Aerospace Engineering Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/2737

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    DIRECT AND LARGE-EDDY SIMULATION AND ANALYSIS OF SHOCK-SEPARATED FLOWS
    (2021) Helm, Clara Marie; Martin, Pino; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The in-house CRoCCo code is used to generate a database of high-fidelity direct numerical simulation (DNS) and large eddy simulation (LES) data of shock wave and turbulent boundary layer interactions (STBLI) at supersonic to hypersonic conditions. The DNS data is employed in the validation of the LES method and the assessment of the sub-grid-scale (SGS) models in application to the STBLI flow problem. It is determined that, under hypersonic conditions, a scale similar model term in both the shear stress and heat transfers SGS terms is necessary to produce the correct STBLI separation flow. The use of the dynamic eddy viscosity term alone produced as much as 30% error in separation length. The high grid-resolving efficiency (equivalently the practicality over the DNS) of the CRoCCo code LES method for the simulation of STBLI flows is also demonstrated with a typical reduction of 95% grid size and 67% in number of time steps as compared to the DNS, a feature that makes spectral convergence of the STBLI low-frequency cycle feasible. The thorough documentation of DNS-validated, high-fidelity LES solutions of hypersonic STBLI flows is a unique contribution of this work. Thanks to the detail in the turbulence data afforded by the LES, an extensive and novel characterization of the separation shear layer in the STBLI flows is possible and the results are related to compressible mixing layer theory. In addition, visualizations of the numerical data show the form of the inviscid instability in hypersonic shock-separated flows. These visualizations combined with the extended CRoCCo Lab numerical database provide significant insight into the nature of the separation length scaling in STBLI at hypersonic Mach numbers.
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    BARELY IMPLICIT CORRECTION ALGORITHM FOR LOW-MACH-NUMBER FLOWS AND ITS APPLICATION TO VORTEX BREAKDOWN UNDERGOING HEAT ADDITION AND EXTRACTION
    (2020) Zhang, Xiao; Oran, Elaine S; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This thesis presents a new Barely Implicit Correction (BIC) algorithm combined with a modified flux-corrected transport (FCT) algorithm for the simulation of three-dimensional (3D), low-Mach-number flows and then proceeds to apply it to the study of vortex breakdown undergoing heat addition and heat extraction. This new algorithm is based on the original, introduced by a prior work in 1987, which was a solution procedure including an explicit predictor step to solve the convective portion of the Navier-Stokes equations and an implicit corrector step to remove the acoustic limit on the integration time-step. The explicit predictor uses the flux-corrected transport (FCT) algorithm while the implicit corrector solves an elliptic equation for a pressure correction to equilibrate acoustic waves. This thesis introduces a procedure for stabilizing and implementing FCT for 3D flows and extends BIC for 3D with physical diffusion processes. A new filter is introduced to further stabilize the algorithm and the solution procedure is clarified for the inclusion of the diffusion fluxes. The new BIC-FCT algorithm is examined in four test problems with successively increased difficulty. The test problems culminate with calculations of vortex breakdown in 3D swirling flows. All the test problems demonstrate that the algorithm is able to predict accurate and robust solutions using time steps varying from near the explicit stability limit to tens and hundreds of times larger. Excellent agreement is also obtained when compared with results from other algorithms. The algorithm is then used to study how vortex breakdown is affected when heat is extracted from or added to the flow. Two heat release rates are applied to a flow with a bubble mode of breakdown upstream and double-helix mode downstream. The simulations show that heat release causes the double-helix structure to become narrower. With more heat release, the double-helix mode transitions to a columnar vortex. In addition, a lower heat extraction rate causes the columnar vortex to first transition to a spiral mode and then to a double-helix mode. With a higher heat extraction rate, the columnar vortex transitions to a double-helix mode, bypassing the spiral mode. Further investigation show that the density gradient formed by heat addition and extraction is the dominant effect in the transitions. The transition is promoted by changes in viscosity due to temperature changes from heat addition and extraction. The new algorithm presented in this thesis provides a new way to calculate low-Mach-number flows. Such vortex breakdown simulations with heat changes serve as a base for understanding the dynamics of a precessing vortex core in swirl combustors and other vortex flows with changes in heat input.
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    NUMERICAL SIMULATION OF THE BLUE WHIRL: A REACTING VORTEX BREAKDOWN PHENOMENON
    (2019) Chung, Joseph Dong il; Oran, Elaine S; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The blue whirl is a small, stable, spinning blue flame that evolved spontaneously in recent laboratory experiments while studying turbulent, sooty fire whirls. It burns a range of different liquid hydrocarbon fuels cleanly with no soot production, presenting a new potential way for low-emission combustion. This thesis uses numerical simulations to present, for the first time, the flame and flow structure of the blue whirl. These simulations show that the blue whirl is composed of three different flames - a diffusion flame and a premixed rich and lean flame - all of which meet in a fourth structure, a triple flame which appears as a whirling blue ring. The results also show that the flow structure emerges as the result of vortex breakdown, a fluid instability which occurs in swirling flows. This thesis also presents the development and testing of the numerical algorithms used in the simulation of the blue whirl. This work is a critical step forward in understanding how to use this new form of clean combustion.
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    https://aero.umd.edu/graduate/graduate-student-forms
    (2018) Kumar, Rubbel; Oran, Elaine S; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The increased use of explosives in military conflicts has been linked to an increase in the number of traumatic brain injuries (TBIs). Assessing the effectiveness of personal protective equipment to mitigate TBIs requires both the ability to replicate the pressure signatures caused by blast waves and an understanding of the interaction between blast waves and human bodies. Computational Fluid Dynamics (CFD) was used to understand the effect of varying different shock tube design parameters and to propose guidelines for selecting shock tube designs to accurately replicate blast wave pressure signatures representative of free-field explosive events. Additionally, a CFD model was developed to represent a shock tube built to mimic the primary overpressure magnitude and impulse loading on the human head surface as a result of free-field explosive events. This model was used to aid in the understanding of flow within the shock tube, characterize the applied pressure loading to a bare head form, augment experimental findings to fully understand the influence of headborne systems on pressure applied to the human head, and support the design of optimized laboratory test methodologies to represent a broad range of free-field blast events.
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    Application of Uncertainty Quantication of Turbulence Intensity on Airfoil Aerodynamics
    (2017) Salahudeen, Atif; Baeder, James; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Traditional CFD results have a number of freestream inputs. In the physical world, these input conditions often have some uncertainty associated with them. However, this uncertainty is often omitted from the CFD results. The effects of uncertainty in CFD can be determined through application of Uncertainty Quantification (UQ). The primary objective of the present work is to determine the effect of uncertainty in freestream turbulence intensity (FSTI) on the coefficients of lift, drag, and moment for four different airfoils: S809, NACA 0012, SC1095, and RC(4)-10. In this work, the Monte Carlo method is used to calculate the sensitivities of the aerodynamic coefficients to Gaussian distributions of uncertainty in FSTI over a range of angles of attack (AOA) at various Reynolds numbers and Mach numbers. However, the Monte Carlo method would require hundreds of thousands of CFD calculations in order to converge to the correct results. A surrogate surface is therefore generated using a parametric study using the in-house flow solver OVERTURNS. Rather than run a separate CFD run for each Monte Carlo run, all of the results can be attained virtually instantaneously via the surrogate surface. The UQ analysis shows how varying these parameters affects the sensitivies of the aerodynamic coefficients to uncertainty in FSTI. In most cases, the response is nearly Gaussian and the mean response is not too dierent from the discrete FSTI response without uncertainty. However, the output standard deviation for drag and pitching moment can become large when the transition location changes rapidly with changing FSTI.