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VEHICLE AERODYNAMICS

Optimization Technique.docx (Size: 97.86 KB / Downloads: 43)

This study concerns about the airflow around the vehicle body. At a speed of about 70 km/hr aerodynamic drag exceeds to 50% of total resistance to motion and above100 km/hr it is the most important factor.

Aerodynamic Drag

Form drag - 57%

Lift drag - 8%

Surface drag -10%

Interference drag -15%

Cooling and ventilation drag - 10%

Aerodynamic forces and moments

Forces

1. Lift force

2. Side force or cross wind force

3. Drag force

Moments

1. Rolling moment

2. Pitching moment

3. Yawing moment

It is the most useful tool to study the aerodynamic aspects of the vehicle. The

Various forces and moments can be evaluated for the vehicle by using scale models. The instrument used to measure the forces and moments is called a component balance.

Flow pattern can be obtained by using smoke method, Tuft or Oil coating methods. Advantages of wind tunnel test1.

Wind velocity and wind angle can be easily and accurately measured.

2. Flow pattern study can also be made accurately.

3. Forces and moments can be measured simultaneously.

4. Testing time and cost is less.

Form drag

Form drag, profile drag, or pressure drag, arises because of the form of the object. The general size and shape of the body is the most important factor in form drag - bodies with a larger apparent cross-section will have a higher drag than thinner bodies. Sleek designs, or designs that are streamlined and change cross-sectional area gradually are also critical for achieving minimum form drag. Form drag follows the drag equation, meaning that it rises with the square of speed, and thus becomes more important for high speed aircraft.

Profile drag (Pxp): depends on the longitudinal section of the body. A diligent choice of body profile is more than essential for low drag coefficient. Streamlines should be continuous and separation of the boundary layer with its attendant vortices should be avoided.

LIFT FORCE

Lift is produced by the changing direction of the flow around a wing. The change of direction results in a change of velocity (even if there is no speed change, just as seen in uniform circular motion), which is an acceleration. To change the direction of the flow therefore requires that a force be applied to the fluid; lift is simply the reaction force of the fluid acting on the wing.

When producing lift, air below the wing is generally at a higher pressure than atmospheric pressure, while air above the wing is generally at a lower than atmospheric pressure. On a wing of finite span, this pressure difference causes air to flow from the lower surface wing root, around the wingtip, towards the upper surface wing root. This spanwise flow of air combines with chordwise flowing air, causing a change in speed and direction, which twists the airflow and produces vortices along the wing trailing edge. The vortices created are unstable, and they quickly combine to produce wingtip vortices.[5] The resulting vortices change the speed and direction of the airflow behind the trailing edge, deflecting it downwards, and thus inducing downwash behind the wing.

Wingtip vortices modify the airflow around a wing. Compared to a wing of infinite span, vortices reduce the effectiveness of the wing to generate lift, thus requiring a higher angle of attack to compensate, which tilts the total aerodynamic force rearwards. The angular deflection is small and has little effect on the lift. However, there is an increase in the drag equal to the product of the lift force and the angle through which it is deflected. Since the deflection is itself a function of the lift, the additional drag is proportional to the square of the lift.[1]

X/WIND.

Crosswinds can also be a difficulty when traveling on wet or slippery roads (snow, ice, standing water, etc.), especially with gusting conditions and vehicles that have a large side area such as vans and SUV. This can be dangerous for motorists because of the possible lift force created as well as causing the vehicle to change direction of travel. The safest way for motorists to deal with crosswinds is by reducing their speed to reduce the effect of the lift force and to steer into the direction of the crosswind.

When winds are not parallel to or directly with/against the line of travel, the wind is said to have a crosswind component; that is it can be separated into two components, a crosswind component and a headwind or tailwind component. A vehicle behaves as though it is directly experiencing a crosswind in the magnitude of the crosswind component only.

The crosswind component is computed by multiplying the wind speed by the sine of the angle between the wind and the direction of travel. For example, a 10-knot wind coming at 45 degrees from either side will have a crosswind component of 10 knots × sin(45°) or approximately 7.07 knots. The headwind component is computed in the same manner, using cosine instead of sine. To determine the crosswind component in real world flight aviators frequently refer to a chart on which the wind speed and angle are plotted and the crosswind component is read from a reference line.

Interference drag

A characteristic that is dominant in bodies in transonic flow is the concept of interference drag. One can imagine two bodies of the aircraft (e.g. horizontal and vertical tail) that intersect at a particular point. Both bodies generate high supervelocities, possibly even supersonic. However, at the intersection there is less physical space for the flow to go and even higher supervelocities are generated resulting in much stronger local shock waves than would be expected if either one of the two bodies would be considered by itself. The stronger shock wave induces an increase in wave drag that is termed interference drag. Interference drag plays a role throughout the entire aircraft (e.g. nacelles, pylons, empennage) and its detrimental effect is always kept in mind by designers. Ideally, the pressure distributions on the intersecting bodies should complement each other’s pressure distribution. If one body locally displays a negative pressure coefficient, the intersecting body should have positive pressure coefficient. In reality, however, this is not always possible. Particular geometric characteristics on aircraft often show how designers have dealt with the issue of interference drag. A prime example is the wing-body fairing which smooths the sharp angle between the wing and the fuselage. Another example is the junction between the horizontal and vertical tail plain in a T-tail . Often, an additional fairing (acorn) is positioned to reduce the added supervelocities. The position of the nacelle with respect to the wing is a third example of how interference-drag considerations dominate this geometric feature. For nacelles that are positioned beneath the wing, the lateral and longitudinal distance from the wing is dominated by interference-drag considerations. If there is little lateral space available between the wing and the nacelle (because of ground clearance) the nacelle is usually positioned much more in front of the win