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.
Full scale CFD simulations of the Generic Conventional Model GCM , a simplified model of a Class 8 truck, were used to explore passive devices for improving the drag performance of the trailer base. Significant improvements over conventional straight base flaps were achieved using an Extended Bent EB flap that stays within the length limits imposed by US federal law.
An additional boat tail device for the cab bogie base was also found to yield improvements in the base drag in that region. An under-trailer scoop to generate air for pressurizing the trailer base or for use in active flow control devices was found to add too much drag to be effective. Due to increasing environmental constraints it is crucial for the heavy duty transport sector to find solutions to stay sustainable in this aggressive and fast changing market. Improving aerodynamic quality, i. With numerical analysis of a standard tractor semi-trailer combination a general overview of the flow has been obtained: a highly complex flow and large separated regions can be observed.
Wind tunnel experiments with a scaled truck model are performed to experimentally analyze the effect of aerodynamic aids to reduce the drag of the tractor with trailer. Full-scale prototypes are built and road tests are conducted. The aerodynamic skirts, called SideWings showed an averaged fuel economy increase 1. During one year four different boat tail configurations are tested. 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 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. Compared with the baseline case, the passive flow control fails to reduce the total fuel consumption. Comparison studies have been conducted on a th scale model and a full scale tractor trailer of a variety of sealed aft cavity devices as a means to develop or enhance commercial drag reduction technology for class 8 vehicles. Eight base cavity geometries with pressure taps were created for the scale model. Drag data were acquired on the models using a 6-axis internal force balance for a range of yaw sweeps and at three Reynolds numbers for each base cavity.
Pressure surveys for selected base cavities were also completed for the same yaw angles and Reynolds numbers to quantify the change in base pressure. Moreover, drag computed from pressure data implied that although the base cavities decreased the base drag due to a pressure increase, the drag may have increased elsewhere on the model. Full-scale tests on the same geometry indicated a fuel savings of over 6. Overall, the use of these devices shows to be a viable, effective and economical way to reduce fuel consumption on ground transport vehicles.
The trucking industry is moving into a new era of development brought on by governmental concerns over energy independence as well as the realities of increasing fuel costs. This has renewed interest in optimizing the aerodynamics of Class 8 tractor-trailer trucks. Research continues in order to better understand the aerodynamics of these vehicles and further improve their efficiency. Scale model rolling road testing has been around for several decades. In fact, the earliest rolling road wind tunnel test of a Class-8 truck that the authors are aware of occurred in the late s [ 1 ].
In order to define the performance of a heavy duty truck, it has been well established that the use of wind averaged drag coefficients are required. To achieve wind averaged drag coefficients, it is necessary to measure data with a model in yaw for either a static floor or rolling road tunnel. The authors previously published results comparing a generic truck model tested using both a moving ground plane and a static ground plane.
This paper builds on the work using a more detailed and modern truck model. The improvements in aerodynamic drag by fitting trailer skirts are discussed. These drag reductions are converted into fuel efficiency improvements and are compared to SAE type II testing. The reader will be able to appreciate the difficulty associated with attempting to correlate wind tunnel results to SAE Type II results.
Some of the issues related to scale model rolling road testing such as Reynolds number dependency and yaw over a rolling road will be explored. Due to the size and nature of heavy duty trucks, cross winds must be taken into account. In addition to track or on-road testing, static tunnels have been the primary experimental tool for developing heavy duty trucks. Using the static floor tunnel method, heavy duty truck models have fixed non-rotating tires and are yawed via a turn table mounted in the floor of a tunnel.
Multiple studies have been published which illustrate the necessity for rotating the tires in order to achieve improved correlation to real-world results, for examples see [ 1 , 3 — 6 ]. However, these wind tunnel experiments lack a moving belt or a suction system to remove the boundary layer of the wind tunnel floor. Therefore, the use of a towing tank which is available at the TU-Berlin was considered and realized. In order to gain experience with the towing tank as a tool for vehicle aerodynamic studies, preliminary experiments with a bluff body with out any ground effect were carried out.
The box-shaped body corresponds to a simplified European trailer at a scale of Comparative measurements in the wind tunnel and the towing tank were carried out with the box. The effect of passive drag reducing devices, so called base flaps, were measured and also their positive effect was well reproduced in the towing tank. The towing tank has thus been established as an aerodynamic test facility.
The paper discusses an appropriate usage of large eddy simulation LES in external vehicle aerodynamics. Three different applications including wheelhouse flow, gusty flow and active flow control, are used to demonstrate how LES can be used to obtain new knowledge about vehicle flows.
The three examples illustrate the information that can be extracted using LES in vehicle aerodynamics. Large-eddy simulation LES was used to study the flow around a simplified tractor-trailer model. The model consists of two boxes placed in tandem. The front box represents the cab of a tractor-trailer road vehicle and the rear box represents the trailer. Two variants of the model were studied, one where the leading edges on the front box are sharp and one where the edges are rounded.
One small and one large gap width between the two boxes were studied for both variants. Two computational grids were used in the LES simulations and a comparison was made with available experimental force measurements. The results of the LES simulations were used to analyze the flow field around the cab and in the gap between the two boxes of the tractor-trailer model. Large vortical structures around the front box and in the gap were identified.
The flow field analysis showed how these large vortical structures are responsible for the difference in the drag force for the model that arises when the leading edges on the front box are rounded and the gap width is varied. Aerodynamic development of a full-scale truck presents a challenge for experimental testing due to the scale of the vehicle relative to most wind-tunnel test facilities.
Numerical simulation is becoming more prevalent for assessing design changes and improving vehicle aerodynamic drag. In this process, the cumulative effects of small design changes are needed. Furthermore, the drag must be considered both at zero crosswind and with five degrees crosswind yaw angle in order to properly represent typical driving conditions. It is well-known that the aerodynamics of heavy trucks are complicated by a very transient wake flow that causes large fluctuations in base pressure, and therefore in the drag coefficient. This effect is often even more prevalent at non-zero yaw angles.
The transient wake flow presents a challenge for effectively using simulation tools to predict the drag effects of small design changes, which may have some influence on the wake flow and base pressure. A square back Ahmed body is used to mimic a heavy vehicle, bus or truck, in order to derive control processes that yield a significant drag reduction. The first step is to analyse carefully the structures in the flow that have a strong impact on the drag forces. Then, active and passive controls using blowing jets or porous medium layers are presented and discussed.
The results are analyzed showing the direct impact of the control on the flow behaviour. This paper describes a development program taking small scale Aerodynamic laboratory experimental technology to full-scale road tests. A full-scale conceptual prototype was designed and characterized by TAU and adopted as a full-scale adjustable and cost-effective prototype by ATDynamics.
Bench-top tests at TAU validated the performance of the prototype as sufficient to warrant full-scale test success. A positive displacement pump operated by a gasoline engine supplied the compressed air. Two identical trucks were tested. Gauges located just downstream of the pump and at 5 locations along the supply ducts measured the supply pressures. Portable sensors measured the device suction pressure and pulsed blowing frequency.
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Out of the 81 possible configurations, determined by a 3 by 3 parameter space, 5 configurations were actually tested with valid results. This translates to a 1. Furthermore, it is still open how close to optimal is this device configuration. The expected road transport demand in the next twenty years and the increasing environmental constraints together with the rising fuel prices has renewed the interest in truck design; any reduction in truck fuel consumption can be associated with large annual fuel cost reduction and considerable emission savings.
Within the development of aerodynamic solutions numerical analysis tools, based on RANS equations, are often used to indicate flow phenomena and characteristics to design low drag bluff bodies. The presented work will discuss the similarities, but mainly the differences between wind tunnel experiments and the time-averaged numerical analysis. Rear pressure distributions are completely different when the numerical outcome is compared with the wind tunnel experiments. The CFD analysis of the boundary layer thickness is within acceptable resemblance with the wind tunnel measurements and the analytical power law model results.
Stereoscopic PIV results show different wake structures. Gandert M. The results of different approaches to quantitative flow visualization with large-scale capability are presented. The techniques were applied on the occasion of a measurement campaign in a medium-sized subsonic wind tunnel that addressed a passive wake control problem on an excursion boat. The obtained data was compared qualitatively with numerical results. Fluorescent tufts attached to the surface in the area of interest were filmed by a digital camera. This allowed for both, the identification of local flow direction and regions of detached flow on the surface of the model.
The sensor is recording temporal changes in intensity only and has a high dynamic range which made it possible to track the bubbles in ambient light conditions. The recordings yielded local flow velocity and the streamlines around the model. The pressure field or local flow direction were recorded by tracking hand-held probes in 3D space with a stereo vision system. By moving the probe in the volume of interest and with suitable post-processing applied, a quick quantitative assessment of the pressure field and flow topology was possible. The application of the different techniques confirmed the potential of quantitative flow visualization in large-scale testing.
Leading scientists and engineers from industry, universities and research laboratories, including truck and high-speed train manufacturers and operators were brought together to discuss computer simulation and experimental techniques to be applied for the design of more efficient trucks, buses and high-speed trains in the future.
This conference was the third in the series after Monterey-Pacific Groove in and Lake Tahoe in The presentations address different aspects of train aerodynamics cross wind effects, underbody flow, tunnel aerodynamics and aeroacoustics, experimental techniques , truck aerodynamics drag reduction, flow control, experimental and computational techniques as well as computational fluid dynamics and bluff body, wake and jet flows.
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