Wind Flashcards
What is the concept of atmospheric boundary layer (ABL)?
As the wind approached the Earth’s surface, frictional forces caused by the terrain become more important. Friction makes wind a turbulent flow that varies in a random way both in space and time. The height where the surface effects are zero is called gradient height. The lowest part of the atmosphere, below the gradient height, is named ABL and typically extends up to 500-1000m
What are the general characteristics of wind close to the ground?
From a wind time history, we can notice that wind varies in an irregular pattern ,revealing the turbulent nature of the wind. We can identify a change in the typical (mean) value of the velocity over the day duration. Wind speed varies in a limited range over the mean value: This is measurable and it is characteristic of tubulence. Fluctuations cover a wide range of frequencies.
The average wind velocity increased with the height from the ground. There is similarity between pattern though, revealing the spatial correlation of the wind turbulence.
What is Taylor’s hypothesis?
Turbulence can be visualized as consisting of irregular swirls of motion called eddies. Taylor suggested that the turbulence can be considered to be frozen. Mean wind is used to translate turbulence measurement as a function of time to their corresponding measurement in space. T = L/U
Describe the long term wind spectrum (Van der Hoven)
The wind spectrum gives an idea of the frequency distribution of the fluctuations. The low frequency energy is mainyl around periods of 4-5 days, which corresponds to movements of large pressure systems (wind climate) and is called macro-metrological peak. The next peak, at 24 hours, shows the diurnal increase of wind during day and decrease at night. The high frequency fluctuations are associated with atmospheric turbulent luctuations and have a peak at a period of about 1min. The amount of energy of the high frequency hump depends on the mean wind speed.
The spectral gap means that the wind climate and the turbulence are mutually independent, so they can be treated independently and superimposed.
What does ABL depend mostly on?
It depends on the effects of the surface roughness. The mean wind velocity depends on the type of roughness elements on the ground Frictional drag causes the wind speed to become zero close to the ground.
Describe roughness length.
The surface roughness can be characterized by the aerodynamic roughness length parameter z0. Higher roughness elements -> larger aerodynamic roughness lengths. Physically, z0 is the size of the vortex that is created from the interaction of wind/ground.
What is the logarithmic profile?
By using Buckingham Pi Theory, it can be obtained that U(z) = u/k ln(z/z0), where k is the von Karmans constant and u = sqrt(τ0/ρ) is the friction velocity.
By using a reference wind speed measured close to the ground:
U(z) = Vbkrln(z/z0) when zmin<z<200
U(z) = U(zmin) z<zmin , where kr is the terrain factor that depends on the roughness length
What is the power law?
Another practical profile used in wind engineering is the power law:
U(z) = Uref(z/zref)^a, where a changes with terrain roughness and subsequently roughnless length
What about change in terrain roughness?
The ABL is developed after about 50-100km of uniform roughness. If the wind profile doesnt fully develop, numerical models should be employed.
What are some effects on ABL?
1) Directionality effect
Different ABL, depending on the direction of wind.
2) Topography
Topography is a large displacement of the ground surface which accelerated, decelarates or separates the flow, modifying the velocity profile.
What is Kolmogorovs similarity theory?
A low frequency fluctuation implies a large eddy and a small frequency fluctuation implies a small eddy.
1) A production range at low frequency where turbulence is generated as large eddies
2) An inertial range over the middle range frequency where large eddies break up transferring their energy to smaller scales; then smaller and smaller ones form
3) A dissipation range where eddies are very small so that viscosity is important and energy is dissipated by heat.
What are the parameters needed to characterize the atmospheric turbulence?
1) Single point analysis (time)
- Turbulence Intensity (energy)
- Peak estimation (gust factor)
- Power spectral distribution of wind velocity flucuation (energy)
- Time scales and integral length scales (vortex size)
2) Multi points analysis (space)
- Cross spectreal density
- Integral length scales
What is turbulence intensity?
Turbulence intensity is defined as the ratio between the standard deviation and the mean velocity, both at height z. Turbulence intensity is related to the surface roughness z0.
What is wind spatial correlation.
Peak forces on structures due to wind gusts strongly depends on the wind spatial correlation.
Autocorrelation or autocovariance, describes the general dependancy of x(t) with its value at a short time later x(t+τ);
What are integral length scales?
Integral length scale are a measure of the sized of the vortices in the wind, or in other words the average size of gust in a given direction. It is defiens through the spatial correlation function and its units are [m]
Lu = Tu(z)U(z), where Tu(z) is equal with thecharacteristic time of memory of the component u.
Lu depends on the height above the ground and on roughness length z0.
