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- - **UDF for the critical rate of strain.**
(*http://www.cfd-online.com/Forums/fluent/74175-udf-critical-rate-strain.html*)

UDF for the critical rate of strain.Hi,
I am trying to implement the following expression in fluent for the critical rate of strain: gcr = 0.5*lam_flame_speed^2/unburnt_therm_diff I tried to do it first using DEFINE_PROPERTY: DEFINE_PROPERTY(udf_critical_strain_rate,c,t) { real gcr; real ul; real therm_diff; real fmean; real CA0 = 3.84e-5; real CA1 = -2.4e-6; real CA2 = 4.48e-7; real CA3 = -6.1e-8; fmean = C_FMEAN(c,t); therm_diff = CA0 + CA1*fmean + CA2*pow(fmean,2.0) + CA3*pow(fmean,3.0); ul = C_LAM_FLAME_SPEED(c,t); gcr = 0.50*pow(ul,2.0)/therm_diff; return gcr; } In the above, I'm using mean mixture fraction polynomial fits. I obtained the coefficients from Fluent. However, when I run my case, the data are off the charts with a minimum of 1.E+8 (which is the default value set by fluent). So I tried to do it using DEFINE_ADJUST: DEFINE_ADJUST(udf_critical_strain_rate,d) { Thread *t; cell_t c; real ul; real therm_diff; real fmean; real gcr; real CA0 = 3.84e-5; real CA1 = -2.4e-6; real CA2 = 4.48e-7; real CA3 = -6.1e-8; thread_loop_c(t,d) { begin_c_loop(c,t) { fmean = C_FMEAN(c,t); therm_diff = CA0 + CA1*fmean + CA2*pow(fmean,2.0) + CA3*pow(fmean,3.0); ul = C_LAM_FLAME_SPEED(c,t); gcr = 0.50*pow(ul,2.0)/therm_diff; C_CRITICAL_STRAIN_RATE(c,t) = gcr; } end_c_loop(c,t) } } Bu this time, when I interpret it I get the following error message: Invalid lvalue in assignment at the line where I set C_CRITICAL_STRAIN_RATE(c,t) = gcr. Going through the UDF documentation, I realized that DEFINE_)ADJUST is only valid to modify variables that are not passed as arguments, which is not the case here. So DEFINE_ADJUST is not suitable. Can anyone tell me if there is something wrong in my first UDF, or if there is a more elegant way to compute the unburnt thermal diffusivity? Thanks, Ace |

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