UFR 3-31 Evaluation: Difference between revisions
| Line 32: | Line 32: | ||
and turbulent kinetic energy) are shown, but other parameters can of course be | and turbulent kinetic energy) are shown, but other parameters can of course be | ||
compared, such as dissipation or Reynolds-stress if using second-moment closure. | compared, such as dissipation or Reynolds-stress if using second-moment closure. | ||
The streamwise evolution of the wall shear stress <math>{\tau_{w,x}}</math> and the pressure coefficient | |||
are shown in Fig. \ref{fig:RANS_CfCp}. As mentioned in the section (\S \ref{sec:RANS}), | |||
the inflow condition was designed to match the behaviour of the wall shear stress up | |||
to <math>{x/H=0}</math> (including the peak near <math>{x/H=0}</math>), and this is indeed the case for all the | |||
models. The difference between the models occurs just downstream of the peak: | |||
both $k-\varepsilon$ models return a slightly too late separation, while the two | |||
$k-\omega$ models are in much better agreement with the LES data, up to $x/H=3$. Inside | |||
the separated region, all four models predict more or less the same wall shear stress. The only | |||
model to return a correct reattachment point is the standard $k-\varepsilon$ model, but | |||
the recovery region downstream (from $x/H=7$) is much better predicted by both $k-\omega$ | |||
models. Corresponding values for the separation and the reattachment points are | |||
given in table \ref{tab:RansResults}. These observations are also valid for the | |||
pressure coefficient $C_p$: before separation, the flow is fairly well predicted by | |||
all four models, compared to the LES data, with the same difference (too little | |||
flow acceleration) compare to the experimental data, included in the plot. | |||
Differences between the models only arise close to the separation point, with a | |||
slightly larger flow acceleration predicted by the $k-\varepsilon$ models. Overall, any of the | |||
two $k-\omega$ models return a much better qualitative picture than the $k-\varepsilon$ | |||
formulations. However, table \ref{tab:RansResults} shows that the separation bubble | |||
is much larger (up to 35\%) than in the LES. Other, more advanced, $k-\varepsilon$ | |||
models were found to give better results for this test case, but once again, the | |||
objective of this document is to provide guidelines, not an assessment of model capabilities. | |||
<br/> | <br/> | ||
Revision as of 12:44, 4 June 2012
Flow over curved backward-facing step
Semi-confined flows
Underlying Flow Regime 3-31
Evaluation
Comparison of LES results with experiments
Comparisons of the simulation results with the experimental measurements of Zhang and Zhong \cite{zhang2010experimental} are shown in Figs. \ref{fig:wallValidation}-\ref{fig:reynoldsStressValidation}. Only results obtained with the fine mesh (ie. corresponding to the data provided), are shown here. Comparisons with the coarse mesh results are given in Lardeau and Leschziner \cite{lardeau2011interaction}. General agreement of the profiles is very good, both for mean flow velocity (Fig. \ref{fig:velocityValidation}) and for the three components of the Reynolds stress tensor (Fig. \ref{fig:reynoldsStressValidation}). The major discrepancies are found on the wall-measurements: the skin-friction coefficient is slightly overestimated close to the separation, with a large peak of not present in the experimental results. The experimental skin friction data should be taken with care, as they were computed from the measured velocity field (bound to greater error in the near-wall region) rather than from direct measurements at the wall.
Comparison of RANS results with LES results and experiments
Results obtained with the four RANS models are shown in Figs. \ref{fig:RANS_CfCp}-\ref{fig:Tke_KOmega}. Only four quantities (wall shear stress, pressure coefficient, streamwise velocity and turbulent kinetic energy) are shown, but other parameters can of course be compared, such as dissipation or Reynolds-stress if using second-moment closure.
The streamwise evolution of the wall shear stress and the pressure coefficient are shown in Fig. \ref{fig:RANS_CfCp}. As mentioned in the section (\S \ref{sec:RANS}), the inflow condition was designed to match the behaviour of the wall shear stress up to (including the peak near ), and this is indeed the case for all the models. The difference between the models occurs just downstream of the peak: both $k-\varepsilon$ models return a slightly too late separation, while the two $k-\omega$ models are in much better agreement with the LES data, up to $x/H=3$. Inside the separated region, all four models predict more or less the same wall shear stress. The only model to return a correct reattachment point is the standard $k-\varepsilon$ model, but the recovery region downstream (from $x/H=7$) is much better predicted by both $k-\omega$ models. Corresponding values for the separation and the reattachment points are given in table \ref{tab:RansResults}. These observations are also valid for the pressure coefficient $C_p$: before separation, the flow is fairly well predicted by all four models, compared to the LES data, with the same difference (too little flow acceleration) compare to the experimental data, included in the plot. Differences between the models only arise close to the separation point, with a slightly larger flow acceleration predicted by the $k-\varepsilon$ models. Overall, any of the two $k-\omega$ models return a much better qualitative picture than the $k-\varepsilon$ formulations. However, table \ref{tab:RansResults} shows that the separation bubble is much larger (up to 35\%) than in the LES. Other, more advanced, $k-\varepsilon$ models were found to give better results for this test case, but once again, the objective of this document is to provide guidelines, not an assessment of model capabilities.
Contributed by: Sylvain Lardeau — CD-adapco
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