Sorry for the long post but it is somewhat of a complicated subject so I wanted to explain it as simply as I could.
Most of this post will pertain to coup class or full bodied vehicles, as streamliners have a super efficient body design and handle the airflow around them and the wake behind them quiet well with little pressure drag.
Several posts ago I mentioned vehicle surface shape and texture having an influence on drag and “maj” from Australia later asked what I thought about the “sharkskin” and its effect on boundary layers, its not an easy answer and like many things will depend on the shape, size and speed of the vehicle it is applied to.
Shark scales are called placoid and have tiny ridges on them that are parallel with the direction the shark swims in, and differ from one shark species to the next, the faster the particular shark species could swim determined the size of the riblets.
The sharkskin, riblets, chevrons, bumps or V’s, I prefer to call them riblets, whatever you wish to refer to them as, do there work in turbulent boundary layers. The size of the riblets will depend on the thickness of the boundary layer at the vehicles surface and the speed of the vehicle through the air.
In LSR racing, pressure drag is more detrimental than friction drag.
Air has mass and it is displaced and moves around the vehicle as the vehicle passes through it, the force produced and the amount of air displaced is determined by the vehicle shape, speed and air density. Air moving past the vehicle sticks to the surface slows down and forms a layer, the air layer adjacent to the vehicle surface remains attached to the body surface, the air above that layer slows a little less and so on until there are several slower than free stream air velocity layers and this is called the boundary layer. How thick the boundary layer is depends on the vehicle speed, shape and air density. As a vehicle accelerates it disturbs more air than it does at slower speeds. Air moving in the boundary layer causes friction drag on the vehicle. Movement between the air layers remove energy and convert it to heat, lost energy from the boundary layer will cause a transition to turbulent flow and can take place even over a smooth vehicle surface.
The boundary layer can be laminar or turbulent. Laminar layers have less vehicle skin friction than a turbulent layer and drag will be less. If the boundary layer is laminar, changes in the speed of the air in the layer is gradual as it moves away from the vehicles surface. If the boundary layer is turbulent the air speed is chaotic and vortices form inside the boundary layer and will separate away from the vehicles surface because it has lost energy and is acted up on by the free stream air pressure.
Turbulent air has a quick change in speed and pressure and forms vortices that react with each other, increasing friction drag at the vehicle surface.
As the boundary layer moves from the front to the back of the vehicle it looses energy and will thicken and become turbulent or an abrupt change in the body shape or vehicle surface texture can cause the flow to totally separate from the vehicle and form a turbulent area behind the vehicle called a wake. The wake will cause a lower air pressure immediately behind the vehicle than there is at the front of the vehicle and that is what creates the pressure drag, think of it as a large suction pulling on the back of the vehicle and slowing its acceleration rate. Vortices are formed in the wake and will continue far behind the vehicle until the energy is overcame by the air viscosity and turned into heat. That is why I stated in many of my earlier posts that it is best to dump all of the air and heat that you can into the wake behind the vehicle, to help reduce the low pressure area and lessen the pressure drag on the vehicle.