## 4 Discussion  
In this paper, theoretical and experimental investigations aimed at obtaining the aerodynamic performance of the F-111 \"Aardvark\" aircraft were carried out. First, the aerodynamic coefficients were calculated and the pathline visualization of the vortex was obtained with use of ANSYS Fluent software. Then, the same results were achieved experimentally using a hydrodynamic tunnel. The calculation output was compared with the experiment in two main parts, i.e., as qualitative and quantitative results. For qualitative analysis, studies were conducted for angles of attack ranging from 25 to 50 degrees with a 5-degree increment. Quantitative analysis in the hydrodynamic tunnel, due to limitations of the strain gauges, was performed for angles of attack ranging from 0 to 30 degrees with a 2.5-degree increment. In the CFD analysis, angles ranging from 0 to 50 degrees with a 5-degree increment were used. Figures 13 and 14 depict a comparison of the qualitative results obtained using ANSYS Fluent software and the hydrodynamic tunnel. The figure was created by overlaying graphics generated using computational software from a photo derived from the water tunnel testing. A detailed description of the elements being compared is provided in Figure 13.  
In Figure 13, an overlay of two images is presented, one from the CFD simulation and the other from the visual study at a 25-degree angle of attack. Due to the use of the same CAD model, the F-111 model matches 100%. The dashed lines labeled as \"Streamlines\" represent the results obtained from CFD. The area labeled as \"Turbulent flow\" depicts the results obtained in the hydrodynamic tunnel. From the above figures, it can be concluded that there is a very high level of agreement in the obtained qualitative results. Figure 15 presents a comparison of the quantitative results (non-zero component of the force and moment coefficients) obtained using ANSYS Fluent software and the hydrodynamic tunnel. In Figure 16, a comparison of the drag force coefficients is shown, and Figure 17 shows a comparison of the pitching moment coefficients. Figure 12: Reference system for the model installed on the support. In Figure 15, a comparison of the quantitative results for the lift force coefficients obtained in CFD and the hydrodynamic tunnel is presented. The difference in results fluctuates within the range of 8% up to an angle of 25 degrees and 7% at an angle of 30 degrees. For the drag force coefficients shown in Figure 16, the difference is 9% for 25 degrees and 10% for 30 degrees. On the other hand, the deviation for the moment coefficients falls within the range of 9% (Figure 17). The differences between the CFD studies and hydrodynamic tunnel tests in the lift coefficient (Figure 15) are rather small, and could be both the effect of the CFD modeling (without a sting and with inadequately resolved vortices or boundary layer) and the experiment (rather than being an effect of the too-low model mount stiffness, it appears to be a difference in the zero position and some difference proportional to the measured force). The difference in the drag coefficient (Figure 16) is constant along the increase in the angle of attack and is most probably an effect of not including the bottom drag in tunnel tests or not modeling the sting in CFD simulation; however, the consistency of both results suggests that the CFD modeling of the flow was adequate in terms of the modeled phenomena (sudden increase at the same angle of attack). The moment characteristic (Figure 17) shows that the main factor of differences is rather a difference in the 0 angle of attack between CFD computation and tunnel tests. The increase in the difference between the results with the increase in the angle of attack is small, so the difference source is not at the nose or tail of the aircraft. The force measurement test in the hydrodynamic tunnel was conducted at 30 degrees due to the presence of significant forces on the strain gauge, which posed a risk of damaging it.