A. James Clark School of Engineering
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Nonlinear Interactions in Planar Jet Flow with High Frequency Excitation(2016) Kreutzfeldt, Timothy; Chopra, Inderjit; Glaz, Bryan; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An experimental active flow control study was conducted involving excitation of a tabletop planar turbulent jet with a high frequency piezoelectric actuator. The excitation frequencies considered corresponded to the dissipative subrange of turbulent kinetic energy and were orders of magnitude greater than classical shear layer instability modes. Single-wire and dual-wire hot wire probes were used to determine how excitation induces alterations to bulk flow quantities as well as nonlinear interactions. Differences in flow receptivity to high frequency excitation were investigated by varying the development length of the turbulent jet at a Reynolds number of 8,700 and Strouhal number of 21.3. Excitation of developed turbulent flow yielded larger increases in the energy dissipation rate and higher magnitude velocity power spectrum peaks at the forcing frequency than undeveloped turbulent flow. Further tests with excitation of reduced mean velocity flow at a Reynolds number of 6,600 and a Strouhal number of 27.8 demonstrated that high frequency forcing resulted in transfer of energy from large to small scales in the turbulent kinetic energy spectrum. This phenomenon appeared to support past literature that indicated that the mechanics of high frequency forcing are fundamentally different from conventional instability-based forcing. Theoretical arguments are presented to support these experimental observations where it is shown that coupling between the applied forcing and background turbulent fluctuations is enhanced. An eddy viscosity model first proposed under the assumption of instability-based forcing was shown to be an effective approximation for the experimental measurements presented here in which the flow was forced directly at turbulence scales. Dimensional analysis of the coupling between the induced oscillations and the turbulent fluctuations supported experimental findings that receptivity to excitation was increased for forced flow with higher turbulent kinetic energy, higher excitation amplitude, and lower energy dissipation rate. This study is the first to present such results which validate a model that offers theoretical insight into flow control mechanics when directly forcing small scale turbulent fluctuations.Item MODELING AND EXPERIMENTAL ANALYSIS OF PHASED ARRAY SYNTHETIC JET CROSS-FLOW INTERACTIONS(2014) HASNAIN, ZOHAIB; Flatau, Alison B; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Synthetic Jet Actuators (SJAs) are fluidic devices capable of adding momentum to static or non-static bodies of fluid without adding mass. They are therefore categorized as zero-net-mass-flux (ZNMF) momentum source. In its simplest compact form a SJA consists of an oscillatory surface connected to a cavity with a single exit orifice through which the fluid enters and exits. SJA technology has been utilized in applications ranging from boundary layer control over aerodynamic surfaces to fluidic mixing in dispersion applications. The ZNMF nature of the technology means it is not subject to constraints experienced by traditional momentum sources that require the addition of mass in order to impart momentum. The momentum that can be added by a single SJA is limited by the energy transfer capabilities of the oscillating surface. In modern SJAs this surface usually is a piezoceramic/metal composite subjected to a high voltage AC signal. For applications such as flow control over aerodynamic surfaces, modern SJAs are used in an array configuration and are capable of altering the flow momentum by values ranging from 0.01-10%. While it is possible to build larger actuators to increase this value the benefits associated with the compact size would be lost. It is therefore desirable to tune other parameters associated with SJA arrays to increase this value. The specific motivation for this study comes from the desire to control the momentum addition capacity of a specific SJA array, without having to alter any geometric parameters. In a broader sense this study focuses on understanding the physics of SJA interaction in array configuration through experiments which are then used to guide in the design of modeling technique that predicts SJA array behavior in cross-flows. The first half of the project focused on understanding SJA behavior through modeling. Numerical techniques were initially used to model SJA and SJA arrays in cross-flows. Reduced numerical models were then developed from the full momentum equations. Analytical methods to solve these reduced order models were then implemented in order to cut down on solution time. A wave equation based solution to the stream and vorticity formulation of the momentum equations was implemented to predict SJA behavior. For the experimental component of the project, a finite span high aspect ratio orifice SJA was designed and characterized through Constant Temperature Anemometry (CTA). Two of these SJA were then placed in close proximity to one another. The relative phase of operation between the two jets was altered and the resulting flow field was measured through Particle Image Velocimetry (PIV). This process was repeated for different sets of array spacing, and SJA to cross-flow velocity ratio. For specific choices of these parameters a 40% increase in momentum addition was observed. The experimental results were used to validate the modeling techniques. In general reasonable agreement between the modeling and experiment was observed in specific domains of the flow field.Item Studies in Tip Vortex Formation, Evolution and Control(2005-04-01) Duraisamy, Karthikeyan; Baeder, James D; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A high resolution computational methodology is developed for the solution of the Compressible Reynolds Averaged Navier Stokes (RANS) equations. This methodology is used to study the formation and evolution of tip vortices from fixed wings and rotary blades. The numerical error is reduced by using high order accurate schemes on appropriately refined meshes. For vortex evolution problems, the equations are solved on multiple {\em overset} grids that ensure adequate resolution in an efficient manner. For the RANS closure, a one equation wall-based turbulence model is used with a correction to the production term in order to account for the stabilizing effects of rotation in the core of the tip vortex. A theoretical analysis of the accuracy of high resolution schemes on stretched meshes is performed as a precursor to the numerical simulations. The developed methodology is validated with an extensive set of experimental measurements ranging from fixed wing vortex formation studies to far-field vortex evolution on a two bladed hovering rotor. Comparisons include surface pressure distributions, vortex trajectory and wake velocity profiles. During the course of these validations, numerical issues such as mesh spacing, order of accuracy and fidelity of the turbulence model are addressed. These findings can be used as guidelines for future simulations of the tip vortex flow field. A detailed investigation is conducted on the generation of tip vortices from fixed wings. Streamwise vorticity is seen to originate from the cross-flow boundary layer on the wing tip. The separation and subsequent roll-up of this boundary layer forms the trailing vortex system. The initial development of the vortex structure is observed to be sensitive to tip shape, airfoil section and Reynolds number. While experimental comparison of the computed vortex structure beyond a few chord lengths downstream of the trailing edge is lacking in the literature, for a single bladed hovering rotor, good validations of the vortex velocity profiles are achieved upto a distance of 50 chord lengths of evolution behind the trailing edge. For the two bladed rotor case, the tip vortex could be tracked upto 4 revolutions with minimal diffusion. The accuracy of the computed blade pressures and vortex trajectories confirm that the inflow distribution and blade-vortex interaction are represented correctly. Finally, utilizing a surface boundary condition to represent a spanwise jet, the effect of tip blowing on the vortex structure is investigated. The interaction of the jet with the cross-flow boundary layer is shown to reduce the vortex strength with a marginal loss in performance. Overall, this level of consistent performance has not been demonstrated previously over such a wide range of test cases. The accuracy achieved in the validation studies establishes the viability of the methodology as a reliable tool that can be used to predict the performance of lift generating devices and to better understand the underlying flow physics.