Hello,
I perform simulations for turbulent flows over bluff bodies and I calculate the Drag coefficient .
I don't know what state to choose ( steady or transient ).
When the steady state is sufficient to obtain correct result and when we should run simulations with transient state?
Thanks.
Turbulence is always unsteady. What can be prescribed as "steady" is the statistically averaged variable, provided that the flow has an energy equilibrium condition. In such a case one correct formulation is the steady RANS.
But other pure unsteady statistical formulations exist such as the URANS.
On a totally different ground, LES and DNS solves the unsteady features without using any statistical averaging.
In your specific flow problem, flow around a bluff body, it is difficult to get a good solution in terms of drag using RANS. I suggest to see the literature, this problem is better approached by the LES or hybrid DES formulations.
I do not have literature results to compare my results and know if they are good results
Amine Agriss this is also part of the CFD work, before to use the code on a specific flow problem for which no existing solution can be compared, you have to assess your setting using problems as similar as possible.
You will find a lot of literature about the flow around a bluff body, is one of the most investigated problem.
Filippo Maria Denaro in this case, I think that we should do an experimental study than compare with CFD results. In fact, if it is a geometry that does not exist in the literature, we have no way of knowing if our CFD simulation gives good results..
Amine Agriss what you stated is an old debate between numerical and experimental people. As a matter of fact, when experimental people do their measurements on some new flow problem, they ask to CFD people for numerical results to know if they get good measurements....
In conclusion, start from a test case in literature, similar as much as possible to your flow problem. When you are able to get the same results you can afford your flow problem. Spend some time thinking about the correct grid resolution and start to analyse if your solution is physicall reasoneble. Test with different grids and models. That is the work of CFD people.
Dear Doctor
Go To
As computers have evolved, transient simulators for realistic field operations have become much more accessible, even in real-time. More importantly, transient simulations may even be required under steady-state conditions!
By Torgeir Vanvik, Principal Project Engineer - September 24, 2020
"Steady state models are based on the assumptions that all flow conditions and properties of the system are constant with respect to time. Transient models, however, can handle conditions that change with time and assess the time-dependent impact on thermal and flow predictions.The difference between transient and steady state calculations can be illustrated using a simple example: taking a shower. Given the characteristics of the faucet, the dimensions of the hose, inlet and ambient temperatures, a steady state simulation will only provide the resulting shower water temperature and rate. A transient simulation will, in addition, tell you how long it takes before the shower temperature is pleasant, even if you make adjustments to the valves during warm-up. "
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