Browsing by Subject "computational fluid dynamics"
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Item Computational Evaluation of a Transonic Laminar-Flow Wing Glove Design(2012-07-16) Roberts, Matthew WilliamThe aerodynamic benefits of laminar flow have long made it a sought-after attribute in aircraft design. By laminarizing portions of an aircraft, such as the wing or empennage, significant reductions in drag could be achieved, reducing fuel burn rate and increasing range. In addition to environmental benefits, the economic implications of improved fuel efficiency could be substantial due to the upward trend of fuel prices. This is especially true for the commercial aviation industry, where fuel usage is high and fuel expense as a percent of total operating cost is high. Transition from laminar to turbulent flow can be caused by several different transition mechanisms, but the crossflow instability present in swept-wing boundary layers remains the primary obstacle to overcome. One promising technique that could be used to control the crossflow instability is the use of spanwise-periodic discrete roughness elements (DREs). The Flight Research Laboratory (FRL) at Texas A&M University has already shown that an array of DREs can successfully delay transition beyond its natural location in flight at chord Reynolds numbers of 8.0x10^6. The next step is to apply DRE technology at Reynolds numbers between 20x10^6 and 30x10^6, characteristic of transport aircraft. NASA's Environmentally Responsible Aviation Project has sponsored a transonic laminar-flow wing glove experiment further exploring the capabilities of DRE technology. The experiment will be carried out jointly by FRL, the NASA Langley Research Center, and the NASA Dryden Flight Research Center. Upon completion of a wing glove design, a thorough computational evaluation was necessary to determine if the design can meet the experimental requirements. First, representative CAD models of the testbed aircraft and wing glove were created. Next, a computational grid was generated employing these CAD models. Following this step, full-aircraft CFD flowfield calculations were completed at a variety of flight conditions. Finally, these flowfield data were used to perform boundary-layer stability calculations for the wing glove. Based on the results generated by flowfield and stability calculations, conclusions and recommendations regarding design effectiveness were made, providing guidance for the experiment as it moved beyond the design phase.Item Computational fluid dynamics (CFD) simulations of aerosol in a u-shaped steam generator tube(2009-05-15) Longmire, PamelaTo quantify primary side aerosol retention, an Eulerian/Lagrangian approach was used to investigate aerosol transport in a compressible, turbulent, adiabatic, internal, wall-bounded flow. The ARTIST experimental project (Phase I) served as the physical model replicated for numerical simulation. Realizable k-? and standard k-? turbulence models were selected from the computational fluid dynamics (CFD) code, FLUENT, to provide the Eulerian description of the gaseous phase. Flow field simulation results exhibited: a) onset of weak secondary flow accelerated at bend entrance towards the inner wall; b) flow separation zone development on the convex wall that persisted from the point of onset; c) centrifugal force concentrated high velocity flow in the direction of the concave wall; d) formation of vortices throughout the flow domain resulted from rotational (Dean-type) flow; e) weakened secondary flow assisted the formation of twin vortices in the outflow cross section; and f) perturbations induced by the bend influenced flow recovery several pipe diameters upstream of the bend. These observations were consistent with those of previous investigators. The Lagrangian discrete random walk model, with and without turbulent dispersion, simulated the dispersed phase behavior, incorrectly. Accurate deposition predictions in wall-bounded flow require modification of the Eddy Impaction Model (EIM). Thus, to circumvent shortcomings of the EIM, the Lagrangian time scale was changed to a wall function and the root-mean-square (RMS) fluctuating velocities were modified to account for the strong anisotropic nature of flow in the immediate vicinity of the wall (boundary layer). Subsequent computed trajectories suggest a precision that ranges from 0.1% to 0.7%, statistical sampling error. The aerodynamic mass median diameter (AMMD) at the inlet (5.5 ?m) was consistent with the ARTIST experimental findings. The geometric standard deviation (GSD) varied depending on the scenario evaluated but ranged from 1.61 to 3.2. At the outlet, the computed AMMD (1.9 ?m) had GSD between 1.12 and 2.76. Decontamination factors (DF), computed based on deposition from trajectory calculations, were just over 3.5 for the bend and 4.4 at the outlet. Computed DFs were consistent with expert elicitation cited in NUREG-1150 for aerosol retention in steam generators.Item Direct Numerical Simulation of the Flow in a Pebble Bed(2014-06-24) Ward, PaulThe flow in a tightly packed array of spheres is important to various engineering fields. In nuclear engineering applications, for instance, researchers have proposed core geometries of the pebble bed reactor (PBR) type cooled by gas or molten salt. Proper core cooling, both at operation and during accident conditions, is a key issue that must be addressed in any reactor design; and the limited amount of data available for the complicated geometry of PBR cores makes this task even more complex. A detailed understanding of coolant flow patterns and properties must be developed in order to meet safety requirements and ensure core longevity. We addressed this issue by using the spectral-element computational fluid dynamics code Nek5000, developed at Argonne National Laboratory, to conduct both large eddy simulation (LES) and direct numerical simulation (DNS) of fluid flow through a single face-centered cubic sphere lattice with periodic boundary conditions. Multiple LES were conducted with varying Reynolds numbers in an effort to determine how the Reynolds number affects the development of asymmetries within the flow patterns. The DNS focused on the development of turbulence and were used to compute the turbulent kinetic energy budgets. A set of statistical analyses were also conducted to support the validity of the results.Item Practical Aspects of the Implementation of Reduced-Order Models Based on Proper Orthogonal Decomposition(2012-07-16) Brenner, Thomas AndrewThis work presents a number of the practical aspects of developing reduced- order models (ROMs) based on proper orthogonal decomposition (POD). ROMS are derived and implemented for multiphase ?ow, quasi-2D nozzle ?ow and 2D inviscid channel ?ow. Results are presented verifying the ROMs against existing full-order models (FOM). POD is a method for separating snapshots of a ?ow ?eld that varies in both time and space into spatial basis functions and time coe?cients. The partial di?erential equations that govern ?uid ?ow can then be pro jected onto these basis functions, generating a system of ordinary di?erential equations where the unknowns are the time coe?cients. This results in the reduction of the number of equations to be solved from hundreds of thousands or more to hundreds or less. A ROM is implemented for three-dimensional and non-isothermal multiphase ?ows. The derivation of the ROM is presented. Results are compared against the FOM and show that the ROM agrees with the FOM. While implementing the ROM for multiphase ?ow, moving discontinuities were found to be a ma jor challenge when they appeared in the void fraction around gas bubbles. A point-mode POD approach is proposed and shown to have promise. A simple test case for moving discontinuities, the ?rst order wave equation, is used to test an augmentation method for capturing the discontinuity exactly. This approach is shown to remove the unphysical oscillations that appear around the discontinuityin traditional approaches. A ROM for quasi-2D inviscid nozzle ?ow is constructed and the results are com- pared to a FOM. This ROM is used to test two approaches, POD-Analytical and POD-Discretized. The stability of each approach is assessed and the results are used in the implementation of a ROM for the Navier-Stokes equations. A ROM for a Navier-Stokes solver is derived and implemented using the results of the nozzle ?ow case. Results are compared to the FOM for channel ?ow with a bump. The computational speed-up of the ROM is discussed. Two studies are presented with practical aspects of the implementation of POD- based ROMs. The ?rst shows the e?ect of the snapshot sampling on the accuracy of the POD basis functions. The second shows that for multiphase ?ow, the cross- coupling between ?eld variables should not be included when computing the POD basis functions.Item The incorporation of bubbles into a computer graphics fluid simulation(Texas A&M University, 2005-08-29) Greenwood, Shannon ThomasWe present methods for incorporating bubbles into a photorealistc fluid simulation. Previous methods of fluid simulation in computer graphics do not include bubbles. Our system automatically creates bubbles, which are simulated on top of the fluid simulation. These bubbles are approximated by spheres and are rendered with the fluid to appear as one continuous surface. This enhances the overall realism of the appearance of a splashing fluid for computer graphics. Our methods leverage the particle level set representation of the fluid surface. We create bubbles from escaped marker particles from the outside to the inside. These marker particles might represent air that has been trapped within the fluid surface. Further, we detect when air is trapped in the fluid and create bubbles within this space. This gives the impression that the air pocket has become bubbles and is an inexpensive way to simulate the air trapped in air pockets. The results of the simulation are rendered with a raytracer that includes caustics. This allows the creation of photorealistic images. These images support our position that the simple addition of bubbles included in a fluid simulation creates results that are much more true to life.Item View dependent fluid dynamics(Texas A&M University, 2006-08-16) Barran, Brian ArthurThis thesis presents a method for simulating fluids on a view dependent grid structure to exploit level-of-detail with distance to the viewer. Current computer graphics techniques, such as the Stable Fluid and Particle Level Set methods, are modified to support a nonuniform simulation grid. In addition, infinite fluid boundary conditions are introduced that allow fluid to flow freely into or out of the simulation domain to achieve the effect of large, boundary free bodies of fluid. Finally, a physically based rendering method known as photon mapping is used in conjunction with ray tracing to generate realistic images of water with caustics. These methods were implemented as a C++ application framework capable of simulating and rendering fluid in a variety of user-defined coordinate systems.