Non-Cyclic Turbulent Inflow

Video in TIB AV-Portal: Non-Cyclic Turbulent Inflow

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Non-Cyclic Turbulent Inflow
Turbulent inflow versus laminar inflow for large-eddy simulations
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Knoop, Helge
Gronemeier, Tobias
Inagaki, Atsushi
Keck, Marius
Kanda, Manabu
Raasch, Siegfried
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The animation shows a comparison between two large-eddy simulations (LES) using different inflow boundary conditions. For both simulations the LES model PALM was used, simulating a neutral stratified flow over an array of building cubes. The upper half of this visualization shows a simulation, which uses a laminar inflow at the left boundary while the lower half shows a simulation, which uses a turbulence generator based on a filter method at the left boundary. The size of both domains is 2180m x 720m x 240m with a mean background wind of 6 m/s at the top of the domain blowing from left to right. The rotation of the velocity vector (absolute values) is visualized to show the turbulence structures and intensities. High values are marked red while low values are white. The buildings have a cubic shape with 24m edge length and are packed with a plane area index of 0.25. One tall building sits in the center of the domain with three times the size of a small building in horizontal direction and four times the size in vertical direction. The animation was created using the visualization software VAPOR. Simulations were calculated on the Cray-XC30 of the North-German Supercomputing Alliance as well as on the TSUBAME 2.5 of the Tokyo Institute of Technology. In the simulation with laminar inflow (top), first turbulent motions can be spotted behind the tenth building row. In reality such a laminar flow is almost never observed and hence very artificial. In the simulation with generated turbulent inflow (bottom), turbulence is created at the inflow boundary. This leads to an already turbulent flow above the first building rows. This flow is much more realistic. The flow in close vicinity to the tall building at the center of the domain shows slight differences between the two simulations. These differences can especially be seen at the rooftop of the tall building. Here the arriving flow in the top simulation shows almost no developed turbulence, while the arriving flow in the bottom simulation is already turbulent. At the outflow boundary however, both simulations show nearly equally developed turbulence. The results indicate, that the used turbulence generation method allows legitimate analysis of simulation data much closer to the inflow boundary which can result in significant cost savings due to smaller required domain sizes.
Keywords large-eddy simulation Turbulent inflow turbulence generator High-Performance-Computing PALM


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