|The boxes at the right of each object show the relative total drag. You can see the pressure drag is a much greater impact for the blunt spherical object while the streamlined object experiences far less relative drag, but most of the drag is due to skin friction rather than the pressure drag in the object's wake.
Applying this to a moving car, as the car travels forward it leaves an empty space in its path. Air molecules can't fill this space as quickly as a car can create it, leading to flow separation behind the car and producing a vacuum that hinders the car's forward travel. Depending on shape (how the rear tapers off) and speed, this vacuum creates a pressure drag that is multiple times greater than the surface area drag at the front of the car. Reducing drag can improve a car's performance and its gas mileage.
So why don't we see dimpled cars all over the F1 circuit? Well, race cars already have really efficient designs. The flow of air (barring any sideways off track adventures) is one directional against the body of the car so it can be better designed with more streamlined, fixed edges that promote low drag, similar to an aircraft wing. The car sits so low and tapers off in the rear, and the speeds are too quick to have a huge problem with flow separation until ideally, at the rear wing. Serrated edges, gurney flaps, and vortex generators are sometimes used on race cars and even passenger vehicles because they are better at keeping air flow from separating on streamlined objects. These are similar to how an airplane wing uses raked wing tips and winglets, vanes and bumps. Some F1 teams (Ferrari and McLaren for instance) have been rumored to have experimented with dimpled wing designs and other surfaces but none have been used in competition. Besides, downforce that keeps an F1 car grounded is more important to manage than drag except for in straight line top speed conditions, one of the reasons why last year the FIA modified some rules around adjusting the rear wing.
|This is the Hexa solar powered concept car by Dimitri Bez.
|The car's golf ball patterned roof reduces drag AND heat transfer, increasing its solar efficiency.
What about on passenger cars? Car manufacturers are constantly striving for better gas mileage. It's easier on driver's wallets and protects nature's valuable resources; “Oil that is, black gold, Texas tea.” A low coefficient of drag on a vehicle is a good indication that the flow of air remains attached longer at the rear of the car. Passenger cars require backseat headroom and trunk space, a design that leaves a large void of space behind the car. This air pocket is also why some objects (balls, tie-down straps, beer cans) seem to suspend magically mid-air in a truck bed. One of the advantages of multi-car drafting is the effort required to overcome drag is distributed amongst the cars, reducing the impact of the drag experienced behind the lead car and the frontal drag by all following cars. But you pretty much need to ride each other's bumpers, which tends to get you a middle finger when attempting said drafting maneuver on the road.
MythBusters actually tested the theory of a dimpled car improving gas mileage by applying an inch of clay to the body of a Ford Taurus. They performed five test runs at 65mph with the clay smoothed out and averaged 26mpg. Then they carved out dimples all over the body similar to a golf ball but left the clay in the backseat so the car would weigh the same. The dimpled car achieved 29mpg on average, an increase of 11% in gas mileage. But face it- aesthetics do matter; do you really want to drive something that looks like it's infected with the herp? Let's leave the dimpled look for the thighs of the Jersey Shore cast!
|That is one fugly car! And then they added the clay 😉
Not to get too wrapped up in aerodynamic design, but there are plenty of modifcations used by race cars to be competitve. The rear wing is one of the most effective modifications for producing downforce but at the expense of increased drag. Angling the wing more vertically will apply more downforce but it will also produce a larger area of low pressure behind the wing, or more pressure drag. Gurney flaps are used on the trailing edge of the wing to keep the air flow attached. Gurney flaps themselves also increase drag slightly but they can produce more downforce at a lower angle of attack for less overall drag than setting the wing at a higher angle of attack.
Rear diffusers direct air under the car, increasing its speed and creating a low pressure area under the car which provides a substantial amount of downforce. The diffuser ceiling then slopes upwards and has a cross-sectional area in the direction air is flowing towards the rear of the car. This design slows the air's speed back down near the rear of the car to near free-stream velocity and pressure as it travels through the diffuser.
|Holy body work batman! ”Don't tell me this ish is functional too!?!?” Affirmative – the rear wing of the GTA series GST Impreza L creates downforce but leaves a low pressure region in its wake. Exposing the air exiting from the rear diffuser to the turbulent, low pressure wake area behind the car helps to suck the air out, keeping this monster planted on the track and turning out podium topping times.
So how can we combat all this low pressure behind the car? Dimples on golf balls and bump-shaped vortex generators on cars can energize the air flow and give it more forward momentum, lowering the impact of pressure drag. While dimples and vortex generators create some surface drag themselves, the reduction in pressure drag is many times greater. Vortex generators are often used on both roofs and rear wings for this reason.