Vehicle Aerodynamics | Reducing Drag On A Car

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What makes a vehicle aerodynamics?

Vehicle Aerodynamics is a potential area to minimize the fuel consumption. Because the air drag acting on the vehicle is directly proportional to the square of the vehicle speed. So by thoroughly studying and by providing necessary modification the drag force is reduced and there by the following advantages can be achieved.

01-car aerodynamics, vehicle aerodynamics, automotive aerodynamics

Advantages of Vehicle Aerodynamics

  • Minimum speed and acceleration for the same power output
  • Fuel consumption is reduced, about 35 % of the fuel cost can be reduced by proper stream lining
  • Good appearance
  • By reducing the various forces and moments, good stability and safety can be achieved

01-automotive aerodynamics, reducing drag on a car, aerodynamic drag car

Study of Vehicle Aerodynamics

The study of aerodynamics is carried out in two stages as follows:

  • Making a solid model of the car to suitable scale for wind tunnel testing for body exterior optimisation
  • By conducting a road test to check the drag co-efficient

01-aerodynamic drag car, automotive aerodynamics

Reynolds Model Law

Here the model is made according to the calculation done by the Reynolds model law:

If viscous forces alone are predominant, a model, may be taken to be dynamically similar to the prototype if the ratio of the inertial to the viscous forces is the same in the model and the prototype.

Re = ρVL/μ

The law in which models are based on Reynold’s Number is called as Reynold’s Model Law.

(Re)prototype  =  (Re)mode
ρp Vp LP / μp  =  ρm Vm Lm / μm
ρp Vp LP / ρm Vm Lm  . 1/ μp / μm = 1
ρr Vr Lr / μr = 1

Where scale ratio are
ρr = ρp / ρ’  Vr  =  Vp / Vm
Lr =  Lp / Lm,  μr = μp / μm

Now, Time scale ratio Tr = Lr / Vr

Velocity scale ratio Vr  =  Vr / Lr

Acceleration scale ratio ar  =  V

Force scale ratio
Fr  =  mr ar =  (ρr ArLr) ar
Fr =  (ρr L3r)ar

Discharge scale ratio
Qr =  ρr Ar Vr
Qr =  ρr Lr2 Vr

Road Vehicle Aerodynamics

The road test is carried out by Coast down method, where the vehicle speed is raised to a predetermined value and time elapsed for every 10 kmph deceleration is noted. Here the vehicle is driven on the narrow road without any disturbances such as braking, turning, gradient, obstacles etc. Now the recorded values are substituted in the formulae and drag co-efficient value is calculated. The test is conducted on the vehicle in two conditions.

  • Windows are kept opened
  • Windows are kept closed

In flow visualization the air blower is used to study the flow pattern around the model is photographed and the results were discussed. Finally, the suggestions are given for lowering the air drag.

01-Variation of drag with vehicle speed - characteristics curve of aerodynamics drag

01-resistance drag force vs vehicle speed - variation of drag force with vehicle speed

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