, such as those found in Vertical Take Off and Landing (VTOL) aircraft in near ground hover, produce a complicated, unsteady aeroacoustic field. These two jets are often employed using mixed operating conditions, such as a hot supersonic core jet, and a lower speed lift-fan or jet, generating a turbulent fountain-fl...