Linus Hjelm

Aerodynamic Effects of Roof Deflector and Cab Side Extenders for Truck-Trailer Combinations

Today there are a large variety of drag reducing devices for heavy trucks that are commonly used, for example roof deflectors, cab side extenders and chassis fairings. These devices are often proven to be efficient, reducing the total aerodynamic resistance for the vehicle. However, the drag reducing devices are usually identical for a specific pulling vehicle, independent of the layout of the vehicle combination. In this study, three vehicle combinations were analyzed. The total length of the vehicles varied between 10.10m and 25.25m. The combinations consisted of a rigid truck in combination with one or two cargo units. The size of the gap between the cargo units differed between the vehicle combinations. There were also three configurations of each vehicle combination with different combinations of roof deflector and cab side extenders, yielding a total number of nine configurations. The aim of this investigation was to determine the aerodynamic effects of the roof deflector and cab side extenders as a function of the type of vehicle combination. Important factors were the total length of the vehicle combination and the influence of the drag reducing devices further downstream. The investigation was performed using Computational Fluid Dynamics (CFD). The results from the investigation showed that the effect of the two drag reducing devices analyzed was different depending on the type of vehicle combination. It was established that the roof deflector and cab side extenders were always efficient in reducing drag both in 0° yaw and 5° yaw but the magnitudes differed between the configurations. The largest effects of the drag reducing devices were seen for the truck including the 1st cargo unit; the influence of the drag reducing devices on the 2nd cargo unit was smaller. The aerodynamic reductions were diminishing downstream, why it was concluded that it is of great importance to improve the aerodynamic design of the rest of the vehicle to maintain the positive effects of the drag reducing devices.

Computations and full-scale tests of active flow control applied on a Volvo truck-trailer

Large-eddy simulations and full-scale investigations were carried out that aimed to reduce the aerodynamic drag and thus the fuel consumption of truck-trailers. The computational model is a relevant generic truck-trailer combination, and the full-scale is a corresponding Volvo prototype vehicle. Passive and active flow control (AFC) approaches were adopted in this work and applied at the rear end of the trailer. Flaps were mounted at an angle that induces separation, and synthetic jet actuators were placed close to the corner of the rear end and the flaps. The drag reduction obtained is in the order of \(30\,\%\). The flow was analyzed by comparing the phase-averaged and time-averaged flow field of the unforced and the forced cases. The full-scale prototype is a Volvo truck-trailer. The trailer is mounted by three flaps at the rear sides and top end. The actuators consist of loudspeakers in sealed cavities, connected to amplifiers that are supplied with a frequency generator controlled by LabVIEW. The full-scale test includes passive and active flow control investigations by varying the flap angle, with and without AFC, investigating different frequency and slot angle configurations. The fuel flux was measured during the full-scale test. The test shows a fuel reduction of about \(4\,\%\) in a comparison of two flap angles. The test of active flow control shows a reduction of \(5.3\,\%\) compared to the corresponding unforced case. Compared with the baseline case, the passive flow control fails to reduce the total fuel consumption.

Experimental and Numerical Investigation of Wheel Housing Aerodynamics on Heavy Trucks

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. To reduce blockage effects, the model was sectioned to reduce both height and width. In the experiments, pressure sensors and static pressure taps located in the wheel housing were utilised, and the simulations replicated the boundary conditions of the wind tunnel experiments, both in terms of the geometry of the model and wind tunnel as well as the modelling of the ground simulation. It was found that the wheel wake structures changed significantly when ground simulation was utilised. The main outflow through the wheel housing was influenced by the wheel rotation and took place further upstream, which resulted in large differences in the flow field downstream of the wheel. The influence of different strategies for modelling the wheel rotation in CFD was investigated and it was found that the Sliding Mesh approach was the most accurate method.

Influence of Different Truck and Trailer Combinations on the Aerodynamic Drag

The aim with this investigation was to study the aerodynamic properties of truck-trailer combinations of varying lengths. The aerodynamic properties of the combinations were evaluated in order to study similarities and differences in the flow field between different configurations. By the use of Computational Fluid Dynamics (CFD) six different types of truck-trailer combinations used for long hauling have been evaluated. The combinations have a total length varying between 10.10 m and 25.25 m and consist of either a tractor or rigid truck in combination with one or two cargo units. All of the combinations are commonly found on roads in Sweden and several other countries in Europe. The results from the simulations show that the aerodynamic properties differ significantly for the truck-trailer combinations. It was found that the longer vehicle combinations are much more sensitive to yaw conditions than the shorter combinations. What was also evident is that the gap between the first and second cargo unit plays a significant role for the aerodynamic properties. Furthermore it was established that there is potential for fuel saving by a strategic choice of truck-trailer combination. According to the simplified evaluation made in this study the 25.25 m combinations show a lower power required per loading length (LL) and are therefore shown to be more efficient than the shorter combinations.

Wheel Housing Aerodynamics on Heavy Trucks

Modern trucks have a reasonably optimised cabshape, and there exist several OEM and aftermarket devices for drag reduction for heavy trucks as well. To further reduce the aerodynamic drag major changes to the current layout of the vehicle are required, or the focus must be shifted from the cab and tractor trailer gap to other regions of the vehicle. The drag of the underbody, including wheel housings, wheels and engine compartment, represents a significant proportion of the aerodynamic drag and there has not been much investigation in this specific area on heavy trucks. To be able to reduce the fuel consumption and to fulfil the legislated emission standards for heavy trucks it is important to take all areas of the vehicle under consideration, and even though the individual improvements may be small, the total drag reduction will be substantial. In order to study the flow close to the vehicle underbody it is important to utilise the correct boundary conditions, that is, moving ground and rotating wheels. This work has focused on the flow in the front wheel housings. The flow field around the front wheels under the influence of ground simulation on a heavy truck of standard European configuration was investigated using numerical simulations. The in- and outflow to the wheel housing was located and the vortices originating from the front wheels were identified. This information was then used to identify which areas of the wheel housing having the greatest potential for aerodynamic improvements by changing the front wheel housing design. Furthermore, several wheel housing design parameters were defined, and their influence on the flow field and aerodynamic drag were investigated. Examples of these parameters are the shape of the wheel housing opening and implementation of wheel housing ventilation. It was found that there is potential for reducing the aerodynamic drag by applying these geometric changes to the wheel housing, and several of the configurations could be implemented on current production vehicles.

Heavy Vehicle Wheel Housing Flows - a Parametric Study

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. It was also found that even though there were large differences in the drag force of each component the change in total drag was small.

An Investigation of the Aerodynamic Drag Mechanisms Due to Ground Simulation in Yawed Flow Conditions for Heavy Trucks

The drag from the underbody, including wheels and wheel housings, constitute a significant amount of the total aerodynamic drag of heavy trucks. 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. Earlier studies on passenger cars have described the drag mechanisms involved when implementing proper ground simulation. However, model scale wind tunnel tests of heavy trucks have shown an opposite trend on the drag coefficient with ground simulation compared to passenger cars. An important aspect of truck aerodynamics is the yaw dependency of the drag coefficient when the vehicle is exposed to crosswinds. Therefore it is of outmost importance to evaluate the performance of the vehicle in yawed flow conditions during the development process. In the current study the influence of ground simulation, including moving ground and rotating wheels, on the flow has been investigated at several yaw angles using Computational Fluid Dynamics. For the simulations a tractor trailer geometry corresponding to a standard European configuration was used, and the geometry included a fully detailed underbody and engine compartment. It was found that there was a significant difference in the structure of the wheel wakes and this was mainly due to the implementation of wheel rotation. It was also shown that the drag coefficient increased due to the utilisation of ground simulation in yawed flow conditions.

European Truck Aerodynamics – A Comparison Between Conventional and CoE Truck Aerodynamics and a Look into Future Trends and Possibilities

The aerodynamic situation for trucks on the European market differs from that in North America on a number of points. Perhaps the most significant difference is that in Europe trucks are of the CoE configuration (Cab over Engine) and in North America trucks are of the conventional type with a hood. Another major difference is that trucks in Europe are speed limited to 90 km/h (56 mph) which of course means that aerodynamics as a whole has less of an impact there. These differences are primarily dictated by different legislations, which in turn have a lot of different side effects. This paper will high-light some of the differences and their impact on aerodynamics, as well as taking a look at possible future ideas such as: extended front or short nose, ride height adjustments, convoy driving, etc.