Browsing by Subject "Flow control"
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Item Model-based control of transitional and turbulent wall-bounded shear flows(2013-01) Moarref, RashadItem Reduced-complexity modeling for control and nonlinear analysis of transitional flows(2021-08) Kalur, AnikethThe predominant causes for transition to turbulence in wall-bounded shear flows are transient energy growth and nonlinear interactions resulting from the governing linear and nonlinear dynamics. This work addresses both the problem of (i) minimizing transient energy growth (TEG) arising from non-modal dynamics and (ii) accounting for nonlinear interactions while performing stability analysis. This work aims to delay the transition and perform analysis to predict stability boundaries for the onset of turbulence---both with the help of reduced-complexity modeling. In part I, the first problem is addressed by developing control-oriented reduced-order models (COROMs) for designing controllers that minimize maximum transient energy growth (MTEG). The COROMs are developed on the linearized channel flow system using proper orthogonal decomposition and balanced truncation in conjunction. The COROMs enable the development of MTEG minimizing controllers, which otherwise were intractable due to the large dimension of the channel flow system. The performance of the designed controllers is investigated in response to optimal perturbations. In part II, the second problem is addressed by accounting for the nonlinear interactions for stability analysis. Typically in fluid flows, modeling the behaviour of nonlinear interactions exactly is notoriously difficult. Here, instead of using the exact nonlinear dynamics, the input-output properties of the nonlinearity are written as quadratic constraints. The stability analysis of only the linear dynamics is subject to the aforementioned quadratic constraints resulting in linear matrix inequalities~(LMIs). These LMIs enable stability analysis by estimating the region of attraction and the largest permissible perturbation in the nonlinear setting. The utility of the proposed framework is demonstrated on nonlinear low-order models of transitional flows such as the Couette flow. Further, two algorithms are developed, both these algorithms enable solving LMIs with quadratic constraints such that the predicted stability margins can be further improved. Finally, the application of the proposed framework in analyzing the stability of reduced-order viscous Burgers equations, is demonstrated. The viscous Burgers equation is a canonical yet simple model to demonstrate the applicability of the proposed framework. Since the viscous Burgers system has a quadratic convective nonlinearity which introduces transition and change in stability, similar to the Navier-Stokes. The stability of the Burgers equation is studied using the proposed framework and its results are benchmarked against methods like the sum-of-squares method.Item Separation control in low pressure turbines using plasma actuators with passing wakes(2013-09) Burman, DebashishA Dielectric Barrier Discharge (DBD) plasma actuator is operated in flow over the suction surface of a Pack-B Low Pressure Turbine (LPT) airfoil at a Reynolds number of 50,000 (based on exit velocity and suction surface length) and inlet free-stream turbulence intensity of 2.5%. Preliminary characterization studies were made of the effect of varying actuator pulsing frequency and duty cycle, actuator edge effects, and orientation of the actuator with the flow. Flow control was demonstrated with the actuator imparting momentum opposite to the stream-wise flow direction, showing that it is possible to use disturbances alone to destabilize the flow and effect transition. No frequencies of strong influence were found over the range tested, indicating that a broad band of effective frequencies exists. Edge effects were found to considerably enhance separation control. Total pressure measurements of the flow without passing wakes were taken using a glass total-pressure tube. Corrections for streamline displacement due to shear and wall effects were made, and comparisons with previous hot-wire measurements were used to validate data. Performance features of conventional two-electrode and a novel three-electrode actuator configuration were compared. Hot-wire anemometry was used to take time-varying ensemble-averaged near-wall velocity measurements of the flow with periodic passing wakes. Corrections were made for near-wall effects, temperature effects, and interference of the electric field. The wakes were generated by a wake generator mechanism located upstream of the airfoil passage. The near-suction-surface total pressure field (flow without wakes) and velocity field (flow with wakes) in the trailing part of the airfoil passage, and the wall-normal gradient of these quantities, were used to demonstrate effective prevention of flow separation using the plasma actuator. Both flows (with and without passing wakes) showed fully attached flow (or very thin separation zones) when the actuator was activated. The flow with passing wakes and the actuator on showed relatively little time variation in the boundary layer, and qualitative similarities to the corresponding flow without passing wakes and with the actuator on were noted.