Wind Engineering and Meteorology of Windstorms

What is Wind Engineering?

  • Wind engineering is defined as the rational treatment of interactions between wind in the atmospheric boundary layer and people and their works on the surface of the earth (Prof. Jack Cermak).

  • It is an interdisciplinary field drawing upon:

    • Structural Engineering: Dynamic response, load paths.

    • Meteorology: Atmospheric boundary layers, turbulence, storm dynamics.

    • Aerodynamics: Bluff-body aerodynamics, wind turbine aerodynamics.

    • Aeronautics: Aeroelasticity (e.g., flutter).

    • Fluid Mechanics: Computational fluid dynamics, Fluid-structure interactions.

    • Signal Processing: Frequency and time domain analysis.

    • Computing: Large-scale, parallel systems.

    • Experimentation: Wind tunnels, large-scale testing.

    • Statistics: Extreme value analysis, energy forecasting, climatology. And more.

Historical Context

  • Wind has been significant throughout history, evidenced by:

    • Anemoi (Greek mythology).

    • Njörðr (Viking mythology).

    • Fujin (Japanese Shintoism).

    • Vayu (Hinduism).

    • Fei Lian (Chinese wind spirit).

  • Wind utilization:

    • Transport.

    • Power.

The Beaufort Scale

  • The Beaufort scale estimates wind speed through observation, developed over thousands of years by sailors.

  • Examples:

    • Force 0: 0 Knots, Sea like a mirror, smoke rises vertically.

    • Force 1: 1-3 Knots, Ripples without foam crests, wind direction shown by smoke, 1-6 km/h.

    • Force 2: 4-6 Knots, Small wavelets, glassy crests, wind felt on face, leaves rustle, 7-11 km/h.

    • Force 3: 7-10 Knots, Large wavelets, crests begin to break, leaves and small twigs in motion, 12-19 km/h.

    • Force 4: 11-16 Knots, Small waves becoming longer, fairly frequent white horses, raises dust and loose paper, small branches moved, 20-29 km/h.

    • Force 5: 17-21 Knots, Moderate waves, pronounced long form, many white horses, small trees sway, wavelets form on inland waters, 30-39 km/h.

    • Force 6: 22-27 Knots, Large waves, white foam crests extensive, large branches in motion, whistling in wires, umbrellas difficult, 40-50 km/h.

    • Force 7: 28-33 Knots, Sea heaps up, white foam blows in streaks, whole trees in motion, umbrellas discarded, inconvenience when walking, 51-62 km/h.

    • Force 8: 34-40 Knots, Moderate high waves, crests break into spindrift, twigs break off trees, impedes progress, 63-75 km/h.

    • Force 9: 41-47 Knots, High waves, crests tumble and roll over, spray may affect visibility, slight structural damage, 76-87 km/h.

    • Force 10: 48-55 Knots, Very high waves, sea surface white, visibility affected, trees uprooted, considerable structural damage, 88-102 km/h.

    • Force 11: 56-63 Knots, Exceptionally high waves, sea covered in foam patches, very rarely experienced on land, widespread damage, 103-117 km/h.

    • Force 12: Over 63 Knots, Huge waves, air filled with foam and spray, sea white with driving spray, visibility seriously affected, countryside devastated, over 117 km/h.

Measuring the Wind

  • Early methods:

    • Hooke (17th C): Dynamic pressure anemometers.

    • Huet (1722): Pendulum (pressure plate) anemometers.

    • Dines (1892).

    • Da Vinci (16th C).

  • Modern instruments:

    • Robinson (1846): Cup anemometers.

    • Synchrotac (BOM).

    • Propeller anemometers.

    • Sonic anemometers.

  • Remote sensing:

    • TTU Ka band radar, Sandia Nat. Labs.

    • LiDAR.

    • RADAR.

    • Satellite (e.g., QuikSCAT).

Wind Engineering History

  • Tay Bridge Disaster (1879): Highlighted the importance of wind loading, leading to discovery of correlated loads.

  • First wind tunnels:

    • Kernot (1893) in Melbourne, Australia: Used for testing cubes, pyramids, cylinders, buildings.

    • Irminger (1894) in Copenhagen, Denmark.

Wind Tunnels

  • 1930s-40s at NPL: Full-scale pressure measurements on a shed compared with turbulent and uniform wind tunnel tests.

  • Key findings:

    • Pressure distribution varies greatly with turbulence and upwind characteristics.

    • Wind speeds (and pressures) increase rapidly with height.

    • Measured pressures are lower than in aeronautical wind tunnels.

  • Jensen (1950s, Denmark): Studied the impact of turbulence on flow separation and reattachment to buildings.

    • Model measurements in mm: model: 152, full scale: 1600, h/z0 = 170, Cp = 1.0

Dynamic Wind Loading

  • Examples of structures affected by dynamic wind loading include:

    • Tacoma Narrows Bridge.

    • Golden Gate Bridge.

Wind Engineering Post 1960

  • Key figures:

    • Alan Davenport.

    • Jack Cermak.

  • World Trade Center design was influenced by wind engineering principles.

The Alan G. Davenport Wind Loading Chain

  • Components:

    • Wind Climate → Terrain Effects → Aerodynamics → Dynamic Effects → Structural dynamics.

  • These components influence the wind load.

  • Criteria are applied to both wind and load.

Tools

  • Wind engineer's tools include:

    • Wind loading of structures.

      • Wind tunnels.

      • Codes and Standards.

      • Online databases.

      • Computational fluid dynamics.

    • Measuring the wind

      • Anemometers

      • Doppler radars/Lidars

      • Satellites

Meteorology of Windstorms

  • Readings: Holmes & Bekele - Chapter 1, Simiu & Yeo - Chapter 1.

Why Does the Wind Blow?

Basic Meteorology

  • Without Rotation: Cold, dry air descends at the North and South Poles, creating high-pressure zones. Warm, moist air rises at the Equator, creating a low-pressure zone.

  • With Rotation: The Earth's rotation causes the air circulation to break into cells:

    • Polar Cell: Descending cool, dry air at the poles.

    • Hadley Cell: Rising warm, moist air at the Equator, descending cool, dry air at subtropical regions.

    • Ferrel Cell: Located between the Hadley and Polar cells.

    • Intertropical Convergence Zone (ITCZ): Rising warm, moist air.

Global Winds

  • Equatorial (+/-10°):

    • Frequent thunderstorms and squalls.

    • Low wind speeds.

    • Little tendency for cyclonic systems to develop (due to Coriolis effect).

  • Tropical Region (10°- 30°):

    • Seasonal monsoon rains (not strong winds).

    • Sea surface temperatures and Coriolis forces are high enough for Tropical Cyclones.

  • Sub-Tropical Region (30°- 40°):

    • Central Australia, South Africa, Southern US, Russia, central South America.

    • High-pressure zone.

    • Thunderstorms are common (solar heating).

  • Temperate Region (40°- 60°):

    • Southern South America and Australia, New Zealand, Europe, North America, and Asia.

    • Unstable frontal systems develop with travelling lows.

    • Thunderstorms are common, and extratropical cyclones can occur.

  • Polar Region (60°- 90°):

    • Antarctic, Arctic, Scandinavia, Russia, Canada.

    • Dynamic pressure increased due to higher air density.

    • Strong steady katabatic winds occur on steep slopes.

Localized Weather

  • Synoptic gales (extratropical cyclones)

  • Frontal systems

  • Tropical cyclones

  • Thunderstorms

  • Tornadoes

  • Other winds

Windstorms of Interest to Wind Engineers

Synoptic Gales

  • Large-scale weather systems, also known as extratropical cyclones.

  • Defined by close spacing of isobars indicating a large pressure difference over a small distance.

Synoptic Gales - Structure

Wind - structure:

  • Wind speed and direction fluctuate.

  • Wind speed increases with height.

  • Turbulence decreases with height.

  • Large range of frequencies in the wind.

  • Correlation between locations exists.

  • All depend on type of storm!

Prandtl's Logarithmic Law (Valid <200-300 m)

  • U(z) = \frac{u*}{κ} \ln(\frac{z}{z0})

    • U(z) = Wind speed at height z

    • u_* = Friction velocity

    • z = height

    • z_0 = Aerodynamic roughness

    • κ = Von Karman constant

    • Friction velocity u* = \sqrt{\frac{τ0}{ρ}}

    • τ_0 = Surface shear stress

    • ρ = Fluid density

Frontal Systems

  • Cold Front: Cold air mass replaces a warm air mass, often resulting in thunderstorms and heavy precipitation.

Tropical Cyclones

Types:

  • Hurricanes

  • Typhoons

  • Tropical cyclones

    • tropical depression

    • tropical storm

  • Classified using the Saffir-Simpson Hurricane Scale (categories 1-5).

Tropical Cyclones - Structure

  • Key features:

    • Eye: central clear area with descending air.

    • Eyewall: ring of intense thunderstorms surrounding the eye.

    • Spiral rainbands: outer bands of thunderstorms.

  • Southern hemisphere rotation is opposite.

Tropical Cyclones - Wind Speed and Pressure

  • Relationship between radial distance (km), pressure (mb), and gradient wind speed (m/s).

Thunderstorms

  • Small scale (hundreds of meters to kilometers).

  • Short duration (minutes to hours).

  • Rapidly changing velocity and direction.

  • Require:

    • Lifting mechanism (surface heating, front, sea-breeze, topography).

    • Shear.

    • Moist air.

  • Associated phenomena:

    • Downbursts (outflow).

    • Tornadoes (inflow).

Structure of a Supercell Thunderstorm

  • Key components:

    • Rotating updraft core (mesocyclone)

    • Forward Flank Downdraft (FFD)

    • Rear Flank Downdraft (RFD)

    • Inflow jet

    • Gust fronts

    • Wall cloud

    • Anvil

Downbursts

  • Definition: outflow from a thunderstorm.

Downbursts - Structure

  • Velocity profile can peak below 150 m, then drop off rapidly.

  • Little full-scale evidence.

  • Rapid change in velocity means that mean velocities are less meaningful than in synoptic scale storms.

  • U(z) - ABL

  • U(z) - DB

Tornadoes

Tornadoes - Structure

Other Winds

  • Orographic Winds: Winds influenced by topography (e.g., Foehn, Chinook).

    • Rain shadow effect: dry air descends leeward side of mountains, promoting evaporation.

  • Katabatic Winds: Cold, dense air flows downhill due to gravity (e.g., Antarctic winds).

Summary

  • Wind engineers must consider different types of severe windstorms in the wind-resistant design of structures.

  • These windstorms have different structures (size, duration, wind profiles).

  • Problem: most wind-resistant design codes and standards do NOT account for different types of windstorm.