Browsing by Subject "Boundary Layer Stability"

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    High enthalpy Effects on two boundary layer disturbances in supersonic and hypersonic flow.
    (2012-05) Wagnild, Ross Martin
    The fluid flow phenomenon of boundary layer transition is a complicated and difficult process to model and predict. The importance of the state of the boundary layer with regard to vehicle design cannot be understated. The high enthalpy environment in which high speed vehicles operate in further complicates the transition process by adding several more degrees of freedom. In this environment, the internal properties of the gas can stabilize or destabilize the boundary layer as well as modify the disturbances that cause transition. In the current work, the interaction of two types of disturbances with the high enthalpy flow environment are analyzed. The first is known as a second mode disturbance, which is acoustic in nature. The second type is known as a transient growth disturbance and is associated with flows behind roughness elements. Theoretical analyses, linear stability analyses, and computation fluid dynamics (CFD) are used to determine the ways in which these disturbances interact with the high enthalpy environment as well as the consequences of these interactions. First, acoustic wave are directly studied in order to gain a basic understanding of the response of second mode disturbances in the high enthalpy boundary layer. Next, this understanding is used in interpreting the results of several computations attempting to simulate the flow through a high enthalpy flow facility as well as experiments attempting to take advantage of the acoustic interaction with the high enthalpy environment. Because of the difficulty in modeling these experiments, direct simulations of acoustic waves in a hypersonic flow of a gas with molecular vibration are performed. Lastly, compressible transient growth disturbances are simulated using a linear optimal disturbance solver as well as a CFD solver. The effect of an internal molecular process on this type of disturbance is tested through the use of a vibrational mode. It is the goal of the current work to reinforce the critical importance of accurately capturing the physics of the "real" gas effects in the high enthalpy flow environment in order to understand and predict transition on high speed vehicles.
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    Investigation of Boundary Layer Stability Using the Parabolic Stability Equations on a Coupled Simulation of the Reentry F Flight Experiment
    (2022-07) Rogers, Robert
    Laminar-to-turbulent transition prediction in hypersonic boundary layers is complicatedby the many physical effects present in high-enthalpy flows. For different flight conditions, certain flow phenomena can either stabilize or destabilize a boundary layer, subsequently moving a transition location upstream or downstream. This can have serious effects on surface heating, skin friction drag, and vehicle dynamics. The uncertainty associated with boundary layer prediction is a major limitation in the design of hypersonic flight systems. Previous work has established how thermochemical non-equilibrium, spherical nose tip blunting, surface heating, and Reynolds number variation, among other things, can impact the disturbance environment over canonical geometries like flat plates, wedges, and cones in supersonic and hypersonic flows. Yet, experiments involving all those physics simultaneously are not possible except in flight. Computational studies are a way to couple many flow physics to investigate the effect on boundary layer stability. The objective of this thesis is to couple many of the flight physics and generate laminar base flows to then examine how boundary layer stability is affected on a blunt cone during atmospheric reentry. The coupled physics include ablation chemistry, a realistic wall temperature, altitude variation, and non-spherical nose tip blunting. A new coupled solid-fluid and gas-surface interaction solver called Ares is used to generate base flows for the geometry and flight conditions of Reentry F. The flow physics under investigation are added sequentially to isolate their impact on boundary layer stability. The final simulation includes the effect of shape change, a flight trajectory, a realistic wall temperature profile, and ablation chemistry. Once base flows are generated, the solution is imported into PSE-Chem, a parabolic stability equation (PSE) solver within STABL-2D. PSE analysis then models the most unstable frequencies in the boundary layer and N factor curves are generated to predict how disturbances vary with the additional flow physics and how the transition location is affected. With Ares, the resulting surface temperature profiles match well with Reentry F flight data where it was collected. However, over the graphite nose tip region upstream of 22 cm, no flight temperature data was measured. A study with conjugate heat transfer and gas-surface interactions are presented for the temperature predictions in this range showing that the inclusion of gas-surface chemistry dramatically increases heating and the predicted wall temperature. The boundary layer chemistry also shows CO as the dominant carbonaceous species throughout the boundary layer. The concentrations of other species through the boundary layer are also examined. Carbon species mass loss rates are then used to compute a surface recession rate at the stagnation point. Then, a sensitivity study investigates the effects that non-spherical nose tip blunting has on boundary layer disturbance growth. The stability results show that a realistic wall temperature profile stabilized boundary layer disturbances relative to a cold isothermal wall condition, while increasing Reynolds number destabilizes the boundary layer. In the case of ablation chemistry, the results are mixed with destabilization upstream in some cases, but the overall impact on the predicted transition location is unaffected. Lastly, the non-spherical nose tip blunting has a strong destabilizing effect upstream of the predicted transition location, while changes to N factors around the predicted transition location are negligible.