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Browsing by Subject "Particle image velocimetry"

Now showing 1 - 4 of 4
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    Experimental studies of flow through deformable silicone and tissue engineered valves.
    (2009-12) Amatya, Devesh M.
    Annually, approximately 250,000 repair/replacement heart valve surgeries are performed world-wide. Currently the two options available for valve replacement are mechanical or bioprosthetic valves. Thrombosis (blood clots) and embolic events (movement of the clots through the blood vessels) have been linked with the mechanical valves, so that life-long anticoagulant therapy is required. Deterioration of the structural integrity, in part due to calcification, has been linked with bioprosthetic valves. The current paradigm is to replace a living, but incompetent valve with a non-living valve, be it mechanical or biological prosthetic. A living prosthetic valve grown with patient donor-based tissue engineering paradigm may be a possible solution. The primary objective of this study was to characterize the in vitro performance of the tissue engineered valve equivalents in a cardiovascular pulse duplicator and assess their potential for clinical use as valve replacement prostheses. A second objective was to conduct experiments under different flow conditions with synthetic silicone polymer valves of various geometries and materials similar in mechanical properties to those of the valve equivalents that are more amenable to experimental measurements of velocity and structural deformation using two-dimensional particle image velocimetry of high spatial resolution, three-dimensional velocimetry of volumetric measurements, and hot film anemometry of high temporal resolution. These measurements are needed to validate computational codes incorporating fluid-structure interaction and may be applied towards tissue engineered heart valve design and optimization. All of the silicone materials tested showed a neo-Hookean material response at engineering strains less than 0.5. The silicone linear elastic modulus was similar in order to the values measured in native aortic valve leaflets. The diaphragm valves with an orifice deformed to a concave shape with respect to the upstream flow for both steady and pulsatile flow conditions, along with orifice expansion at increasing flow rates. The orifice expansion (up to 75% increase in area) led to reduced pressure drops as compared with non-expanding or rigid diaphragm valves. A jet with significant inward radial velocity was present immediately downstream of the deformed diaphragm valves for both steady and pulsatile flows. This inward flow was associated with vena contracta. For low Reynolds number, laminar steady upstream flow conditions, the diaphragm valve supported the formation of relatively large scale vortices with passage frequency of St = 0.34. For pulsating flow, a leading vortex ring followed by a trailing jet was present during forward flow acceleration. Phase-averaged velocity measurements show lower fluctuations during the acceleration phase than during the deceleration phase of the flow. The deformation of the transparent bileaflet silicone valve in the pulsating flow showed leaflets deforming in similar concave state with respect to the upstream forward flow of systole and towards the lower pressures during diastole. The bileaflet silicone valve showed asymmetry in root deformation and a slot-like elliptical jet flow profile through the leaflets unlike the circular profile of the diaphragm valve. Downstream flow stagnation and recirculation were present during systole and areas of recirculation were present both upstream and downstream. These flow features were less organized for the latter during diastole. The tissue engineered valve equivalents harvested after development in the bioreactor and placed within a rigid housing were able to withstand pressures of ~50 mmHg, pressure drops of ~40 mmHg, and flow rates of ~25 L/min throughout the loading of the right ventricular cardiac cycle. The temporal pressures and flow signatures replicated right physiological conditions. The flow downstream indicated an elliptical jet during systole similar to the bileaflet silicone valve. The locations of tissue engineered valve equivalent failures were at the leaflet commissure and Dacron cuff-valve root interface.
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    Microalgal swimming in fluid environments: experimental and numerical investigations
    (2013-09) Chengala, Ahammed Anwar
    The objective of this research was to examine the effects of small-scale fluid motion on the kinetic behavior and some key physiological aspects of Dunaliella primolecta Butcher (D. primolecta / Dunaliella). D. primolecta, a fast growing microalga, is a promising organism for alternative energy production because of its capability to accumulate significant amount of "lipids", a major prerequisite for commercial production of microalgal oil-derived biofuel. For kinetic response studies of Dunaliella, flow visualization and quantification techniques such as Particle Image Velocimetry (PIV) and Digital Holographic microscopy were employed. The two-dimensional PIV results showed that Dunaliella were influenced by the fluid flow as soon as the local (or ambient) flow velocities surrounding the cells exceeded the individual (flow subtracted) swimming velocity of Dunaliella. Further inspection of the swimming characteristics of Dunaliella under shear flow in a three-dimensional holography revealed that Dunaliella preferred to swim cross-stream (i.e. also the direction of local vorticity) when the shear flow exceeded a critical value, and this resulted in Dunaliella dispersing in a thin two-dimensional horizontal layer. The cell body rotation was absent during this display in shear flow, although the cell body rotation was evident while swimming in stagnant fluid. A physical model was developed that provided a possible explanation for the cell orienting and swimming in the cross-stream direction in a shear flow while cell body remained irrotational. The experimental swimming data also showed good agreement with the computational results. In order to investigate the biochemical composition and some physiological aspects in Dunaliella under different flow conditions, a laboratory bioreactor equipped with speakers was utilized. The fluid flow velocities in the proximity of the cells generated by the speaker bioreactor are observable in natural water ecosystems. The results showed that the flow condition with the highest turbulence investigated favored the growth and lipid accumulation in Dunaliella.
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    Particle image velocimetry data characterizing flow and turbulence fields at free-flow-porous media interface
    (2020-12-01) Kang, Peter K; Kim, Junsong; pkkang@umn.edu; Kang, Peter K
    The data includes two-dimensional (2D) instantaneous velocity fields and time-averaged 2D flow properties. We obtained the velocity data in an experimental flume, which is composed of an open channel and underlying porous media, at the St. Anthony Falls Laboratory, University of Minnesota using Particle Image Velocimetry (PIV). The PIV is a non-intrusive laser optical measurement technique, which measures flow at the high spatiotemporal resolution by estimating cross-correlations between laser-illuminated subsequent images recorded by a high-speed camera. We used the PIV-measured 2D flow fields to validate the results of numerical flow simulations based on Large Eddy Simulation. The main objective of this study is to investigate pore-scale flow effects on solute transport across open channel- porous media interfaces. The released data would also be useful to researchers who need to validate flow simulation results.
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    Studies in wall turbulence using dual plane particle image velocimetry and direct numerical simulation data.
    (2010-02) Saikrishnan, Neelakantan
    Wall-bounded turbulent flows play a critical role in a variety of engineering applications. A detailed understanding of the fundamental processes underlying these flows is crucial to the accurate modeling and design of efficient and practical systems. The present studies are directed towards demonstrating the utility of experimental and numerical approaches in tandem to elucidate the structure and dynamics of wall-bounded turbulent flows over a range of Reynolds numbers. The first part of this thesis deals with the use of Dual Plane Particle Image Velocimetry (DPPIV) to understand the organization of vortex structures in the logarithmic region of turbulent boundary layer and channel flows. DPPIV provides the full velocity gradient tensor in a plane parallel to the wall and was used to calculate the projection angles of vortex structures with the three coordinate directions. In order to validate the experimental technique for identification of vortex cores, Direct Numerical Simulation (DNS) data at a comparable Reynolds number and higher resolution were used. The DNS data were averaged to the resolution of the DPPIV data and a vortex core identification routine was implemented to compare the raw DNS, averaged DNS and DPPIV data. It was observed that results from the DPPIV data match those from the raw and averaged DNS data very well. This confirms that DPPIV is a robust technique for calculating vortex core statistics, and the resolution of the measurements is sufficient to adequately resolve these structures. The second part of the thesis examines the effect of Reynolds number on the scale energy budget in wall-bounded turbulent flows. An understanding of the distribution of turbulent kinetic energy across the momentum deficit region in wall-bounded turbulent flows is critical to the complete understanding of the energy dynamics in these flows. The scale energy budget provides a tool to simultaneously assess the influence of spatial location and scales in the flow on the distribution of turbulent energy. This analysis was conducted using three DNS data sets across a range of Reynolds numbers, and results from these data were compared to results from an earlier study at a smaller Reynolds number. It is observed that the previous low Reynolds number study did not sufficiently resolve all the quantities of the energy budget in the near-wall region, primarily due of the lack of a distinct logarithmic region in the low Reynolds number simulation. Close to the wall, no effects of Reynolds number were observed on the terms of the scale energy analysis across the datasets studied. Upon moving away from the wall, the turbulent production term did not display any effects of Reynolds number, while the scale transfer term increased with increasing Reynolds number. As a result the cross-over scale, a quantity related to the shear scale in turbulent flows, increased with increasing Reynolds numbers for all datasets. The shear scale provides the scale at which the change from an isotropy-recovering range to an anisotropic region is observed, while the cross-over scale provides a transition between a transfer dominated range to a production dominated range of scales. The plot of cross-over scale versus wall-normal location revealed that the slope of the best fit lines increased with increasing Reynolds number. Further, the slope of the best fit line decreased with increasing wall-normal location. DPPIV data were also used to conduct the same analysis in turbulent boundary layers across a range of Reynolds numbers. The aim of using DPPIV data was to quantify the effects of Reynolds number on the scale energy analysis, using larger Reynolds number experimental data. The effects of resolution and Reynolds number on the DPPIV data were assessed and described in detail. The effect of resolution was most dominant on the wall-normal gradients of the streamwise and spanwise velocities, and hence the transfer of energy in physical space and the transfer of energy in scale space were overestimated in the lower resolution data. The effect of resolution was smallest on the turbulent production term, since it does not contain of any fluctuating velocity gradients. With increasing Reynolds numbers, the production term did not change significantly in the logarithmic layer, while the scale transfer term increased, resulting in a larger range of transfer-dominated scales. The value of the cross-over scale between the effective production and scale transfer term increased with increasing Reynolds number, suggesting a larger range of isotropic-type transfer dominated scales. In conclusion, it was demonstrated that DPPIV in tandem with DNS can be used to reliably assess the scale energy budget in wall-bounded turbulent flows. i