Doctor of Philosophy (PhD)
As flow becomes rarefied, a quantity called as the tangential momentum accommodation coefficient (TMAC) becomes important because it is a measure of the momentum transport from a gas molecule to a surface. Very few experimental measurements of continuum breakdown in boundary layer flows exist. All experimental measurements of the TMAC in macro-scale boundary layer flows have been done in the continuum slip and the transition flow regimes. Moreover the experimental apparatus used by previous researchers cannot accommodate for materials that are planar by nature such as those used in the field of aerospace and microfabrication. The objectives of this research include the experimental measurement of continuum breakdown in a macro-scale boundary layer flow, development of a test facility such that TMAC may be measured in the free molecular flow regime, and the measurement of TMAC for various gases versus material interactions. An experimental facility is built which consists of a disc spin down experiment in various gas pressures from atmospheric pressure through the free molecular flow regime. The real time deceleration torque is measured during the disc spin-down in each ambient pressure. The deceleration torque is non-dimensionalized suitably and is plotted against Reynolds number. The non-dimensional curves show self-similarity and therefore continuity in the viscous flow regime. Self-similarity breaks down when the viscous forces are no longer dominant and therefore it is a measure of continuum breakdown. This is also confirmed through the departure of the CFD and the semi-analytic von Karman curves from the experimental curves. A differential scavenging system is designed and incorporated into the apparatus and it facilitates measurements in the free molecular flow regime. TMAC for interactions between several gases and certain aerospace materials are measured in the free molecular flow regime. While most measurements show TMAC values of 0.7 or above in the different gases, the values are consistently low in carbon dioxide. The results are of significant impact for future space missions to Mars because the Martian atmosphere contains carbon dioxide predominantly and a lower TMAC suggests lower atmospheric drag on space vehicles.
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Acharya, Tathagata, "Measurement of Continuum Breakdown Using a Disc Spin-Down Experiment in Low Pressure Air" (2013). LSU Doctoral Dissertations. 1448.