In the present work, we experimentally study thrust generation from sinusoidally pitched rigid and flexible foils immersed in a uniform flow. The flexible foils are made by attaching a flexible flap of known flexural rigidity and flap length to the trailing edge of a rigid foil. For such thrust generating systems, a propulsive efficiency (η) may be defined as the ratio of the useful work done to the input energy requirement. In the present experiments, the propulsive efficiency (η) of the flapping foil can be determined from direct measurement of the unsteady forces and torque on the foil. The effects of systematic variation of the flexural rigidity of the foil, from highly flexible to rigid, on the thrust and efficiency characteristics of the foil are investigated. Studying such oscillating foils helps one to understand and mimic the efficient thrust generating mechanism in fishes and other creatures that use flapping to locomote themselves. A strain guage based loadcell is used to measure the forces normal to the foil (N) and forces along the chord of the foil (A). With a potentiometer, the instantaneous angular position (θ) is also measured, so that instantaneous lift (L) and thrust (T ) can be calculated. The measured moment (M) is used to calculate the instantaneous power input (P = Mθ˙). The foil is immersed in a uniform flow (u) set in a water tunnel, and the sinusoidal pitching (θ = θmaxsinωt) is provided by a servo motor. The Reynolds number (Re = uc/ν) in the present study is in the range of 103 to 104 . For the case of the rigid foil, the thrust and efficiency characteristics are presented for variation of the non-dimensional flapping frequency called the ‘reduced frequency’ (k = πfc/u), which is varied in the range of 1 to 10. At small reduced frequency (k < 3), the foil experiences a mean drag, while at k > 3, the foil experiences a mean thrust that grows rapidly as the reduced frequency (k) is increased. The thrust characteristics of the rigid foil are decided mainly by the normal force’s phase with respect to θ (φCN ) and its magnitude ([CN ]), as the chord-wise force is very small compared to the normal force (A << N). The measurements show that the non-dimensional mean thrust coefficient (CT ) scales as k2 and non-dimensional mean power (CP ) scales as k3 for k Ҳ 4. The maximum efficiency for rigid foils is found to be 8 % and it occurs at k 6. For the flexible foil case, the effect of making a portion of the total foil flexible by means of attaching a flexible flap of known flexural rigidity (EI) and flap length (cF ) to a rigid foil of length (cR) is studied. Unlike the rigid foils, the chordwise force (A) becomes an important factor in determining the thrust and efficiency characteristics of the flexible foils, due to the bending of the flap. We present results for a broad range of flexural rigidities from highly flexible flaps to stiff flaps, with the extent of flexibility fixed at cF /cR =0.8. We find that there is an optimal flexural rigidity for which the efficiency (η) reaches a maximum of 28 %. This represents a 250 % improvement compared to the rigid foil. The flexible foils with stiff flaps show a strange behavior with all the mean thrust coming from chordwise forces (A), unlike other flexible foils where the contribution to mean thrust come from both normal and chordwise forces. The effect of varying the extent of flexibility (cF/cR) with fixed flexural rigidity has also been studied. We define a non-dimensional flexibility parameter, R∗ = EI/(0.5ρu2sc3F ), which can combine the effect of variations in EI and cF /cR. Using this non-dimensional flexibility parameter (R∗), we find out that mean thrust and efficiency data for both the EI and cF/cR variation study collapse onto a single curve, indicating that R∗ can indeed be a single parameter characterizing flexibility. The present work shows that flexible foils can improve efficiency over rigid foils. Efficiency improvements can come in two ways depending on the R∗ of the flexible foil. Flexible foils with R∗ in the range of 10−2 to 100 show nearly 250% improvement in efficiency, accompanied by nearly 70 % loss in thrust compared to an entirely rigid foil of the same total chord. Flexible foils with R∗ in the range of 100 to 101 show nearly 50 % improvement in efficiency accompanied by nearly 100% increase in thrust.

Oscillating-foil hydrokinetic turbines have gained interest over the years to extract energy from renewable sources. The influence of the sweep angle on the performance of a fully-passive oscillating-plate hydrokinetic turbine prototype was investigated experimentally in the present work. The sweep angle was introduced to promote spanwise flow along the plate in order to manipulate the leading edge vortex (LEV) and hydrodynamically optimize the performance of the turbine. In the present work, flat plates of two configurations were considered: a plate with a 6° sweep angle and an unswept plate (control), which were undergoing fully-passive pitch and heave motions in uniform inflow at the Reynolds numbers ranging from 15 000 to 30 000. The resulting kinematic parameters and the energy extraction performance were evaluated for both plates. Planar (2D) particle image velocimetry (PIV) was used to obtain patterns of the phase-averaged out-of-plane vorticity during the oscillation cycle. The circulation in the wake was then related to the induced-forces on the plate by calculating the moments of vorticity of the LEV with respect to the pitching axis of the plate. Tomographic (3D) PIV was implemented in evaluating the influence of the spanwise flow on the dynamics of the vortex structure in three-dimensional space. The rate of deformation of the vortex length was quantified by calculating the deformation terms embedded in the vorticity equations, then linked to the stability of the vortex. The results show evidence of delay of the shedding of LEV and increased vortex stability, in the case of the swept plate. The manipulation of the LEV by the spanwise flow was related to the induced kinematics exhibited by the prolonged heave forces experienced by the swept plate, which led to the higher power extraction performance at high inflow velocities. In the presence of spanwise flow, positive vortex stretching along the vortex line increased the stabilization of the vortex core and prevented the onset of helical vortex breakdown, observed in the case of the unswept plate. The use of the sweep profile on the plate has led to the improvement of energy extraction performance of the fully-passive hydrokinetic turbine. / Graduate

There is currently a great deal of interest within the aviation community in the design of small, slow-flying but manoeuvrable uninhabited vehicles for reconnaissance, surveillance, and search and rescue operations in urban environments. Inspired by observation of birds, insects, fish and cetaceans, flapping wings are being actively studied in the hope that they may provide greater propulsive efficiencies than propellers and rotors at low Reynolds numbers for such Micro-Air Vehicles (MAVs). Researchers have posited the Strouhal number (combining flapping frequency, amplitude and forward speed) as the parameter controlling flapping wing aerodynamics in cruising flight, although there is conflicting evidence. This thesis explores the effect of flapping frequency and amplitude on forces and wake structures, as well as physical mechanisms leading to optimum propulsive efficiency. Two-dimensional rigid airfoils are considered at Reynolds number 2,000 ??? 40,000. A compressible Navier-Stokes simulation is combined with numerical and analytical potential flow techniques to isolate and evaluate the effect of viscosity, leading and trailing edge vortex separation, and wake vortex dynamics. The wake structures of a plunging airfoil are shown to be sensitive to the flapping frequency independent of the Strouhal number. For a given frequency, the wake of the airfoil exhibits ???vortex lock-in??? as the amplitude of motion is increased, in a manner analogous to an oscillating circular cylinder. This is caused by interaction between the flapping frequency and the ???bluff-body??? vortex shedding frequency apparent even for streamlined airfoils at low Reynolds number. The thrust and propulsive efficiency of a plunging airfoil are also shown to be sensitive to the flapping frequency independent of Strouhal number. This dependence is the result of vortex shedding from the leading edge, and an interaction between the flapping frequency and the time for vortex formation, separation and convection over the airfoil surface. The observed propulsive efficiency peak for a pitching and plunging airfoil is shown to be the result of leading edge vortex shedding at low flapping frequencies (low Strouhal numbers), and high power requirements at large flapping amplitudes (high Strouhal numbers). The efficiency peak is governed by flapping frequency and amplitude separately, rather than the Strouhal number directly.

There is currently a great deal of interest within the aviation community in the design of small, slow-flying but manoeuvrable uninhabited vehicles for reconnaissance, surveillance, and search and rescue operations in urban environments. Inspired by observation of birds, insects, fish and cetaceans, flapping wings are being actively studied in the hope that they may provide greater propulsive efficiencies than propellers and rotors at low Reynolds numbers for such Micro-Air Vehicles (MAVs). Researchers have posited the Strouhal number (combining flapping frequency, amplitude and forward speed) as the parameter controlling flapping wing aerodynamics in cruising flight, although there is conflicting evidence. This thesis explores the effect of flapping frequency and amplitude on forces and wake structures, as well as physical mechanisms leading to optimum propulsive efficiency. Two-dimensional rigid airfoils are considered at Reynolds number 2,000 ??? 40,000. A compressible Navier-Stokes simulation is combined with numerical and analytical potential flow techniques to isolate and evaluate the effect of viscosity, leading and trailing edge vortex separation, and wake vortex dynamics. The wake structures of a plunging airfoil are shown to be sensitive to the flapping frequency independent of the Strouhal number. For a given frequency, the wake of the airfoil exhibits ???vortex lock-in??? as the amplitude of motion is increased, in a manner analogous to an oscillating circular cylinder. This is caused by interaction between the flapping frequency and the ???bluff-body??? vortex shedding frequency apparent even for streamlined airfoils at low Reynolds number. The thrust and propulsive efficiency of a plunging airfoil are also shown to be sensitive to the flapping frequency independent of Strouhal number. This dependence is the result of vortex shedding from the leading edge, and an interaction between the flapping frequency and the time for vortex formation, separation and convection over the airfoil surface. The observed propulsive efficiency peak for a pitching and plunging airfoil is shown to be the result of leading edge vortex shedding at low flapping frequencies (low Strouhal numbers), and high power requirements at large flapping amplitudes (high Strouhal numbers). The efficiency peak is governed by flapping frequency and amplitude separately, rather than the Strouhal number directly.