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Almost everyone appreciates the beautiful (and often sleek) styling of a new automobile model. However, the smooth curves of a new car model are as much to do with aerodynamic necessity as aesthetic pleasure.

A typical sedan cruising on the freeway will use about 18 percent of its fuel energy in overcoming aerodynamic drag*. Since the power required to overcome drag rises with the cube of velocity, the faster you go, the more fuel you use. (If this seems deceptively low, it’s because the process of turning gasoline into the kinetic energy of the vehicle is tremendously inefficient, thanks to the second law of thermodynamics and the fact that entropy sucks).

In order to maximize the driving range of a vehicle, it’s generally a good idea to minimize its drag (and therefore fuel consumption). This is a particular concern for drivers of electric vehicles, whose range is limited by a fixed battery capacity.

Of course, not all of the energy lost to drag is “wasted.” Some of the airflow is deliberately directed through the radiator grill and (under the floor of the car) in order to provide cooling the engine, battery and electronics (dealing with the consequences of the Second Law of Thermodynamics).  Although this increases the drag signature of the vehicle, it is necessary to ensure reliability and durability of the powertrain.

In the past, physical testing was used to determine whether the aerodynamic cooling of the vehicle was sufficient to cope with the heat rejection from the engine. If it was determined that a vehicle would overheat during a particular driving scenario, the only solution was to add bigger heat exchangers, larger fans or additional ducting to force more air through the heat exchangers. This has the consequence of increasing the drag of the vehicle, while also increasing its weight, both of which have a negative impact on the fuel consumption of the vehicle. This type of post-hoc modification, typically used to occur late in the vehicle program (the stage at which physical testing was conducted), at which stage changes to the vehicle design are typically very expensive.

Today physical testing of vehicle aerodynamics and cooling has been almost entirely replaced by computer simulation, which is deployed from the earliest stages of the design process to understand and improve the performance of underhood cooling strategies. Unlike physical tests, computer simulations are able to predict the performance of the vehicle under a wide variety of real-world operating conditions (such as when the vehicle is stuck in traffic or towing a trailer up a hill). By understanding exactly how the energy flows through the underhood and underfloor regions, designers can maximize the cooling influence of the airflow, while minimizing the drag penalty from it.

Computer simulations are also used to predict (and improve) the overall drag signature of the vehicle, including the influence of components such as wipers and the rotating wheels (including the effect of tire tread, wheel rims and overall wheel design), which were previously neglected because of the cost of capturing the unsteady wake interactions, but are now possible with the advent of cloud computing and large network clusters.

With design space exploration, engineers can investigate the trade-offs between different design parameters, such as finding the best compromise between overall vehicle drag and cooling performance.

The lowest wind resistance is typically found with the front grill blocked and no cooling air through the vehicle. However, under those conditions, the engine (or battery) would quickly overheat. Manufacturers are using design exploration techniques to develop dynamic cooling strategies in which the radiator grill adjusts automatically to allow more air in only when extra cooling is required. Grid morphing is also being used to automatically improve the design of the front of the vehicle in order to ensure maximum airflow and minimal cost.

For electric vehicles, the trade-offs can get even more complex, as there is less energy available for ensuring the comfort of passengers (through air conditioning systems). Energy “wasted” in keeping vehicle occupants cool detracts directly from the overall range of the vehicle. Early design simulation can be accomplished by coupling systems using LMS Virtual.Labs Amesim. The system simulation can then be linked to a component, such as the battery, cabin, or aerodynamics to optimize the performance for the configuration, hence quickly reducing the overall energy use of the vehicle.

With the availability of predictive engineering, STAR-CCM+® software is aiding to find Better Designs, Faster. Unsteady aerodynamics is helping users investigate complex interaction from wheels. Red Cedar Technology’s HEEDSTM software helps with design exploration to find better ways to reduce energy.


*For a vehicle with a Cd of 0.3 and a frontal area of 2m2 has an aerodynamic force of 330 N acting upon it at 30 m/s (which is approximately 70 mph), which requires about 10 kW in energy to overcome. If the fuel economy of the vehicle is about 50 mpg (which is about 2 kJ/m). So to travel at 30 m/s requires about 60 kW of chemical energy).

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