CFD quaestion about my sphere OpenFoam case

mostanad · December 16, 2021, 02:53

Hi every one,
I am trying to simulate the flow around a sphere at Re=1. To this end, I have generated a grid over the sphere using snappyHexMesh and a sphere stl file. The grid is something like below:

I know that for a sphere, in physics, only the relative velocity matters. So in my initial case, I just considered the flow over a sphere with free stream velocity equal to +0.015 corresponding to my Re number. You can see the following velocity field.

which the drag value is in about 3% relative to the literature, and the flow field look reasonable.

I another case, I just specified the velocity on the surface patch of the particle equal to U=-0.015 m/sec, as only the relative velocity matters. I should mention that I have remeshed the domain, and I am only giving a fixedValue velocity BC.

The drag value for this is again equal to my freestream case. However, although the sphere has a velocity to the right, the sphere wake region occurs in the front region of sphere.

So can anyone tell me what is happening with my relative velocity cases, when the drag force is completely matches?

Mohammad

mostanad · December 16, 2021, 06:00

So from the CFD point of view, what is the difference between a free-stream case and moving boundary sphere?!
Any help in this regard is appreciated.

mostanad · December 19, 2021, 03:32

Sorry guys. I know it might be hard to understand my problem. But it is only about keeping the sphere stationary and having freestream flow and moving the sphere in the stagnant flow.
What us MRF? Can it be used for this case?

December 16, 2021, 02:53	CFD quaestion about my sphere OpenFoam case	#1
mostanad Senior Member mohammad Join Date: Sep 2015 Posts: 274 Rep Power: 11	Hi every one, I am trying to simulate the flow around a sphere at Re=1. To this end, I have generated a grid over the sphere using snappyHexMesh and a sphere stl file. The grid is something like below: I know that for a sphere, in physics, only the relative velocity matters. So in my initial case, I just considered the flow over a sphere with free stream velocity equal to +0.015 corresponding to my Re number. You can see the following velocity field. which the drag value is in about 3% relative to the literature, and the flow field look reasonable. I another case, I just specified the velocity on the surface patch of the particle equal to U=-0.015 m/sec, as only the relative velocity matters. I should mention that I have remeshed the domain, and I am only giving a fixedValue velocity BC. The drag value for this is again equal to my freestream case. However, although the sphere has a velocity to the right, the sphere wake region occurs in the front region of sphere. So can anyone tell me what is happening with my relative velocity cases, when the drag force is completely matches? Mohammad

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December 16, 2021, 06:00		#2
mostanad Senior Member mohammad Join Date: Sep 2015 Posts: 274 Rep Power: 11	So from the CFD point of view, what is the difference between a free-stream case and moving boundary sphere?! Any help in this regard is appreciated.

December 19, 2021, 03:32		#3
mostanad Senior Member mohammad Join Date: Sep 2015 Posts: 274 Rep Power: 11	Sorry guys. I know it might be hard to understand my problem. But it is only about keeping the sphere stationary and having freestream flow and moving the sphere in the stagnant flow. What us MRF? Can it be used for this case?