aerodynamic drag of car brief.ppt

8/19/2019 Aerodynamic drag of car Brief.ppt

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Vehicle Body EngineeringAerodynamics

Introd!ction "• The #o$er delivered %y the engine is finally

made availa%le at the drive $heels as#ro#!lsive force.

• The #ro#!lsive force or tractive effort,availa%le at the contact %et$een the driving

$heels and road sho!ld %e more than the total

resistance for the motion of a vehicle• The s!r#l!s tractive effort contri%!te for

acceleration, clim%ing gradient, etc.• The total resistance to the motion of a vehicle


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Introd!ction "

is %y Air resistance & 'olling resistance• The vehicle drag is a force $hich resists motion

and is d!e to (

a) the deformation of the $heel and the gro!nd

%) aerodynamic effects of air flo$ over the vehicle• Deformation of the $heel

* the #ne!matic tyres are most s!ita%le for road

trans#ort vehicles.

* the deformation of tyres acco!nt for + to +-

of the rolling resistance of the vehicle


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Introd!ction "

* the distortion of the tyre tread as it #asses

thro!gh the contact area res!lts in a hysteressesloss manifests itself as heat & rise in tem#erat!re

of tyre* The hysteresses loss is #rimarily a f!nction of

deflection ca!sed %y the load it carries

* /ther #arameters affecting rolling resistance are(

tyre tem#., inflation #ress!re, tread thic0ness,

no. of #lies, r!%%er 1!ality, level of tor1!e

transmitted and vehicle s#eed


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Introd!ction "

* The 'olling resistance e2#ressed in terms of

non* dimensional rolling coefficient,3 as

'r 4 3 . 5 $here( 5 * $eight of vehicle

* The relationshi# %et$een rolling resistance and

vehicle $eight is more com#le2 and re1!ire a

detailed 0no$ledge of the soil and gro!ndmaterial,etc.


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Introd!ction "• Aerodynamic effects of air flo$ over the vehicle

* A moving vehicle, in dis#lacing the s!rro!nding

air, has a res!ltant resisting force called

aerodynamic drag or sim#ly air resistance.

* It can %e e2#ressed as resistive force o##osingthe motion of a vehicle thro!gh the air and the

$or0 done in overcoming the force is dissi#atedas energy lost to the air flo$.

* The amo!nt of drag de#ends on the vehicle

sha#e and varies $ith the s#eed of the vehicle.


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* A lo$*drag %ody allo$s vehicle to reach higher

s#eeds for a given #o$er o!t #!t.Conversely,

red!cing the #o$er cons!m#tion at any #artic!lars#eed ma0es it availa%le for acceleration.

* 'ed!cing #o$er re1!irement im#roves f!el

cons!m#tion there%y red!cing on*%oard f!elcarrying re1!irements. This can contri%!te

to$ards red!cing laden $eight of the vehicle


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* Motor vehicles have demonstrated strongaerodynamic infl!ence !#on their design.

* ntil recently flo$ing lines on vehicle %ody $ere#rimarily a statement of style and fashion $ith little

regard for economic 6 environmental %enefits.

* 'ising f!el #rices, triggered %y f!el crisis in 7+8 sand no$ the environmental concerns have #rovidedserio!s attention to$ards aerodynamic designs.


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* 9ignificance of Aerodynamic 9t!dy

7. 'ed!ction of drag force and achieve ma2im!ms#eed & acceleration for the same #o$er o!t #!t

:. 'ed!ction of drag force im#roves f!el economy

;. Good aerodynamic design gives %etter

a##earance and styling


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* 9ignificance of Aerodynamic 9t!dy

-. This hel#s to !nderstand the dirt flo$,e2ha!stgas flo$ #atterns, etc.

>. Good aerodynamic design #rovide #ro#er

ventilation, red!ce noise,etc.


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o##osing the motion of the vehicle %eca!se

se#aration of flo$ at rear of the vehicle res!lts in

lo$ering of #ress!re on rear$ard facing s!rfaces.

* The s0in friction drag and the ind!ced drag are!s!ally small in com#arison to normal #ress!re

drag.


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Aerodynamic orces and Moments "• The #rofile of the vehicle is the #rinci#le com#onent of

aerodynamic drag and is governed %y the $ay in$hich vehicle dist!r%s the air stream.

• Its %ehavior has %een fo!nd not to accord $ithesta%lished aerodynamic theory evolved in aviationsince vehicle has to maintain contact $ith the gro!nd.

•

The im#ortance of a good aerodynamic #arameters inthe design of a vehicle is %eing increasinglyrecogni ed. The designer m!st have a 0no$ledge ofthe forces and the la$s governing them in order to


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Aerodynamic orces and Moments "

#rod!ce %ody sha#es $hich $ill have acce#ta%leaerodynamic characteristics.

• Considering a car #rofile as an aerofoil the

streamlines aro!nd a car %ody is as sho$n %elo$


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Aerodynamic orces and Moments "• The car %ody #rofile sho$n having smooth

streamlines $hich are contin!o!s and $ith nose#aration of %o!ndary layers & vortices. o$ever,

li0e an aerofoil, the streamlines over the !##er #arthave a higher velocity than the streamlines %elo$

the car.

• or com#lete descri#tion of aerodynamic effects onthe motion of the vehicle it sho!ld %e considered

as a mass having si2 degree of freedom and the

aerodynamic forces and moments acting on thevehicle are %alanced %y the $heel reactions.


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Aerodynamic orces and Moments "• The aerodynamic forces on a vehicle in a real

environment is as sho$n %elo$


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* My " Pitching moment ca!sed %y forces a%o!t the

F*a2is

* M " Fa$ing moment ca!sed %y the force, Py a%o!t

the * a2is• Drag force, P2

* The air flo$ over a vehicle is com#le2 and theaerodynamic drag is e2#ressed %y the semi *

em#irical e1!ation to re#resent the aerodynamiceffect. It is defined %y the follo$ing e1!ation.


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DA 4 76: H V : A Cd

$here ( DA 4 aerodynamic drag force P2 ) , gf

H 4 air density , gf. 9ec : 6 m<

V 4 velocity , m 6 sec

A 4 rontal area of the vehicle , m :

Cd 4 aerodynamic drag coefficient• Jift force, P

The lift force is a res!lt of the asymmetrical flo$ of

air a%ove and %elo$ the vehicle. The lift force


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affects the vehicle driving sta%ility. The lift force ismeas!red at the centerline of the vehicle at thecenter of the $heel %ase.

P 4 76: H V : A C , gf

$here ( C 4 Jift coefficient• 9ide orce, Py

The side force is formed %y the asymmetric flo$ of

air aro!nd the %ody of the vehicle d!e to cross $indflo$ forces). The lateral $ind com#onents im#ose a

side force on the vehicle to change its direction .


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* Pitching "

The ang!lar oscillation of the vehicle a%o!t

lateral hori ontal) a2is is called #itching as sho$n

* Pitching Moments

Pitching moment acts to transfer $eight %et$een

the front and rear a2les. The #itching moment is


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!s!ally negative i.e., nose do$n. This ma0es the reara2le lift off the gro!nd and f!rther red!ce theavaila%le traction. The #itching moment arises from

the drag %!t drag itself does not act at the gro!nd#lane. The lifting force may not act e2actly at thecenter of the $heel %ase.

PM 4 76: H V:

A C #m . J , gf.m$here (

C#m 4 #itching moment coefficient

J 4 $heel %ase or characteristic length, m


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* Fa$ing

The ang!lar oscillation of the vehicle a%o!t the

vertical a2is is called ya$ing. It is the vertical

movement of the com#lete vehicle %ody. 9o thecom#lete %ody rises !# and do$n and 0no$ as

%o!ncing as sho$n in the fig!re %elo$


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* Fa$ing Moment

The lateral force ca!sed %y a side $ind does not

act at the mid* $heel %ase #osition. A side $ind $ill

#rod!ce a Fa$ing moment tending to t!rn thevehicle a$ay from the direction of motion. Fa$ing

moment is defined as

FM 4 76: H V : A C ym . J , gf.m$here (

Cym 4 Fa$ing moment coefficient

J 4 $heel %ase or characteristic length, m


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Vehicle Body EngineeringAerodynamicsAerodynamic Drag " ty#es & effects

• Profile or orm DragThe #rofile drag de#ends !#on the longit!dinal

section of the vehicle %ody , and #lays the

most im#ortant #art as its contri%!tion is the

ma2im!m.

A caref!l choice of %ody #rofile, essential for lo$ drag, re1!ires streamlines to %econtin!o!s

and se#aration of %o!ndary layers $ith its

attendant vortices to %e avoided.


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•

Ind!ced or Jift DragA vehicle %ody #rod!ces accelerated air flo$ andthe ind!ced drag is ca!sed %y the vortices formed at

the sides of the vehicle travelling do$n$ards.The #ress!re differential from the to# to the %ottomof the vehicle ca!ses a lift drag.

This lift force de#ends on the !##er s!rfaceses#ecially in areas of the leading edge of the hood,$ind shield corners, leading edges of the co$l and

!nder%ody s!ch as s!s#ension, e2ha!st system&other com#onents #rotr!ding, and the gro!nd


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hi l d i i


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E2terior vehicle %ody #ro=ections s!ch as doorhandles, mirrors,roof l!ggage, $ind shield $i#ers,

etc. and also, #ro=ections %elo$ the vehicle s!ch as

a2les, to$*%ars,etc. contri%!te to interference drag• Cooling & ventilation system drag

The cooling and ventilation systems also contri%!te

significantly to the total drag. Air flo$ #assingthro!gh the radiator im#act on the engine and $all$hich e2erts dynamic #ress!re as drag on the

vehicle


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V hi l B d E i i


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•

'otating $heels & otherThe shar# edges of the $heel c!to!t #rovides

o##ort!nities to ind!ce flo$ in hori ontal #lane,

$hile the rotating $heel tends to ind!ce circ!lationin vertical #lane.

/ther #rot!%erances from the %ody of the vehicle

re#resent an area contri%!ting to increase in dragand $here caref!l design can im#rove.

V hi l B d E i i


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Vehicle Body EngineeringAerodynamicsAerodynamic Drag " im#rovements

9ome of the aerodynamic im#rovements in vehicle %ody $o!ld %e highlighted s!ch that the drag co!ld %e

red!ced %y minor modifications. As an e2am#le, vehicle

sho$s changes that co!ld %e considered in (

A" Air dam, B" hood line, C & D" Pillars, E" 9#oilers

V hi l B d E i i


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A" Air dam

* The air dam area encircled as ?A@ in the fig!re is

the frontal area esta%lishing stream lines,

#artic!larly in !nder%ody and $heel areas.

* A cleverly designed air dam at the front of the

vehicle red!ces the re1!irements of gro!nd

clearance and limits the vol!mes of air #assing

!nder the %ody.

* The #rimary o%=ective of modifications are to red!ce

#ress!re of air stream !nder the vehicle %ody.

* This greatly red!ce the vortices and ind!ced drag

V hi l B d E i i


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B" ood line

* It is the leading edge $hich dist!r%s the air stream

and infl!ence the #rofile drag. The hood line and thesha#e of hood sho!ld g!ide the streams $itho!t

discontin!ity over the $indscreen.The formation ofvortices sho!ld also %e avoided in this region.

C & D" Pillars

* As sho$n in the fig!re the sha#es of the t$o

e2treme #illars can %e effectively a##lied for

o#timi ation of drag. By having a slight conve2

#rofile from front to rear #illar the discontin!ity can

V hi l B d E i i


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%e eliminated and $ith it the associated flo$

se#aration. The #erfectly smooth #rofile from$indscreen to the #illars $o!ld %e com#romised tosome e2tent at the =!nctions of the glass and the

s!rro!nding frames.E" 9#oilers

The s#oilers and air foils on the rear chec0 may

serve several #!r#oses. The rear s#oilers, $hich isattached either to the rear of the roof or the !##eredge of the rear $ings, has the effect of increasingthe #ress!re acting on the rear dec0 area. This

V hi l B d E gi i g


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increase in #ress!re acting on the rear dec0 creates a do$n

force at the most advantageo!s #oint as sho$n in the fig!re.

* The s#oilers may also serve to sta%ili e the vortices in these#aration flo$, th!s red!cing the aerodynamic %!ffeting.


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Vehicle Bod Engineering


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5ind T!nnel Test

and moments etc. can %e made.

* In an o#en circ!it t!nnel the air is dra$n directly from

the atmos#here

* In closed circ!it or ret!rn flo$ t!nnel the air is

re*circ!lated.

* The air flo$ in a $ind t!nnel is controlled for its

velocity and maintained !nder !niform flo$

conditions $ith a lo$ level of t!r%!lence.

* The air flo$s thro!gh fan or com#ressor to cooler to

Vehicle Body Engineering


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5ind T!nnel Test

Test* 9ection to diff!ser and %ac0 to com#ressor or

o#en to atmos#here. ig!re sho$s schematically a

$ind t!nnel



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5ind T!nnel Test

* Basically the $ind t!nnel consists of a set !# to

#rod!ce !niform air flo$ $ith varia%le flo$ velocities.

This is achieved %y accelerating and decelerating flo$

inside the $ind t!nnel and maintaining !niform flo$

over the $or0ing section.

* The fan is #laced in a lo$ s#eed section #roviding a

flo$ of air $hich is accelerated %y #roviding

contraction in the do$n stream as sho$n. The g!ide

valve cascades, honey com% ga! e section, settling



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5ind T!nnel Test

cham%ers are #rovided for directing and dam#ing the

flo$. The significance of t!r%!lence is red!ced.

* The model is #laced in the $or0ing section and is

mo!nted in s!ch a $ay that #ermits meas!rement of

forces and moments %y the transd!cers, %alance

!s!ally mo!nted o!t side the t!nnel, #ittot t!%es $ith manometers, or #ress!re,velocity,load meas!rement

transd!cers.

* The #ress!re distri%!tion on the model can %e fo!nd



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5ind T!nnel Test

* from s!rface #ress!re ta##ings. The diameter of these

sho!ld %e small li0e ca#illary or hy#odermic t!%es.

The ta##ings m!st %e fl!shed $ith the s!rface and

connected to a##ro#riate manometer or transd!cer

s!ch that readings are o%tained and !sing these

val!es of velocity and #ress!re the forces, moments,coefficients,etc. can %e calc!lated.

* In a $ind t!nnel the $ind velocity and $ind angle can

also %e meas!red easily



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Ty#ical 9tatic Press!re Coefficient Distri%!tion



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Drag 'ed!ction %y Changes to ront Body 9ha#e



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91!are%ac0 atch%ac0 6 ast%ac0

large scale flow separation vortex generation



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Infl!ence of %ac0light angle on drag coefficient



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igh tail, lo$ drag design


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igh tail, lo$ drag design



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9ta%ility and cross* $inds

Aerodynamic sta%ilityeel of a car as it travels straight at high s#eed in calm

conditions affected %y lane change mane!vera%ility

Effects of steady cross* $inds and transient g!sts

atmos#heric conditions, em%an0ments, %ridges )

Ca!ses

This is largely d!e to com#le2 interactions %et$een thechassis dynamics and relatively small changes in magnit!deof lift forces and center of #ress!re

9ta%ility degrades $ith increase in overall lift and

differences in lift %et$een the front and rear a2les



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9ta%ility and cross* $inds " Comments

Design vehicles to minimi e the side forces, ya$ing momentsand ya$ rates that occ!r as vehicle is #rogressively and ra#idly

e2#osed to cross $inds

The lo$ drag, ro!nded %ody sha#es are s!sce#ti%le to cross

$inds ( increased ya$ sensitivity, changes of lift distri%!tion!nder the infl!ence of cross*$inds red!ce vehicle sta%ility.

The aerodynamic infl!ence li0ely to %e f!rther e2aggerated %y

the trends to$ards $eight red!ction for im#roved f!el economy

Methods of testing models !nder transient cross*$ind conditions

are !nder develo#ments and relia%le data at %est can %e o%tained

%y f!ll scale testing of #rod!ction and #re*#rod!ction vehicles.



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9ta%ility and cross* $inds " Comments

To eval!ate the transient %ehavior of a vehicle at m!ch earlierstage of its design & develo#ment re1!ire $ind t!nneltechni1!es !sing models and !nderstand the flo$

mechanisms that give rise to transient aerodynamic forcesand moments .

9t!dies s!ggest that forces and ya$ing moments

e2#erienced in tr!e transient case e2ceed those meas!red!nder steady state conditions.



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Aerodynamic Noise

9ome aerodynamic noise is created %y ventilation flo$s thro!ghthe ca%ins

Ma=or o%tr!sive noise is generally created %y the e2ternal flo$aro!nd the vehicle %ody


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Creation Aerodynamic Noise

Mostly associated $ith t!r%!lence at or near the %ody s!rface

Essentially associated $ith the random t!r%!lence that occ!rs

$ithin a t!r%!lent %o!ndary layer

It is the so!nd associated $ith eddy vorte2) creation at s!rfacediscontin!ities that has %oth the greatest magnit!de and also theclearly defined annoying fre1!encies entire a!di%le range ).

/ne of the largest single noise generator is the s!n roof. Its largesi e res!lts in lo$ fre1!encies & large magnit!de. Poorly designed!nits may even lead to discerni%le lo$ fre1!ency #ress!re #!lsingin the ca%in.



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'ed!ction in Aerodynamic Noise

'ed!ction in noise can #artly %e thro!gh O im#roved air flo$s $ithred!ced noise creation and %y im#roved sealing red!cing %oth itscreation & ins!lation of occ!#ants from the so!rce.

Considerations to red!ce drag inevita%ly #rovide %enefits of noisered!ction

Im#rovements in (

rain g!tter designs,

#ositioning of $indscreen $i#ers,

man!fact!ring techni1!es & 1!ality control of im#roved #anelfit,

#rotr!sions s!ch as $ing mirrors, small s!rface radii s!ch as at?A@ #illar



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nderhood Ventilation

– nderhood flo$ restrictions arising from engine cooling systemand ever*increasing vol!me of ancillary e1!i#ments !nder the

%onnet has foc!sed attention on cooling air flo$s.

– Evidence from n!mero!s researchers s!ggests that the enginecooling system is res#onsi%le for 7 to 7- of overall vehicle drag.

– Considera%le effort are %eing foc!sed on o#timisation of theseflo$s incl!ding develo#ment of com#!tational flo$ sim!lation

codes.



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nderhood Ventilation

– Many of the so!rce of cooling drag are( resistance created %y thedense radiator matri2 and the drag associated $ith the tort!o!sflo$ thro!gh the engine %ay.

– Jess o%vio!s %!t significant is the interaction %et$een the!ndercar flo$s and the cooling flo$s at the e2it $here high

t!r%!lence levels and flo$ se#arations may occ!r

Caref!l design to control the cooling e2it flo$ in terms of its s#eedand direction can red!ce the drag associated $ith the merging

flo$s



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Potential for nderhood drag red!ction

Greatest red!ction #ossi%le if air flo$ can %e controlled %y !se ofd!cting to g!ide the air into and o!t from the radiator core.

The high %loc0age ca!sed %y the radiator core has the effect ofdramatically red!cing the air velocity thro!gh the radiator andm!ch of the air s#ills aro!nd it.

The slo$* flo$ d!cted in a controlled and efficient manner %ycaref!l design, enhancing the degree of diff!sion, red!ces %oth thedrag force and the heat transfer. This re1!ire larger radiator core

s!rface area. A caref!l com#romises are necessary.



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Ca%in Ventilation

9ealing %et$een the %ody #anels and aro!nd doors %enefits inred!ction of noise and aerodynamic drag. B!t almost com#leteelimination of lea0age re1!ire #assenger com#artment ventilation.

Ca%in ventilation flo$ re1!ire attention to inta0e and e2it locationsas $ell as the velocity and #ath of fresh air thro!gh the #assengercom#artment.

The inta0e sho!ld %e located in a one of relatively high #ress!renot too close to the road to avoid #artic!lates and #oll!tants. Theregion ahead of $indscreen ade1!ately meets s!ch re1!irementsand conveniently located for air entry for ventilation and air

conditioning.



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Ve c e ody g ee gAerodynamics

Ca%in Ventilation

or effective e2traction of ventilation air, a one of lo$er #ress!resho!ld %e so!ght. A location at the rear of the vehicle is !s!allyselected. Air is directed thro!gh the #arcel shelf and %oot to e2it

thro!gh a %leed in the %oot seal.

Increasing the #ress!re difference %et$een the inta0e and e2it

#rovides the #otential for high ventilation air flo$. This can %e

noticed in heated*air conditions inside the ca%in.

'ecent trends has %een to !se lo$ #ress!re difference co!#led

$ith fan6 %lo$er assistance for sim#le ventilation or air

conditioning systems.


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B!s & Tr!c0 Aerodynamics



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Bac0gro!nd

Commercial Vehicles are less streamlinedIn general aerodynamic drag is lesssignificant d!e to lo$er s#eeds

'olling resistance is high d!e to vehicle$eight and #ayload

Drag Coefficient of a tr!c0 or a %!s may %et$ice that of a car d!e to frontal area :*;times higher)



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Total resistance rolling aerodynamic) can%e of considera%le magnit!de at high

s#eeds

Aerodynamic im#rovements desira%le for

saving engine #o$er,

red!ction in f!el cons!m#tion & emissions



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Methods of 'ed!cing Aerodynamic Drag inTr!c0s

Aerodynamic %ehavior is additionally morecom#le2 %eca!se of tractor trailer com%ination

Tractor trailer com%ination has air ga# %et$een

the !nits

Air Ga# Bet$een Tractor & Trailer



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As air ga# e2ists the flo$ a%ove tractor roofdivides $ith some do$n$ard flo$ associated$ith t!r%!lence %et$een the tractor trailer

Drag red!ction can %e ca!sed %y tractorsmoothing the air flo$ on to the trailer

Drag increases as se#aration increases d!e todo$n flo$



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Methods of 'ed!cing Aerodynamic Drag inTr!c0s

Air lo$ $itho!t Deflector



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This increase in drag can %e virt!allyeliminated !sing hori ontal #late to inhi%it thedo$n flo$

By fitting deflectors, foils or fins to the Ca% to#

streamlining of the air flo$ as it #asses overand aro!nd the ca%6 tractor, deflectors6 foils orfins, the drag on the vehicle red!ces

9ome f!rther streamlining can %e achieved %yfitting air dams to the lo$er #arts of the ca%front and nose #anels to the front of the trailer%o2


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Air lo$ $ith Deflector


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Air Vane O air flo$ g!ide

Air vane devices as the air flo$ g!ide aredesigned and #erfected %y e2tensive $ind

t!nnel testing

It hel#s to sooth o!t the air flo$ over a

vehicle

It is a light $eight aerodynamic device thatcan %e installed easily on the front of any

vehicle %ody.


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A ty#ical tr!c0 or trailer creates the 0ind of airt!r%!lence as sho$n at high$ay s#eeds

Tr!c0 $itho!t Air Vane Airflo$ G!ide



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Increased air drag act!ally increases theeffective frontal area leading to e2cessive#o$er & f!el cons!m#tion

5ith com#lete air vane installed the airflo$ on the to# and the sides of a vehicleis smoothened greatly to red!ce

t!r%!lence and air drag as sho$n.


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Jo$ drag affects(

'ed!ce f!el cons!m#tion

'each to higher s#eed $ith a given #o$er

o!t #!t'ed!ction in #o$er at any lo$er s#eed the

availa%le greater #o$er s!r#l!s gives %etteracceleration

nder %ra0ing, lo$ drag im#air #erformanceoffering less resistance of contin!ed motion

aerodynamic drag of car brief.ppt

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