## 3 CFD Applied to Compost-Bedded Pack Barn Systems
### Post-Processing and Model Validation  
In the post-processing stage, the user can visualize and analyze the simulation results using contour graphs, vectors, lines, etc. [23]. One of the most common visualizations that yields the results in studies of animal facilities using CFD is the contour planes (horizontal and/or vertical), which is applied in several studies [20; 36; 42; 46]. In this case, the horizontal and/or longitudinal profiles of the interest variable (temperature, humidity, air velocity, thermal comfort indexes, gas concentrations, etc.) are described, making it possible to reach good detail levels in the variable distribution. Another visualization form is via the isosurface and isovolume maps, which make it possible to proportionally quantify the area/volume with values above or below the variable/index of interest, in addition to its distribution [14; 45].  
For air speed, in particular, in many studies, the results are visualized using air speed vectors [13; 15; 28; 36; 46; 49] and air current lines [13; 42], allowing us to understand in more detail how the air flow occurs. Typically, mass-weighted averages are used to sample the air velocity magnitude and calculate its average value over the interest sections [14].  
From the scalar fields of the modeled variables (air temperature, relative humidity, air velocity, etc.), it is possible to proceed with the calculation of thermal comfort indexes applicable to the evaluation of the thermal environmental conditions to which the animals are submitted. Among the indexes that can be used, the Temperature and Humidity Index (THI) can be highlighted, which relates the temperature to the relative humidity of the air [85, 86], and the Equivalent Temperature Index for Dairy Cattle (ETIC), which includes the effects of air velocity and solar radiation, in addition to temperature and relative humidity [19, 29, 87].  
If is desired to calculate the achieved ventilation rate, the reduction method can be used. One of the ways to apply this method is to use a tracer gas with known concentration and with the same properties as the ventilation air. This gas can be treated as a passive scalar and solved after obtaining the flow field solution, that is, decoupling the momentum equations. Initially, all the internal cells that make up the facility mesh have a fixed unit concentration (C\({}_{0}\)), and the external cells have zero concentration. The gas concentration inside the facility reduces at a rate that depends on local air velocity values, and, when all the gas has been removed to the outside of the facility, the volumetric gas concentration (C) can be tabulated in a time-dependent exponential function t (C = C\({}_{0}\cdot\)e\({}^{-\text{nt}}\)), where the scalar n describes the ventilation rate of studied control volume [13].  
The validation of the CFD modeling using experimental data is also particularly important, as it will make it possible to understand whether the simulated results adequately represent the real facility environment. When it comes to modeling animal facilities, whenever possible, experimental data for validation should be collected at different heights and facility sections to represent the animals' occupied zones and headspace in regions of inlet, central, and air outlets [44]. In relation to air velocity, in particular, its magnitude is usually measured in experimental data collections, and in some cases, its direction is also recorded [28].  
To assess the agreement between simulated results and experimental data collected in animal facilities, several metrics are applicable, such as normalized mean square error --NMSE [41, 42, 45], root mean square error--RMSE [29], relative error [44, 80], among others.