This thesis introduces the Deterministic Disturbance Generator (DDG) and its development process. The DDG performs two motions and five pitch rates. The flap motion, which rotates the airfoil from 0◦ to 20◦ and back, and the ramp motion, which rotates it from 0◦ to 20◦ with a dwell of 1s before returning to 0◦. To determine the angle of attack, a Matlab function converted thrust rod displacement into the assumed angle, validated against true angle of attack measurements on the DDG. Mean angular displacements were plotted, and standard deviations of the 95% confidence intervals were calculated within ±1.3◦ for all motions. The mechanical force on the actuator was computed to be 77N. Aerodynamic forces on the DDG were determined to be 15N and 19N for flap and ramp motions respectively. The total force on the system did not exceed 100N in any case, staying below the peak force capacity, while acceleration reached its limit. Flow velocimetry in the Virginia Tech Stability Wind Tunnel (VTSWT) employed a time-resolved Particle Image Velocimetry (PIV) to study the effects of 20◦ flap and ramp motions, with mean actuation times of 63ms and 37ms. Flap motion showed a significant deficit in mean streamwise velocities, and the ramp motion exhibited similar behavior until its dwell position, generating a large wake region due to airfoil stall after its peak. Comparison of data from the Goodwin Hall Subsonic Tunnel (GHST) with VTSWT data for overlapping domains revealed similar flow field features when normalized based on the boundary layer velocity (43mm plane from wall) of the latter. Considering actuation time differences, the freestream normalized GHST data was combined with VTSWT data. The cohesive PIV domain offered a broader perspective on the missing flow features. / Master of Science / A Deterministic Disturbance Generator (DDG) was designed to generate consistent largescale transversal transient disturbances in the wall boundary layer of the Virginia Tech Stability Wind Tunnel. It comprises an airfoil connected to an actuator through a rotating mechanism. The rotating mechanism can be controlled by manipulating the actuator to induce motion. The rotational speed of the airfoil is regulated by a program provided to the actuator. The DDG motions were validated to achieve nearly identical motion profiles to ensure it produced consistent turbulence wakes. The linear displacement of the actuator and airfoil was measured using a laser sensor, and a code was developed to convert this data into the observed angle of attack. Tests were conducted to verify repeatability and fine-tune the system's motions. A comprehensive description of the fabrication process, hardware and software setup, and calibration procedures involved in developing the DDG are provided. Using aerodynamic models, a computational study is performed to determine the forces associated with the airfoil and actuator. Subsequently, the DDG was subjected to testing in two wind tunnels: the Goodwin Hall Subsonic Tunnel for preliminary characterization and error mitigation and the Virginia Tech Stability Wind Tunnel for final assessment of the DDG's performance. Flow velocimetry data obtained from both tests are analyzed, revealing similarities in the induced motions. Mean flow fields and turbulence values are determined, and the effects of different pitch rates are also assessed. Finally, the mean flow fields corresponding to identical motion types from both datasets were integrated into a cohesive plot. This resulted in a comprehensive understanding of the flow field.

Separated flows about single and multi-element airfoils are featured in many scenarios of practical interest, including: stall of fixed wing aircraft, dynamic stall of rotorcraft blades, and stall of compressor and turbine elements within jet engines. In each case, static and/or dynamic stall can lead to losses in performance. More importantly, modeling and analysis tools for stalled flows are relatively poorly evolved and designs must completely avoid stall due to a lack of understanding. The underlying argument is that advancements are necessary to facilitate understanding of and applications involving static and dynamic stall. The state-of-the-art in modeling stall involves numerical solutions to the governing equations of fluids. These tools often either lack fidelity or are prohibitively expensive. Ever-increasing computational power will likely lead to increased application of numerical solutions. The focus of this thesis is improvements in numerical modeling of stall, the need of which arises from poorly evolved analysis tools and the spread of numerical approaches. Technical barriers have included ensuring unsteady flow field and vorticity reproduction, transition modeling, non-linear effects such as viscosity, and convergence of predictions. Contributions to static and dynamic stall analysis have been been made. A hybrid Reynolds-Averaged Navier-Stokes/Large-Eddy-Simulation turbulence technique was demonstrated to predict the unsteadiness and acoustics within a cavity with accuracy approaching Large-Eddy-Simulation. Practices to model separated flows were developed and applied to stalled airfoils. Convergence was characterized to allow computational resources to be focused only as needed. Techniques were established for estimation of integrated coefficients, onset of stall, and reattachment from unconverged data. Separation and stall onset were governed by turbulent transport, while the location of reattachment depended on the mean flow. Application of these methodologies to oscillating flapped airfoils revealed flow through the gap was dominated by the flap angle for low angles of attack. Lag between the aerodynamic response and input flap scheduling was associated with increased oscillation frequency and airfoil/flap gap size. Massively separated flow structures were also examined.

The aim of this study is to simulate the unsteady flow around rotating or oscillating airfoils. This will help to understand the rotor aerodynamics, which is essential in turbines and propellers. In this study, a pre-existing Euler solver with finite volume method that is developed in the Mechanical Engineering Department of Middle East Technical University (METU) is improved. This structured pre-existing code was developed for 2-D internal flows with Lax-Wendroff scheme. The improvement consist of firstly, the generalization of the code to external flow / secondly, implementation of first order Roe&rsquo / s flux splitting scheme and lastly, the implementation of rotation with the help of Arbitrary Lagrangian Eulerian (ALE) method. For the verification of steady and unsteady results of the code, the experimental and computational results from literature are utilized. For steady conditions, subsonic and transonic cases are investigated with different angle of attacks. For the verification of unsteady results of the code, oscillating airfoil case is used. The flow is assumed as inviscid, unsteady, adiabatic and two dimensional. The gravity is neglected and the air is taken as ideal gas. The developed code is run on computers housed in METU Mechanical Engineering Department Computational Fluid Dynamics High Performance Computing (CFD-HPC) Laboratory.

In nature, many swimming and flying creatures use the principle of oscillatory lift-based propulsion. Often the flapping element is flexible, totally or partially. The flow dynamics because of a flexible flap is thus of considerable interest. We are interested especially in lunate fish propulsion. The present work investigates the effect of trailing edge flexibility on the flow field created by an oscillating airfoil in an attempt to mimic the flow around the flexible tails often found in fish. A flexible flap with negligible mass and stiffness is attached at the trailing edge of NACA0015 airfoil. The flap length is 75% of the rigid chord length. The airfoil oscillates about a hinge point at 30% chord from the leading edge and at the same time it moves in a circular path in stationary water. The parameters varied are frequency, amplitude of oscillation and forward speed. For a given combination of amplitude and frequency of oscillation, the forward speed is chosen such that the Strouhal number comes around 0.3, which falls in the gamut of Strouhal numbers for maximum propulsive efficiency. We visualize the flow with dye and particles and measure velocities using Particle Image Velocimetry (PIV). We use shadow technique and image processing to study the flap dynamics. We do a qualitative and quantitative comparison of the wake flow generated by two airfoil models, one with rigid trailing edge (model -B) and the other with flexible trailing edge (model -A) i.e. with a flexible flap fixed to the trailing edge. We study the flap dynamics, the flow around the flap, evolution of vortices, wake width, circulations around airfoil and vortices, momentum and energy in the wake (which is measure of propulsion efficiency), vortex geometry in the wake in terms of vortex spacing, etc. We also conduct a parametric study for both the models. Flap dynamics plays a prominent role in defining the signature of the wake. The observed flap deflections are quite large and the flap exhibits more than one mode of deflection; this affects the vortex-shedding pattern. The flap tip also executes a near sinusoidal motion with a phase difference between the trailing edge and the flap tip. The dye visualization studies show that a flexible trailing edge induces multiple vortices while in case of a rigid trailing edge, large vortical structures are shed. In case of flexible trailing edge (model -A), the vortices are shed away from the mean path of motion and are arranged in a ‘reverse Karman vortex street’ pattern producing an undulating jet representing a thrust on the airfoil. For the same Strouhal number, in case of rigid trailing edge (model -B), the vortices are shed nearly along the mean path of motion indicating a momentumless wake. The wake structures, particularly in case of model -A, are nearly insensitive to variations in amplitude and frequency. The wake of model -B shows some variable flow patterns for different amplitudes of oscillation. Although the total chord of model -A is 1.75 times more than the chord of model -B, the wake width is nearly the same for the two models when the amplitude of oscillation is same. The addition of the flap to the airfoil keeps the wake flow two-dimensional or symmetric about the center plane for longer times and longer downstream distances in comparison with the wake flow generated by rigid trailing edge. For 15o and 20o amplitudes of oscillations, the flow separates over the airfoil itself; the interaction of the separated flow with the flexible flap is quite interesting, which needs further investigations. The wake generated by the airfoil with flexible flap at the trailing edge has some common features with the wakes generated by the flow over a flapping filament (which is the one-dimensional representation of a fluttering flag), an accelerating mullet fish (a carangiform swimmer) and a steadily swimming eel (an anguilliform swimmer).