# D'Alembert paradox + kutta condition

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 January 4, 2006, 02:31 D'Alembert paradox + kutta condition #1 snegan Guest   Posts: n/a hi all, I am presenting 2 questions to the CFD commmunity 1. According to the D'Alembert paradox, for an inviscid incompressible flow, the drag for a body is zero. But through experiments this is a non zero quantity. Suppose if one is doing a numerical simulation of this what will cause the non-zero drag in the compuational methods. Is it a numerical dissipation associated with the scheme is responsible? pls do clarify me in this regard. 2. How will u incorporate the kutta condition at the trailing edge for the difference scheme, Finite volume scheme and the finite element scheme? -snegan

 January 4, 2006, 04:24 Re: D'Alembert paradox + kutta condition #2 Tian_FB Guest   Posts: n/a i will say some thing about first q, the drag for a body is zero in the direction of incoming flow velocity vector.so,does the drag in your simulation parallel to the velocity direction? if the non-zero drag in the compuational methods is very small,it is,maybe, a numerical dissipation associated with the scheme is responsible.

 January 6, 2006, 11:45 Re: D'Alembert paradox + kutta condition #4 AnotherCFDUser Guest   Posts: n/a Although a Kutta condition is required for inviscid simulations most people choose not to apply this condition, instead relying on the inherent numerical dissipation of the scheme (from truncation errors) or added artificial viscosity. This means that they are relying on the fact that the discrete problem we are actually solving is related to an advection-diffusion equation rather than the advection problem described by the continuum governing equations. As we are now dealing with a 'viscous' problem there is now no paradox - although there will be drag. Thus the solution for the drag forces should tend towards zero asymptotically as the mesh is refined (a good verification test for a CFD code). Alarmingly a number of well known commercial CFD codes appear to asymptote to a small but non-zero value for drag! If we are dealing with a potential (irrotational) flow then even in 3D there should be no drag force. Although the governing equations convect vorticity, there is no mechanism for its generation.

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