Left ventricular flows in the human heart are very complex and in the presence of a diseased condition, such as unhealthy or prosthetic heart valves, the complexity of the flow is further increased. The intricacy of the heart geometry combined with the pulsatile character of the flow, the interaction of high-speed jets with the flexible walls, and the unsteady motion of the heart valve leaflets generate inherently complicated flow fields. It is therefore essential that we study and understand the complex cardiac energetics and physics of blood flow in both healthy and diseased hearts. Although artificial heart valves, mechanical and biological, have evolved to a level of universal acceptance, they have never reached a level of performance comparable to that of the natural valves of the heart. Many of the problems are directly related to the fluid mechanics. Considering that mechanical heart valves (MHV) are more commonly implanted because of their durability, it is imperative to better understand their hemodynamic behavior. Yet to date, no study has documented in depth the complex hemodynamic characteristics of left ventricular flows and assessed the intricate structures that are generated in the left ventricle (LV) due to vortex formation (roll-up of shear layers shed past the valve leaflets), turbulence characteristics, and energetics. The flow through pivoted leaflets of MHVs induces a combination of flow characteristics that are dependent on the specific valve design and orientation. The aim of the present study is to provide new insight into the spatio-temporal dynamics of the flow distal to a mitral MHV by employing a state-of-the-art, high resolution, flow diagnostic method, Time Resolved Digital Particle Image Velocimetry (TRDPIV) in a flexible, transparent LV documenting the evolution of eddies and turbulence during a complete period of the heart cycle. The broad impact of the proposed research extends beyond the hemodynamics of heart valve prosthesis. The research herein will enable the development of a tool for application in all cardiac energetic studies (unhealthy valves, tissue engineered valves, cardiac remodeling stages, and even congestive heart failure) and aid in better diagnosis of the efficiency and performance of the heart. The last component of the dissertation involved the translation of my dissertation research into an engineering educational tool for undergraduate engineering students. / Ph. D.

Left ventricular diastole (filling) is a complex process with many features and coupled compensatory mechanisms which coordinate to maintain optimal filling and ejection of the left ventricle. Diastolic filling is controlled by the left ventricular recoil, relaxation, and compliance as well as atrial and ventricular pressures making left ventricular diastolic dysfunction very difficult to understand and diagnose. An improved understanding of these unique flows is important to both the fundamental mechanics of the cardiac diastolic filling as well as the development of novel and accurate diagnostic techniques. This work includes studies of in-vivo and in-vitro vortex rings. Vortex rings created in the left ventricle past the mitral valve during diastole are produced in a confined domain and are influenced by the left ventricular walls. Therefore, an in-vitro analysis of the formation and decay of vortex rings within confined cylindrical domains using particle image velocimetry was conducted. Varying mechanisms of vortex ring breakdown were observed over a wide range of Reynolds numbers, and an analytical model for vortex ring circulation decay of laminar vortex rings was developed. Also, in this work a novel method for analyzing color M-mode echocardiography data using a newly developed automated algorithm is introduced which examines the pressure gradients and velocities within the left ventricle. From this analysis, a new diagnostic filling parameter is introduced which displays a greater probability of detection of diastolic dysfunction over the conventionally used diagnostic parameter. / Ph. D.

An experimental study of an aerodynamic ramp (aeroramp) injector was conducted at Virginia Tech. The aeroramp consisted of an array of two rows with two columns of flush-wall holes that induce vorticity and enhance mixing. For comparison, a single-hole circular injector with the same area angled downstream at 30 degrees was also examined. Test conditions involved sonic injection of helium heated to 313 K, to safely simulate hydrogen into a Mach 4 air cross-stream with average Reynolds number 5.77 e+7 per meter at a jet to freestream momentum flux ratio of 2.1. Sampling probe measurements were utilized to determine the local helium concentration. Pitot and cone-static pressure probes and a diffuser thermocouple probe were employed to document the flow. The main results of this work was that the mixing efficiency value of this aeroramp design which was optimized at Mach 2.4 for hydrocarbon fuel was only slightly higher than that of the single-hole injector at these flow conditions and the mass-averaged total pressure loss parameter showed that the aero-ramp and single-hole injectors had the same overall losses. The natural extension of the investigation was then to look in detail at two major physical phenomena that occurs in a complex injector design such the Aeroramp: the jet-shock interaction and the interaction of the vortical structures produced by the jets injection into a supersonic cross flow. Experimental studies were performed to investigate the effects of impinging shocks on injection of heated helium into a Mach 4 crossflow. It was found that the addition of a shock behind gaseous injection into a Mach 4 crossflow enhances mixing only if the shock is closer to the injection point where the counter-rotating vortex pair (always associated with transverse injection in a crossflow) is not yet formed, and the deposition of baroclinic generated of vorticity is the highest. The final investigation concerned with the interaction of the usual vortex structure produced by jet injection into a supersonic crossflow and an additional axial vortex typical of those that might be produced by the inlet of a scramjet or the forebody of a vehicle to be controlled by jet interaction phenomena. The additional axial vortices were generated by a strut-mounted, diamond cross-section wing mounted upstream of the injection location. The wing was designed to produce a tip vortex of a strength comparable to that of one of the typical counter-rotating vortex pair (CVP) found in the plume of a jet in a crossflow. The profound interaction of supersonic vortices supported by a quantitative description and characterization of the flowfield has been demonstrated. / Ph. D.

Experiments were performed to document the turbulent flowfield produced downstream of an airfoil encountering an intense streamwise vortex. This type of perpendicular airfoil--vortex interaction commonly occurs in helicopter rotor flows. The experiments presented here thus provide useful information for the prediction of helicopter noise, particularly BWI noise. Three-component velocity and turbulence measurements were made in unprecedented detail using a computerized miniature four-sensor hot-wire probe system; revealing much about the structure and behavior of this flow over a range of conditions. The interaction between the vortex and the airfoil wake leaves the vortex surrounded by a large region of intense turbulence unlike the turbulence surrounding an isolated vortex. Even for close separations, the vortex core passes the airfoil virtually unchanged. However, vorticity of opposite sign is shed by the airfoil in response to the angle of attack distribution induced by the vortex resulting in an unstable circulation distribution according to Rayleigh's criterion. Simple theoretical models adequately describe the shed vorticity distribution of the airfoil and the unstable circulation distribution it imparts on the vortex. As the flow develops, the vortex continuously distorts the airfoil wake. The strain rates imparted by the vortex on the spanwise vorticity contained in the airfoil wake result in an anisotropic, turbulence producing stress field. For several chord lengths downstream, the vortex core remains laminar and little change is seen in the unstable circulation distribution. While the vortex core is laminar, turbulent fluctuations measured in the core are the result of inactive wandering motions and the characteristic length and velocity scales of the flat portion of the vortex wake appear to be appropriate scales for the fluctuations. Eventually, the vortex core becomes turbulent as indicated by an increase in high frequency velocity fluctuation levels of more than an order of magnitude. Subsequently, the circulation distribution reorganizes to a stable distribution. A loss in core circulation occurs due to a decrease in the peak tangential velocity which is proportionately larger than the increase in the vortex core radius. The peak tangential velocity decreases to the point where it is exceeded by the axial velocity deficit---another unstable situation. These effects increase with decreased separation between the vortex and the airfoil, but appear to be largely independent of airfoil angle of attack an only weakly dependent upon vortex strength. / Ph. D.

Detailed measurements were taken downstream of several modified trailing edge configurations designed to impart streamwise velocity into the flow behind a cascade of GE Rotor B fan blades. These measurements were conducted in the Virginia Tech Low Speed Linear Cascade wind tunnel. The trailing edge configurations tested utilized passive techniques for producing streamwise vorticity, which in turn causes downstream wake diffusion and increased mixing. A more diffuse wake, when it impinges on the downstream stator, will produce lower noise levels as a result of this rotor-stator interaction. Furthermore, increased mixing in the flow will reduce the levels of turbulence kinetic energy observed downstream of the blade trailing edge. Thus, this project seeks to identify which passive techniques of imparting streamwise vorticity are most effective at improving the flow characteristics responsible for some of the noise production in modern jet aircraft. The three trailing edge configurations tested in detail for this project showed significant ability to widen and stretch the downstream wake by utilizing vorticity generation techniques. The TE-8 configuration was the most effective at increasing the wake width downstream of the trailing edge. Additionally, each configuration was able to successfully reduce some of the turbulence kinetic energy levels observed downstream when compared to the baseline blade, the most effective configuration being TE-8. Finally, the momentum thickness of each configuration was measured. When compared to the baseline, the TE-1 configuration showed an increased momentum thickness, TE-8 showed little change, and TE-7 actually showed an improved momentum thickness value. / Master of Science

The wake of a bluff body is a representative issue in vortex dynamics that plays a central role in civil engineering, ocean engineering and thermal engineering. In this work, a flowing soap film was used to investigate the wakes of multiple stationary circular cylinders and of a single oscillating cylinder. Corresponding computer simulations were also conducted. Vortex formation of a stationary circular cylinder was analyzed by proper orthogonal decomposition (POD). The POD analysis was used to define an unsteady vortex formation length, which suggests a relationship between the vortex formation length of a single cylinder and the critical spacing of two cylinders in a tandem arrangement. A systematic parametric study of the wake structure was conducted for a controlled transversely oscillating cylinder. Neural network and support vector machine codes assisted the wake classification procedure and the identification of boundaries between different wake regimes. The phase map of the vortex shedding regimes for the (quasi) two-dimensional experiment qualitatively agrees with previous three-dimensional experiments. The critical spacings of two identical tandem circular cylinders in a flowing soap film system were determined using visual inspections of the wake patterns and calculations of the Strouhal frequencies. The dimensionless spacing was both increased and decreased quasi-statically. Hysteresis was observed in the flow patterns and Strouhal numbers. This study appears to provide the first experimental evidence of critical spacing values that agree with published computational results. The wake interaction between a stationary upstream circular disk and a free downstream circular disk was also investigated. With the ability to tie together the wake structure and the object motion, the relationship between energy generation and flow structure in the simplified reduced order model system was studied. The research results find the optimal efficiency of the energy harvesting system by a parametric study. / PHD / The wake of a bluff body is a classic issue in vortex dynamics that has been the subject of much research in civil engineering, ocean engineering and thermal engineering. Bluff bodies, especially circular cylinders, can be found extensively in heat exchangers, cooling systems and offshore structures. Flow-induced vibration of a bluff body due to the formation of a wake is an important problem in many fields of engineering. Flow-induced vibration determines the oscillation of flexible pipes that transfer oil from the seabed to the surface of the ocean, for example [71]. In civil engineering, flow-induced vibration affects the design of bluff structures in wind such as bridges, chimneys and buildings [62]. Flow-induced vibration caused by vortices being shed from a bluff body is also a promising way to extract energy from geophysical flows [10]. FIV energy harvesting systems are especially suitable for slow flow speeds in the range 0.5-1.5m/s which cannot be efficiently harvested by traditional hydroelectric power technologies. When a pair of tandem cylinders is immersed in a flow, the downstream cylinder can be excited into wake-induced vibrations (WIV) due to the interaction with vortices coming from the upstream cylinder. In this work, a flowing soap film was used to investigate the flow-induced vibration of the downstream cylinder of a tandem pair. With the ability to tie together the wake structure and the object motion, we investigate the relationship between energy generation and flow structure in the reduced order model system. The research results find the optimal efficiency of the energy harvesting system by a parametric study. To get deep physical understanding of the flow-induced vibration, wake structures of a circular cylinder undergoing controlled motion and the critical spacing of two identical tandem circular cylinders were also investigated in this research. These research results can help not only the optimization of energy harvesting systems based on flow-induced vibration of the circular-cylinder system, but also will benefit the understanding of wake interactions between multiple bluff bodies such as schooling fish, natural draft cooling towers and wind turbine farms.

The ability to generate a crossflow vortex pattern on a swept flat plate, that is typical of swept-wing flows, is presented. A swept flat plate with an elliptic leading edge is mounted vertically in the VPI&SU Stability Wind Tunnel opposite to a swept wall-bump; along with floor and ceiling fairings that duplicate the inviscid streamlines. The resulting pressure gradient over the plate produces a crossflow vortex structure. Detailed three-dimensional measurements, made within the boundary layer using hot-wire anemometry, are supplemented with different flow-visualization techniques. Freestream measurements are carried out to find the variation of the velocity vector along the model and comparisons are made with the theory showing good agreement. Spanwise measurements are conducted within the boundary layer and show a steady vortex structure. Boundary-layer profiles are taken using both a straight-wire and slant-wire probe in order to obtain the variation of the velocity vector in the region. The components parallel and perpendicular to the freestream velocity vector are extracted, and the perpendicular component is called the crossflow profile. These profiles are compared to the theory. Shape factors and crossflow Reynolds numbers are obtained and analysed. Two types of flow-visualization techniques are used in the tests. One is the "smoke-wire" technique, where streaklines inside the boundary layer are visualized. This method is not successful in visualizing any vortex structure in the boundary layer. Therefore, the second method was employed: a sublimation technique using trichlorethane and naphthalene is used to visualize the variation of the surface shear stress. A uniform pattern of streaks aligned approximately in the potential flow direction is observed. This pattern has a wavelength on the order of one centimeter which agrees quite well with the theoretical model. / Master of Science

An experimental investigation was conducted to find effective methods of controlling a junction vortex system around and in the wake of a NACA 0020 body with a 1.5:1 elliptical nose mounted normal to the flat surface. Control of the junction vortex system was investigated using more slender noses and leading edge fillets. The study includes surface flow visualizations, and extensive mean flow measurements. The mean flow measurements consist of velocity, total pressure, and static pressure distributions. The results of the mean flow measurements around and in the wake of a NACA 0020 body with a 1.5:1 elliptical nose show the presence of a large, dominant vortex, or a primary horseshoe vortex, in the junction between the body and the flat surface. A smaller, co-rotating vortex relative to the primary horseshoe vortex was observed at the 100 percent chord position, which is believed to be identified with the corner separation at the junction near the trailing edge. Mean flow measurements of the flow field around and in the wake of a NACA 0020 body with more slender noses were acquired. The results indicate that the more slender nose generated a weaker primary horseshoe vortex. The results also suggest that the weak primary horseshoe vortex developed by the more slender nose was dissipated alongside the body to the point where it had a very small effect on the flow field at the 100 percent chord position. The results of the mean flow measurements around and in the wake of a NACA 0020 body with a 1.5:1 elliptical nose with leading edge fillets indicate that the leading edge fillets were very effective in reducing the strength and size of the primary horseshoe vortex. The results suggest that an effective fillet should be about one boundary layer thickness high and two body thicknesses long. The vorticity identified with the corner separation vortex was observed in the wake for both leading edge shape changes and fillets. The results show that the corner separation vortex dominates the flow in the wake due to the weak effects of the primary horseshoe vortex. A trailing edge fillet was used to investigate its effects on the corner separation vortex in the wake of the NACA 0020 body with the 1.5:1 elliptical nose with a leading edge fillet. The results indicate a small decrease in the strength of the comer separation vortex at the 150 percent chord position, but the effects on the wake pattern were small. / Ph. D.

A nonlinear three-dimensional vortex-lattice method which treats the steady separated flow over prolate bodies with open separation moving through an inviscid incompressible fluid is developed. The strength and position of the body wake are found as part of the solution. Specifically, flows with smooth-surface separation are considered as opposed to flows with sharp-edge separation treated· with the vortex-lattice concept in the past. To demonstrate the technique, results for the flow over an inclinded ogive-cylinder are presented. In the case of attached flow, comparsions are presented of the results from the vortex-lattice method using optimal and average control point locations with the results of the source-distribution method and with experimental data. Significantly, the same panel arrangement is used in the calculations for both methods. The results demonstrate that the results of the present method is somewhat more sensitive to panel arrangement than are those of the source-distribution method. Also, the effect of control point location varies dramatically as the incidence of the body is changed. In the case of separated flow, comparsions of the results of the vortex-lattice method are made with experimental data and with the results of a typical two-dimensional analogy. The results demonstrate that the present method agrees most favorably with the experimental data windward of a separation line. / Doctor of Philosophy

Approved for public release, distribution is unlimited / When a burst of air is produced in water, the result can be a toroidal bubble. This thesis is concerned with experimental investigations of three acoustical properties of toroidal bubbles: (i) propagation through high-intensity noise, (ii) emission, and (iii) scattering. In (i), an attempt to observe a recent prediction of the acoustic drag on a bubble is described, which is analogous to the Einstein-Hopf effect for an oscillating electric dipole in a fluctuating electromagnetic field. No effect was observed, which may be due to insufficient amplitude of the noise. In (ii), observations of acoustic emissions of volume oscillations of toroidal bubbles are reported. Surprisingly, the emission occurs primarily during the formation of a bubble, and is weak in the case of very smooth toroidal bubbles. In (iii), we describe an experiment to observe the effect of a toroidal bubble on an incident sound field. In addition to the acoustical investigations, we describe the construction of a large hallway apparatus for further investigations and for hands-on use by the public. The tank has cross section 2 feet by 2 feet and height 6 feet, and the parameters of reservoir pressure and time between air bursts are adjustable by the observer. / Lieutenant, United States Navy