Browsing by Subject "shear stress"
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Item Determining an Appropriate Method to Simulate Pump Shear on the Diatom Nitzschia sp. and a Methodology to Quantify the Effects(2012-12-13) Lassig, JarrettWhen cultivated properly in bioreactors, microalgae have been found to produce vast amounts of biomass. In the case of diatom cultivation where the organisms will fall out of suspension quite easily, paddle wheels or pumps are the primary means to maintain the necessary velocity in the raceway. This study will focus on the potentially harmful shear stress these devices may impart onto the organisms. The system used to impart shear stress to a diatom culture was a cone and plate viscometer. Cells were counted using a fluorescein diacetate staining method with a fluorescent and brightfield microscope. Under the white light all cells were visible while only the healthy cells were visible under fluorescent light. The sample was exposed to shear stress with the cone and plate viscometer at 6 Pascals for 10 minutes and compared against a non-sheared sample. For each sample, 5 pairs of white and fluorescent light images were captured, counted, and averaged. A non-sheared sample was paired with a sheared sample to calculate the decrease in cell viability. The slope was calculated from the plot of shear stress and cell viability for 9 strains. In each case shear stress resulted in a significant decrease in cell viability; however, there was no statistical difference between strains. While effective, this method would be impractical for a commercial algae cultivation facility as the viscometer in this study costs approximately $100,000. Therefore, tests were performed to determine if a rotary mixer could be substituted for the viscometer. The hypothesis was that the cell damage was a product of shear stress and exposure time. For the viscometer test, the shear exposure was 3600 Pa s. Two rotational mixer tests were performed, one at 1250 RPM for 7 hours and one at 313 RPM for 28 hours, providing the same 3600 Pa s shear exposure. After staining, cell viability decreased 35.62% and 11.07% in the 1250 RPM and 313 RPM test, respectively. This difference was significant compared to the 6.04% decrease in the viscometer test. The increased cell damage was attributed to turbulence in the mixer tests and the basis for further study.Item Lymphatic Fluid Mechanics: An In Situ and Computational Analysis of Lymph Flow(2012-10-19) Rahbar, ElahehThe lymphatic system is an extensive vascular network responsible for the transport of fluid, immune cells, proteins and lipids. It is composed of thin-walled vessels, valves, nodes and ducts, which work together to collect fluid, approximately 4 L/day, from the interstitium transporting it back to the systemic network via the great veins. The failure to transport lymph fluid results in a number of disorders and diseases. Lymphedema, for example, is a pathology characterized by the retention of fluid in limbs creating extreme discomfort, reduced mobility and impaired immunity. In general, there are two types of edema: primary edema, being those cases that are inherited (i.e. genetic predisposition), and secondary edema, which develop post-trauma or injury of the lymphatic vessels. With the onset of breast cancer and radiation therapies, the prevalence of secondary edema is on the rise. Clinical studies have shown that up to 80% of women who undergo nodal-dissection surgery develop lymphedema in their arms within 3-5 years of the surgery. Unfortunately, there is no cure or remedy for lymphedema stemming from our lack of understanding of the lymphatic system. The goal of this study was to evaluate lymph flow both experimentally and analytically to better understand the mechanisms regulating lymph transport. In particular we investigated the effects of pressure, volume loads and valve resistance on lymphatic function in the rat mesentery. Our experimental results were then used to develop computational and constitutive models to emulate the dynamic behavior of lymph transport. Collectively, the data illustrate the mechanics of lymphatic contractility and lymph flow. In particular, lymph flow and pumping significantly increased post edemagenic stress in the rat model. Furthermore, lymphangions exhibited highly nonlinear pressure-diameter responses at low pressures between 3-5 cmH2O. These experimental results strongly suggest the regulation of lymph flow via changes in pressure, shear stress and vessel diameter. Furthermore, the computational and constitutive models from this study provide great insight into lymphatic function characterizing the mechanical properties of a single pumping unit (i.e. lymphangion). These models will serve as valuable tools to further lymphatic research.Item Wall-pressure and PIV analysis for microbubble drag reduction investigation(Texas A&M University, 2005-11-01) Dominguez Ontiveros, Elvis EfrenThe effects of microbubbles injection in the boundary layer of a turbulent channel flow are investigated. Electrolysis demonstrated to be an effective method to produce microbubbles with an average diameter of 30 ??m and allowed the placement of microbubbles at desired locations within the boundary layer. Measurement of velocity fluctuations and the instantaneous wall shear stress were carried out in a channel flow facility. The wall shear stress is an important parameter that can help with the characterization of the boundary layer. This parameter can be obtained indirectly by the measurement of the flow pressure at the wall. The wall shear stress in the channel was measured by means of three different independent methods: measurement of the pressure gradient by a differential pressure transducer, Particle Image Velocimetry (PIV), and an optical wall shear stress sensor. The three methods showed reasonable agreement of the wall shear stress values for single-phase flow. However, differences as skin friction reductions were observed when the microbubbles were injected. Several measurements of wall-pressure were taken at various Reynolds numbers that ranged from 300 up to 6154. No significant drag reduction was observed for flows in the laminar range; however, a drag reduction of about 16% was detected for turbulent Reynolds numbers. The wall-pressure measurements were shown to be a powerful tool for the measurement of drag reduction, which could help with the design of systems capable of controlling the skin friction based on feedback given by the wall-pressure signal. The proposed measurement system designed in this work has capabilities for application in such diverse fields as multiphase flows, drag reduction, stratified flows, heat transfer among others. The synchronization between independent systems and apparatus has the potential to bring insight about the complicated phenomena involved in the nature of fluid flows.