Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
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Item One-Dimensional Analytical Model Development of a Plasma-Based Actuator(2014) Popkin, Sarah Haack; Flatau, Alison B; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation provides a method for modeling the complex, multi-physics, multi-dimensional processes associated with a plasma-based flow control actuator, also known as the SparkJet, by using a one-dimensional analytical model derived from the Euler and thermodynamic equations, under varying assumptions. This model is compared to CFD simulations and experimental data to verify and/or modify the model where simplifying assumptions poorly represent the real actuator. The model was exercised to explore high-frequency actuation and methods of improving actuator performance. Using peak jet momentum as a performance metric, the model shows that a typical SparkJet design (1 mm orifice diameter, 84.8 mm3 cavity volume, and 0.5 J energy input) operated over a range of frequencies from 1 Hz to 10 kHz shows a decrease in peak momentum corresponding to an actuation cutoff frequency of 800 Hz. The model results show that the cutoff frequency is primarily a function of orifice diameter and cavity volume. To further verify model accuracy, experimental testing was performed involving time-dependent, cavity pressure and arc power measurements as a function of orifice diameter, cavity volume, input energy, and electrode gap. The cavity pressure measurements showed that pressure-based efficiency ranges from 20% to 40%. The arc power measurements exposed the deficiency in assuming instantaneous energy deposition and a calorically perfect gas and also showed that arc efficiency was approximately 80%. Additional comparisons between the pressure-based modeling and experimental results show that the model captures the actuator dependence on orifice diameter, cavity volume, and input energy but over-estimates the duration of the jet flow during Stage 2. The likely cause of the disagreement is an inaccurate representation of thermal heat transfer related to convective heat transfer or heat loss to the electrodes.Item Contributions to the Measurement and Analysis of Helicopter Blade Tip Vortices(2004-10-18) Ramasamy, Manikandan; Leishman, John G; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The physical structure and the evolution of helicopter blade tip vortices, particularly the effect of the filament strain and flow rotation effects on the turbulent structure, were investigated through model-scale hovering rotor experiments as well as by developing a mathematical model from the Navier-Stokes (N--S) equations. The flow properties were measured using a high-resolution, three-component Laser Doppler Velocimetry (LDV) system. Images of the tip vortices were also obtained using laser sheet flow visualization. A strain field on the tip vortices was introduced by placing a solid boundary downstream of the rotor. As the vortices approached this boundary, they were convected radially outwards enabling measurements to be obtained in a known strain field. Comparing these measurements with vortex measurements in free-air provided considerable insight into the interdependence of strain and diffusion and their impact on the evolution of rotor tip vortices. The measurements also served to validate a mathematical model developed to predict the core growth of the tip vortices, which included the effects of both filament strain and diffusion. The measurements also laid the groundwork for the development of a comprehensive engineering tip vortex core growth model, which combined the effects of vortex filament strain and Reynolds number effects. Laser sheet flow visualization of the vortex core structure indicated the presence of three distinct regions --- an inner laminar region, an intermediate transition region and an outer turbulent region. Analysis of the velocity profiles measured across the vortex core at various wake ages further supported this hypothesis. The effects of flow rotation on the turbulence present inside the vortex core was quantified based on a Richardson number concept. This results in a laminar flow structure until a particular distance from the vortex center that correlated with the location where the Richardson number fell below a threshold value Based on these observations an eddy viscosity intermittency function was developed, which modeled the flow transition within the three regions of the core. This function was incorporated into a comprehensive tip vortex model formulated in terms of vortex Reynolds number. The empirical constants used in the present model were obtained from the present measurements, as well as several other sources. The dependence of the results on vortex Reynolds number ensured that the model successfully predicted the core growth of tip vortices for both sub-scale as well as full-scale rotors.