To test various turbulence models for flow over a building, a standard scaled down case of a building, i.e. surface mounted cube, has been used. Experimental data for such a case is widely available in scientific literature, making it convenient to compare the results obtained from CFD simulations.
Several commonly used turbulence models for external flow in the built-environment have been tested:
- RNG: This is a k-e based turbulence model, derived using renormalized group theory. This refines and improves the standard k-e in case of rapidly strained flows.
- SST: This is a hybrid model with both k-omega and k-e. The k-omega model is well suited for simulating flow in the viscous sub-layer. The k-epsilon model is ideal for predicting flow behavior in regions away from the wall. Therefore it can account for the transport of the turbulent shear stress and can accurately predict flow separations under adverse pressure gradients.
- SST-RM: The SST model (like most RANS models) can underpredict turbulent stresses in separating shear layers. This can lead to lower mixing and hence longer separation zones. The SST model is capable of predicting the separation onset well but to improve the prediction of the separated shear layer modification are made to the original SST model. This model is called SST-RM (RM: Reattachment Modification), it introduces additional production of turbulence in the separated shear layer.
- SAS: The Scale-Adaptive Simulation (SAS) is derived by introducing the von Karman length-scale into the turbulence scale equation. The Scale-Adaptive Simulation can then dynamically change to resolved structures in a URANS simulation, as a consequence an LES-like behavior can be observed in unsteady regions of the flow field while still keeping RANS capabilities in stable flow regions.
- DES: This model is developed to overcome the shortcomings of RANS by using LES in certain areas away from the wall boundary layer and using RANS in the near wall regions. This allows the use of LES without having to refine the near wall mesh layer. A blending function is used in the solver which acts as a switch between RANS and LES resolved regions.
A comparison of the results obtained using the above mentioned models with experimental data is presented in the slider below.
- A cube is mounted on a surface with top of the domain as a no slip wall, the flow enters from the left (inlet colored green), the sides (colored yellow) are given a symmetry boundary condition.
- The flow velocity at the center line: 4 m/s and Reynolds number: 40,000
Velocity contours side
- The velocity contours and streamlines from the side view show that DES shows best agreement in wake length to the experimental and LES results from literature. All RANS models like RNG, SST and SST-RM predict a much longer wake, typical for RANS models.
- The streamlines and the velocity curl contours from the top view show the horse shoe vortex around the cube. It could be observed that the DES and SAS demonstrate higher vorticity compared to RANS models. However, amongst the RANS models the SST-RM model under-predicts vorticity in the horse show vortex.
Velocity profile: Line 1
- As marked in the figure, line 1 is at 0.5*H from the windward side of the cube.
- In the separated boundary layer close to the wall (z/H < 1.1, where z/H =1 is the wall), all turbulence models fail to predict the velocity profile.
- For z/H > 1.1, most turbulence models predict the velocity well. However, it can be observed that SST-RM captures the profile best in the region 1.1 < z/H < 1.3. The SST-RM model introduces additional turbulence production, which helps the reattachment of the separated boundary layer on the top surface of the cube and hence produced better results.
- The results from DES simulations do not match well to the experimental values close to the wall, this is expected as DES only uses LES away from the walls. Close to the wall a RANS model is used, also the switch between LES and RANS is based on a blending function. Hence the overall effect of a RANS model and a switch function does not help in resolving the flow near the wall accurately.
Velocity profile: Line 2
- As marked in the figure, line 2 is at the trailing edge of the cube.
- In the reattached boundary layer zone close to the wall (z/H < 1.1, where z/H =1 is the wall), SST-RM predicts the profile with high accuracy as it is designed to capture reattachment better.
- For 1.1 < z/H < 1.4, all turbulence models under-predict predict the velocity well. However, it can be observed that SST-RM captures the profile with the least error.
Velocity profile: Line 3
- As marked in the figure, line 3 is in the wake at distance H from the trailing edge of the cube.
- In the reattached boundary layer zone close to the wall (z/H < 0.5, where z/H =0 is the surface), SAS model predicts the profile with high accuracy as it is designed to capture reattachment better. Out of the RANS models, SST-RM seems to be the most reasonable in this regards.
- For 0.6 < z/H < 1.4, all turbulence models under-predict the velocity. However, it can be observed that SST-RM captures the profile with the least error