While external aerodynamic analysis using CFD and wind tunnel approaches has become a regular tool for passenger cars, its use is far more profound to evaluate aerodynamic stability of heavy vehicles such as trucks, buses and trailers. With increasing fuel costs and government implications on energy conservation, heavy vehicle manufacturers are being consistently pushed to improve designs and reduce fuel consumption.
While weight reduction and engine performance optimization are two of the major concerns, the effects of aerodynamic forces are also of prime interest, as it has a significant impact on fuel economy. To achieve an aerodynamically optimized body designed for heavy vehicles, the wind tunnel approach can turn out extremely costly. On the other hand a numerical approach is quite challenging as an accurate flow prediction is necessary.
Aerodynamic Study on Heavy Vehicles
The prime role for any aerodynamicist working on vehicle design is to minimize the effect of drag forces that resist the vehicle motion. Having a direct impact on fuel consumption, it is extremely crucial to find an exterior body design that has a least drag co-efficient (Cd). A common thumb rule used in the industry is that to reduce the fuel consumption by 1%, a decrease of 2% in aerodynamic drag at 60mph is required. A table below shows this relationship at different speeds.
Considering an example of a truck-trailer unit, the design consists of several regions where the chance of an increase in drag is eminent. The image below demonstrates the same, requiring modification in the design. The preliminary resistance to motion occurs at the front grille, provided to cool the engine but increases the drag co-efficient greatly. The gap between truck and trailer is also a critical region where formation of turbulence can disrupt the flow field and increase drag. Additionally, the air flow underbody and past the trailer has every chance to get disturbed and generate wake, restricting the vehicle motion even further.
Reducing Drag Using CFD
To achieve desired reduction in Cd values, addressing the critical regions by providing aerodynamic flaps, sealing the gap between truck-trailer or optimizing the body design would solve the problem to some extent. However, evaluating the efficiency of these modifications using wind tunnel experiments can be costly. Thankfully, the proliferation and advancement in computational tools to simulate complex flow behavior finds an excellent application for such aerodynamic cases.
With validated turbulence models, flow stagnation regions, wake formation and drag effects can be quickly examined. CFD has the capability to simulate real world conditions and provides detailed flow visualization, which is helpful in predicting the performance of aerodynamic components such as flab or seals right before any actual modification. Predicting drag right in the early design stage prevents excessive manufacturing costs and the time associated with it.