I have a question on wind flow around the building using CFD. Why the most of the people are using steady RANS (RANS) models instead of Transient (URANS and LES). The flow will be stochastic turbulence in nature, but most of the reputed papers on wind flow around the building, ventilation, pedestrian comfort etc.., are used SRANS (STEADY RANS), models.
Thanks advance
looking forward the answers.
Rajasekarababu,
In most of one phase industrial applications, RANS is a pragmatic choice. This is explained by the robustness, ease of use and efficiency of the RANS.
However most of multiphase flows are highly transient and steady state approach does not work.
Some industrial applications expose limitations to the RANS modelling approach. For these applications, LES and DES provide a viable alternative at the expense of increased complexity and simulation time.
Regards
When you use the RANS approach that implies that the flow you are simulationg is statistically steady. This is exploited by solving the steady equations for the statistical velocity field and allow for approaching a simplified model that, for complex engineering flows, is largely adopted. As a counterpart, the closure turbulence model must be properly tuned for each problem.
Dear Rajasekarababu, I call you to see my discussion with Pr Markatos on this issue see link: https://www.researchgate.net/messages/485265837
Dear Rajasekarababu,
while results from LES simulations on urban canopy flows and dispersions outperformed those from RANS, they have two main disadvantages compared to RANS: i) the computational costs and ii) treatment with boundary conditions. Indeed, these both disadvantages are case-dependent, and from this point of view an LES, or DNS, serve rather as a research tool. For practical solutions of engineering problems RANS simulations are "good enough", and for this reason many guidelines (see, e. g., COST 732 action) for their use and validation in cases of urban canopy flows have been developed. Nevertheless, I would like to recommend the very recent paper (open access) about this topic from B. Blocken: https://link.springer.com/article/10.1007/s12273-018-0459-3
Besides what is already mentioned it is also a question of the costs. LES simulations require significantly more computational power compared to steady RANS. Also URANS (depending on the application) requires more iterations due to a small time step if you are interested in time dependent effects.
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