Biomimicry as a Tool for Aerodynamic Drag Reduction of Class 8 Heavy Vehicle Trailers
A computational analysis and wind tunnel study investigating boxfish-inspired biomimetic design principles for reducing aerodynamic drag in heavy-duty trucking, published in Stanford Intersect Journal.
Abstract
In August 2021, the Environmental Protection Agency (EPA), through its Clean Truck Plan, proposed new standards to promote clean air and reduce pollution from heavy-duty vehicles starting in model year 2027. One of the recommended approaches is for heavy-duty vehicles to improve fuel economy by 40% by the year 2027.
Class 8 trucks achieve fuel economy in the range of 6-8 miles per gallon of diesel. At speeds of 70 mph, 65% of the energy is spent in overcoming aerodynamic drag, making aero-drag the largest opportunity for improving efficiency. Inspired by the Boxfish hydrodynamics, different add-on shapes were used in CFD simulations to provide a streamlined shape to the trailer and its corresponding drag measured. A 3D-printed model of the cab and trailer with add-on attachments was tested in a wind tunnel to validate the simulation.
When comparing the bio-inspired to a standard trailer, a drag reduction of 13.8% in computational and 16.7% in the wind tunnel experiments was achieved. These results translate to 14%-16% efficiency gains of Class-8 trailers by bio-mimicking the Boxfish.
Problem Statement
Class 8 trucks achieve fuel economy in the range of 6-8 miles per gallon of diesel. At speeds of 70 mph, 65% of the energy is spent in overcoming aerodynamic drag, making aero-drag the largest opportunity for improving efficiency. A truck consists of a cab in front and a trailer in the back. Cab aerodynamics is well understood and modeled, but the trailer has remained as cubical containers that are designed more for stacking and storage than being aerodynamic.
Current state of the art trucks deal with a speed averaged drag coefficient of 0.6 while they can broadly range from 0.6 to 1.0. Reducing the drag coefficient from 0.6 to 0.3 for a typical Class 8 tractor-trailer would result in a mileage improvement from 6.1 miles per gallon to 8.7 miles per gallon—a 43% savings.
Technical Implementation
Biomimetic Design Process
- • Boxfish (Ostraciidae) morphology and hydrodynamic analysis
- • Four main features: sharp snout, curved vertical sides, convex top/bottom, tapered tail
- • 3D modeling in Autodesk Fusion360 at 1:120 scale
- • Standard semi dimensions: 48 ft length, 8 ft width, 14 ft height
- • Add-on shapes for trailer modification while preserving cargo capacity
Computational Analysis
- • Autodesk CFD software with incompressible, laminar equivalent air flow
- • Speed range: 20-70 mph in 5 mph increments
- • Material: Stainless-Steel 304 simulation
- • Virtual wind tunnel with surface wrap feature
- • 200 iterations for flow stabilization (typically at 160 iterations)
Research Methodology
Biological Inspiration
- • Boxfish: non-streamlined (box-like) appearance with drag coefficient of 0.2
- • Comparable to efficiently designed aerofoils used for airplanes
- • Much lower than cuboid drag coefficient of 1.5 or current trucks at 0.6
- • Eliminates low pressure (flow reversal) around back edges
- • Low drag despite large payload capacity
Wind Tunnel Validation
- • Table top high precision wind tunnel with 120x scaled model
- • VIVOSUN T6 6 Inch 390 CFM Inline Duct Fan with variable airspeed
- • 3D printed honeycombs for laminar flow channeling
- • One kilogram load cell with HX711 analog to digital weight amplifier
- • 10 trials per speed for statistical validation
Results & Performance
Both computational simulation and experimental wind tunnel analysis show comparable results with a 13.8% (computational) and 16.7% (experimental) reduction in speed averaged drag force when the trailer model is adapted to the bio-inspired design of the Boxfish.
Aerodynamic Performance
- • Drag coefficient reduction: from 0.65 (standard) to 0.56 (bio-inspired)
- • Significantly smaller wake region for bio-inspired model
- • Reduced flow reversal at trailer edges
- • Decreased vacuum along bottom of truck and wheels
- • Exponential drag force increase with velocity (validates drag equation)
Comparative Analysis
- • Comparable to cab fairings: 19.1% reduction
- • Better than base flap device: 15% reduction
- • Superior to side skirts: 10% reduction
- • Experimental results higher than simulation due to smoother 3D printed surfaces
- • Valid comparison despite wind tunnel wall effects
Technologies Used
Publication
Stanford Intersect Journal
Published research paper on biomimetic approaches to aerodynamic drag reduction in heavy vehicle transportation, detailing the computational analysis methodology, wind tunnel validation, and economic implications of boxfish-inspired trailer design.
Journal: Stanford Intersect Journal
Volume: Vol 16, No 3 (2023)
Research Area: Transportation Engineering & Biomimicry
Environmental Impact
The 13.8-16.7% drag reduction achieved through biomimetic design translates to 14-16% efficiency gains for Class-8 trailers. When applied to all Class 8 trucks across the US alone, this could result in 60-85 million tons of carbon emissions reduction. To put this number into perspective, the countries of Norway and Sweden together emitted 55 million tons of carbon in 2019.
Future Work
There are two variants of class 8 trucks: unibody trucks where the container is completely attached to the chassis (mail trucks, moving and delivery vehicles), and detachable rectangular containers. The concept can be directly applied to unibody trucks, but for detachable containers, add-on shapes can be used that could be attached during transport and detached post transport.
This mechanism of adaptation during movement would allow for scalability of this solution across a wider fleet of vehicles. Future improvements can address wind tunnel limitations by applying friction reducing paint and increasing the size of the flow chamber to minimize streamline compression.