(Flu 12 Introduzione aerodinamica veicoli [modalità...

INTRODUZIONE ALL’AERODINAMICA DEI VEICOLI

SUMMARY

• INTRODUCTION TO ROAD VEHICLE AERODYNAMICS• HISTORY OF VEHICLE AERODYNAMICS• VEHICLE ATTRIBUTES INFLUENCED BY AERODYNAMICS• AERODYNAMICS CHARACTERISTIC

DragLift and Piching MomentYawing Moment and Side ForceDetailed Surface Flows

• DEVELOPMENT PROCESS AND TOOLSAerodynamic development of a vehicleExperimental test facilitiesLimitations of wind-tunnel testingNew experimental test facilitiesMeasurement techniquesCFD Computational Fluid mechanics

Road Vehicles are bluff bodies in very close proximity to the ground.

Their detailed geometry is extremely complex. Internal and recessed cavities which communicate freely with the external flow (i.e. engine compartment and wheel wells, respectively) and rotating wheels add to their geometrical and fluid mechanical complexity.

The flow over a vehicle is fully three-dimensional.

Boundary layers are turbulent.

Flow separation is common and may be followed by reattachment.

Large turbulent wakes are formed at the rear and in many cases contain longitudinal trailing vortices.

INTRODUCTION TO ROAD VEHICLE AERODYNAMICS

As is typical for bluff bodies, drag (which is a key issue for most roadvehicles--but far from the only one) is mainly pressure drag.

The avoidance of separation or, if this is not possible, its control are among the main objectives of vehicle aerodynamics.


Depending on the specific purpose of each type of vehicle, the objectives of aerodynamics differ widely.

While low drag is desirable for all road vehicles, other aerodynamic properties are also significant.

Negative lift is decisive for the cornering capability of race cars, but is of no importance for trucks.

Cars and, even more so, vans are sensitive to cross wind, but heavy trucks are not.

Wind noise should be low for cars and buses, but is of no significance for race cars.

With the race car being the only exception, the shape of a road vehicle is not primarily determined by the need to generate specific aerodynamic effects

To the contrary, a road vehicle’s shape is primarily determined byfunctional, economic and, last but not least, aesthetic arguments.


HISTORY OF VEHICLE AERODYNAMICS

1. The recognition that the pattern of flow around half a body of revolution is changed significantly when that half body is brought close to the ground (Klemperer 1922).

2. The truncation of a body’s rear end (Koenig-Fachsenfeld et al 1936,Kamm et al 1934).

3. The introduction of "detail-optimization" into vehicle development (Hucho et al. 1976)

4. The deciphering of the detailed flow patterns at car rear ends

5. The application of "add-ons" like underbody air dams, fairings, and wings to passenger cars, trucks, and race cars.


In general, there are different types of drag variation that lead to such an optimum:

(a) minimum; (b) jump; (c) saturation.


Detail-optimization (Hucho et al, 1976)


Detail-optimization



VEHICLE ATTRIBUTES AFFECTED BY AERODYNAMICS

PERFORMANCE AND FUEL ECONOMY

Fuel economy and, increasingly, global warming are the current keyArguments for low drag worldwide.

In Europe, particularly Germany top speed is still considered an important sales feature.

An analysis of the factors affecting automobile fuel economy can best be made if the driving pattern is prescribed.

In the U.S., two speed variations of particular relevance are the Environmental Protection Agency (EPA) Urban and Highway schedules that are the basis for the fuel-economy and exhaust-emissions regulations.

In E.U. in the past regulation is based on the EUROMIX cycle, and now on MVEG cycle (ECE +EUDC)

where FTR is the tractive force, R the tire rolling resistance, D the aerodynamic drag, M the vehicle mass, g the acceleration of gravity, and teta is the inclination angle of the road.

The tractive power is

The tractive energy required for propulsion

The main reason that fuel is consumed in an automobile is to provide this tractive energy.



The Benefits of Reduced Drag are

reduced fuel consumption

increased acceleration capability

increased top speed.

When maximum fuel-economy benefit is the objective the increased acceleration and top-speed capabilities can be converted to additional reductions in fuel consumption

For a typical Midsize car at Steady state cruising at 120 Km/h speed

CD = 0.30 => 16.6 KW => 7 l / 100 KmCD = 0.15 => 10 KW => 4.2 l / 100 Km

CD = 0.30 => Top speed 200 Km/hCD = 0.15 => Top speed 249 Km/h





CD Drag coefficient (= D/q∞∞∞∞A) CL Lift coefficient (= L/q∞∞∞∞A) CS Side force coefficient (= S/q¥A) CPM Pitching moment coefficient (= PM/q∞∞∞∞A.WB)CYM Yawing moment coefficient (= YM/q∞∞∞∞A.WB) CRM Rolling moment coefficient (= RM/q∞∞∞∞A.WB)

D Drag (= -Fx)L Lift (= -Fz)S Side force (= Fy)PM Pitching moment (= My)YM Yawing moment (= Mz)RM Rolling moment (= Mx)q∞∞∞∞ Dynamic pressure (= 1/2ρρρρV∞∞∞∞

2)ρρρρ Mass density of air V∞∞∞∞ Relative wind velocityA Projected frontal areaWB Wheelbase



HANDLING (vehicle’s behavior in still air)

The flow over a vehicle moving through still air is nominally symmetricabout the vehicle’s plane of symmetry

Lift, pitching moment and, of course, drag are therefore the only aerodynamic components.

the vertical force on a bluff body close to ground is positive, i.e. it tends to lift the vehicle

For a typical European car with a typical lift coefficient of 0.3, lift amounts to less than 3% of the vehicle’s weight at a speed of 100 km/h, and only 10% at 180 km/h

It is the pitching moment rather than total lift that counts in vehicle dynamics because it changes the load distribution between the front and rear axles, which alters the steering properties of a vehicle


CROSSWIND SENSITIVITY (vehicle’s behavior in the presence of a crosswind)

In a crosswind, and while passing another vehicle in still air, the flow around a vehicle becomes asymmetric and so a side force, a yawing moment, and a rolling moment are produced.

The components of drag, lift, and pitching moment are altered

The main parameter of a vehicle’s behavior in crosswind are: yawing moment and side force.

The yawing moment referred to a vehicle’s center of gravity gives a first indication of sensitivity to crosswind. For almost any vehicle, the yawing moment is unstable, i.e. it tends to twist the vehicle further away from the wind.

With regard to yawing moment, it is increased by the rounded rear-end shape

The flow over a vehicle not only produces aerodynamic forces andmoments, but also many other effects that can be summarized under the term functionals.

FORCES ON BODY PARTS

WIND NOISE

BODY-SURFACE WATER FLOW AND SOILING

INTERIOR FLOW SYSTEMS


FUNCTIONALS

AERODYNAMIC CHARACTERISTICS

DRAG

Vehicle perspective

D= Dp+Df

Measurement of only total drag (D) per se does not provide information on their relative magnitudes, nor on their distributions over the body surfaces.

The origin and nature of drag contributions can be and has been gained by making systematic, parametric changes in the body surfaces

Direct evaluation of Dp and Df requires knowledge of the detailed stress distributions over all vehicle surfaces

This is not practicable from experimental point of view for bodies with the complex geometries of typical road vehicles

On the contrary, detailed surface-stress distributions are the specific output of computational fluid dynamics (CFD)


DRAG

Stream perspective

The wind-axis drag acting on a vehicle can be determined by applying the streamwise momentum equation to a large control volume containing the vehicle

This method of drag evaluation and breakdown requires data from extensive and detailed traverses behind vehicles.

By reducing the cross flow velocities to a vorticity field, vortex structures are better discriminated facilitating the process of tracking them back to their origin. This technique it is used as an aid in car-shape development. (Pininfarina wind tunnel)


DRAG

Vortex drag

airfoil thinking is used to suggest that the inadvertent lift of passenger cars produces an induced-drag component


DRAG

Critical After body Geometry

For any current car that has received aerodynamic attention, the contribution of the forebody to drag is usually small.

The major aerodynamic problem is at the rear


DRAGCritical After body Geometry


DRAG

Critical After body Geometry


DRAG

Ambient wind typical of a flat environment


DRAG

average repartition of drag for a modern car


DRAG

Underbody

Audi A4 “aereofloor”


DRAG

The road traffic

The integration of the maps gives, for the 406 C alone (S= 2.04 m2), a drag coeff. Cd= 0.369 and for the 406 C in the wake of the 1st car, a Cd=0.414( +12.2%).


LIFT and PITCHING MOMENT

A Venturi channel in the body’s underside. The effectiveness of this measure is enhanced by preventing lateral flow with side skirts. A large downforce can be generated with only a small penalty in drag. (Ground effect, Pistolesi 1935)

Negatively cambered wings at the car’s front and rear, even equipped with double flaps. In this case, there is a large increase in drag due to the induced drag of the wings, and mainly from the wing at the rear



Downforce and drag coefficient versus ground clearance for an inverted LS(1)-0413 airfoil.





Lift and drag coefficients for the rear wing of a generic open-wheel race car. AR = 1.5, and coefficients are based on planview area. AR, aspect ratio.



Lift and drag coefficient variation with ground clearance for a generic model with underbody diffuser




YAWING MOMENT AND SIDE FORCE


YAWING MOMENT AND SIDE FORCE


DETAILED SURFACE FLOW

Detailed surface flows are investigated for a number of reasons:

• to properly locate openings for air inlets and outlets; • to ascertain the forces on particular body parts;• to determine the sources of wind noise;• to find means for controlling water flow and soil deposition on surfaces• to individuate source of drag.


DETAILED SURFACE FLOW

DEVELOPMENT PROCESS AND TOOLS

AERODYNAMIC DEVELOPMENT OF A VEHICLE

Aerodynamic development of a is vehicle performed in a closed loop containing aesthetic, packaging, and aerodynamic considerations.

Development of a road vehicle can proceed from two different and opposite starting points.

The designer makes the first proposal, the aerodynamicist tries to improve the shape (detail optimization), if the drag coefficient of vehicles was still fairly high the designer take in accounts the aerodynamicist request,

The opposite approach starts with a simple body of very low drag which has the desired overall dimensions of the subsequent car. This so-called basic body is modified step by step, in cooperation with the exterior designer, until a final shape materialize (shape-optimization).

All vehicle characteristics influenced by aerodynamics are finally evaluatedon the road.


AERODYNAMIC DEVELOPMENT OF A VEHICLE

The typical design of a fluid-dynamic machine is accomplished in threemajor steps:

The main overall dimensions of the machine ( the plan form and span of an aircraft wing, the diameter and number of stages of a turbine or compressor)

The detailed design of the machine’s components follows an iterative process between analytical and numerical design (CFD) and experimental verification

The separately optimized components are put together in a prototype system which is then tested. (Experiments e/o CFD)


EXPERIMENTAL TEST FACILITIES

Full Scale Wind TunnelHigh operative cost

Reduced Scale Wind Tunnel (1:4 or 1:5)• Reduced operative cost• Models are cheaper than full-scale ones, are easy to handle, and can

be quickly modified.• Not reproduce full-scale values with the accuracy needed.

(lack of geometric similarity in the model, effects of Reynolds number)• Shapes in small scale cannot be adequately assessed aesthetically.


LIMITATIONS OF WIND-TUNNEL TESTING

BLOCKAGE As carryover from aircraft terminology, the ratio of a car’sfrontal area to the cross-sectional area at the tunnel-nozzle exit is calledthe blockage ratio.

Blockage ratios up to 20%

For a typical car and a blockage ratio (5%) , this would lead to a test-section cross-sectional area of 40 m2.





Influence of test-section length L (relative to the hydraulic diameter DN of thenozzle) and the ratio of collector area (Ac) to nozzle area (AN) on measured drag; Porsche 22 m2 wind tunnel, open test section


LIMITATIONS OF WIND-TUNNEL TESTINGRoad representation and wheel rotation





The “Ground Effect Simulation System” and the new Fan-Drive System of Pininfarina wind tunnel


NEW EXPERIMENTAL FACILITIES

Aero-acoustic wind tunnels

Aerodynamic noise can be investigated in a wind tunnel only if there is asufficient difference in sound-pressure level between the noise produced bythe flow around (and through) a vehicle and the background noise of thetunnel.

The formerly required 10 dB(A) difference (Buchheim et al 1983)may be sufficient for objective measurement of the external sound pressurefield, but it is inadequate for subjective assessment of wind noise insidea car.

Aero-acoustic wind tunnels have now been built which provide a 30dB(A) sound-pressure-level difference



Aero-acoustic wind tunnels

Comparison of background noise in various aeroacoustic wind tunnels

Out-of-flow frequency spectrum at 160 kph of the Audi aeroacoustic wind tunnel



Unsteady aerodynamic wind tunnels

For most of the driving time, there is some light atmospheric wind (V< 10 m/s) and its corresponding turbulence.

The road traffic, i.e. the turbulence generated by other vehicles running on the road

The atmospheric wind can be something between these two limits: • The wind typical of a flat environment, or • A wind heavily modified by roadside obstacles

The turbulence generated by the traffic may depend on: • The size and the shape of the other vehicles on the road. In particular, different shapes mean different wakes, different vortices and different turbulence intensities. • The distances of the upstream vehicles, and their alignment with the following vehicles. • The vehicle’s crossing on the opposite lanes.




Model scheme for the roadside obstacles simulation.

Turbulence Intensity, Longitudinal Iu, Lateral Iv, and Total I, computed in the presence of roadside obstacles.




The Five Vortex Generators in the nozzle of Pininfarina Wind tunnel


MEASUREMENT TECHNIQUES

The motivations for improving measurement techniques have been, andstill are, threefold: time saving, increased accuracy, and more-detailedinformation for reaching the ever-higher aerodynamic targets.

Standard techniquesforces and moments;pressure distribution on specific sectors of the surface;flow visualization by various techniques

Advanced techniquesHot-wiresLaser-Doppler anemometry (LDA), Wake surveys are being made with multi-hole probes,Fast-response pressure probes,Acoustic mirrorParticle Image Velocimetry (PIV).



Acoustic mirror


MEASUREMENT TECHNIQUESAcoustic mirror



Particle Image Velocimetry (PIV).



Particle Image Velocimetry (PIV).


COMPUTATIONAL FLUID DYNAMICS

The primary reason for the automobile industry’s interest in numericalmethods is to save time during product development. The ability to quicklyreact to the ever-changing needs of the market has even higher prioritythan cost saving, which is very important in its own right.

questionsCFD reproduce the related physics with adequate accuracy;CFD have to be faster than experiment.CFD is less expensive.

Main CFD techniquesPanel MethodReynolds-averaged Navier-Stokes equations (RANS),Large Eddy Simulation (LES)Direct Numerical Simulation (DNS)





Simulations carriedout by VOLVO in order to optimise the drag on one of their vehicle. The result of thecalculation, validated in Volvo wind tunnel, shows a decrease of 0.003 of the drag coefficient with the introduction of a little wheel spoiler in front of thewheels.



Streamlines under a stock car

(Flu 12 Introduzione aerodinamica veicoli [modalità...

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