Dimensionless scaling method for extending CFD simulation results for mechanically ventilated animal housing located above manure pits.

A trend observed during analysis of CFD simulation results for a given manure pit and barn shape and ventilation configuration (the same pit-safety fan location and flow rate with the same barn ventilation rate) with different uniform initial manure pit hydrogen sulfide (H2S) gas concentrations (C0) suggested that the ratio of concentration (C) at each time step during ventilation was equal to the ratio between the initial concentrations inside the manure pit. It was determined that C/C0 scaling could be used to expand the results from one CFD simulation at one C0 value to a wide range of C0 values. The benefit of using this dimensionless method was to reduce the total number of simulations needed to estimate the effects of pit-safety ventilation for a range of initial concentrations inside the manure pit. A single simulation was run, and the resulting gas distribution everywhere in the computational domain was deduced for any multiple of the initial concentration used. The coefficient of variation (cv), or ratio of standard deviation to the mean, was used to demonstrate the accuracy of this method when compared to actual CFD simulations run for different initial pit concentrations. The maximum error when comparing simulated to estimated C/C0 values was ± 2.5% for the global maximum H2S gas concentration over time. With an error level of ± 2.5%, the estimation uncertainty would be ± 12.5 ppm when the simulated initial H2S concentration is 500 ppm.