# Scalar transport equation dimension problem

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 February 27, 2007, 05:46 Hi How to use 'rho' in a sca #1 litonx Guest   Posts: n/a Hi How to use 'rho' in a scalar transport equation. If i use like following, ddt(rho, T)+div(phi,T)+laplacian(K,T), I have dimension problem.. T=dimLess ddt(rho, T)= 1/s kg/m3 phi=linearInterpolate(rho*U) & mesh.Sf() div(phi,T)=1/m kg/m3 m/s m2 =kg/m-s Therefore different dimensions. In rhoTurboFoam momenturm equation is working with this definitaion. I could not understand where is my mistake. Anyone help me please !! Regards -Liton

 February 27, 2007, 12:24 hi, i think if you using th #2 Senior Member   Stephan Gerber Join Date: Mar 2009 Location: Germany Posts: 118 Rep Power: 10 hi, i think if you using the same argument for the momentum equation in rhoTurbFoam you will find the same problem. it's the m^2-term which is not there: writing the transport equations with div for the convection implies that this term is an volume integral (gauss:int (dui/dxi ) dV =int ui dAi) i guess the problem with the dimension comes from the laplacian term. what dimension has k in your set up (there should be no kelvin in the dimension, [k]=kg/(s*m)) ? stephan

 February 27, 2007, 18:13 Hi Stephan Gerber, Thanks for #3 litonx Guest   Posts: n/a Hi Stephan Gerber, Thanks for your reply. actually i wrote the equation in a wrong fashion. When I printed the phi, I found its dim is 1/s .. as mesh.Sf() is area vector I thought the dimension of area must be multiplied with the U. one more question. if I declare a volScalarField like following: volScalarField XXX ( IOObject( .. .. ), c1+c2 ) where c1 and c2 dimensionedScalar. If c1 or c2 update, XXX won't update automatically. How to get the update of XXX. I used like following XXX=c1+c2; It works, but it does not seem to me very convenient. Any tips!! Regards - Liton

 March 2, 2007, 07:49 hi, you may combine calcula #4 Senior Member   Stephan Gerber Join Date: Mar 2009 Location: Germany Posts: 118 Rep Power: 10 hi, you may combine calculating XXX=c1+c2 with calling XXX (some kind of special-output routine for XXX) which calculates the equation - so you wont have to calculate explicitly. maybe there is a nice way using pointers or references? stephan

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