Out of Thin Air Review Part 1

Driving Aerodynamic Forces

Lift and Drag

Newton’s Law of Motion

Objects with mass behave when subject to forces: Air has mass → aerodynamics forces can be explained by Newton’s principles

1. An object in at rest will stay at rest or an object moving at a speed will remain a constant speed unless some force acts upon them (∆V requires introducing force)

2. Force required to accelerate a body is proportional to the mass of the body being accelerated (more massive car req more force to accelerate, slow, corner) - reluctance to change aka inertia

3. Every force has an equal an opposite reactive force

Effect of lift generation on road Cars

Reduces effective weight felt by the tires → In turn reduces the amount of available grip

Cause of aerodynamic Forces

• Properties of Air

• Disturbance to air caused by vehicle movement through the air

Factors Affecting Air Mass Disturbance from a Car

1. Size of car (and shape)

2. Density of air

3. Speed of car through the air

Connecting Air Mass to Newton’s Laws

Air has mass and as car passes through air, air is accelerated and deflected around the car - Newton’s Laws say that the ∆V and ∆direction sets up force and reactions that will be experienced by the car

Importance of Density in Aerodynamics

Number of molecules in a given volume

• At a given air pressure and temperature → DENSITY is CONSTANT

Air is a gas and thus compressible (subject to significant changes in density)

• Air speeds in motorsports - air can be treated as incompressible (also as constant)

• Air still subject to density changes based on temp/atm pressure changes (weather, altitude)

Car Size Affect on Mass Disturbance

Larger car will disturb more air → produces greater reaction forces from air (as opposed to smaller car moving at same speed)

Car Speed Affect on Mass Disturbance

Car moving faster creates greater disturbance of the air → creates greater reaction forces by air

Principle of Relative Motion

Air moving over a stationary body is the same as a body moving through the air

• Commonly seen with wind tunnel analysis

(Cars move close to the ground - makes the two not entirely the same)

Viscosity

“Stickiness” of a fluid

• More viscous the fluid, the more force that is required to push a body through the fluid

• Viscosity resists a body’s motion through a fluid

Boundary Layer Interactions

Fluid often flows around body as if in thin layers (laminae)

• Because of viscosity: layer immediately adjacent to body surface remains attached to body

• Layers of fluid moving normal to surface are held back slightly by slower layer beneath them, until a distance away from surface where the static layer/viscous forces no longer felt (free stream - no relative motion between fluid layers)

Velocity Gradient

Characterizes the change in velocity of a flow moving from surface outward normally

• Shows viscous force affects forming the boundary layer

Analysis of Viscous Boundary Layer Effects

Relative motion between layers → shear forces between layers due to viscosity

• Viscous shear force analogous to friction forces between solid surfaces (resists motion of body relative to fluid passing through it)

• Viscosity steals KE (movement energy) from airflow near surface and converts it to heat (AIRFLOW LOSES ENERGY DUE TO VISCOSITY - loses to entropic form, uncapturable)

Mean paths/streamlines

Laminar vs. Turbulent

Can have average velocity of same magnitude and direction

• Turbulent: fluid particles trace out non-parallel erratic paths

Boundary Layer Transition

Small differences in velocity of the boundary sub-layers → layers slide over each other with little interaction → Gives laminar boundary layer

• As energy is removed from the flow (via viscous forces in BL) → flow begins to become more random → transitions into Turbulent BL

Boundary Layer Thickness

BL tends to thicken along a surface (as viscous effects remove energy of flow near surface)

• Non-parallel components of velocity of fluid particles w/in turbulent BL → increases thickness of BL, causes it to grow more rapidly

As flow loses energy, it becomes more turbulent (less parallel, organized), it has more fluid particles with velocity components that are normal, normal flow increases the size of the BL

Factors Afffecting Boundary Layers

Change of:

• Surface curvature

• Surface roughness

• Distance along body

• Speed?