Open-wheels race cars are the sort of category that has a wide range of models or categories. However, these can be splitted in just two, Formula One and single make formula series, inside this last one there are Formula 2, Formula 3, Formula 4, Super Formula and Indycar. This last one needs a more detailed approach since it races in oval and road course tracks. The reason behind this division is that the product proposal for a Formula One car has a different approach with respect to the others. Usually these have to deal with two important aspects, the overtakeability and the raceability. In addition there are the decisions taken regarding the costs, which are also very different between these two open-wheels cars. Therefore, this article proposes a review about the main aspects regarding the aerodynamic features of general open-wheels (OW) race cars.

Cost

The OW race car design has three main pillars, the cost, the time frame and the performance. These usually are the same for any race car, but what is different is the orientation of the design process. In case for single make racing series, the time frame and the cost are most important. The first is usually very tight, but it allows more creative designs. The design process of an OW race car for one make series is mainly constrained by the technical requirements with respect to overtakeability and raceability. This means that the aerodynamic performance is agreed and defined in contract between the series organization and the car maker. The cost is the most important between these two. Hence, it is not a design process oriented by performance, instead it is guided by the general costs of the car. The car maker should provide a sustainable product with respect to cost since these racing series require spare parts and technical assistance.

Technical requirements

The aerodynamic performance is defined on the contract, but in terms of technical requirements these are described as:

• Smooth ride heigth sensitivity;
• Smooth angle sensitivity;
• No diffuser stall;
• Stable rear wing.

These requirements mean that the vehicle should be easy to drive and predictable in order to allow the driver to exploit its limits. Hence, the vehicle behavior does not vary excessively with ride height variation.

Tire wake

In OW race cars one of the main concern of the aerodynamic department is the management of the wake produced by the wheels. Since these are uncovered, the airflow is massively disturbed by the wheel. In addition these are rotating, which increases their impact on the flow. The main effect caused by wheels is the wake, but there other effects or sub effects, these are summarized below.

• Tire jet or tire squish;
• Upper wake;
• Lateral wake;
• Wake body (lower and upper).

During the development, the CFD environment is used to analyze the impact of the wheels and the modifications proposed tries to reduce the wake produced by those. Actually, the tire make management is an approach to confine the wheel wake and guide the coherent structures to interesting sections of the car. The wheel wake is an unsteady and low energy flow, that is able to reduce the efficiency (Eff) of the aerodynamic devices if it goes in the direction of them. For instance, the tire squish has a great tendency to flow to the underfloor, which reduces a lot the underfloor and rear diffuser efficiencies.

Front wing

The front wing is one of the main aerodynamic devices, it faces the fresh air which results in its great efficiency (Figure 2). Hence, FW is a component that stabilizes the car and provides a huge downforce with a very low cost of drag. This is valid for close and open wheels race cars. Another important aspect of FW is its impact on the airflow.

Normally, FW results in a characteristic up-wash (flow deviated upwards), while its end plate generates an in-wash (flow deviated into the body) vortex that sucks the tire squish to the underfloor leading edge. In addition, it is possible to design wings in order to generate also an out-wash (flow deviated out of the body) as an alternative to deviate the wake from the wheels.

As can be noticed, these are effects generated not only due to the wing flaps, actually the entire wing assembly has a great influence on the wake generated. Therefore, a front wing should not be developed as single part, but the entire wing assembly should be developed together since each part of it has an impact on the wake. The front wing design defines most of the characteristics of the flow field to the rest of the car.

Rear wing

The rear wing (RW) defines the total downforce provided to the car. However, this cames at cost of high drag since the rear wing has a lower efficiency with respect to the front wing. Usually the RW adjustment is done to perform a re-drag or a iso-balance method in order to balance the car according to the iso-time of the track.

The RW design aims to guarantee no stall during operations and the main flap shape in order to have a powerful effect when DRS is activated. Hence, the main requirement of the RW design is the rear wing stability, high downforce generation and optimized wake management.

Underfloor

Although open-wheels race cars have significant smaller area with respect to the closed-wheels race cars, this still is the most important aero device since it is the most efficient. The underfloor has two important zones, the leading edge and the kink-line zone, which is near the rear diffuser. These are two suction zones and their areas must be improved in order to improve the efficiency. Hence, the underfloor development is usually splitted in two zones, front floor and rear floor, which is basically the diffuser.

Front floor

The main point of the front floor is the suction zones. However, since the tire jet can lead some vortices to the underfloor and this reduce its efficiency, it is common to see in some OW race cars the use of barge boards (Figure 14).

These deviates this flow creating some out-wash. In addition, the pressure difference between the front and the rear part of the barge board creates a coherent vortices that can be used as suction generation for the underfloor.

Overtakeability

Basically this is a term that defines the capability of the car to run close to another one. As higher this feature, more easily the car will be able take advantage of drafting and perform an overtake. Actually, for the race cars with drag reduction system (DRS), this term means the ability of the car to run close to another in order to use DRS to perform an overtake. However, the wake generated is a low energy and unsteady flow that results in a significant drop on the aerodynamic performance. In terms of number, C_XS, C_ZS and F_bal can drop to a range of 5 to 15 %, 20 to 30 % and 5 to 10 %, respectively. Hence, the following car has too much trouble to race close the leading one. This reduces the competitivity of the race and it is not good in terms of entertainment factor.

The modifications proposed on the bodywork to improve overtakeability are done on the aerodynamic devices. The objective is to clean the wake, to limit the out-wash and the amount of vorticity. Hence, the wake generated will be more contained, in other words, short and thin. However, even though the wake is reduced on the straights, this is not enough to improve the overtakeability. Actually, this should be improved in three important situations, straight line acceleration under wake, high speed corner under wake and braking at straight line. It is important to keep in mind that some modification done in other to reduce wake is beneficial for the following car, but not good for the leading one.

Front wing

The front wing is the first device of the car faces the airflow, thus it usually encounters fresh air and must take advantage of this at maximum. However, when inside the wake, the front lose its efficiency. Hence, its design must change to better adequate it to this situation. These improvements are the reduction of the out-wash, the reduction of the vorticity and the increase of the in-wash.

In term of geometry, the front wing usually a great wing span, but now this more distributed to the end plates. The objective is use this zone to generated drag since the wake, at a distance of 1.5L_ref is concentrated at the center of bodywork. In this way, the front wing is able to release the wake energy and the out-wash.

As can be seen, the optimized FW has a significant reduction on the vorticity generated at the end plates. It is also possible to notice by Figure 8, 9 and 10 how simpler the end plates of the optimized FW are respective to the baseline one. Another clear diference is the wake width, which reduced despite the fact that the height is almost the same.

Rear wing

Since the rear wing is the aerodynamic device that generates the highest amount of downforce, its optimization requires more attention from the engineers. Actually, the main point of the rear wing optimization for low wake is the drag reduction system (DRS). In this one the main flap is pivoted at its top. When DRS is activated, the flat rotates around the pivot and opens the passage to flow. This results in huge drag reduction, consequently also in downforce reduction.

In terms of the rear wing optimization with exception of DRS, the geometry of the wing span is modified. The objective is the lower amount of vortices generated at the end plates, the confinement of the wake and the increase of the height of the bulk wake. For this the wing span distribution increases towards the plane Y₀, in other words, at the center of the wing.

As can be seen, the optimized RW can generate a reduced vorticity, promote a better suction of the outboard wake and reduced the wake width. Although, the bulk of the wake still is high, it is narrow. Hence, combined with an optimized FW it is possible to improve the overtakeability.

References

• This article was written based on the lecture notes written during the Industrial Aerodynamic course at Dallara Accademy.