UFR 4-19 Evaluation: Difference between revisions
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|align="center"|'''Figure 14:''' Strong shock-wave case: Velocity distributions | |align="center"|'''Figure 14:''' Strong shock-wave case: Velocity distributions | ||
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From the first two measurements stations it is evident that the experimental data show a | |||
recirculation region formed after the ?strong? shock-wave on the top wall of the diverging | |||
part of the diffuser. | |||
From fig.14 it can be seen that the EVMs are able to capture the boundary layer separation | |||
but with a smaller negative velocity resulting in a smaller recirculation region. | |||
Additionally,the LEVM computes also a small recirculation zone on the bottom wall, | |||
which has an impact on the development of the attached boundary layer further downstream. | |||
As a result,the attached boundary layers of the LEVM are thicker in comparison with the | |||
computed boundary layers of the other two turbulence models and in relation to the | |||
experiment,fig.14. | |||
For the first station at x/hthroat | |||
Revision as of 06:50, 13 April 2016
Converging-diverging transonic diffuser
Confined flows
Underlying Flow Regime 4-19
Evaluation
Comparison of CFD Calculations with Experiments
For both "weak" and "strong" Mach number cases,for comparison with the calculations, experimental data for four measurements stations after the shock-wave (h1, h2, h3, h4) are selected for the longitudinal velocity as well as pressure distributions along the bottom and the top wall of the diffuser. A representative figure of the shock-wave positions and the measurement stations is shown in fig.9.The x-location of the axial measurement stations are non-dimensionalized with the diffuser throat height,resulting in the following four stations: h1 = 2.822, h2 = 4.611, h3 = 6.340 and h4 = 7.493.
Figure 9: Shock-waves positions and experimental measurements |
The weak Mach number case
The modelling results of the three adopted turbulence models are compared with the available experimental data for the "weak" Mach number case in figs.10 and 11.
All three turbulent models are able to capture the maximum velocity value. The main differences are observed in the boundary layer in the wall regions. The EVMs have similar behavior as they underpredict the axial velocity,especially in the top wall region of the diverging part of the diffuser. On the other hand,the RSM overpredicts the axial velocity on both walls providing thinner boundary layers. The pressure coefficient distributions along both walls of the diffuser are plotted in fig.11.
All the turbulence models capture the pressure coefficient distribution in the converging and the diverging part of the diffuser with a very good accuracy.However,in the region near the shock-wave (the region where the minimum pressure value is computed),the good level of accuracy is not maintained. The EVMs compute a less intense shock-wave,as it can be concluded from fig.11. Regarding the RSM,it computes a shock-wave with a greater value for the maximum Mach number,giving a lower pressure coefficient value.
The static pressure (Pa) and the Mach number contours for all the turbulence models are presented in figs.12 and 13. The flow field representation is almost the same for all the models with the RSM providing the largest Mach number value in comparison with the EVMs.
Figure 12: Weak shock wave case: Contours of static pressure |
Figure 13: Weak shock wave case: Contours of Mach number |
The strong Mach number case
The computed velocity distributions for the EVMs and RSM in comparison with the experimental data are shown in fig.14.
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Figure 14: Strong shock-wave case: Velocity distributions |
From the first two measurements stations it is evident that the experimental data show a recirculation region formed after the ?strong? shock-wave on the top wall of the diverging part of the diffuser. From fig.14 it can be seen that the EVMs are able to capture the boundary layer separation but with a smaller negative velocity resulting in a smaller recirculation region. Additionally,the LEVM computes also a small recirculation zone on the bottom wall, which has an impact on the development of the attached boundary layer further downstream. As a result,the attached boundary layers of the LEVM are thicker in comparison with the computed boundary layers of the other two turbulence models and in relation to the experiment,fig.14. For the first station at x/hthroat
Contributed by: Z. Vlahostergios, K. Yakinthos — Aristotle University of Thessaloniki, Greece
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