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Transcript of Automobile English
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Fahrzeug- undWindradaerod namik
Dr.-Ing. A. Henze, Prof. Dr.-Ing. W. Schrder
Institute of Aerodynamics, RWTH Aachen University
Automobiles
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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
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Command variables
Tasks for automobile aerodynamics
Photo: Daimler-Chrysler AG
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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
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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
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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
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Automobiles
Design vs. aerodynamics
Droplet, Rumpler 1921 2 profiles, Jaray 1921 Profile + Rotational body, Jaray 1933 2 horizontal profiles, Lange 1937
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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
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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
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Automobiles
Development of cw coefficient of european cars compared with low drag bodies
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Automobiles
Cw histogramm of european passenger cars, 2003
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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
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Driving performance
Drag coefficient and facing surface fordifferent types of automobiles
Facing surface Aa) Different types of automobilesb) Typical values for passenger cars
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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
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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
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Reduction of fuel consumption
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
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Reduction of fuel consumption
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
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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:
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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
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Drag, flow around the car
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
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Drag, flow around the car
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
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Drag, flow around the car
Fundamental relationship between modifications of shape detailsand air drag
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Drag, flow around the car
Typical functions: saturation, asymptote, minimum, jump
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Blunted leading edge
Flow around the sharp edge of a flat platea) Streamlinesb) Pressure distributionc) Velocity profiles
Influence of the edge radius on cw
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Blunted leading edge
Influence of the edge radius onto theflow pattern and the drag of a cuboid
Influence of the radius on the critical Reynolds number
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Blunted leading edge
Influence of the edge radius on the critical Reynolds number
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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
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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
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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
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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)
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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
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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
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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
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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
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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
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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
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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
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Inclined incoming flow
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)
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Inclined incoming flow
Ford Werke GmbH (2003)
Pressure distribution for symmetric flow and for an angle of 20
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Inclined incoming flow
Flowfield through the gap between the cabinAnd the containe at inclined flow
Increase of the optimum noseradius at inclination
Lif d i hi
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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
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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
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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
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Linearised car model
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
Linearised car model
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Linearised car model
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
Linearised car model
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Linearised car model
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
Linearised car model
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Linearised car model
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
Linearised car model
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Linearised car model
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
Linearised car model
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Linearised car model
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
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O e a d U de stee g
Drivers model
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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
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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
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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
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frequency range natural side wind ration of the yaw angle velocity and the side wind
Aerodynamics at inclination
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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
Aerodynamics at inclination
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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
Aerodynamics at inclination
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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
Aerodynamics at inclination
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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
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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
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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
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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
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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
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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
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Single tones are very aggravating cylindrical and prismatic parts
Acoustics on a telescope antenna, BMW
Multi phase flow
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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
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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
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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