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The effect of blockage due to the presence of the wind tunnel walls has been known since the early days of wind tunnel testing. Today there are several blockage correction methods available for correcting the measured aerodynamic drag. Due to the shape of the test object, test conditions and wind tunnel dimensions the effect on the flow may be different for two cab variants. This will result in a difference in the drag delta between so-called open-road conditions and the wind tunnel. This makes it more difficult to evaluate the performance of two different test objects when they are both tested in a wind tunnel and simulated in CFD. A numerical study where two different cab shapes were compared in both open road condition, and in a digital wind tunnel environment was performed.

Scania AB has opened the new CD7 climatic wind tunnel test facility, located at the Scania Technical Center in Södertälje, Sweden. This facility is designed for product development testing of heavy trucks and buses in a range of controllable environments. Having this unique test environment at the main development center enables Scania to test its vehicles in a controlled repeatable environment year round, improving lead times from design to production, producing higher quality and more reliable vehicles, and significantly improves the capability for large vehicle performance research. This state-of-the-art facility provides environmental conditions from -35°C to 50°C with humidity control from 5 to 95 percent. The 13 m2 nozzle wind tunnel can produce wind speeds up to 100 km/h. The dynamometer is rated at 800 kW for the rear axle and 150 kW for the front axle, which also has ±10° yaw capability.

Wheel and underbody aerodynamics have become important topics in the search to reduce the aerodynamic drag of the heavy trucks. This study aims to investigate, experimentally as well as numerically, the local flow field around the wheels and in the wheel housing on a heavy truck; and how different approaches to modelling the wheel rotation in CFD influences the results. Emphasis is on effects due to ground simulation, and both moving ground and wheel rotation were requirements for this study. A 1:4-scale model of part of a heavy truck geometry has been developed. During the model design numerical simulations were used to optimise the shape, in order to replicate the flow field near the wheel of a complete truck. This was done by changing the flow angles of the incoming and exiting flows, and by keeping the mass flow rates in to, and out of, the wheel housing at the same ratios as in a reference full size vehicle.

The drag from the underbody, including wheels and wheel housing, constitutes a significant amount of the total aerodynamic drag of heavy vehicles. A correct simulation of the underbody boundary conditions, including rotating wheels and moving ground, has turned out to be of great importance in the minimising of the aerodynamic drag. In the current study several front wheel housing design parameters have been evaluated using Computational Fluid Dynamics (CFD). Design concepts, like enclosed inner wheel housings, underbody panel and wheel housing ventilation, were evaluated by flow analysis and comparison of the drag force contribution. It was shown that changes to the wheel housing geometry had an important impact on the local flow field and force distribution. The total drag of the vehicle decreased with reduced wheel housing volume and wheel housing ventilation can reduce the aerodynamic drag significantly provided it is designed properly.

There are now several wind tunnel facilities within Europe for testing passenger cars with and without moving ground and rotating wheel conditions (henceforth abbreviated to MVG&RW conditions). Within these facilities, the drag of a car under MVG&RW conditions is typically less than the drag of a car under stationary ground and stationary wheel conditions. This drag difference has been found to vary from a decrease of about 25 drag counts to a small drag increase according to published sources. A drag reduction of 10 to 20 drag counts is more typical, however.

Numerical simulations of the flow inside a passenger compartment are compared with experimental data obtained from velocity field measurements using Particle Image Velocimetry (PIV). Comparisons are made in the front part of the passenger compartment with the air-distribution system operated in a ventilation mode. The sensitivty of the CFD-model to the boundary conditions was investigated and two different turbulence models were tested. Computations and experiments resulted in similar results for the overall flow field, however, rather large differences were found in the vertical spreading of the jet from the dashboard nozzle. The width of the jet was lower in the measurements than in the simulations. This difference is believed to be caused by the high diffusivity obtained when using a k-epsilon model in combination with an unstructured grid.