Discretization Error vs Residual Error?

dancfd · March 20, 2012, 23:09

Hello all,

I just ran a sensitivity study on my case to see if I could improve my accuracy by using SFCD (2nd order scheme for convection terms) vs linearUpwind (1st order accurate). All else being equal, the SFCD case had a p-residual of 5e-1 and the linearUpwind case had a p-residual of 10e-3. I expected better accuracy from the 2nd order accurate case, but the residuals appear to show that was not the case. I am running simpleFoam on a 2D airfoil. Could anyone please suggest why the residual is higher for the 2nd order scheme?

More strange behaviour: the SFCD took LESS time to execute the case, and resulted in values of Cd and Cm that were farther from the experimental data than the upwind scheme. I expected different. Can anyone explain this?

Thanks,

Dan

akidess · March 21, 2012, 03:49

I always thought linearUpwind is 2nd order upwind. Is plain "upwind" what you were intending to compare to?

dancfd · March 26, 2012, 21:05

Hello Anton,

Thanks for finding my error, I appreciate it.

Regards,
Dan

March 20, 2012, 23:09	Discretization Error vs Residual Error?	#1
dancfd Senior Member Daniel Join Date: Jul 2009 Location: Montreal, Canada Posts: 156 Rep Power: 17	Hello all, I just ran a sensitivity study on my case to see if I could improve my accuracy by using SFCD (2nd order scheme for convection terms) vs linearUpwind (1st order accurate). All else being equal, the SFCD case had a p-residual of 5e-1 and the linearUpwind case had a p-residual of 10e-3. I expected better accuracy from the 2nd order accurate case, but the residuals appear to show that was not the case. I am running simpleFoam on a 2D airfoil. Could anyone please suggest why the residual is higher for the 2nd order scheme? More strange behaviour: the SFCD took LESS time to execute the case, and resulted in values of Cd and Cm that were farther from the experimental data than the upwind scheme. I expected different. Can anyone explain this? Thanks, Dan

March 21, 2012, 03:49		#2
akidess Senior Member Anton Kidess Join Date: May 2009 Location: Germany Posts: 1,377 Rep Power: 30	I always thought linearUpwind is 2nd order upwind. Is plain "upwind" what you were intending to compare to? __________________ On twitter @akidTwit Spend as much time formulating your questions as you expect people to spend on their answer.

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March 26, 2012, 21:05		#3
dancfd Senior Member Daniel Join Date: Jul 2009 Location: Montreal, Canada Posts: 156 Rep Power: 17	Hello Anton, Thanks for finding my error, I appreciate it. Regards, Dan