## 2 Models and Methods
### Numerical Model  
In order to study the flow field around vehicles and phenomena of flow separations, in 1984 [PERSON] et al. proposed a simplified geometry of a vehicle [9]. The model is characterized by a rectangular shaped body with a rounded front and a slanted top part of the rear. The most common model also has four cylindrical pillars under the geometry (Figure 1). To this day the model is in use as a reference geometry for scientist all over the world, and it is known as the Ahmed body.  
The Ahmed body has many applications in the scientific community. Many scientific works are focused on the numerical investigation of aerodynamic parameters depending on the turbulence model used. [PERSON] et al. [10] compared \(k-\omega\) SST, the Explicit Algebraic Stress Model (EARSM), Detached Eddy Simulation (DES) and Improved Delay Detached Eddy Simulation (IDDES) models. The last two are called hybrid models due to joining the advantages of the RANS model with the complexity of transient models. Investigation has been conducted for [PERSON] body geometry with two different slant angles, 25 and 35 degrees. The best results were received from the IDDES approach. [PERSON] et al. [11] presented studies on a reference structure with a rear slant angle of 25 degrees. They performed four simulation processes using LES and DES models, achieving overestimates of the drag coefficient between 6-16%. Additionally, the mesh used in the simulation depends on the used turbulence model. [PERSON] et al. [12] used a \(k-\epsilon\) realizable model with a tetrahedral mesh. The studies concentrate on drag and lift coefficients depending on the rear slant angle with an inlet velocity of 40 m/s. Ten different angles were tested ranging from 0 to 40 degrees. The minimum drag coefficient of 0.235 was obtained for the slant angle of 7.5 degrees. [PERSON] et al. [13] presented studies on aerodynamic parameters for slant angles of 25 and 35 degrees. Numerical simulations used the Reynolds Stress Model (RSM) of turbulence for a velocity range of 10 to 40 m/s. The cut-cell mesh was implemented. Results were compared with experimental data. [PERSON] et al. [14] compared five different turbulence models to compute airflow around an Ahmed body with a rear slant angle of 25 degrees. Turbulence models employed in these work were: \(k-\epsilon\) standard, \(k-\epsilon\) realizable, \(k-\omega\) SST, Spalart-Allmaras (SA) and Wray-Agarwal (WA). The WA model is a one-equation turbulence model implemented in the ANSYS Fluent software as a user-defined function [14; 15]. The advantage of this turbulence model is less simulation time required per iteration.  
In this paper, an [PERSON] body is used in CFD and CAA analyses. In the first stage of this work the numerical model is validated. Experimental data from the literature are compared with the results received from CFD simulations. It is an important step in this work because the accuracy of the calculated acoustic field depends on the quality of the CFD solution ([PERSON] et al. [16]). The next and main step is the calculation of the acoustic field around the reference structure and on this basis the prediction of the acoustic field around real road vehicles in the future.