Physics of Drag 2
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Studying Drag

  • Force of Drag

  • Horsepower vs Highway speed

  • Reynolds Number

 

It is important to define the equation of the force of drag in order to understand it.  Cd is defined as follows for road vehicles (Bib. #8):

 

 

Where        D = Aerodynamic drag force D(V, ρ, L, μ)

                    L = vehicle size

                    μ = air property

                    ρ = density of air

                    V = velocity of the vehicle

                    Af = the surface area the air hits on the vehicle

Now that Cd is defined we can use it to graph vehicle horsepower vs highway speed and be able to interpret the results.  See below. (Bib. #3)

We now need to study the effect of the Reynolds number. 

Reynolds Number

 

lRecall the definition of Reynolds number.  –Reynolds number is a comparison of inertial and frictional forces. (Bib. #3)

–

U = Free Stream Velocity

 

–L = Characteristic Length or the square root of the surface area (of the surface the air is hitting)

 

–n = viscosity

Now that we have defined the Reynolds number we can use it to study drag.

Physics of Drag 3, back