Automobile English

8/12/2019 Automobile English

1/67

Fahrzeug- undWindradaerod namik

Dr.-Ing. A. Henze, Prof. Dr.-Ing. W. Schrder

Institute of Aerodynamics, RWTH Aachen University

Automobiles


2/67

Automobiles

blunt bodies small ground distance complex geometries

open cavities

rotating weels internal channel flow

three-dimensional flow turbulent boundary layers

flow through the auto body flow inside of aggregates coupled external and internal flow flowfield in the engine


3/67

Command variables

Tasks for automobile aerodynamics

Photo: Daimler-Chrysler AG


4/67

Automobiles

Coordinate system and definition

Variety of shapes in automobile aerodynamics

Usually Drag reduction fuel consumption maximum speed pollutant emission

Lateral forces, yaw mamentum, lift

directional stability behaviour in curves sensitivity against cross wind


5/67

Automobiles

Visualization of the flowfield on a VW-Golf 1 Streamlines with smoke smoke in separated region

Flowfield around Mercedes-Benz A-Modell Visualization with smoke computed simulation


6/67

Automobiles

Flow around single elements sound creation at sharp edges

open windows lighter doors, roofs, etc. flutter Cooling air inlet (position) air condition acoustic level in the cabin acoustic level in the surrounding raindrops, mud

Simple shapes of automobiles

racing cars

low drag

negative lift optimum for each racing track


7/67

Automobiles

Design vs. aerodynamics

Droplet, Rumpler 1921 2 profiles, Jaray 1921 Profile + Rotational body, Jaray 1933 2 horizontal profiles, Lange 1937


8/67

Automobiles

slight entering without separation

cut off the contour immediately beforethe flow separation smaller tail area pressure increase smaller air drag

Kamm tail, 1938-1939


9/67

Automobiles

Streamlined Opel GT, 1969, cw = 0.41, A = 1.51 m2

Detail optimized VW Scirocco, 1974, cw = 0.41, A = 1.73 m2

2 possibilities

initial start from design, aerody-namic optimization contours gettingrounder and fluent, drag is decreased

base body with low drag, changes fromdesigner contours getting stronger,drag increased


10/67

Automobiles

Development of cw coefficient of european cars compared with low drag bodies


11/67

Automobiles

Cw histogramm of european passenger cars, 2003


12/67

Driving performance

fuel consumption maximum speed ability to accelerate

Traction force between the wheels and the road surface

W: Air drag forceR: Rolling drag focem: mass of the car

V: velocityt: timeg: gravitational acceleration: slope of the road

Power Plane road without acceleration

Air drag Facing surface


13/67

Driving performance

Drag coefficient and facing surface fordifferent types of automobiles

Facing surface Aa) Different types of automobilesb) Typical values for passenger cars


14/67

Driving performance

Rolling drag

o ng rag coe c en o passenger car yre

as a function of the velocity

Air and rolling drag as a function of thevelocity for a BMW 520 iCw = 0.27, A = 2.18 m

2, G = 1570 kg

Air drag is dominantAt 200 km/h: air drag is 80 % of the total drag


15/67

Reduction of fuel consumption

If the drag coefficient can be reduced from cw0 aboutcw there are 2 possibilities

1. increase of the maximum velocity Vmax2. reduction of fuel consumption b

Number 2 should be modern, but number 1 is usual

Efficiency of the drag reduction with respect to the reduction of fuel consumption

: efficiency coefficient

The efficiency coefficient depends on type of car fuel consumption map of the engine gear box adjustment operation profile


16/67


Comparison between the old 1/3 mix and the new EU test cyclus (1996) City cycle is the same for both test curves high speed range is different

800 s constant 90 km/h and 120 km/h

Curve with accelerations and decelerations

High speed part is only 50% of the city cycle Average velocity for 1/3 mix is 76.2 km/h Average velocity for new EU mix is 32.5 km/h

Result:10 % reduction of c w

3 % reduction of fuel consumption: = 0.3for the 1/3 mix

2 % reduction of fuel consumption: = 0.2for the new EU mix


17/67


More realistic results can be found, if if the efficiency of a drag reduction is measuredin a customer oriented driving operation.Fast highway part of the cycle increases with the class of the passenger carHigh class cars are often driving fast.

Efficiency is higher for high class cars

Typical values of for different car typesschwerer Kurs: mountanious track for passenger cars, city traffic for bussesAssumption: smaller drag, but the same maximum speed weaker engine


18/67

Increase of the maximum velocity

P: PowerF: Traction forceV: Velocity

Efficiency between entry of gearbox and thecontact area of the wheel

P B does not include the power ofauxiliary units

Simple rule of thumb:


19/67

Drag, flow around the car

a) Edges which are normal to the flow directionedges which are inclined against the localflow direction

b) Different separations in the tail region Fullback: dead water with large volume

edges are normal to the localflow direction

Fastback: strong vortex pair comingthe obli ue C-columns

Notchback: combination of both


20/67


Three longitudinal vortex systems strong vortex pair from theC-column with inward rotating

direction counterrotating vortices fromthe A-column

third vortex pair coming from thecowl. The flow separates at the

,others


21/67


Assumption:Aober = flat plate of length lFully turbulent boundary layerU locally higher velocity

Approximation of the friction drag for a passengercar and an overland bus

Overland bus: cw = 0.5: friction is 8%

Passenger car: cw = 0.3: friction i2 13%

Cwf is an invariant for the friction coefficient


22/67


Fundamental relationship between modifications of shape detailsand air drag


23/67


Typical functions: saturation, asymptote, minimum, jump


24/67

Blunted leading edge

Flow around the sharp edge of a flat platea) Streamlinesb) Pressure distributionc) Velocity profiles

Influence of the edge radius on cw


25/67


Influence of the edge radius onto theflow pattern and the drag of a cuboid

Influence of the radius on the critical Reynolds number


26/67


Influence of the edge radius on the critical Reynolds number


27/67

Saturation

Saturation for the small bus VW LT 1City bus: U = 50 km/h: r = 140 mmTravel bus: U = 80 km/h: r = 89 mm

Optimal radius when the curve is saturated


28/67

Asymptote

inclination angle of front and rear window height of boot width of boot

Examples for Asymptote for AUDI 100 III1983

No clear optimumConflict between design and

aerodynamics


29/67

Asymptote

Mechanisms:

inclination of the front window

more fluid over the roof and lessaround the A-column, weaker vortices smaller drag

inclination of the rear windowsimilar to a wing profile with smaller

,becomes weaker, smaller under pressureon the rear window smaller drag,higher lift

height and length of the bootbackward facing step, benefit forreattachment


30/67

Minimum

Two different effects that work in opposite direction

Front spoiler and lower side of the car

a) Lift and drag

Type Minimum, schematic

ressure str ut on

Spoiler: normal plateLower side: rough flat plate

The spoiler protects the rough plate.

Decrease of the static pressure reduction ofthe lift on the front axisStronger pressure gradient increase of thevolume flux of cooling air (important for racing cars)


31/67

Jump

Typical example: Hatchback

Fullback: = 0:Separation on all 4 edges,large dead water

Small angle:Vortex pair induces down windFlow coming from the roof movesmownward and reattaches on the rearwindow, for a well rounded edge theseparation bubble vanishes, the dead

water becomes smaller with the base area

The intensity of the vortices increases withthe angle . The under pressure in-creases larger drag.At 30: vortex break down of the vortices.

The downwind also breaks down. Theseparation jumps from the lower edgeto the upper edge


32/67

Flow through channels

Cooling unit, water, oil, charge air air condition in the passenger cabin cooling unit for front wheel brakes additional drag influence on lift measurement with open inlet and with closed inlet drag of the cooling system interference drag

different flow around the front art(can be negative)

stronger inclination of the front wheels

higher velocity on the lower surface

Face velocity uffor free and for covered

cooling inlet as a function of theloss coefficient


33/67

Interference

multiple bodies tractor-trailer passenger car with caravan geometric quantity

width of gap

Limited total length < 18 m manoeuverable

height difference

Influence of the correlation betweendrivers cab and container on the drag


34/67

Interference

Head drag coefficient of a longitudinal cylinderwith a circular plate with different diameter ratiosand distances

Head: circular plate plus support stick plus

small distances: cw = 0.72 (coaxial cylinder) cw decrases with increasing distance minimum for all diameter ratios

minimum depends on the diameter ratio absolute minimum for d1/ d2 = 0.75 cylinder with rounded leading edge for larger distances: sum of both single values


35/67

Interference

I) extremely small drag cw* < 0.1Distance diameter of the plate are optimalideal body with rounded nose

II) Distance too large, strong oscillationsIII) Plate is too small

IV) Distance is too close

A) Plate too small, distance too largeB) Plate still too small, distance too largeC) Plate and distance too smallD) Plate too large

Triple-Point CBD is optimum


36/67

Interference

Influence of a baffle on the driverscab of a tractor trailer, example for Minimum, butdifferent from front spoiler

Comparison between the flowfields on a tractor-trailer with

and without a baffle on the drivers cab


37/67

Inclined incoming flow

Wind velocity Side slip (Yaw) angle different from air planes tangential force, coefficient c T

Influence of the yaw angle on thetangential force coefficient of a tractor-trailer

with rounded and with sharp edge drivers cabin


38/67


The flow is going through the complex underbody underinclination Inclination in the gap flow between the cabin and thecontainer vertical plate in the center at the front of the container

easy to handle if the truck is always in combination with thesame container fairing on both sides of the gap the tangential force is not very important in practice high yaw angles at high velocities are seldom high yaw angles at lower velocities are not relevant for fuel

consumption Estimation:

~ 5for passenger cars on highways ~ 8for trucks (lower velocity)


39/67


Ford Werke GmbH (2003)

Pressure distribution for symmetric flow and for an angle of 20


40/67


Flowfield through the gap between the cabinAnd the containe at inclined flow

Increase of the optimum noseradius at inclination

Lif d i hi


41/67

Lift and pitching moment

Blunted bodies adjacent to the ground Upward lift momentum around the transverse axis additional effect: positive curvature positive lift (profile theory)

only important for passenger cars unimportant for busses and trucks reduction of the lift force on both axes positive lift is usually unwanted lower side forces unbalanced forces on the axes self-steering properties

Lift and pitching moment on a passenger car, schematic

Di ti f ti t bilit


42/67

Direction of motion, stability

inclined flow nonsymmetric flowfield side forces and additional moments change of course, if the driver does not navigate against these forces

2 aspects for the reaction of a vehicle on side winds comfort safety

o annoying and exhausting for the driver to react permanentlyon side winds

o for larger course deviation it becomes dangerous, if thereactions during short gusts of wind or during outdistancehappens at the wrong moment

Rear engine front engine: smaller side wind sensitivity ( center of gravity moved forward if the total weight of cars becomes smaller side wind sensitivity becomes more important

Di ti f ti t bilit


43/67

Direction of motion, stability

Driver plus car: closed loop system command variable: road the closed loop system should be stableunder all practical conditions type of control of the driver cannot bechanged easily (by training) type of control of the car itself is important

mathematical model for the car mathematical model for the driver forces and momentums for all

combinations of wind velocity and yawangle

Linearised car model


44/67


Small cross acceleration ay/ g < 0.5linearised model All angles are small except the yaw angle cosine = 1 sine and tangent = angle one lane model No longitudinal forces (no acceleration, nobrake) variation of vertical forces is neglected (at first) model with 2 degrees of freedom (swimming

neglection of side winds



45/67


Movement of the center of gravity in cross direction

Rotation of the vehicle around the vertical axis

Slip angles on front and rear axes

Angle

Side forces on front and rear axis

cV and cH include the side stiffnes ofthe tyresand the elastic kinematicproperties of the axesand the steering mechanism

Acceleration

Equations of the movement



46/67


Sidewind included unsymmetric flow change of tangential force T, lift A, pitching moment M new: side force S, yawing moment N, rolling moment R

Wind velocity U has two parts wind velocity VW

Simplified one-lane model including side wind



47/67


New components

Yawing moment : side force x lever arm

To keep the course with steady side wind

Resulting equations of movement

Elimination of leads to the steer angle

Side force and yawing moment

Equations of the movement



48/67


The steering angle remains small, if

the aerodynamic coefficient k is small, can be influenced by the shape of the body

Interesting: steer angle is zero



49/67


Usually: passenger cars understeer

Distance xs0 between the pressure point and the center of

gravity: preasure point must be behind cog.

Only possible with a large finMovement of the center of gravity is limitedEven cars with front engine have a positive x

http://en.wikipedia.org/wiki/Understeer

Linear only for small anglesStiffness = slope of the curves,

decreases if thewheel load decreases

Lift influences the wheel loadPitching moment distributes the liftSwitch from understeering to over-steering more dangerous

Over- and Understeering


50/67

O e a d U de stee g

Drivers model


51/67

2-plane model normal tour without disturbances, anticipating, control stabilising, stochastic disturbances (wind), reduction of course deviation , adjustment control

Control

Compensatory partCritical es eciall for drastical chan es s uall wind

T1 : reaction time

stable

lateral deviation from course yaw angle yaw angle velocity yaw angle acceleration

roll angle (high drivers positions in buses or vans

T , Ty , reaction times A , Ay, driver coefficients, experiments

Side winds


52/67

fixed steering wheelL = 0 released steering gear active control

Open loop: only the characteristicsof the car itselfSub ective feelin b interviewin

V: driving speedVW : Wind velocity Small yaw angle linear behaviour of cS and cN

Measurement of the yaw angle velocityIntegration: y(t)Differentiation: yaw angle accelertaion

Side winds


53/67

From many experiments:Large side wind sensitivity from yaw angle acceleration at the beginning

From interviews with test persons Large cN high side wind sensitivity large c

Swithout yawing is good to be handled

For Aerodynamic people: on a conflict side force vs. Yaw moment smaller yawing moment and higher side force

System driver + carTry to keep the direction constantMeasurement of average and RMS ofL (t)

usually no standard experimental conditions different length of the test section different velocitites experiments cannot be compared

Side winds


54/67

frequency range natural side wind ration of the yaw angle velocity and the side wind

Aerodynamics at inclination


55/67

Main values: Yawing moment andside force important for side instabilities where do they come from how can they be influenced

Similar to lift force and pitchingmoment on airfoils

Yaw angle = angle of attack

Overpressure on the windward

side (luv)

Underpressure on the leeward side

Side force (car) = lift (airfoil)

For well rounded front part (topview) attached flow with aleeside pressure peak Yawing or pitching moment

Comparison of the pressure distribution on an airfoil witha horizontal cut through a car



56/67

Usually the aerodynamic yaw moment isunstableTends to increase the yaw angle

a) streamlined body:

no separation, small side force, large yawingmomentb) Sharp tail:

flow separation, approximately the same

Dependency of the yawing moment of the yaw angle

,large side force and smaller moment

c) Sharp nose and tail:separation at the nose, larger sider force,

smaller moment, but very large drag

All three are unstable

d) Tail fin on the slender tail:large side force and stable moment,but, tail fins are not suitable for passenger

cars, because they make the car longer



57/67

Side view is also important for side forces and yawing moment

hatchback: small side force, large yawing moment, sensitive against side winds variant: large side force, smaller yawing moment, relatively unsensitive notchback: moderate force and moment



58/67

Smaller yaw moment for an intended flow separation at the nose for large inclination

Large inclination are only important for short times (squally wind)Drag force is not very important

Optimal rounded nose attached floaw and small drag for small yaw angles

Real flow


59/67

Except for calm air: athmospheric boundary layer twisted velocity profile unsteady flow

Overtake

squally wind coming up against another car side wind jets

Stochastic and deterministic time dependency

From high to lower frequencies transition from laminar to turbulent, modifiedseparation changed forces and moments by aeroelasticeffects excitation of eigenfrequencies (yawing atsqually wind additional wind noise

Real flow


60/67

Overshooting of side forceand yawing moment atimmersion into a sidewind jet

Typical problems in windtunnels andcomputers steady flow thin boundary layer

non twisted boundary layer degree of turbulence is small in windtunnels the question of the errors due to thestron idealization are not et answered

Flow on surfaces


61/67

Not only forces and moments, i.e. integral valuesDetails: location of vents in the car body forces on single parts of the car body; leak tightness

keep windows and mirrors free from mud and water reduction of wind noise

Vents: cooling air for engine, aggregates, brakes combustion air for motor fresh air for air condition in the cabin

Opening


62/67

Sun roofs large scale connection between innerroom and outer flow low frequent resonance

humming can be reduced with kerfs in the winddeflector but slightly increased sound at higherfre uencies

Cabriolets: avoid the backward flow behind the

seats wind separation

Acoustics


63/67

wind noise becomes more important, since theengine and the tyres have already been dampedin the last years noise in the cabin is in the focus, but outer noisebecomes more important

leakages

whistling noise

optimization ofseals turbulent boundary layers air frame noise A-columns outer mirror opt m ze t e s ape o t e m rror an ts x ng tokeep the turbulent small and to avoid that it hitsthe side windows; windows have a smallerdamping

Acoustics


64/67

Single tones are very aggravating cylindrical and prismatic parts

Acoustics on a telescope antenna, BMW

Multi phase flow


65/67

self contamination air includes particles, wet and dry particles are heavier than air cannotfollow the streamlines

settle down on surfaces 2 Aspects: safety and aesthetics drip moulding at A-column or on top ofthe tail window

Photo: Daimler Chrysler AG

Multi phase flow


66/67

Avoiding of mud sedimentation is more complex stagnation points, head light side windows, tail aerodynamic forces are not strong enough

windshield wiper, water jets

a) Sedimentation of mud, particles are dispersed from the rear wheels,enter the dead water region, sediment on the base

b) Dust shield across the total width increase of the drag force, isreasonable for city buses, because of the small velocity

c) Additional wing, avoids mud in the upper part of the window, butmore mud in the lower region (licence tag) , effective wing must beoutside of the boundary layer increased drag force

Multi phase flow


67/67

Contamination of other motorists and cyclists important for passenger cars covering of uncontrolled wheels free moving space for front wheels reduced drag, especially for inclined flow angles protection for the underbody

Automobile English

Documents

Transcript of Automobile English