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Drag Coefficient (Cylinder 2D)

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

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    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.


    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.


    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


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


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


    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

    drag given by the code is already the drag coefficient

    , it should be equal to the 0.47 for Re = 10 000 ( 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.


    Dear Alina,rnrnsome years ago we simulated the flow around a falling ball (cf. 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


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


    SuperLatticePhysDrag3D computes the drag coefficient. Best Mathias

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