Air flows past a golf bail of 20 mm radius. It is observed that the bo...
Reynolds Number and Transition to Turbulent Flow:
The Reynolds number (Re) is a dimensionless quantity that characterizes the flow of a fluid. It is defined as the ratio of inertial forces to viscous forces and is given by the formula:
Re = (ρ * V * L) / μ
Where:
- ρ is the density of the fluid
- V is the velocity of the fluid
- L is a characteristic length of the flow (in this case, the diameter of the golf ball)
- μ is the dynamic viscosity of the fluid
When the Reynolds number is below a certain critical value, the flow is laminar, which means it is smooth and orderly. However, when the Reynolds number exceeds this critical value, the flow becomes turbulent, characterized by chaotic and unpredictable fluctuations.
Given Information:
- Radius of the golf ball (r) = 20 mm = 0.02 m
- Kinematic viscosity of air (ν) = 1.5 x 10^-5 m^2/s
- Critical Reynolds number for transition to turbulence (Re_critical) = 2 x 10^5
Calculating the Speed of Air:
To find the speed of air when the flow becomes turbulent, we need to rearrange the Reynolds number formula and solve for V:
Re = (ρ * V * L) / μ
V = (Re * μ) / (ρ * L)
Substituting the given values:
V = (2 x 10^5 * 1.5 x 10^-5) / (ρ * 0.04)
Now, we need to determine the density of air (ρ). At standard conditions (25°C and 1 atm), the density of air is approximately 1.184 kg/m^3.
V = (2 x 10^5 * 1.5 x 10^-5) / (1.184 * 0.04)
V ≈ 75 m/s
Therefore, the speed of air when the flow becomes turbulent is approximately 75 m/s, which corresponds to option C.