UFR 2-11 Evaluation: Difference between revisions
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The same is to a major extent correct regarding the PSD of the lift | The same is to a major extent correct regarding the PSD of the lift | ||
coefficient and mean pressure distributions over the airfoil shown in | coefficient and mean pressure distributions over the airfoil shown in | ||
[[UFR_2-11_Evaluation#figure7|Figure 7]]. | [[UFR_2-11_Evaluation#figure7|Figure 7]]. | ||
Figure 7(a) | [[UFR_2-11_Evaluation#figure7|Figure 7(a)]] also suggests that all | ||
the simulations are | |||
capable of predicting the experimental spectra, particularly the main | capable of predicting the experimental spectra, particularly the main | ||
shedding frequency and its harmonic, fairly well, whereas | shedding frequency and its harmonic, fairly well, whereas | ||
[[UFR_2-11_Evaluation#figure7|Figure 7(b)]] | |||
reveals a systematic difference between the predicted and measured | reveals a systematic difference between the predicted and measured | ||
pressure on the suction side. Note also that SAS results somewhat | pressure on the suction side. Note also that SAS results somewhat | ||
deviate from those of SA DES and are closer to the experiment. The same | deviate from those of SA DES and are closer to the experiment. The same | ||
trend is observed for the integral lift and drag forces | trend is observed for the integral lift and drag forces | ||
concrete reason | ([[UFR_2-11_Evaluation#table5|Table 5]]). | ||
A concrete reason for the difference between the SAS and DES predictions | |||
is not clear but, in any case, it is not significant when compared to | is not clear but, in any case, it is not significant when compared to | ||
e.g. the differences between DES and URANS or between different URANS | e.g. the differences between DES and URANS or between different URANS |
Revision as of 10:36, 7 September 2011
High Reynolds Number Flow around Airfoil in Deep Stall
Flows Around Bodies
Underlying Flow Regime 2-11
Evaluation
Comparison of CFD Calculations with Experiments
A dramatic improvement in solution fidelity for DES compared to URANS, first reported by Shur et al. [22], was observed in the extensive cross-validation exercise carried out in the EU FLOMANIA project [4]. Figure 4 depicts the relative deviation from experimental drag achieved by DES and URANS within this work.
Figure 4: Comparison of URANS and DES for the prediction of mean drag coefficient for the NACA0012 airfoil at α = 60°. Results of 11 different simulations conducted by different partners with different codes and turbulence models within the EU FLOMANIA project [4]. Experimental data cited by Hoerner [6] are used as reference. |
The effect of spatial and temporal numerical schemes on DES was
investigated for the NACA0012 case at α = 45° by
Shur et al. (2004)
[23]. Using a localised "hybrid" convection
scheme [29] (in which 4th
order central differences are applied within the vortical wake region)
and a 2nd order temporal integration was seen to resolve fine turbulent
structures to a scale near to that of the local grid spacing. Switching
the convection scheme to 3rd order upwind or, to a lesser extent, the
temporal scheme to 1st order was seen to strongly damp the fine
vortices in the wake (Figure 5). Correspondingly, a strong under-
prediction of the Power Spectral Density (PSD) of the drag and lift
forces at higher frequencies was observed. The effect on the mean
forces and pressure distributions was however comparatively mild for
this case.
Figure 5: Effect of different spatial and temporal numerical schemes on the resolved wake structures of the NACA0012 at α = 45° [23]. "Hybrid" refers to the localized blending between 4th order central and 3rd or 5th order upwind convection schemes proposed by Travin et al. [29] |
Having clearly demonstrated the benefits of DES compared to URANS [4, 22]
(Figure 4),
no further URANS computations were carried out in the
successor EU project DESider [5], and the
focus shifted to cross-comparison of different turbulence-resolving approaches.
Figure 6
compares flow visualizations from 3 simulations carried out with the
use of different approaches (k – ω
SST SAS and DES based on SA and CEASM
RANS models) in the form of instantaneous fields of the vorticity
magnitude. They reveal quite similar flow and turbulent structures thus
supporting a marginal sensitivity of the simulations to the turbulence
modelling approach and numerics used.
Figure 6: Comparison of snapshots of vorticity magnitude from three simulations |
The same is to a major extent correct regarding the PSD of the lift
coefficient and mean pressure distributions over the airfoil shown in
Figure 7.
Figure 7(a) also suggests that all
the simulations are
capable of predicting the experimental spectra, particularly the main
shedding frequency and its harmonic, fairly well, whereas
Figure 7(b)
reveals a systematic difference between the predicted and measured
pressure on the suction side. Note also that SAS results somewhat
deviate from those of SA DES and are closer to the experiment. The same
trend is observed for the integral lift and drag forces
(Table 5).
A concrete reason for the difference between the SAS and DES predictions
is not clear but, in any case, it is not significant when compared to
e.g. the differences between DES and URANS or between different URANS
approaches (see Figure 4).
This justifies the above conclusion on the
weak sensitivity of the predictions to the turbulence modelling
approach and numerics used in different turbulence-resolving
simulations.
Contributed by: Charles Mockett; Misha Strelets — CFD Software GmbH and Technische Universitaet Berlin; New Technologies and Services LLC (NTS) and Saint-Petersburg State University
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