148 Commercial vehicle drag reducing devices

14.8.1 Cab roof deflectors (Figs 14.54(a and b), 14.55(a and b) and 14.56(a-c)) To partially overcome the large amount of extra drag experienced with a cab to trailer height mismatch a cab roof deflector is commonly used. This device prevents the air movement over the cab roof impinging on the upper front of the trailer body and then flowing between the cab and trailer gap, see Fig. 14.54(a). Instead the air flow is diverted by the uptilted deflector surface to pass directly between the cab to trailer gap and then to flow relatively smoothly over the surface of the trailer roof, see Fig. 14.54(b). These cab roof deflectors are beneficial in reducing the head on air flow but they do not perform so well when subjected to side winds. Slight improvements can be made to prevent air flowing underneath and across the deflector plate by enclosing the sides; this is usually achieved

Flow re-attachment
Drag Reducing Devices

(a) Cab to trailer height mismatch (b) Cab to trailer height mismatch bridged with roof deflector

Fig. 14.54(a and b) Air flow between cab and trailer body with and without cab roof deflector

(a) Cab to trailer height mismatch (b) Cab to trailer height mismatch bridged with roof deflector

Fig. 14.54(a and b) Air flow between cab and trailer body with and without cab roof deflector by using a fibre glass or plastic moulded deflector, see Fig. 14.55(b).

If trailers with different heights are to be coupled to the tractor unit while in service, then a mismatch of the deflector inclination may result which will again raise the aerodynamic drag. There are some cab deflector designs which can adjust the tilt of the cab deflector to optimize the cab to trailer air flow transition (see Fig. 14.55(a)), but in general altering the angle setting would be impractical. How the cab roof deflector effectiveness varies with deflector plate inclination is shown in Fig. 14.56(c) for both a narrow and a wide cab to trailer gap, representing a rigid truck and an articulated vehicle respectively (see Fig. 14.56(a and b)). These graphs illustrate the general trend and do not take into account the different cab to trailer heights, cab to trailer air gap width and the width to height ratio of the deflector plate. It can be seen that with a rigid truck having a small cab to trailer gap the

Moulded

Moulded

Fig. 14.55 (a and b) Moulded adjustable cab roof deflector
Aticulated Lorry Deflector

(b) Articulated truck Deflector inclination angle(G) deg

Fig. 14.56(a-c) Optimizing roof deflector effectiveness for both rigid and articulated trucks

(b) Articulated truck Deflector inclination angle(G) deg

Fig. 14.56(a-c) Optimizing roof deflector effectiveness for both rigid and articulated trucks

reduction in the drag coefficient with increased deflector plate inclination is gradual, reaching an optimum minimum at an inclination angle of 80° and then commencing to rise again, see Fig. 14.56(c). With the articulated vehicle having a large cab to trailer gap, the deflector plate effectiveness increases rapidly with an increase in the deflector inclination angle until the optimum angle of 50° is reached, see Fig. 14.56(c). Beyond this angle the drag coefficient begins to rise steadily again with further increase in the deflector plate angle; this indicates with the large gap of the articulated vehicle the change in drag coefficient is much more sensitive to deflector plate inclination.

14.8.2 Yaw angle (Figs 14.57 and 14.58) With cars the influence of crosswinds on the drag coefficient is relatively small; however, with much larger vehicles a crosswind considerably raises the drag coefficient therefore not only does the direct air flow from the front but also the air movement from the side has to be considered. It is therefore necessary to study the effects crosswinds have on the vehicle's drag resistance, taking into account the velocity and angle of attack of the crosswind relative to the direction of motion of the vehicle and its road speed. This is achieved by drawing to scale a velocity vector triangle, see Fig. 14.57. The vehicle velocity vector line is drawn, then the crosswind j CZJ

"TT 11 11

Vehicle

Vehicle velocity

Vehicle velocity

Resultant angle relative to direction of motion (yaw angle)

Resultant angle relative to direction of motion (yaw angle)

Relative flow' air velocity

Wind angle relative to directioi of motion

Wind direction &

velocity

Wind angle relative to directioi of motion

Wind direction &

velocity velocity vector at the crosswind angle to the direction of motion; a third line representing the relative air velocity then closes the triangle. The resultant angle made between the direction the vehicle is travelling and the resultant relative velocity is known as the yaw angle, and it is this angle which is used when investigating the effect of a crosswind on the drag coefficient.

In addition to head and tail winds vehicles are also subjected to crosswinds; crosswinds nearly always raise the drag coefficient, this being far more pronounced as the vehicle size becomes larger and the yaw angle (relative wind angle) is increased. The effect crosswinds have on the drag coefficient for various classes of vehicles expressed in terms of the yaw angle (relative wind angle) is shown in

Fig. 14.58. Each class of vehicle with its own head on (zero yaw angle) air flow drag coefficient is given a drag coefficient of unity. It can be seen using a drag coefficient of 1.0 with zero yaw angle (no wind) that the drag coefficient for a car reaches a peak of 1.08 at a yaw angle of 20°, whereas for the van, coach, articulated vehicle and rigid truck and trailer the drag coefficient rose to 1.18, 1.35, 1.5 and 1.7 respectively for a similar yaw angle of 20°.

14.8.3 Cab roof deflector effectiveness versus yaw angle (Fig. 14.59(a and b)) The benefits of reducing the drag coefficient with a cab roof deflector are to some extent cancelled out when the vehicle is subjected to crosswinds. This can be demonstrated by studying data taken from

Aerodynamic Trucks From The

Influence of yaw angle upon aerodynamic drag

Influence of yaw angle upon aerodynamic drag

Fig. 14.58

one particular vehicle, see Fig. 14.59(a and b), which utilizes a cab roof deflector; here with zero yaw angle the drag coefficient reduces from 0.7 to 0.6 as the deflector inclination changes from 90° (vertical) to 50° respectively. With a 5° yaw angle (relative wind angle) the general trend of drag coefficient rises considerably to around 0.9 whereas the tilting of the deflector from the vertical over an angle of 40° only shows a marginal decrease in the drag coefficient of about 0.02; with a further 10° inclination decrease the drag coefficient then commenced to rise steeply to about 0.94. As the yaw angle is increased from 5 to 10° the drag coefficient rises even more to 1.03 with the deflector in the vertical position, however this increase in drag coefficient is not so much as from 0 to 5°. Hence the reduction in the drag coefficient from 1.03 to 0.98 as the deflector is tilted from the vertical to 40° is relatively small compared to the overall rise in drag coefficient due to crosswind effects. However, raising the yaw angle still further from 10 to 15° indicates on the graph that the yaw angle influence on the drag coefficient has peaked and is now beginning to decline: both the 10 and 15° yaw angle curves are similar in shape but the 15° yaw angle curve is now below that of the 10° yaw angle curve. Note the minimum drag coefficient deflection inclination angle is only relevant for the dimensions of this particular cab to trailer combination.

14.8.4 Comparison of drag resistance with various commercial vehicle cab arrangements relative to trailer body height (Fig. 14.60(a-e)) The drag coefficient of a tractor-trailer combination is influenced by the trailer body height and by different cab configurations such as a conventional low cab, low cab with roof deflector and high sleeper cab, see Fig. 14.60(a-c). Thus a high cab arrangement (see Fig. 14.60(c, d and e)) is shown to be more effective in reducing the drag coefficient than a low cab (see Fig. 14.60(a, d and e)) and therefore for long distance haulage the sleeper compartment above the driver cab has an advantage in having the sleeper area behind the driver's seat. Conversely with a low cab and a roof deflector which has an adjustable plate angle (see Fig. 14.60(b, d and e)), the drag coefficient can be kept almost constant for different trailer body heights. However, it is not always practical to adjust the deflector angle, but fortunately a great many commercial vehicle

40 50 60 70 80 90

Deflector angle (6) deg

Fig. 14.59(a and b) Effect of yaw angle upon drag reducing effectiveness of a cab roof deflector

Airstream n i

(a) Low cab and high trailer body

ie ici

Adjustable deflector

(d) Effects of different cab roof configurations relative to trailer body height

Cab Trailer Drag Efficient

r~ n

n sz

ui4

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(b) Low cab with deflector and high trailer body

(e) Articulated truck with different trailer body heights

(c) High cab and trailer body

Fig. 14.60 (a-e) Methods of optimizing air flow conditions with different trailer body heights cab-trailer combinations use the same size trailer bodies for one particular application so that the roof deflector angle can be pre-set to the minimum of drag resistance. With a low cab the drag coefficient tends to increase as the cab roof to body roof step height becomes larger whereas with a high cab the drag coefficient tends to decrease as trailer body height rises, see Fig. 14.60(d and e).

14.8.5 Corner vanes (Fig. 14.61(a-c)) The cab of a commercial vehicle resembles a cube with relatively flat upright front and side panels, thus with well rounded roof and side leading edges the cab still has a blunt front profile. When the vehicle moves forward the cab penetrates the surrounding air; however, the air flow passing over the top, underneath and around the sides will be far from being streamlined. Thus in particular the air flowing around the side leading edges of the cab may initially separate from the side panels, causing turbulence and a high resistance to air flow, see Fig. 14.61(a).

One method of reducing the forebody drag is to attach corner vanes on each side of the cab (Fig. 14.61(c)). The corner vane is set away from the rounded vertical edges and has several evenly spaced internal baffles which bridge the gap between the cab and corner vane walls. Air meeting the front face of the cab moves upwards and over the roof, while the rest flows to the left and right hand side leading edges. Some of this air also flows around the leading edge through the space formed between the cab and corner vanes (see Fig. 14.61 (b and c)); this then encourages the airstream to remain attached to the cab side panel surface. Air drag around the cab front and side panels is therefore kept to a minimum.

14.8.6 Cab to trailer body gap (Fig. 14.62) Air passing between the cab and trailer body gap with an articulated vehicle due to crosswinds significantly increases the drag resistance. As the crosswind angle of attach is increased, the flow through the cab-trailer gap produces regions of flow separated on the sheltered side of the trailer body, see Fig. 14.62. This flow separation then tends to spread rearwards, eventually interacting with and enlarging the trailer wake, the net result being a rise in the rearward pull due to the enlarged negative pressure zone.

(a) Air flow without corner vanes

(b) Air flow with corner vanes

Flow ' separation

Flow ' separation

Corner Vanes

Fig. 14.61 (a-c) Influence of corner vanes in reducing cab side panel flow separation

Flow Seperation Corner

14.8.7 Cab to trailer body gap seals (Fig. 14.63 (a and b), 14.64, 14.65(a and b) and 14.66(a-d)) Simple tilt plate cab roof deflectors when subjected to side winds tend to counteract the gain in head on airstream drag resistance unless the deflector sides are enclosed. With enclosed and streamlined cab roof deflector sides, see Fig. 14.55(a and b), improvements in the drag coefficient can be made with yaw angles up to about 20°, see Fig. 14.63(a and b). Further reductions in the drag coefficient are produced when the cab to trailer gap is sealed by some sort of partition which prevents air flowing through the cab to body gap, see Fig. 14.63 (a and b). The difficulty with using a cab to trailer air gap partition is designing some sort of curtain or plate which allows the trailer to articulate when manoeuvring the vehicle-trailer combination.

Cab to trailer gap seals can be divided into three basic designs:

1 Cab extended side panels

2 Centre line gap seals (splitter plate seal)

3 Windcheater roller edge device (forebody edge fairing).

Cab extended side panels (Fig. 14.64) These devices are basically rearward extended vertical panels attached to the rear edges of the cab which are angled towards the leading edges of the trailer body. This type of gap fairing (side streamlining) is effective in reducing the drag coefficient with increasing crosswind yaw angle. With zero and 10° yaw angles a drag coefficient reduction of roughly 0.05 and 0.22 respectively have been made possible.

Deflector ,Gap seal --

re ar p

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(a) Low cab with roof deflector and gap seals

d2 e

(b)

/

- /

1 1

Fig. 14.63(a and b) Drag reductions with crosswinds when incorporating a roof deflector and gap seal

Extended Minimum side panel / turning

IZZ)

King-pin

Trailer body

Fig. 14.64 Cab extended side panels

Standing vortices

Fig. 14.64 Cab extended side panels

Standing vortices

Minimum turning circle

Single splitter plate »

King-pin

(a) Centre line gap seal

Fig. 14.65 (a and b) Cab to trailer body gap seals

Standing vortices vri

King-pin

Double flexibe vertical plates

(b) Offset flexible gap seals d2 e

Flow separation

Recirculating air bubble

Crosswind

Head on wind

Flow separation

Crosswind

Flow Seperation Corner

Flow separation

Re-attachment

Crosswind

Flow separation

Re-attachment

Crosswind

Flow Seperation Corner
Head on wind

(a) Sharp corner with and without crosswind

(b) Rounded corner with and without crosswind

Roof panel

Extended quadrant section

Roof panel

Air flow stream

Civil Engineering Truck Turning Diagrams

Extended quadrant section

Air flow stream

Trailer body front panel

(c) Extended quadrant corner (windcheater)

Drag The The Partition

¿B\ Semicircular

Elongated semicircular Quadrant

Yaw angle deg.

(d) Effectiveness of various forebody edge sections upon drag coefficient

Fig. 14.66 (a-d) Trailer forebody edging fairings

Centre line and offset double gap seal (vortex stabilizer) (Fig. 14.65(a and b)) A central vertical partition or alternatively a pair of offset flexible vertical plates attached to the trailer body (see Fig. 14.65(a and b)) is effective in not only preventing crosswinds passing through the cab to trailer gap but also stabilizes the air flow entering the gap by generating a relatively stable vortex on either side of the plate or plates. The vortex stabilizer is slightly less effective than the extended side panel method in reducing the drag coefficient when side winds prevail.

Rolled edge windcheater (Fig. 14.66(a-d)) This device consists of an extended quadrant section moulding attached to the roof and both sides of the leading edges of the forebody trailer panel. When there are sharp leading edges around the trailer body air flowing through the cab to trailer space tends to overshoot and hence initially separate from the side panels of the trailer body (see Fig. 14.66(a)) and even with rounded edging there is still some overshoot and flow separation (Fig. 14.66(b)). The effectiveness of different sectioned forebody edge fairings are compared corresponding to a yaw angle (relative wide angle) variation from 0 to 20°, see Fig. 14.66(d). Here it can be seen that there is very little difference between the semi circular and elongated semi circular moulding but there is a moderate improvement in the drag coefficient at low yaw angles from 0 to 10° for the quadrant section; however, with the extended-quadrant moulding there is a considerable improvement as the yaw angle is increased from 0 to 20°. With the extended quadrant moulding (see Fig. 14.66(c)) the air flow tends to move tangentially between the cab to trailer air gap; some of the air then scrubs along the flat frontage of the trailer body until it reaches the extended-quadrant step, is then deflected slightly rearwards and then again forwards before closely following the contour of the curved corner. This makes it possible for the air flow to remain attached to the side panel surface of the trailer body, therefore keeping the drag resistance on the sheltered trailer panel side to the minimum.

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