Effect of inlet turbulence generation methods on large-eddy simulation results

Wang, Yi ORCID: 0000-0002-7288-9465 (2022). Effect of inlet turbulence generation methods on large-eddy simulation results. University of Birmingham. Ph.D.

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Abstract

For large eddy simulation, it is critical to choose the suitable turbulent inlet boundary condition as it significantly affects the calculated flow field. In the thesis, the effect of different inlet boundary conditions including random method (RAND), Lund method and divergence-free synthetic eddies method (DFSEM) on the flow in a channel with a hump are investigated through large-eddy simulation. The simulation results are further compared with experimental data. It has been found that turbulence is nearly fully developed in the case based on Lund method, not fully developed in the case based on DFSEM and not developed in the case based on RAND method. In the flow region before the hump, mean velocity profiles in the case applying Lund method gradually fit the law of the wall as main flow moves towards the hump, but the simulation results based on RAND and DFSEM methods cannot fit the wall function. In the flow region after the hump, cases applying Lund and DFSEM methods could relative precisely predict the size of turbulent bubble and turbulent statistics profiles. While the case based on RAND method cannot capture the positions of flow separation and re-attachment point and overestimates the turbulent bubble size. From this part, it could be found that different turbulent inflow generation methods have a manifested impact on the flow separation and re-attachment after the hump. If the coherent turbulence is maintained in the approach flow, even though turbulent intensity is not large enough, the simulation can still predict the flow separation and turbulent bubble size relative precisely. From the results, even if the simulation based on the Lund method and DFSEM have a better performance than the simulation based on the RAND method, the results still cannot agree well with the experimental data. There are some possible reasons that result in the big difference. Firstly, the spanwise width of the simulation domain is relatively small. Additionally, the LES sub-grid scale model has a slight impact on the results according to the previous research. Finally, the DFSEM is sensitive to the surface normal gradient schemes.
Beside the choice of the turbulent inlet boundary condition (IBC) methods, the settings of each turbulent IBC method are critical as well. In this thesis, the effect of setting different IBC methods in LES using the Lund method, the divergence-free synthetic eddies method (DFSEM) and the digital filter method (DFM) on the simulation of the boundary layer over a flat plate is investigated. This research also fully explained the influences of different IBC methods on the results of turbulent kinetic energy budget terms, and it is found that the DFM and the DFSEM both have good performance. In addition, for the DFM and DFSEM, the input parameter such as turbulent length scales are hard to set generally without prior knowledge of the flow field. It is found that the simulation results based on these two methods with constant turbulent length scales of 0.4-1.0 times of the value of boundary layer thickness could agree well with the DNS results to a great extent after about 10 boundary layer thickness along the streamwise direction. Overall, it could be recommended that the DFM and the DFSEM can be used in this case with the constant input turbulent length scale about 0.4-1.0 times of the value of boundary thickness.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):