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Title:Active and hybrid flow control in S-ducts and diffusers
Author(s):Kale, Nachiket
Director of Research:Dutton, J. Craig; Elliott, Gregory S.
Doctoral Committee Chair(s):Dutton, J. Craig; Elliott, Gregory S.
Doctoral Committee Member(s):Bodony, Daniel J.; Glumac, Nick G.
Department / Program:Aerospace Engineering
Discipline:Aerospace Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
flow control
S-duct inlets
plasma-based active flow control
pneumatic hybrid flow control
Abstract:This dissertation documents the research endeavor conducted by the author in the field of aerodynamic flow control. The objectives were to develop flow control actuators, investigate their mechanism, and study their application on different flow-fields. Two flow control actuators - a pneumatic and a plasma-based one - were developed. On an applied level, the control authority of the actuators on flow separation was tested on multiple flow-fields and geometries. On a fundamental level, the basic control mechanism was studied using multiple state-of-the-art diagnostic techniques. The pneumatic actuator developed is a combination of steady blowing and vortex-generators (VGs), christened the "pneumatically enhanced/deployed actuator (PEDA)". The control authority of the PEDAs was studied on the Glauert-Goldschmied wall-mounted hump, a canonical geometry over which flow undergoes separation, using multiple diagnostics, including wall-static pressure measurements, surface oil flow visualization and two-dimensional particle image velocimetry (PIV) measurements. Two variants of the PEDA - one with membrane-deployable VGs, another with always-deployed VGs - were designed and studied (at a freestream velocity of 30 m/s). Further, for the membrane-deployable PEDA variant, two streamwise locations were investigated. A reduction in the size and streamwise extent (by as much as 60.3% of the baseline length) of the recirculation region was observed with increasing jet actuation. The VGs, without jet actuation, reduced the separation bubble length by 12.3%. A plasma-based flow control actuator called the "localized arc lament plasma actuator (LAFPA)" was investigated in this research. The primary working mechanism of the LAFPA is the formation of DC electric arcs, at a controlled frequency, between two electrodes embedded inside a cavity. This investigation sought to extend the understanding of the flow control method of high-density energy deposition using LAFPAs to boundary layer control in separated flows. Initially, the LAFPA actuator and the system driving it were developed and the actuator was studied on a fundamental level in quiescent conditions. Subsequently, a four-actuator LAFPA quad-array was developed and its effect on the boundary layer and separated flow over a Glauert-Goldschmied ramp (freestream velocity approximately 35 m/s) was studied. The investigation employed multiple diagnostics, including electrical measurements, schlieren imaging, surface flow visualization and particle image velocimetry. Two important features of the actuator - the blast wave (traveling at nearly sonic speeds) and the heated plume - were observed via schlieren imaging. At each actuation (average power 76 W), ejection of fluid (maximum velocity of 13.5 m/s, 0.5 mm above the cavity, at 10 ms delay, for an actuation frequency of 1 kHz) was observed. The actuation created a perturbation in the boundary layer that propagated downstream at a streamwise velocity of approximately 380 m/s, indicating the source of the perturbation to be the blast wave. This perturbation was strongest for the actuation frequency range of 1 kHz to 10 kHz. The perturbation modified the boundary layer, primarily by increasing its fullness. It dampened as it moved downstream, and died out at approximately 17 mm (10 times the cavity width) downstream of the actuator. The boundary layer did not seem receptive to amplifying the instability over the wide range of frequencies (100 Hz to 100 kHz) and modes tested, and the global flow-field and the massive separation region stayed largely unaffected. Nevertheless, the actuator has a substantial effect on the boundary layer, and retains its potential as a boundary layer control device for perhaps more responsive flow-fields. The primary focus of this research endeavor was the investigation of ow control and performance enhancement of an S-duct inlet diff user. The objective was to improve the flow quality (distortion and pressure recovery) in the S-duct exit plane - the "aerodynamic interface plane" (AIP). The distortion and loss of pressure recovery in the inlet adversely affect the performance of the engine by reducing its e fficiency, its aerodynamic envelope and increasing fatigue and operational costs. The primary goal of this research was to improve the quality of the flow through the S-duct by increasing the pressure recovery and reducing distortion. A new wind tunnel facility was designed and constructed to perform the experiments, with the S-duct test-section as an integral part of it. The performance of eight PEDA variants (embedded inside the duct) at two flow conditions (inlet Mach numbers of 0.73 - 0.77 and 0.30) was studied by wall-static pressure measurements, surface oil flow visualization and AIP pitot-static measurements. The dominant flow-features - the bottom wall flow separation and the counter-rotating vortices at the first bend - that lead to performance degradation of the duct were identified and addressed. A parametric variation study was performed (two streamwise actuator locations, three VG configurations, and two wall-jet angles). All actuators performed well in eliminating the bottom-wall separation, weakening the twin vortices, reducing the circumferential distortion (by as much as 53.1% of the baseline) and increasing pressure recovery (by as much as 1.18% of the baseline). Due to the differences in the targeted ow-features by different actuator variants, and effectively the differences in their performance and AIP total pressure distributions, the set of actuators offers an engineered choice based on the goal (or a combination of goals), including lowering distortion, increasing pressure recovery and decreasing specific harmonic content.
Issue Date:2013-05-24
Rights Information:Copyright 2013 Nachiket Vinayak Kale
Date Available in IDEALS:2013-05-24
Date Deposited:2013-05

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