Turbulent flow — which force dominates over viscosity in defining turbulence?

Introduction / Context:
In fluid mechanics, the nature of flow (laminar vs. turbulent) is determined by the balance of forces acting on the fluid. Turbulence is characterized by chaotic eddies and strong mixing caused by inertial effects being much larger than viscous damping.

Given Data / Assumptions:

• Incompressible Newtonian fluid for conceptual discussion.
• Characteristic velocity U, length scale L, and kinematic viscosity ν.
• High Reynolds number regime (Re = U * L / ν ≫ 1).

Concept / Approach:
The Reynolds number represents the ratio of inertia to viscous forces: Re ∝ Inertia/Viscous. Turbulent flow occurs when inertia forces dominate viscous forces (high Re), leading to instability of laminar layers and energy cascade from large to small scales.

Step-by-Step Solution:

Verification / Alternative check:
Classic experiments (e.g., pipe flow) show transition around Re ≈ 2300 for smooth pipes; well above this, turbulence appears as inertial effects overpower viscous smoothing.

Why Other Options Are Wrong:

• Elastic and surface tension forces are relevant in compressible and capillary phenomena respectively, not the primary cause of turbulence.
• Viscous force dominates in laminar (low Re) flows.

Common Pitfalls:
Confusing the trigger of turbulence (instability) with effects such as pressure fluctuations; also misstating that viscosity is irrelevant—viscosity still dissipates energy but does not dominate the dynamics in turbulence.

Final Answer:
Inertia force