Search results for "Fluid dynamics"
showing 10 items of 1005 documents
Boundary-layer Flows Past an Hemispherical Roughness Element: DNS, Global Stability and Sensitivity Analysis
2015
Abstract We investigate the full three-dimensional instability mechanism arising in the wake of an hemispherical roughness element immersed in a laminar Blasius boundary layer. The inherent three-dimensional flow pattern beyond the critical Reynolds number is characterized by coherent vortical structures called hairpin vortices. Direct numerical simulation is used to analyze the formation and the shedding of hairpin packets inside the shear layer. The first bifurcation characteristics are investigated by global stability tools. We show the spatial structure of the linear direct and adjoint global eigenmodes of the linearized Navier-Stokes operator and use structural sensitivity analysis to …
Physical Aspects of the Atmospheric Aerosol
1982
The atmospheric aerosol is a trace substance of the troposhere as well as the trace gases. Therefore a number of similiarities should be expected. As an example a common model on residence time is developed and discussed in its results. The residence time connects the content of a reservoir (size distribution for aerosols) with its source strength or removal rate. This is presented for the aerosol. In addition other physical properties of the tropospheric aerosol like optical, electrical and water uptake properties are discussed.
A dynamic subgrid-scale tensorial eddy viscosity model
1999
In the Navier-Stokes equations the removal of the turbulent fluctuating velocities with a frequency above a certain fixed threshold, employed in the Large Eddy Simulation (LES), causes the appearance of a turbulent stress tensor that requires a number of closure assumptions. In this paper insufficiencies are demonstrated for those closure models which are based on a scalar eddy viscosity coefficient. A new model, based on a tensorial eddy viscosity, is therefore proposed; it employs the Germano identity [1] and allows dynamical evaluation of the single required input coefficient. The tensorial expression for the eddy viscosity is deduced by removing the widely used scalar assumption of the …
Analysing longitudinal turbulence intensity in vegetated channels
2007
Vegetation altering hydrodynamic conditions of an open channel flow controls the exchanges of sediment, nutrients and contaminants. In this paper the turbulence structure of open-channel flow over a flexible grass vegetation is investigated. Velocity measurements were carried out, in a rectangular flume, using a 2D-ADV (Acoustic Doppler Velocimeter) for different values of stem concentration, water discharge, and bed slope. The experimental results showed that: 1) the maximum value of the longitudinal turbulence intensity occurs at a water depth close to the bent vegetation height; 2) the turbulence intensity damps for increasing values of the stem concentration; 3) above the vegetation hei…
A mathematical model of the self-averaging Pitot tube
2005
Abstract Flowmeters with self-averaging Pitot tubes are more and more often applied in practice. Their advantages are practically no additional flow losses, usability in the case of high temperature of fluids and simplicity of fitting. A mathematical model of a self-averaging Pitot tube including the influence of the probe shape, selected constructional features and flow conditions on the quantity of differential pressure gained has been given in this paper. The values and ranges of variations of the coefficients established for the model have been assessed on the basis of the numerically computed velocity and pressure fields around and inside the probe. Velocity and pressure fields were ca…
Simulation of the Propagation of Tsunamis in Coastal Regions by a Two-Dimensional Non-Hydrostatic Shallow Water Solver
2017
Due to the enormous damages and losses of human lives in the inundated regions, the simulation of the propagation of tsunamis in coastal areas has received an increasing interest of the researchers. We present a 2D depth-integrated, non- hydrostatic shallow waters solver to simulate the propagation of tsunamis, solitary waves and surges in coastal regions. We write the governing continuity and momentum equations in conservative form and discretize the domain with unstructured triangular Generalized Delaunay meshes. We apply a fractional- time-step procedure, where two problems (steps) are consecutively solved. In the first and in the second step, we hypothesize a hydrostatic and a non-hydro…
CFD predictions of turbulent two-phase flow in helical coils
2012
Turbulent two-phase flow in a helical pipe was simulated by means of CFD using the ANSYS-CFX code, based on a finite-volume multi-fluid approach. The focus was on flow patterns and frictional pressure drops; inter-phase heat and mass transfer and heat exchange with the pipe walls were not included and physical properties were assumed constant for each phase. The computational domain included a single, vertical axis, coil turn; the flow was assumed to be fully developed. Geometry and physical conditions were representative of the IRIS nuclear reactor steam generators (saturated water at 58 bar, pressure gradient of 0.1 bar/m). Void fraction and computational options (homogeneous vs. inhomo…
Autecology, lifestyle and hydrodynamics of early vertebrates
2018
Capítulo 1. Introducción general Los vertebrados constituyen uno de los grupos de animales más exitosos, contando con más de 60000 especies distintas y ocupando la gran mayoría de ecosistemas marinos y continentales. Su origen y evolución han sido temas centrales para biólogos y paleontólogos durante décadas, pero las discusiones en este sentido se han visto obstaculizadas porque los grupos actuales (ciclóstomos, condrictios y osteíctios) constituyen solo una pequeña representación de toda la diversidad que existió en el pasado. Por lo tanto, el registro fósil es crucial para arrojar luz sobre estos aspectos permitiendo desvelar el momento y el tempo de la aparición de los diferentes caract…
A Simulation Based Analysis of an Multi Objective Diffusive Load Balancing Algorithm
2018
In this paper, we presented a further development of our research on developing an optimal software-hardware mapping framework. We used the Petri Net model of the complete hardware and software High Performance Computing (HPC) system running a Computational Fluid Dynamics (CFD) application, to simulate the behaviour of the proposed diffusive two level multi-objective load-balancing algorithm. We developed an meta-heuristic algorithm for generating an approximation of the Pareto-optimal set to be used as reference. The simulations showed the advantages of this algorithm over other diffusive algorithms: reduced computational and communication overhead and robustness due to low dependence on u…
Convective Roll Instabilities of Vertical Throughflow with Viscous Dissipation in a Horizontal Porous Layer
2009
Published version of an article from the journal: Transport in Porous Media. The original publication is available at Spingerlink. http://dx.doi.org/10.1007/s11242-009-9417-y The vertical throughflow with viscous dissipation in a horizontal porous layer is studied. The horizontal plane boundaries are assumed to be isothermal with unequal temperatures and bottom heating. A basic stationary solution of the governing equations with a uniform vertical velocity field (throughflow) is determined. The temperature field in the basic solution depends only on the vertical coordinate. Departures from the linear heat conduction profile are displayed by the temperature distribution due to the forced con…