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We had a discussion regarding local and macroscale wake effects in Cyprus 5. that discussion's in the past, but thought it important to give a visual of what wake effects mean.

Vestas 2MW turbines at Horns Reef (Horns Rev) from 2004

"Life shrinks or expands in proportion to one's courage." - Ana´s Nin

by Crazy Horse on Wed Mar 27th, 2013 at 02:21:04 PM EST
Fab photo

keep to the Fen Causeway
by Helen (lareinagal at yahoo dot co dot uk) on Wed Mar 27th, 2013 at 02:28:11 PM EST
[ Parent ]
you can see the turbulence in the wakes like in a smoke tunnel

and the fact that there isn't much interaction between rows.

by stevesim on Wed Mar 27th, 2013 at 02:41:12 PM EST
[ Parent ]
The photo is a wonderful visualization of the turbulence associated with wind generators. Is there data on the output power of successive generators back from the first? That, along with wind speed, would be a useful way to quantify wake effects. Also, might it be possible to maintain an angle of attack into the wind by the generator heads? If so that would allow the wake effect to be distributed more widely through the array.

As the Dutch said while fighting the Spanish: "It is not necessary to have hope in order to persevere."
by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Mar 27th, 2013 at 02:39:42 PM EST
[ Parent ]
no, because the turbulence in the wake, maintaining an angle of attack is rather difficult

however, the wake does cause efficiency -  as exemplified in cars driving in the wake of a car in front of theirs to minimize drag and wind resistance

by stevesim on Wed Mar 27th, 2013 at 02:44:52 PM EST
[ Parent ]
I would think the race car analogy would be more an example of the detrimental effects of wake turbulence. It takes power to push the car through the air and that power, IIRC, is linear. In the case of race cars the following cars require less power for the same speed as the lead car does a disproportionate amount of the work of moving the air around the vehicle, setting up the wake in which others can follow. In the windmill example the blades use the wind energy to generate the lift to make them turn. Less total wind energy would be available to turn the second and third sets of blades due to losses to turbulence, rather like a competition yacht spoiling the wind of a competitor by turning in front of it.

What my brilliant possibility requires is either the ability to dither the wind direction back and forth by a few degrees or to rotate the array at a similar rate and I cannot imagine a feasible way to do that. I wrote before I thought.  

As the Dutch said while fighting the Spanish: "It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Mar 27th, 2013 at 03:34:54 PM EST
[ Parent ]
On reflection the 'angle of attack' seems improbable. :-)

As the Dutch said while fighting the Spanish: "It is not necessary to have hope in order to persevere."
by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Mar 27th, 2013 at 02:44:57 PM EST
[ Parent ]
I just reviewed my basic aerodynamics.

Reynolds number is the ratio of inertial forces to viscous forces

Laminar flow has a Reynolds number below 2000

Turbulent flow usually is somewhere above that because thtere is a transition state, it is not exactly at 2000

The Coefficient of Lift is defined as the lift force over the inertial forces so if the inertial forces are high (as in turbulent flow), the Coefficient of Lift is larger

The Coefficient of Drag on the other hand, is defned as the forces of drag over the viscous forces, so a lower Reynolds number (in laminar flow) would have more viscous forces and a higher Coefficient of Drag.

Therefore, turbulent flow should provide a more efficient ratio of Lift to Drag.

by stevesim on Wed Mar 27th, 2013 at 03:16:15 PM EST
[ Parent ]

Is there data on the output power of successive generators back from the first?

Yes, there is, but the results are slightly counter-intuitive. From memory, the highest losses are in the second row, and decrease beyond that.

Distance between rows is typically longest in the prevailing wind conditions, not unsurprisingly - typically 7-8 rotor diameters.

Wind power

by Jerome a Paris (etg@eurotrib.com) on Wed Mar 27th, 2013 at 04:49:51 PM EST
[ Parent ]
Counter-intuitive? I would expect the second row, which is the first to encounter wake turbulence, to have the biggest drop and then for drops to increase slowly WRT the first row as one moves back in the array. The first row is the only one not to encounter wakes. What is considered the maximum acceptable drop allowed for the last row of an array? Any rule of thumb?

As the Dutch said while fighting the Spanish: "It is not necessary to have hope in order to persevere."
by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Mar 27th, 2013 at 04:59:56 PM EST
[ Parent ]
I just did some quick reading on this.

The drop in efficiency has to do with the wind speed not the effect of turbulence.  

by stevesim on Wed Mar 27th, 2013 at 05:20:07 PM EST
[ Parent ]
I suspect that the drop in wind speed IS the result of turbulence. If energy is extracted by the windmill the total energy of the moving air has to decrease somehow, according to conservation of energy and entropy has to increase. The increased entropy is likely what we see visualized in part. That would be the condensation of water previously present as water vapor as a result of a pressure transient in the vicinity of the blade.

As the Dutch said while fighting the Spanish: "It is not necessary to have hope in order to persevere."
by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Mar 27th, 2013 at 08:51:48 PM EST
[ Parent ]
This recent article seems relevant
by mustakissa on Tue Apr 2nd, 2013 at 04:30:48 PM EST
[ Parent ]

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