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Trailer Skirt Aerodynamics: Impacted by Crosswinds

NULL WINDS Technology has road tested numerous aerodynamic concepts with MVT Solutions in Pecos, Texas over three separate occasions between 2017 and 2021. From these various tests we determined what works and what does not work, and under what conditions, including headwind conditions.

(For example, surprisingly underbelly deflector panels directing air flow under the rear axle actually dramatically increased vehicle drag.)

(And, surprising to us, arranging trailer skirts to direct air flow laterally inward under the rear axle between the wheel sets also actually dramatically increased vehicle drag.)

Trailer skirts reduce the lateral displacement of air under the vehicle which otherwise impinges upon the wheels to produce more turbulence behind the vehicle. Increased turbulence behind the vehicle increases overall vehicle drag.

Turbulent swirling winds developed behind the vehicle include a momentum component of force within the moving air currents, thereby reducing the effective static pressure component in the swirling air. Reduced static air pressure developed behind the vehicle then increases the effective differential pressure developed between the front and rear of the vehicle, dramatically increasing overall vehicle drag.

For this reason, reducing turbulence developed behind the vehicle is also critical to reducing overall vehicle drag. Trailer skirts are effective under null wind conditions since they shield the wheels and guide air along the outside, reducing the lateral displacement of air under the vehicle, which otherwise also increases turbulence developed behind the vehicle.

Power is required to push air around the front of the forward moving vehicle. Similarly, power is also required to push air laterally underneath the truck as it passes by. By limiting the lateral displacement of air under the truck, trailer skirts then reduce the power lost in pushing this air laterally inward underneath the truck and then laterally outward apart from the moving rear wheels and axle assemblies.

Instead, trailer skirts guide the air to flow down the outsides of the vehicle. However, as previously discussed, the air flowing down the side of the vehicle can still impinge the outside of the wheels, including the sidewalls, thereby still reducing vehicle efficiency, especially when crosswinds are present. Even moderate crosswinds can cause dramatically increased the turbulence developed behind the vehicle, by increasing the exposure of unshielded portions of the wheel sets to headwinds.

(As previously discussed, exposed wheels tested inside wind tunnels will not yield an accurate measurement of overall vehicle drag contributed by the wheels.)

Shorter trailer skirts deployed far forward of the rear wheels especially expose the wheel sets to crosswinds, which then increases turbulence developed behind the vehicle. Thus, crosswinds can negate much of efficiency gains otherwise obtained under null wind conditions, the standard protocol that industry typically utilizes for testing both on the road and also inside wind tunnels.


(Shorter, more inexpensive trailer skirts expose wheels to crosswinds, negating much of the gains obtained under null wind test conditions.)

(Our Inner Wheel Skirt invention as tested on two separate occasions in Pecos, Texas, as well as in the ARC wind tunnel. It was shown especially effective in crosswinds.)

NULL WINDS Technology's Inner Wheel Skirt invention can counter much of air flow otherwise diverted laterally inward between the wheel sets that then otherwise increases the turbulence developed behind the vehicle, which would then increase the effective drag on the vehicle. The forward moving wheels must displace air to either lateral side of the wheel. Inner wheel skirts simply direct that displaced air to flow laterally to the outside of the vehicle, reducing the turbulence developed immediately behind the vehicle.

 NULL WINDS Technology Inner Wheel Skirts are currently under further product development.

See more about industry testing protocols on trucks in the next section.
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