Particle Reynolds number calculation in Lagrangian tracking?
Dear foamers
I am trying to implement the Srudhar and Katz bubble motion equation in the OpenFOAM. Hence, I looked at the exisiting Sphere Drag force model. I found there might be a possible mistake in calculating the particle Reynolds number. Particle Reynolds number is defined as Re_p = U_rel*d_p/nu_c, where U_rel is the relatvie velocity between the particle and carrying phase, d_p is the particle diameter and nu_c is the kinematic viscosity of the carrying phase. However, I found OpenFOMA uses the particle velocity (U_) instead of the relative velocity to calculate the particle Reynolds number (see line 110 in http://foam.sourceforge.net/docs/cpp/a05113_source.html). I am not sure whether this makes sense to everyone. Did anyone have this problem before? Thanks a lot. Jie 
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// Reynolds number Code:

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Thanks for the quick reply.This means the U_rel is defined as "U_p  U_c". The drag force is defined as F_d = 0.5*Cd*rho_c*A_p*U_cU_p*(U_cU_p). If OpenFOAM uses the U_rel = U_p  U_c. Then, should F_d = 0.5*Cd*rho_c*A_p*U_pU_c*(U_pU_c) instead? Jie 
I think I found the answer: F = Sp(U  Up) + Su
When they evaluate the force with the value.Sp, it is multiplied by (U_cU_p), hence, there should not be a negative sign in the front. Jie 
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I have a similar problem. In dieselFoam all non dimensional numbers are calculated in Parcel.H but none of them gets written in the output data fields. Please can you elaborate how to get these non dimentional numbers at the output. e.g. in parcel.H scalar We ( const vector& U, const scalar rho, const scalar sigma ) const; And more over there is parcelFunctions.C which calculates theses nondimensional numbers, is there any way to benefit from these. 
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