Cranfield
College of Aeronautics R o. 8622
October, 1986
Scale Model Wind Tunnel Measurements on the
Leyland T45 and D A F 3300 Vehicles used for the T.R.R.L.
Spray Dispersion Programme
by N A Cowperthwaite
College of Aeronautics
Cranfield Institute of Technology
Cranfield, Bedford MK43 OAL, England
TEC?
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K l u y v e r w e g 1 - 2 6 2 9 H S DELFT
Cranfield
College of Aeronautics Report No. 8622
October, 1986
Scale Model Wind Tunnel Measurements on the
Leyland T45 and DAF 3300 Vehicles used for the T.R.R.L.
Spray Dispersion Programme
by N A Cowperthwaite
College of Aeronautics
Cranfield Institute of Technology
Cranfield, Bedford MK43 OAL, England
ISBN 0 947767 53 3
£7.50
Research programme 'Improved Aerodynamics for Large Articulated Road Vehicles' sponsored by Department of the Environment; Department of Transport,
Transport and Road Research Laboratories, Crowthorne, Berkshire.
"The views expressed herein are those of the authors alone and do not necessarily represent those of the Institute."
SUMMARY
The dispersion of water spray by large articulated road vehicles, together with the degree of buffetting experienced by smaller vehicles in their proximity, is recognised as a source of nuisance to other road users. Both these problems are considered to be closely related to the aerodynamic characteristics of this type of vehicle.
In order to increase current understanding and to improve the future aerodynamic characteristics of these vehicles, an extensive experimental programme has been coordinated using both wind tunnel models and full scale vehicles. This report deals specifically with the scale model wind tunnel measurements on the two vehicles chosen to carry out the full scale programme ; a Leyland T45 'Roadtrain' and a DAF 3300, each fitted with a standard 40 ft articulated trailer.
Analysis is concentrated on variations in aerodynamic drag due to the use of several 'add-on' devices intended primarily to prevent water spray dispersion. Such modifications include: 'skirts', cab-container gap seals, container fairings, cab roof deflectors, etc. The two vehicles were chosen specifically as having fundamentally different baseline aerodynamic characteristics and this feature is seen to influence the comparative effectiveness of some add-on devices considerably. Measurements were made over a range of simulated crosswind angles and the corresponding wind averaged drag coefficients are presented in order to simplify assesment of potential performance variations.
CONTENTS
SUMMARY
NOTATION
LIST OF FIGURES
INTRODUCTION
1. EXPERIMENTAL PROCEDURE AND DATA REDUCTION
2. DESCRIPTION OF ADD-ON DEVICES
2.1 Flate plate cab roof deflector 2.2 Curved plate roof deflector
2.3 Three dimensional cab roof deflector (CoA type) 2.4 DAF three dimensional cab roof deflector
2.5 T45 three dimensional cab roof deflector 2.6 Tractor front wheel skirts
2.7 Short tractor skirts 2.8 Long tractor skirts 2.9 Tractor chassis fairing 2.10 Partial trailer skirts 2.11 Full trailer skirts 2.12 Base seal
2.13 Central gap seal 2.14 Side gap seals 2.15 Horizontal gap seal 2.16 Gap scoop
2.17 Total forebody moulding 2.18 Windcheater
2.19 Boatail
Figures 1 - 2 1
SECTION 1: SOLO TRACTOR UNITS
1.2 The effect of aerodynamic modifications on the DAF 3300 solo tractor unit
1.3 The effect of aerodynamic modifications on the Leyland T45 solo tractor unit
1.4 Comparison of the two tractor units 1.5 Summary of results
1.6 Conclusions
Figures 1.1 - 1.15
Tables 1.1 - 1.3
SECTION 2: DAF 3300 TRACTOR-TRAILER COMBINATION
2.1 Description of model
2.2 Effect of aerodynamic modifications 2.2.1 Cab mounted roof deflectors 2.2.2 Tractor trailer skirts 2.2.3 Base seal
2.2.4 Tractor chassis fairing 2.2.5 Gap scoop
2.2.6 Horizontal gap seal 2.2.7 Vertical gap seals
2.2.8 Container forebody fairings 2.2.9 Boatail
2.2.10 Cumulative effect of devices
2.2.11 Comparison of different degress of modification
2.2.12 General trends in CdA vs. yaw angle for different configurations
Figures 2.1 - 2.22
Tables 2.1 - 2.2
SECTION 3: LEYLAND T45 ROADTRAIN TRACTOR-TRAILER COMBINATION
3.1 Description of model
3.2.1 Cab mounted roof deflectors
3.2.3 The influence of removing engine cooling simulation 3.2.3 Front under run bar
3.2.4 Tractor-trailer skirts
3.2.5 Container forebody mouldings 3.2.6 Central gap seal
3.2.8 Boatail
3.2.9 Cumulative effect of devices
3.2.10 Good modern practice and best configuration 3.2.11 General trends in CdA vs. yaw angle
Figures 3.1 - 3.12
Tables 3.1 - 3.2
SECTION 4: COMPARISON OF RESULTS AND CONCLUSIONS
4.1 Comparison of the DAF 3300 and the Leyland T45 tractor-combinations
4.1.1 Baseline vehicles
4.1.2 Vehicles with three-dimensional deflectors 4.1.3 Flatbed configurations
4.1.4 Good modern practice
4.1.5 Variation of sideforce with yaw 4.1.6 Variation of yawing moment with yaw
4.2 Conclusions
Figures 4.1 - 4.7
Tables 4.1 - 4.3
REFERENCES
ACKNOWLEDGEMENTS
N O T A T I O N 2 A R e f e r e n c e p r o j e c t e d a r e a {m ) Cd D r a g c o e f f i c i e n t Cs S i d e f o r c e c o e f f i c i e n t CyM Y a w i n g m o m e n t c o e f f i c i e n t F o r c e c o e f f i c i e n t . C f = 0.5^V^A M o m e n t c o e f f i c i e n t , C m = M_ 0 . 5 ^ V ^ A 1 1 R e f e r e n c e c h a r a c t e r i s t i c l e n g t h (m) R e . W o . R e y n o l d s n u m b e r = o 1 V V F l o w v e l o c i t y (m/s) ^ A n g l e r e l a t i v e to f r e e s t r e a m p D e n s i t y (kg/m ) >i V i s c o s i t y (kg/ms)
LIST OF FIGURES
1. The College of Aeronautics 8ft (2.4m) x 6ft (1.8 m) low speed wind tunnel
2. DAF 3300 baseline vehicle
3. Leyland T45 'Roadtrain' baseline vehicle
4. Schematic layout of wind tunnel model mounting technique - solo tractors
5. Schematic layout of wind tunnel model mounting technique - tractor-trailer vehicles
6. Repeatability of baseline drag measurements - DAF 3300 7. Repeatability of baseline drag measurements - Leyland T45 8. Influence of Re. No. on drag coefficient - DAF 3300
9. Influence of Re. No. on drag coefficient - Leyland T45 10. Force and moment sign convention
11. Curved plate cab roof deflector
12. Three-dimensional cab roof deflector (CoA type) 13. DAF three-dimensional cab roof deflector
14. Leyland three-dimensional cab roof deflector 15. Tractor front wheel skirts
16. Tractor-chassis fairing
17. Central gap seal (90% upper, 55% lower) 18. Side gap seals
19. Horizontal gap seals 20. Total forebody moulding
21. Boatail (includes base seal and trailer skirts)
1.1 Comparison of CdA vs. yaw angle for the baseline tractor units 1.2 Influence of cab roof deflectors on the DAF 3300 tractor
1.3 Influence of tractor skirts on the DAF 3300 tractor
1.4 Effectiveness of tractor skirts and 3D roof deflector on the DAF 3300 tractor
1.5 Effectiveness of total tractor skirts and 3D roof deflector on the DAF 3300 tractor
1.6 Variations in sideforce due to total tractor skirts and 3D roof deflector for the DAF 3300
1.7 Variations in yawing moment due to total tractor skirts and 3D roof deflector for the DAF 3300
1.8 Influence of cooling flow simulation on the T45 tractor 1.9 Influence of the front under-run bar on the T45 tractor 1.10 Influence of cab roof deflectors on the T45 tractor 1.11 Effectiveness of tractor skirts on the T45 tractor
1.12 Effectiveness of tractor skirts and 3D roof deflector on the T45 tractor 1.13 Effectiveness of total tractor skirts and 3D roof deflector on the T45
tractor
1.14 Variations in sideforce due to total tractor skirts and 3D roof deflector for the T45 tractor
1.15 Variations in yawing moment due to total tractor skirts and 3D roof deflector for the T45 tractor
2.1 Effectiveness of cab roof plate deflectors 2.2 Comparison of flat plate and 3D roof deflectors 2.3 Effectiveness of 3D roof fairings
2.4 Effectiveness of plate cab roof deflector together with other devices 2.5 Influence of roof deflector location on cab
2.6 Effectiveness of tractor skirts 2.7 Effectiveness of front wheel covers 2.8 Effectiveness of trailer skirts
2.9 Effectiveness of short tractor and trailer skirts 2.10 Comparison of full and partial trailer skirts 2.11 Influence of rear trailer skirts
2.12 Effectiveness of a base seal together with tractor-trailer skirts 2.13 Effectiveness of the tractor-chassis fairing
2.14 Effectiveness of the gap-scoop
2.15 Effectiveness of a horizontal gap seal 2.16 Comparison of side and central gap seals 2.17 Influence of the extent of central gap seals
2.18 Comparison of Windcheater and total forbody fairings 2.19 The effectiveness of the boatail
2.20 Cumulative effect of various add-on devices
2.21 Comparison of different degrees of modification to the baseline vehicle
2.22 Comparison of four 'basic' DAF 3300 configurations
3.2 Influence of engine cooling simulation 3.3 Influence of front under run bar
3.4 Effectiveness of tractor and trailer skirts 3.5 Effectiveness of tractor and trailer skirts 3.6 Influence of rear trailer skirts
3.7 Comparison of total forebody moulding and 'Windcheater' 3.8 Effectiveness of a central gap seal
3.9 Influence cf trailer streamlining and the effectiveness of a 'boatail' 3.10 Cumulative effect of various add-on devices
3.11 Good modern practice compared to best configuration tested 3.12 Comparison of four basic T45 Roadtrain configurations
4.1 Variation of CdA with yaw angle for T45 and DAF tractor 4.2 Effectiveness of 3D cab roof fairings
4.3 Variation of CdA with yaw angle for T45 and flatbed trailer vehicles 4.4 Differences in CdA between solo tractor and tractor-trailer vehicle
configurations
4.5 Comparison of 'Good Modern Pratice' for T45 and DAF vehicles 4.6 Comparison of sideforce characteristics for T45 and DAF vehicles 4.7 Comparison of yawing moment characteristics for T45 and DAF vehicles
LIST OF TABLES
1.1 Configurations tested for T45 and DAF cabs in isolation
1.2 Summary of drag changes produced for each cab in isolation configuration tested
1.3 Effectiveness of individual devices tested on the cabs in isolation
2.1 Summary of DAF 3300 vehicle configurations tested with corresponding changes in CdA
2.2 Effectiveness of specific modifications to the DAF 3300 vehicle
3.1 Summary of Leyland T45 vehicle configurations tested with corresponding changes in CdA
3.2 Effectiveness of specific modifications to the Leyland T45 vehicle
4.1 Comparison of component drag for the baseline vehicles 4.2 Comparison of component drag for the baseline vehicles
4.3 Variation of trailer component drag with yaw angle for both vehicles
Note: Tables 2.1 and 3.1 are identical and have been reproduced within their respective sections for convenience. Similarly, tables 2.2 and 3.2.
INTRODUCTION
Over the past decade, an increasing volume of freight has been carried by heavy goods vehicles (H.G.V.'s). As the size and number of these vehicles has increased so too have the adverse effects associated with them - perhaps the most concerning being the amount of spray produced on wet roads and the degree of buffetting experienced by other road users.
Both these problems can to some extent be associated with the aerodynamic characteristics of this type of vehicle and consequently a long terra research programme, broadly labelled 'Improved aerodynamics for large articulated road vehicles' was initiated, sponsored by the Transport and Road Research Laboratory (T.R.R.L). The primary aim of the programme was to increase understanding of the flowfield associated with this type of vehicle, and to assess the potential for various modifications that could be made to the vehicle to reduce the degree of water spray dispersion and external buffetting.
It was decided at an early stage that in view of the difficulties involved with simulating water spray dispersion using conventional wind tunnel models and test techniques, the programme would utilise both model and full scale vehicles monitored on the T.R.R.L test track. Two vehicles were made available for the full scale spray measurements; a Leyland T45
'Roadtrain' and a DAF 3300. The corresponding scale wind tunnel models were assessed on the basis of the measured aerodynamic forces/moments, surface pressures and the vehicle's wake geometry, together with smoke flow visualisation of the flowfield. This report deals only with the wind tunnel measurements intended to evaluate variations in aerodynamic forces/moments on these two vehicles due to a variety of modifications or 'add-on' devices.
The add-on devices tested were chosen primarily as those most likely to reduce water spray dispersion, namely tractor and trailer 'skirts', wheel covers, etc. Other modifications included the more conventional cab roof deflectors and container fairings, as well as more advanced concepts such as cab-container gap seals and container boatailmg. The analysis concentrates almost entirely on variations in drag coefficient. Tests were carried out over a range of simulated crosswind angles and calculations of
'wind averaged' drag were made to simplify an assessment of likely performance variations.
The body of the report is divided into four sections; the first dealing with measurements made on both tractors in isolation, i.e. without the standard 40 ft trailer/container. Section [2] analyses the measurements made on the DAF 3300 tractor-trailer vehicle, and similarly section [3] considers the Leyland T45 'Roadtrain'. A comparison of the performance of both vehicles, together with relevant conclusions is given in section [4].
1. EXPERIMENTAL PROCEDURE AND DATA REDUCTION
The experiments described in this report were carried out in the Cranfield College of Aeronautics 8' x 6' (2.4 m x 1.83 m) low speed general purpose wind tunnel, see Figure 1. The tests were performed using l/8th scale models of a DAF 3300 articulated tractor unit, and Leyland T45
'Roadtrain' articulated tractor together with 40' x 8' x 8' (12.19 m x 2.44 m x 2.4 m) box trailer unit shown in Figures 2 and 3. These models were tested using the mounting technique developed by Garry (1) shown in Figure 4 for solo tractor units and Figure 5 for tractor-trailer combinations. The aerodynamic forces and moments acting on the models were measured using the 8' x 6' wind tunnel's six-component electro-mechanical balance, the data from which was recorded on micro computer for subsequent analysis.
Each vehicle configuration was tested using the following procedure -the wind tunnel was 'run up' to an airspeed of 45 m/s with -the model at zero yaw, & = 0 deg, and then the yaw angle varied in the following sequence: P = 20, -15, -12, -9, -6, -3, -2, -1, 0, 1, 2, 3, 6, 9, 12, 15, 20, and 0 deg. The tunnel was then run down and modifications to the model carried out. Whenever a model was removed from the tunnel the procedure described above was repeated for the un-modified (baseline) configuration enabling accurate comparison of results. Figures 6 and 7.
The test section flow velocity of 45 m/s gave a Reynolds number (based on square root of frontal area) Re = 1,000,000 compared to full scale Re = 5,500,000 (60 mph). Preliminary experiments indicated that at wind tunnel airspeeds above 35 m/s the influence of Re. No. was not significant, see Figures 8 and 9, and when Re. No. effects did appear (usually on bodies with leading edge curvature) roughness strips were used to fix separation points.
After collection the raw data was processed on a micro-computer giving force and moment coefficients in the axes system shown in Figure 10. In order to facilitate comparison of results for the DAF 3300 and Leyland T45 the drag measurements have been expressed in terms of the product of drag coefficient and projected frontal area, CdA, which eliminates the effect of
the models' different frontal areas. The results were corrected for blockage using the method of Carr (2). To estimate the effectiveness of devices under 'on road' conditions the aerodynamic drag data was reduced to wind-averaged form using a computer programme based on the work of Ingram
2. DESCRIPTION OF ADD-ON DEVICES
The following devices were used for tests on both the T45 and DAF 3300 tractor vehicles; unless otherwise stated the layout was identical for both vehicles, apart from minor modifications in mounting/installation.
2.1 Flate plate cab roof deflector
A simple thin flat plate cab roof mounted device, inclined at an angle ( 6 ) to the horizontal irith the support structure mounted a distance (x) downstream of the cab leading edge.
Dimensions (full scale): width (w) = 1.72 m, height (h) = 0.61 m
Rat Rate Deflector
2.2 Curved plate roof deflector
This device is essentially the same size as the flat plate deflector described above, but has thickness and edge radii as detailed below, to streamline the device, see figure 11.
Dimensions (full scale): width = 1.72 m, height (h) = 0.61 m
2.3 Three dimensional cab roof deflector (CoA type)
A simple moulded cab roof mounted device having both planform and sideface curvature as detailed in figure 12, underbody designed to fit on the roof of the DAF 3300 tractor unit.
2.4 DAF three dimensional cab roof deflector
A scale reproduction of the deflector commercially available from DAF, see figure 13.
Dimensions (full scale): height (h) = 1.08 m
2.5 T45 three dimensional cab roof deflector
A scale reproduction of the deflector commercially available from Leyland Vehicles, see figure 14.
2.6 Cab front wheel skirts
These skirts effectively seal the front wheel arch, fitting flush with the external body work and leaving a 0.45 m (full scale) ground clearance. See figure 15.
2.7 Short tractor skirts
These skirts seal the various ancillary components mounted between the front and rear tractor wheel arches, leaving a ground clearance of 0.45 m
2.8 Long tractor skirts
These skirts are effectively the same as the short tractor skirts (above) with an extension to cover the tractor rear wheels.
2.9 Cab chassis fairing
This fairing is a large horizontal and vertical plate assembly which is mounted on the tractor chassis, effectively streamlining the ancilliary components housed directly behind the cab and between the wheel arches see figure 16.
2.10 Partial trailer skirts
These skirts extend from the rear of the trailer to the tractor wheel mudguards, running parallel to the trailer body leaving a minimum ground clearance of 0.30 m (full scale). The trailer's rear wheels are left uncovered.
Partiol Trailer Skirts
2.11 Full trailer skirts
Identical to the partial trailer skirts (above) but with the trailer's rear wheels completely enclosed by the skirt.
Full Trailer Skirts
2.12 Base seal
This device is mounted in the same way as side skirts but along the base of the trailer effectively sealing the flow into the wake -from underneath the vehicle.
2.13 Central gap seal
This device is mounted vertically on the container forebody to restrict flow though the cab-container gap in crosswinds. Two sizes were tested sealing~91% and 55% of the cab-container gap respectively, see figure 17.
2.14 Side gap seals
These devices (always fitted in pairs) operate on the same principles as the central gap seal; they are simple vertical plates, fitted to the container forebody, effectively extending the sides of the container, see figure 18.
2.15 Horizontal gap seal
A simple flat plate mounted horizontally on the container forebody to restrict downward flow into the cab-container gap. Positioned at approximately cab roof height and extending over 55% of the gap width, see figure 19.
2.16 Gap scoop
This container forebody mounted device is similar to the horizontal gap seal described above but has curvature intended to direct the downward flow in the cab-container gap onto the rear of the tractor cab.
2.17 Total forebody moulding
The container forebody mounted device is intended to streamline the container's leading edges, see figure 20.
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2.18 Windcheater
This commercially available device is mounted on the container forebody, effectively streamlining the vertical and horizontal leading edges of the container.
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^ .5/? w = h = 304- mm r = 19mm 2.19 BoatailThis device is mounted to the rear of the container and is simply a chamfered (side and upper edges only) extension to the base intended to reduce the base area and entrain flow from along the sides of the vehicle into the wake, see figure 21.
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Figure 10
FORCE AND
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SIGN CONVENTION - VEHICLE
Figure 11
110 mm
side view
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Figure 12
THREE DIMENSIONAL CAB ROOF
DEFLECTOR
Figure 13
front view side view
F i g u r e 14
LEYLAND VEHICLES' THREE DIMENSIONAL
Figure 15
TRACTOR FRONT ^HEEL SKIRTS
Figure 16
rigure 17
CENTRAL GAP SEALS
Figure 18
SIDE GAP SEALS
Figure 19
Figure 20
TOTAL FOREBODY MOULDING
Figure 21
BOATTAIL (photograph also ehovys base seal
and trailer skirts)
SECTION 1: SOLO TRACTOR UNITS
1.1 Description of the Wind Tunnel Models
As described earlier the wind tunnel models are l/8th scale reproductions of vehicles which have been used for fullscale experiments. The only significant difference in scaling the models is that the cooling flow simulation on the DAF 3300 is not reproduced, otherwise the wind tunnel models may be considered accurate.
The two vehicles are, as can be seen in Figures 2 and 3, significantly different, the DAF 3300 being a much more'aerodynamically bluff' body. This feature considerably affects the aerodynamic characteristics of the DAF as the sharp leading edges lead to flow separation all around the cab, particularly over the roof where a lip at the windscreen-roof juncture creates a large region of separated flow. This characteristic of the flowfield explain; why the CdA does not rise significantly as the model is yawed relative :o the oncoming airstream, as shown in Figure 1.1. The asymmetry m this figure is assumed to be due to the arrangement of ancillaries, such as spare wheels and fuel tanks in the space between the front and rear tractor wheels.
Figure 1.1 also shows the variation of CdA with yaw angle for the Leyland T45 (with engine cooling blocked). The T45 has a lower CdA because the leading edges of this cab are designed to maintain attached tlow aioniul the sides of roof. On the T45 at small yaw angles the flow is attached around all edges except the lower edges and the windscreen pillars. In this case Figure 1.1 shows that CdA does change considerably with yaw, although the values always remain significantly lower than those for the DAF. CdA increases as the boundary layer around the lower sides of the cab separate as the pressure gradients become more unfavourable with increasing yaw angle. Over the roof, however, the leading edge radius is such that the flow remains attached for all yaw angles, except for a small region near the windward edge. This explains the lower values of CdA for the Leyland T45. A number of different configurations of both the above vehicles were tested, these are listed in Table 1.1 and diagrams of each configuration given in Figure 1.2. The wind tunnel results for each configuration are summarised in Table 1.2 where values of Cdmin, C d m m , %
Cdmin, CdWA, CdWA and %CdWA are presented. The effects of adding individual devices, either alone or in ron]unction with others are summarised in Table 1.3. A discusion of the devices tested and a more detailed presentation of the results follows.
1.2 The Effect of Aerodynamic Modifications on the DAF 3300 Solo Tractor Unit
Figure 1.2 shows the effects of mouning cab roof deflectors on the solo tractor unit. Results for the 'two-dimensional' curved plate deflector are as e.xpected; drag increases as the effective frontal area of the vehicle is increased. With the three-dimensional DAF deflector a completely different result arises, this is assumed to be due to the tact that this type of deflector considerably alters the nature of the separated region over the roof and the wake behind the cab. At small yaw angles, -7 < ^ < 7 (deg), the deflector appears to generate areas of low pressure or suction over its forward facing surfaces that are greater than the suction on the base, hence developing a net thrust, which appears as a drag reduction. As the yaw angle increases, this behaviour breaks down and the drag increases, eventually producing larger overall drag increases than the curved plate deflector - as might be expected due to the larger effective frontal area at larger yaw angles.
In general, adding any form of skirts to the DAF tractor reduces drag for moderate yaw angles as demonstrated in Figure 1.3; the change m draq produced, however depends on the extent of the skirts and their position. Comparison of the performance of short and long skirts suggests that there is no advantage in shielding the tractor's rear wheels; in fact at larger yaw angles -7 < f < 7 (deg), shielding the rear wheels produces a note-worthy detriment. If the front wheels are shrouded there is a benefit, as drag is lower than that for other arrangements until -12 < ^ < 12 (deg). This is presumably due to improved flow around the front wheel arches, but this effect might be considerably changed by rotation of the wheels, which was not simulated in this set of experiments. Overall for yaw angles m the range -8 < ^ < 8 (deg), fitting side skirts improves the solo tractors aerodynamic performance, mainly due to a 'smoothing' of the airflow. With fron*; wheel covers this effect is increased whereas rear wheel shielding shows a slight decrease. Initial drag reductions due to skirts become drag
increases as the yaw angle is increased and the skirts are exposed to the freestream flow, in this case, rear wheel shielding proves a poor option. The asymmetry in Figure 1.3 is assumed to be due to the arrangement of ancillaries as explained previously, see section [1.1].
Using tractor skirts and a three dimensional deflector together produced the results shown in Figure 1.4. This clearly demonstrates the inteference between different devices when used in conjunction. At yaw angles in the range -3 < ^ < 3 (deg), the addition of another device decreases the drag reduction produced by each device used in isolation, whereas for other yaw angles this effect is reversed. In some cases the interference results in drag variations greater than the summation of the effect of individual devices. The reduction in device effectiveness for small yaw angles is probably due to the deflector inducing a greater amount of air to flow over the roof and so the smoothing effect of skirts is reduced, whereas at larger yaw angles the reduced underbody flow decreases the drag on the side skirts.
The effect of using a three dimensional deflector and total tractor skirts together is produced in a different form in Figure 1.5. This figure is part of a summary of the effects of changing the tractors aerodynamics on drag, side-force, Figure 1.6 and yawing moment. Figure 1.7.
1.3 The Effect of Aerodynamic Modifications on a Leyland T46 Solo Tractor Unit
As described previously the wind-tunnel model of the Leyland T45 incorporates cooling flow simualtion. To investigate the significance of this flow on the vehicle's aerodynamic characteristics, experiments were carried out with and without cooling for two different configurations; the results of these tests are shown in Figure 1.8. It can be seen that blocking the cooling flow does change the model's aerodynamic behaviour, in general producing a slight reduction in drag which is assumed to be due to increased suction on the cab leading edges adjacent to the cooling ducts together with reduced internal drag. On the baseline vehicle the drag reduction occurs at smaller yaw angles -10 < P < 9 (deg) while for larger yaw angles a slight increase in drag results as the flow separates around the leading edges. When using the T45 three dimensional deflector the
influence of cooling flow does change, mainly due to the increased flow around the sides of the tractor produced by the deflector.
The Leyland T45 tractor being used for full scale tests by T.R.R.L. has been fitted with a front under-run bars, it was therefore decided to ascertain what effect such a device has on drag. The results presented in Figure 1.9 demonstrate that under-run bars do have an effect on drag, this effect being highly dependant on yaw angle. Before considering these results it should be noted that the wind-tunnel test technique does not provide an exact simulation of the ground and so tests conducted for modifications placed near the ground such as the under-run bar should take this into consideration. It can be seen from figure 1.9 that the bar reduces drag for lower yaw angles, -5 < ^ < 11 (deg) presumably because this device restricts air passing beneath the mdoel and so reduces underbody drag, this effect falling off as the yaw angls increases. For
> 11 (deg) there is little effect whereas for ^ < -5 (deg) the under-run increases drag, this asymmetry can be explained, as with the DAF, by the arrangement of ancillary devices on the chassis.
In the case of the Leyland T45 the effect of adding roof deflectors, Figure 1.10, is as would be expected - increased drag for all yaw angles. This results from the increased frontal area presented to the oncoming airstream and the increased pressure loss produced in the model's wake. The two dimensional curved plate deflector produces worse results due to its greater "bluffness" when compared to the more aerodynamically shaped T45 three dimensional deflector. The drag increases associated with these deflectors decrease with yaw as other areas of cab geometry become more important, although the rate of decrease for the three-dimensional deflector is less because more of its aerodynamic side profile is exposed.
Figure 1.11 demonstrates that fitting tractor skirts to the model produces drag reductions for all yaw angles, for yaw angles in the range -9
< ^ < 9 (deg) increasing the area of skirts producing greater reductions. On the DAF the results indicated that front wheel covers were the 'best' type of skirt and here again this appears to be the case, another similarity between the two sets of results is the reduction in effectiveness of rear wheel covers as the yaw angle increases.
The cumulative effect of adding devices is presented in Figure 1.12 where tractor skirts are progressively added to a model already fitted with a three dimensional deflector. It is again obvious that using different devices at the same time produces interference effects which alter the performance of devices. As in Figure 1.11 there are only small differences in the performance of long tractor skirts and short tractor skirts and again fitting front wheel covers produces the largest drag reductions.
The effects of adding a combination of devices on a vehicle are summarised in Figures 1.13, 1.14 and 1.15 where the more significant aerodynarr.ic components drag, side-force and yawing moment are plotted against yaw angle.
1.4 Comparison of the two tractor units
As described previously the effects of adding devices to the two different tractors are briefly sum.marised in Figures 1.5, 1.6 and 1.7 for the DAF and Figures 1.13, 1.14 and 1.15 for the T45. Comparing Figures 1.5 and 1.13 the CdA versus yaw angle curves as before it is evident that the drag of the baseline DAF and T45 are considerably different, due mainly to the differences in leading edge geoemtry. When '3-dimensional' roof deflectors (different for the two vehicles) and total trailer skirts (the same for both vehicles) are added, completely different results occur. The drag of the DAF is reduced except for y8 > 10 deg, whereas for the T45 the drag increases for all yaw angles. The major reason for these results is the change in the nature of the flow over the roof on the DAF when a roof deflector is fitted.
The variation of side-force coefficient Cs with yaw is presented in Figure 1.6 for the DAF and Figure 1.14 for the T45, direct comparison of these results should take into account the different frontal areas of the two vehicles (DAF 3300:A = 0.1025 m T45:A=0.0980m ) which can be accounted for by subtracting 3% from the results for the T45. In Figure 1.6 the effect of adding the deflector and skirts is to increase the magnitude of the side force for all yaw angles. If the effect of the deflector and the skirts are examined individually then it appears that both devices actually reduce side-force when used alone. The increases demonstrated in Figure 1.6 must therefore be the product of interference between the different
devices, although the results could be explained by the increased 'solidity' of the model.
A more predictable build up of side-force coefficient occurs in the case of the Leyland T45. Examination of the results in Appendix I shows that admg a deflector in isolation increases theside force, adding total tractor skirts again in isolation gives a greater increase and using the devices together gives even more side-force. Figure 1.14 (although the increase in side-force measured is not as large as that produced by adding the effects of individual devices). This increase in side-force produced by adding these devices can be put down to the increased solidity of the model when viewed from the side, and the increased areas of low pressure on the leeward side of the model.
Comparing the results for both cabs shows that the DAF baseline produces more crosswind resistance than the T45 baseline, again due to the differences in leading edge geometry. Adding a 3-dimensional roof deflector and total tractor skirts increases the magnitude of side-force for both vehicles but by different amounts.
In Figures 1.7 and 1.15 the variation is yawing moment with yaw angle for modified and unmodified versions of both DAF and T45 are plotted. For the Leyland T45 Figure 1.15 the addition of the deflector and skirts produces a large increase in the magnitude of the yawing moment, which corresponds with the increased drag and sideforce discussed previously although the movement of the centre of pressure (the point at which the resultant aerodynamic force acts) is also important. The magnitude of the increase m CyM suggests that most significant changes are due to (i) forward movement of the centre of pressure due to fitting the roof deflector (the effect of the tractor skirts is roughly distributed about the centre of the wheeibase, the moment reference point) and (ii) the increases in sideforce.
The results for the DAF 3300 are not as simple, see Figure 1.7. The addition of the devices completely changing the pattern of behaviour. Considering the baseline vehicle first as there are no major changes in sideforce and drag. Figures 1.6 and 1.5 the sinusoidal nature of the yawing moment variation with yaw must be due to movement of the centre of pressure
along the vehicles longitudinal axis. When the vehicle is fitted with a deflector and tractor skirts the behaviour is again dominated by centre of pressure movement, although comparison with Figure 1.6 indicates that there are discontinuities around ^ = -5 deg and ? = 10 deg, presumably due to unsteady separations.
Because the variation of yawing moment with yaw angle appears quite different for the two vehicles no attempt at comparison will be made. Although the results do indicate that sharp leading edges with flow separation, as on the DAF lead to greater movement of the centre of pressure and the direction of movement varies with yaw angle.
1.5 Summary of Results (Effectiveness of devices on the two models)
In the preceding section 1.1 and 1.4, the differences between the two tractor units, a DAF 3300 and a Leyland T45, have been explained, the most signficiant being that the DAF 3300 has much sharper leading edges than the T45 and flow separation from these edges dominates the flow field. The cab roof juncture, where there is a prominent lip, appears to be the most significant. Here a large separated region over the whole roof is created, resulting in large drag forces. The T45 has much rounder leading edges and the flow is attached over the roof (for all yaw angles) and around the sides of the cab below the windscreen. These smooth edges result in a lower drag, due to lower pressures acting on the whole front face, suctions on the attached corners and reduced turbulence. These important geometric features also considerably influence the performance of aerodynamic devices fitted to the cabs.
In Table 1.2 the variation in drag coefficients produced by changing from the baseline configuration are shown, it is immediately apparent from these figures that the effectiveness of a particular device is considerably influenced by the vehicle to which it is fitted. Careful examination of the data given in Table 1.3 reveals an interesting trend which is true for all configurations tested - for a given device, the magnitude of the drag change increases in proportion to the drag of the original configuration
(NB this is not necessarily true if percentage reductions are used).
fitted to the DAF 3300 will give a larger reduction than on a Leyland T45. For certain devices it is possible to explain why this is the case - cab roof devices fitted to the T45 will in general result in large drag increases because the flow remains attached around the edges of the roof. So any device on the roof of the T45 (unless it promotes earlier separation from these edges) will be in a relatively high energy airstream, which gives rise to large pressure differentials on the front and back faces of the device and hence high drag. Any deflector type device fitted to the roof will also increase the size of the tractor wake, and the consequent increase in momentum loss will also increase drag. However, on the DAF 3300, the separation from the roof leading edges creates a low energy airstream which will reduce the drag of devices fitted on the roof, compared to the T45. In the case of the three dimensional roof deflector on the roof of the DAF, as described earlier this device significantly changes the nature of the flow over the roof and in doing so reduces the overall vehicle drag.
For other devices, such as tractor skirts, explaining why the drag reductions on the DAF are greater than those of the T45 is more difficult. Presumably, on the DAF with its sharp leading edges there is a greater amount of turbulent flow along the sides of the vehicle and so side skirts which reduce this turbulence give greater benefits on the DAF. The design of wheel arches, the arrangement of ancillaries on the chassis and the flow underneath the tractor will also influence the performance of side skirts.
Of the side skirts tested, the front wheel covers appear to give the greatest advantage (although they have not been tested in isolation). On the DAF, short tractor skirts are more effective than long tractor skirts, particularly at yaw, whereas on the T45 there is little to choose between them. This suggests that on solo tractors there is little to gain, in terms of drag, by shielding the rear wheel housings.
The other tests performed were conducted on only the Leyland T45. The results of the test on cooling indicated that having cooling flow simulation reduces drag, this reduction decreasing with yaw angle. Blocking the cooling flow,cobmined with a 3-dimensional roof deflector, showed that the cooling flow could effect the performance of add-on devices, blocking the cooling increased the drag produced by fitting the
deflector. Following this all subsequent tests were conducted with the cooling flow simulation open. On the DAF, which has no cooling flow, the effect would probably not be as significant because of the sharp leading edges. The tests with a front under-run bar demonstrated that this device does influence the aerodynamic characteristics, but as explained previously, devices fitted in positions such as this one could be very sensitive to the method of ground simulation.
1.6 Conclusions
These experiments on solo tractor units have shown the significance of leading edge geometry on aerodynamic performance, for a low drag tractor unit the leading edge radii should be such that the flow is attached wherever possible. (NB. This does not mean that the T45 is more aerodynamic than the DAF when used to tow box trailers, as will be shown later).
On both tractor units the experiments demonstrated that two-dimensional deflectors give severe penalties when used on solo tractor units, if possible these devices should be removed when running solo, or at least adjusted to present as small a frontal area as possible to the oncoming airstream. For the T45, fitting a 3-dimensional roof fairing produced significant drag increases and as with 2-dimensional deflectors the device should be run as infrequently as possible on solo tractors. Fitting a 3-dimensional roof fairing to the DAF surprisingly produced an overall reduction in drag, this result should not encourage operators to fit such devices to solo cabs as in general any roof deflctor will significantly increase the drag of solo tractor units.
Using tractor skirts on a solo tractor should give drag reductions except in extreme crosswinds when small increases in drag may arise. Of the arrangements tested the front wheel covers gave the best results whilst in general there is little to choose between short tractor skirts and long tractor skirts in terms of drag.
The results of the cooling flow simualtion demosntrated that drag is reduced if the cooling is blocked, and blocking cooling does influence the performance of other devices.
If front under-run bars are used the results obtained here suggest an overall drag reduction would arise, although tests with a more realistic ground simulation technique are necessary before accurate conclusions can be drawn.
Fitting several devices simultaneously does not give the cumulative benefits of the devices in isolation and the inteference effects are in the majority of cases, detrimental. The effect of devices on other aspects of solo tractor performance have been shown to be important. In general both side skirts and roof deflectors increase the magnitude of the side-force on the vehicles whereas for yawing moment the results are significantly influenced by vehicle configuration.
The results presented in this section are not intended to be used on their own, but in conjunction with the following sections of the report to enable operators to choose aerodynamic devices to suit their particular applications. They are also aimed at showing what effects spray reducing devices have on the perofrmance of solo tractor units.
0.12 e.10
0.08
0.06
T.<
ÜJ o: o o0.04
0.02
0
-20 -15 -10
-5
YAU ANGLE CDEGREES)
T
10 1r
20
A - LEYUND T45 • - DAF 3300
Figure 1. 1
-10
OQ
o:o
txJ co O oc Ui o.-30
-40
DAF 3300 SOLO TRACTOR
• - WITH 3-D DEFLECTOR
• - WITH 2-D DEFLECTOR
- 1 0 - 5 0 YAU ANGLE (DEGREES)
10
15
20Figure 1. 2
-J0
O M I-OQ
CO co I —s
-30
-40
DAF 3300 SOLO TRACTOR
• _ SHORT TRACTOR SKIRTS
• - LONG
• - TOTAL
-20 -15 - 1 0 - 5 0
YAW ANGLE (DEGREES)
20
Figure 1, 3
20
10
z
o
o
CD UJo
o
UJ O.-10
-20
-30
-40
-50
s.
> — ^ /y
• • * - • . // j ^ '
f w
/ /J /
y '
r
^ • ^ • * i t . ^ DRAS DECREASE l \[ X
\ \ \ \V
k I V » 1 ^ DRAG I Iv >
^ ^ ^ % -^ ^ ; ^CREASE -20 -15 -10 -5 0 YAU ANGLE (DEGREES)10
15 20OAF 3300 SOLO TRACTOR
I - 3-D DEFLECTOR
• - PLUS.TOTAL TRACTOR SKIRTS
Figure 1.4
EFFECTIVENESS OF TRACTOR SKIRTS AND 3D ROOF DEFLECTOR ON THE
DAF 3300 TRACTOR
OC < O C_) 0.12 0.10 0.88 0.86 0.94 0.82 -2Ö- - 1 5 - 1 0 - 5 8 YAU ANGLE (DEGREES)
18 —r-15
• - BASELINE DAF 3300
• -WITH TOTAL TRACTOR SKIRTS
& 3-D ROOF DEFLECTOR
20
Figure 1. 5
EFFECTIVENESS OF TOTAL TRACTOR SKIRTS AND 3D ROOF DEFLECTOR ON
0.8 0.6 0.4 0.2 co o UI M O UI o CJ cc a 1 u. CO -0.2 UI - 0 . 4 -0.6 - 0 . 8 r^l^^ » ' " 1 •
1 /
1 / •y 4
r
i r4 /
iy
-BASELINE DAF 3300
-WITH TOTAL TRACTOR SKIRTS
& 3-D ROOF DEFLECTOR
-20 -15 - I YAW 0 - 5 8 ANGLE (DEGREES) 10 15 2 0
Figure 1. 6
VARIATIONS IN SIDEFORCE DUE
TO TOTAL TRACTOR
SKIRTS AND 3D ROOF
DEFLECTOR FOR THE DAF
3300 TRACTOR
0.2 UI M O UI O o o CD 3 >-- 0 . 2 \ >^--\ y t ^ / \ / \ / \ / \ / \ i f • \
V
' I I t t 1 / / / / / J \ / \-V \
o -20 -15 -10 - 5 0 YAU ANGLE (DEGREES)10 15 20
-BASELINE DAF 3300
-WITH TOTAL TRACTOR SKIRTS
& 3-D ROOF DEFLECTOR
Figure 1.7
VARIATIONS
IN
YAWING MOMENT DUE
TO
TOTAL
TRACTOR
SKIRTS AND 3D ROOF
20
10
M t-UQ
co a UI co •< t—s
o
CU-10
-20
-30
-40
-50
r — '
m-i 0RA6 DECREAS ^ ^ ^ ' ' ^ - ^E
_ ^ J
x ^ _ ^ ^ - 4
T45 SOLO TRACTOREEFECT OF BLOCKING C00LIN8
• - BASELINE
• - WITH 3-D DEFLECTOR
- 2 0 - 1 5 - 1 0 - 5 0 YAW ANGLE (DEGREES)
10 15 20
Figure 1. 8
T45 SOLO TRACTOR • - WHH CRASH BAR
- 1 5 - 1 0 - 5 0 YAU ANGLE (DEGREES)
10 20
Figure 1. 9
-20 -IS -10 -5 0 YAU ANGLE (DEGREES)
10
15
20
T45 SOLO TRACTOR
• - W H H 3-D DEFLECTOR
• - WITH 2-0 DEFLECTOR
Figure 1. 10
z
o
o
e
CU CD•<
OC UJ co • <o
e
o. -10 -20 -30 -40 T45 SOLO TRACTOR+ - WITH SHORT TRACTOR SKIRTS
• - - LONG
• - - TOTAL
- 2 0 - 1 5 - 1 0 - 5 0 YAU ANGLE (DEGREES)
10 15 20
Figure 1.11
z
a
o
e
oc co <. oco
u
CS-10
5 -30
o
es
o.
-40
-20
T4S SOLO TRACTOR + 3-D OEFLECTOR
• - AS ABOVE
• - SHORT TRACTOR SKIRTS
X - LONG
• - TOTAL
- 1 0 - 5 0 YAU ANGLE (DEGREES)
f- igure 1.12
EFFECTIVENESS OF TRACTOR SKIRTS AND 3D ROOF DEFLECTOR
0.12 0.10 0.08 0.06 0.04 0.02 -2Ö -15 - 1 8 - 5 (3 YAW ANGLE ( D E G R E E S ) —T— 18
• - BASELINE LEYLAND TA5
• -WITH TOTAL TRACTOR SKIRTS
& 3-D ROOF DEFLECTOR
20
Figure 1.13
EFFECTIVENESS OF TOTAL TRACTOR SKIRTS AND 3D ROOF DEFLECTOR
CO o UI M O UI O CJ UJ o Qi O u. I UI M CO 0.8 5 0.6 5 0.4 5 0.2 5 0.0 5 -0.1 5 -8.3 5 -0.5 5 -0.7 5
y]
^ '
y _^ <• • — ^ ^ — — I 1 T ^ l » ^ 1 I^ ^ J P H - y^
i r Jf•-BASELINE LEYLAND TA5
• -WITH TOTAL TRACTOR SKIRTS
& 3-D ROOF DEFLECTOR
-20 -15 -10 -5 0 YAU ANGLE (DEGREES)
10 15 20
Figure 1. 14
VARIATIONS IN SIDEFORCE DUE TO TOTAL TRACTOR SKIRT AND 3D ROOF
DEFLECTOR FOR THE T45 TRACTOR
0.2 z: >-CJ z UJ M O M ü -U . UI O O z UI o z: CD Z >-- 0 . 2 — _ -^ I j — — —
u\
I I — V i ^ I 1 \ \ \ \ \ 'te.-BASELINE LEYLAND TA5
-WITH TOTAL TRACTOR SKIRTS
& 3-D ROOF DEFLECTOR
-20 - ! 5 -10 - 5 0 YAU ANGLE (DEGREES)
10 15 20
h igure 1. 15
VARIATIONS IN YAWING MOMENT DUE
TO TOTAL TRACTOR
SKIRTS AND 3D
ROOF DEFLECTOR FOR THE T45
TRACTOR
OAF 330 Snlü Tiactor Unit C o n f i g u r a t i o n 1 2 3 4 5 6 7 Code DAF01 nAF02 DAF 03 DAI 04 UAf 05 DAF 06 DAF07 D e s c r i p t i o n Baseline Vehicle
Baseline + short call s k i r t s Baseline f long call s k i r l s Basel ine t f u l l call s k i r l s B a ' e l i n e i 3 - d i m e i r j i u n j | del l e c t o r Baseline » 3-diniensional d e f l e c t o r -i f u l 1 cab s k i r t s
Baseline -t curved p l a t e d e f l e c t o r
Leyland T45 Solo Tractor Uni t
C o n f i g u r a t i o n Code 1 2 3 4 5 6 7 8 9 10 11 12 T45/00 r45/01 T45/02 T45/03 T45/04 145/05 T45/06 145/07 T45/08 145/09 145/10 145/11 D e s c r i p t i o n Baseline Vehicle
Baseline t f r o n t under-run bar Baseline w i t h engine c o o l i n g blocked
Baseline ( c o o l i n g blocked) t 3-dimensional deflector-Baseline t 3-dimensional d e f l e c t o r
Baseline + 3-dimensional d e f l e c t o r f long tab s k i r t s Baseline t 3-dimensional d e f l e c t o r t f u l l cab s k i r t s Basel ine < f u l 1 call s k i r l s
Baseline + long cab s k i r l s Basel ine r short i ali s k i r t s
Baseline i 3-dimeiisii)nal d e l l e c t o r » shori i ali s k n l s Baseline i curved p l a t e i l e f l e c t u r
T a b l e 1. 1
CONFIGURATIONS TESTED FOR T45 AND DAF CABS IN
T a b l e 1.2
SUMMARY OF DRAG CHANGES PRODUCED FOR EACH CAB IN ISOLATION
CONFIGURATION TESTED
CONFIGURATION Baseline
Baseline with cooling blocked Baseline * short cab skirts Baseline + long cab skirts Baseline * total cab skirts Baseline + 3-dimensional deflector Baseline + 3-dimensional deflector (cooling blocl<ed)
Baseline + curved plate deflector Baseline + 3DD + short cab skirts Baseline - 3DD + long cab skirts Baseline + 3DD + total cab skirts Baseline + front under-run bar
DAF 3300 ^ A "Omin • (m-l .1005 .0939 .0946 .0897 .0886 .1095 .0831 «C„ . A DmiR Im^j -0.0065 -0.0059 -0.0103 -0.0119 +0.0090 -0.0174 ''^Smin -6.5 -5.9 -10. • -ll.f. +8.9 -17. < "DWA'* .1024 .0960 .1007 .0920 .0958 .1090 .0376 ^'^DWA'^ (m^) -0.0064 -0.0017 -0.0104 -0.0056 +0.006 -0.0148 '^^DWA -6.3 -1.7 -10.2 -5.5 r-6.4 -14.5 Leyland T45 ^Dmin* .0704 .0665 .0689 .0682 .0648 .0934 .0905 .0991 .0914 .0917 .0893 .0675 fiCrv„. A umin -0.0039 -0.0015 -0.0022 -0.0056 +0.0230 +0.0240 -0.0029 +0.0287 +0.0210 +0.0213 +0.0189 -0.0029 ^^^Dmin -5.5 -2.1 -3 1 -8.0 + 32.7 +36.1* -4.1** +40.8 +29.8 + 30.3 +26.5 -1."; '" A "DWA .0737 .0716 .0725 .0721 .0694 .0978 .0957 .1029 .0953 .0953 .0931 ,0''18 ^SWA'^ (mn -0.0021 -0.0012 -0.0016 -0.0043 +0.0241 +0.0241 -0.0021 +0.0292 +0.0216 +0.0216 -0.0194 -0.0019 •^•^'^DUA -2.8 -1.6 -2.2 -5.8 + 3 2 . ' + 3 2 . 7 * - 2 8 * * +39.6 +29.3 +29.3 +26.3 -2.6 " A - C A C A - C A "D 5dd - cool ing D 3dd ^^.p _ D 3aa - cooling D oaseiine cooling
T a b l e 1.3
EFFECTIVENESS OF INDIVIDUAL DEVICES TESTED ON THE CABS IN ISOLATION
Devices Short Cab S k i r t s a) on b a s e l i n e b) w i t h 3-d d e f l e c t o r Long Cab S k i r t s a) on b a s e l i n e b) w i t h 3-d d e f l e c t o r F r o n t Wheel Covers a) w i t h long cab s k i r t s b) w i t h long cab s k i r t s S 3-d d e f l e c t o r T o t a l Cab S k i r t s a) on b a s e l i n e b) w i t h 3-d d e f l e c t o r 3-Dimensiona' D e f l e c t o r a) on b a s e l i n e b) w i t h short cab s k i r t s c) w i t h long cab s k i r t s d) w i t h t o t a l cab s k i r t s e) w i t h c o o l i n g blocked Curved P l a t e D e f l e c t o r a) on b a s e l i n e v e h i c l e B l o c k i n g Cool ing a) on b a s e l i n e b) w i t h 3-d d e f l e c t o r Front Under-run Bar a) on baseline OAF 3300 ^'^D.in'^ (m^^ -0.0066 -0.0059 -0.0049 -0.0108 -0.0055 -0.0119 -0.0066 +0.0090 ^^^Dmir - 6 . 6 - 5 . 9 - 4 . 9 - 1 0 . 7 - 5 . 5 - 1 1 . S - 6 . 6 +8.9 ^^DWA* -0.0064 -0.0017 -0.0087 -0.0104 -0.0092 -0.0055 -0.0044 +0.0066 •'^-DWA - 6 . 3 - 1 . 7 - 8 . 5 - 1 0 . 2 -9.C - 4 . 3 + 6 . ' Leyland T45 1 '^^Dniin'^ -0.0015 -0.0020 -0.022 -0.0017 -0.0034 -0.0024 -0.0056 -0.0041 +0.0230 +0.0225 +0.0235 +0.0245 +0.0240 +0.0287 ^^^D^in - 2 . 1 - 2 . 8
tl
- 4 . 8 - 3 . 4 - 8 . 0 - 5 . 8 + 32.7 +32.0 +33.4 +34.8 +36.1 +40.8 -0.0039 , - 5 . 5 -C.0029 I - 4 . 1 1 -0.0029 j - 4 . 1 '^'^DWA -0.0012 -0.0025 -0.0016 -0.0025 -0.0027 -0.0022 -0.0043 -0.0047 +0.0241 +0.0228 +0.0232 +0.0237 +0.0241 +0.0292 -0.0021 -0.0021 -0.0019 '-•^^DUA - 1 . 5 - 3 . 4 - 2 . 2 - 3 . 1 - 3 . 7 - 3 . 0 -5 .8 - 5 . 4 + 32.7 +30.9 + 31.5 +32.2 + 32.7 + 39.6 - 2 . 8 - 2 . 8 - 1 . 7SECTION 2: DAF 3300 TRACTOR-TRAILER COMBINATION
2.1 Description of model
This model, shown in Figure 2, consists of the DAF 3300 tractor unit described in the previous section and a l/8th scale model of a 40' x 8' x 8' (12.19 m X 2.4 m x 2.44 m) box container on a 40 ft twin axle Crane Fruehof trailer. The major dimensions of the cab and its position relative to the box-trailer are shown in Figure 2. A list of the different configurations tested and a summary of the drag changes produced can be found in Table 2.1 while Table 2.2 presents the drag changes associated with the individual devices tested on the various configurations. Line drawings to help identify the different configurations examined are presented in Section 2; Figures 11 to 21 inclusive.
2.2 Effect of Aerodynamic Modifications
2.2.1 Cab mounted roof deflectors
The effect of 'two dimensional' cab roof mounted deflectors on the drag of the DAF 3300 tractor-trailer combination is shown in Figure 2.1. The deflector used has been described previously and in this case was set an an angle of 75 deg to the horizontal, its bottom edge aligned with the front of the cab roof. The general behaviour of the two deflectors (one has a flat front face and the other a convex front face of radius 24" full scale) is similar at small yaw angles, they produce a drag reduction, whereas at greater yaw angles the drag increases. As explained in section 1 there is a large region of separated flow over the roof of the DAF cab, which effectively shields the front face of the container from the oncoming airstream, providing an inherent drag reduction.
The CdA versus yaw angle curve in Figure 6 shows the drag of the DAF tractor-trailer configuration increasing rapidly as the model is yawed, this suggests that the shielding of the container provided by the separated region on the cab breaks down quite rapidly.
This shielding effect explains why the efficiency of the deflectors increases from the trough at ^ = 1 deg shown in Figure 2.1. At this yaw
angle the separated region from the cab is having the greatest effect on the flow; consequently the extra shielding provided by the deflectors has little influence. However, as the vehicle is yawed the shielding provided by the cab reduces and the deflectors have a greater effect. Further
increases in yaw reduce the shielding of the container by the deflectors and the drag of the deflectors themselves increasingly becomes the dominant feature until at high yaw angles deflectors actually increase the drag of the combination.
The differences between the two devices is assumed to be a combination of two effects; the curvature on the curved plate deflector, which reduces this device's own drag and also changes the shape of the separated region from its edges. At the smaller yaw angles the reduced device drag of the curved deflector dominates but as the yaw angle is increased the reduced size of the wake from this device,which gives less container shielding, becomes more important.
The performance of the curved plate deflector described above is compared with that of a DAF 3-dimensional roof fairing in Figure 2.2. In section 1 it was seen that fitting a 3-dimensional roof fairing to the DAF cab actually reduced the solo tractor's drag at small yaw angles, so drag reductions obtained with the 3-dimensional deflector will be largely due to the possible reduction of tractor drag, reduction of container forebody drag and reduction in drag due to decreasd separation over the container. (pressure measurements on the base of the vehicle have shown that the effect of roof deflectors on base drag are minimal, Cowperthwaite (4)). Pressure measurements on the container forebody by the author (4) gave the following reductions in forebody drag, when the DAF 3-dimensional deflector was fitted
-deg
0
-5
-10
-15
-20
CdAForebody (m^) -0.0134 -0.0267 -0.0329 -0.0390 -0.0390CdA whole vehicle (m^)
-0.0113 -0.0144* -0.0136* -0.0148 -0.0134 Difference (m^) 0.0021 0.0123 0.0193 0.0187 0.0256 * interpolated results
These results show that the actual reductions in forebody drag produced by the deflector are always greater than the drag reduction on the whole vehicle, and this difference appears to increase with yaw. This suggests that as the vehicle is yawed the drag of the deflector becomes increasingly important, which agrees with the results for the solo DAF 3300 tractor unit. At small yaw angles, where on the solo tractor the deflector gave drag reductions, the opposite effect occurs with the tractor-trailer vehicle, when container forebody drag is taken into account. It, therefore appears that the container influences the flowfield around the tractor, particularly over the roof, as would be expected.
Comparing the performance of this deflector with the 2-dimensional curved plate deflector described previously it is apparent that adding depth and shape to roof deflectors considerably improves their performance particularly at larger yaw angles, see Figure 2.2.
In Figure 2.3 two different 3-dimensional deflectors are compared; (i) a commercially available design (DAF) and, (ii) a College of Aeronautics design (CoA). The results shown in this figure indicate that the variation of drag reduction with yaw is similar for both devices, although the DAF design becomes more effective as the model is yawed.
Figures 2.4 and 2.5 show the effect of using the curved plate deflector in conjunction with other devices and in different positions on the cab roof, (the angle of the deflector ^ = 75 deg in both cases). Figure 2.4 shows that there is a considerable asymmetry in the results for configuration 16, this is assumed to be due to the side gap seals, which interfere with the shielding of the container by the separated region produced by the cab (this is demonstrated later in Figure 2.16). When the curved plate deflector is fitted in its forward position the asymmetry is reduced which confirms the above assumptions. This is because the curved plate deflector, which is situated in the separated region over the cab roof, reduces the three-dimensional nature of the separation. Apart from reducing drag in the range -12 < ^ < 0 (deg), the curved plate deflector offers no advantage and increases drag at higher yaw angles. In the region -12 < ^ < 0 (deg), the improvements shown would be removed by using a central gap seal (55% of the gap) which does not produce the asymmetry of
