Should prop wake dissipate before reaching outlet domain? (Transient))

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 March 8, 2022, 01:19 Should prop wake dissipate before reaching outlet domain? (Transient)) #1 New Member   Varun V P Join Date: Jun 2020 Posts: 11 Rep Power: 4 Hello, We are simulating some test cases for a propeller, which requires that we do a transient simulation. The common advice on sizing of the mesh is that the pressure outlet should be far enough so that the wake dies out before it reaches it. But since this is a transient simulation, it takes a lot of time steps for the wake to fully reach the outlet. And even when it is doing so, I notice on my lift and drag monitors that the forces are converged. Therefore, I was thinking if I should run a steady state simulation first, wait for wake to fully develop, and then switch to transient?

March 8, 2022, 05:16
#2
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Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,290
Rep Power: 67
Quote:
 Originally Posted by VarunV Hello, We are simulating some test cases for a propeller, which requires that we do a transient simulation. The common advice on sizing of the mesh is that the pressure outlet should be far enough so that the wake dies out before it reaches it. But since this is a transient simulation, it takes a lot of time steps for the wake to fully reach the outlet. And even when it is doing so, I notice on my lift and drag monitors that the forces are converged. Therefore, I was thinking if I should run a steady state simulation first, wait for wake to fully develop, and then switch to transient?

Well, it depends on what you really want to simulate. Let me clarify:

1) you want to simulate the whole transient, that is the flow problem starting from some suitable physical initial and boundary conditions.
2) you are interested only in the final fully developed unsteady flow when an equilibrium is reached.

Clearly, only case 2 can be started from arbitrary initial conditions and you can use a converged steady solution.

Be aware that, depending on the formulation you are using (DNS/LES/DES/URANS) the things can be more complex.

March 8, 2022, 08:57
#3
New Member

Varun V P
Join Date: Jun 2020
Posts: 11
Rep Power: 4
Quote:
 Originally Posted by FMDenaro Well, it depends on what you really want to simulate. Let me clarify: 1) you want to simulate the whole transient, that is the flow problem starting from some suitable physical initial and boundary conditions. 2) you are interested only in the final fully developed unsteady flow when an equilibrium is reached. Clearly, only case 2 can be started from arbitrary initial conditions and you can use a converged steady solution. Be aware that, depending on the formulation you are using (DNS/LES/DES/URANS) the things can be more complex.
I want to simulate the forces acting on the propeller when it is exposed to oblique flow. Barring the initial wake development, I would be interested in the force variation as the blades go through a complete revolution. So I guess case 2 is what I'm looking for.

So when I go with case 2, if I understand correctly, the steady state simulation will converge with a full wake, which the transient simulation will finally refine. Am I correct?

 Tags propellers, transient 3d, wake modeling