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Experimental and Computational Investigation of a Fractal Grid Wake

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Fractal grids generate turbulence by exciting many length-scales of different sizes simultaneously. The interest in these grids finds its origin in the attempt to create ideal experiments on turbulence with well controlled conditions and very high Reynolds numbers [7, 4]. This interest has been further building-up since the surprising findings stemming from the experimental and compu- tational studies conducted on these grids [2, 8, 3]. These results show that in contrast to the power-law decay of standard turbulence grids, the turbulence intensity in the wake of a fractal grid increases to reach a peak at some distance from the grid before it decays following an exponential law [2, 6]. In this work we present experimental and computational studies of the turbulent wake generated by a space-filling square fractal grid. The experimental work includes Particle Image Velocimetry (PIV) and hot-wire measurements. The numerical simulations are Delayed Detached Eddy Simulations (DDES) conducted using the open source package OpenFOAM [5]. Our goal is to provide a better insight on the turbulence generated by fractal grids and the suitability of DDES in characterizing it. To the authors knowledge this is the first time that DDES simulations are used to simulate the wake generated by a fractal grid. Additionally, the generated multi-scale turbulence will be used in future simulations as inflow condition. This will permit the use of more realistic inflow conditions to simulate flows over airfoils, wind turbine blades and whole wind turbines.

Author(s):

Wided Medjroubi    
ForWind, Institut of Physics, University of Oldenburg, Ammerländer Heerstr. 136, 26111 Oldenburg
Germany

Hannes Hochstein    
ForWind, Institut of Physics, University of Oldenburg, Ammerländer Heerstr. 136, 26111 Oldenburg
Germany

Andre Fuchs    
ForWind, Institut of Physics, University of Oldenburg, Ammerländer Heerstr. 136, 26111 Oldenburg
Germany

Gerd Gülker    
ForWind, Institut of Physics, University of Oldenburg, Ammerländer Heerstr. 136, 26111 Oldenburg
Germany

Joachim Peinke    
ForWind, Institut of Physics, University of Oldenburg, Ammerländer Heerstr. 136, 26111 Oldenburg
Germany