L2: Wind Modelling II Flashcards
What kind of wind speed variations are there?
Temporal and spatial
What does wind shear cause?
Wind speed variations, from 0 at ground level to undisturbed wind speed at reference height
What is the equation for the log wind speed profile law?
U(z) = u/k(a) * ln(z/z(0))
Where U is wind speed at desired height z, k(a) is the Von Karman constant, z(0) is the terrain roughness parameter, u is the friction velocity
Give the definition for u* (friction velocity)
u* = sqrt(k) * U(10) Where U(10) is the wind speed at H = 10m and k is the surface friction coefficient
What is the equation for the power law profile?
U(z)/U(zr) = (z/z(r))^alpha Where z(r) is reference height, U(z(r)) is reference speed, alpha is the power law exponent
What does an anemometer record?
A wind speed U(i) every delta t seconds, and collects N samples
What does a probability density function for wind show?
The probability that the wind speed u will be between u - deltau and u + deltau
How is the cumulative probability density function F(u) calculated?
Integrating f(u) between 0 and ~^u to derive the probability that u is equal to or less than ~^u
What are the names of the two common PDFs used to model wind speed distributions?
Weibull and Rayleigh
What do k and c represent in the Weibull distribution? How is the Rayleigh distribution obtained from this?
k = shape parameter
c = scale parameter
Rayleigh: when k = 2
How can the shape parameter k be estimated?
From the mean and variance of the wind speed measured at the site
k = (sqrt(variance)/mean) ^ -1.086
Describe the difference between upwind and downwind HAWTs
Upwind: blades face wind ahead of main nacelle body
Downwind: main nacelle body faces wind ahead of blades
What are the 3 turbine configurations?
Horizontal axis, vertical axis, airborne
What is the main advantage of HAWTs?
They tend to be more aerodynamically efficient
What is the main advantage of VAWTs?
They tend to be more mechanically simple
