147 Commercial vehicle aerodynamic fundamentals

14.7.1 The effects of rounding sharp front cab body edges (Fig. 14.50(a-d)) A reduction in the drag coefficient of large vehicles such as buses, coaches and trucks can be made by rounding the front leading edges of the vehicle.

Fluctuating

Fluctuating

Over flow

I Side flow

Flow separation

I Side flow

(a) Coach with sharp leading edges

(b) Coach with rounded leading edges Flow remains attached

(c) Coach with rounded edges and backsloping front 1.0

(d) Effect of rounding vehicle leading edges § upon the aerodynamic drag eg a h C

(d) Effect of rounding vehicle leading edges § upon the aerodynamic drag eg

R

a

Leading edge radius (R) mm

Fig. 14.50(a-d) Forebody coach streamlining

Simulated investigations have shown a marked decrease in the drag coefficient from having sharp forebody edges (see Fig. 14.50(a)) to relatively large round leading edge radii, see Fig. 14.50(b). It can be seen from Fig. 14.50(d) that the drag coefficient progressively decreased as the round edge radius was increased to about 120 mm, but there was only a very small reduction in the drag coefficient with further increase in radii. Thus there is an optimum radius for the leading front edges, beyond this there is no advantage in increasing the rounding radius. The reduction in the drag coefficient due to rounding the edges is caused mainly by the change from flow separation to attached streamline flow for both cab roof and side panels, see Fig. 14.50(a and b). However, sloping back the front profile of the coach to provide further streamlining only made a marginal reduction in the drag coefficient, see Fig. 14.50(c).

14.7.2 The effects of different cab to trailer body heights with both sharp and rounded upper windscreen leading edges (Fig. 14.51(a-c)) A generalized understanding of the air flow over the upper surface of an articulated cab and trailer can be obtained by studying Fig. 14.51(a and b). Three different trailer heights are shown relative to one cab height for both a sharp upper windscreen leading edge (Fig. 14.51(a)) and for a rounded upper windscreen edge (Fig. 14.51(b)). It can be seen in the case of the sharp upper windscreen leading edge cab examples (Fig. 14.51(a)) that with the low trailer body the air flow cannot follow the contour of the cab and therefore overshoots both the cab roof and the front region of the trailer body roof thereby producing a relatively high coefficient of drag, see Fig. 14.51(c). With the medium height trailer body the air flow still overshoots (separates) the cab but tends to align and attach itself early to the trailer body roof thereby producing a relatively low coefficient of drag, see Fig. 14.51(c). However, with the high body the air flow again overshoots the cab roof; some of the air then hits the front of the trailer body, but the vast majority deflects off the trailer body leading edge before re-attaching itself further along the trailer body roof. Consequently the disrupted air flow produces a rise in the drag coefficient, see Fig. 14.51(c).

In the case of the rounded upper windscreen leading edge cab (see Fig. 14.51(b)), with a low trailer body the air flowing over the front windscreen remains attached to the cab roof, a small proportion will hit the front end of the trailer body then flow between the cab and trailer body, but the majority flows over the trailer roof leading edge and attaches itself only a short distance from the front edge of the trailer roof thereby producing a relatively low drag coefficient, see Fig. 14.51(c). With the medium height trailer body the air flow remains attached to the cab roof; some air flow again impinges on the front of the trailer body and is deflected between the cab and trailer body, but most of the air flow hits the trailer body leading edge and is deflected slightly upwards and only reattaches itself to the upper surface some distance along the trailer roof. This combination therefore produces a moderate rise in the drag coefficient, see Fig. 14.51(c). In the extreme case of having a very high trailer body the air flow over the cab still remains attached and air still flows downwards into the gap made between the cab and trailer; however, more air impinges onto the vertical front face of the trailer body and the deflection of the air flow over the leading edge of the trailer body is even steeper than in the case of the medium height trailer body. Thus re-attachment of the air flow over the roof of the trailer body takes place much further along its length so that a much larger roof area is exposed to air turbulence; consequently there is a relatively high drag coefficient, see Fig. 14.51(c).

14.7.3 Forebody pressure distribution

With both the conventional cab behind the engine and the cab over or in front of the engine tractor unit arrangements there will be a cab to trailer gap to enable the trailer to be articulated when the vehicle is being manoeuvred. The cab roof to trailer body step, if large, will compel some of the air flow to impinge on the exposed front face of the trailer thereby producing a high pressure stagnation region while the majority of air flow will be deflected upwards. As it brushes against the upper leading edge of the trailer the air flow then separates from the forward region of the trailer roof before re-attaching itself further along the flat roof surface, see Fig. 14.52(a). As can be seen the pressure distribution shows a positive pressure (above atmospheric pressure) region air spread over the exposed front face of the trailer body with its maximum intensity (stagnant region) just above the level of the roof; this contrasts the negative pressure (below atmospheric pressure) generated air flow in the forward region of the trailer roof caused by the air flow separation turbulence. Note the negative pressure drops off towards the rear of the roof due to air flow re-attachment.

Highest CD

Lowest Cn

Medium C

Low

y/r

body h

height

-ix

Medium body height lg

Medium body height

Medium C

High body height

(a) Tractor cab with sharp windscreen/roof leading edge (flow separation over cab roof)

Lowest C

Low body height

Medium CD

Medium CD

Highest C,

/TTTy vif^

Medium body height

Highest C,

High body height

1X2.

High body height

(b) Tractor cab with rounded windscreen/roof leading edge (attached air flow over cab roof)

Low body

Medium body

High body

Low body

Medium body

High body

Attached air flow over roof

Air flow separation over roof

(c) Influence of cab to body height and cab shape upon the drag coefficient

Fig. 14.51 (a-c) Comparison of air flow conditions with both sharp and rounded roof leading edge cab with various trailer body heights

Fig. 14.52 (a and b) Trailer flow body pressure distribution with and without cab roof deflector

By fitting a cab roof deflector the pattern of air flow is diverted upwards and over the roof of the trailer body, there being only a slight degree of flow separation at the front end of the trailer body roof, see Fig. 14.52(b). Consequently the air flow moves directly between the cab roof deflector and the roof of the trailer body; it thus causes the air pressure in the cab to trailer gap to decrease, this negative pressure being more pronounced on the exposed upper vertical face of the trailer, hence the front face upper region of the trailer will actually reduce that portion of drag produced by the exposed frontal area of the trailer. Conversely the negative pressure created by the air flow over the leading edge of the roof falls rapidly, indicating early air flow re-attachment.

14.7.4 The effects of a cab to trailer body roof height step (Fig. 14.53(a and b)) Possibly the most important factor which contributes to a vehicle's drag resistance is the exposed area of the trailer body above the cab roof relative to the cab's frontal area (Fig. 14.53(a)). Investigation into the forebody drag of a truck in a windtunnel has been made where the trailer height is varied relative to a fixed cab height. The drag coefficient for different trailer body to cab height ratios (t/c) were then plotted as shown in Fig. 14.53(b). For this particular cab to trailer combination dimensions there was no noticeable change in the drag coefficient C of 0.63 with an increase in trailer body to cab height ratio until about 1.2, after which the drag coefficient commenced to rise in proportion to the increase in the trailer body to cab height ratio up to a t/c ratio of 1.5, which is equivalent to the maximum body height of 4.2 m; this corresponded to a maximum drag coefficient of 0.86. Hence increasing the trailer body step height ratio from 1.2 to 1.5 increases the step height from 0.56 m to 1.4 m and in turn raises the drag coefficient from 0.63 to 0.86. The rise in drag coefficient of 0.23 is considerable and therefore streamlining the air flow between the cab and trailer body roof is of great importance.

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