CAR AERODYNAMICS

• Aerodynamics is the study of how gases interact with moving bodies. Because the gas that we encounter most is air, aerodynamics is primarily concerned with the forces of drag and lift, which are caused by air passing over and around solid bodies. Engineers apply the principles of aerodynamics to the designs of many different things, including buildings, bridges and even soccer balls; however, of primary concern is the aerodynamics of aircraft and automobiles. This is a study concerns about the airflow around the vehicle body.

Aerodynamics of automobiles

• Automobiles started using aerodynamic body shapes in the early part of their history. As engines became more powerful and cars became faster, automobile engineers realized that wind resistance significantly hindered their speed. The first cars to adopt improved aerodynamics, or streamlining, were racing cars and those attempting to break the land speed record. 

Aerodynamics impacts in the automobile:

⦁ Fuel Consumption (pollution)

⦁ Styling
⦁ Noise & Vibration
⦁ Control and Handling

 1)Fuel Consumption:

⦁ Drag forces increase exponentially with velocity
⦁ More fuel is consumed to counter aerodynamic forces than any other factor
⦁ Better air flow management reduces fuel consumption and pollution 

2) Styling:

⦁ Aerodynamically efficient body surfaces may interfere with the styling intent.

3)Noise & Vibration:

⦁ Air flow can create wind noise
⦁ Turbulent airflow can deposit dirt on windows and automobile’s body
(e.g., rear window)
⦁ Turbulent airflow can cause undesirable vibrations

4) Control and Handling:

⦁ Lift - dangerous at high speeds
⦁ Drag - decreases performance
⦁ Down force - better handling
⦁ Side force - better cornering

Terminologies

i)Streamlines:
 
⦁ Curves associated with a pictorial representation of air flow
⦁ Smoke is commonly used in wind tunnels to represent the streamlines
⦁ Streamlines are used to study air flow Velocity Distribution

ii)Velocity Distribution:

 

iii)Laminar flow:

⦁ Fluid motion that is "well organized"
⦁ Fluid with parallel velocity vectors
⦁ Generally, laminar flow has the ideal aerodynamic properties
 
 iv)Turbulent Flow:

⦁ Fluid motion that is not "well organized"
⦁ Fluid with parallel and other velocity vectors
⦁ Generally, turbulent flow has undesirable properties

v)Viscosity:

⦁ It is the fluid’s resistance to motion
⦁ Internal fluid forces at the molecular level
⦁ Where, F= fluid viscosity force, μ = coefficient of viscosity, V = fluid velocity, 
    h=separation distance, and A= contact area

Reynolds Number:

⦁It is a dimensionless number used in fluid mechanics to indicate whether fluid flow past a body or in a duct is steady or turbulent.
⦁ Represents the ratio between inertial and viscous forces


Where, is fluid density, μ is the viscosity, V is the velocity, and L is the length of the object

• Laminar flow if the Reynolds Number (based on diameter of the pipe) is less than 2100 and is turbulent if it is greater than 4000.

Force coefficients:

Rolling : The angular oscillation of the vehicle about longitudinal axis.
Pitching : The angular oscillation of the vehicle about lateral (horizontal) axis.
Yawing: The angular oscillation of the vehicle about the vertical axis.

TYPES OF AERODYNAMIC DRAGS:

1. Form drag
2. Lift drag
3. Surface drag
4. Interference drag
5. Internal flow drag

Profile (or) FORM DRAG:

• FORM DRAG is directly affected by basic shape of the vehicle body as created by body engineer.
• Body shapes that minimize positive aerodynamic forces or pressure on the front of the vehicle and minimize negative aerodynamic forces or suction on the rear of the vehicle will exhibit low
form drag.
• It is about 55% of the total drag.


Induced (or) Lift drag:

• Lift Drag is the result of any lift force that is generated by the moving vehicle.
• The magnitude of the lift force is primarily a function of the basic body shape.
• The magnitude of the lift and its distribution to the front and rear wheels is a function of the ground clearance, the contours of the body and under-body, and the angle of attack of the body to the air.
• It is about 7% of the total drag.

Surface (or) frictional drag: 

• Surface Drag is a frictional resistance that results from air passing tangentially along the body.
• The velocity of air produced a thin layer called the boundary layer next to the vehicle body,
    which slows the velocity of air due to tangential friction forces.
• The viscous friction losses in the boundary layer and the drag on small surface imperfections within this layer are considered as surface drag.
• It is a small part of the total aerodynamic drag i.e., about 9%.


INTERFERENCE DRAG:

• Interference Drag is caused by the projection and protuberances that exist on the basic body.
• The exterior vehicle body projections, such as hood ornament, windshield wipers, radio aerial, rear-view mirror, air scoop, roof pillars, rain gutters, all contribute to the total interference drag.
• The various component projecting under the vehicle, such as engine pan, the suspension
arms, exhaust system and rear suspension also contribute to the interference drag.
• It is about 17% of the total drag.

Internal flow or Cooling and Ventilation drag:

 
• Internal Flow Drag is the sum of all energy losses produced when air passes into, through, and out of all vehicle systems requiring or permitting air flow.
• The engine cooling flow (which is the primary internal flow component) plus passenger
ventilation flow and any internal flow required to cool brakes or other mechanical components contribute to internal flow drag.
• It is about 12% of the total drag.


Wind Tunnel:

• Wind tunnels are used to simulate air flow in laboratories under controlled conditions. 
• These find vast application in automobile and aircraft industry to test the prototype for aerodynamic conditions specially drag force on the prototype.
•  School laboratories utilize them to study flow around small objects like a sphere or a wedge.
• Wind tunnels are designed for specific purposes. A wind tunnel designed for school lab is generally between 2m to 12 meters long while the wind tunnels in automobile are large enough to accommodate whole car in its test section.These days wind tunnels are being replaced by CFD tools.

Types of Wind Tunnel:

 Wind tunnels can be classified based on air flow speed in test section and based on shape.

• Based on Flow Speed: 

1. Subsonic or low speed wind tunnels
2. Transonic wind tunnels
3. Supersonic wind tunnels 
4. Hypersonic wind tunnels

• Based on Shape

1. Open circuit wind tunnel
2.  Suck-down tunnel
3.  Blower tunnel
4.  Closed circuit wind tunnel

Subsonic or low speed wind tunnels:

• Maximum flow speed in this type of wind tunnels can be 135m/s. 

• Flow speed in wind tunnels is generally preferred in terms of Mach number which comes out to be around 0.4 for this case.

•  This type of wind tunnels are most cost effective due to the simplicity of the design and low wind speed.

•  Generally low speed wind tunnels are found in schools and universities because of low  budget.

Transonic wind tunnels:

• Maximum velocity in test section of transonic wind tunnels can reach up-to speed of sound ie 340m/s or Mach number of 1.

•  These wind tunnels are very common in aircraft industry as most aircraft's operate around this speed.

Supersonic wind tunnels:

•  Velocity of air in test section of such wind tunnels can be upto Mach 5(1715m/s).

•  This is accomplished using convergent - divergent nozzles.

• Power requirements for such wind tunnels are very high.

Hypersonic wind tunnels:

• Wind velocity in test section of such type of wind tunnels can measure between Mach 5 and Mach 15 (18522Km/hr).

• This is also achieved using convergent - divergent nozzles.

Open circuit wind tunnel:

• This type of wind tunnel is open at both ends. The chances of dirt particles entering with air are more so more honeycombs (mesh to clean incoming air) are required to clean the air.

•  Open type wind tunnels can further be divided into two categories:

  a) Suck down tunnel:

• With the inlet open to atmosphere, axial fan or centrifugal blower is installed after test section. This type of wind tunnels are not preferred because incoming air enters with significant swirl.

b) Blower tunnel:

•  A blower is installed at the inlet of wind tunnel which throws the air into wind tunnel. swirl is a problem in this case as well but blower tunnels are much less sensitive to it.

Closed circuit wind tunnel:

•  Outlet of such wind tunnel is connected to inlet so the same air circulates in the system in a regulated way.

•  The chances of dirt entering the system are also very low.

• closed wind tunnels have more uniform flow than open type. This is usually a choice for large wind tunnels as these are more costlier than open type wind tunnels.

AERODYNAMIC FORCES & MOMENTS:

AERODYNAMIC LIFT, YAW & PITCHING MOMENT:
 

• Vertical component of the resultant of the pressure distribution – Lift.
• Streamline body – higher velocity at the upper part & lower velocity below the vehicle.
• Aerodynamic lift is applied through the center of pressure of the body profile and, since this point does not correspond with the centre of gravity, it creates a pitching movement about the lateral axis.
• Influence of force Px on Pitching moment is usually small, as the vertical separation between CG & CP is not great.

• Both Lift & Pitching moment have undesirable effects.

• Lift tend to reduce the pressure between wheels and ground.

               1. Loss of steering on the front wheels
               2. Loss of traction on the rear axle.

• Pitching moment is usually negative i.e nose down.

• Lift force on the rear axle lifts the rear axle off the ground which leads to further loss of traction.

 

PX = Force of air drag in the direction of motion with wind along longitudinal axis.
PY = Side wind force or Cross-wind force.
PZ = Lift force.
MX = Rolling moment about longitudinal axis caused by the force PY.
MY = Pitching moment about lateral axis caused by force PZ.
MZ = Yawing moment about vertical axis caused by the force PY.

EFFECT OF FAIRING:

 
• If a fairing is used to cover the cockpit there will be an increase in both lift and pitching  moment.
• However if a fairing is not used there will be an advantageous effect on lift and pitching movement but increase in the drag coefficient CX  With the fairing fitted, the large area of negative pressure is toward the rear of the car.
• It is this negative pressure which causes the increase in lift and negative pitching moment.

                                            


EFFECT OF CROSS WIND:

 

 

• It indicating that the lift coefficient increases parabolically with the increase in the wind angle, up to two or three times its value when there is no side wind.

EFFECT OF VEHICLE PROFILE:

• Three box construction has the greatest spread of lift coefficients (from 0.4 to 1.0)
• Flat fronted type of vehicle has the smallest range (0.15 to 0.55)

• Saloon cars can reach a value of 100 kg, or 8 to 10 per cent of the total weight.
• Sports or racing cars the lift can reach values of 130 kg, which is 15 to 25 per cent of the
total weight.

SIDE FORCE, YAWING MOMENT AND ROLLING MOMENT:

• Side force is formed by asymmetric flow round the vehicle body when the wind angle is not equal to zero.This force acts at CP & creates moment about CG-Yawing moment about Z-axis.

FIG 1

FIG 2

• It try to turn the vehicle away from the direction of motion-loss of directional control.

• Fig 1 and 2 shows the pressure distribution for two wind angles around two different vehicles, measured at a constant height from the ground.The diagrams clearly shows that the maximum change in pressure occurs at the front and rear of the vehicle(causing large changes in the yawing moment)

• Values of the cross wind coefficient CY are shows in above fig 3. for different vehicle body shapes.The lowest values of the cross wind force coefficient are obtained with the streamlined bodies of sports cars.
•  Saloons, vans, the pseudo-aerodynamic vehicles(styles with fore and aft projection wings).
•  And lastly, old models with box-like bodies have increasingly large values.

Fig 4

• The above fig 4 gives values of Cmz Obtained from wind tunnel tests. It is worthy of note that vehicle bodies with good aerodynamic direct force coefficients (low values of Cx CY and CZ ) have the largest values of Cmz.
• The vehicle with poor aerodynamic properties will have a better( that is, lower) yawing moment coefficient.
•  A side wind will produce a yawing moment tending to turn the vehicle away from the direction of the motion.

•  The use of stabilisers or fins at the rear of the vehicle gives very good results.Without a stabiliser (curve a) thevehicle is unstable.
• Simple stabiliser (curve b) reduces the yawing moment coefficient and, at large crosswind angles, actually provides a stable condition.
• The center of the aerodynamic forces is usually above the center of gravity so that the
  cross wind force PY will produce a rolling moment Mx about longitudinal axis.
• Rolling moment generated by cross winds has a sizeable effect on the weight distribution on the wheels.

FIG 3

• Fig 3 shows that the wheel load on the same axle can vary by up to 100kg. This effect is dangerous for coaches and particularly for tall vans, where the side force acts a long way above the center of gravity. The only real solution here is an increase in wheel track

VARIOUS OPTIMIZATION TECHNIQUE FOR MINIMUM DRAG:

• Modification

           • Modification of Fore body
           • Modification of Windshield
           • Modification of Roof
           • Modification of Vehicle rear end

•  Add on device
• General Improvements

Modification of Fore body:

• Frontal shape is designated as “forebody”. 
• A small correction on front edge alone reduced the drag marginally. 

A: Air dam, B: hood line, C & D: Pillars, E: Spoilers

• The air dam area encircled as ‘A’ in the figure is the frontal area establishing stream lines, particularly in underbody and wheel areas. 
• A cleverly designed air dam at the front of the vehicle reduces the requirements of ground clearance and limits the volumes of air passing under the body.
• The primary objective of modifications are to reduce pressure of air stream under the vehicle body. This greatly reduce the vortices and induced drag.

B: Hood line

• It is the leading edge which disturbs the air stream and influence the profile drag. The hood line and the shape of hood should guide the streams without discontinuity over the windscreen.The formation of vortices should also be avoided in this region.

Modification of Windshield:

• Flow separation occurs at the Cowl.
• Reattachment occurs at windshield.
• Point of separation S is displaced towards front.
• Point of reattachment towards rear.
• If windshield angle γ becomes steeper.

Figure show the measurement made on research automobiles.

C & D: Pillars

• As shown in the figure the shapes of the two extreme pillars can be effectively applied for optimization of drag. By having a slight convex profile from front to rear pillar the discontinuity can be eliminated and with it the associated flow separation. The perfectly smooth profile from windscreen to the pillars would be compromised to some extent at the junctions of the glass and the surrounding frames.
• Direct influence of Windshield inclination on drag is only moderate.
• Windshield inclination of more than 60deg are not practical because of light diffusion.
• Increased solar heating of the passenger compartment.
• The inclination of the engine hood also has an effect upon the drag.
• Once the slope is steep enough to keep the flow attached, further sloping does not reduce drag any further.
• The optimum slope angle α depends on the leading edge radius and on the windscreen rake.

Modification of Roof:

• Roofs are designed with convex shape to ensure rigidity for stylistic reasons it is kept minimum.
• Increased convexity reduce drag co-eff.However frontal area is increased which increases drag.
• However original roof is kept constant, front & rear windows must be curved into the roof for reducing drag co-eff.

Modification of Vehicle rear end:

• Pressure recovery is obtained by tapering the bottom upwards.
• Long diffuser gives notable reduction with reduced angle β.
• However smooth underside must be assured.
• Lift at the rear axle is also reduced by long diffuser.

ADD ON DEVICE:

• When there is a gentle rear end body profile curvature change, it will be accompanied with a relatively fast but smooth streamline air flow over this region which does not separate from the upper surface. This results in lower local pressures which tend to exert a lift force ( upward suction) at the rear end of the car.

• A lip or small aerofoil spoiler attached to the rear end of the car boot interrupts the smooth streamline air flow thereby raising the upper surface local air pressure which effectively increase the downward force known as negative lift..

• A typical relationship between rear lift, front lift and drag coefficients relative to the spoiler lip height is shown in graph.

General Improvements:

1- Front spoiler
2- Ducted engine cooling
3- Shrouded windshield wiper arms
4- Aerodynamic mirrors
5- Smooth windshield transitions
6- Smooth side window transitions
7- Smooth rear window transition
8- Optimized trunk corner radii
9- Optimized lower rear panel
10 - Smooth fuel tank and underbody
11- Optimized rocker panels
12- Flush wheel covers
13- Elimination of the rain gutter

• The spoilers and air foils on the rear check may serve several purposes. The rear spoilers, which is attached either to the rear of the roof or the upper edge of the rear wings, has the effect of increasing the pressure acting on the rear deck area. 

• This increase in pressure acting on the rear deck creates a down force at the most advantageous point as shown in the figure. 

• The spoilers may also serve to stabilize the vortices in the separation flow, thus reducing the aerodynamic buffeting.

Flow visualization:

Because air is transparent it is difficult to directly observe the air movement itself.Instead, multiple methods of both quantitative and qualitative flow visualization methods have been developed for testing in a wind tunnel.

Qualitative methods

•  Smoke
•  Tufts

Tufts are applied to a model and remain attached during testing. Tufts can be used to
gauge air flow patterns and flow separation.

 

Compilation of images taken during an alpha run starting at 0 degrees alpha ranging to 26 degrees alpha. Images taken at the Kirsten Wind Tunnel using fluorescent mini-tufts. Notice how separation starts at the outboard wing and progresses inward. Notice also how there is delayed separation aft of the nacelle.

 

Fluorescent mini-tufts attached to a wing in the Kirsten Wind Tunnel showing air flow direction and separation. Angle of attack ~ 12 degrees, speed ~120 Mph.

 Evaporating suspensions

• Evaporating suspensions are simply a mixture of some sort or fine powder, talc, or clay mixed into a liquid with a low latent heat of evaporation. When the wind is turned on the liquid quickly evaporates leaving behind the clay in a pattern characteristic of the air flow.

 

China clay on a wing in the Kirsten Wind Tunnel showing reverse and span-wise flow.

Oil

When oil is applied to the model surface it can clearly show the transition from laminar to
turbulent flow as well as flow separation.

 

Oil flow on straight wing in the Kirsten Wind Tunnel. Trip dots can be seen near the leading edge.

 Sublimation

• If the air movement in the tunnel is sufficiently non-turbulent, a particle stream released into the airflow will not break up as the air moves along, but stay together as a sharp thin line. Multiple particle streams released from a grid of many nozzles can provide a dynamic three-dimensional shape of the airflow around a body. As with the force balance, these injection pipes and nozzles need to be shaped in a manner that minimizes the introduction of turbulent airflow into the air-stream.

• High-speed turbulence and vortices can be difficult to see directly, but strobe lights and film cameras or high-speed digital cameras can help to capture events that are a blur to the naked eye. High-speed cameras are also required when the subject of the test is itself moving at high speed, such as an airplane propeller. The camera can capture stop-motion images of how the blade cuts through the particulate streams and how vortices are generated along the trailing edges of the moving blade.

Measurement of aerodynamic forces:

Ways that air velocity and pressures are measured in wind tunnels:

⦁ Air velocity through the test section is determined by Bernoulli's principle. Measurement of the dynamic pressure, the static pressure, and (for compressible flow only) the temperature rise in the airflow
⦁ Direction of airflow around a model can be determined by tufts of yarn attached to the aerodynamic surfaces 

⦁ Direction of airflow approaching an aerodynamic surface can be visualized by mounting threads in the airflow ahead of and aft of the test model
⦁ Dye, smoke, or bubbles of liquid can be introduced into the airflow upstream of the test model, and their path around the model can be photographed (see particle image velocimetry)
pressures on the test model are usually measured with beam balances, connected to the test model with beams or strings or cables
⦁ Pressure distributions across the test model have historically been measured by drilling many small holes along the airflow path, and using multi-tube manometers to measure the pressure at each hole
⦁ Pressure distributions can more conveniently be measured by the use of pressure sensitive
paint, in which higher local pressure is indicated by lowered fluorescence of the paint at that point
⦁ Pressure distributions can also be conveniently measured by the use of pressuresensitive
pressure belts, a recent development in which multiple ultraminiaturized pressure sensor modules are integrated into a flexible strip. The strip is attached to the aerodynamic surface with tape, and it sends signals depicting the pressure distribution along its surface.
⦁ Pressure distributions on a test model can also be determined by performing a wake survey, in which either a single pitot tube is used to obtain multiple readings downstream of the test model, or a multiple-tube manometer is mounted downstream and all its readings are taken (often by photograph).

How it works

Six-element external balance below the Kirsten Wind Tunnel

• Air is blown or sucked through a duct equipped with a viewing port and instrumentation where models or geometrical shapes are mounted for study. Typically the air is moved through the tunnel using a series of fans. For very large wind tunnels several meters in diameter, a single large fan is not practical, and so instead an array of multiple fans are used in parallel to provide sufficient airflow. Due to the sheer volume and speed of airmovement required, the fans may be powered by stationary turbofan engines rather than electric motors.

• The airflow created by the fans that is entering the tunnel is itself highly turbulent due to the fan blade motion (when the fan is blowing air into the test section - when it is sucking air out of the test section downstream, the fan-blade turbulence is not a factor), and so is not directly useful for accurate measurements. The air moving through the tunnel needs to be relatively turbulence-free and laminar. To correct this problem, closely-spaced vertical and horizontal air vanes are used to smooth out the turbulent airflow before reaching the subject of the testing.

• Due to the effects of viscosity, the cross-section of a wind tunnel is typically circular rather than square, because there will be greater flow constriction in the corners of a square tunnel that can make the flow turbulent. A circular tunnel provides a smoother flow.

• The inside facing of the tunnel is typically as smooth as possible, to reduce surface drag and turbulence that could impact the accuracy of the testing. Even smooth walls induce some drag into the airflow, and so the object being tested is usually kept near the center of the tunnel, with an empty buffer zone between the object and the tunnel walls. There are correction factors to relate wind tunnel test results to open-air results.

• Lighting is usually recessed into the circular walls of the tunnel and shines in through windows. If the light were mounted on the inside surface of the tunnel in a conventional manner, the light bulb would generate turbulence as the air blows around it. Similarly, observation is usually done through transparent portholes into the tunnel. Rather than simply being flat discs, these lighting and observation windows may be curved to match the cross-section of the tunnel and further reduce turbulence around the window. 

• Various techniques are used to study the actual airflow around the geometry and compare it with theoretical results, which must also take into account the Reynolds number and Mach number for the regime of operation.

Pressure measurements

• Pressure across the surfaces of the model can be measured if the model includes pressure taps. This can be useful for pressure-dominated phenomena, but this only accounts for normal forces on the body.

Force and moment measurements

• A typical lift coefficient versus angle of attack curve.With the model mounted on a force balance, one can measure lift, drag, lateral forces, yaw, roll, and pitching moments over a range of angle of attack. This allows one to produce common curves such as lift coefficient versus angle of attack (shown).

• Note that the force balance itself creates drag and potential turbulence that will affect the
  model and introduce errors into the measurements. The supporting structures are
  therefore typically smoothly shaped to minimize turbulence.