UFR 3-30 Description: Difference between revisions
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between the hills seven and eight. These investigations became basis for a testcase of the ERCOFTAC/IAHR-Workshop in 1995 [Rodi et al. (1995)]. It turned out that the comparability of this experiment and the 2D numerical simulations was limited because of strong 3D effects and doubts about the periodicity of the flow. | between the hills seven and eight. These investigations became basis for a testcase of the ERCOFTAC/IAHR-Workshop in 1995 [Rodi et al. (1995)]. It turned out that the comparability of this experiment and the 2D numerical simulations was limited because of strong 3D effects and doubts about the periodicity of the flow. | ||
Mellen et al. (2000) used the advantages of this geometry and adjusted it to meet numerical needs. The channel height was reduced to save computational time though the distance between the hills was doubled to achieve natural reattachment. Periodicity was applied in the streamwise and in the spanwise direction to keep the numerical cost affordable, however the Reynolds number had to be reduced to | Mellen et al. (2000) used the advantages of this geometry and adjusted it to meet numerical needs and introduced the flow case which is subject to this investigation. The channel height was reduced to save computational time though the distance between the hills was doubled to achieve natural reattachment. Periodicity was applied in the streamwise and in the spanwise direction to keep the numerical cost affordable, however the Reynolds number had to be reduced to | ||
Re≈ 10<sup>4</sup>. | Re≈ 10<sup>4</sup>. | ||
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Temmerman et al. (2003) state that the challenge of this case is to prediction the separation point, which has a strong impact on the recirculation length. According to the authors the grid resolution of the LES in the vicinity of the separation point has got a not negligible influence on the recirculation length. | Temmerman et al. (2003) state that the challenge of this case is to prediction the separation point, which has a strong impact on the recirculation length. According to the authors the grid resolution of the LES in the vicinity of the separation point has got a not negligible influence on the recirculation length. | ||
A detailed review of | A detailed review of the flow physics was undertaken by Fröhlich et al. (2005) who conducted LES at Re=10,595. Mean and RMS-values, spectra and anisotropy measures are being presented whilst they found phenomena such as the so-called 'splatting effect' on the windward side of the hill. Moreover they studied the size of the largest structures by two-point correlations of the streamwise velocity component. Temmerman (2004) investigated the impact of the number of periods on the flow. | ||
A recent publication comprises cross comparisons of numerical and experimental results up to a Reynolds number of 10,595 (Breuer et al. 2009). A Cartesian (MGLET) and a curvilinear code (LESOCC, Breuer and Rodi 1996, Breuer 2002) are checked with thoroughly validated PIV data. These data are presented here. | A recent publication comprises cross comparisons of numerical and experimental results up to a Reynolds number of 10,595 (Breuer et al. 2009). A Cartesian (MGLET) and a curvilinear code (LESOCC, Breuer and Rodi 1996, Breuer 2002) are checked with thoroughly validated PIV data. These data are presented here. | ||
Revision as of 08:28, 9 February 2010
2D Periodic Hill
Semi-Confined Flows
Underlying Flow Regime 3-30
Description
The two-dimensional flow over periodically arranged hills separates from a curved surface, recirculates in the leeward side of the hill and reattaches naturally at the flat channel bottom. The challenge of this case is to predict the point of separation from that curved surface which has got a strong impact on the point of reattachment. The length and height of the main recirculation bubble varies with the Reynolds number. Furthermore a tiny recirculation zone has been detected on the hilltop at Re=10,595 and a minor one can be found for various Re at the windward foot of the hill. Fig. 2.1 depicts streamlines of the flow and shows the turbulent kinetic energy with its maximum right above the mean recirculation zone.
Review of UFR studies and choice of test case
Zilker et al. (1977) conducted experiments on small amplitude sinusoidal waves in a water channel. Zilker and Hanratty (1979) modified the channel and investigated the flow over large amplitude waves. A periodic behavior of the flow in the streamwise direction was assumed from the eighth out of ten wave trains. They recorded the wall shear stress by electro-chemical probes and measured velocities through thermal coated films. The same channel was used by Buckles et al. (1984) to investigate the flow phenomena separation from a curved surface, recirculation and reattachment with Laser Doppler Anemometry and high resolution pressure cells.
Almeida et al. (1993) published an article in 1993 on the flow over two-dimensional hills that correspond to the symmetry axis of a three-dimensional hill used by Hunt and Snyder (1980). The hills of height h (defined by the six polynomials shown above) were 3.857h long and confined the 6.07h channel by about one sixth. Almeida et al. chose an inter-hill distance of 4.5h and a lateral extent of the domain of 4.5h as well. The measurements with an LDA system were carried out at Re=6.0⋅104 between the hills seven and eight. These investigations became basis for a testcase of the ERCOFTAC/IAHR-Workshop in 1995 [Rodi et al. (1995)]. It turned out that the comparability of this experiment and the 2D numerical simulations was limited because of strong 3D effects and doubts about the periodicity of the flow.
Mellen et al. (2000) used the advantages of this geometry and adjusted it to meet numerical needs and introduced the flow case which is subject to this investigation. The channel height was reduced to save computational time though the distance between the hills was doubled to achieve natural reattachment. Periodicity was applied in the streamwise and in the spanwise direction to keep the numerical cost affordable, however the Reynolds number had to be reduced to Re≈ 104.
Several collaborative studies have followed because various research initiatives such as a DFG-CNRS group have chosen the case to benchmark their codes. Temmerman et al. (2003) state that the challenge of this case is to prediction the separation point, which has a strong impact on the recirculation length. According to the authors the grid resolution of the LES in the vicinity of the separation point has got a not negligible influence on the recirculation length.
A detailed review of the flow physics was undertaken by Fröhlich et al. (2005) who conducted LES at Re=10,595. Mean and RMS-values, spectra and anisotropy measures are being presented whilst they found phenomena such as the so-called 'splatting effect' on the windward side of the hill. Moreover they studied the size of the largest structures by two-point correlations of the streamwise velocity component. Temmerman (2004) investigated the impact of the number of periods on the flow.
A recent publication comprises cross comparisons of numerical and experimental results up to a Reynolds number of 10,595 (Breuer et al. 2009). A Cartesian (MGLET) and a curvilinear code (LESOCC, Breuer and Rodi 1996, Breuer 2002) are checked with thoroughly validated PIV data. These data are presented here.
Contributed by: (*) Christoph Rapp, (**) Michael Breuer, (*) Michael Manhart, (*) Nikolaus Peller — (*) Technische Universitat Munchen, (**) Helmut-Schmidt University Hamburg
© copyright ERCOFTAC 2009