A Summary of the Scale Model Wind Tunnel Measurements and Full Scale Surface Pressure Tests on the Leyland T45 and D AF3300 Vehicles Used for the TRRL Spray Dispersion Programme

Cranfield

College of Aeronautics Report No. 8707

April 1987

A Summary of the Scale Model Wind Tunnel Measurements and

Full Scale Surface Pressure Tests on the Leyland T45 and D AF3300 Vehicles

Used for the TRRL Spray Dispersion Programme

by Kevin P. Garry

College of Aeronautics

Cranfield Institute of Technology

Cranfield, Bedford MK43 OAL, England

Kluyverweg 1 - 2629^HS DELFT

Cranfield

College of Aeronautics Report No. 8707

April 1987

A Summary of the Scale Model Wind Tunnel Measurements and

Full Scale Surface Pressure Tests on the Leyland T45 and D AF3300 Vehicles

Used for the TRRL Spray Dispersion Programme

by Kevin P. Garry

College of Aeronautics

Cranfield Institute of Technology

Cranfield, Bedford MK43 OAL, England

ISBN 0 947767 61 4

£7.50

"The views expressed herein are those of the authors alone and do not necessarily represent those of the Institute."

SUMMARY

Aerodynamic streamlining of commercial vehicles is assumed to both improve fuel efficiency, by reducing aerodynamic drag, and reduce the dispersion of water spray by limiting the extent of the vehicles' external

flowfield.

The results from a programme of wind tunnel scale model and full scale experiments are presented, intended to correlate the drag reducing performance of a range of modifications to two existing vehicles with their spray reducing effectiveness. This report serves to summarise the salient features of the research programme, already presented in two separate reports, in one document. Emphasis is placed on the reductions in wind averaged drag coefficient that are possible with the use of streamlining modifications, and the correlation between surface pressure measurements on the box container between wind tunnel and full scale.

CONTENTS

SUMMARY

LIST OF FIGURES

1. INTRODUCTION

2- EXPERIMENTAL PROGRAMME

2-1 Wind Tunnel Models and add-on devices 2.2 Wind Tunnel Facilities

2.3 Measurement Techniques

3. AERODYNAMIC DRAG MEASUREMENTS

3.1 Tractor Units in Isolation 3.2 Tractor-trailer Vehicles

3.2.1 Summary of effectiveness of various devices. DAF 3300 3.2.2 Summary of effectiveness of various devices. Leyland T45 3.3 Discussion of aerodynamic drag measurements

4. SURFACE PRESSURE MEASUREMENTS

4.1 Wind Tunnel Measurements 4.2 Full Scale Measurements

4.2.1 Container base pressures 4.2.2 Container forebody pressures

4.3 Discussion of Full Scale/Model Measurements

CONCLUSIONS

REFERENCES

ACKNOWLEDGEMENTS

CONTENTS continued

TABLES

APPENDIX A THE WIND AVERAGED DRAG COEFFICIENT

APPENDIX B DEFINITION OF SURFACE PRESSURE COEFFICIENT (Cp)

LIST OF FIGURES

1. Leyland T45 'Roadtrain' baseline vehicle 2. DAF 3300 baseline vehicle

3. Schematic layout of the wind tunnel pressure measuring and data recording system

4. Schematic layout of the full scale vehicle pressure measuring and data recording recording system

5. Box container surface pressure tapping locations

6. Summary of the effectiveness of various devices on the tractor in isolation

7. Variation of C D A with yaw angle (^), for the Baseline and 'Good Modern Practice' configurations

8. Effectiveness of various tractor roof devices: DAF 3300 9. Effectiveness of various trailer skirts: DAF 3300

10. Comparison of the effectiveness of various tractor-trailer skirts: DAF 3300 and T45 vehicles

11. Effectiveness of various tractor-trailer gap seals: DAF 3300 12. Effectiveness of container forebody devices: DAF 3300

13. Effectiveness of container forebody devices: Leyland T45

14. Comparison of cab roof mounted deflectors on the T45 and DAF 3300 vehicles

15. Container forebody pressure distribution: Leyland T45 Baseline Vehicle

16. Container forebody pressure distribution DAF 3300 baseline vehicle 17. Container forebody pressure distribution at zero yaw with a cab roof

mounted 3D deflector on both vehicles

18. Comparison of model and full scale surface pressure coefficient distribution Leyland T45 container forebody

19 Comparison of model and full scale surface pressure coefficient distribution DAF 3300 container forebody

1. INTRODUCTION

Interest in the aerodynamic characteristics of heavy commercial vehicles (HGVs) has concentrated on the economic advantages to be gained by improving fuel economy through reductions in aerodynamic drag. As a result, foundations for the design of low drag HGVs have been established, but other factors relating to the aerodynamics of this type of vehicle have received comparatively little attention, among them

being:-(1) the dispersion of water spray generated by the vehicle's tyres, and (2) the buffetting experienced by other road users in close proximity to

the vehicle.

Both these factors are assumed to be closely related to the structure of the vehicle's external flowfield and it is recognised that streamlining can significantly reduce the size of this region as well as changing the vehicle's overall aerodynamic characteristics. However, very little information is available linking the changes that are likely to occur in the external flowfield as a result of using add-on devices that are intended to reduce aerodynamic drag. Similarly, little is known about the influence of vehicle geometry on the effectiveness of such modifications.

The conventional technique for experimental investigations into the aerodynamic characteristics of this type of vehicle, is to use a scale model in a wind tunnel. However, since it is recognised that correct scale simulation of water spray dispersion is likely to be very difficult, wind tunnel programmes intended to monitor spray dispersion should ideally be supported by full scale correlation tests. This presents an opportunity to correlate not only the external flowfield measurements but also the aerodynamic forces and surface pressures acting on the vehicle itself, since there is very little data available to verify wind tunnel experimental techniques.

In view of this apparent defficiency of available data in two key areas, the Transport and Road Research Laboratory (T.R.R.L) proposed an experimental programme, using both full scale and wind tunnel model tests with the intention of;

(1) correlating the possible reductions in aerodynamic drag and water spray dispersion through the use of various modifications to existing vehicles.

(2) assessing any changes in the vehicle's external flowfield - and consequently the degree of water spray dispersion - as a result of these modifications.

(3) analysing the differences between surface pressure measurements on the wind tunnel model and the full scale vehicle.

Clearly, the programme called for careful coordination of wind tunnel and full scale measurements. In order to simplify this as much as possible, it was decided to concentrate on two vehicles; a Leyland T45 and DAF 3300 tractor unit, each coupled with a standard 40 ft articulated trailer. The choice of tractor units was made specifically to highlight an important aerodynamic characteristic of this type of vehicle which is discussed in section (3.1). Within the framework of the overall programme, the wind tunnel measurements can be broken down as

follows:-(i) aerodynamic force and moment measurements on both vehicles fitted with a range of streamlining devices.

(ii) container surface pressure measurements on both vehicles. (iii) external flowfield measurements.

The wind tunnel programme was supplemented throughout by full scale smoke flow visualisation tests on the T.R.R.L. test track. This enabled a qualitative assessment of the various add-on devices to be made and served as an important correlation exercise.

Each phase of the programme, as listed above, is the subject of a separate CoA Report. The purpose of this summary is to present the salient results from; (i) the aerodynamic force measurements and (ii) the surface pressure measurements, in one concise document. It is hoped to highlight the principle findings of the research and refer the reader to relevant sections of the more comprehensive reports wherever necessary.

2. EXPERIMENTAL PROGRAMME

2.1 Wind Tunnel and Add-on Devices

The models used for the wind tunnel experiments were l/8th scale reproductions of the Leyland T45 'Roadtrain' and DAF 3300 tractor units coupled to a Crane Freuhof articulated twin axle trailer with 40' x 8' x 8' box container, see figures (1) and (2).

The modifications made to these vehicles in the form of add-on devices, were chosen to reflect the current and anticipated trend in aerodynamic streamlining for this type of vehicle, with the emphasis on devices most likely to prevent water spray dispersion.

A characteristic feature of the flow around this type of vehicle is assumed to be responsible for the high levels of spray dispersion - flow over the cab roof results in high pressures on the container forebody, and consequently a downflow in the cab-container gap. This flow mixes with that from underneath the cab and the pressure increases due to the tractor rear axle and ancillaries impeding the flow. Eventually the flow moves sideways, past the tractor rear wheels towards the low pressure area along the side of the vehicle. This lateral movement of air carries with it water droplets brought from the road surface by the tyres. These droplets are atomised into a fine spray when they meet the high speed external flow and are dispersed by the vehicle's flowfield.

Consequently any add-on devices intended to modify the flow in the cab-container gap or along the sides of the vehicle can be expected to have the additional effect of reducing spray dispersion as well as reducing aerodynamic drag.

The add-on devices chosen for this series of tests can be categorised into three broad groups as

follows:-(1) streamlining of the cab-container and control of the flowfield in the cab-container gap.

(a) cab roof plate deflector (b) cab roof '3D deflector' (c) central gap seal

(d) side gap seal

(e) horizontal gap seal (f) container forebody

streamlining.

(2) Streamlining of the tractor and trailer chassis/wheels with side skirts.

(a) short tractor skirts (b) long tractor skirts (c) total tractor skirts (d) partial trailer skirts (e) full trailer skirts (f) tractor chassis fairing.

(3) Base flow control. (a) base seal

(b) boattail.

Detailed specification of each of these devices is given in Cowperthwaite (1) and a summary in Appendix (C).

2.2 Wind Tunnel Facilities

(i) The 8 ft (2.43 m) x 6 ft (1.80 m) Wind Tunnel.

This facility has a closed return, closed working section layout having a maximum flow velocity of 55 metres/second (125 m.p.h.) at near ambient temperature and pressure. It is equipped with a six component overhead electro-mechanical balance, data from which is returned on-line for subsequent analysis.

In the case of tests involving commercial vehicles, the model is usually mounted on a 'ground board plate'. This technique detailed by the author (2), reduces the influence of the wind tunnel wall boundary layer, and allows the model to be yawed relative to the freestream (to simulate crosswind effects) - whilst at the same time mounting the model through the wheels (which eliminates the interference effects of struts and mounting

(ii) The 8 ft (2.43 m) x 4 ft (1.2 m) Boundary Layer Wind Tunnel

This is a closed working section, open return facility with a maximum flow velocity of approximately 20 m/sec (45 m.p.h). It is equipped with a microcomputer controlled, 3 axis probe traversing mechanism, which permits wake surveys to be made within a control volume 1.9 m x 1.9 m x 0.6 m.

For the purposes of these tests, boundary layer suction slots were incorporated into the floor of the wind tunnel's 40 ft long working section. This enables the model to be positioned at various locations upstream of the probe traverse mechanism, whilst in the same incidence floor boundary layer. This permits wake surveys to be made at up to 5.0 body lengths downstream of the model.

2.3 Measurement Technique

(i) Aerodynamic force and moment measurements

The 8' X 6' wind tunnel's typical operating speed for these tests was 45 m/sec, which gives Reynolds numbers (Re, based on A^/*) of i,o5 x 10», compared to that of a full size vehicle at 60 m.p.h. of 5.49 x 10*. Measurements of 6 forces and moments were made over yaw angles in the range ±20 degrees, following the procedure laid out in SAE recommended practice ref. (4). In order to simplify the analysis, the majority of the data presented in this report is in the form of wind averaged drag ( C D W A ) as defined by Ingram (3). A summary of the procedure used to evaluate wind averaged data is given in Appendix (A). The analysis concentrates on drag, being the component of aerodynamic force which directly influences fuel efficiency.

(ii) Aerodynamic surface pressure measurements

The surface pressure at a specific location of the box container is measured by connecting a small static pressure tapping via pneumatic tubing to a differential pressure transducer.This connection is usually made via a Scanivalve" pneumatic connector which enables up to 48 different tappings to be sequentially switched into the same transducer. A schematic layout of the systems used for both the wind tunnel model and the full scale

vehicle pressure measurements is given in figures (3) and (4). Essentially the same technique was used in each case, the principal differences being in the data collection and control of the scanivalve full scale.

Surface static pressures reveal considerable detail of the flow around the vehicle and for simpler interpretation are presented in the form of isobar plots. The contours in each case are based on a technique of linear interpolation between each point in the grid of surface pressure tappings shown in figure (5).

3. AERODYNAMIC DRAG MEASUREMENTS

3.1 Tractor Units in Isolation

The two cab units chosen for the programme have fundamentally different aerodynamic characteristics. The Leyland T45 Roadtrain uses considerable streamlining, and the full scale smoke visualisation showed that the flow is seen to remain attached to the surface around the forward leading edges. This characteristic ensures that the region of turbulent flow or 'wake' behind the cab is a minimum and the drag of the cab itself is relatively low. As a consequence the degree of shielding this cab offers to the trailer it tows is reduced and the forward facing areas of the box container are exposed to the freestream flow, increasing its drag. In contrast the DAF 3300 cab is a much more aerodynamically bluff design, the flow is seen to separate from the surface at nearly all the forward facing corners. Consequently the wake behind the cab is larger than the cab itself and this effectively shields the container from the freestream flow.

The relative merits of these different approaches to vehicle design rest on the concept of flowfield 'matching' between cab and container which is discussed more fully by Gilhaus (5) and others. The interest, in terms of this research programme, was to assess the different results obtained when using the same 'add-on' devices on the different vehicles. In support of this, a series of tests were carried out on both cabs in isolation, fitted with a range of devices, to supplement the very little aerodynamic data that is available to operators who, for operational reasons, occasionally have to operate their vehicles 'tractor only'.

A summary of the results for both vehicles is given, see table (1) and figure (6), in terms of S C D W A A as a percentage of the baseline (tractor only) vehicle. In both cases, tractor skirts were seen to reduce wind averaged drag, their effectiveness (with one exception) increasing as the extent of skirt cover is increased. Tractor skirts appear to be marginally more effective on the DAF 3300. Cab roof mounted devices gave different results for each vehicle. On the Leyland T45, both 3D roof and curved plate deflectors gave very significant increases in wind averaged drag. As expected this type of device disturbed the otherwise streamlined flow over

the roof resulting in a large component drag and significantly increasing the size of the wake. On the DAF 3300 however, while the plate deflector gave the expected drag increase, the 3D deflector was seen to reduce the wind averaged drag. This is a surprising result, believed to be due to an interaction between the deflector and the separated shear layer over the cab roof. These and other tests on the tractor units in isolation are discussed further by Cowperthwaite (1); section 1.

3.2 Tractor-Trailer Vehicles

Aerodynamic drag data for each of the model configurations tested is given in table (2), in terms of CDmin and C D W A . In order to simplify the analysis and presentation, discussion here is concentrated on wind averaged drag measurements ( C D W A ) , a more detailed appraisal is given by Cowperthwaite (1), sections 2 to 4.

The variation of C D A with yaw angle for both baseline vehicles is shown in figure (7). This clearly illustrates the aerodynamic features of the vehicle discussed in the previous section; the bluff cab of the DAF shielding the container at low yaw angles , resulting in a lower overall drag, while the more streamlined T45 exposes more of the container forebody to the freestream at zero yaw, increasing its drag.

The objective, in terms of reducing CDWA is to;

(1) reduce the zero yaw drag (usually equal to CDnm for a symmetric vehicle) and

(2) reduce the rate of increase in drag with yaw angle.

This concept is highlighted by the Co vs P data in figure (7) for the "good modern practice" configurations, D34 and Til, where C D A is seen to be virtually the same for both vehicles, particularly at low yaw angles.

Summary of the effectiveness of various devices - DAF 3300

(a) Cab roof deflectors [figure 8] - the 3D deflectors appear to be marginally more effective than the flat plate type, mainly because their drag does not rise as rapidly with yaw angle.

(b) Tractor skirts [figure 9] - there appears to be some interaction between various types of tractor skirt, i.e. front wheel covers in isolation give a 5 C D W A of -3% while the addition of front wheel covers to long tractor skirts does not vary the wind averaged drag.

(c) Trailer skirts [figure 10] - no apparent benefit arises from covering the trailer wheels, otherwise, progressively increasing the degree of skirts gives a corresponding reduction in C D W A .

(d) Tractor-Trailer gaps seals [figure 11] - the effectiveness of various gap seals was tested on a vehicle already fitted with; total skirts, a base seal and tractor chassis fairing (config. D15). The side and central (55%) gaps seals gave similar S C D W A A

of -17% and -16% respectively. Increasing the size of the central gap seal to 90% further reduced the S C D W A A to -19%. A horizontal gap seal had very little influence on this vehicle configuration, presumably because the tractor chassis fairing has a similar influence on the tractor-trailer flowfield.

(e) Container forebody devices [figure 12] - the effectiveness of two container forebody fairings was tested on config. D15. The total forebody moulding proved considerably more efficient than the commercial 'Windcheater' device, reducing the 5 C D W A A to -17% compared to -13% with 'Windcheater'. This is assumed to be due to (i) the increased leading edge radius offered by the total forebody moulding and (ii) the susceptibility of the Windcheater design to the type of tractor-trailer gap flowfield.

2 Summary of the effectiveness of various devices - Leyland T45

effectiveness, the 3D deflector giving a -19% S C D W A A and the plate deflector -9%.

(b) Trailer skirts [figure 10] - a progressive increase in S C D W A A is obtained as the extent of the skirts is increased, giving -9%

S C D W A A for full trailer and total tractor skirts.

(c) Container forebody devices [figure 13] - the effectiveness of two container forebody fairings was tested on the baseline T45 vehicle. The total forebody moulding (-17%, S C D W A A ) gave significantly better drag reduction than the commercial

•Windcheater' design (-9%, S C D W A A ) .

3.3 Discussion of the Aerodynamic Drag Measurements

Results serve to confirm the well established objective of matching the cab and container flowfield as being the most effective way of reducing the drag of HGVs. The techniques for correcting a 'miss-matched' flow obviously depend very much on the geometry of the cab, but nevertheless cab roof 3D deflectors appear to be the most effective add-on device available.

However well matched the cab-container flow, prevention of cross flow through the cab-container gap at yaw is possibly the preferred next stage of modification. A simple vertical gap seal is shown to reduce drag in proportion to the amount of the gap it covers. No significant differences between the effectiveness of side or central gap seals were detected.

Tractor and trailer side skirts achieve similar objectives to gap seals in preventing cross flow at simulated crosswind angles. They also serve to shield chassis protuberances from the free stream flow, although their true effectiveness in this respect may not be apparent from the wind tunnel tests, as they were conducted with stationary wheels. This may well explain the relatively small gains that were measured for skirts that were extended to cover the tractor and trailer wheels. In terms of spray suppression it is assumed that enclosing the wheels would contribute significantly to the effectiveness of skirts, but confirmation of this will require full scale trials.

4. SURFACE PRESSURE MEASUREMENTS

4.1 Wind Tunnel Measurements

Surface static pressure measurements were made on the exposed faces of the box container at the locations given in figure (5), for three model

configurations:-(1) baseline

(2) baseline + 3D deflector

(3) baseline + 3D deflector + full trailer skirts.

Measurements were made at yaw angles 0, 5, 10, 15 and 20 degrees in the 8ft X 6ft wind tunnel and a comprehensive set of isobar plots is given in Cowperthwaite (6). Discussion here is restricted to pressure changes on the container forebody and base A definition of the aerodynamic surface static pressure coefficient (Cp) on which the isobar plots are based, is given in Appendix (B).

The variation of forebody pressure distribution with yaw angle for both vehicles is given in figures (15) and (16). By integrating these pressure distributions we obtain a drag force due solely to the container forebody which can be expressed as a 'component' C D A in the same way as the drag force on the complete vehicle. This technique clearly illustrates the contribution of the container forebody drag to the overall drag, and again emphasizes the significance of tractor-trailer

'matching':-Table (3): BASELINE VEHICLE

DEGREES 0 5 10 15 20

DAF 3300 CONTAINER FOREBODY

CDA (m2) -0.005 0.016 0.026 0.031 0.037 % of Total -4 11 14 14 15

LEYLAND T45 CONTAINER FOREBODY

CDA (m«) 0.047 0.045 0.045 0.047 0.043 % of Total 36.1 32.1 25.7 21.8 19.1

The use of cab roof mounted 3D deflectors changes the container forebody pressure distributions significantly, as shown in figure (17). Very little change was seen in the container base pressures. Consideration of the corresponding container forebody C D A values with a 3D deflector, given in the table below, show the significant reductions in drag that can be achieved. However, a comparison of the S C D A / C D A T O T A L ( % ) values and the overall CoA(%) values taken by Cowperthwaite (1), (given in brackets in the table below), suggest that the drag of the deflector itself is significant at yaw, this reducing the overall effectiveness. Careful optimisation of the shape of the deflector throughout the yaw range would therefore improve the efficiency of this type of device still further.

Table (4): BASELINE VEHICLE WITH 3D DEFLECTOR

DEGREES 0 5 10 15 20 0 5 10 15 20

DAF 3300 CONTAINER FOREBODY

CDA (m2) -0.019 -0.011 -0.007 -0.003 -0.002 SCDA (m2) 0.013 0.027 0.033 0.034 0.039 5CDA/CDATOTAL(%) 10.18 [11] 20.0 [10] 18.3 [8] 15.1 [7] 16.2 [6]

LEYLAND T45 CONTAINER FOREBODY

-0.013 -0.004 0.009 0.014 0.012 0.060 0.049 0.039 0.032 0.031 44.4 [30] 35.0 [15] 22.2 [6] 15.2 [3] 13.7 [-1]

NB Figures in [ ] refer to the S C D A / C D A T O T A L values measured during the force/moment wind tunnel measurements, Cowperthwaite (1).

The addition of full trailer skirts did not appear to change the container forebody pressure distribution of either vehicle, as would be expected. However, small increases in base pressure were apparent, particularly over the lower area of the base, at yaw angles 5 and 10 degrees. This suggests that the skirts may be reducing drag not only by shielding the protuberances around the chassis and wheels, but also by modifying the wake formation region.

4.2 Full Scale Measurements

Surface static pressure measurements were made on the full size box container, using both tractor vehicles, on the T.R.R.L. Test Track, Crowthorne, Berkshire. The pressure measuring system, essentially the same system as used in the wind tunnel experiments, is shown in figure (4). Obviously, it is not practical to conduct full scale tests at a specified crosswind angle and therefore an attempt was made to conduct all the full scale measurements in nil-wind conditions. Emphasis throughout the full scale programme was on the validation and repeatability of the test technique, in preference to maximising the number of different vehicle configurations tested. Consequently comparison here is restricted to that of the container forebody and base for the baseline vehicle configurations, further details of the full scale programme are given in Cowperthwaite (6).

4.2.1 Container base pressures

Differences in base pressure were very small for the three test speeds at which measurements were taken, 40, 50 and 60 m.p.h. and repeatability was acceptable. Comparison between the full scale and model base pressure distributions is difficult, because the unsteady nature of the flow means there is no established 'pattern' to the pressure contours. Inspite of this, it is possible to see that the surface pressure falls marginally from the top of the base to the bottom of the base in both cases. However, the magnitude of the base pressure coefficient is very different between model and full scale. Typical model base Cp's for the T45 vehicle at zero yaw were, -0.10 to -0.16 while the corresponding full scale values were -0.01 to -0.08.

measurements are over-estimating the contribution of the base drag, which has been shown by Cowperthwaite (1) to be approximately 9% of the total vehicle drag.

4.2.2 Container forebody pressure

There were no significant changes in forebody pressure coefficients for either vehicle at the three test speeds of 40, 50 and 60 m.p.h., and repeatability of measurement, particularly at the higher speeds is seen to be good.

Important differences are apparent between full scale and model measurements according to the vehicle configuration: Correlation for the Leyland T45 is good, see figure (18) - the forebody stagnation region is well defined and only minor differences in magnitude of the pressure peaks are apparent. However, the DAF 3300 shows significant differences between tunnel and full scale, see figure (19). Model measurements suggest that the separated flow region above the roof of the DAF cab, deflects the freestream flow over the container forebody, effectively shielding it from the high pressure flow. Full scale measurements however show a large near stagnation pressure region on the container forebody, similar to that seen on the T45 vehicle.

4.3 Discussion of Full Scale/Model Pressure Measurements

A comprehensive assessment of the pressure differences between full scale and model tests is given in Cowperthwaite (6). Discussion here is limited to possible reasons for the significantly different flowfield observed on the DAF 3300.

The shape and extent of the separated flow region above the cab roof can be assumed to be dependant on;

(1) the position of flow separation on the cab roof

(2) the freestream Reynolds number and turbulence intensity

(3) the pressure/flow regime in the gap between cab and container (4) the relative wind direction.

Considering each of these

points:-The relative wind direction is seen from figure (16) to progressively increase the container forebody pressure as the effective yaw angle increases The flow field depicted by the 5 degrees yaw case in the above figure begins to resemble the full scale pressure distribution given in figure (19). It is possible therefore that ambient winds may have contributed to the differences seen in the full scale tests.

Variations in freestream turbulence intensity between the wind tunnel and full scale will influence the shape of the separated flow region above the cab roof, see for example Bearman (7).

Similarly, changes in freestream Reynolds number have been shown by the author (2), chapters 5 and 7, to influence container forebody pressure distributions, particularly at near "optimum" cab-container shielding. This may well explain why a similar effect was not seen in the case of the T45 vehicle for which the cab and container flowfields are not 'matched'. A reason for the dependance of a near 'optimum' cab-container flowfield to Reynolds number is given in ref. (2) as; the influence of flow changes in the cab-container gap at chassis level. Clearly, without moving ground wind tunnel tests, the flow under the vehicle may well be different to that at full scale. Whilst it is normally assumed that the typical ground clearances for HGVs are such as to reduce the significance of 'ground effect' simulation, it may well be that for the finely balanced flow between cab and container at optimum interference, even such small differences are significant.

The differences in pressure distribution between model and full scale on the DAF 3300 may be important to the aerodynamic optimisation of many future vehicles. The data made available by this research programme is not sufficient to isolate the cause of these differences and the problem of correlation between full scale and wind tunnel simulation techniques is seen to warrant further investigation.

CONCLUSIONS

1. Wind tunnel tests on l/8th scale models of DAF3300 and Leyland T45 tractors towing a standard articulated trailer have shown that significant reductions in wind averaged drag can be achieved using suitable combinations of simple add-on devices.

2. The drag reducing effectiveness of tractor and trailer side skirts -the devices recognised as being most likely to reduce water spray dispersion - is seen to be unaffected by tractor geometry. This suggests side skirts to be an effective addition to any vehicle of this class irrespective of individual design or layout.

3. The reduction of wind averaged drag provided by various combinations of side skirt varied in proportion with the extent of cover, reaching a maximum reduction of C D W A A (at 60 m.p.h.) of 12% for the completely skirted vehicle.

4. Measurements of container forebody pressure suggest that whilst cab roof mounted 3D deflectors are effective in reducing drag, the drag of the deflector itself can be considerable at yaw angles greater than 10 degrees. Whilst the statistical weighting of drag at these relatively high crosswind angles is low, this nevertheless does reduce the overall efficiency of the device and careful optimisation is therefore essential.

5. Comparison between full scale and model surface pressure measurements showed better correlation for the Leyland T45 than for the DAF 3300 tractor. This may be due to the susceptibility of the 'near matched' cab-container flow on the DAF to external influences such as; turbulence level, freestream flow direction, ground simulation and Reynolds number. These differences are such as to warrant further investigation.

REFERENCES

1. Cowperthwaite, N.A.

Scale model wind tunnel measurements on the Leyland T45 and DAF 3300 vehicles used for the T.R.R.L. spray dispersion programme.

College of Aeronautics, Cranfield Report 8622, October 1986.

2. Garry, K.P.

Wind tunnel techniques for reducing commercial vehicle aerodynamic drag.

College of Aeronautics, Cranfield Report 8220, September 1982.

3. Ingram, K.C.

The wind averaged drag coefficient applied to heavy goods vehicles. T.R.R.L. Supplementary Report 392, 1978.

4. SAE Wind Tunnel test procedure for trucks and buses. SAE J.1252

August 1979.

5. Gilhaus, A.

The influence of cab shape on the air drag of trucks. Jn. Wind Eng. and Ind. Aero., 1981, 9.

6. Cowperthwaite, N.A.

Full scale and wind tunnel model surface pressure measurements on the T.R.R.L. spray dispersion programme vehicles.

CoA Cranfield Report 8623, 1986.

7. Bearman, P.W.

Some effects of freestream turbulence and the presence of the ground on the flow around bluff bodies.

Aerodynamic drag mechanisms of bluff bodies and road vehicles. Plenum Press 1978.

ACKNOWLEDGEMENTS

Financial support for this programme has been provided by the Department of the Environment and Department of Transport, coordinated by the Transport and Road Research Laboratory, Crowthorne, Berkshire, under agreement TRR/842/424 - "Improved aerodynamics for large articulated road vehicles".

The author would like to thank Mr Nigel Cowperthwaite who carried out the majority of the experimental work, and members of the contract steering committee for their assistance during the preparation of this report, in particular Mr Alan Naysmith (T.R.R.L.).

2.800 m

Figure 1.

Pressure Tapped

Container

Scanivalve

Pressure

Transducer

Scanivalve

Controller

Computer Interface

Microcomputer

Data storage System

Figure 3 .

Schematic layout of the wind tunnel pressure measuring

and data recording system

Pressure Tapped

Container

Scanivalve

Pressure

Transducer

static Pressure Reference

Pressure

Transducer

Total

Pressure

(Kiel Tube)

(a), on'board data recording system.

Magnetic

Tape Recorder

Pre-set Scanivalve

Control unit

Frequency

Modulating

Circuit

Magnetic

Tape Recorder

Frequency

Demodulating

Circuit

Afxilogue to Digital

Converter

Microcomputer

(b).dato transfer /processing system.

Figure 4.

Forebody

Upper surface

- 6

- 6

Side

Figure 5

Short tractor

skirts

Long tractor

skirts

Total tractor

skirts

3D roof

deflector

Curved plate

deflector

24 16 8

Figure 6.

0.25

0.20

E

o

O

0.15

0.10

0.05

-20

Baseline vehicle

*Good modern practice'

configs. 034 and T i l

DAF 3300

-.-A.-LEYLANO T45

O 10 20

Yaw angle , degrees

Figure 7.

I I I I I I I I

Flat plate deflector (D2)

Curved plate deflector (D3)

3 D deflector, C of A design (D 30)

3D deflector, Commercial

design (D31)

0 - 4 - 8 -12 -16 -20

Figure 8.

Effectiveness of various tractor roof mounted devices - DAF3300

T 1

Front wheel cover (D 5)

Short tractor skirts (D 8)

Long tractor skirts (D6)

Total tractor skirts (D7)

I I I I I

-4 - 8

^

_{DAF 3300}

_{LEYLAND T45}

I I 1 1 1 I

Partial trailer skirts (012)

Full trailer skirts (D9),(T12)

Partial trailer plus short

tractor skirts (013)

Full trailer plus long

tractor skirts (D10),(T13)

Full trailer plus total

tractor skirts (D11),(T14)

Base seal ( 0 4 )

- 4 - 8 -12

-16

"/oACnwA A

Figure 10.

Comparison of the effectiveness of various skirts on the

DAF3300 and Leyland T45 vehicles.

I I

- 4

- 8

Total skirts , base seal and

tractor chassis fairing (015)

Config. (015) plus side

gap seals (55%) (016)

Config. (D 15) plus central

gap seal (90'/o) (018)

Config. (015) plus central

gap seal (55%) (019)

Config. (D15) plus horizontal

gap seal (Dt7)

-12 -16

•/o AC DWA A

-20

Figure 11.

- 8

Config. 015 plus total

forebody moulding (022)

Total skirts y base seal and

tractor chassis fairing (015)

Config. 015 plus

'Windcheater' (023)

Config. 015 plus total forebody

moulding and boattail (024)

-L.

Config. 015 plus total

forebody moulding,

central gap seal (73%)

and curved plate

_i I deflector (027)

12 -16

•/OACDWAA

-20

-24

-28

Figure 12.

I l l I I I I I 1 I 1 I I I I

-4

Front under-run bar

Windcheater (T18)

Total forebody

moulding (T20)

Boattail (T21)

8 -12 -16 -20 -24 - 2 8

Figure 13.

Effectiveness of container forebody devices - Leyland T45

OAF 3300

-I 1 r I I

Curved plate deflector ( 0 3 )

LEYLAND T45

DAF 3300

Curved plate deflector (T17)

3D deflector y Commercial design (031)

LEYLAND T45

3D deflector,Commerial

design (T4)

- 4 - 8 -12

-16

20

/oACnwA A

Figure 14.

Figure 15

Container forebody pressure

distribution

Figure 16.

Container forebody pressure

distribution.

Leyland T45 DAF 3300

Figure 17.

Container forebody pressure distribution at zero yaw with a cab roof mounted

3D deflector on both vehicles.

Wind tunnel model, zero yaw

Wind tunnel model,zero yaw

Full scale vehicle . 40 mph.

Figure 19.

Devices Short Tractor S k i r t s a) on baseline b) w i t h 3-d d e f l e c t o r Long Tractor S k i r t s a) on baseline b) w i t h 3-d d e f l e c t o r Front Wheel Covers a) w i t h long t r a c t o r s k i r t s b) w i t h long t r a c t o r s k i r t s &3-d d e f l e c t o r Total Tractor S k i r t s a) on baseline b) w i t h 3-d d e f l e c t o r 3-Diniensional Deflector a)on baseline b ) w i t h short t r a c t o r s k i r t c j w i t h l o n g t r a c t o r s k i r t s d ) w i t h t o t a l t r a c t o r s k i r t ; e j w i t h cooling blocked Curved Plate Deflector a) on baseline vehicle Blocking Cooling a) on baseline b) w i t h 3-d d e f l e c t o r Front Under-run Bar a) on baseline DAF 3300 -0.0066 -0.0059 -0.0049 -0.0108 -0.0055 -0.0119 -0.0066 +0.0090 *^'^I>.in -6.6 - 5 . 9 - 4 . 9 -10.7 -5.5 -11.8 - 6 . 6 +8.9 ^^DWA'* -0.0064 -0.0017 -0.0087 -0.0104 -0.0092 -0.0056 -0.0044 +0.0066 ^'^'^DWA -6.3 - 1 . 7 -8.5 -10.2 - 9 . 0 -5.5 -4.3 +6.4 ' Leyland T45 1 Umin -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 -0.0039 -0.0029 -0.0029 ''^m. - 2 . 1 - 2 . 8 - 3 . 1 -2.4 - 4 . 8 -3.4 - 8 . 0 - 5 . 8 +32.7 +32.0 +33.4 +34.8 + 36.1 +40.8 -5.5 - 4 . 1 - 4 . 1 ^•^DWA - 0 . 0 0 1 2 - 0 . 0 0 2 5 - 0 . 0 0 1 6 -0.0025 - 0 . 0 0 2 7 - 0 . 0 0 2 2 - 0 . 0 0 4 3 - 0 . 0 0 4 7 +0.0241 +0.0228 +0.0232 +0.0237 +0.0241 +0.0292 - 0 . 0 0 2 1 - 0 . 0 0 2 1 - 0 . 0 0 1 9 ^'^SWA -1.5 -3.4 -2.2 -3.4 -3.7 -3.0 - 5 . 8 - 6 . 4 +32.7 +30.9 +31.5 +32.2 +32.7 +39.6 - 2 . 8 - 2 . 8 -1.7

Table 1.

CONFIGURATION

Baseline (1)

Baseline + flat plate deflector Baseline + curved plate deflector Baseline + base seal

Baseline + front wheel covers Baseline + long tractor skirts Baseline + total tractor skirts Baseline + short tractor skirts Baseline + full trailer skirts Config. 9 + long tractor skirts Baseline + total skirts

Baseline + partial trailer skirts Baseline + partial trailer skirts + short tractor skirts Baseline + total skirts and base

seal

Baseline + total skirts, base seal and tractor/chassis fairing Config. 15 + side gap seals (55%) Config. 15 + horizontal gap seal (55%) Config. 15 + central gap seal (90%) Config. 15 + central gap seal (55%) Config. 16 + gap seal

Config. 15 + side gap seals (55%) and curved plate deflector Config. 15 + total forebody moulding Config. 15 + 'Windcheater'

Config. 15 + total forebody moulding and boat-tail

Config. 15 + gap fairing (45%) and rearbody moulding Config. 15 + total forebody moulding,

boat-tail and central gap seal (73%)

Config. 15 + total forebody moulding, central gap seal (73%) and curved plate deflector Baseline (2)

Flatbed

Baseline + CoA 3D deflector Baseline + commercial 3D deflector Baseline + 3D deflector, central gap

seal (55%) and short trailer skirts

Baseline + 3D deflector, central gap seal (55%) and partial trailer skirts 'GOOD MODERN PRACTICE'

Baseline + 3D deflector, central gap seal (55%), short trailer skirts and short tractor skirts

Baseline + total skirts, total forebody moulding, central gap seal

(91%) and 3D deflector CODE Dl. D2. 03. D4. D5. 06. D7. D8. 09. 010. Oil. D12. 013. 014 D15. 016. 017. 018. D19. D20 D21. D22. D23. D24. D25. 026. D27. 028. 029. 030. 031. 032. 033. 034. D35. DAF m m 0.1118 0.1094 0.1063 0.1136 0.1059 0.1082 0.1047 0.1110 0.1026 0.1015 0.0977 0.1035 0.1035 0.1016 0.1051 0.1023 0.1100 0.1026 0.1026 0.1078 0.1016 0.0989 0.1002 0.0983 0.0979 0.0962 0.0988 0.1089 0.1085 0.0960 0.0976 0.0937 0.0917 0.0932 0.0858 3300 TRACTOR-TRAILER %AC, min --2 -5 +2 -5 -3 -6 -1 -8 -9 -13 -7 -7 -9 -6 -8 -2 -8 -8 +4 -9 -12 -10 -12 -12 -14 -12 0 -12 -10 -14 -16 -14 -21 C A ^WA (m') 0.1490 0.1400 0.1394 0.1501 0.1439 0.1420 0.1410 0.1456 0.1373 0.1321 0.1306 0.1379 0.1360 0.1346 0.1369 0.1232 0.1348 0.1208 0.1247 0.1340 0.1219 0.1244 0.1290 0.1215 0.1220 0.1119 0.1124 0.1460 0.1215 0.1361 0.1320 0.1148 0.1132 0.1123 0.1023 %AC "WA --6 -6 + 1 -3 -5 -5 -2 -8 -11 -12 -8 -9 -10 -8 -17 -9 -19 -16 -10 -18 -17 -13 -19 -18 -25 -25 -17 -7 -10 -21 -22 -23 -30 CODE Tl T17 T12 T13 T14 T16 T15 T4 Til LEYLAND T45 TRACTOR-TRAILER ^0 . ' min 0.1375 0.1035 , 0.1336 0.1287 0.1264 0.1381 0.0824 0.0956 mm --25 -3 -6 -8 -40 -13 Cp A "^WA ( m ^ 0.2544 0.1435 • 0.1451 0.1411 0.1393 0.1584 0.0927 0.1259 %AC ^WA --9 -6 -9 -10 -40 -19

Table 2.

CONFIGURATION

Baseline + front under-run bar Baseline with cooling flow blocked Baseline + 3D deflector and total

forebody moulding Baseline + 3D deflector, total

forebody moulding and central gap seal (80%) Config. T6 + full trailer skirts Config. T7 + long tractor skirts Config, T8 + front wheel covers Baseline + 3D deflector, central

gap seal (55%), partial trailer skirts and short tractor skirts

Baseline + 'Windcheater'

Baseline + total forebody moulding and boat-tail

Baseline + total forebody moulding Baseline + boat-tail

CODE DAF 3300 TRACTOR-TRAILER

Cp A %ACn C„ A %AC

"min "min '^WA V

. M _ . _ , , . , CODE T2 T3 T5 T6 T7 T8 T9 TIO T18 T19 T20 T21 LEYLAND T45 TRACTOR-TRAILER Cp A %AC C A %AC

"min "min "wA "wA

0.1337 -3 0.1509 -2 0.1309 -5 0.1495 -3 0.0945 -31 0.1197 -23 0.0941 -32 • 0.1164 -25 0.0878 -36 0.1056 -32 0.0864 -37 0.1016 -34 0.0846 -39 0.0997 -35 0.0900 -35 0.1082 -30 0.1230 -11 0.1436 -9 0.1026 -26 0.1232 -22 0.1070 -23 0.1310 -17 0.1322 -4 0.1499 -5

Table 2 . (continued )

APPENDIX A

The Wind Averaged Drag Coefficient

In order to make realistic calculations of the effect of aerodynamic drag on a vehicle's fuel consumption, it is necessary to take into account the effect of crosswinds. Insight into these conditions can be achieved in the wind tunnel by measuring the aerodynamic forces on the vehicle at an angle of yaw to the freestream.

However, the true effect of crosswinds on a vehicle's power output is dependent on two

factors:-(1) the forward velocity of the vehicle.

(2) the wind velocity (at the vehicle height), and its direction relative to the line of travel.

Both these factors are taken into account in the evaluation of a wind averaged drag coefficient ( C D W A ) , see Ingram (3).

V = local wind velocity VT = vehicle speed

VR = relative speed

From the wind vector diagram above we can obtain expressions for the relative wind speed (VR) and relative wind angle (P) in terms of the; wind velocity (V), vehicle speed (VT) and the wind direction relative to the direction of travel (0). This enables the definition of a wind averaged drag coefficient as;

2tr Vmax

CDWA = C D ( P ) [ 1 + ( V / V R ) 2 + 2 ( V / V T ) C O S ^ ] p(V,0).dÓ.dV

o O

where C D ( P ) = body axis drag coefficient, obtained for each yaw angle by wind tunnel tests.

p(V,0) = probability of a wind speed V blowing at an angle 0 relative to the freestream.

It can be seen that evaluation of wind averaged drag, at a specified vehicle cruising speed (in this case 60 m.p.h.) depends on the statistical prediction of wind speed (V) and direction 0.

This prediction is made from wind data at 19 meteorological stations in the U.K. for the years 1966-1975. Each set of data is weighted according to the vehicle population density in its particular area, and the direction of nearby motorways. Consequently the probability function p(V,0) can be used and a wind averaged drag evaluated for a particular vehicle, at a specified cruising speed on a U.K. motorway.

In practical terms, wind averaged drag coefficients are a statistical guide to the likely effect of changes in aerodynamic drag. Experience suggests that between l/3rd and 1/2 of the % change in CDWA can be translated into a change in fuel economy. This obviously depends on numerous other factors, for example vehicle all up weight, gearing, driving cycles etc.

APPENDIX B

Definition of Surface Pressure Coefficient (Cp)

Measurements of surface static pressure on both the model and full size box container are expressed in the form of a pressure coefficient

(Cp), defined as

follows:-Cp = p - po where p = pressure (H/m^}

l/2pVo* Po = freestream static pressure (N/m^)

p = air density (kg/m^) Vo = velocity (m/s)

The theoretical maximum value of Cp is 1.0. This corresponds to a 'stagnation' point in the flow, i.e. a point at which the freestream flow is brought to rest by the body.

APPENDIX C

Description of Add-on Devices

long tractor

full trailer

skirts

short tractor

skirts.

partial trailer

skirts

cab front wheel

skirt

height ( h ) , 0.610 m width ( w ) , 1.720m

cab roof mounted plate

deflector

F

_v

cab roof mounted 3D

deflector

central gap seal

boattail

container forebody

moulding / Windcheater

^FP-^

IZ

tractor chassis

f a i r i n g ( **« photograph overleef )

container forebody moulding

2-43 m

r\

L/

r\—i"