written answer Flashcards
Explain the effect of a high-pressure system on the weather in a short sentence .
A high-pressure system is where the air ‘sinks’, the wind rotates in an anticlockwise direction (in the southern hemisphere) and the associated weather is generally calm and settled.
Explain the effect of a low-pressure system on the weather in a short sentence .
A low-pressure system is where the air rises, the wind rotates in a clockwise direction (in the southern hemisphere). They are often associated with rain and clouds.
Explain the effect of a cold front on the weather in a short sentence .
In a cold front, a cold air mass replaces a warm air mass and the air behind it is colder than
ahead of it.
Explain the effect of a trough on the weather in a short sentence .
A trough is an extended area of low atmospheric pressure.
Explain the effect of an occluded cold front on the weather in a short sentence .
An occluded cold front is where a cold front takes over a warm front often resulting in cyclones or thunderstorms.
Explain the features and implications
associated with a 1 year timescale.
1 year timescale is associated with long term weather patterns and climate, macro-meteorological effects such as el nino or la nina. These would affect a wind farm operator as la nina can be associated with less wind.
Explain the weather features and implications
associated with a 4 days timescale.
4 day timescale is associated with movement of high and low pressure systems. Low pressure systems can sometime cause cyclones which would have adverse affects for wind farms.
Explain the weather features and implications
associated with a 1 day timescale.
1 day timescale is associated with land and sea breezes, a wind farm operator could adjust his turbines to account for the days wind patterns.
Explain the weather features and implications
associated with a 1 minute timescale.
1 minute timescale is associated with wind gusts. This feature is undesirable for wind farm operator as they can cause speed and energy fluctuations.
Explain what
“geostrophic balance’’ means with reference to the Navier Stokes equations. What are the three major assumptions required to
simplify the Navier Stokes equations during geostrophic balance?
Geostrophic balance refers to the balance of pressure-gradient force and Coriolis force on a synoptic or planetary scale. The wind that results from this balance is parallel to isobars. The three major assumptions required to simplify the Navier Stokes equations during geostrophic balance are:
- Zero acceleration
- Zero internal friction
- Velocity gradients are negligible
Explain the relationship between the spacing of the isobars and wind speed with
reference to the geostrophic balance.
The closer the isobars are together, the higher the wind speed. This is due to a greater pressure difference in a smaller amount of space. These wind speeds are directly proportional to pressure gradients.
Give two reasons why wind data available from a weather station should not be directly
used when predicting the wind resource for a nearby wind farm (justify your reasoning).
Wind data available from a weather station should not be used when predicting the wind resource for a wind
farm as the wind data is not normally collected at the same height (i.e. hub height) of a wind farm. Height
affects wind speeds greatly so the wind speeds at the different heights would be very different. Also, the land
roughness is not necessarily the same, i.e. the land near the weather station could be populated with houses
and trees whereas the wind farm could be on grasslands. The different roughness of the land affects the wind
speeds, more densely populated areas have a higher ‘roughness’ and thus slow the wind down.
Comment on the implications of any differences for predicting the wind power
potential.
The wind power potential can vary due to different terrain types, and thus different surface roughness. The accuracy of scale factor c can alter the wind power potential prediction, the assumed air density can also affect the result. Air density is normally assumed to be 1.2 kg/m3 although it might not necessarily have been the case. Humidity and temperature can both affect the air density.
What angle is relevant for determining the lift and drag coefficients and why?
The angle relevant for determining the lift and drag coefficient is the angle of attack (𝛼). The angle of attack alters the way the turbine moves through air, thus affecting the magnitude of lift and drag.
What angle is relevant for determining the resulting thrust force dFn and dFt and why?
The angle relevant for determining the resulting thrust force dFn and dFt is the relative wind angle (𝜑).
The angle between the aerodynamic forces and normal and tangential directions, required to resolve them into the normal and tangential components.
