title: Complex flow physics &amp; active plasma flow control in convoluted ducts
Creators: name: Wojewodka, Michael M.
Publisher: name: University of Glasgow
Year: 2020
Subjects: subject: convoluted ducts, decomposition techniques, CFD, plasma actuators, flow control.

description: Convoluted, s-shaped ducts form an integral part of many subsystems in engineering applications and specifically the aviation industry. They are used, for example, as inlet ducts for fuselage embedded jet engines and as connector pipes between high and low pressure turbine or compressor stages. With a strong curvature and a diffusive nature, the geometry acts on the through-flow making it prone to separate and experience significant cross-stream pressure gradients. The geometry and resulting flow phenomena lead to a non-uniform and highly unsteady flow field in the duct aft the inflection point. Those effects are detrimental to the overall performance of the convoluted duct, reducing the pressure recovery and increasing the distortion parameters. S-shaped ducts have been studied by a large number of researchers for many years. Traditionally, many studies rely on steady state simulations and time averaged experimental data to characterise the flow in convoluted ducts and analyse their performance. However, more recent findings point to the need of transient data to fully understand the dynamic nature of the through-flow and discuss the complex flow physics. This is something that is lacking from many studies reported in the current literature. This is addressed with computational fluid dynamics (CFD) studies of the through-flow in the s-duct using the open source tool OpenFOAM. First low fidelity, steady state simulations are set up before higher fidelity, transient delayed detached eddy simulations (DDES) are conducted. Baseline s-duct through-flow computations are validated against experimental data from literature with very good agreement of pressure recovery values, wall static pressure contours, and flow structures. CFD data is next post processed with statistical and modal decomposition methods. Coherent structures and phase information are obtained from the proper orthogonal decomposition (POD) and the dynamic mode decomposition (DMD) methods. Modal decomposition analysis of DDES data confirms the existance of the horizontal shifing mode. Contrary to previous findings, the presence of a second vertical shifting mode is observed from DDES data. Occurance rates and phase information are determined from the DMD analysis. The recent surge of interest in plasma actuators is clearly illustrated by the high research output that has been reported in literature. Dielectric barrier discharge (DBD) plasma actuators have been studied and successfully applied to control external aerodynamics on aerofoils and bluff bodies. However, successful flow control in convoluted ducts has not been reported with this technology for realistic Reynolds numbers. The DBD plasma characterisation is conducted on two types of actuators: alternating current (ac) and nanosecond (ns) DBD plasma actuators. The Schlieren imaging technique is used with ns-DBD plasmas to record density changes and establish the shock front strength and propagation speed with changing ambient pressure. Higher ambient pressures result in stronger shock waves; this has been observed irrespective of the actuator thickness. This might be explained with fewer air molecules to ionize at lower ambient pressures and hence a lower temperature from the exothermal recombination reactions. For ac-DBD actuators, thinner dielectric materials outperformed thicker ones in terms of ionisation strength with constant voltage input. The smallest dielectric constant of the materials tested resulted in higher induced velocities. Using particle image velocimetry (PIV), a high gradient of velocity reduction with streamwise distance was recorded in the plasma jet. This is significant, as it shows plasma actuators have mostly localised effects. Experimental campaigns are set up such that the DBD experiments are coherent studies in their own right. However, the main purpose of plasma experiments in the context of this thesis is to collect data to validate numerical plasma models. Those phenomenological plasma models are subsequently used for numerical flow control studies on the s-shaped duct. Phenomenological plasma models match the experimental data well when tuned. However, the flow control studies did not show a performance improvement in the convoluted duct.