Question

Among the following non-dimensional numbers, which one characterizes periodicity present in a transient flow?

• A. Froude number
• B. Strouhal number
• C. Peclet number
• D. Lewis number

Hint:

If the question mentions {frequency} or {unsteadiness} in fluid flow, think $St=fL/U$. Gravity waves $\Rightarrow Fr$, heat/mass transport $\Rightarrow Pe$ or $Le$.

Answer 1

The Correct Option is B

Step 1: Recall the physical meaning of each number.
- Froude $Fr=\dfrac{U}{\sqrt{gL}}$ compares inertia to gravity (surface/gravity-wave problems).
- Strouhal $St=\dfrac{fL}{U}$ (or $\omega L/U$) is a {dimensionless frequency} quantifying unsteadiness/periodicity.
- Peclet $Pe=Re\,Pr=\dfrac{UL}{\alpha}$ compares advection to diffusion of heat (or mass).
- Lewis $Le=\dfrac{\alpha}{D}$ compares thermal to mass diffusivity.
Step 2: Match to “periodicity in transient flow.”
Only Strouhal number contains the oscillation frequency $f$ and measures periodic unsteadiness relative to convection. Final Answer: \[ \boxed{\text{Strouhal number (B)}} \]