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Measuring velocity in electronic systems
Published in Kaveh Azar, in Electronic Cooling, 2020
It is noted that some words in common usage: “laminar, turbulent, vortex, eddy,” defy precise verbal definitions. This is true for “laminar-turbulent” because the instantaneous motion for both is described by exactly the same governing equations. Regarding the terms: “vortex” or “eddy”, the literature does not provide unambiguous definitions for these words. It is best to recognize that these four words do not permit the user nor the listener/reader to communicate or infer anything of precision about the flow by their use.
Fundamentals of Pathway Transport Assessment
Published in Jack Daugherty, Assessment of Chemical Exposures, 2020
Turbulent diffusion is another matter. The turbulent motion of any fluid is called eddies, or eddy movement, or eddy motion. Eddies transport mass as they move in a fluid. Intuitively, large eddies cause rapid mass transport, while small eddies cause slower mass transport. Mass transport by eddies is called diffusivity, D. The values of this property for any species of concern is dependent on the fluid through which it disperses and cannot be estimated, but must be determined experimentally. Literature values, according to Keil, range from 0.05 to 11.5 m2/min, with typical values ranging from 0.3 to 3 m2/min. Keil calls the study of diffusivity one of the last great frontiers of Newtonian mechanics. Most of the work being done on the subject is limited to restricted fluid flow.
Thermal Radiation and Energy Closure Assessment in Evapotranspiration Estimation for Remote Sensing Validation
Published in Ni-Bin Chang, Yang Hong, Multiscale Hydrologic Remote Sensing, 2012
John H. Prueger, Joe Alfieri, William Kustas, Lawrence Hipps, Christopher Neale, Steven R. Evett, Jerry Hatfield, Lynn G. McKee, Jose L. Chavez
The turbulent process responsible for the exchange and transport of vertical fluxes of heat and water vapor contains spectral information that describes the contribution of various frequencies to the observed velocity variances that are ultimately component turbulent kinetic energy or covariances. Turbulent flows typically found in the boundary layer (near a surface) are a superposition of many eddies varying in scale and frequency. These eddies that are energy containing interact continuously with the mean wind flow from where most of their energy is derived from and with each other (i.e., eddy interaction at local and regional scales). To better understand the turbulent processes associated with the vertical transport of fluxes (flux covariance), it is necessary to evaluate the cospectrum of turbulent motions that exist in the boundary layer of the atmosphere. This can be accomplished with the following expression (Garratt 1975): () w′,χ′¯=∫0∞Swχ(ω)dω,
Designing a Two-Level Steel Cable-stayed Bridge against Fires
Published in Structural Engineering International, 2023
Zhi Liu, Guobiao Lou, Jing Hou, Guoqiang Li
The Fire Dynamics Simulator (FDS) was employed to reproduce fire environments. FDS is an open-source computational fluid dynamics (CFD) program developed by the National Institute of Standards and Technology. It establishes a large-eddy model to simulate the fire-driven fluid flow by numerically solving the Navier–Stokes equations. It can characterize the transient temperature field more realistically with a high-fidelity gradient. FDS fire models necessitate defining the domain of meshes in which the simulation will be carried out. The domain has boundary conditions on its edges and is discretized into multiple rectilinear cells. Obstructions that can potentially affect the fire-driven gas flow need to be incorporated along with the fire source with its combustion characteristics. In the performed study, fire environments were modeled inside the space with the sizes of 40.5 m wide (lateral bridge direction), 20 m long (longitudinal bridge direction), and 9 m high (vertical direction). Cube cells with a 0.5-m side length were adopted to discretize the numerical space. Fire sources were reproduced according to HRRs per unit areas, footprint areas, and lasting times defined previously.
Evaluation of tidal stream energy at major tidal inlets of Goa, India
Published in ISH Journal of Hydraulic Engineering, 2022
Vikas Mendi, Jaya Kumar Seelam, Subba Rao
The two-dimensional MIKE21 Flow Model FM (Flexible Mesh) was used for this study. The MIKE FM modelling suite is based on a cell-centered finite volume method, with an unstructured mesh to permit accurate representation of complex coastal areas. The momentum equations used are the incompressible, Reynolds averaged form of the Navier-Stokes equations, using hydrostatic pressure and the Boussinesq assumption as to the representation of turbulence by eddy viscosity. Horizontal eddy viscosity is represented by the Smagorinsky formulation. The bed resistance is specified as a Manning number that varies across the domain. A comprehensive description of the model can be MIKE21 (2012). The model domain for Chapora includes high tidal flow regions. It structures higher resolution in areas where the flow is high, and lower resolution in areas where the currents are weaker. The unstructured mesh triangles in coarse areas have a maximum characteristic length of 2500 m, mesh triangles in the finer zones have a characteristic length of 50 to 200 m.
An alternative Vorticity based Adaptive Mesh Refinement (V-AMR) technique for tip vortex cavitation modelling of propellers using CFD methods
Published in Ship Technology Research, 2022
In the numerical simulations, the three different simulation methods, RANS, DES and LES, and the associated solvers were used to solve the flow around the cavitating propeller. The RANS solver is based on the solution of the time-averaged equations in the fluid domain, whereas LES is based on filtered differential equations. DES is a hybrid method by combining the RANS in the boundary layer and LES in the free field region. The improved formulation of DES (i.e. Delayed DES) was used in this study. The k-ω SST turbulence model was selected both for the RANS and DDES solvers. Additionally, the WALE (Wall-Adapting Local-Eddy viscosity) subgrid-scale model was used to close the filtered Navier-Stokes equations for the LES. Also, the large scales of the turbulence are directly resolved everywhere in the flow domain in the LES while the small scales are modelled. Despite its several limitations, the RANS method is still used for many engineering problems due to its lower computational cost compared to scale resolving simulations (i.e. DES and LES), particularly in the design stage. The detailed information about the solvers can be found in Spalart et al. 2006 and Star CCM+ 14.06 (2019).