|
[Sponsors] |
High Residual values for converged steady state computations |
|
LinkBack | Thread Tools | Search this Thread | Display Modes |
November 16, 2018, 00:59 |
High Residual values for converged steady state computations
|
#1 |
New Member
CFD
Join Date: Jul 2016
Posts: 2
Rep Power: 0 |
An equation of the form given below:
du/dt+R=0 reaches a steady state solution computationally, when the residual (R) goes to zero. In my CFD simulations, I have observed that in many cases, the Residuals (R) are as high as 0.0004 - 0.01, but the solution has converged to steady state (based on Cl, Cd values and surface pressure (Cp)). However, as per the above equation, since R is not close to zero, we should still see a changing solution. How does one explain this discrepancy? |
|
November 16, 2018, 03:34 |
|
#2 | |
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,865
Rep Power: 73 |
Quote:
There is no discrepancy. First, Cl, Cd, Cp are integrals of local values over a body and do not determine a steady state that implies a norm on all the values of the acceleration field. Second, the values means nothing if the problem is not properly scaled (non-dimensionalize) in such a way that all the terms are of O(1). You can get 0.01 max value acceptable if the fields are dimensional and the order of the velocity is not unitary |
||
November 16, 2018, 08:15 |
|
#3 |
Senior Member
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,750
Rep Power: 66 |
The solution converges when u is (asymptotically) constant regardless of what du/dt is and regardless of what R is. R need not be zero for u to converge. This is a technicality but important in CFD because you solve not the continuous du/dt = 0 problem but you take your governing equation and discretize it which introduces eventually non-zero du/dt and R. Note that your governing equation is satisfied as long as du/dt + R = 0, which means that any du/dt = -R is a solution to your problem.
In floating point arithmetic there is no zero regardless but only small numbers. So to say du/dt is 0 is simply imposible. You can only say du/dt is less than some tolerance (often labeled TOL) for u. This same problem shows up in even more basic arithmetic like solving for 3*x = 2. Try doing this and you will find that x is not exactly the analytical 2/3 but 0.6666 with funny terminating digits. Does this mean that computers cannot do any arithmetic? No, it means they do a particular type of arithmetic. Last edited by LuckyTran; November 16, 2018 at 11:37. |
|
Tags |
convergance, residuals, steady state |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Suppress twoPhaseEulerFoam energy | AlmostSurelyRob | OpenFOAM Running, Solving & CFD | 33 | September 25, 2018 18:45 |
Free surface issues with interDyMFoam for hydroturbine | oumnion | OpenFOAM Running, Solving & CFD | 0 | October 6, 2017 15:05 |
Compressor Simulation using rhoPimpleDyMFoam | Jetfire | OpenFOAM Running, Solving & CFD | 107 | December 9, 2014 14:38 |
Micro Scale Pore, icoFoam | gooya_kabir | OpenFOAM Running, Solving & CFD | 2 | November 2, 2013 14:58 |
SLTS+rhoPisoFoam: what is rDeltaT??? | nileshjrane | OpenFOAM Running, Solving & CFD | 4 | February 25, 2013 05:13 |