Learning Forum

[ahmed.hassan902]

Hello everyone!

I am still beginner using ansys fluent.

I was simulating the flow over a semi-circle obstacle and I was expecting to find symmetrical velocity contour over the obstacle ,as I was not expecting to find recirulation zone downstream.

I don't know what's wrong with my setup.

[SRP]

 

Hi,

you might be encountering flow separation and the formation of a recirculation zone behind the semi-circle, which is quite common for flow over bluff bodies

 

[ahmed.hassan902]

Yeah I know that the formation of a recirculation zone behind a bluff body is common ,but I don't think it is common for inviscid flow.

[Petros Diakoumakos]

Hi,

Reynolds number must be less than 5 in order for the flow to remain attached. For this case based on the contour and assuming air as the medium, the cylinder diameter should be ~5μm to have a Re<5, which I guess it is not. Thus you are in the turbulent regime and flow recirculation is expected.

[ahmed.hassan902]

Since I have chosen the inviscid model, which means viscosity is zero, the Reynolds number (Re) is approximately infinity. Therefore, by changing the diameter, I don't think it will change anything. Does this mean that I will always expect separation in inviscid flow?

[Petros Diakoumakos]

Even if flow is inviscid, recirculation will be introduced due to artificial viscosity (from numerical schemes and spatial discretization) which in turn will create vorticities. So there will be "separation" behind objects.

Solving the potential equation will yield the results you expect as irrotationality of the flow is assumed. Since you have a simple case, to get an idea of what the flow field would look like you can initialize using hybrid initialization which does that; solves Laplace's equation to obtain the velocity field.

Otherwise you can limit this artificial viscosity by further refining the grid and using higher order discretizations, which might still not completely give you what you want.