Browsing by Subject "Model Validation"

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    Air-Carbon Ablation for Hypersonic Flow Environments
    (2022-05) Prata, Krishna Sandeep
    Recent molecular beam experiments of high-velocity O, N, and O2 impacting carbon material at high temperatures produced detailed surface chemistry data relevant for carbon ablation processes. New data on O and N reactions with carbon has been published using a continuous molecular beam with lower velocity (2000 m/s) and approximately 500 times higher beam flux than previous pulsed-beam experiments. This data is interpreted to construct a new air-carbon ablation (ACA) model for use in modeling carbon heat shield ablation. The new model comprises 20 reaction mechanisms describing reactions between impinging O, N, and O2 species with carbon and producing scattered products including desorbed O and N, O2, and N2 formed by surface-catalyzed recombination, as well as CO, CO2, and CN. The new model includes surface-coverage-dependent reactions and exhibits non-Arrhenius reaction probability in agreement with experimental observations. All reaction mechanisms and rate coefficients are described in detail and each is supported by experimental evidence or theory. The model predicts pressure effects and is tested for a wide range of temperatures and pressures relevant to hypersonic flight. Model results are shown to agree well with available data and are shown to have significant differences compared to other models from the literature. A preliminary step towards validating the ACA model required simulating the plasma flows in the plasma chamber of von Karman Institute's (VKI's) Inductively Coupled Plasma (ICP) facility called Plasmatron using US3D, a 3D unstructured Navier-Stokes equations solver developed at the University of Minnesota. First, a parameter study of transport properties and the wall-catalycity of a catalytic probe used to characterize the plasma flow was conducted. It was found that the Gupta-Yos mixing rule with collision cross-section data performed better than Wilke's mixing rule with Blottner curve fits and Eucken relation to compute mixture viscosity and thermal conductivity. Also, the wall-catalycity had a strong effect on the boundary layer edge properties along the stagnation line for lower pressure flows in the Plasmatron. It was also found that the Self Consistent Effective Binary Diffusion (SCEBD) model predicted higher stagnation line enthalpy at the boundary layer edge for a flow over a non-catalytic wall when compared to the Fickian diffusion model that attributes a single diffusion coefficient to all the species in the mixture. Then, a series of iterative US3D simulations were performed to characterize the plasma flows over a fully-catalytic wall for seven air-plasma experiments in the Plasmatron. The simulations matched the experimentally measured cold wall heat flux and agreed relatively well with the boundary layer edge properties predicted by VKI's own analysis, giving confidence that the plasma freestream was well characterized in the seven experiments. Then, preliminary simulations of carbon ablation using the seven plasma flows were performed. Two ablation models called the ZA model and the MURI model gave comparable carbon mass loss rates with the experiments. However, the ZA model predicted lower surface heat flux than the MURI model due to the presence of spurious gas-surface reactions. Further experiments of carbon ablation measuring the surface heat flux are suggested in addition to mass-loss measurements. Finally, an analytical framework was developed that characterizes a flight mission or an experimental condition as reaction-limited or diffusion-limited with respect to carbon ablation. The framework uses the flight conditions such as the velocity, altitude, nose radius of the vehicle, and the surface temperature of the ablating heat shield to calculate time scales for diffusion and gas-surface chemical reactions. A new Damkohler number for ablation, defined as the ratio of diffusion time scale to the time scale for gas-surface chemical reactions was proposed. The framework was applied to several flight conditions in the existing literature. It was found that ablation of larger heat-shields like the Stardust re-entry capsule and Orion space-crew vehicle falls under a diffusion-limited regime, while the ablation of smaller objects like the nose tip of the Re-entry F vehicle falls under a reaction-limited regime. Future CFD simulations of ablation on various heat shields using the existing ablation models in the literature are recommended to establish a reference Damkohler number for the classification of the ablation regimes.
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    Experimental Testing, Subsystem Model Validation, and Design of a Variable Displacement Hydraulic Motor
    (2022-05) Larson, Jacob
    The design, testing and model validation of a novel variable displacement hydraulic motor is presented in this Thesis. This novel, radial piston, low-speed high-torque motor achieves variable displacement using a 7-bar 2-degree of freedom mechanism that infinitely varies the piston stroke. This motor is called the Variable Displacement Linkage Motor (VDLM). One potential application of the VDLM is in the propulsion drive of off highway equipment such as a compact track loader. Efficiency is improved at a component level by designing a motor that is inherently efficient and at the system level by adding the additional degree of freedom of variable displacement. Varying motor displacement allows reduced flow demand in the hydrostatic loop at high motor rotational speeds while maintaining high torque output at low rotational speeds. This novel architecture is beneficial because currently available solutions for traction drives either do not feature continuous displacement variation or require a gearbox. Nearly all off-highway equipment is currently fueled with diesel or gasoline engines. With the push to decarbonize these energy sources, fluid power systems that can more seamlessly integrate into hybrid electric or fully electric powertrains are in demand. The VDLM technology has significant potential to fill this market gap. A series of models were created to thoroughly predict the performance of the VDLM. A Multi-Objective Genetic Algorithm (MOGA) was utilized to create a Pareto front of potential mechanism geometries and a comprehensive Matlab code analyzed each solution for validity. From the Pareto front, a prototype was selected from which to build a single cylinder prototype. Displacement of this prototype was 42cc/rev and featured 6 discrete displacement settings from 50% to 100% fractional displacement. The purpose of this single cylinder prototype was to collect experimental data to validate the models. Modeling is a closed-loop process, and experimental data was required to validate the models. In the case of the single cylinder VDLM, mechanical losses were the primary focus. Losses were modeled and grouped into five primary sources: main taper roller bearings, case windage, shaft seal, valve actuation, and linkage. A staged-assembly method was developed that allowed individual loss components to be isolated and each model was validated independently. Two test stands were designed and assembled for the purpose of collecting the energy loss data. Instrumentation, such as pressure sensors, a torque transducer, and a piston position sensor, collected data at a sample rate of 10kHz and was recorded on a DAQ system for future analysis. Low magnitude torque tests were conducted on the test stand at the University of Minnesota and high magnitude torque tests were conducted on the Milwaukee School of Engineering test stand. Model predictions and experimental results were compared side-to-side. Correlation was improved by altering physics-based model parameters such as the coefficient of friction, viscosity, and radial clearance in the valve. The final validated model prediction achieved correlation to the experimental data to within 10% error when averaged through an entire piston cycle. Data analysis of the pressure dynamics inside the cylinder was conducted to qualify the valve design. Design revisions were required during the testing phase to make the single-cylinder prototype operational. The valve return spring force was increased, a linkage over-travel stop was designed and installed, and the valve timing was corrected. A failure of the VDLM occurred at 19.3 MPa pressure differential testing. A thorough teardown of the VDLM was conducted following the failure to determine the root cause. The valve was seized in its bore which was attributed to deflection or debris in the valve bore. Valve timing was interrupted due to the valve sticking and a hydro lock condition occurred. This pressure spike exceeded the clamping capacity of threaded fasteners in the cylinder block and caused an O-ring to fail. The failed O-ring allowed high pressure fluid to enter the VDLM housing which overwhelmed the case drain port and caused the main shaft seal to blow. Linkage components also failed due to the over pressurization. Wear throughout the motor was analyzed during the teardown. From the testing and tear down, design guidelines for future VDLM’s were drawn. Utilizing the validated models from generation one, a second generation VDLM was designed. The goals for the generation two VDLM were to be smaller in overall diameter, include live, continuous displacement adjustment control, and feature a full array of pistons and cylinders. The purpose of this motor was to conduct hardware in the loop testing. Constraints such as constant top dead center through all displacements and the requirement for the roller follower and rocker pivot axis of rotation to be coincident were relaxed to achieve smaller motor diameters. A Pareto front of potential motor geometries was developed, and a 5-piston 7-lobe individual was selected. This motor featured continuously adjustable displacement from 50% to 100% fractional displacement and displaced 300 cc/rev. The detailed design of the mechanism is described in this thesis from an early conceptual phase to a final phase where part prints are developed. The preliminary VDLM solution was reviewed for validity; layouts were determined, bearing sizes were validated and a CAD model was developed. Fits and tolerances for the linkage were determined and materials and surface finishes selected. Linkage design was validated using Ansys Finite Element Analysis (FEA). Because the VDLM is a complex assembly, geometric simplifications were made to create a more basic CAD model of the linkage for the purpose of the FEA study. Simplifications included that a single cylinder block and linkage assembly was created, a symmetric boundary condition was applied, and the position where the linkage experienced the highest internal loads was determined. 35 MPa of fluid pressure was applied to the piston and an FEA solution was determined. Two metrics were used to determine design validity, deflection, and stress. Finally, detailed 2D part drawings were developed, and custom manufactured parts were ordered and assembled. Part assembly and fit-up was critiqued to close the solution development to implementation loop.