WISE Research Group

Wind engineering glossary

Key terms used in the study of wind and the built environment: aerodynamics, structural dynamics, and the meteorology and statistics of extreme winds.

Aerodynamics and fluid flow

17 terms.

Aerodynamics
The study of how air moves and of the forces it exerts on bodies in or near the flow.
Atmospheric boundary layer
The lowest part of the atmosphere, often on the order of a kilometer deep, where friction with the land or sea surface slows the wind and generates turbulence.
Blockage
Distortion of wind-tunnel measurements when the tunnel walls constrain the flow around a model, raising local velocities and altering measured forces and pressures.
Bluff body
A body whose shape makes the flow separate from its surface and leave a wide wake. Most buildings and many bridge decks are bluff bodies.
Boundary layer
The region of slowed flow next to a surface, whether the ground or the surface of a body.
Boundary-layer wind tunnel
A wind tunnel whose long upstream section of roughness elements and spires reproduces, at model scale, the mean wind profile and turbulence of the atmospheric boundary layer.
Computational fluid dynamics (CFD)
Numerical solution of the equations of fluid motion to simulate a flow and its interaction with surfaces. Common approaches include Reynolds-averaged Navier–Stokes (RANS) models and large-eddy simulation (LES).
Drag
The component of aerodynamic force in the direction of the mean wind.
Lift
The component of aerodynamic force perpendicular to the mean wind, often but not always vertical.
Open-jet testing
Testing a model in a free jet of air, without walls around the test section, which allows larger models at higher Reynolds numbers. This is the approach of the LSU WISE open-jet facility.
Pressure tap
A small hole in a model's surface, connected by tubing to a pressure sensor, used to measure surface pressures.
Reynolds number
The ratio of inertial to viscous forces in a flow, Re = UL/ν, where U is a velocity, L a length, and ν the kinematic viscosity. Differences between model and full-scale Reynolds numbers can change how flow separates, especially from curved surfaces.
Separation
Detachment of the boundary layer from a surface, typically at sharp edges or where pressure rises in the flow direction, producing eddies and a wake.
Strouhal number
The dimensionless vortex-shedding frequency, St = nD/U, where n is the shedding frequency, D a body dimension, and U the wind speed.
Turbulence
Random fluctuations of the wind velocity about its mean value. In wind engineering it is also called gustiness.
Vortex shedding
The periodic formation and release of vortices from alternate sides of a bluff body, which produces fluctuating cross-wind forces.
Wake
The region of slower, turbulent flow downstream of a body.

Structural response and dynamic effects

14 terms.

Aerodynamic damping
Aerodynamic forces that depend on the velocity of a moving structure and therefore add to, or subtract from, its structural damping.
Aeroelasticity
The study of the interaction among inertial, elastic, and aerodynamic forces on flexible structures.
Cauchy number
The ratio of elastic forces in a structure to inertial forces in the airflow. Aeroelastic model tests must match it.
Fluid–structure interaction (FSI)
Two-way coupling in which flow loads deform or move a structure and the structure's motion in turn changes the flow. Aeroelastic phenomena such as flutter and galloping are examples.
Flutter
An aeroelastic instability, usually involving torsion, in which aerodynamic forces feed energy into the motion so that it grows. It can involve one degree of freedom or coupled bending and torsion.
Galloping
A single-degree-of-freedom, cross-wind instability of flexible structures with certain cross sections, such as square, rectangular, or D-shaped sections. The Den Hartog criterion indicates whether a section is susceptible.
Gust response factor
The ratio of the expected peak structural response to the mean response.
Lock-in
Synchronization of vortex shedding with the motion of a vibrating body near its natural frequency, which strengthens the fluctuating forces.
Magneto-rheological (MR) damper
A semi-active damper filled with a fluid whose resistance changes within milliseconds when a magnetic field is applied, so its damping force can be adjusted in real time with little power.
Scruton number
A dimensionless mass–damping parameter that combines the ratio of structural mass to air mass with the structural damping. Low values indicate susceptibility to vortex-induced vibration.
Semi-active control
Vibration control that adjusts the properties of a device, such as a damper's force, in real time without adding energy to the structure, combining much of the adaptability of active control with the reliability of passive devices.
Smart structure
A structure with sensors, a controller, and adjustable devices or materials that let it change its response, such as its damping, as loads change.
Spectral density
The distribution of a fluctuating quantity's variance over frequency, showing how much each frequency band contributes.
Tuned mass damper
A mass attached to a structure through a spring and damper and tuned near one of the structure's natural frequencies, so that it moves out of phase with the structure and absorbs vibration energy.

Meteorological and statistical terms

19 terms.

Correlation
A statistical measure of how closely two fluctuating quantities vary together.
Froude number
The ratio of inertial forces in the flow to gravity forces. It matters in aeroelastic tests where gravity affects the response, as for cables and long-span bridges.
Gust factor
The ratio of the expected maximum to the mean value of wind speed, pressure, or force over a stated period.
Integral length scale
A measure of the average size of the turbulent eddies, or gusts, in the flow.
Jensen number
The ratio of a building dimension, usually its height, to the roughness length of the terrain. Model tests in boundary-layer flow aim to match it.
Logarithmic law
A description of how mean wind speed increases with height near the ground, in proportion to the logarithm of height divided by the roughness length.
Mean wind speed
The average wind speed over a stated period, such as one minute, ten minutes, or one hour. Its value depends on the averaging time.
Peak factor
The difference between the maximum and mean values of a fluctuating quantity, divided by its standard deviation.
Peak gust
The highest wind speed within a stated period.
Pressure coefficient
Surface pressure divided by the dynamic pressure of a reference wind speed, which makes it dimensionless.
Return period
The average time between occurrences of an extreme value; the inverse of its annual probability of exceedance.
Roughness length
A measure of the aerodynamic roughness of the terrain, which governs the shape of the wind profile above it.
Saffir–Simpson scale
A one-to-five hurricane category scale based on maximum sustained wind speed, defined as a one-minute average at 10 m height.
Stationary
Describes a random process whose statistical properties do not change over time.
Three-second gust
The peak wind speed averaged over three seconds. Design wind speeds in the United States (ASCE 7) are specified on this basis.
Thunderstorm
A convective storm driven by thermal instability, capable of strong downdraft winds.
Tornado
An intense, rotating column of air that extends from a thunderstorm to the ground, with very high winds around a narrow vortex core.
Tropical cyclone
A rotating storm that forms over warm tropical oceans; the general term for hurricanes and typhoons.
Wind axes
Coordinate axes aligned with, and perpendicular to, the mean wind direction.

References

For fuller treatment of these terms.