13-11-2012, 05:53 PM
Basics of vehicle aerodynamics
Vehicleaerodyn.pdf (Size: 6.2 MB / Downloads: 178)
Influence of flow characteristics on the
operation of vehicles
Objectives of improvement of flow past vehicle bodies:
• reduction of fuel consumption
• more favourable comfort characteristics (mud
deposition on body, noise, ventilating and cooling of
passenger compartment)
• improvement of driving characteristics (stability,
handling, traffic safety)
Vehicle aerodynamics includes three interacting flow fields:
• flow past vehicle body
• flow past vehicle components (wheels, heat exchanger,
brakes, windshield),
• flow in passenger compartment
Characteristics of flow past vehicle bodies
• Front: stagnation point, overpressure, accelerating flow
• Side walls, roof: boundary layer separation depending
on the rounding up of leading edges around the front.
• Rear wall: in separation bubble nearly constant pressure
below the ambient, strong turbulent mixing
• Underbody gap: surrounded by „rough” and moving
surfaces, decreasing velocities downstream, sideward
outflow
Cause of drag forces at streamlined
and bluff bodies
Streamlined bodies are
characterized by attached flow.
The share of pressure forces in
drag force (component of
aerodynamic force parallel to
undisturbed flow) is small. Drag is
caused mainly by shear stresses.
Since shear forces are small cD is
relatively small.
Bluff bodies are characterized by
boundary layer separation and
separation bubbles. Drag is caused
mainly by pressure forces, since
p-p0>> τ cD is relatively big.
Conclusion
Optimisation of vehicle bodies results in
• considerable reduction of fuel consumption
• improvement of comfort characteristics and
• more favourable driving characteristics of
ground vehicles.
In optimisation besides wind tunnel
investigations numerical simulation of flow
field has become more and more important.