Browsing by Subject "Direct numerical simulation"

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    Euler-Lagrangian simulations of turbulent bubbly flow.
    (2011-03) Mattson, Michael David
    A novel one-way coupled Euler-Lagrangian approach, including bubble-bubble collisions, coalescence and variable bubble radius, was developed in the context of simulating large numbers of cavitating bubbles in complex geometries using direct numerical simulation (DNS) and large-eddy simulation (LES). This dissertation i) describes the development of the Euler-Lagrangian approach, ii) outlines the novel bubble coalescence model derived for this approach and iii) describes simulations performed of bubble migration in a turbulent boundary layer, bubble coalescence in a turbulent pipe ow and cavitation inception in turbulent flow over a cavity. The coalescence model uses a hard-sphere collision model is used and determines coalescence stochastically. The probability of coalescence is computed from a ratio of coalescence timescales, which are dynamically determined from the simulation. Coalescence in a bubbly, turbulent pipe ow (Re#28; = 1920) in microgravity was simulated with conditions similar to experiments by Colin et al. [1] and excellent agreement of bubble size distribution was obtained. With increasing downstream distance, the number density of bubbles decreases due to coalescence and the average probability of coalescence decreases due to an increase in overall bubble size. The Euler-Lagrangian approach was used to simulate bubble migration in a turbulent boundary layer (420 < Re#18; < 1800). Simulation parameters were chosen to match Sanders et al. [2], although the Reynolds number of the simulation is lower than the experiment. The simulations show that bubbles disperse away from the wall as observed experimentally. Mean bubble diffusion and profiles of bubble concentration are found to be similar to the passive scalar results, except very near the wall. The carrier-fluid acceleration was found to be the reason for moving the bubbles away from the wall. The one-way coupled Euler-Lagrangian approach was applied to simulate the experiment of cavitating turbulent ow over a cavity by Liu and Katz [3]. The classical Rayleigh-Plesset equation is integrated using adaptive time-stepping to accurately and efficiently solve for the change of the bubble radius over time. The one-way coupled Euler-Lagrangian model predicts cavitation inception at the trailing edge of the cavity and also in the vortices shed from the leading edge, in qualitative agreement with experiment.
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    High-fidelity unstructured overset simulation of complex turbulent flows
    (2023-05) Morse, Nicholas
    The goal of this dissertation is to provide insight into the underlying physics of two sets of complex flows: (i) the axisymmetric and appended DARPA SUBOFF and (ii) tabbed jets in crossflow. The accurate simulation of the flow around marine vessels such as the DARPA SUBOFF is critical for maneuvering predictions, which are inherently challenging due to the characteristically large Reynolds numbers, the complex geometries of the hull, appendages, and propeller, and the unsteady flow-fields, which consist of turbulent boundary layers with pressure gradients, curvature, junction flows, and separations. The understanding of jets in crossflow (JICFs) is important for a variety of applications, and there has been significant interest in designing passive devices to control the mixing and penetration characteristics of the jet, although the specific effects of these devices are not well understood. The unstructured overset method of Horne and Mahesh [1,2] provides the flexibility to perform large-eddy simulations (LES) and direct numerical simulations (DNS) to extract valuable physical insights from these flows.First, wall-resolved LES is performed to study flow about the axisymmetric DARPA SUBOFF hull at a Reynolds number of 1.1×10^6 based on the hull length and free stream velocity. To gain an understanding of the streamline curvature and pressure gradient effects of the hull’s turbulent boundary layer (TBL), the axisymmetric Reynolds-averaged Navier-Stokes equations are derived in an orthogonal coordinate system aligned with streamlines, streamline-normal lines, and the plane of symmetry. Analysis in this frame of reference provides a new perspective on curved TBLs, and has numerous practical benefits, including the orthogonality of the streamline-normal coordinate to the hull surface and to the free stream velocity far from the body, which is critical for studying bodies with concave streamwise curvature. In the potential flow outside the boundary layer, the momentum equations in the streamline coordinate frame naturally reduce to the differential form of Bernoulli’s equation and the s-n Euler equation for curved streamlines. In the curved laminar boundary layer at the front of the hull, the streamline momentum equation represents a balance of the streamwise advection, streamwise pressure gradient, and viscous stress, while the streamline-normal equation is a balance between the streamline-normal pressure gradient and centripetal acceleration. At the mid-hull TBL, the curvature terms and streamwise pressure gradient are negligible, and the results conform to traditional analysis of flat plate boundary layers. Finally, the thick stern TBL causes the curvature and streamwise pressure gradient terms to reappear to balance the turbulent and viscous stresses. This balance is used to explain the characteristic variation of static pressure observed for thick boundary layers at the tails of axisymmetric bodies. Next, trip-resolved LES of the DARPA SUBOFF is performed to investigate the extent to which the details of tripping affect the development of TBLs in model-scale studies, which are limited to moderate Reynolds number TBLs. In particular, four cases are studied at length-based Reynolds numbers of 1.1×10^6 and 1.2×10^6: the bare hull and appended SUBOFF with a resolved experimental trip wire geometry, and the same cases tripped using a simple numerical trip (wall-normal blowing), which serves as an example of artificial computational tripping methods often used in practice. When the trip wire height exceeds the laminar boundary layer thickness, LES reveals that shedding from the trip wire initiates transition, and the near field is characterized by an elevation of the wall-normal Reynolds stress and a modification of the turbulence anisotropy and mean momentum balance. This trip height also induces a large jump in the boundary layer thickness, which affects the rate at which the TBL grows and how it responds to pressure gradients and curvature. In contrast, a trip wire height shorter than the laminar boundary layer thickness is shown to initiate transition at the reattachment point of the trip-induced recirculation bubble. The artificial trip reasonably replicates the resolved trip wire behavior. For each case, the inner layer collapses rapidly in terms of the mean profile, Reynolds stresses, and mean momentum balance. This is followed by the collapse of the Reynolds stresses in coordinates normalized by the local momentum thickness, which proves to be a more robust outer scale than the 99% thickness due to its lower sensitivity to the over-tripped wake at the edge of the boundary layer. The importance of tripping the model appendages is also highlighted, due to their lower Reynolds numbers and susceptibility to laminar separations. Finally, DNS of a JICF with a triangular tab at two positions are performed at jet-to- crossflow velocity ratios of R = 2 and 4 with a jet Reynolds number of 2000 based on the jet’s bulk velocity and exit diameter. DNS and dynamic mode decomposition reveal that a tab on the upstream side of the jet produces Lambda-shaped streamwise vortices in the upstream shear layer (USL), while a tab placed 45 degrees from the upstream side produces a tertiary vortex for R = 4, which is not present at R = 2. For the upstream tab, the presence of streamwise vortices curled around the spanwise USL vortices provides an explanation for the improvements in mixing and spreading associated with an upstream tab. This streamwise vortex structure shows remarkable similarities to the ‘strain- oriented vortex tubes’ observed for disturbed plane shear layers. In contrast, the tab placed 45 degrees from the upstream position produces significantly different effects. At R = 4, the jet cross-section is significantly skewed away from the tab and a tertiary vortex is formed, as observed in past experiments on round JICFs at relatively high R and low Reynolds numbers. The 45 degree tab produces asymmetric effects in the wake of the jet at R = 2, but the effect on the jet cross-section is much smaller, highlighting the sensitivity of jets at high R to asymmetric perturbations.
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    Studies in wall turbulence using dual plane particle image velocimetry and direct numerical simulation data.
    (2010-02) Saikrishnan, Neelakantan
    Wall-bounded turbulent flows play a critical role in a variety of engineering applications. A detailed understanding of the fundamental processes underlying these flows is crucial to the accurate modeling and design of efficient and practical systems. The present studies are directed towards demonstrating the utility of experimental and numerical approaches in tandem to elucidate the structure and dynamics of wall-bounded turbulent flows over a range of Reynolds numbers. The first part of this thesis deals with the use of Dual Plane Particle Image Velocimetry (DPPIV) to understand the organization of vortex structures in the logarithmic region of turbulent boundary layer and channel flows. DPPIV provides the full velocity gradient tensor in a plane parallel to the wall and was used to calculate the projection angles of vortex structures with the three coordinate directions. In order to validate the experimental technique for identification of vortex cores, Direct Numerical Simulation (DNS) data at a comparable Reynolds number and higher resolution were used. The DNS data were averaged to the resolution of the DPPIV data and a vortex core identification routine was implemented to compare the raw DNS, averaged DNS and DPPIV data. It was observed that results from the DPPIV data match those from the raw and averaged DNS data very well. This confirms that DPPIV is a robust technique for calculating vortex core statistics, and the resolution of the measurements is sufficient to adequately resolve these structures. The second part of the thesis examines the effect of Reynolds number on the scale energy budget in wall-bounded turbulent flows. An understanding of the distribution of turbulent kinetic energy across the momentum deficit region in wall-bounded turbulent flows is critical to the complete understanding of the energy dynamics in these flows. The scale energy budget provides a tool to simultaneously assess the influence of spatial location and scales in the flow on the distribution of turbulent energy. This analysis was conducted using three DNS data sets across a range of Reynolds numbers, and results from these data were compared to results from an earlier study at a smaller Reynolds number. It is observed that the previous low Reynolds number study did not sufficiently resolve all the quantities of the energy budget in the near-wall region, primarily due of the lack of a distinct logarithmic region in the low Reynolds number simulation. Close to the wall, no effects of Reynolds number were observed on the terms of the scale energy analysis across the datasets studied. Upon moving away from the wall, the turbulent production term did not display any effects of Reynolds number, while the scale transfer term increased with increasing Reynolds number. As a result the cross-over scale, a quantity related to the shear scale in turbulent flows, increased with increasing Reynolds numbers for all datasets. The shear scale provides the scale at which the change from an isotropy-recovering range to an anisotropic region is observed, while the cross-over scale provides a transition between a transfer dominated range to a production dominated range of scales. The plot of cross-over scale versus wall-normal location revealed that the slope of the best fit lines increased with increasing Reynolds number. Further, the slope of the best fit line decreased with increasing wall-normal location. DPPIV data were also used to conduct the same analysis in turbulent boundary layers across a range of Reynolds numbers. The aim of using DPPIV data was to quantify the effects of Reynolds number on the scale energy analysis, using larger Reynolds number experimental data. The effects of resolution and Reynolds number on the DPPIV data were assessed and described in detail. The effect of resolution was most dominant on the wall-normal gradients of the streamwise and spanwise velocities, and hence the transfer of energy in physical space and the transfer of energy in scale space were overestimated in the lower resolution data. The effect of resolution was smallest on the turbulent production term, since it does not contain of any fluctuating velocity gradients. With increasing Reynolds numbers, the production term did not change significantly in the logarithmic layer, while the scale transfer term increased, resulting in a larger range of transfer-dominated scales. The value of the cross-over scale between the effective production and scale transfer term increased with increasing Reynolds number, suggesting a larger range of isotropic-type transfer dominated scales. In conclusion, it was demonstrated that DPPIV in tandem with DNS can be used to reliably assess the scale energy budget in wall-bounded turbulent flows. i