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A preliminary study of configuration effects on the drag of a tractor-trailer combination Wacker, Thomas 1985

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A PRELIMINARY STUDY OF CONFIGURATION EFFECTS ON THE DRAG OF A TRACTOR-TRAILER COMBINATION By THOMAS WACKER B.A.Sc. (Mechanical Engineering), U n i v e r s i t y of B r i t i s h Columbia, 1980 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF ,THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of Mechanical Engineering) We accept t h i s t h e s i s as conforming to fche Esquired standard THE UNIVERSITY OF BRITISH COLUMBIA October, 1985 © Thomas Wacker, 1985 I In presenting t h i s thesis i n p a r t i a l f u l f i l l m e n t of the require-ments for an advanced degree at the University of B r i t i s h Columbia, I agree that the library shall make i t freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Depart-ment or by his or her representatives. It i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Thomas Wacker Department of Mechanical Engineering The University of British Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date: October, 1985 ABSTRACT The e f f e c t of configuration changes and add-on devices on the drag reduction of a t r a c t o r - t r a i l e r i s studied through wind tunnel tests using two 1/12-scale models. The configuration changes involve ground clearance, tractor-trailer gap, roof angle and back i n c l i n a t i o n while add-on devices include flow deflectors, s k i r t s and gap seals. Moving surface boundary layer control as a means of drag reduction i s also attempted. Both drag and pressure data are obtained to help i d e n t i f y l o c a l contributions. Results suggest that an optimum combination of configuration parameters can reduce drag up to 17% while the add-on devices resulted in a further decrease by a modest amount. The results with moving surface boundary layer control proved to be inconclusive. i i i TABLE OF CONTENTS Page Abstract i i List of Tables v List of Figures v i List of Symbols v i i i Acknowledgement ix 1. INTRODUCTION 1 1.1 Background 1 1.2 A Brief Review of Previous Work 1 1.3 Objectives 8 2. MODELS AND TEST PROCEDURES 9 2.1 Models 9 2.2 Add-on Devices 13 2.2.1 Tractor spoiler 13 2.2.2 Tractor roof deflector 13 2.2.3 Gap seals 13 2.3 Wind Tunnel 13 2.4 Wind Tunnel Balance 17 2.5 Instrumentation and Test Procedure 17 iv Page 3. RESULTS AND DISCUSSION 19 3.1 Configuration Changes 19 3.1.1 Ground clearance and tractor-trailer gap 19 3.1.2 Trailer back angle 23 3.1.3 Trailer roof angle 28 3.1.4 Tractor roof angle 28 3.2 Reynolds Number Effects 32 3.3 Add-on Devices 32 3.3.1 Tractor spoiler 32 3.3.2 Tractor roof deflector 32 3.3.3 Gap seals 36 3.4 Moving Surface Boundary Layer Control 36 4. CONCLUDING REMARKS 42 4.1 Conclusions 42 4.2 Recommendations for Future Work 43 REFERENCES 44 V LIST OF TABLES Page 3-1 Drag coefficient for various gap sealing configurations 38 3-2 Summary of drag reductions with various configurations and add-on devices 41 v i LIST OF FIGURES Page 1-1 Drag coefficient of several common bluff bodies, automobiles and a typical tractor-trailer configuration 2 1-2 A schematic diagram of Roshko and Koenig's [1] experimental set-up 3 1-3 Add-on devices used by Wong et a l . [4] 5 1-4 Add-on devices and configuration changes used i n the SAE study [6] 6 1- 5 Correlation of f u e l consumption and drag c o e f f i c i e n t for f u l l scale trucks as reported by Rose [7] 7 2- 1 Photograph of the 1/12-scale model used in the configuration changes and add-on devices test program 10 2-2 A schematic diagram showing parameters studied during the configuration changes and add-on devices test program 11 2-3 Photograph of the 1/12-scale model used in the moving sur-face boundary layer control study 12 2-4 Photograph of the tractor spoiler 14 2-5 Photograph of the tractor roof deflector 15 2- 6 A schematic diagram of the U.B.C. boundary layer wind tunnel 16 3- 1 Drag c o e f f i c i e n t versus ground clearance (H/L) for three different tractor-trailer gaps (G/L) 20 3-2 Variation of drag coefficient with tractor-trailer gap for a fixed ground clearance H/L =0.1 21 3-3 Effect of ground clearance on drag coefficient for a fixed tractor-trailer gap G/L = 0.1 22 3-4 Typical pressure d i s t r i b u t i o n on a conventional t r a c t o r -t r a i l e r configuration (G/L = 0.1875, H/L = 0.0875) 24 3-5 Pressure distribution on the optimum tractor-trailer config-uration (G/L = 0.1, H/L = 0.1) 25 3-6 Drag coefficient versus t r a i l e r back angle 0 26 3-7 Pressure d i s t r i b u t i o n on the optimum t r a c t o r - t r a i l e r c o n f i g -uration with 20° back angle 3-8 V a r i a t i o n of drag c o e f f i c i e n t with t r a i l e r roof angle a f o r forward sloping roofs 3-9 V a r i a t i o n of drag c o e f f i c i e n t with t r a i l e r roof angle a f o r backward sloping roofs 3-10 E f f e c t of t r a c t o r roof angle 3 on drag c o e f f i c i e n t 3-11 V a r i a t i o n of drag c o e f f i c i e n t with Reynolds number f o r the optimum t r a c t o r - t r a i l e r configuration 3-12 Drag c o e f f i c i e n t versus non-dimensional t r a c t o r s p o i l e r width Hs/Hc 3-13 V a r i a t i o n of drag c o e f f i c i e n t w i t h t r a c t o r r o o f d e f l e c t o r angle y f o r a given d e f l e c t o r length C/Lc = 0.33 3-14 V a r i a t i o n of drag c o e f f i c i e n t w i t h t r a c t o r r o o f d e f l e c t o r angle y f o r a given d e f l e c t o r length C/Lc = 0.5 3-15 E f f e c t of the t r a i l e r leading edge c y l i n d e r r o t a t i o n on drag c o e f f i c i e n t L I S T O F S Y M B O L S projected frontal area of model tractor roof deflector length drag coefficient, D/(l/2pTJ2A) pressure coefficient, (P-Po)/(l/2pTj2) drag tractor-trailer gap t r a i l e r ground clearance tractor height spoiler width t r a i l e r length tractor roof length pressure on model surface static pressure Reynolds number, UL/v free stream velocity rotating cylinder surface velocity t r a i l e r roof angle tractor roof angle tractor roof deflector angle t r a i l e r back angle air viscosity air density ix ACKNOWLEDGEMENT This study was supported by the Natural Sciences and Engineering Research Council of Canada, Grant No. A-2181. Thanks go to Mr. Ed Abell for construction of the wind tunnel models and to Mr. Andrew Kwok for his help with the wind tunnel tests. A s p e c i a l thank you i s extended to Dr. V. J. Modi for his time and guidance throughout the project and to my wife, Meg, for her patience and encouragement. 1 1. INTRODUCTION 1.1 Background With the ever r i s i n g price of f u e l , i t has become increasingly important to reduce road vehicle f u e l consumption. As a substantial portion of goods i n Canada are transported by trucks, even a small reduction in aerodynamic drag (and hence fuel consumption) can result in a significant yearly saving in transportation costs. Since trucks function as bulk load carriers, they necessarily have a large frontal area. Frontal area cannot be significantly reduced due to length and height constraints, hence a reduction i n the drag co-e f f i c i e n t becomes the only p r a c t i c a l method of minimizing the aerodynamic resistance. It would be useful at t h i s point to compare the drag of a t y p i c a l tractor-trailer configuration to that of some common bluff bodies and automobiles. Figure 1-1 shows such a comparison. 1.2 A Brief Review of Previous Work Given the importance of this topic, there has obviously been much research done. At a fundamental l e v e l , Roshko and Koenig [1] studied the i n t e r -action between two bl u f f bodies placed i n tandem. A disk was located upstream of a f l a t faced cylinder over a range of gap sizes and the forebody drag measured. Figure 1-2 shows a s i m p l i f i e d layout of the model used. In the optimum configuration (dl/d2 = 0.75, g/d2 = 0.375) a drag reduction of 97% with reference to the cylinder alone was observed. 2 Figure 1-1. Drag c o e f f i c i e n t of s e v e r a l common b l u f f bodies, a u t o -mobiles and a t y p i c a l t r a c t o r - t r a i l e r c onfiguration. f o r e b o d y g d 1 / d 2 = 0 . 7 5 g / d 2 = 0 . 3 7 5 R e = 0 . 5 x 1 0 6 62 C d ( c y l i n d e r ) = 0 . 7 5 C d ( c o m b i n a t i o n ) = 0 . 0 2 Figure 1-2. A schematic diagram of Roshko and Koenig's [I] experimental set-up. A The resu l t s are l a t e r compared to those obtained i n the present study with tractor-trailer separation. In essentially qualitative studies, Ahmed and Baumert [2], and Mair [3] examined flow patterns behind road vehicles and axi-symmetric bodies with various rear end shapes. The object was to reduce the wake and hence the drag. Similar ideas are explored in this work. Wong et a l . [A], Mason and Beebe [5] and a study sponsored by the Society of Automotive Engineers [6] have conducted wind tunnel tests on tractor-trailer models. Wong et al., using a 1/18-scale model, reported a 28% drag reduction with the configuration shown in Figure 1-3. Mason and Beebe tested several 1/7-scale models and reported a 3A% drag reduc-tion using a roof f a i r i n g alone. The SAE study involved 1/8-scale models with several add-on devices. Figure 1-A shows the different con-figurations tested. A maximum drag reduction of 3A% was obtained using a combination of roof fairing and t r a i l e r corner rounding. Fu l l scale tests using add-on devices were conducted by Rose [7]. Drag reductions were then correlated with fuel savings. Figure 1-5 shows these results. The highest measured drag reduction of 36% corres-ponded to a 16% reduction in fuel consumption. In the f i e l d of boundary layer control, Modi et a l . [8] studied the e f f e c t s of momentum i n j e c t i o n on the performance of an a i r f o i l while Catalano et a l . [9] attempted to reduce the wake of a road vehicle by introduction of unsteady vortex shedding into the flow f i e l d at the rear of the vehicle. Both the studies showed quite promising r e s u l t s and possible application of the concept to tractor-trailer combinations. 5 TRAILER CENTRAL VERTICAL PLATE TOP FAIRING •AIR DUCT ^AIRFLOW tcORNER VANES TRACTOR SECTION Q-Q Cd (base) = 0.93 Cd (optimum) = 0.674 Figure 1-3. Add-on devices used by Wong et a l . [4], 6 Figure 1-4. Add-on devices and configuration changes used in the SAE study [6], 7 K E Y i ARTICULATED VEHICLE t RIGID VEHICLE } DENOTES REPEAT TESTS 80 km/h S 10r-<,8km/h 02 04 06 MEASURED DRAG COEFFICIENT, C 0 0 8 10 Figure 1-5. C o r r e l a t i o n of f u e l consumption and drag c o e f f i c i e n t f o r f u l l scale trucks as reported by Rose [7] . 8 1.3 Objectives The objectives of t h i s study are fourfold: (i) None of the prev-ious investigators have attempted to a r r i v e at an optimum base configuration of a t r a c t o r - t r a i l e r , i.e., the combination of ground clearance and tractor-trailer gap producing the least drag. ( i i ) One cannot accurately compare the results of previous investigations with add-on devices since they were used on d i f f e r e n t base configurations. In the present study the effectiveness of add-on devices i s assessed using the same base configuration to get an accurate estimate of their performance, ( i i i ) The study explores the f e a s i b i l i t y of using moving surface boundary layer control to reduce the drag of road vehicles, (iv) Results obtained by different investigators using different models (size and geometrical details), test f a c i l i t i e s (tunnel boundary layer, turbulence intensity, blockage) and conditions (Reynolds number) seldom compare due to complex corrections involved. The main objective of this test program i s to provide a sound database through a systematic study with configuration changes and add-on devices using the same model and test conditions. 9 2. MODELS AMD TEST PROCEDURES 2.1 Models Two 1/12-scale Plexiglas models were used in the wind tunnel tests. Both were somewhat idealized, lacking wheels, detailed undersides and other minor components (mirrors, exhaust stacks, etc.). The models were f i t t e d with pressure taps to give some appreciation as to the l o c a l contribution to the overall drag. The f i r s t model (Figure 2-1) was used in tests with various add-on devices and with different tractor-trailer configurations. This model was constructed in modular form, to permit the use of different t r a i l e r roofs and backs. Ground clearance and tractor-trailer gap could also be varied. Figure 2-2 shows the various parameters studied during the test program. The second model (Figure 2-3) was used during the boundary layer control study. Cylinders were mounted at the leading and t r a i l i n g edges of the tractor and t r a i l e r roofs. Each cylinder was driven by an inter-n a l l y mounted D.C. motor, allowing for i n d i v i d u a l r o t a t i o n a l speed control. Power was transmitted via an o-ring f r i c t i o n drive. For this model, ground clearance and tractor-trailer gap were fixed at the opt-imum values found in tests with the f i r s t model. Figure 2-1. Photograph of the 1/12-scale model used i n the configuration changes and add-on devices test program Figure 2-2. A schematic diagram showing parameters studied during the configuration changes and add-on devices test program Figure 2-3. Photograph of the 1 /12-scale model used i n the moving surface boundary layer c o n t r o l study 13 2.2 Add-on Devices 2.2.1 Tractor spoiler This device (Figure 2-4) consists of a front airdam and side s k i r t s attached to the bottom of the tractor. The s k i r t s extend the f u l l length of the tractor. Three spoiler widths (Hs) were tested: 2.5 cm, 3.8 cm and 5.1 cm. 2.2.2 Tractor roof deflector The deflector (Figure 2-5) consists of a f l a t plate attached to the front edge of the tractor roof and extending i t s f u l l width. Tests were conducted with deflector lengths (C) of 4.5 cm and 6.75 cm at various angles of inclination (y). 2.2.3 Gap seals The gap seals are in the form of f l a t plates extending from the back-edge of the tractor roof, sides or bottom to the corres-ponding front edge of the t r a i l e r . The seals were tested alone and in combinations.' The side seals were always tested together. 2.3 Wind Tunnel The tests were conducted in the U.B.C. boundary layer wind tunnel (Figure 2-6). The tunnel i s an open-circuit type powered by an 80 kW three phase motor which drives an axial flow fan at a constant 700 rpm. Velocity i s varied using a pneumatic c o n t r o l l e r to a l t e r the blade pitch. Velocity range i s 2.5 to 25 m/s with an undisturbed turbulence F i g u r e 2-5. Photograph of the t r a c t o r roof deflector Axivane Series 2000 Rotor, 2.44 m dia., 16 Cast aluminum blades, 80 kW electric motor, 175,000 cfm at 700 rpm, Fisher 480-60 pneumatic variable pitch control 1 honeycomb and 4 screens in 4 x 4 m settling section Figure 2-6. A schematic diagram of the U.B.C. boundary layer wind tunnel 17 l e v e l of less than 1%. Spatial variation of mean vel o c i t y i n the test section i s less than 2%. The s e t t l i n g section contains a honeycomb and four screens to smooth the flow as i t enters a 4.7 to 1 contraction which accelerates the flow and improves i t s uniformity. The test section i s 24.4 m long with a cross section of 2.44 m x 1.62 m at i t s entrance. The adjustable test section roof was set for a zero pressure gradient. 2.4 Wind Tunnel Balance Force measurements were carried out using an Aerolab strain gauge balance. I t i s capable of measuring the three p r i n c i p a l forces and moments acting on a model. During a typical test, only drag was record-ed, although side force was monitored to ensure alignment with the tunnel axis (zero yaw angle). 2.5 Instrumentation and Test Procedure Balance signals were amplified and read with a digital voltmeter. The amplifier was calibrated so that the reading in volts corresponded to the drag in pounds-force. Wind tunnel dynamic head was measured with a p i t o t s t a t i c tube connected to a Lambrecht manometer. Atmospheric pressure and wind tunnel temperature were also recorded to translate the dynamic head into velocity. Due to the small area ratio (Amodel/Atunnel = 0.014), drag readings were not corrected for blockage effects. To compensate for d r i f t i n the balance sig n a l a m p l i f i e r , the 18 f o l l o w i n g t e s t procedure was used. An i n i t i a l 'drag' reading was taken with the tunnel on but at zero v e l o c i t y . The tunnel v e l o c i t y was then s e t to the d e s i r e d value and a drag r e a d i n g noted once the f l o w was s t a b i l i z e d . The tunnel was then returned to zero v e l o c i t y and a f i n a l 'drag' r e a d i n g taken. As an a d d i t i o n a l check, the c l e a n c o n f i g u r a t i o n (no add-on d e v i c e s ) drag was measured at the beginning and the end of each s e r i e s of t e s t s . 19 3. RESULTS AND DISCUSSION 3.1 Configuration Changes 3.1.1 Ground clearance and tractor-trailer gap Figure 3-1 shows the va r i a t i o n of drag c o e f f i c i e n t versus non-dimensional ground clearance (H/L) for three d i f f e r e n t non-dimensional gaps: G/L = 0.0875, 0.1 and 0.1125. Reynolds number was 0.64 x 106 for a l l the tests. These r e s u l t s established the optimum (i.e., minimum drag) base configuration on which a l l sub-sequent modifications were based. A l l three curves show the same trend with minimum drag occurring at approximately the same ground clearance of H/L = 0.1. The tests with G/L = 0.1 resulted i n the lowest drag value of Cd = 0.668. This then established the optimum combination of gap and ground clearance at G/L = 0.1 and H/L = 0.1. It should be pointed out here that a typical configuration found on today's tractor-trailers i s G/L = 0.1875 and H/L = 0.0875. Testing this configuration resulted in a drag coefficient of Cd = 0.71. To i l l u s t r a t e the above r e s u l t s i n clearer terms, the two parameters were varied separately. Figure 3-2 shows the effects of varying gap with ground clearance fixed at H/L = 0.1 and Figure 3-3 shows the effects of varying ground clearance with gap fixed at G/L = 0.1. A O 0 . 69 -C d 0 . 68 0 .67 -G/L = 0.1125 G/L = 0.0875 o G/L = 0.1 0 . 0 9 4 0.1 A O Re=0.64x106 • IS 0 . 1 0 6 0 . 1 1 2 0 . 1 1 8 H/L Figure 3-1. Drag c o e f f i c i e n t versus ground clearance (H/L) for three d i f f e r e n t t r a c t o r - t r a i l e r gaps (G/L) KJ o 0 . 6 9 -0 .68 -0 . 6 7 -H/L=0.1 Re=0.64x106 1 1 1 0 . 0 8 7 5 0.1 0 . 1 1 2 5 G/L Figure 3-2. Variation of drag coefficient with tractor-trailer gap for a fixed ground clearance H/L =0.1 G/L = 0.1 Re=0.64x106 • IS 0.094 0.1 0.106 0.112 0.11 H/L Figure 3-3. Effect of ground clearance on drag coefficient for a fixed tractor-trailer gap G/L = 0.1 23 To obtain better appreciation as to the local contribution to the drag, pressure d i s t r i b u t i o n s were measured on the con-ventional (G/L = 0.1875, H/L = 0.0875) and optimum (G/L = 0.1, H/L = 0.1) configurations. The results are shown in Figures 3-4 and 3-5. The major difference between the two i s a lager negative pressure region at the rear of the conventional configuration. This i s clearly a contributing factor leading to a higher drag. 3.1.2 Trailer back angle With optimum gap and ground clearance established, other configuration parameters could now be varied to explore the poss-i b i l i t y of further reduction i n drag. Figure 3-6 shows the variation of drag coefficient with the t r a i l e r back angle (6) at a Reynolds number of 0.64 x 10^. Minimum drag was observed at an angle of 20° with a very sharp increase at angles above 30°. The decrease i n drag from 0° to 20° i s attributed to the flow being able to negotiate the shallower angle at the top edge. The i n -crease i n drag at higher angles i s presumably due to separation from the sides and bottom edge. To verify the mechanism of drag reduction, a pressure dis-t r i b u t i o n was charted with 0 = 20°. Figure 3-7 c l e a r l y shows a smaller rear separation bubble compared to the base configuration shown in Figure 3-5. 0.09 G / L = 0 . 1 8 7 5 H / L = 0 . 0 8 7 5 0.09 C d = 0 . 7 1 R e = 0 . 6 4 x 1 0 6 Figure 3-4. Typical pressure distribution on a conventional tractor-trailer configuration (G/L = 0.1875, H/L = 0.0875) Figure 3 - 5 . Pressure distribution on the optimum tractor-trailer configuration (G/L = 0 . 1 , H/L = 0 . 1 ) 0 .86 0 .67 -0 . 66 -o 0 .65 -0 .64 -G/L = 0.1 H/L=0.1 Re=0.64x106 • 0 .63 10 i — 2 0 3 0 4 0 0 ( Figure 3-6. Drag c o e f f i c i e n t versus t r a i l e r back angle 6 Figure 3-7. Pressure d i s t r i b u t i o n on the optimum t r a c t o r - t r a i l e r configuration with 20° back angle 28 3.1.3 T r a i l e r roof angle Two s e t s of t e s t s were run w i t h a v a r i a b l e t r a i l e r r o o f angle, a. Figure 3-8 shows drag c o e f f i c i e n t versus a f o r forward s l o p i n g r o o f s at a Reynolds number of 0.59 x 10^. Corresponding r e s u l t s f o r backward s l o p i n g r o o f s are presented i n F i g u r e 3-9. The forward sloping roofs show a steady increase i n angle while the trend i s reversed f o r backward sloping roofs. This i s explained by the f a c t t h a t f o r backward s l o p i n g r o o f s , the f l o w encounters a s h a l l o w e r angle at the back w h i l e f o r forwa r d s l o p i n g r o o f s , the angles become greater with increasing roof slope. There could also be more f a v o u r a b l e gap geometry w i t h backward s l o p i n g r o o f s a l -though t h i s would require a further study to v e r i f y . 3.1.4 Tractor roof angle Figure 3-10 shows the v a r i a t i o n of drag c o e f f i c i e n t w i t h t r a c t o r r o o f angle (6) a t a Reynolds number of 0.56 x 10 6. A l -though the drag i s always g r e a t e r than the base va l u e (B = 0°), th e r e i s a c o n s i d e r a b l e d i p i n the curve at 8 = 20°. T h i s would imply that 20° i s close to the angle required f o r smooth reattach-ment of the f l o w to the f r o n t of the t r a i l e r . Other i n c l i n a t i o n s r e s u l t e d i n a l a r g e r s e p a r a t i o n bubble and hence a hi g h e r drag c o e f f i c i e n t . 0 . 8 3 -0.8 -0.77 -0 .74 -G/L = 0.1 H/L=0.1 Re=0.59x106 c 0.71 -0 .68 -> o T ~T~ 2 3 T -4 5 Figure 3-8. Vari a t i o n of drag c o e f f i c i e n t with t r a i l e r roof angle a for forward sloping roofs 0 .68 -> G/L = 0.1 H/L=0.1 Re = 0.6x106 3 - r -4 5 a 1 Figure 3-9. Variation of drag coefficient with t r a i l e r roof angle for backward sloping roofs o 0 .75 G/L = 0.1 H/L=0.1 Re=0.56x106 0.72 A 0 .69 5 10 15 2 0 2 5 0C Figure 3-10. Effect of tractor roof angle 3 on drag c o e f f i c i e n t 32 3.2 Reynolds Number Effects Preliminary tests suggested drag of the base configuration (G/L = 0.1, H/L = 0.1) to be dependent on the Reynolds number. To establish the dependence more precisely a systematic study was under-taken. Figure 3-11 shows the results. A steady decrease i n drag c o e f f i c i e n t with increasing Reynolds number i s apparent. This i s i n agreement with the results of the con-figuration tests. 3.3 Add-on Devices 3.3.1 Tractor spoiler Figure 3-12 shows drag c o e f f i c i e n t versus non-dimensional spoiler width (Hs/Hc) at a Reynolds number of 0.56 x 106. A steady increase i n drag with an increase i n s p o i l e r width i s observed. For a model with a detailed underside, this trend would be, prob-ably, reversed as the s p o i l e r reduces the separated flow on the underside. With a smooth underside, however, the effect of adding a s p o i l e r i s the same as reducing the ground clearance, which results in an increase in drag. 3.3.2 Tractor roof deflector The tractor roof deflector tests were conducted for two deflector lengths, C/Lc. Figure 3-13 shows variation of the drag coefficient with deflector angle (y) for C/Lc = 0.33 at a Reynolds number of 0.568 x 10^, while Figure 3-14 presents results for C/Lc 0.74 0.72 0.7 - o Cd 0.68 - G/L = 0.1 H/L=0.1 0.66-D i 0.64 0.4 0.5 0.6 0.7 Re (x10" 6) Figure 3-11. Variation of drag coefficient with Reynolds number for the optimum tractor-trailer configuration 00 0.72 A G/L = 0.1 H/L=0.1 Re= 0.56x106 He 0.71 A I H s 0.7 A 0.69 A 0.1 0.2 Hs/Hc 0.3 Figure 3-12. Drag c o e f f i c i e n t versus non-dimensional t r a c t o r s p o i l e r width Hs/Hc 0.73-G/L = 0.1 H/L=0.1 C/Lc=0.33 Re = 0.568x106 10 20 - 1 — 30 Figure 3-13. Variation of drag coefficient with tractor roof deflector angle y for a given deflector length C/Lc = 0.33 36 = 0.5 at two Reynolds numbers; 0.545 x 10° and 0.623 x 106. Note a very slight drag reduction at y = 30° (Figure 3-13). On the other hand, Figure 3-14 shows a substantial decrease in Cd at y = 30° for both Reynolds numbers tested. It i s interesting to recognize that in this test, the deflector angle corresponding to a minimum drag (i.e., smooth flow reattachment at the t r a i l e r roof) i s the same for both the Reynolds numbers. 3.3.3 Gap seals Table 3-1 summarizes the results obtained using various com-binations of gap-seals. As can be seen, a l l combinations had l i t t l e or no beneficial effect on the drag. 3.4 Moving Surface Boundary Layer Control Figure 3-15 shows drag coefficient versus non-dimensional surface velocity for the cylinder located at the top leading edge of the t r a i l -er. The Reynolds number was 0.55 x 10°. At zero velocity, the model had a lower drag than the one tested earlier. This i s due to the rounded edges at the front and back of the tractor and t r a i l e r roofs (This modification, incidentally, works quite well, yielding a 12% reduction i n drag.). The r e s u l t s with increasing cylinder v e l o c i t y showed an i n i t i a l increase in drag followed by a reduction at a velocity ratio of V/U = 1.8. Beyond this value, the drag increases again, although higher velocities (V/U >3) would seem to be required to realize the f u l l bene-f i t s of this approach to boundary layer control through moving surfaces [8]. Unfortunately, due to slippage of the o-ring, power capability of 0 . 7 5 -0 . 7 3 -0 .71 -\ R e • 0.545x106 © 0.623x10s G/L = 0.1 H/L=0.1 C/Lc = 0.5 C d 0 . 6 9 -0 . 6 7 -0 . 65 10 2 0 3 0 — i — 4 0 Figure 3-14. Variation of drag coefficient with tractor roof deflector angle y for a given deflector length C/Lc = 0.5 LO Table 3-1 Drag coefficient for various gap sealing devices Configuration Drag Coefficient No Seal 0.680 Top Only 0.680 Sides Only 0.690 Bottom Only 0.687 Top and Bottom 0.679 Top and Sides 0.687 Sides and Bottom 0.693 A l l Seals 0.705 Re - 0.6 x 10 6 0.64 H 0 .63 H o o o U G/L = 0.1 H/L=0.1 Re = 0.55x106 D Q 0 .62 H 0.61 ^ 0.6 0 .59 0.5 1.5 2 2.5 V / U Figure 3-15. E f f e c t of the t r a i l e r leading edge cyl i n d e r r o t a t i o n on drag c o e f f i c i e n t 40 the motor and space c o n s t r a i n t s i t was not p o s s i b l e to a t t a i n h igher c y l i n d e r rpm. Due to the extensive modifications required to the model and the large number of variables involved i t was decided to assess the v a l i d i t y of the concept through a separate project. For ease of comparison, Table 3-2 summarizes r e s u l t s of the e n t i r e t e s t program. Table 3-2 Summary of drag reductions with various configurations and add-on devices Configuration Cd Cd Reduction (%) Conventional 0.71 -Optimized (Base) 0.67 (= Cdo) 5.6 Base + 20° Back 0.64 9.9 Base + 5° Roof 0.60 (Cdo = 0.68) 16.9 Base + Roof Deflector 0.66 1.5* Trailer Leading Edge Rotating Cylinder 0.597 (Cdo = .611) 2.3* •These drag reductions are with respect to Cdo 42 4 . CONCLUDING REMARKS 4.1 Conclusions Based on the results and observations during the test program, the following general conclusions can be drawn: (i) Configuration changes have a greater e f f e c t on drag than add-on devices. With respect to the base configuration, a 5° backward sloping roof resulted i n a 12% drag reduction while the tractor roof deflector resulted in a reduction of only 1.5%. ( i i ) Relatively minor configuration changes can result in large drag reductions. For example, a 2° backward sloping roof yields a 6% decrease i n drag coefficient. ( i i i ) Not a l l add-on devices have a beneficial effect and those that do are geometry dependent. For instance, addition of a roof deflect-or (C/Lc = 0.5) at 30o inclination resulted in a 1.5% reduction in drag c o e f f i c i e n t while the same deflector at 20o i n c l i n a t i o n resulted in a 5% increase. (iv) The maximum drag reduction achieved through configuration changes and add-on devices i s 16.9%. (v) Moving surface boundary layer control looks promising, however, i t s effectiveness needs to be verified through a carefully planned experiment. A3 A.2 Recommendations f o r Future Work As with a l l preliminary studies, several areas are open to further study: ( i ) Because the d e v i c e s and c o n f i g u r a t i o n changes would be used a t highway speeds, t e s t s should be performed at the c o r r e s p o n d i n g Reynolds number (2 x 10? f o r 100 km/h). The use of wind t u n n e l f a c i l i t i e s at the National Research Council should be explored to t h i s end. ( i i ) E f f e c t i v e n e s s of a d d i t i o n a l add-on d e v i c e s such as gap s p l i t t e r p l a t e s , t r a i l e r r e a r f a i r i n g s and t r a c t o r r o o f d e f l e c t o r s of d i f f e r e n t geometries should be explored. ( i i i ) D e t a i l s should be added to the models to gauge t h e i r influence on the add-on devices and configuration changes. ( i v ) The t r a c t o r - t r a i l e r model should be modified to provide p o s i t i v e drive and higher speed to a l l four cylinders. This can be accom-plished with more powerful motors transmitting power through b e l t , chain or gear drives. The boundary layer c o n t r o l should be tested with the c y l i n d e r s operating i n d i v i d u a l l y and i n various combin-ations over a range of surface v e l o c i t y . 44 R E F E R E N C E S [1] Roshko, A. and Koenig, K., "Interaction E f f e c t s on the Drag of Bluff Bodies in Tandem," Proceedings of the Symposium on Aero- dynamic Drag Mechanisms of Bluff Bodies and Road Vehicles,Ed. G. Sovran, T. Morel and W.T. Mason, Jr., Plenum Press, New York, 1978. Ahmed, S.R. and Baumert, W., "The Structure of Wake Flow Behind Road Vehicles," Aerodynamics of Transportation, Ed. T. Morel and C. Dalton, ASME, New York, 1973. Mair, W.A., "Drag-Reducing Techniques for Axi-Symmetric Blu f f Bodies," Proceedings of the Symposium on Aerodynamic Drag Mech- anisms of Bluf f Bodies and Road Vehicles, Ed. G. Sovran, T. Morel and W.T. Mason, Jr., Plenum Press, New York, 1978. [4] Wong, H.Y., Cox, R.N. and Rajan, A., "Drag Reduction of T r a i l e r -Tractor Configuration by Aerodynamic Means," Proceedings of the 4th  Colloquium on I n d u s t r i a l Aerodyanmics, Ed. C. Kramer and H.J. Gerhardt, Fotodruck Mainz, Aachen, 1980. [5] Mason, W.T., Jr. and Beebe, P.S., "The Drag Related Flow F i e l d Characteristics of Trucks and Buses," Proceedings of the Symposium  on Aerodynamic Drag Mechanisms of Bluff Bodies and Road Vehicles, Ed. G. Sovran, T. Morel and W.T. Mason, Jr., Plenum Press, New York, 1978. [6] "Truck and Bus Aerodynamics Investigated," Automotive Engineering, Vol. 88, No., 11, 1980, pp.50-57. [7] Rose, M.J., "Commercial Vehicle Fuel Economy - The Correlation Between Aerodynamic Drag and Fuel Consumption on a Typical Truck," Proceedings of the 4th Colloquium on Industrial Aerodyanmics, Ed. C. Kramer and H.J. Gerhardt, Fotodruck Mainz, Aachen, 1980. [8] Modi, V.J., Swinton, P.G., McMillan, K., Lake, P., Mullins, D., and Akutsu, T., "Moving Surface Boundary layer Control for A i r c r a f t Operation at High Incidence," Journal of Aircraft, AIAA, Vol. 18, No. 11, November 1981, pp.963-968. [9] Catalano, G.D., Viets, H. and Bougine, D., "Reduction of the Turbulent Wake of a Road Vehicle by use of Unsteady Vortex Shedding," AIAA 21st Aerospace Sciences Meeting, Reno, Nevada, USA, January 1983, paper No. AIAA-83-0427. [2] [3] 

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