Advanced Vehicle Technology Episode 3 Part 12 doc

14.6.3 Hatchbackdrag Figs 14.41, 14.44 and 14.45 Cars with a rear sloping surface angle ranging from 50 to 25 are normally referred to as hatchback a High speed low pressure Reduced spee

and therefore there is virtually no variation in the

afterbody drag (see Fig 14.41) With a parallel

sided squareback rear end configuration, the

whole rear surface area (base area) becomes an

almost constant low negative pressure wake region

Tapering the rear quarter side and roof of the body

and rounding the rear end tends to lower the base

pressure In addition to the base drag, the

after-body drag will also include the negative drag due to

the surrounding inclined surfaces

14.6.2 Fastbackdrag (Figs 14.41 and 14.43)

When the rear slope angle is reduced to 25or less

the body profile style is known as a fastback, see

Fig 14.43 Within this much reduced rear end

inclin-ation the airstream flows over the roof and rear

downward sloping surface, the airstream

remain-ing attached to the body from the rear of the roof

to the rear vertical light-plate and at the same time the condition which helps to generate attached and trailing vortices with the large sloping rear end is

no longer there Consequently the only rearward suction comes from the vertical rear end projected base area wake, thus as the rear end inclined angle diminishes, the drag coefficient decreases, see Fig 14.41 However, as the angle approaches zero there

is a slight rise in the drag coefficient again as the rear body profile virtually reverts to a squareback style car

14.6.3 Hatchbackdrag (Figs 14.41, 14.44 and 14.45)

Cars with a rear sloping surface angle ranging from

50 to 25 are normally referred to as hatchback

(a)

High speed low pressure

Reduced speed increase in pressure 20°

Lip spoiler

Turbulent wake

Negative lift tendency

h

Front lift Drag

(c)

CCLD

–0.4

–0.3

–0.2

–0.1

0

0.1

Lip height (mm)

Rear lift

High speed low pressure

Negative lift tendency

Reduced speed increase

in pressure Aerofoil spoiler

Turbulent wake (b)

Fig 14.39 (a±c) Effect of rear end spoiler on both lift and drag coefficients

style, see Fig 14.44 Within this rear end

inclin-ation range air flows over the rear edge of the roof

and commences to follow the contour of the rear

inclined surface; however, due to the steepness of

the slope the air flow breaks away from the surface

At the same time some of the air flows from the

higher pressure underfloor region to the lower

pres-sure roof and rear sloping surface, then moves

slightly inboard and rearward along the upper

downward sloping surface The intensity and

direc-tion of this air movement along both sides of the

rear upper body edging causes the air to spiral into

a pair of trailing vortices which are then pushed

downward by the downwash of the airstream

flowing over the rear edge of the roof, see Fig 14.45 Subsequently these vortices re-attach themselves on each side of the body, and due to the air's momentum these vortices extend and trail well beyond the rear of the car Hence not only does the rear negative wake base area include the vertical area and part of the rearward projected slope area where the airstream separates from the body profile, but it also includes the trailing conical vortices which also apply a strong suction pull against the forward motion of the car As can be seen in Fig 14.41 there is a critical slope angle range (20 to 35) in which the drag coefficient rises steeply and should be avoided

Direction

of

motion

Slower airstream higher pressure

Airstream Aerofoil

Faster airstream lower pressure

Direction of air flow

Higher pressure

Lower pressure Down-thrust (negative lift)

Drag

Resultant reaction

(a) Air streamlining for an inclined

negative lift aerofoil wing

(b) Lift and drag components on

an inclined negative lift wing

Front negative

lift wing

Negative

lift

(down-thrust)

Lf

Drag

Resultant wheel load (W)

Wheel base (L)

note W =F h

L D

Negative lift (downthrust)

Lr

Drag(F ) D

Rear negative lift wing

(c) Racing car incorporating negative lift wings

Fig 14.40 (a±c) Negative lift aerofoil wing considerations

Slope angle (deg)

CD

+

Critical angle 0

–

Fig 14.41 Effect of rear panel slope angle on the afterbody drag

Flow attachment and

separation

Flow reattachment Rear screenpanel Flow

separation 90 –50°°

Base area wake

Negative pressure

Fig 14.42 Squareback configuration

Rear screen panel

Flow separation 22°–10°

Base area wake

Negative pressure

Fig 14.43 Fastback configuration

14.6.4 Notchbackdrag (Figs 14.46, 14.47(a and b)

and 14.48(a and b))

A notchback car style has a stepped rear end body

profile in which the passenger compartment rear

window is inclined downward to meet the horizontal

rearward extending boot (trunk) lid (see Fig 14.46)

With this design, the air flows over the rear roof

edge and follows the contour of the downward

sloping rear screen for a short distance before

separating from it; however, the downwash of the

airstream causes it to re-attach itself to the body

near the rear end extended boot lid Thus the

base-wake area will virtually be the vertical rear boot

and light panel; however, standing vortices will be

generated on each side of the body just inboard on

the top surface of the rear window screen and boot

lid, and will then be projected in the form of trailing

conical vortices well beyond the rear end of the

boot, see Fig 14.19(b) Vortices will also be created along transverse rear screen to boot lid junction and across the rear of the panel light

Experiments have shown (see Fig 14.47(a)) that the angle made between the horizontal and the inclined line touching both the rear edges of the roof and the boot is an important factor in deter-mining the afterbody drag Fig 14.47(b) illustrates the effect of the roof to boot line inclination; when this angle is increased from the horizontal the drag coefficient commences to rise until reach-ing a peak at an inclination of roughly 25, after which the drag coefficient begins to decrease From this it can be seen that raising the boot height or extending the boot length decreases the effective inclination angle e and therefore tends to reduce the drag coefficient Conversely a very large effective inclination angle ewill also cause a reduction in the

Flow

attachment

and separation

Flow reattachment

Rear screen panel

Flow separation 50 –22°°

Base area wake

Negative pressure

Fig 14.44 Hatchback configuration

Side vortex

Low pressure Transverse

standing vortices

Trailing vortex cone

Airstream

Fig 14.45 Hatchback transverse and trailing vortices

drag coefficient but at the expense of reducing the

volume capacity of the boot The drag coefficient

relative to the rear boot profile can be clearly

illus-trated in a slightly different way, see Fig 14.48(a)

Here windtunnel tests show how the drag

coeffi-cient can be varied by altering the rear end profile

from a downward sloping boot to a horizontal

boot and then to a squareback estate shape It

will be observed (see Fig 14.48(b)) that there is a

critical increase in boot height in this case from 50

to 150 mm when the drag coefficient rapidly

decreases from 0.42 to 0.37

14.6.5 Cabriolet cars (Fig 14.49)

A cabriolet is a French noun and originally referred

to a light two wheeled carriage drawn by one

horse Cabriolet these days describes a car with

a folding roof such as a sports (two or four seater)

or roadster (two seater) car These cars may be driven with the folding roof enclosing the cockpit

or with the soft roof lowered and the side screen windows up or down Streamlining is such that the air flow follows closely to the contour of the nose and bonnet (hood), then moves up the windscreen before overshooting the screen's upper horizontal edge (see Fig 14.49) If the rake angle of the wind-screen is small (such as with a high mounted off road four wheel drive vehicle) the airstream will be deflected upward and rearward, but with a large rake angle windscreen the airstream will not rise much above the windscreen upper leading edge

as the air flows over the open driver/passenger

Flow

attachment and

separation

Flow reattachment

Flow separation

Flow attachment and separation

Base area wake

Negative pressure

Fig 14.46 Notchback configuration

Various boot heights

φ e

CD 0.8

0.4

0.0

– 0.4

(b)

Φ e = rear effective slope angle

Critical angle (25°)

Effective slope angle ( Φ e ) deg (a)

Fig 14.47 (a and b) Influence of the effective slope angle on the drag coefficient

compartment towards the rear of the car A

separ-ation bubble forms between the airstream and

the exposed and open seating compartment, the

downstream air flow then re-attaches itself to the

upper face of the boot (trunk) However, this

bub-ble is unstabub-ble and tends to expand and burst in a

cyclic fashion by the repetition of separation and

re-attachment of the airstream on top of the boot

(trunk), see Fig 14.49 Thus the turbulent energy

causes the bubble to expand and collapse and the

fluctuating wake area (see Fig 14.49), changing

between h1and h2produces a relatively large drag

resistance With the side windscreens open air is

drawn into the low pressure bubble region and in

the process strong vortices are generated at the side

entry to the seating compartment; this also

there-fore contributes to the car drag resistance Typical

drag coefficients for an open cabriolet car are given

as follows: folding roof raised and side screens up

CD0.35, folding roof down and side screens up CD

0.38, and folding roof and side screens down CD 0.41 Reductions in the drag coefficient can be made by attaching a header rail deflector, stream-lining the roll over bar and by neatly storing or covering the folding roof, the most effective device

to reduce drag being the header rail deflector

14.7 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

(a) Squareback

estate

Notchback horizontal boot

Fastback downward sloping boot

CD

3 2 1

0.42

0.40

0.38

0.36

1

2

3 (b)

Boot (trunk) height (h) mm

Fig 14.48 (a and b) Effect of elevating the boot (trunk) height on the drag coefficient

Flow attachment Sideflow

Header rail deflector

Side screen

Separation bubble Roll over

bar separationFlow

Fluctuating venting bubble

1 h2

Fig 14.49 Open cabriolet

Flow separation

CD= 0.88

(a) Coach with sharp leading edges

Flow almost remains attached

CD= 0.36

(b) Coach with rounded leading edges

Flow remains attached

CD= 0.34

(c) Coach with rounded edges and backsloping front

CD 1.0

0.8

0.6

0.4

0.2

0

Leading edge radius (R) mm

R

(d) Effect of rounding vehicle leading edges

upon the aerodynamic drag

Over flow

Side flow

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

round-ing 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

coeffi-cient 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

produ-cing 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

wind-screen 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 re-attaches 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

(Fig 14.52(a and b)) 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 separ-ates 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 nega-tive pressure (below atmospheric pressure) gener-ated 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

Low body height

Medium body height

High body height

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

Low body height

Medium body height

High body height

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

0.8

0.7

0.6

0.5

0.4

CD

Body height (h) m

Low body

Medium body

High body

(b)

(a)

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

Trailer roof pressure distribution

Trailer front panel pressure distribution +ve

–ve

Airstream

(a) Cab without roof deflector

Roof deflector

Airstream

(b) Cab with roof deflector

–ve

–ve

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