UFR 2-12 Evaluation: Difference between revisions
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As a result, sensitivity of predictions of the major characteristics of the flow in the wake of the downstream cylinder | As a result, sensitivity of predictions of the major characteristics of the flow in the wake of the downstream cylinder | ||
to the value of ''L''<sub>z</sub>/''D'' turns out to be marginal (see Figure 5 and Table 4). | to the value of ''L''<sub>z</sub>/''D'' turns out to be marginal (see [[UFR_2-12_Evaluation#figure5|Figure 5]] | ||
At the same time, as seen in Figure 6, the flow features directly related to the | and [[UFR_2-12_Evaluation#table4|Table 4]]). | ||
At the same time, as seen in [[UFR_2-12_Evaluation#figure6|Figure 6]], the flow features directly related to the details | |||
of the flow past the upstream cylinder (its boundary layers separation and shear-layers roll-up) vary with ''L''<sub>z</sub>/''D'' | |||
variation rather significantly. | |||
Other than that, [[UFR_2-12_Evaluation#figure5|Figures 5]], [[UFR_2-12_Evaluation#figure6|6]] suggest that the effect of | |||
''L<sub>z</sub>/''D'' within different turbulence modelling approaches is different and is stronger pronounced for IDDES than for DDES. | |||
These findings should be kept in mind when analyzing agreement with the experiment of the simulations carried out at | |||
''L<sub>z</sub>/''D'' = 3 with the use of different approaches to turbulence representation presented in the next section. | |||
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Revision as of 09:09, 29 October 2012
Turbulent Flow Past Two-Body Configurations
Flows Around Bodies
Underlying Flow Regime 2-12
Evaluation
Comparison of CFD Calculations with Experiments
This section is organized as follows. First (Section 6.1), results of some sensitivity studies are presented and briefly discussed. These include evaluation of such effects as span-size of the domain, compressibility, time sample used for computing the mean flow and turbulent statistics, and numerical dissipation of the method used. Then, in Section 6.2, a comparison with the experimental data is shown for the main body of simulations carried out within the ATAAC project with the use of the physical and computational problem setups outlined in Section 5.
RESULTS OF SENSITIVITY STUDIES
Effect of span size of domain
As mentioned in Section 4, the aspect ratio of the CT configuration Lz/ D in the BART facility is equal to 12.4. Strictly speaking this demands carrying out simulations exactly at this value of Lz/ D and imposing no-slip boundary conditions on the floor and ceiling of the test section (see Figure 2). However such simulations would be very expensive. Considering this and, also, recommendations of the BANC-I Workshop based on simulations at different Lz/ D with periodic boundary conditions in the spanwise directions, most of the simulations in the ATAAC project were performed at Lz/ D = 3 assuming spanwise periodicity. In order to get an idea on how strong the effect of such a simplification could be, NTS conducted a series of simulations at different Lz/ D. Some results of these simulations are presented below [3].
Figure 4 compares flow visualisations from the SA DDES carried out in the "mandatory"
(Lz/ D = 3) and the widest of the considered domains (Lz/ D = 16)
in the form of instantaneous isosurface of the magnitude of the second eigenvalue of the velocity gradient tensor or
"swirl" quantity, λ2.
The figure is reassuring in the sense that it visibly displays that the narrow-domain simulation resolves not only
fine-grained turbulent eddies but also large, nearly coherent, structures and exhibits all the complex flow features
observed in the visualization of the wide-domain simulation, except for the initial region of the free shear-layer
separated from the upstream cylinder, where a noticeable difference between the two flow-visualizations is observed.
 |
|
| Figure 4: Isosurface of λ2 = 4.0(U0 /D ) from incompressible SA DDES at Lz = 3D and 16D. | |
As a result, sensitivity of predictions of the major characteristics of the flow in the wake of the downstream cylinder
to the value of Lz/D turns out to be marginal (see Figure 5
and Table 4).
At the same time, as seen in Figure 6, the flow features directly related to the details
of the flow past the upstream cylinder (its boundary layers separation and shear-layers roll-up) vary with Lz/D
variation rather significantly.
Other than that, Figures 5, 6 suggest that the effect of
Lz/D within different turbulence modelling approaches is different and is stronger pronounced for IDDES than for DDES.
These findings should be kept in mind when analyzing agreement with the experiment of the simulations carried out at
Lz/D = 3 with the use of different approaches to turbulence representation presented in the next section.
Contributed by: A. Garbaruk, M. Shur and M. Strelets — New Technologies and Services LLC (NTS) and St.-Petersburg State Polytechnic University
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