# Numerical Scheme Correlation

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 March 15, 2014, 09:35 Numerical Scheme Correlation #1 New Member   Matt Holford Join Date: Oct 2013 Posts: 4 Rep Power: 12 Hi all, I have quick question. I am correlating the flow performance of a water valve with CFD simulations. The target flow rate I am trying to achieve is 52L/min. 1st order numerical scheme, CFD tells me I get 82L/min. 2nd order I get 53L/min 3rd order is 62L/min The software I'm using is Autodesk Simulation CFD 2014 which is FEM based. The mesh is at an independent state with a low y+ on average, thus the mesh isn't a problem. In this case, why would the 1st and 3rd order schemes be yielding me odd results?

 March 15, 2014, 10:46 #2 Senior Member   cfdnewbie Join Date: Mar 2010 Posts: 557 Rep Power: 20 on the same mesh, different numerical schemes will give you (in under resolved scenarios usually) different results. Are you perfectly resolved? Is your flow laminar or turbulent? What is your Reynolds number? What time discretization do you use? Do you have cavitation going on? You need to study the source of the discrepancies you are seeing, and it is a very common effect to see a difference when changing the spatial discretization, as you are doing.

 March 15, 2014, 10:59 #3 New Member   Matt Holford Join Date: Oct 2013 Posts: 4 Rep Power: 12 By resolved do you mean low magnitude converged residuals? Reynolds number is on average 10^5. It's a steady simulation. I may have cavitation, there's a very small gap, 0.4mm.

 March 15, 2014, 11:40 #4 Senior Member   cfdnewbie Join Date: Mar 2010 Posts: 557 Rep Power: 20 By resolved I mean DNS, not LES or RANS...do you resolve all the flow features? I doubt that Re=10^5 is steady, if you are solving it with a RANS approach, it will strongly depend on the interaction of the numerical scheme and the turbulence model. What equation of state model do you have for water? FMDenaro likes this.

 March 15, 2014, 11:46 #5 New Member   Matt Holford Join Date: Oct 2013 Posts: 4 Rep Power: 12 The turbulence model is k-e, petrov galerkin numerical scheme. By steady, I meant it's not a transient solution, no time dependence.

 March 15, 2014, 11:47 #6 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,793 Rep Power: 71 first and third order accurate schemes have different action of the local truncation error compared to second order one

 Tags correlation, discretization, higher order, numerical scheme, separation