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Unsteady aerodynamic modeling is indispensable in the design process of rotary air vehicles, flapping flight and agile unmanned aerial vehicles. Undesirable vibrations can cause high-frequency variations in motion variables whose effects cannot be well predicted using quasi-steady aerodynamics. Furthermore, one may exploit the lift enhancement that can be generated through an unsteady motion for optimum design of flapping vehicles. Additionally, undesirable phenomena like the flutter of fixed wings and ensuing limit cycle oscillations can be exploited for harvesting energy. In this dissertation, we focus on modeling the unsteady nonlinear aerodynamic response and present various applications where unsteady aerodynamics are very relevant. The dissertation starts with experiments for measuring unsteady loads on an NACA-0012 airfoil undergoing a plunging motion under various operating conditions. We supplement these measurements with flow visualization to obtain better insight into phenomena causing enhanced lift. For the model, we present the frequency response function for the airfoil at various angles of attack. Experiments were performed at reduced frequencies between 0.1 and 0.95 and angles of attack up to 65 degrees. Then, we formulate an optimization problem to unify the transfer function coefficients for each regime independently to obtain one model that represents the global dynamics. An optimization-based finite-dimensional (fourth-order) approximation for the frequency responses is developed. Converting these models to state-space form and writing the entries of the matrices as polynomials in the mean angle of attack, a unified unsteady model was developed. In the second set of experiments, we measured the unsteady plunging forces on the same airfoil at zero forward velocity. The aim is to investigate variations of the added forces associated with the oscillation frequency of the wing section for various angles of attack. Data of the measured forces are presented and compared with predicted forces from potential flow approximations. The results show a significant departure from those estimates, especially at high frequencies indicating that viscous effects play a major role in determining these forces. In the second part of this dissertation, we consider different applications where unsteady loads and nonlinear effects play an important role. We perform a multi-objective aerodynamic optimization problem of the wing kinematics and planform shape of a Pterosaur replica ornithopter. The objective functions included minimization of the required cycle-averaged aerodynamic power and maximization of the propulsive efficiency. The results show that there is an optimum kinematic parameter as well as planform shape to fulfill the two objectives. Furthermore, the effects of preset angle of attack, wind speed and load resistance on the levels of harvested power from a composite beam bonded with the piezoelectric patch are determined experimentally. The results point to a complex relation between the aerodynamic loading and its impact on the static deflection and amplitudes of the limit cycle oscillations as well as the level of power harvested. This is followed by testing of a centimeter scale micro wind turbine that has been proposed to power small devices and to work as a micro energy harvester. The experimental measurements are compared to predicted values from a numerical model. The methods developed in this dissertation provide a systematic approach to identifying unsteady aerodynamic models from numerical or experimental data that may work within different regimes. The resulting reduced-order models are expressed in a state-space form, and they are, therefore, both simple and efficient. These models are low-dimensional linear systems of ordinary differential equations so that they are compatible with modern flight dynamic models. The specific form of the obtained added force model, which defines the added forces as a function of plunging velocity and drag forces, guarantees that the resulting model is accurate over a range of high frequencies. Moreover, presented applications give a sense of the broad range of application of unsteady aerodynamics. / Ph. D.

The increased use of infrared imaging as a flow visualization technique and as a validation technique for computational fluid dynamics (CFD) codes has led to an in-depth study of infrared band models. The ability to create fast and accurate images of airframe and plume infrared emissions often depends on the complexity of the band model. An infrared band model code has been created based largely on the band model published in NASA SP-3080, Handbook of Infrared Radiation from Combustion Gases. Improvements to the NASA SP-3080 model using the N I RA T AM data files have been made. The model and its theoretical basis are thoroughly described. Results are presented and are compared with results from the band models contained in SCORPIO and LOIR. / Master of Science

The present investigation is a two-part study of the mean flow and turbulence structure of isolated vortices and counter-rotating vortex pairs. In the first part, the turbulence structure of an isolated vortex was studied using three-component velocity measurements. Vortices were generated using two symmetrical airfoils. Measurements were made in cross-sectional grids and profiles over a range of Reynolds numbers and downstream distances. Contours of axial normal stress were high-pass filtered to remove the contributions of wandering to the velocity fluctuations. This process reveals a vortex core which is laminar and is surrounded by a region of high turbulence. Core velocity profiles reveal that maximum tangential velocity increases with Reynolds number and decreases with distance downstream. Core radius increases with distance downstream and decreases with Reynolds number. In the second part, flow visualizations of the wake behind a delta wing model were made for a range of Reynolds numbers and lift coefficients. These visualizations reveal the near-instantaneous turbulence structure of the wing wake which is dominated by a vortex pair and a connecting "braid" wake. The braid spacing decreases with increasing Reynolds number and is independent of lift coefficient. The extent of the braid downstream of the wing increases with lift coefficient and decreases with increasing Reynolds number. The large turbulence scales in the wing wake were found to increase in discrete jumps indicating some sort of reorganization of turbulence such as pairing. This reorganization of turbulence was found to occur more quickly as Reynolds number is increased. / Master of Science

Flow visualization movies of flow through a cascade of compressor blades showed propagating stall at stagger angles of 36.5 and 45 degrees for angles of attack of 20 degrees and higher. At a stagger angle of 25 degrees, the development of a steady, separated boundary layer occurred with no propagation. The observed propagating stall process was the development of a vortex in the boundary layer and its subsequent shedding. The shedding mechanism was observed to be the interference by the reverse flow from the previously stalled passage with the vortex flow in the stalled passage. This dissipated the vortex in the blade passage and the incoming flow then flushed the stagnated vortex out of the passage. Measurements of propagation speeds showed that the propagation speed is related to the blockage of the passage, that stagger angle has an insignificant effect on propagation speed, and that propagation speed is proportional to the relative velocity. Based on the observations, a geometric model was developed to predict the onset of propagating stall. This model showed that increased solidity, decreased stagger angles, and operation at low angles of attack make a cascade more resistant to propagating stall inception. The model shows the relation of the operating point of a compressor to the stall inception point. When expanded to include all significant aspects of blade geometry, the model may provide a basis for controlling propagating, and hence, rotating, stall inception based on the blade row·geometry. / Master of Science

Active pneumatic flow control methods as applied to aerospace applications have shown noteworthy improvements in lift compared to traditional means. The General Aviation Circulation Control (GACC) concept currently under investigation at NASA's Langley Research Center (LaRC) is an attempt at addressing some of the fundamental obstacles related to the successful development and implementation of such techniques. The primary focus of research in the field of high lift pneumatic devices is to investigate ways of obtaining significant improvements in the lift coefficient without resorting to moving surfaces. Though it has been demonstrated that the lift coefficient can be amplified in a variety of ways, the chosen method for the current work is via enhanced circulation stemming from a trailing edge Coanda jet. A secondary objective is to reduce the amount energy expenditure used in these pneumatic techniques by implementing time-variant flow. This paper describes experimental observations of the flow behavior at the trailing edge of a modified water tunnel based supercritical airfoil model that exploits both steady and pulsed Coanda driven circulation control. A total of 10 sets of data, excluding a baseline case of no Coanda jet, were sampled with five cases each for steady and pulsed flow, the latter at a reduced frequency, f+, of 1. Two cases of equal momentum coefficient but with varying forced frequencies were isolated for further study in an attempt to accurately compare the resultant flow dynamics of each method. All measurements were taken at a zero-lift angle of attack by means of a non-invasive time accurate flow visualization technique (DPIV). Vorticity behavior was investigated using Tecplot® and a MATLAB® program was developed to quantify the Strouhal Number of time-averaged velocity fluctuations moving aft of the Coanda surface for each case. / Master of Science

Aerodynamic measurements were taken over models of the Boeing 777 high fidelity isolated landing gear in the 6- by 6-foot Virginia Tech Stability Wind Tunnel (VT-SWT) at a free-stream Mach number of 0.16. Noise control devices (NCD) were developed at Virginia Tech [9] to reduce noise by shielding gear components, reducing wake interactions and by streamlining the flow around certain landing gear components. Aerodynamic measurements were performed to understand the flow over the landing gear and also changes in the flow between "Baseline" and "NCD" configurations (without and with Noise Control Devices respectively). Hot-film, Pitot-static measurements and flow visualization using tufts were performed over an isolated 26% scale-model high fidelity landing gear for the "Baseline" and "NCD" configurations. Contours of turbulence intensity, normalized wake velocity and normalized total pressure loss for both configurations are compared. The "Baseline" configuration was also compared with the NASA Ames study conducted by Horne et al [7]. Hot-film measurements are also compared to Microphone Phased Array results which were acquired at Virginia Tech by Ravetta [8]. A novel technique for processing hot-film measurements by breaking turbulence into octave bands as acoustic measurements is presented. Particle Image Velocimetry (PIV) measurements were taken at six different locations over the 13% scale-model landing gear with no door and at a truck angle of zero degrees. Results are compared to PIV measurements taken over the wheels of a four-wheel landing gear by Lazos [10-12]. PIV results such as average velocity contours and vectors, streamlines and instantaneous velocity contours and vectors are presented. Results presented from PIV and flow visualization are in good agreement with results from Lazos [10-12]. / Master of Science

A hydraulic collapse mechanism was incorporated into a recirculating pulsatile flow system to simulate the physiologic problem cf coronary artery vasospasm. A dimensional analysis of the hemodynamic problem provided the basis for i) the specifications for elastic test sections (both straight and branching), ii) the determination of the flow modelling parameters, Reynolds number and unsteady Reynolds number, and iii) the determination of the dimensionless collapse parameters. The models were collapsed in a controlled manner while changes in volumetric flow rate into and out of the models as well as axial pressure drop were monitored. It was found that the driven collapse of the vessel acts as a pump, the effectiveness of which is dependent on upstream and downstream resistance. There was noted a difference in the volumetric flow curves representing fluid leaving the pre- and the post-collapse models under the same inflow conditions. This was due to both the elastic properties of the models and to the post-collapse shape of the models (curved walls and non-circular cross-section). Time-exposed photographs of tracer particle displacements within the model indicate increased volumetric flow in each branch during the initial phase of the collapse process. Moreover, it was seen that the radial gradient of the axial velocity at each wall surface varied in magnitude (and possibly in sign) during the collapse. The in vitro results do not substantiate the coronary spasm/myocardial ischemia connection, but do further implicate vasospasm as a factor in atherogenesis. / Ph. D.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: The purpose of this project is to investigate the method and accuracy of simulating axial ow fans with three-dimensional axisymmetric CFD models. Two models are evaluated and compared with experimental fan data. Veri cation data is obtained from a prototype fan tested in a facility conforming to the BS 848 standards. The ow eld over the blade surfaces is investigated further with a visualization experiment comprising of a stroboscope and wool tufts. Good correlation is found at medium to high ow rates and recommendations are made for simulation at lower ow rates as well as test guidelines at the fan test facility. The results and knowledge gained will be used to amend currently used actuator disc theory for axial ow fan simulation. / AFRIKAANSE OPSOMMING: Die doel van hierdie projek is om die metode en akkuraatheid om aksiaalvloeiwaaiers met drie-dimensionele BVM modelle te simuleer, te ondersoek. Twee modelle word geëvalueer en met eksperimentele waaiertoetse vergelyk. Veri- kasie data is verkry vanaf 'n prototipe waaier wat in 'n fasiliteit getoets is en wat aan die BS 848 standaarde voldoen. Die vloeiveld oor die lemoppervlaktes word ondersoek met 'n visualisering eksperiment wat uit 'n stroboskoop en wolletjies bestaan. Goeie korrelasie word gevind vir medium tot hoë massavloeie en aanbevelings word gemaak vir die simulasie by laer massavloeie met riglyne vir toetswerk in die toets-fasiliteit. Die resultate en kennis opgedoen sal gebruik word in die verbetering van huidige aksieskyfteorie vir numeriese aksiaalvloeiwaaier simulasies.