Early researchers applied visualization techniques based on smoke and dye injections in order to describe coherent structures in turbulent flows. Generally speaking, visualization techniques have substantially evolved in the last few decades, spanning all disciplines. In recent times, Virtual Reality (VR) has revolutionized the way that visualization is carried out.
In this study, we are performing fully immersive visualization of high-fidelity numerical results of supersonic spatially-developing turbulent boundary layers (SDTBL) under strong concave and concave curvatures and Mach = 2.86. The selected numerical tool is Direct Numerical Simulation (DNS) with high spatial/temporal resolution. The comprehensive DNS information sheds important light on the transport phenomena inside turbulent boundary layers subject to strong deceleration or Adverse Pressure Gradient (APG) caused by concave walls as well as to strong acceleration or Favorable Pressure Gradient (FPG) caused by convex walls at different wall thermal conditions (i.e., cold, adiabatic and hot walls). Another fluid dynamics example to be discussed is the high-speed crossflow-jet problem. We are extracting vortex core iso-surfaces via the Q-criterion to convert them to a file format readable by the HTC Vive VR and Varjo toolkit.
Amidst the backdrop of cutting-edge progressions in both capabilities and User Interface (UI) enhancements of the VWT, researchers are now poised to delve into a realm of comprehensive understanding concerning SDTBL. Within this dynamic, fully immersive environment, the intricacies of flow development unfold before their eyes. The elevated UI refinements have bestowed users with remarkable freedom of movement across six directions and database selection, effectively amplifying their capacity for meticulous observation and incisive analysis of the animated flow phenomena
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Assessment of turbulent boundary layer detachment due to wall-curvature-driven pressure gradient
The present study provides fundamental knowledge on an issue in fluid dynamics that is not well understood: flow separation and its association with heat and contaminant transport. In the separated region, a swirling motion increases the fluid drag force on the object. Very often, this is undesirable because it can seriously reduce the performance of engineered devices such as aircraft and turbines. Furthermore, Computational Fluid Dynamics (CFD) has gained ground due to its relatively low cost, high accuracy, and versatility. The principal aim of this study is to numerically elucidate the details behind momentum and passive scalar transport phenomena during turbulent boundary layer separation resulting from a wall-curvature-driven pressure gradient. With Open- FOAM CFD software, the numerical discretization of Reynolds-Averaged Navier-Stokes and passive scalar transport equations will be described in two-dimensional domains via the assessment of two popular turbulence models (i.e., the Spalart-Allmaras and the K-w SST model). The computational domain reproduces a wind tunnel geometry from previously performed experiments by Baskaran et al. (JFM, vol. 182 and 232 “A turbulent flow over a curved hill.” Part 1 and Part 2). Only the velocity and pressure distribution were measured there, which will be used for validation purposes in the present study. A second aim in the present work is the scientific visualization of turbulent events and coherent structures via the ParaView toolkit and Unity game engine. Thus, fully immersive visualization approaches will be used via virtual reality (VR) and augmented reality (AR) technologies. A Virtual Wind Tunnel (VWT), developed for the VR approach, emulates the presence in a wind tunnel laboratory and has already employed fluid flow visualization from an existing numerical database with high temporal/spatial resolution, i.e., Direct Numeric Simulation (DNS). In terms of AR, a FlowVisXR app for smartphones and HoloLens has been developed for portability. It allows the user to see virtual 3D objects (i.e., turbulent coherent structures) invoked into the physical world using the device as the lens.
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- NSF-PAR ID:
- 10415231
- Editor(s):
- Advisor: Dr. Guillermo Araya
- Date Published:
- Journal Name:
- Theses
- ISSN:
- 2532-1668
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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In this video, we show high-fidelity numerical results of supersonic spatially-developing turbulent boundary layers (SDTBL) under strong concave and concave curvatures and Mach = 2.86. The selected numerical tool is Direct Numerical Simulation (DNS) with high spatial/temporal resolution. The prescribed concave geometry is based on the experimental study by Donovan et al. (J. Fluid Mech., 259, 1-24, 1994). Turbulent inflow conditions are based on extracted data from a previous DNS over a flat plate (i.e., turbulence precursors). The comprehensive DNS information sheds important light on the transport phenomena inside turbulent boundary layers subject to strong deceleration or Adverse Pressure Gradient (APG) caused by concave walls as well as to strong acceleration or Favorable Pressure Gradient (FPG) caused by convex walls at different wall thermal conditions (i.e., cold, adiabatic and hot walls). In this opportunity, the selected scientific visualization tool is Virtual Reality (VR) by extracting vortex core iso-surfaces via the Q-criterion to convert them to a file format readable by the HTC Vive VR toolkit. The reader is invited to visit our Virtual Wind Tunnel (VWT) under a fully immersive environment for further details. The video is available at: https://gfm.aps.org/meetings/dfd-2022/6313a60c199e4c2da9a946bcmore » « less
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In the study, a series of wind tunnel tests were conducted to investigate wind effects acting on dome structures (1/60 scale) induced by straight-line winds at a Reynolds number in the order of 106. Computational Fluid Dynamics (CFD) simulations were performed as well, including a Large Eddy Simulation (LES) and Reynolds-Averaged Navier–Stokes (RANS) simulation, and their performances were validated by a comparison with the wind tunnel testing data. It is concluded that wind loads generally increase with upstream wind velocities, and they are reduced over suburban terrain due to ground friction. The maximum positive pressure normally occurs near the base of the dome on the windward side caused by the stagnation area and divergence of streamlines. The minimum suction pressure occurs at the apex of the dome because of the blockage of the dome and convergence of streamlines. Suction force is the most significant among all wind loads, and special attention should be paid to the roof design for proper wind resistance. Numerical simulations also indicate that LES results match better with the wind tunnel testing in terms of the distribution pattern of the mean pressure coefficient on the dome surface and total suction force. The mean and root-mean-square errors of the meridian pressure coefficient associated with the LES are about 60% less than those associated with RANS results, and the error of suction force is about 40–70% less. Moreover, the LES is more accurate in predicting the location of boundary layer separation and reproducing the complex flow field behind the dome, and is superior in simulating vortex structures around the dome to further understand the unsteadiness and dynamics in the flow field.
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An incoming canonical spatially developing turbulent boundary layer (SDTBL) over a 2-D curved hill is numerically investigated via the Reynolds-averaged Navier–Stokes (RANS) equations plus two eddy-viscosity models: the K−ω SST (henceforth SST) and the Spalart–Allmaras (henceforth SA) turbulence models. A spatially evolving thermal boundary layer has also been included, assuming temperature as a passive scalar (Pr = 0.71) and a turbulent Prandtl number, Prt, of 0.90 for wall-normal turbulent heat flux modeling. The complex flow with a combined strong adverse/favorable streamline curvature-driven pressure gradient caused by concave/convex surface curvatures has been replicated from wind-tunnel experiments from the literature, and the measured velocity and pressure fields have been used for validation purposes (the thermal field was not experimentally measured). Furthermore, direct numerical simulation (DNS) databases from the literature were also employed for the incoming turbulent flow assessment. Concerning first-order statistics, the SA model demonstrated a better agreement with experiments where the turbulent boundary layer remained attached, for instance, in Cp, Cf, and Us predictions. Conversely, the SST model has shown a slightly better match with experiments over the flow separation zone (in terms of Cp and Cf) and in Us profiles just upstream of the bubble. The Reynolds analogy, based on the St/(Cf/2) ratio, holds in zero-pressure gradient (ZPG) zones; however, it is significantly deteriorated by the presence of streamline curvature-driven pressure gradient, particularly due to concave wall curvature or adverse-pressure gradient (APG). In terms of second-order statistics, the SST model has better captured the positively correlated characteristics of u′ and v′ or positive Reynolds shear stresses (more » « less
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The interaction of a turbulent, spatially developing crossflow with a transverse jet possesses several engineering and technological applications such as film cooling of turbine blades, exhaust plumes, thrust vector control, fuel injection, etc. Direct Numerical Simulation (DNS) of a jet in a crossflow under different streamwise pressure gradients (zero and favorable pressure gradient) is carried out. The purpose is to study the physics behind the transport phenomena and coherent structure dynamics in turbulent crossflow jets at different streamwise pressure gradients and low/high-velocity ratios (0.5 and 1). The temperature was regarded as a passive scalar with a molecular Prandtl number of 0.71. The analysis is performed by prescribing accurate turbulent information (instantaneous velocity and temperature) at the inlet of a computational domain. The upward motion of low-momentum fluid created by the “legs” of the counter-rotating vortex pair (CVP) encounters the downward inviscid flow coming from outside of the turbulent boundary layer, inducing a stagnation point and a shear layer. This layer is characterized by high levels of turbulent mixing, turbulence production, turbulent kinetic energy (TKE) and thermal fluctuations. The formation and development of the above-mentioned shear layer are more evident at higher velocity ratios.more » « less
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