Flow Pattern Analysis

Although convergence was not optimal, many of the main flow features can be discussed with the final solution obtained, namely surface pressure and velocity distributions.

Surface Pressure Distribution

The pressure distribution on the car’s surface is shown in Figure 4.29, in dimensionless form as pressure coefficientCp:

Cp= P−P_ref

1 2ρU²

On (1) it is possible to see that the flow stagnation zone (maximum pressure, displayed in red) is small due to the shape of the front structure (bonnet and front bumper), with a small leading edge and a smooth curvature throughout the bonnet, resulting in low drag force. It is also important to note the front radiators and brake duct, displayed in light blue, are positioned in order to receive an undisturbed flow.

Figure 4.29: Surface pressure coefficient.

The splitter and front bumper junction creates a concave shape generating a high pressure zone close to a small radius corner, indicated by (2), leading to the front radiator duct entrance. The steep curvature and the pressure gradient results in flow separation, displayed in dark blue, which reattaches before the front radiator entrance.

On the other hand, the large blue zones indicated by (3) represents high velocity regions near the car’s surface. These are related to the car’s shape, where the cockpit and the wheel arch are the highest sections, relative to the ground and transversal to the flow, therefore the streamlines curvature are more intense and the velocity is, by definition, higher.

Underfloor

Figure 4.30 shows that the splitter’s leading edge creates a high velocity and low pressure zone (4) with the objective of generating downforce at the front of the car. However, the rest of the splitter has higher pressure than expected. The reason for this pressure loss is the flat shape of the bottom of the splitter and, more importantly, the huge gap between the latter and the underfloor. This gap does not allow for a gradual pressure recovery, resulting in an abrupt flow expansion, which leads to a velocity decrease, pressure increase and downforce loss on the entire splitter and underfloor. A more detailed analysis of the flow and velocity distribution is made posteriorly.

Figure 4.30: Underfloor pressure coefficient.

As expected, the imposed rotation on the wheels and floor movement result in a very high pressure on the front section of the front wheel (5), especially near the ground. Adding this to the closed wheel arches results in a high turbulence zone around the wheel envelops.

As referred above, the underfloor’s downforce is seriously compromised as most of the flow escapes through the gap between the underfloor and the splitter. Therefore it was expected that the diffuser effect would be very poor since there is no gradual acceleration of the flow throughout the underfloor and so there is no low pressure areas. The upstream flow pressure coefficient was almost zero and so the diffuser does not have any pressure to recover. The side channel has almost no effect while the one in the centre still generates a small zone of low pressure at the beginning (6). In conclusion, the underfloor and diffuser combination does not generate any significant downforce.

Rear Wing

Below, Figure 4.31 shows the rear wing pressure side, from the top view with the endplate located on the right hand side. As expected this is one of the highest pressure zones on the car’s surface.

However, it is possible to see a lower pressure zone on the right side (7). Since the endplate has a significant thickness it creates a high pressure zone². At the endplate leading edge curvature the flow accelerates and in combination with the high pressure on the top of the wing, generates a vortex that

2In order to reduce excessive high pressure openings on the top part of the endplate are sometimes adopted, in the streamwise direction, to allow for a slight pressure drop

reduces the pressure near the wing tip.

Figure 4.31: Pressure Coefficient of the Rear Wing’s Pressure Side.

Figure 4.32, shows the bottom view of the same wing, with the endplate on the left hand side. The large zone in light blue (8) has a pressure coefficient close to 0.5 where it was expected lower pressure, similar to the rest of the suction side, in dark blue. There is flow separation, resulting in increased drag and a lost of downforce. This inefficient performance is probably due to the bodywork’s and rear radiator proximity, just 150mmbetween both, and the disturbed flow that reaches the wing.

Figure 4.32: Pressure coefficient of the rear wing’s suction side.

Bodywork by Sections

In this section, the velocity fields will be evaluated in the cross sections displayed in Figure 4.33.

Figure 4.33: Cross sections to be analysed.

Figure 4.34: Velocity stream lines on the front. Section A, y = 0.25 m.

The front of the car as several features that require some attention. In (9), Figure 4.34, it is possible to see the flow accelerating after the leading edge for a short distance to reach a maximum across the longitudinal direction of the splitter. Note that the velocity near splitter’s surface is lower than the freestream due to the ground effect.

In zone (10) it is possible to notice a small recirculation bubble due to the fast acceleration after the splitter’s leading edge, where the flow detaches from the wall and reattaches a few centimetres further, similar to what is described in (2).

Between the splitter and underfloor there is a significant gap (11) which creates a low velocity region with recirculation due to the abrupt expansion of the flow. The sudden transition from the flat bottom surface of the splitter and the gap to the underfloor does not allow any pressure recovery, resulting in a flow behaviour similar to a free jet and consequently all the downforce in the splitter is lost.

As shown in Figure 4.29 , the radiators location (12) is a good performance decision. The intake section presents a uniform velocity profile.

Figure 4.35: Velocity distribution overview. Section A, y = 0.25m

In figure 4.35, and according to (13), the zone of high velocity near the surface of the bodywork correspond to more intense streamlines curvatures and so, by definition, to a low pressure zone which explain the lift generated by the car’s bodywork.

Cross section A, Figure 4.36(a), reveals a low velocity zone near the top surface of the rear radiator duct (14). The sharp edge at the duct’s start leads to flow separation and, consequentially a reduction of the radiator’s cooling performance. This could be reduced with a smoother curvature to the duct’s entrance.

In both cross sections there is a large recirculation zone (15) at the rear of the car and in both chan- nels of the diffuser. This not only has a huge impact on the diffuser’s performance but also generates pressure drag.

Figure 4.36(b) shows the proximity between the wing and bodywork increases the velocity in the suction side of the wing (16) resulting, not only, in stall and flow separation, due to the increase of the adverse pressure gradient, but also in lift generation on the bodywork’s surface. Therefore, the shark fin and the endplate height should be modified so that undisturbed flow reaches the wing. A translation in the X direction, downstream, would also be beneficial.

The stalled wing creates a freestream recirculation (17) zone which reduces downforce and con- tributes to drag force increase.

(a) Section A, y = 0.25 m.

(b) Section B, y = 0.45 m.

Figure 4.36: Velocity distribution on the rear.