Density- vs. Pressure-Based Solvers

Dwayne · July 5, 2013, 13:05

Hey Guys,

I know the difference between density-based and pressure-based flow solvers and I know their extensions to regimes of arbitrary Machnumbers. So preconditioning and density-velocity-pressure-coupling.

My question is how I decide wich solver I should use in what situation. What are the advantages and disadvantages of both? I only found out that density-based solvers are better in solving shocks.

Thanks for help!

immortality · July 7, 2013, 04:07

there may be not a clear boundary between them.
Density based solvers are originated for high velocity flows,although pressure based have improved a lot for high velocities too but still density based's are better in mach around ond and higher,i think it depends on the problem if others add something is nice.
I'd like to know preconditioning if you tell me a bit.

Dwayne · July 7, 2013, 06:51

Quote:

Originally Posted by immortality

there may be not a clear boundary between them.
Density based solvers are originated for high velocity flows,although pressure based have improved a lot for high velocities too but still density based's are better in mach around ond and higher,i think it depends on the problem if others add something is nice.
I'd like to know preconditioning if you tell me a bit.

Thanks, in compressible flows information is carried with flow velocity and also with speed of sound. So you have to consider both velocities is your stability consideration. For explicit methods the CFL-Condition must be fulfilled wich causes very small timesteps because of the high speed of sound.

A system is than called bad conditioned. In other words: A system is bad conditioned because of the large disparity of the eigenvalues of the jacobian. By multiplying the timederivation with an preconditioning-matrix you can force the eigenvalues to be of similiar order. This also solves the accuracy problem of density based solvers at low mach numbers.

immortality · July 7, 2013, 08:13

thanks,may multiplying a matrix to time derivation cause an inaccuracy in each time step in real unsteady problems?
and what nSweep is better in your opinion?

Dwayne · July 7, 2013, 08:53

That's right the preconditioned equations have only the stationary solution in common with the orginal equations. But the time accuracy can be restored by dual time stepping. I just found out that this is really time consuming so this is an disadvantage of preconditioned density-based solvers. Thanks for the hint.

What do you mean with nsweeps? Density-based solvers don't need iterative methods I thought or am I wrong?

July 5, 2013, 13:05	Density- vs. Pressure-Based Solvers	#1
Dwayne New Member Join Date: Jul 2013 Posts: 11 Rep Power: 12	Hey Guys, I know the difference between density-based and pressure-based flow solvers and I know their extensions to regimes of arbitrary Machnumbers. So preconditioning and density-velocity-pressure-coupling. My question is how I decide wich solver I should use in what situation. What are the advantages and disadvantages of both? I only found out that density-based solvers are better in solving shocks. Thanks for help! immortality likes this.

July 7, 2013, 04:07		#2
immortality Senior Member Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,208 Rep Power: 26	there may be not a clear boundary between them. Density based solvers are originated for high velocity flows,although pressure based have improved a lot for high velocities too but still density based's are better in mach around ond and higher,i think it depends on the problem if others add something is nice. I'd like to know preconditioning if you tell me a bit. __________________ Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked.

July 7, 2013, 08:13		#4
immortality Senior Member Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,208 Rep Power: 26	thanks,may multiplying a matrix to time derivation cause an inaccuracy in each time step in real unsteady problems? and what nSweep is better in your opinion? __________________ Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked.

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July 7, 2013, 08:53		#5
Dwayne New Member Join Date: Jul 2013 Posts: 11 Rep Power: 12	That's right the preconditioned equations have only the stationary solution in common with the orginal equations. But the time accuracy can be restored by dual time stepping. I just found out that this is really time consuming so this is an disadvantage of preconditioned density-based solvers. Thanks for the hint. What do you mean with nsweeps? Density-based solvers don't need iterative methods I thought or am I wrong?