OpenLB – Open Source Lattice Boltzmann Code Forums on OpenLB General Topics Drag Coefficient (Cylinder 2D)

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• #1840

Hi, rnrnFirst of all, this is an amazing project! Congratulations to all people that some how collaborated with it. rnrnI am a new user of the OpenLB and I just finished to run one example simulation (cylinder2d). From the books that I read, using the momentum exchange approach, we can calculate the drag coefficient using the following equation: cd = (2*Fx)/(rho*D*(u^2)). Where D = diameter of the cylinder, rho = density, u = LBM velocity. rnrnThe example simulation (Re=20) gave me Drag = 5.63043, but using the equation above I can not reach a reasonable drag coefficient for Re=20. Could somebody help me?rn rnBest,rnPedro Paulo.

#2398

Hi Pedro Paulo,rnrnas you can see, the drag coefficient is calculated by the force “”Fx””. Can you tell me which value you used to the force?rnrnBest,rnLiliana.

#2401

I assumed that the 5.63043 (drag given by the code) was the Fx in the equation. This clearly did not gave me a good (reasonable) result when I also assumed the diameter of the cylinder in number of links/nodes and u = 0.02. rn

#2402

Dear Pedro Paulo,rnrnActually, the drag given by the code is already the drag coefficient (CD).rnrnBest,rnLiliana.

#2403

Hi Liliana,rnrnMakes sense when I consider the force factor (physForce=0.5). rnThank you. rnrnBest, rnPedro Paulo.

#2434
aliiab
Member

Hi, Liliana, Pedro!rnrnI am intrested in drag force too. I simulate the fluid flow in a rectangular channel with sphere in its center. So if

Quote:
drag given by the code is already the drag coefficient

, it should be equal to the 0.47 for Re = 10 000 (https://en.wikipedia.org/wiki/Drag_coefficient). But in my simulation it’s about 5.0, but it is not a constant value (fluid velocity and density are constant in this simulation). Can you explain the reason of such difference?rnAlso can you tell me, how OpenLB calculates the Reynolds number?rnrnThanks,rnAlina.

#2438
mathias
Keymaster

Dear Alina,rnrnsome years ago we simulated the flow around a falling ball (cf. http://optilb.org/openlb/others 3d falling ball). We got good results for Re=10,100,1000 and 5000 with D3Q19 BGK-Boltzmann model and also for Re=10000 and 100000 with D3Q19 Smago BGK-Boltzmann model.rnrnMaybe your boundary conditions are not right or your resulotion is not fine enough.rnrnBestrnMathias

#2440
aliiab
Member

Mathias, thanks. But it’s still not clear: SuperLatticePhysDrag3D (like it’s used in cylinder3d example) presents drag force or drag coefficient?

#2442
mathias
Keymaster

SuperLatticePhysDrag3D computes the drag coefficient. Best Mathias

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