Factors Affecting Vehicle Drag on Trucks:
Null Winds Technology has road tested numerous aerodynamic concepts with MVT Solutions in Pecos, Texas over three separate occasions between 2017 and 2021. As a result, we have honed in on numerous factors affecting vehicle efficiency of semi-trucks. While some concepts proved viable, unexpectedly several did not. From these tests and including additional testing in the ARC wind tunnel in 2021, several basic aerodynamic factors can be concluded:
1. Since power lost in drag results from both friction (skin) drag and form (pressure) drag, a balance must be obtained between losses from both of these sources of drag, an aerodynamic systems problem.
2. A moving vehicle must push relatively heavy air out of the way, requiring considerable power at highway speeds where losses are maximized. Minimizing the back-and-forth displacement of air caused by the moving vehicle can then minimize overall vehicle drag.
3. Turbulent air includes losses due also to the rotational displacement of the air (creating eddies), further increasing the power lost through even further displacement of the air over the more simple translational displacement of air produced in laminar air flow. Where possible, any displacement of air should be limited to being mostly simple laminar (lateral displacement) flow.
4. Inducing air to flow laterally inward from the outside of the vehicle to flow between the wheel sets actually increases vehicle drag, since it also increases the lateral displacement of air by the moving vehicle. This displaced air also disturbs the otherwise relatively static air naturally passing under the vehicle between the wheels, inducing rotational displacements as well. Static air is higher in pressure (not having a momentum component), and therefore should be preserved as static air developed behind the vehicle as much as possible.
5. Blocking air flowing in-between the wheel sets is also counterproductive, since diverting this airflow also increases the lateral and rotational displacements of air by the moving vehicle, requiring increased power to do so, while also reducing the static pressure developed behind the vehicle, thereby also greatly increasing vehicle drag. This unexpected result was observed from all road tests that employed a shield disposed immediately in front of the rear axle. It is only the wheel portion of the undercarriage that should be shielded, and preferably only on the uppermost portions thereof.
6. Trailer skirts perform well by inhibiting the lateral displacement of air under the vehicle, thereby improving the static air pressure developed behind the vehicle, while also minimizing the overall displacement of air by the moving vehicle, minimizing the input power required to propel the vehicle. Trailer skirts should extend downward close to the ground, and along most of the length of the open portion of the vehicle, making them more extensive and therefore more expensive.
7. Exposed rear wheels in crosswinds dramatically increase vehicle drag, since both wheel drag is highly magnified at the top of the wheel, and crosswinds increase turbulent air developed behind the vehicle. Only the uppermost portion of the exposed wheels should be shielded. Short trailer skirts expose wheels to crosswinds, largely negating their effectiveness under windy conditions.
8. Inner Wheel Skirts inhibit the lateral displacement of air by the forward moving wheels toward the inside of the wheel sets, thereby stabilizing the relatively static central air column passing between the wheels under the axle. Increased static air passing under the vehicle increases the static pressure developed immediately behind the vehicle, to thereby decrease overall vehicle drag. While this reduced vehicle drag effect is minimal in null wind conditions, the effective reduction in vehicle drag is greatly enhanced in the presence of even minor crosswinds. Inner wheel skirts were wind-tunnel tested and shown to improve vehicle drag reduction over the null crosswind angle by a factor of 10 or more on both a semitrailer as well as a pickup at only a 4 degree yaw angle.