# Definition of strain rate magnitude for axisymmetric case in FLUENT

**2**

The strain rate magnitude(STRAIN_RATE-MAG(c,t)) is defined in header files as:

sqrt(ND_SUM(ND_DOT(C_DUDX(c,t), C_DUDY(c,t), C_DUDZ(c,t),(C_DUDX(c,t) + C_DUDX(c,t)),(C_DUDY(c,t) + C_DVDX(c,t)),(C_DUDZ(c,t) + C_DWDX(c,t))),ND_DOT(C_DVDX(c,t), C_DVDY(c,t), C_DVDZ(c,t),(C_DVDX(c,t) + C_DUDY(c,t)),(C_DVDY(c,t) + C_DVDY(c,t)),(C_DVDZ(c,t) + C_DWDY(c,t))),ND_DOT(C_DWDX(c,t), C_DWDY(c,t), C_DWDZ(c,t), (C_DWDX(c,t) + C_DUDZ(c,t)),(C_DWDY(c,t) + C_DVDZ(c,t)),(C_DWDZ(c,t) + C_DWDZ(c,t))))

+ (rp_axi ? (2.*SQR(C_V(c,t)/C_AVE_Y(c,t)) + (sg_swirl ? (SQR(C_AVE_Y(c,t))*NV_MAG2( C_STORAGE_R_NV(c,t,SV_OMEGA_G))): 0.)): 0.))

I understand the first part of the definition which is basically the second invariant of the strain rate tensor. But for axisymmetric case part(rp_axi part) I don't get how those terms appear. Can anyone explain/refer to any theory resource to learn how those terms appear for axisymmetric case? Also, I can't find definition of the terms too in the manual.

## Answers

11,714Forum CoordinatorWe can only point you at the documentation, which won't cover the contents of the header files. Search for the various parts in the UDF manual, that'll explain what some of the terms are.