the science of winds - archlord · the science of winds ... vortex generators are simply small...

Erasmus LLP Intensive Programme

Vehicles Aerodynamics

The Science of Winds

Dr.-Ing. G. Deliporanides

Powering the Future With Zero Emission and Human Powered Vehicles – Terrassa 2011

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Contents

� Short Historical Review

� Fundamental Principles

� Viscous Effects – Drag

� Downforce – Drag Reduction

� Conclusions


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I Short Historical Review


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Jenatzy

speed 105.88 km/h on April 29th 1899

Streamlined body !!!

No one cares about:

the wheels / driver !


Alfa Romeo 1914Ricotti


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Streamlined body

Round windows (constr. detail)

No open air driver

Free wheels

Steamlined back !!!


Audi 14/35 PSAlpensieger (no ground effects cons.)


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Rumpler Tropfenwagen - (Auto) 1920 teardrop car (1922) Cd=0.28


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Edmund Rumpler (1872-1940),

Maschinenbauingenieur und

Automobilkonstrukteur

http://www.bredow-web.de/Autos/Rumpler_Tropfenwagen/rumpler_tropfenwagen.html


Rumpler’s Life


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•Aerodynamics Engineer

• Aerodynamics shape

• Motor built in front of the 1st axle

Problems§ motor cooling,§ access to driving wheel,§ front wheels vibration,§ little baggage place


Rumpler’s Small Production(ca. 100 cars contsructed)

� Body shaped as

2-D Wing Profile

� Cambered Roof

� Smooth underbody

� Astonishingly low CdPowering the Future With Zero Emission and Human Powered Vehicles – Terrassa 2011

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Jarray’s Steps to streamlined Cars …1922


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Fundamental principles

� Cars’ Shape based on streamlined subtotals

� First attempts to take account of body effects

� Extremely Low Cd(..0,3)

� Long tails (to avoid sep.)

� Commercially poor


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Porsche Carrera (1939)designed by Ferdinand Porsche

� Car’s Shape based on two Horizontal Profiles

� Achievement of Cd=0.14


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Kamm’s Truncated modelCd = 0.39

� First attempts to get away from streamlined tails

� Small Separation effects by truncating the body just upsteram of S.P.

� More available place in car (commercially plus)


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Development of Cars in 20th Century

� Cars look like Carriages

Cd = 0.7 – 0.8 (.. 1940)

� After Second World War

3 Volume Shape pred.

Cd = 0.45 (.. 1940)

� Oil crises, emphasis in

Aerodynamics


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Cd Histogramm froma Sample of 303 cars (1995)

� Average Cd =0.35

� Small dispersion

� Today’s average similar


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II. Fundamental Principles ofBluff Bodies Aerodynamics

� Viscous Drag is made up of:

� “skin friction drag”

&

� “pressure drag”


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Skin friction

� Skin friction is the frictional force associated with the air rubbing against the surface of the vehicle.

� Generally, the skin friction component of the viscous drag becomes larger at higher Reynolds number.

� This is because the turbulent boundary layer has higher velocity and causes more vigorous “rubbing” of the air on the surface.


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Pressure Drag

� Pressure drag occurs when the pressures between the fore and aft facing surfaces do not balance:

� Generally, the pressure drag decreases as the Reynolds number increases.

� This is because the turbulent boundary layer has more energy and can remain attached to the body surface more easily than the laminar boundary layer.

� Streamlining helps to reduce pressure drag.


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Pressure Drag vs Skin Friction Drag


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� DRAG ON A GOLF BALL comes mainly from pressure drag.

� The only practical way of reducing pressure drag is to design the ball so that the point of separation moves back further on the ball.

� The golf ball's dimples increase the turbulence in the boundary layer, increase the inertia of the boundary layer, and delay the onset of separation.

� The effect is plotted in the chart, which shows that for Reynolds numbers achievable by hitting the ball with a club, the coefficient of drag is much lower for the dimpled ball.


Air Flow Separation – Impact on Drag

� Separation occurs when the velocity gradient is zero at the wall and shear stress at wall is zero

� Necessary condition for Flow Separation:

adverse pressure gradient dP/dx >0 i.e the pressure is increasiing in flow directionPowering the Future With Zero Emission and Human Powered

Vehicles – Terrassa 201119


Air Flow Separation – Impact on Drag cont.

� Fluid in turbulent boundary layer has appreciably more momentum than the flow of a laminar B.L. Thus a turbulent B.L can penetrate further into an adverse pressure gradient without separation

Smooth ball Rough ball

� There is an increase in drag as a result of separation as it prevents pressure recovery

� There is low pressure in separated region and it persists in the entire region.

� Turbulent eddies formed due to separation can not convert their rotational energy back into pressure head. So there is no pressure recovery (increase).

� The difference between high pressure at the front and low pressure at rear increases the drag.


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Air Flow Separation – Control cont.� Vortex generators are

simply small rectangular plates that sit above the surface. § As air moves past them,

vortices are created off the tips of the generators. These vortices interact with the rest of the air moving over the surface to speed it up and help prevent separation.

§ Vortex Generators reduce overall aerodynamic drag, rear-end air flow separation, turbulent air flow, and allows for faster clean air at the vehicle's rear end.


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Air Flow Separation – Control cont.


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� the VGs that are installed for generating streamwise vortices bring drag by itself. The actual effectiveness of installing VGs is therefore deduced by subtracting the amount of drag by itself from the amount of drag reduction that is yielded by shifting the separation point downstream.

� Larger-sized VGs increase both the effect of delaying the flow separation and the drag by itself. The effect of delaying the flow separation point, however, saturates at a certain level,which suggests that there must be an optimum size for VGs.


Air Flow Separation – Control cont.

� No reverse flow occurs at Point A, located further upstream of Point C because the momentum of the boundary layer is prevailing over the pressure gradient (dp/dx).

� Between Points A and C, there is separation Point B, where the pressure gradient and the momentum of the boundary layer are balanced.


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Air Flow Separation – Control cont.� The purpose of adding VGs is to

supply the momentum from higher region where has large momentum to lower region where has small momentum by streamwise vortices generated.

� This allows the separation point to shift further downstream. Shifting the separation point downstream enables the expanded airflow to persist proportionately longer, the flow velocity at the separation point to become slower, and consequently the static pressure to become higher.


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Shifting the separation point downstream, provides dual advantages in drag reduction:§ one is to narrow the separation region in which low pressure constitutes the cause of drag;§ another is to raise the pressure of the flow separation region. A combination of these two effects reduces the drag acting on the vehicle.


Air Flow Separation – Control cont. not applicable to cars!!

Riblets� Drag reduction device

used to trip boundary- layer into controlled turbulence.

� Size on the order of tenths of a millimeter.

� Prevents large scale vortex formation.

� Naturally present on sharks.


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Air Flow Separation – Control cont. not applicable to cars!!

� By supplying additional energy to fluid particles in the boundary-layer that are low in energy, flow can remain attached to the surface.

� Two ways of accomplishing this are§ blowing high velocity

fluid from inside the body &

§ sucking low energy fluid from the boundary-layer into the body.


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a – blowingb - sucking


Sources of Drag

� The wheels area has a contribution of

~ 30 %

� The upper part contributes ~ 45 %

� The unter part completes with

~ 25%


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Downforce

§ Main sources of Downforce:

• Front wing 25 %

• Rear Wing 35 %

• Diffusor 40 %


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Downforce from Diffusor

§ Diffusor’s Main Principle• Accelerate the air in

the rear underbody thus creating vacuum

• The Upper pressure remains essentially the same


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Downforce from the Wings

§ The Wings generate around 60 % of the Downforce• The rear wing allows 20

possible settings• The front wing complex

allows 100 settings

§ So the ideal task of get closer to the ideal is reached


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Front Wing main Functions

§ Calming the Air, as any disturbance will travel further to the car

§ Generating Downforce 25 % which easily can be reduced to 10% by air turbulence of the front car.

§ Cooling the Brakes


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Unterbody Air Flow

§ Schematic Unterbody Air Flow

indicating


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Downforce


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Downforce vs Drag


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Downforce vs Drag


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Means of Drag Reduction


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Means of Drag Reduction


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Conclusions Ø Low emissions can be achieved by reducing

CdA� Cd can be lowered by streamlining main car body

andoptimizing the details

� A (frontal area) has in effect reached the limits

Ø Downforce has positive effects in car racing (F1 & high performance vehicles)� increases the Automobile’s Grip on the road� allows higher cornering speeds� Improves the directional stability / increases the

emissions

Ø Focus of Aerodynamics research is concentrated on viscous details of Air Flow

many thanks for your attention

Dr.-Ing. G. Deliporanides


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the science of winds - archlord · the science of winds ... vortex generators are simply small...

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