Car rear diffuser

In recent years, whether cutting-edge spacecraft such as spacecraft, space shuttles, rockets, or conventional vehicles such as airplanes, high-speed rails, cars, and yachts, have become more and more demanding in appearance, such as aerodynamics, bionics, etc. Fluid mechanics and biological elements are more and more frequently manifested. It has to be said that it is a gift from nature to mankind, which has played a very important role in improving vehicle performance and fuel economy. As a killer that stimulates adrenal hormones in the human body, the tail diffuser also emerged in such an environment. So how powerful is it, let's find out next.

The air pressure inside the diffuser gradually recovers from low pressure to normal pressure (ambient pressure at the outlet). The air flow rate under the chassis in front of the diffuser will increase and the pressure will decrease. The pressure difference between the upper and lower surfaces of the car will bring down pressure.

It can be counted. Do CFD simulation. Simply put, put the car on the diffuser and count it again. If you don't install it, the difference is the benefit of the diffuser. In fact, the aerodynamics of the car affects the whole body, so it is difficult to disassemble it. However, the downforce generated by the diffuser and the low pressure area of the chassis has the highest lift-to-drag ratio, so a large part of the downforce of the car comes from this (about half of the downforce in F1 comes from the chassis).

According to inertia, the airflow moves in a straight line, but because the diffuser is narrow in front and wide in the rear, the airflow tends to leave along the normal direction of the upper surface of the diffuser (that is, the bottom of the vehicle), which leads to the generation of negative pressure. When negative pressure is generated, the atmospheric pressure on the roof (one atmosphere, positive pressure relative to negative pressure) will push the car down, which will increase the grip of the car and increase the stability of the car.

As shown in the figure, the red arrow is the speed direction of the airflow at point A of the diffuser. From the speed direction, the gap between the airflow at point A and the bottom of the vehicle is small, and the gap between the airflow at point B and the bottom of the vehicle is large, so the airflow is in The tendency of the normal direction of the bottom of the vehicle to leave the bottom of the vehicle, so that negative pressure is generated under the vehicle.

The diffuser creates a vacuum area at the bottom of the car, which means that the airflow at the bottom of the car will accelerate to this area. According to Bernoulli's principle, a greater negative pressure will be generated at the bottom of the car. At the same time, due to the Venturi effect, the airflow is the fastest when it flows through the throat of the diffuser, and an area of minimum negative pressure appears at this position. Therefore, the diffuser increases the speed of the airflow at the bottom of the car, forming a low pressure zone, and achieves the effect of enhancing the downforce of the car.

The direct function of the rear diffuser is to quickly evacuate the air under the vehicle to form a near-vacuum environment under the vehicle. Then the air pressure acts on the upper surface of the car body to generate downward pressure. The diffuser itself does not generate downforce, which acts on the entire body.

If the diffuser is designed and the diffuser design is effective, the effect is shown below the resistance in the above figure. As the cross-sectional area of the lower part of the car body decreases, the overall flow rate increases. According to Bernoulli's equation, the pressure at the lower part of the car body will decrease, thereby increasing the downforce of the car. The role of the diffuser will also reduce the wake area of the car body, increase the pressure at the rear of the car body, thereby reducing aerodynamic drag.

If the diffuser is designed, there will be a sudden drop in air pressure where the diffuser and the floor of the vehicle meet, which will significantly increase the downforce on the body, thereby reducing aerodynamic lift or increasing aerodynamic downforce. Most of the diffuser area is still a negative pressure zone, which can generate downforce. As the diffuser moves away from the body, the underbody pressure gradually increases.

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