No description available.

The self-similar evolution of a turbulent Rayleigh-Taylor (R-T) mix is investigated through experiments and numerical simulations. The experiments consisted of velocity and density measurements using thermocouples and Particle Image Velocimetry techniques. A novel experimental technique, termed PIV-S, to simultaneously measure both velocity and density fields was developed. These measurements provided data for turbulent correlations, power spectra, and energy balance analyses. The self-similarity of the flow is demonstrated through velocity profiles that collapse when normalized by an appropriate similarity variable and power spectra that evolve in a shape-preserving form. In the self-similar regime, vertical r.m.s. velocities dominate over the horizontal r.m.s. velocities with a ratio of 2:1. This anisotropy, also observed in the velocity spectra, extends to the Taylor scales. Buoyancy forcing does not alter the structure of the density spectra, which are seen to have an inertial range with a -5/3 slope. A scaling analysis was performed to explain this behavior. Centerline velocity fluctuations drive the growth of the flow, and can hence be used to deduce the growth constant. The question of universality of this flow was addressed through 3D numerical simulations with carefully designed initial conditions. With long wavelengths present in the initial conditions, the growth constant was found to depend logarithmically on the initial amplitudes. In the opposite limit, where long wavelengths are generated purely by the nonlinear interaction of shorter wavelengths, the growth constant assumed a universal lower bound value of

The tendency of aortic bileaflet mechanical heart valves (BiMHVs) to promote thrombosis has been well documented in the literature. The relationship of thrombosis to valve fluid dynamics has prompted numerous studies of aortic BiMHV flow. In this study, steady flow was investigated downstream of a model Carbomedics No. 25 BiMHV in an axisymmetric aortic sinus using stereoscopic particle image velocimetry (SPIV). The Reynolds number based on inlet diameter was 7600, and the measurement plane was perpendicular to the leaflet axes at the centerline of the aortic sinus. The typical formation of three jets was observed: the upper and lower lateral orifice jets, and the central jet. Flow separation from the valve ring was seen, and large scale vortices were identified in both the upper and lower sinus regions. An asymmetry in the reverse flow was found, and possible causes were discussed. All three jets were seen to decay similarly to free rectangular jets, with zero decay initially, followed by a 'linear' decay rate in which Umax^2~X. The central jet was also seen to be self similar in the linear decay region. Analysis of the out-of-plane velocity yielded two alternate explanations of streamwise vortex (i.e. Wx) structure, with either a four-cell or an eight-cell streamwise vortex structure being present in the mean velocity field. Organization of large scale three dimensional flow structures was thus apparent. Calculation of in-plane Reynolds stresses showed that values were highest in the outer shear layers of the lateral orifice jets. Elevated Reynolds shear stress values were also found in the leaflet wake regions, and the shear layers of the central jet.

The tendency of aortic bileaflet mechanical heart valves (BiMHVs) to promote thrombosis has been well documented in the literature. The relationship of thrombosis to valve fluid dynamics has prompted numerous studies of aortic BiMHV flow. In this study, steady flow was investigated downstream of a model Carbomedics No. 25 BiMHV in an axisymmetric aortic sinus using stereoscopic particle image velocimetry (SPIV). The Reynolds number based on inlet diameter was 7600, and the measurement plane was perpendicular to the leaflet axes at the centerline of the aortic sinus. The typical formation of three jets was observed: the upper and lower lateral orifice jets, and the central jet. Flow separation from the valve ring was seen, and large scale vortices were identified in both the upper and lower sinus regions. An asymmetry in the reverse flow was found, and possible causes were discussed. All three jets were seen to decay similarly to free rectangular jets, with zero decay initially, followed by a 'linear' decay rate in which Umax^2~X. The central jet was also seen to be self similar in the linear decay region. Analysis of the out-of-plane velocity yielded two alternate explanations of streamwise vortex (i.e. Wx) structure, with either a four-cell or an eight-cell streamwise vortex structure being present in the mean velocity field. Organization of large scale three dimensional flow structures was thus apparent. Calculation of in-plane Reynolds stresses showed that values were highest in the outer shear layers of the lateral orifice jets. Elevated Reynolds shear stress values were also found in the leaflet wake regions, and the shear layers of the central jet.

No description available.

No description available.

An experimental research was performed to study the effects of surface roughness on the characteristics of separated and reattached turbulent flows in an open channel. A backward facing step was used to induce flow separation. The rough surfaces comprised wire mesh grit-80 and sand grains of average diameter 1.5 mm. In each experiment, the Reynolds number based on the step height and freestream velocity of approach flow was fixed at 3240 and the Reynolds number based on the approach flow depth and freestream velocity was kept constant at 25130. Particle image velocimetry (PIV) technique was used to measure the flow velocity. The results showed that roughness effects on the mean and turbulent quantities are evident only in the recovery region. Moreover, roughness effects on the flow dynamics are dependent on the specific roughness element.

The research focus of this thesis is on modelling techniques for river kinetic turbines, to develop predictive numerical tools to further the design of this emerging hydro technology. The performance benefits of enclosing the turbine in a shroud are quantified numerically and an optimized shroud design is developed. The optimum performing model is then used to study river kinetic turbines, including different anchoring systems to enhance performance. Two different turbine numerical models are studied to simulate the rotor. Four different computational fluid dynamics (CFD) turbulence models are compared against a series of particle image velocimetry (PIV) experiments involving highly-separated diffuser-flow and nozzle-flow conditions. The risk of cavitation is briefly discussed as well as riverbed boundary layer losses. This study is part of an effort to develop this emerging technology for distributed power generation in provinces like Manitoba that have a river system well adapted for this technology.

An adjustable systemic-pulmonary artery (SPA) shunt is being developed that consists of apolytetrafluoroethylene (PTFE) graft with a screw plunger mechanism. This device would allowfull control of flow through SPA shunts used to augment pulmonary blood flow in neonates bornwith single ventricle physiology. The objective of this study is to evaluate the changes this mechanismhas on flow fields for a 4 mm and 5 mm adjustable SPA shunt. Two in vitro models wereexamined; an idealized model with an axisymmetric constriction and a model developed from 3-Dreconstruction of the actual shunt under asymmetric constriction. These models were used to measurethe instantaneous velocity and vorticity fields using Particle Image Velocimetry (PIV) underboth steady and pulsatile flow conditions. Recirculation regions and maximum values of velocity,vorticity, and shear stress are compared between the 4 mm and 5 mm models. The results indicatethat for the idealized model of both shunts, separation regions are much smaller, persistingfor approximately 0-1.75 diameters downstream of the constriction, while for the realistic modelsseparation regions of 2.5 diameters downstream were observed. Additional models of a 4 mm and5 mm shunt were tested under pulsatile conditions matching Re of 1061 and 849 and a Womersleynumber of 4.09 and 5.12, respectively, as seen in vivo. The maximum shear rates observed in bothshunts are within an allowable range without inducing platelet aggregation or hemolysis. However,regions of reverse flow exist distal to the throat, leading to possible concerns of plaque formation.Further in vivo testing will be needed to address this concern. This work is part of an extensiveeffort in developing a completely implantable adjustable systemic-pulmonary artery shunt.

A model of a gas turbine blade cooling channel equipped with turbulators and a backward facing step geometry was examined. Up to four turbulators oriented cross-stream and inclined 45° to the flow direction were mounted in the channel. The blockage ratio b/H of the turbulators and the height h/H of the backward facing step was 0:125 and 0:14 respectively. The number of turbulators as well as their size was varied. In a preliminary investigation, hot-wire and pressure measurements were taken for three different Reynolds numbers (5,000, 12,000, 18,000)in the center plane of the test section. Subsequently, particle image velocimetry (PIV) measurements were made on the same geometries. Results of PIV measurements for a Reynolds number range of Reb=600 to 5,000 for the turbulators and Reh=1,500 to 16,200 for the backward facing step are presented, where Reynolds numbers are based on turbulator height b and step height h, respectively. Plots of the velocity field, vorticity, reverse flow probability and RMS velocity are shown. The focus is on the steady flow behavior but also the unsteadiness of the flow is discussed in one section. Also reattachment lengths were obtained and compared among the various turbulator arrangements and the backward facing step geometry. It was found that the flow becomes periodic after three or four ribs. For one turbulator, a very large separation region was observed. The magnitude of the skin friction factor was found to be the highest for two ribs. If the first rib is replaced by a smaller rib, the skin friction factor becomes the lowest for this case. Compared to the backward facing step, the flow reattaches earlier for multiple turbulators. A dependency of reattachment length on Reynolds number was not observed.