Drain from a Basin — Identify the Flow Type Water draining through a hole at the bottom of a wash basin commonly exhibits which type of flow pattern?

Introduction / Context: Household sinks and basins often show a whirlpool-like motion as water drains. Classifying this vortex helps connect everyday observations with fluid mechanics concepts used in hydraulic engineering and mixing processes.

Given Data / Assumptions:

• Draining through an orifice in a thin plate (bottom hole).
• No external device is forcing rotation (no agitator or rotating container).
• Viscous effects exist but the dominant rotation arises from circulation conservation.

Concept / Approach: In a free vortex, fluid rotates due to conservation of angular momentum with essentially zero external torque. Tangential velocity varies as vθ ∝ 1/r and the free surface forms a dip. In contrast, a forced vortex requires an external agency to enforce solid-body rotation (vθ ∝ r).

Step-by-Step Identification:

Verification / Alternative check: If the container were physically rotated, a nearly solid-body rotation (forced vortex) would appear, contradicting the observed 1/r behavior of a natural drain whirl.

Why Other Options Are Wrong: “Steady” and “uniform” describe time/space variation, not the rotational character; “forced vortex” needs external torque, which is absent here.

Common Pitfalls: Assuming any rotation is “forced”; ignoring that small asymmetries or residual swirl seed a free vortex upon draining.

Final Answer: Free vortex.