Evaluation AC6-15: Difference between revisions
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===Application Challenge AC6-15=== | ===Application Challenge AC6-15=== | ||
=Evaluation= | =Evaluation= | ||
==COMPARISON OF TEST DATA AND CFD== | |||
We present first a summary of the performance of various URANS models on a typical RANS grid of 2M nodes and compare with the LES on 6M and DES on 2M grid for the low part-load of . All RANS calculations were performed in unsteady mode. The time-averaged streamline patterns and colour contours of the fields of the vertical velocity in Fig. 7 clearly show that the RSM, DES and LES all give very similar results; whereas the linear eddy viscosity models (LEVM) show quite a different and, as shown below, erroneous flow pattern. This is substantiated by a quantitative comparison with experiments of the profiles of the axial and tangential velocity components and the rms of their fluctuations at the 3 mm cross-section, Figs 8 and 9. It is noted that the LES on the 6M grid reproduced best the experimental data, but the DES and RSM (both using the coarse, 2M grid) show also good agreement with the measurements. Since the DES employs LES in the flow bulk, it is likely that the LES would also reproduce well the helical vortex pattern on the 2M grid provided the near-wall region (with the average ) is treated by some wall-function, or using a near-wall RANS approach as practiced by DES. This option was not tested here since, as already stated above, the near-wall resolution on the 2M grid is far too coarse for a proper LES. In contrast, both EVMs notably failed, as shown by typical (unphysical) solid-body profiles of the tangential velocity and a wrong recirculation pattern. Moreover, the k-? realizable and k-? SST, LEVMs on the 2M grid could not maintain the unsteadiness, most probably due to excessive numerical diffusion that suppressed the natural instabilities, and resulted in stationary solutions failing to capture the twin rope precessing structures. | |||
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Revision as of 15:05, 27 November 2018
Vortex ropes in draft tube of a laboratory Kaplan hydro turbine at low load
Application Area 6: Turbomachinery Internal Flow
Application Challenge AC6-15
Evaluation
COMPARISON OF TEST DATA AND CFD
We present first a summary of the performance of various URANS models on a typical RANS grid of 2M nodes and compare with the LES on 6M and DES on 2M grid for the low part-load of . All RANS calculations were performed in unsteady mode. The time-averaged streamline patterns and colour contours of the fields of the vertical velocity in Fig. 7 clearly show that the RSM, DES and LES all give very similar results; whereas the linear eddy viscosity models (LEVM) show quite a different and, as shown below, erroneous flow pattern. This is substantiated by a quantitative comparison with experiments of the profiles of the axial and tangential velocity components and the rms of their fluctuations at the 3 mm cross-section, Figs 8 and 9. It is noted that the LES on the 6M grid reproduced best the experimental data, but the DES and RSM (both using the coarse, 2M grid) show also good agreement with the measurements. Since the DES employs LES in the flow bulk, it is likely that the LES would also reproduce well the helical vortex pattern on the 2M grid provided the near-wall region (with the average ) is treated by some wall-function, or using a near-wall RANS approach as practiced by DES. This option was not tested here since, as already stated above, the near-wall resolution on the 2M grid is far too coarse for a proper LES. In contrast, both EVMs notably failed, as shown by typical (unphysical) solid-body profiles of the tangential velocity and a wrong recirculation pattern. Moreover, the k-? realizable and k-? SST, LEVMs on the 2M grid could not maintain the unsteadiness, most probably due to excessive numerical diffusion that suppressed the natural instabilities, and resulted in stationary solutions failing to capture the twin rope precessing structures.
Contributed by: A. Minakov [1,2], D. Platonov [1,2], I. Litvinov [2], S. Shtork [2], K. Hanjalić [3] —
[1] Institute of Thermophysics SB RAS, Novosibirsk, Russia,
[2] Siberian Federal University, Krasnoyarsk, Russia,
[3] Delft University of Technology, Chem. Eng. Dept., Holland.