Master of Science / Department of Mechanical and Nuclear Engineering / B. Terry Beck / Mohammad H. Hosni / Cavitation is the formation of vapor phase from the liquid phase by reduction in its absolute pressure below the saturation pressure. Unlike boiling, where the temperature of the liquid is increased to cause vaporization, the reduction in the pressure alone can cause the liquid to turn into vapor. Cavitation is undesirable in many engineering applications as it is associated with reduction in efficiency and is known to cause damage to pump and propeller components. However, the endothermic nature of cavitation could be utilized to create a region of low temperature that could be utilized to develop a new refrigeration cycle. The work presented in this thesis is part of ongoing research into the potential cooling capacity of cavitation phenomena, where the cavitation in a converging-diverging nozzle is being investigated. Due to the constricting nature of the throat of the converging-diverging nozzle, the liquid velocity at the throat is increased, obeying the continuity law. With an increase in velocity, a reduction in absolute pressure is accompanied at the throat of the nozzle according to the Bernoulli’s principle. The local absolute pressure at the throat can go lower than the saturation vapor pressure, thereby causing the fluid to cavitate. The effect of water temperature on the flowrates, the onset of cavitation within the nozzle, and the resulting length of the cavitation region within the nozzle are the subject of this thesis. Experimental results and analysis are presented which also show that near the onset of cavitation, the flowrate can go beyond the choked flowrate, causing the local pressure in the throat to go well below zero for an extended amount of time in the metastable state, before nucleating (cavitating) into a stable state. Flow visualization using a high speed digital camera under different operating conditions was aimed at investigating the region of cavitation onset, which appears to be associated with boundary layer separation just downstream of the nozzle throat. In order to delay the boundary layer separation point in the downstream section of the nozzle, the diffuser region of the nozzle was modified to enable two flow paths, where one path would suck the flow near the inner walls of the nozzle and the other would allow the bulk of the flow to pass through. This was achieved with the use of inserts. Various inserts were tested in an attempt to capture the effect of inserts on the cavitation phenomena. Their effect on the flowrates, length of two phase region, and cavitation onset are presented in this thesis.

No description available.

Microhemodynamics is the study of blood flow in small vessels, usually on the order of 50 to 100 µm. The in vitro study of blood flow in small channels is analogous to the in vivo study of the microcirculation. At this scale the Reynolds and Womersly numbers are significantly less than 1 and the viscous stress and pressure gradient are the main determinant of flow. Blood is a non-homogeneous, non-Newtonian fluid and this complex composition and behavior has a greater impact at the microscale. A key parameter is the shear stress at the wall, which is involved in many processes such as platelet activation, gas exchange, embryogenesis and angiogenesis. In order to measure the shear rate in these blood flows the velocity profile must be measured. The measured profile can be characterized by the maximum velocity, the flow rate, the shear rate at the wall, or a shape parameter reflecting the bluntness of the velocity profile. The technique of micro-particle image velocimetry (µPIV) was investigated to measure the velocity profiles of blood microflows. The material of the channel, the type of tracer particles, the camera used, and the choice in data processing were all validated to improve the overall accuracy of µPIV as a blood microflow measurement method. The knowledge gained through these experiments is of immediate interest to applications such as the design of lab-on-a-chip components for blood analysis, analysis of blood flow behavior, understanding the shear stress on blood in the microcirculation and blood substitute analysis. Polymer channels were fabricated from polydimethylsiloxane (PDMS) by soft lithography in a clean room. PDMS was chosen for ease of fabrication and biocompatibility. The contacting properties of saline, water, and blood with various polymer channel materials was measured. As PDMS is naturally hydrophilic, surface treatment options were explored. Oxygenated plasma treatment was found to be less beneficial for blood than for water. The choice of camera and tracer particles were validated. Generally, for in vivo studies, red blood cells (RBCs) are used as tracer particles for the µPIV method, while for in vitro studies, artificial fluorescent micro particles are added to the blood. It is demonstrated here that the use of RBCs as tracer particles creates a large depth of correlation (DOC), which can approach the size of vessel itself and decreases the accuracy of the method. Next, the accuracy of each method is compared directly. Pulsed images used in conjunction with fluorescing tracer particles are shown to give results closest to theoretical approximations. The effect of the various post-processing methods currently available were compared for accuracy and computation time. It was shown that changing the amount of overlap in the post-processing parameters affects the results by nearly 10%. Using the greatest amount of correlation window overlap with elongated windows aligned with the flow was shown to give the best results when coupled with a image pre-processing method previously published for microflows of water. Finally the developed method was applied to a relevant biomedical engineering problem: the evaluation of blood substitutes and blood viscosity modifiers. Alginate is a frequently used viscosity modifier which has many uses in industry, including biomedical applications. Here the effect of alginate on the blood rheology, i.e., the shape of the velocity profile and the maximum velocity of blood flow in microchannels, was investigated. Alginate was found to blunt the shape of the velocity profile while also decreasing the shear rate at the wall. Overall, the accuracy of µPIV measurements of blood flows has been improved by this thesis. The work presented here has extended the known methods and accuracy issues of blood flow measurements in µPIV, improved the understanding of the blood velocity profile behavior, and applied that knowledge and methods to interesting, relevant problems in biomedical and biofluids engineering.

No description available.

A slit die with a rectangular slot placed transverse to the flow direction has been used for both flow visualization and direct pressure measurements of the hole pressure (Ph). The results from these measurements have been used to evaluate the Higashitani-Pritchard-Baird (HPB) equation which has been proposed for determining the magnitude of the primary normal stress difference (N1) from the values of the hole pressure. The slit die was run at higher shear rates than those used by Pike. Both the tracer method and flow birefringence technique were applied to visualize the streamlines and stress field, respectively, in the slot region . Effects of the slot on the flow pattern and on the stress field were examined by changing the slot width and by rounding the corners of the slot. The validity of the HPB equation, which is derived from the Higashitani-Pritchard theory (H-P theory), was tested by comparing values of N1 predicted by the HPB equation and slit die data with that obtained from the cone-and-plate rheometer. The validity of the HPB equation was also tested by changing the slot dimensions. Flow visualization experiments were performed for polystyrene (Styron-678) and polycarbonate (Lexan) melts. It was found that both the flow and stress fields are asymmetrically distributed about the slot centerline and that secondary flows exist in the upper part of the slot. However, no visible vortices was found for LOPE even though σ_w was increased to 70 Kpa. The shear rate at which the vortices became visible is lower for a wider slot. Rounding both corners of the slot seems to have an effect on smoothing the streamlines across the slot. Polycarbonate, which exhibits lower fluid elasticity than polystyrene does, the streamlines and stress field are more symmetrically distributed about the slot centerline. Five polymer melts were used in measuring Ph . It was found that the values of N1 predicted from the HPB equation correlate well with those obtained by the C&P apparatus at low shear rates. The predicted values of N1 also agree well with 2G' even though the shear rate was increased to 70 sec⁻¹ for most of the polymers investigated. Changing the width of the slot did not have a significant effect on the magnitude of Ph , whereas the magnitude of Ph depended largely on the polymers investigated. The measured Ph was nearly zero for polycarbonate at σ_w = 40KPa and was about 70 KPa for polystyrene at σ_w = 8O KPa. This was attributed to the significantly lower fluid elasticity of polycarbonate relative to polystyrene. / M.S.

The single-plate interferometer is a powerful flow visualization and aerodynamic measurement tool. It can provide full-field data for the density distribution in a non-intrusive manner, and it can be used for highly unsteady flows. While the device itself represents a large decrease in complexity over other forms of interferometry, the data reduction procedure has traditionally been laborious and difficult. To remove these difficulties and to improve the accuracy of the Virginia Tech interferometer setup, the software has been revamped into a black box design removing the need to handle the code directly. Furthermore, the software has been made to be platform independent by implementing the algorithms using the Java programming language. New hardware has also been added which further simplifies the setup procedure. The improved setup and the new software is used to study the flow around a film cooled turbine blade in the Virginia Tech cascade wind tunnel. The study of this flowfield is used as a validation for the new algorithms and to illustrate the ease of use of the system. Through this analysis, the density distribution for the entire flowfield is acquired. Furthermore the use of Plexiglas as window material was tried. This proved to work, however the manufacturing processing of these windows proved relatively difficult. Studying the film layer close to the surface proved difficult because of inherent limitations with the single-plate interferometer. / Master of Science

The flowfield in a swirling combustor with a Confinement Ratio of 1.6 was simulated in water. Qualitative measurements were made from flow visualization in a horizontal water rig with the help of Pliolite beads and liquid dye. Velocity and turbulence measurements were made in a cold air flow rig. Mean velocity measurements were made using a five hole Pitot probe and turbulence measurements were obtained from a hot wire anemometer. All the flow visualization tests were made with Reynold's number between 10,000 and 47,500, and quantitative measurements in the cold air flow were made at a Reynold's number of 7.1x10⁵. The experiment was conducted for two configurations of the combustor system inlet- 1) Straight inlet and 2) S-inlet. Both configurations were tested with swirling and non-swirling flows. The straight inlet configuration with non-swirling flow produced a uniform corner recirculation near the wall of the combustion chamber. The S-inlet made the corner recirculation zones unequal. The introduction of swirling in the flow created a central recirculation zone in addition to the corner recirculation zone. It was found that the central recirculation zone was most prominent in the configuration with the S-inlet and swirling flow. An oscillating central core was seen in the straight inlet case. The average turbulence level was higher in the configuration with the s-inlet. / M. S.

Entrainment can be defined as the thickening of a shear layer in the streamwise direction due to an increase in the volume occupied by the vorticity-containing fluid. The extent to which this process occurs depends heavily upon the geometry of the shear layer, with an elliptical jet, for example, exhibiting much greater entrainment than a circular one. It was desired to characterize the entrainment process and to explore the means by which the entrainment might be further enhanced by using the tool of numerical flow visualization to explore the results of numerical simulations and linear stability analysis. Because the results of preliminary numerical flow simulations were so topologically complex, linear stability analysis (LSA) was employed to generate simpler flows. These results provided important insight into the early stages of the (near field) mixing process and provided a means for testing the accuracy of the visualization tool. / Master of Science

Modeling of internal flow fields with hot, compressible fluids and sometimes combustion using cold flow techniques is discussed. The flow in a gas generator has been modeled using cold air. Experimental set up was designed and fabricated to simulate the unsteady flow with different configurations of inlet tubes. Tests were run for flow visualization and measurement of axial velocity at different frequencies ranging from 4 Hz to 12 Hz. Flow visualization showed that the incoming flow was a complex jet flow conformed to a cylindrical enclosure, while the outgoing flow resembled the venting of a pressurized vessel. The pictures show a complex flow pattern due to the angling of the jet towards the wall for the bent tube configurations and straightened flows with straight tube and other configurations with straighteners. Velocity measurements were made at an inlet Re of 8.1 x 10⁴ based on maximum velocity and inlet diameter using a single sensor hot wire anemometer at several locations in the plane of the inlet tube at 4 Hz, 8 Hz and 12 Hz for the straight tube and bent tube inlet configurations. The axial velocity near the entrance showed a strong component of the forcing frequency. Phase averaged mean velocities were observed to be well defined during charging and diminished during venting inside the cylinder. The jet flow penetrated most for the 4 Hz and least for the 12 Hz case. For the straight tube inlet comparison with a steady flow measurement of sudden expansion flow showed a qualitative similarity of the mean axial velocity distribution and centerline velocity decay during the charging phases. For the bent tube inlet case the contour plots showed the flow tendency towards the wall. Two cells were seen in the contours for the 8 Hz and 12 Hz cases. The deviation of the point of occurrence of maximum velocity in a radial profile was found to be about 6.5°. Entrance velocity profiles showed symmetry for the straight tube inlet while were skewed for the bent tube inlet. Contour plots of the phase averaged axial turbulence intensity for bent tube cases showed higher values in the core and near the wall in the region of impingement. Axial turbulence intensity measured for the straight tube case showed features as observed in an axisymmetric sudden expansion flow. / Ph. D.

Fluid flows past single and multiple cylinders in different configurations are investigated both experimentally and numerically. Three and five in-line cylinders and in-line and staggered bundles of cylinders with different pitch-to-diameter ratios are considered. Experimental work comprises of laser-Doppler velocimetry and flow visualization obtained in a water tunnel and skin friction, pressure, lift, drag and hot-wire measurements obtained in a wind tunnel. Both steady and pulsed flows are considered. Numerical work consists of finite element analysis of Navier-Stokes and energy equations governing viscous fluid flow past single and multiple cylinders. Detailed measurements of the fluid dynamic quantities for flow past cylinders reveal that flow pulsation at frequencies which induce lock-on increases the organization of the flow in gaps between cylinders. A new pattern of flow field is found for flow past a triad and a pentad of cylinders with a pitch-to-diameter ratio of l.8. The numerical analysis generates important integral characteristics like flow resistance and heat transfer. A staggered square arrangement of finite bundle of cylinders is found to have better heat transfer characteristics compared to the in-line or staggered equilateral-triangular arrangements. / Ph. D. / incomplete_metadata