A pressure-driven three-dimensional turbulent boundary layer flow upstream and around a junction vortex was experimentally studied and is offered for use as a benchmark flow for testing and evaluating the predictive ability of state-of-the-art three-dimensional turbulent boundary layer codes. The pressure-driven flow and junction vortex system was generated by a streamlined cylinder placed normal to a flat surface. Measurements of wall static pressure, wall shear stress, mean velocity, and Reynolds stress tensor field are reported at several stations in the three-dimensional turbulent boundary layer region. Documentation of the flow edge conditions is provided as well as upstream initial conditions along a plane with measured mean velocity and Reynolds stress tensor to permit the testing of intermediate and higher order turbulence models. Measurements of wall shear stress magnitude were made with a Preston tube and the wall shear stress directions were taken from an oil streak flow visualization. These results are compared with earlier direct force wall shear measurements of both magnitude and direction. Mean velocity magnitude and direction were measured with a single hot film probe. Measurements of the complete Reynolds stress tensor were carried out with three hot film x-array probes. Supporting work includes a wind tunnel calibration which examined the sensitivity and effects of spanwise nonuniformities and a two-dimensional momentum integral calculation along the tunnel center plane; the development of a calibration technique to determine individual sensor yaw characteristics in more complex probe geometries; and a generalized response analysis for a sensor with arbitrary orientation to the flow which allows for the use of an arbitrary yaw cooling law, allows for modest amounts of probe misalignment and yields a precise definition of matched sensors, geometric guidelines for constructing x-array probes, and a general mean velocity correction for turbulence where several existing formulas are compared. In addition, two popular cooling laws are studied, comparisons are made with other response equations, and an extensive discussion of the errors associated with the matched sensor approximations is given. Comparisons are made of several mean velocity measurements using different probes and redundant normal and shear stresses measured by the different x-array film probes, a single wire, and single film probe are compared. / Ph. D.

Thesis (Ph. D.)--Joint Program in Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Department of Mechanical Engineering; and the Woods Hole Oceanographic Institution), 2015 / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (pages 183-193). / This thesis is motivated by a series of biological experiments that display the harbor seal's extraordinary ability to track the wake of an object several seconds after it has swum by. They do so despite having auditory and visual cues blocked, pointing to use of their whiskers as sensors of minute water movements. In this work, I elucidate the basic uid mechanisms that seals may employ to accomplish this detection. Key are the unique ow-induced vibration properties resulting from the geometry of the harbor seal whisker, which is undulatory and elliptical in cross-section. First, the vortex-induced vibration (VIV) characteristics of the whisker geometry are tested. Direct force measurements and ow visualizations on a rigid whisker model undergoing a range of 1-D imposed oscillations show that the geometry passively reduces VIV (factor of > 10), despite contributions from eective added mass and damping. Next, a biomimetic whisker sensor is designed and fabricated. The rigid whisker model is mounted on a four-armed flexure, allowing it to freely vibrate in both in-line and crossflow directions. Strain gauges on the flexure measure deflections at the base. Finally, this device is tested in a simplified version of the sh wake { seal whisker interaction scenario. The whisker is towed behind an upstream cylinder with larger diameter. Whereas in open water the whisker exhibits very low vibration when its long axis is aligned with the incoming ow, once it enters the wake it oscillates with large amplitude and its frequency coincides with the Strouhal frequency of the upstream cylinder. This makes the detection of an upstream wake as well as an estimation of the size of the wake-generating body possible. A slaloming motion among the wake vortices causes the whisker to oscillate in this manner. The same mechanism has been previously observed in energy-extracting foils and trout actively swimming behind bluff cylinders in a stream. / by Heather Rachel Beem. / Ph.D.

A modified source and dipole panel method to calculate the flow properties around an oscillating arbitrary body in the presence of a free surface is proposed. To demonstrate the feasibility of the method the problem of a pulsating line source submerged under a free surface is treated. The technique chosen is based on Green's identity whereby the boundary-value problem is expressed as a boundary integral equation which is solved numerically. The near field of the water surface is represented by singularity panels with constant strength. The work was motivated by the reported large computing times for existing programs using Green's functions satisfying the free surface boundary condition. The present approach uses free-space Green's function. The free surface boundary condition is applied to surface singularity panels using Green's theorem. Thus free surface effects are included in the solution while panel integrations are simplified considerably by the use of simpler Green's function. The matrix equations resulting from Green's identity were solved by using IMSL routines for Gaussian Elimination, and the behavior of the influence coefficient matrix was tested by using LINPACK routines. The depth of the submerged-source and wave number was kept constant while the length of near field and the number of panels per wavelength was varied systematically. A minimum of 10 panels per wavelength and paneled water surface length of 2 wavelengths gives good agreement with the known exact solution. Computing times were low, indicating the feasibility of the technique for application to unsteady ship problems. / Ph. D.

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).

Experiments are done using a smooth solid rigid homogeneous acrylic sphere rolling on an inclined plane which is submerged in water. The motivation for these experiments comes from a need to understand a class of solid-fluid interaction problems that include sediment transport, movement of gravel on ocean floor and river bed due to water currents. Experiments are performed in a glass water tank 15 cm wide by 14 cm deep by 61 cm long which can be tilted to desired angle. The sphere is released from rest on the inclined false bottom of the tank in quiescent water. Our experimental study has twofold aim: (1)to study the boundary layer separation, the three-dimensional eddying motion in the wake and the near-wake structure and(2) to establish hydrodynamic force coefficients by analyzing kinematical data of the sphere motion from start to till it attains terminal velocity. Experiments are carried out at moderate Reynolds number Rearound1500. Previous studies on the first problem exist in the literature for Reup to 350. Previous studies on the second problem do not clearly define the added-mass coefficient and the influence of the water tank side-walls on the drag coefficient. In the first study, the characterization of the wake is done using flow visualization methods (fluoresce in dye visualization and particle streak visualization) and Particle Image Velocimetry (PIV). Laser light sheet obtained from an argon ion continuous laser beam is taken in different orientations to illuminate the fluoresce in dye or 14 m silver-coated hollow glass spheres. These experiments show that the wake behind the rolling sphere up to 1.6 diameters (or 1.6D) downstream is confined within height 1.2Dand width1.2D. At about 1.8Ddownstream, the wake sways alternately on either side of the equatorial plane, moving in lateral-vertical direction and moving out of the confining region; this gives zigzag appearance to the wake. Also in these experiments, we observe that the flow separations from the surface of the rolling sphere show three separation zones. The eddies shed from the primary separation surface on the upper hemisphere are symmetrical about the equatorial plane with Strouhal number St=1.0. The primary separation is affected by the symmetrical secondary separations on the rear surface in the piggyback region — it is the region near the upper rear surface of the sphere behind the transverse equatorial plane and below the primary separation surface. The lower eddies below the primary separation zone are shed alternately on either side of the equatorial plane with shedding frequency St=0.5. Our experiments show that there is a viscous blockage of width 0.4Dat the crevice near the point of contact. On either side of the viscous blockage at the crevice, we see weak symmetric eddies. Based on our experimental observations, we proceed to build a simple physical model of the separated flow on the surface of the rolling sphere. In the second study, the motion of the sphere is photographed and paired data of the displacement and time is obtained for the sphere motion from the start of motion till terminal velocity is reached at about 4.5 sphere diameters from the point of release of the sphere. Equation of motion of the sphere is solved numerically treating added-mass coefficient Ca and drag coefficient Cd as parameters. Experimental data is fitted on these solutions and the best fit gives the values of the force coefficients. Theoretical value of Ca equal to 0.621 is confirmed experimentally. Value of Cd is found to be 1.23 at Re=990 and it is 1.06 at Re= 1900. Side-wall effects become important for ratio of diameter of sphere to width of tank greaterthan0.20.

"Fluid-structure interaction arises in a wide array of technological applications including naval and marine hydrodynamics, civil and wind engineering and flight vehicle aerodynamics. When a fluid flows over a bluff body such as a circular cylinder, the periodic vortex shedding in the wake causes fluctuating lift and drag forces on the body. This phenomenon can lead to fatigue damage of the structure due to large amplitude vibration. It is widely believed that the wake structures behind the structure determine the hydrodynamic forces acting on the structure and control of wake structures can lead to vibration control of the structure. Modeling this complex non-linear interaction requires coupling of the dynamics of the fluid and the structure. In this thesis, however, the vibration of the flexible cylinder is prescribed, and the focus is on modeling the fluid dynamics in its wake. Low-dimensional iterative circle maps have been found to predict the universal dynamics of a two-oscillator system such as the rigid cylinder wake. Coupled map lattice (CML)models that combine a series of low-dimensional circle maps with a diffusion model have previously predicted qualitative features of wake patterns behind freely vibrating cables at low Reynolds number. However, the simple nature of the CML models implies that there will always be unmodelled wake dynamics if a detailed, quantitative comparison is made with laboratory or simulated wake flows. Motivated by a desire to develop an improved CML model, we incorporate self-learning features into a new CML that is trained to precisely estimate wake patterns from target numerical simulations and experimental wake flows. The eventual goal is to have the CML learn from a laboratory flow in real time. A real-time self-learning CML capable of estimating experimental wake patterns could serve as a wake model in a future anticipated feedback control system designed to produce desired wake patterns. A new convective-diffusive map that includes additional wake dynamics is developed. Two different self-learning CML models, each capable of precisely estimating complex wake patterns, have been developed by considering additional dynamics from the convective-diffusive map. The new self-learning CML models use adaptive estimation schemes which seek to precisely estimate target wake patterns from numerical simulations and experiments. In the first self-learning CML, the estimator scheme uses a multi-variable least-squares algorithm to adaptively vary the spanwise velocity distribution in order to minimize the state error (difference between modeled and target wake patterns). The second self-learning model uses radial basis function neural networks as online approximators of the unmodelled dynamics. Additional unmodelled dynamics not present in the first self-learning CML model are considered here. The estimator model uses a combination of a multi-variable normalized least squares scheme and a projection algorithm to adaptively vary the neural network weights. Studies of this approach are conducted using wake patterns from spectral element based NEKTAR simulations of freely vibrating cable wakes at low Reynolds numbers on the order of 100. It is shown that the self-learning models accurately and efficiently estimate the simulated wake patterns within several shedding cycles. Next, experimental wake patterns behind different configurations of rigid cylinders were obtained. The self-learning CML models were then used for off-line estimation of the stored wake patterns. With the eventual goal of incorporating low-order CML models into a wake pattern control system in mind, in a related study control terms were added to the simple CML model in order to drive the wake to the desired target pattern of shedding. Proportional, adaptive proportional and non-linear control techniques were developed and their control efficiencies compared."