Glossary term
Turbulence
Irregular fluid motion dominated by fluctuations, mixing, and multi-scale vortical structures.
Definition
phenomenonTurbulence is irregular, three-dimensional fluid motion with fluctuating velocity, pressure, vorticity, and enhanced mixing across a wide range of scales.
Turbulence occurs when inertial effects dominate viscous damping enough for flow disturbances to grow, interact, and cascade from large eddies to small dissipative scales. It affects drag, lift, heat transfer, pressure loss, noise, combustion, mixing, sediment transport, weather, turbomachinery, and hydraulic systems.
Turbulence is not simply random motion. It has coherent structures, vortices, intermittent bursts, and a transfer of kinetic energy from large scales to smaller scales where viscosity dissipates it as heat. The instantaneous velocity is often decomposed into mean and fluctuating components:
Those fluctuations increase momentum, heat, and mass transfer compared with laminar flow, but they also increase drag, pressure drop, vibration, acoustic noise, and unsteady loading.
Engineering use
Turbulence affects pipe sizing, pumps, compressors, aircraft drag, heat exchangers, combustion chambers, wind-tunnel testing, building aerodynamics, river flows, and mixing equipment. It is often correlated through Reynolds number, roughness, boundary conditions, and empirical coefficients because direct resolution of all turbulent scales is too expensive for most engineering design.
Common modelling approaches include Reynolds-averaged Navier-Stokes models, large eddy simulation, direct numerical simulation, and empirical correlations. Each has different cost, assumptions, mesh requirements, and sensitivity to boundary conditions.
Model selection
The right turbulence treatment depends on the decision being made. A pressure-drop estimate in a long pipe may be served by a validated correlation, while separated aerodynamic flow, combustor mixing, or acoustic loading may require higher-fidelity simulation or testing. Wall treatment, inlet turbulence intensity, roughness, transition modelling, and validation data often matter more than the name of the turbulence model alone.
Common mistakes
A common mistake is calling any unsteady or noisy flow turbulent. Flow can be periodic, transitional, separated, cavitating, or externally forced without being fully turbulent. Another is trusting a CFD turbulence model without checking mesh near walls, inlet turbulence quantities, roughness, separation, validation data, and whether the model is suitable for the flow physics. A strong turbulence review states Reynolds number, geometry, roughness, boundary conditions, measurement bandwidth, turbulence intensity, model choice, and validation evidence.