# Wind Engineering Terminology

**Aerodynamic damping:**Aerodynamic forces proportional to the velocity of a structure, and additional to (or subtractional from) the structural damping.**Aerodynamics:**The study of the motion of air and its interaction with a solid object.**Aeroelasticity:**The study of the interactions among inertial, elastic and aerodynamic forces.**Atmospheric boundary layer:**The lowest layer of air in the troposphere which is about one km thick and forms the layer where the ground surface (land or sea) influences the behavior of the atmospheric flows.**Blockage effect:**Distortion effect of wind tunnel walls on measurements, particularly force and pressure measurements.**Bluff body:**Body with a large frontal dimension, from which the airflow separates.**Boundary layer:**Region of reduced air velocities near the ground or the surface of a body.**Boundary layer wind tunnel:**It is a testing section with flow characteristics that mimics a target full-scale boundary layer used in aerodynamic research to study the effects of wind on the built environment.**Cauchy number:**Ratio of internal forces in a structure to inertial forces in the air.**Computational fluid dynamics (CFD):**It is a branch of fluid mechanics that uses numerical analysis and algorithms to solve and analyze problems that involve fluid flows. Computers are used to perform the calculations required to simulate the interaction of fluids with surfaces defined by boundary conditions. With high-speed supercomputers, and capable turbulence closures, for instance, large eddy simulations (LES), better solutions can be achieved. Initial experimental validation of such software is performed using a wind tunnel with the final validation coming in full-scale testing, e.g. wind loadings on structures.**Correlation:**Statistical relationship between two fluctuating random variables.**Drag:**Along-wind force.**Flutter:**One-, or two-, degree-of-freedom aeroelastic instability, involving rotational motion.**Froude number:**Ratio of inertial forces in the air to gravity forces.**Galloping:**Single-degree-of-freedom aerodynamic instability found with flexible structures with special cross-sectional shapes; typically square and rectangular sections, or D-sections characteristic of ice-coated cables. There is a potential for galloping if (dCL/da + CD) < 0 in which CL and CD are the mean left and drag coefficients, and a = angle of attack in radian and dCL/da is the rate of change of the lift with angle of attack (Den Hartog criterion). The critical speed or the onset of galloping depends upon the magnitude of the structural damping.**Gust factor:**Ratio of expected maximum to mean value of wind speed, pressure or force.**Gust response factor:**Ratio of expected maximum to mean structural response.**Integral scale (L):**A characteristic length which represents a measure of the average size of turbulent eddies or gusts present in the atmospheric boundary layer. The integral scale of a particular component of turbulence corresponds to the product of the mean wind speed and the integral time scale of that component obtained from the integration of the autocorrelation function.**Jensen number:**Ratio of building dimension (usually height) to roughness length in atmospheric boundary-layer flow.**Lift:**Cross-wind force, usually but not necessarily, vertical.**Lock-in:**The enhancement of fluctuating forces produced by vortex shedding due to the motion of the vibrating body.**Logarithmic law:**A mathematical representation of the profile of mean velocity with height in the lower part of the atmospheric boundary layer.**Mean wind speed:**The average speed of wind flow expressed with a variety of averaging times: fastest mile, three-second gust, one minute mean velocity, and mean hourly velocity, typically measured at 10 m height.**Peak factor:**Ratio of maximum minus mean value, to standard deviation, for wind velocity, pressure, force or response.**Peak gust:**Maximum value of wind speed in a defined time period.**Pressure coefficient:**Surface pressure made non-dimensional by the dynamic pressure in the wind flow.**Return period:**Inverse of probability of exceedence of an extreme value.**Reynolds number:**Ratio of inertial forces to viscous forces in fluid flow.**Roughness length:**A measure of the aerodynamic roughness of a surface, which affects the boundary-layer flow over it.**Saffir-Simpson scale:**It is a one to five rating based on a hurricane's sustained wind speed (one-min average measured over water at ten meters height). Hurricanes reaching Category three and higher are considered major hurricanes, with a potential to bring a significant loss of life and damage to property. See more information from the National Hurricane Center.**Separation:**in bluff body aerodynamics, objects exposed to a moving fluid acquire a boundary layer of fluid around them; flow separation occurs when the boundary layer travels far enough against an adverse pressure gradient that the speed of the boundary layer falls almost to zero and the fluid flow becomes detached from the surface of the object and instead takes the forms of eddies and vortices.**Scruton number:**A non-dimensional parameter incorporating the ratio of structural mass to fluid mass, and structural damping, which is a measure of the propensity of a structure to resonant dynamic response.**Spectral density:**A measure of the contribution to a fluctuating quantity (e.g. wind velocity, wind pressure, deflection) within a defined frequency bandwidth.**Stationary:**Description of a random process whose statistical properties do not change with time.**Strouhal number:**Non-dimensional vortex-shedding frequency.**Three-second gust:**highest sustained gust over a three second period. In the United States the design wind speed is taken as the three-second gust, which has a probability of being exceeded per year of 2%.**Thunderstorm:**Thermally driven local storm capable of producing strong downdraft winds.**Tornado:**Local intense storm formed from thunderclouds, with intense winds rotating around a vortex structure.**Tropical cyclone:**An intense tropical storm which can occur over warm tropical oceans. A generic name which incorporates ‘hurricane' (used for Caribbean and northwest Atlantic storms) and ‘typhoon' (used in the north-west Pacific).**Turbulence:**Fluctuations in fluid flow. In meteorology and wind engineering the term ‘gustiness' is also used.**Vortex shedding:**The periodic shedding of eddies formed from the rolling-up of the boundary shed from a bluff body.**Wake:**The region of low velocity and turbulent flow in the region downstream of a body.**Wind axes:**Axes parallel and normal to the mean wind direction.

## References

- Holmes, D.J. (2015). Wind Loading of Structures. Third Edition. CRC Press.
- Simiu, E. and Scanlan, R.H. (1996), Wind Effects on Structures: Fundamentals and Applications to Design, John Wiley & Sons, Inc., New York.