J. Cent. South Univ. (2015) 22: 4645−4652 DOI: 10.1007/s11771-015-3015-7
Aerodynamic drag reduction of heavy vehicles using append devices by CFD analysis Mehrdad khosravi1, Farshid Mosaddeghi2, Majid Oveisi3, Ali khodayari-b4 1. Young Researches and Elite Club, Borujerd Branch, Islamic Azad University, Borujerd, Iran; 2. Faculty of Mechanical Engineering, University of Tabriz, Tabriz, Iran; 3. Lecturer Department of Marine Engineering, Chabahar, Maritime University, Iran; 4. Department of Mechanical Engineering, Urmia University, Urmia, Iran © Central South University Press and Springer-Verlag Berlin Heidelberg 2015 Abstract: Improving vehicle fuel consumption, performance and aerodynamic efficiency by drag reduction especially in heavy vehicles is one of the indispensable issues of automotive industry. In this work, the effects of adding append devices like deflector and cab vane corner on heavy commercial vehicle drag reduction were investigated. For this purpose, the vehicle body structure was modeled with various supplementary parts at the first stage. Then, computational fluid dynamic (CFD) analysis was utilized for each case to enhance the optimal aerodynamic structure at different longitudinal speeds for heavy commercial vehicles. The results show that the most effective supplementary part is deflector, and by adding this part, the drag coefficient is decreased considerably at an optimum angle. By adding two cab vane corners at both frontal edges of cab, a significant drag reduction is noticed. Back vanes and base flaps are simple plates which can be added at the top and side end of container and at the bottom with specific angle respectively to direct the flow and prevent the turbulence. Through the analysis of airflow and pressure distribution, the results reveal that the cab vane reduces fuel consumption and drag coefficient by up to 20 % receptively using proper deflector angle. Finally, by adding all supplementary parts at their optimized positions, 41% drag reduction is obtained compared to the simple model. Key words: aerodynamics; computational fluid dynamic (CFD); append device; drag reduction; fuel consumption
1 Introduction Nowadays, demand for reducing the fuel consumption of vehicles within the automotive industry is increasing. Increased fuel price and development of more fuel-efficient vehicles have raised this issue. Heavy commercial vehicles in comparison to other ground vehicles, due to high aerodynamic drag, have low fuel efficiency. Recent researches [1−4] about fuel reduction technologies for trucks show that aerodynamic improvement is one of the most important technologies when it comes to fuel saving. A large commercial vehicle travelling at 100 km/h consumes about 50% of the total fuel to provide the power required to overcome the aerodynamic drag [5]. It has been investigated that on average a heavy commercial vehicle’s annual mileage varies between 130000 km and 160000 km [6]. Due to such a high mileage, any reduction of aerodynamic drag will result in significant fuel savings and reductions in greenhouse gas emission. In Ref. [7], it is found that over 1.3 trillion litters of petrol and diesel is consumed by road vehicles. This also relates to high levels of pollution (CO2), from the burning of fossil fuels. KASSIM and
FILIPPONE [8] used various aerodynamic retrofitting techniques to reduce heavy vehicle drag and fuel consumption. They numerically simulated realistic on-road operations to represent the effectiveness of these retrofits on various vehicle weights and driving cycles. Their results demonstrate that fuel economy improvement could be achieved from less than 1% to almost 9% of an annual mileage. HYAMS et al [9] proposed numerical solutions to investigate unsteady aerodynamic flows affecting the fuel economy of Class 8 trucks and validated their results with comparison to experimental data. Nowadays, modern trucks are equipped with a range of drag reducing mechanisms. A lot of numerical and experimental studies about the heavy vehicle aerodynamics have been carried out. Most of the research today is done on reducing drag on newly designed trucks [10−15]. As a result, little work is done on current designs. Due to the high number of trucks already on the road today, as well as the fact that many of these older designs are still sold, it is imperative to find ways to reduce the drag on these designs. McCALLEN et al [16] developed a method for reducing the aerodynamic drag of heavy vehicles by numerical
Received date: 2015−01−16; Accepted date: 2015−05−03 Corresponding author: Mehrdad khosravi; Tel: +98−9120880967; E-mail:
[email protected]
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simulation and analysis of aerodynamic flow along with experiments on a model of an integrated tractor-trailer. They developed advanced computational models that use an LES approach, in addition to the use of state-of-theart RANS modeling. ENGLAR [17] studied the effect of the gap between the tractor and trailer. He used a generic truck model for wind tunnel tests. MIRALBES and CASTEJON [18] studied the aerodynamics of boat tails to reduce the aerodynamic drag coefficient in heavy vehicles using FLUENT package and comparing the results with the ones of the vehicle without them. HÅKANSSON and LENNGREN [19] tested different aerodynamic trailer devices and aerodynamically shaped trailers by means of computational fluid dynamics (CFD), in order to investigate their influence on the flow around the truck. In addition, drag contribution from different regions was analyzed to see where it is possible to gain most drag. Furthermore, devices that were implemented in the undercarriage and base region presented the best results, which indicate that these regions are most susceptible for drag improvements. They also showed the importance of integrating the tractor and trailer development for further aerodynamic improvements. SELENBAS et al [20] carried out a CFD study in order to design and optimize the cabin geometry and its various parts for drag reduction including the side deflectors, the mirrors and the sun visor. For the validation of computational results, an experimental investigation using a 1/5 scale truck model was conducted in a wind tunnel. Both steady and unsteady CFD simulations were performed. The comparison of steady/unsteady results revealed that the time-averaged unsteady flow characteristics were practically the same as the steady calculations for design purposes. In Ref. [21], the effectiveness of adding a fairwater on the top of truck’s head was investigated for different shrink angles. In this work, a comprehensive CFD study was conducted to investigate the influences of supplementary parts on drag reduction of heavy commercial vehicles using ANSYS CFX package.
2 Modeling and simulation At first, a 15.85 m semi-trailer is modeled as two boxes by using CATIA and then the meshing is performed and boundary conditions are defined. The vehicle is assumed to have speed of 27 m/s at 25 °C, at pressure of 105 Pa. The vehicle model was imported to the ANSYS CFX environment. The main parameters of this model are illustrated in Fig. 1. Also, the supplementary parts studied here are classified as 1) parts that have been added to the cab, 2) parts that have been added to the container, and 3) parts that have been added to the chassis.
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Fig. 1 Schematic of semi-trailer (Unit: mm)
The most popular supplementary part is deflector that is a flat and formed plate fixed on the top of cab at different angles. It is added to the simple box model and then it is meshed. Finally, current flow passes over it to evaluate the effect of this set on aerodynamic drag reduction. The same procedure is performed for cab vanes. In the next step, the gap between cab and container is changed to find its effect on aerodynamic drag variations. Other popular supplementary parts are two types of side panels, one of which is installed under the container and fills the space between wheels, and the other is installed at both sides of container and covers the wheels to regulate the current flow around wheels and then reduces the drag. Front fairings are simple plates that are formed to the quarter cylinder and are riveted to the narrow edges of the direct flow. Back vanes are some simple rectangular plates that are installed at the top end and side end of container with angles toward the flow; this part has a considerable effect on drag reduction. In this work, the length and angles of this part have been changed to find the optimized length and angle for the drag reduction [22−23]. Base flaps are simple trapezoidal plates installed at the bottom of container and converge together to postpone the separation and regulate the turbulence. It is known from literature that the effect of base flaps depends on Reynolds number and the relations are so complicated. In this work, it is supposed that the flow is laminar and the effect of base flap in drag reduction in different widths and angles is investigated by using CFX. At the end, all these supplementary parts are added to the base model to compare the drag coefficient of this set with simple box model. The schematic diagrams of all supplementary parts are illustrated in Fig. 2. The model and mesh of semi-trailer are depicted in Fig. 3. Definition of wind tunnel boundary conditions is shown in Fig. 4. The boundary conditions of the trailer with deflector in 105 Pa and 25 °C and steady state are defined and after setting the CFX-Solver, the results may be drawn.
3 Results and discussion 3.1 Deflector effect Deflector is a plate or other attachment for deflecting
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Fig. 2 Various supplementary parts
a flow of air. Deflector angle is defined with the step between deflector and container height (H1−H2). The drag coefficient may vary by changing the angle of deflector from 5° to 35° and subjecting the CAD models (without and with deflector) to the defined current flow (27 m/s, horizontal, laminar), as illustrated in Fig. 5. Also, the results are summarized in Table 1 for comparing deduced forces and drag coefficient (Cd) by changing the angle of deflector. Table 1 Effect of deflector angle on drag coefficient
Fig. 3 Model (a) and mesh (b) of semi-trailer
Fig. 4 Wind tunnel boundary conditions in ANSYS CFX
(H1−H2)/mm
Degree/(°)
Force/N
Cd
−758
5
1796
0.813085
−562.5
10
1673
0.757401
−361
15
1544
0.699
−150
20
1402
0.634714
0
23
1450
0.656444
74.5
25
1490
0.674553
318.5
30
1623
0.734765
588.5
35
1760
0.796787
According to Fig. 5, the best angle of deflector is 19.6° and when the angle exceeds or lessens this angle, the drag coefficient increases because of disturbing the airflow. If the angle lessens the optimized angle, the pressure distribution on container causes a drag increase and if the angle exceeds this angle, the distributed airflow causes a drag increase too, so the optimized angle related to the mentioned height is calculated to be 19.6°. The step between deflector and container is a more
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appropriate criteria than deflector angle to compare the drags because this is independent of vehicle dimension (Fig. 6).
Fig. 7 Flow streamlines over vehicle with various steps: (a) Optimized step; (b) Minimum step
Fig. 5 Effect of deflector angle on drag coefficient
Fig. 6 Effect of step between deflector and container on drag coefficient
From both analyses, it is found that the best angle is about 20° and the best step is H1−H2=−150 mm. The flow streamlines for both cases are shown in Fig. 7. Figure 8 demonstrates the pressure distribution at various parts of the vehicle for models with and without deflector. As can be seen, utilization of deflector has a profound effect on drag reduction. 3.2 Cab vane corner effect Cab vane corner is a broad blade attached to the corners of cab front side that pushes the wind to change the airflow and makes the vehicle have better
Fig. 8 Comparison of pressure distribution at various parts of vehicle: (a) Without flap; (b) With flap
aerodynamic. The effect of cab vane corners is studied in presence of deflector to find the final drag coefficient. These small parts cause a great change in airflow and turbulence around the edges of cab front side with postponing the start of turbulence. The velocity streamlines are shown in Fig. 9. As shown, the turbulence around both frontal edges of trailer is considerably decreased. CFX analysis of the simple model leads to the drag coefficient of 0.8521 with deflector at 19.6°, drag coefficient of 0.639 and the drag coefficient of 0.589 with deflector and cab vane corner. As listed in Table 2, by adding cab vane corners to the previous trailer, about 7.8% drag reduction is achieved.
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Fig. 9 Comparison of effect of cab vane corner on drag reduction: (a) Without cab vane corner; (b) With cab vane Table 2 Drag reduction in presence of cab vanes Model
Cd
Reduction/%
Simple model
0.8161
0
With deflector in best position
0.639
21
With deflector and cab vanes
0.589
7.8
3.3 Cab/trailer gap The gap between the cab and container is normalized with G/L to investigate its effect, where G is the distance between cab and container, L= Af (frontal area). In order to evaluate the drag coefficient, the trailer is tested in various positions of gap at defined area of 22 m/s current flow, as shown in Fig. 10. The existence of gap between cab and container is inevitable, despite the fact that increasing the gap would increase the drag coefficient, there is a critical point (G/L=0.5) that the turbulence flow inside this gap changes its phase from steady to unsteady and a sudden drag reduction occurs. 3.4 Trailer side panels There are two types of side panel, one of them is a box installed below the container and between wheels, and the other is a cover installed at the both sides of container and hides the space between wheels. These two types of side panel are assembled to the CAD model and do the same at 27 m/s current flow with CFX. Drag reduction would be 0.8% and 3.9% relative to the trailer with deflector when side panel is a box and a cover, respectively. As can be seen in Fig. 11, when the side panel is a cover, the current flow is more regular than the other.
Fig. 10 Effect of gap between cab and container on drag reduction: (a) Schematic diagram; (b) Trailer; (c) Cabin
Fig. 11 Comparison of effect of side panels on flow streamlines around vehicle: (a) Without side panel; (b) With side panel
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3.5 Front fairing The same simulation is accomplished with front fairing once in comparison with simple CAD model and once in comparison with the trailer with deflector, as shown in Fig. 12. In simple model, the drag coefficient has been reduced from 0.778 to 0.639, and in the second model, no drag reduction occurs in this case because the effect of front fairing is dependent on the deflector angle.
Fig. 12 Comparison of effect of front fairing on flow streamlines around vehicle: (a) Without front fairing; (b) With front fairing
The effect of back vanes in regulating the turbulence at the back of the vehicle is shown in Fig. 14. It is obvious that turbulent flow streams are much less for the vehicle with back vanes.
Fig. 13 Effects of back vanes on air flow streamlines around vehicle: (a) Schematic view; (b) Velocity streamline
3.6 Back vane The same simulation considering different sizes of back vanes is accomplished. As shown in Fig. 13, with increasing the length of back vane the drag coefficient would decrease; however, with increasing the width of back vane, because increasing effective frontal area is subjected to the flow, the negative effects are observed. As shown in Table 3, four different back vanes with various dimensions are investigated and the drag coefficient reduction is obtained. Table 3 Effects of back vane size on drag reduction Initial Cd without Dimension Cd with back wane back vane 50 mm×20 mm 0.639 0.617 100 mm×20 mm
0.639
0.601
150 mm×20 mm
0.639
0.586
200 mm×20 mm
0.639
0.528
It should be noted that as the length of backs increases, the total weight of vehicle would be increased and it is a negative effect. So, this parameter should be optimized to obtain the optimal size of back vane. Figure 14 shows a comparison to the air flow streamlines in two cases for the vehicle with and without back vanes.
Fig. 14 Comparison of air flow streamlines at end of vehicle with (a) and without (b) back vanes
3.7 Base flap In this stage, the effects of base flaps in different sizes and angles are investigated. The effect of this part is completely dependent of Reynolds number. According to other research works [12], the best length of this part is 1/4 of container width. So, to clarify the optimum base flap, according Fig. 15, just its angle has been changed from 10° to 34°, the same with CFX. Figure 16 represents that the best angle would be about 25° at length of 62 cm. At this angle, drag coefficient decreases by about 4.85% (from 0.639 to 0.608).
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drag coefficient with CFX is computed to compare with simple model. A considerable drag coefficient reduction from 0.778 to 0.46 (41% drag reduction) has been achived. Figure 18 depicts the air flow around the trailer with all supplemtary parts and the simple trailer. It can be seen that the air flow is much regular for the first case.
Fig. 15 Length and angle of base flap
Figure 17 shows the air streamlines over the vehicle with and without use of base flaps. As can be seen, utilization of base flap has a considerable effect on drag reduction. 3.8 Addition of all supplementary parts Finally, all suplementary parts in their best condition are added, this CAD model is subjected to the defined current flow (27 m/s), and then this set is meshed and the
Fig. 16 Effect of flap angle on drag coefficient
Fig. 17 Comparison of effect of utilization of base flap on drag reduction: (a) Front view without base flap; (b) Front view with base; (c) Top view without base flap; (d) Top view with base flap
Fig. 18 Comparison of air flow around trailer with and without supplementray parts: (a) Front view without supplementary parts; Front view withsupplementary parts; (c)Top view without supplementary parts; (d)Top view with supplementary parts
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Figure 19 illustartes the pressure distrubition contour around the trailer with all supplemtary parts and the simple trailer. [7]
[8]
[9]
[10] [11]
[12]
Fig. 19 Comparison of pressure distribution around trailer with (a) and without (b) supplementray parts
4 Conclusions 1) The most effective supplementary part is deflector, by means of which, the drag coefficient decreases considerably at a special angle. 2) Significant drag reduction is noticed by adding two cab vane corners at the both frontal edges of cab. 3) The analyses of airflow and pressure distribution indicate that deflector angle and the cab vane corner should carefully be optimized. 4) It is shown that back vanes and base flap have noticeable effect on drag reduction. 5) With installing all supplementary parts at their optimized positions, about 41% drag reduction is enhanced compared to the simple model.
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