Froude number (open-channel and free-surface flows):\nFroude’s number compares inertia force to which other characteristic force?

Introduction / Context:
Dimensionless numbers encapsulate the competition between forces in fluid flow. Froude number Fr is central to free-surface flows such as rivers, spillways, and ship hydrodynamics. Recognizing the force pairing behind each number helps decide dynamic similarity and interpret scale models correctly.

Given Data / Assumptions:

• Characteristic velocity V and length L (e.g., depth).
• Gravitational acceleration g acts on the free surface.
• Inertia force represents ρ * L^2 * V^2 scaling.

Concept / Approach:
Froude number is defined as Fr = V / sqrt(g * L). Interpreting via force ratios, Fr^2 ∝ (inertia forces) / (gravity forces). When Fr < 1, the flow is subcritical; Fr = 1 marks critical; Fr > 1 indicates supercritical regimes with distinct wave propagation behavior. This controls hydraulic jump formation, specific energy relationships, and surface wave patterns.

Step-by-Step Solution:

Verification / Alternative check:
In spillway design, matching Fr between model and prototype ensures similar wave and jump behavior since gravity dominates surface dynamics; other numbers like Reynolds may be unmatched without breaking similarity critically for free-surface patterns.

Why Other Options Are Wrong:

• Pressure force: Leads to Euler number, not Froude.
• Elastic force: Associated with Mach number.
• Surface tension force: Associated with Weber number.

Common Pitfalls:
Confusing Froude with Reynolds (inertia/viscous) or Weber (inertia/surface tension); forgetting that appropriate characteristic length for Fr is usually depth in open-channel flow.

Final Answer:
gravity force