Dr. B Dayal

Dr. B Dayal

AERODYNAMICSINTRODUCTIONA car body with the aerodynamic shape passes with least resistance through the air, as a consequence the fuel economy is improvedFor a vehicle without aerodynamic body, a lot of engine power is required to drive through the air.An automobile is a small object submerged amid vast surrounding of air. The motion of the vehicle takes place through a large mass of either stationary air, or air in motion.The air exerts force on the vehicle.It is the super structure (body) of the vehicle which is mainly exposed to air.An arbitrary shaped body will experience a large air resistance which implies that there is more loss of engine power. Consequently less power will be available to propel the automobile thereby causing less load carrying capacity and slow speed for the same fuel consumption.Thus, there exists a need to profile aerodynamically suitable body.

AERODYNAMICSAERODYNAMIC FORCESTHE force exerted by air on a moving automobile has two components:The one in the direction of motion called drag FDThe one in the direction perpendicular to the motion is called lift FL.The body profile of an automobile should be such that the lift force FL is zero or negligible, and then the total force on the body is drag force FD.The viscosity and density of air are mainly responsible for drag on the body. Area = AAir density = Air velocity = vairObject velocity = vobjV = vair - vobj

AERODYNAMICSAERODYNAMIC FORCESTHE MASS FLOW RATE OF AIR = . A . VASSUMING dV AS CHANG IN VELOCITYTHEN RATE OF CHANGE IN MOMENTUM DUE TO ATTACK ON THE OBJECTIVE M = . A . V . dVBut rate of change of momentum = force applied = dFdF = . A . V . dVdF = .A.V.dVF = .A.V2 / 2This force may be same or fraction of this due to shape of the body, body profile and surface finish.Thus it is multiplied by a coefficient called as drag coefficient Cd.Therefore,air resistance = drag force FD = .A.V2. Cd / 2Similarly,lift force = Fl = .A.V2. CL / 2Where CL is the lift coefficient

DRAG AND LIFT FORCES ON A BODY

AERODYNAMICSAERODYNAMIC FORCESTHE ARBITRARY SHAPED BODY OF AN AUTOMOBILE, HELD STATIONARY IN A STREAM OF AIR MOVING AT RELATIVE VELOCITY V, EXPERIENCES SHEAR FORCE ALONG ITS TANGENTIAL DIRECTION AND PRESSURE FORCE IN THE NORMAL DIRECTION.THE SHEAR FORCE ARE CALLED FRICTION DRAG FORCES, FDFTHE PRESSURE FORCES ARE KNOWN AS PRESSURE DRAG FROCES, FDPTHE TOTAL DRAG ON THE BODY IS THEREFORE SUM OF FRICTION AND PRESSURE DRAGS.FD = FDF + FDPTHE MAGNITUDE OF FRICTION DRAG AND PRESSURE DRAG DEPEND ON SHAPE OF THE BODY.FOR EXAMPLE A FLAT PLATE PORTION EXPERIENCES ONLY FRICTION DRAG (FDP = 0)WHILE THE FLAT PORTION PERPENDICULAR TO THE DIRECTION OF FLOW, FEELS ONLY PRESSURE DRAG DUE TO PRESSURE DIFFERENCE ON THE UP STREAM AND DOWN STREAM SIDE OF THE BODY. (FDF = 0)

DRAG

AERODYNAMICSDRAG CAN BE CALCULATED FROM:FD = . . A . V2 . CD FL = . . A . V2 . CLWHERE CD IS THE COEFFICIENT OF DRAG AND CL IS THE COEFFICIENT OF LIFT.DRAG COEFFICIENT DRAG COEFFICIENT IS THE FRACTION OF THE FULL FORCE, WHICH CAN BE EXERTED BY THE WIND ON SOME AREA OF MOVING VEHICLE. THE DRAG COEFFICIENT IS RELATED TO THE ANGLE OF ATTACK BY;CD = 2 SINWHERE = ANGLE OF ATTACK

DISTRIBUTION OF PRESSURE ON A CAMBERED WING AT DIFFERENT ANGLES OF ATTACK

DRAG FORCES ON A CAR BODY

FLOW PATTERN AROUND A STREAMLINED CAR BODY

WAKEQUITE OFTEN WE SEE ON THE DOWN STREAM SIDE OF A FAST MOVING VEHICLE THAT THE SMALL AND LIGHT OBJECTS SUCH AS PAPERS, PEBBLES ETC. LIFT UP AND MOVE IN A HAPHAZARD WAY. IT IS DUE TO PHENOMENA CALLED WAKE.SEPERATION OF FLOW AND THE DIFFERENCE OF PRESSURE ON THE UPSTREAM AND DOWNSTREAM SIDES OF THE MOVING VEHICLE ARE RESPONSIBLE FOR THE WAKE.WAKE IS AN UNDESIRED SITUATION. IT SHOULD BE AVOIDED OR MINIMISED BY PROPER PROFILING OF THE BODY.THE CONTOURING OF THE BODY SHOULD BE SUCH THAT THE SEPERATION OF FLOW DOES NOT OCCUR, AND THE PRESSURE DIFFERENCE IS NOT MUCH ON THE UPSTREAM AND DOWNSTREAM SIDES.TO ACHIEVE IT, THE MODERN CARS EMPLOY A REAR SPOILER THAT ADDS TO AERODYNAMIC STYLING OF THE BODY.

WAKEThe formed wakes can be of different sizes according to shape of the body. The magnitude of pressure drag depends on the size of the wake. The size of the wake will be large in a body such as circular disc, having sharp edges than well round bodies.The wake and therefore the drag force is extremely small in case of stream lined body.Ina well streamlined object, the friction drag is larger than the pressure drag; even though the total drag is about 0.02 to 0.03 only to that of the circular disc.The term v2 / 2 is called the dynamic pressure of the flowing air.

RACING CAR PROFILINGThe COEFFICIENT OF DRAG DEPENDS ON SHAPE OF THE BODY IN HIGH VELOCITY AIR STREAMS. AS COMPARED TO FLAT HEADED BODY IN WHICH CD = 0.85 AT 300 KMPH, THIS VALUS IS ONLY 0.15 IN SHARP POINTED PROJECTION OF RACING CARS. HENCE RACING CARS ARE MADE OF THE PROFILE WITH SHARP POINTED NOSE.AS WE DESIRE CL TO BE MINIMUM, PREFERABLY ZERO, HENCE THE BODY SHOULD BE ALIGNED IN FRONT OF THE UPSTREAM AIR SO THAT = 0. THE DRAG COEFFICIENT VARIES LITTLE WITH THE ANGLE OF ATTACK.

BODY PROFILE OF A FIAT UNO CAR

AERODYNAMIC DRAG COEFFICIENT FOR DIFFERENT TYPES OF VEHICLESRACING CARS0.25 0.30PASSENGER CARS0.30 0.60CONVERTIBLES0.40 0.65BUSES0.60 0.70TRUCKS0.80 1.00TRACTORS AND TRAILERS1.25 1.35MOTOR CYCLES1.75 1.85

DRAG COEFFICIENT OF SOME VEHICLESEthos 2 electric car0.190Yamaha yzf thunder coat racing motor cycle 0.275Yamaha fzr 600 racing motor cycle0.300Saab 92 (sweden) car0.300Citreon ds 19 car0.311Mercedez 300 se0.387Ford falcon futura0.416Ford mustang0.475

DRAG REDUCTION: DESIGN CONSIDERATIONSA major drag source occurs at the very front of the car where the maximum pressure is recorded. The lowering and rounding of the sharp front corner together with the reduction or elimination of the flat, forward facing surface at the very front of the car addresses both of these drag sources.A second seperation zone is observed at the base of the wind screen.The research has clearly demonstrated the benefits of the shallow screens but the raked angles desired for aerodynamic efficiency lead to problems of reduced space, driver head room and internal optical reflections from the screen. An angle of 360 from the horizontal has been found to give optimum reduction and compromise with other drawbacks.A bonnet slope of 50 to 100 also provides the better results in reduction of drag coefficient.

DRAG REDUCTION: DESIGN CONSIDERATIONSrear bodythe airflow over the rear surfaces of the vehicle is more complex and the solution required to minimise the drag for practical shapes are less intuitive.an angle of 150 from horizontal for backlight gives the minimum drag coefficient. but it will reduce the luggage compartment space. it is necessary to raise the boot lid and this has been a very clear trend in the design of the medium and large saloon cars. this has further benefit in terms of luggage space although rearward visibility is generally reduced.rear end boot lid spoilers have a similar effect without the associated practical benefits.one of the most effective drag reduction technique is the adoption of boat tailing, which reduces the effective cross sectional area at

DRAG REDUCTION: DESIGN CONSIDERATIONSrear bodythe rear of the car and hence reduces the volume enclosed within the wake. in its extreme configuration this results in the tail extendibg to a fine point, thus eliminating any wake flow. practical considerations prevent the adoption of such designs but truncation of the tail for 100 inclined towards center gives optimum aerodynamic efficiency.under body smoothing.smooth outside visible surfaces.shaping of the floor pan at the rear of the car for diffused flow.

Typical static pressure coefficient distribution

DRAG REDUCTION BY CHANGES TO FRONT BODY SHAPE

WAKE

INFLUENCE OF BACKLIGHT ANGLE ON DRAG COEFFICIENT

HIGH TAIL LOW DRAG DESIGN

BOAT TAILING REDUCED WAKE

REAR UNDER BODY DIFFUSION

PRACTICE QUESTIONthe effective frontal area of a car is 1.6m2. it is moving at a speed of 60 kmph in calm air surrounding. profile of its body is such that the coefficient of lift and drag are 0.75 and 0.15 respectively. taking weight density of air as 1.15 kg/m3, determinethe drag forcethe lift forcethe resultant force and angle of the resultant force with respect to horizontal.power exerted by the air stream on the car.the expected pressure distributions on bonnet, front and rear wind screens, roof, rear hood, front and back faces of the car.

PRESSURE AND SUCTION DISTRIBUTION IN A TYPICAL CAR BODY

STREAM TUBES FLOWING OVER AN AERODYNAMIC BODIES

PRESSURE AND VELOCITY GRADIENTS IN THE AIR FLOW OVER A BODY

DEVELOPMENT OF A BOUNDARY LAYER

FLOW SEPARATION IN AN ADVERSE PRESSURE GRADIENT

VORTEX SHEDDING IN FLOW OVER A CYLINDRICAL BODY

PRESSURE DISTRIBUTION ALONG THE CENTERLINE OF A CAR

VORTEX SYSTEMS IN THE WAKE OF A CAR

AERODYNAMIC LIFT AND DRAG FORCES WITH DIFFERENT VEHICLE SYLES

EFFECT OF SEPARATION POINT ON DIRT DEPOSITION IN THE REAR

AERODYNAMIC FORCES AND MOMENTS ACTING ON A CAR

INFLUENCE OF REAR END INCLINATION ON GRAG

INFLUENCE OF FRONT END DESIGN ON DRAG

INFLUENCE OF WIND SHIELD ANGLE ON DRAG

AIR FLOW RECIRCULATION IN A WHEEL WELL

AIR FLOW INSIDE A TYPICAL ENGINE COMPARTMENT

INFLUENCE OF COOLING SYSTEM ON DRAG

INFLUENCE OF A SPOILER ON THE FLOW OVER THE REAR

OPTIMISATION OF BODY DETAIL

DRAG COEFFICIENT OF VARIOUS BODIES

RELATIVE WIND SEEN BY A MOTOR VEHICLE ON THE ROAD

AIR FLOW AROUND A TRACTOR SEMITRAILER WITH 300 WIND ANGLE

INFLUENCE OF YAW ANGLE ON DRAG COEFFICIENT OF TYPICAL VEHICLE TYPES

SIDE FORCE COEFFICIENT AS A FUNCTION OF YAW ANGLE FOR A TYPICAL VEHICLE

VARIATION OF PITCHING MOMENT COEFFICIENT WITH BODY PITCH ANGLE

YAW MOMENT COEFFICIENT AS A FUNCTION OF YAW ANGLE FOR A TYPICAL VEHICLE

CORRELATION OF SUBJECTIVE RATINGS WITH NORMALISED RMS YAW RATE RESPONSE

CORRELATION OF SUBJECTIVE RATINGS WITH NORMALISED RMS YAW RATE RESPONSE (CONTD)

VEHICLE CONFIGURATIONS:BASELINEFORWARD CPREARWARD CP320 LB REAR BALLASTBASELINE AND REDUCED ROLL STIFFNESSFORWARD CP AND REDUCED ROLL STIFFNESSFORWARD CD, REAR BALLAST AND REDUCED ROLL STIFFNESS

CROSS WIND LATERAL ACCELERATION RESPONSE WITH VARIATION OF CP LOCATION

ROLLING RESISTANCE

MECHANISMS RESPONSIBLE FOR ROLLING RESISTANCE:Energy loss due to deflection of the tyre sidewall near the contact areaEnergy loss due to deflection of tread elementsScrubbing in the contact patchTyre slip in the longitudinal and lateral directionsDeflection of the road surfaceAir drag on the inside and outside of the tyreEnergy loss on bumpsTotal resistance is the sum of the resistances from all the wheels.Rx = Rxf + Rxr = fr WWhere,Rxf = rolling resistance of the front wheelsRxr = rolling resistance of the rear wheelsfr = rolling resistance coefficient or specific rolling resistanceW = weight of the wheels

FACTORS AFFECTING ROLLING RESISTANCE

Tte coefficient of rolling resistance, fr, is a dimensionless factor that expresses the effects of the complicated and interdependent physical properties of tyre and ground.Tyre temperature. In the typical situation where a tyre begins rolling from a cold condition, the temperature rises and the rolling resistance will diminish over a first period of travel. The tyre must roll at least 30 km before the system approaches the stable rolling resistance.RELATIVE TYRE TEMPERATUE AND ROLLING RESISTANCE DURING WARM UP

FACTORS AFFECTING ROLLING RESISTANCE

Tyre inflation pressure / load. The tyre inflation determines the tyre elasticity and in combination with the load, determines the deflection in the side walls and contact region. The overall effect on rolling resistance also depends on the elasticity of the ground.COEFFICIENT OF ROLLING RESISTANCE VERSUS INFLATION PRESSUREOn soft surfaces like sand, high inflation pressures result in increased ground penetration work and therefore higher coefficient. Conversely, lower inflation pressure, while decreasing the ground penetration, increases tyre flexture work. Thus, the optimum pressure depends on the surface deformation characteristics

FACTORS AFFECTING ROLLING RESISTANCE

Velocity. The coefficient is directly proportional to speed because of increased flexing work and vibration in the tyre body, although the effect is small at moderate and low speeds and often assumed to be constant for calculation.ROLLING RESISTANCE VERSUS SPEEDfr = A + BVfr = f0 + 3.24fs (V / 100)2.5Where v = speed in mph; f0 = basic coefficient; fs = speed effect coefficient

COEFFICIENT FOR ROLLING RESISTANCE ON A CONCRETE SURFACE

FACTORS AFFECTING ROLLING RESISTANCE

Velocity. At the university of michigan transportation research institute:fr = (0.0041 + 0.000041V) Chradial tyres fr = (0.0066 + 0.000046V) Chbias-ply tyresWhere,V = speed in miles per hourCh = road surface coefficient = 1.0 for smooth surface = 1.2 for worn concrete, brick, cold black top = 1.5 for hot black top

FACTORS AFFECTING ROLLING RESISTANCE

Tyre material and design. The materials and thickness of both tyre sidewalls (usually expressed in plies) and the tread determine the stiffness and energy loss in the rolling tyres. Worn out, smooth tread tyres show coefficient values up to 20% lower than new tyres. ROLLING RESISTANCE VERSUS TEMPERATURE OF TYRES WITH DIFFERENT POLYMERSFine laminations, on the other hand, increase the coefficient as much as 25%. The cord material in the sidewall has only a small effect, but the cord angle and tyre belt properties have a significance influence.

FACTORS AFFECTING ROLLING RESISTANCE

Tyre slip. Wheels transferring tractive or braking forces show higher rolling resistance due to wheel slip and the resulting frictional scuffing. Cornering forces produce the same effects. At a few degree of slip, equivalent to moderate high cornering accelerations, theROLLING RESISTANCE COEFFICIENT VERSUS SLIP ANGLERolling resistance coefficient may nearly double in magnitude. The effect is readily observed in normal driving when one will scrub off speed in a corner.Fr = Rx / W = CW / D (ht / w)0.5

FACTORS AFFECTING ROLLING RESISTANCE

Where,Rx = rolling resistance forceW = weight on the wheelC =constant reflecting loss and elastic characteristics of the tyre materialD = outside diameterht = tyre section heightw = tyre section widthROLLING RESISTANCE COEFFICIENT FOR VARIOUS SOILS

VEHICLESURFACECONCRETEMEDIUM / HARDSANDPASSENGER CARS0.0150.080.30HEAVY TRUCKS0.0120.050.25TRACTORS0.020.040.20

GRADIENT RESISTENCEA vehicle, while climbing uphill, in addition to the rolling resistances also faces gradient resistance. This gradient resistance keeps increasing with increase in gradient and a vehicle with adequate power would keep climbing till it starts slipping back (Because of lack of ground traction.RG = W sin If we happen to see the performance curve for self propelled vehicles, on the fine grained soils for 50 passes we can see that maximum slope for vehicles is just 60% or 0.60.Hence Sin = 0.60 or = 36.80PWD roads can not have gradient more than 18. Border Roads can have max gradient upto 22. The vehicle (Light trucks and Cars) are tested for 300 gradients.

TOTAL ROAD LOADSRRL = fr W + 0.5 A V2 Cd + W sin Power = RRL . V = (fr W + 0.5 A V2 Cd + W sin ) . V

EXAMPLE PROBLEMS1. A heavy truck weighing 72,500 Ib rolls along I70 in Denver at a speed of 67 mph. The air temperature is550F ad the barometric pressure is 26.01 in Hg. The truck is 8 wide by 13.5 high, and has an aerodynamic drag coefficient of 0.65. The truck has radial-ply tires. Calculate the aerodynamic drag, the rolling resistance(according to the SAE equations) and the road load horsepower at there conditions. 2. A passenger car has a frontal area of 2I square feet and a drag coefficient of 0.42. It is traveling a long at 55mph. Calculate the aerodynamic drag and the associated horsepower requirements if it is driving into a 25 mph headwind and with a 25 mph tailwind.