# How to calculate dissipation rate in OpenFOAM

 Register Blogs Members List Search Today's Posts Mark Forums Read November 3, 2021, 15:35 How to calculate dissipation rate in OpenFOAM #1 Senior Member   Farzad Faraji Join Date: Nov 2019 Posts: 119 Rep Power: 4 Hello all I hope you are well. I want to calculate dissipation rate in LES simulation and I find a good code for calculating TKE budget. https://github.com/AndreaDesan/pimpleTKEBudgetFoam But I can not use it since some definitions are different, so I decided to use the definitions in the code(tkeBudget.H); https://github.com/AndreaDesan/pimpl...er/tkeBudget.H Code: ``` volScalarField kSGS = turbulence->k(); //Instantaneous subgrid-scale tke volScalarField kRes = 0.5*(UPrime & UPrime); //Instantaneous resolved tke kTot = kRes + kSGS; volScalarField epsilonSGS = turbulence->epsilon(); volSymmTensorField SRes = symm(fvc::grad(UPrime)); //Instantaneous resolved fluctuating rate of strain volScalarField epsilonRes=-2*nuLam*(SRes && SRes); //Instantaneous resolved dissipation rate epsilonTot = mag(epsilonRes + epsilonSGS); LESResIndex = kRes/max(kSmall,kTot); //Instantaneous ratio of resolved to total (Resolved + SGS) tke``` Now, lets return to my main discussion; a) The dimension of tke is [m2s(-2)] and "tke rate" must be [m2s(-3)] b) the dimension of dissipation rate must be the same, so I calculate for both epsilonSGS and epsilonRes as below; 1- epsilonSGS = Ce*pow(k,3/2)/delta====>[Ce]*[k^3/2]/[delta]====>*[m3s(-3)]/[m]====>m2s(-3) 2- epsilonRes = nu * (gradU && gradU)====>[m2s(-1)]*[?????]=== I calculate "gradU && gradU" in OpenFoam and dimension was s(-2), so we have [m2s(-1)]*[s(-2)]=m2s(-3). basically epsilonSGS and epsilonRes has the same dimension m2s(-3) which is the same as tke rate m2s(-3). From this I think I have calculated the [?????] correctly. Am I correct?   November 3, 2021, 16:28 #2
Senior Member

Join Date: Nov 2019
Posts: 119
Rep Power: 4 Maybe this question is a little stupid one. I always read and heard dissipation rate [m2s(-3)], but I never heard about TKE rate. TKE is [m2s(-2)], so TKE rate must be [m2s(-3)] which is the same as dissipation rate. am I correct?

Quote:
 Originally Posted by farzadmech Hello all I hope you are well. I want to calculate dissipation rate in LES simulation and I find a good code for calculating TKE budget. https://github.com/AndreaDesan/pimpleTKEBudgetFoam But I can not use it since some definitions are different, so I decided to use the definitions in the code(tkeBudget.H); https://github.com/AndreaDesan/pimpl...er/tkeBudget.H Code: ``` volScalarField kSGS = turbulence->k(); //Instantaneous subgrid-scale tke volScalarField kRes = 0.5*(UPrime & UPrime); //Instantaneous resolved tke kTot = kRes + kSGS; volScalarField epsilonSGS = turbulence->epsilon(); volSymmTensorField SRes = symm(fvc::grad(UPrime)); //Instantaneous resolved fluctuating rate of strain volScalarField epsilonRes=-2*nuLam*(SRes && SRes); //Instantaneous resolved dissipation rate epsilonTot = mag(epsilonRes + epsilonSGS); LESResIndex = kRes/max(kSmall,kTot); //Instantaneous ratio of resolved to total (Resolved + SGS) tke``` Now, lets return to my main discussion; a) The dimension of tke is [m2s(-2)] and "tke rate" must be [m2s(-3)] b) the dimension of dissipation rate must be the same, so I calculate for both epsilonSGS and epsilonRes as below; 1- epsilonSGS = Ce*pow(k,3/2)/delta====>[Ce]*[k^3/2]/[delta]====>*[m3s(-3)]/[m]====>m2s(-3) 2- epsilonRes = nu * (gradU && gradU)====>[m2s(-1)]*[?????]=== I calculate "gradU && gradU" in OpenFoam and dimension was s(-2), so we have [m2s(-1)]*[s(-2)]=m2s(-3). basically epsilonSGS and epsilonRes has the same dimension m2s(-3) which is the same as tke rate m2s(-3). From this I think I have calculated the [?????] correctly. Am I correct?   November 8, 2021, 07:27 #3 Senior Member   Andrea Join Date: Feb 2012 Location: Leeds, UK Posts: 179 Rep Power: 14 Not sure from where within code you got "tke rate". Andrea   January 11, 2022, 03:44 #4
Member

Josh Williams
Join Date: Feb 2021
Location: Scotland
Posts: 65
Rep Power: 3 Quote:
 Originally Posted by farzadmech Hello all I hope you are well. I want to calculate dissipation rate in LES simulation and I find a good code for calculating TKE budget. https://github.com/AndreaDesan/pimpleTKEBudgetFoam But I can not use it since some definitions are different, so I decided to use the definitions in the code(tkeBudget.H); https://github.com/AndreaDesan/pimpl...er/tkeBudget.H Code: ``` volScalarField kSGS = turbulence->k(); //Instantaneous subgrid-scale tke volScalarField kRes = 0.5*(UPrime & UPrime); //Instantaneous resolved tke kTot = kRes + kSGS; volScalarField epsilonSGS = turbulence->epsilon(); volSymmTensorField SRes = symm(fvc::grad(UPrime)); //Instantaneous resolved fluctuating rate of strain volScalarField epsilonRes=-2*nuLam*(SRes && SRes); //Instantaneous resolved dissipation rate epsilonTot = mag(epsilonRes + epsilonSGS); LESResIndex = kRes/max(kSmall,kTot); //Instantaneous ratio of resolved to total (Resolved + SGS) tke``` Now, lets return to my main discussion; a) The dimension of tke is [m2s(-2)] and "tke rate" must be [m2s(-3)] b) the dimension of dissipation rate must be the same, so I calculate for both epsilonSGS and epsilonRes as below; 1- epsilonSGS = Ce*pow(k,3/2)/delta====>[Ce]*[k^3/2]/[delta]====>*[m3s(-3)]/[m]====>m2s(-3) 2- epsilonRes = nu * (gradU && gradU)====>[m2s(-1)]*[?????]=== I calculate "gradU && gradU" in OpenFoam and dimension was s(-2), so we have [m2s(-1)]*[s(-2)]=m2s(-3). basically epsilonSGS and epsilonRes has the same dimension m2s(-3) which is the same as tke rate m2s(-3). From this I think I have calculated the [?????] correctly. Am I correct?

You use UPrime to calculate epsilonRes, but from equation 14 in this paper , it appears this would give you epsilonSGS. Instead, based on , you should use epsilonRes_ij = nu * (gradU & gradU). Then you can probably get it as a scalar as you appear to have it by taking the trace?
I have also seen epsilonRes been calculated as epsilonRes = dkRes/dt .

 Wacławczyk, M., Pozorski, J. and Minier, J.P., 2004. Probability density function computation of turbulent flows with a new near-wall model. Physics of Fluids, 16(5), pp.1410-1422.
 Jayaram, R., Jie, Y., Zhao, L. and Andersson, H.I., 2020. Clustering of inertial spheres in evolving TaylorGreen vortex flow. Physics of Fluids, 32(4), p.043306.  Tags dissipation rate, les, tke les Thread Tools Search this Thread Show Printable Version Email this Page Search this Thread: Advanced Search Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are Off Pingbacks are On Refbacks are On Forum Rules Similar Threads Thread Thread Starter Forum Replies Last Post wyldckat OpenFOAM Installation 2 December 8, 2021 00:18 Jibran OpenFOAM Announcements from Other Sources 2 November 4, 2019 08:51 elowfoam Main CFD Forum 2 October 9, 2019 14:31 aishk OpenFOAM 5 May 27, 2019 07:03 wyldckat Site Help, Feedback & Discussions 20 October 28, 2014 09:04

All times are GMT -4. The time now is 13:18.