Tuesday, December 23, 2008

Drag Terminology Defined

Adverse Pressure Gradient: The Adverse Pressure Gradient (APG) is the rate at which the air pressure builds around a moving object. Many vehicles in the United States have an extreme APG; there is a direct correlation between extreme APG and poor gas mileage.

Flowfield: A “flowfield” represents all the airflow surrounding a vehicle while the vehicle is in motion. When oncoming air hits the vehicle, the air enters into and becomes part of the flowfield.

Within a vehicle’s flowfield resides a “boundary layer.” The boundary layer is an ultra thin layer of air that lies very close to the surface body of the vehicle while the vehicle is in motion. When the flowfield tightly surrounds the surface contours of the vehicle, the “boundary layer” is “attached.” When the flowfield does not tightly surround the surface contours of the vehicle, the boundary layer is “separated.” The break between the “attached” and “separated” boundary layer is called the “point of separation.” Following the “point of separation”, circulating vortices form and force the flowfield into an unstable “wake.”

Laminar Boundary Layer: In a laminar state, the airflow in the boundary layer flows smoothly and is considered ideal for reducing drag. The laminar state, however, is fragile and quickly separates when it is subjected to adverse pressure.

Turbulent Boundary Layer: In the turbulent state, the airflow in the boundary layers interacts or mixes at the surface of the vehicle. This “mixing” should technically result in higher drag, however, it actually results in less drag because the “mixing” increases the speed of the airflow within the boundary layer, which creates more forward momentum making the boundary layer more resistant to the adverse pressure thus extending the “point of separation.”

Reynolds Number (Re): The Reynolds Number (Re) is an equation that allows for the cross referencing of aerodynamic characteristics (including drag coefficient) between objects of different sizes that may move at different rates of speed. In sum, two different types of vehicles going at different speeds that experience the same Re will have the same drag coefficient. A detailed indexing of Re shows that there is a noticeable change in a smooth surface vehicle’s drag at a Re 3x105. Above this Re, the drag coefficient is roughly a constant 0.1; below this Re, the drag coefficient is roughly a constant at 0.5. Re 3x105 is thus considered a critical maker, where air pressure around a smooth surfaced vehicle begins to change dramatically.

= atmospheric density; V = velocity; l = reference length (in the case of a sphere, this variable is defined as the diameter);  = viscosity (or friction)

Transition Point: When the Re rises above 3x105, the boundary layer naturally/automatically changes from “laminar” to “turbulent.” The point of this naturally occurring change is known as the “transition point.”

Tripping the Boundary Layer: The process of artificially changing the boundary layer from laminar to turbulent without reaching the transition point of Re 3x105. This can be accomplished by apply FastSkinz MPG-Plus™ to a vehicle.