Physics · Class 11

Trace buoyancy-driven circulation

Heating from below makes lower fluid less dense, creating buoyancy. Viscosity and thermal diffusion oppose the instability. The circulation appears only when the Rayleigh number exceeds the ideal onset in this schematic.

Open the interactive experiment →

Trace buoyancy-driven circulation

கீழே சூடாக்கப்படும் திரவத்தின் அடர்த்தி குறைந்து மேலே எழும். குளிர்ந்த திரவம் கீழே வந்து வெப்பச் சலனத்தை உருவாக்கும்.

Key notes, relationships and model limits

Key relationship

Ra=gβΔTL³/(να); ideal onset Ra≈1708; buoyancy speed scale √(gβΔTL).

Remember

Common trap

The circulation illustration is not a numerical velocity solution.

Model limits

Qualitative circulation in a fluid heated from below. Rayleigh-number onset is for an ideal horizontal layer; speed is a dimensional buoyancy scale, not a computed fluid velocity. No CFD or turbulence is modeled.

What you will learn

Connect buoyancy, diffusion and geometry to convection onset.

Try the experiment

Try both scenarios, change one control, and predict the result before checking the graph and readouts.

Think of it this way

Change one input while keeping the others fixed, as you would in a controlled experiment.

Worked example

For β=0.001/K, ΔT=10 K, L=0.01 m, ν=10⁻⁵ and α=10⁻⁶ m²/s, Ra=9810, above the ideal 1708 threshold.

Why this matters

The model connects trace buoyancy-driven circulation with an observable reading. Check the equation and assumptions before applying it.

Before you start

SI units · algebra · graph reading

Check your understanding

What does increasing layer depth do to Rayleigh number with other inputs fixed?

It increases linearly with depth
It increases as the cube of depth
It decreases with depth

Read the explanation

Ra contains L³; doubling depth multiplies Ra by eight.

Course connection: Class XI Chapter 10. Consult the roadmap for CBSE, NEET, JEE Main and Advanced scope.