Shrouded wind turbine - Revision history

4ndy: Added link to image of japanese wind-lens turbines in operation

2012-09-23T21:39:13Z

Added link to image of japanese wind-lens turbines in operation

← Older revision		Revision as of 21:39, 23 September 2012
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	Longer ducts have shown to induce a stronger vortex effect, and so increase power, but are more difficult to construct and mount.<ref name="wind lens energies study"/>		Longer ducts have shown to induce a stronger vortex effect, and so increase power, but are more difficult to construct and mount.<ref name="wind lens energies study"/>

			==External Links==
			[http://en.wikipedia.org/wiki/File:Windlens1.jpg Downwind radially-mounted wind lens turbines at Kyushu University Ito campus - Image on Wikipedia]. Kyushu is a coastal town and so likely gets strong winds; it would be good to know how these turbines manage heat dissipation in such weather.

	==References==		==References==
	{{reflist}}		{{reflist}}

4ndy: Fixed some links

2012-09-21T10:01:16Z

Fixed some links

← Older revision		Revision as of 10:01, 21 September 2012
Line 1:		Line 1:
	A 'shrouded', '[http://en.wikipedia.org/wiki/Cowling cowled]', '[http://en.wikipedia.org/wiki/Ducted_fan ducted]' or '[http://en.wikipedia.org/wiki/Wind_lens wind lens]' turbine is one that is housed in a ring-shaped aerofoil that increases airflow through the turbine blades' swept area by generating a localised [http://en.wikipedia.org/wiki/Vortex_ring ring vortex].		A 'shrouded', '[http://en.wikipedia.org/wiki/Cowling cowled]', '[http://en.wikipedia.org/wiki/Ducted_fan ducted]' or '[http://en.wikipedia.org/wiki/Wind_lens wind lens]' turbine is one that is housed in a ring-shaped aerofoil that increases airflow through the turbine blades' swept area by generating a localised [http://en.wikipedia.org/wiki/Vortex_ring ring vortex].

	Such systems can increase the power of a [[Horizontal-Axis Wind Turbine]] by 2-5 times<ref name="wind lens energies study">[http://www.mdpi.com/1996-1073/3/4/634 A Shrouded Wind Turbine Generating High Output Power with Wind-lens Technology - Yuji Ohya and Takashi Karasudani, Energies, volume 3 issue 4] (open access, PDF, has diagrams of duct profiles tested)</ref>, for a given blade diameter and windspeed. While the use of an engine duct in aircraft design is considered a trade-off of additional weight for efficiency, the weight that it adds to ground-based wind turbine systems is of negligible importance, and a carefully designed diffuser can not only increase power output, but also cut out downstream turbulence and noise by inhibiting or removing [http://en.wikipedia.org/wiki/Wingtip_vortices blade-tip vortices] and by inducing rapid mixing of the high and low-speed air flows behind the turbine<ref>Elements of Gas Turbine Propulsion - Jack D. Mattingly, McGraw Hill International Editions, 1996, pp.804, Figure 10-56.</ref><ref>[http://www.youtube.com/watch?v=WB5CawKfE2M FloDesign wind turbine] - might as well mute the video, since the narration is full of 'marketing'.</ref><ref name="patent US8021100B2">[http://www.google.com/patents?hl=en&lr=&vid=USPAT8021100&id=IAPwAQAAEBAJ&oi=fnd&dq=US+6877960+B1&printsec=abstract ''Wind turbine with mixers and ejectors''] -		Such systems can increase the power of a [[Wind_Turbine/Research_Development#Horizontal-axis_wind_turbines\|Horizontal-Axis Wind Turbine]] by 2-5 times<ref name="wind lens energies study">[http://www.mdpi.com/1996-1073/3/4/634 A Shrouded Wind Turbine Generating High Output Power with Wind-lens Technology - Yuji Ohya and Takashi Karasudani, Energies, volume 3 issue 4] (open access, PDF, has diagrams of duct profiles tested)</ref>, for a given blade diameter and windspeed. While the use of an engine duct in aircraft design is considered a trade-off of additional weight for efficiency, the weight that it adds to ground-based wind turbine systems is of negligible importance, and a carefully designed diffuser can not only increase power output, but also cut out downstream turbulence and noise by inhibiting or removing [http://en.wikipedia.org/wiki/Wingtip_vortices blade-tip vortices] and by inducing rapid mixing of the high and low-speed air flows behind the turbine<ref>Elements of Gas Turbine Propulsion - Jack D. Mattingly, McGraw Hill International Editions, 1996, pp.804, Figure 10-56.</ref><ref>[http://www.youtube.com/watch?v=WB5CawKfE2M FloDesign wind turbine] - might as well mute the video, since the narration is full of 'marketing'.</ref><ref name="patent US8021100B2">[http://www.google.com/patents?hl=en&lr=&vid=USPAT8021100&id=IAPwAQAAEBAJ&oi=fnd&dq=US+6877960+B1&printsec=abstract ''Wind turbine with mixers and ejectors''] -
	Walter M. Presz, Jr. et al, US Patent number: 8021100, Filing date: Mar 24, 2008, Issue date: Sep 20, 2011, Application number: 12/054,050</ref>		Walter M. Presz, Jr. et al, US Patent number: 8021100, Filing date: Mar 24, 2008, Issue date: Sep 20, 2011, Application number: 12/054,050</ref>

Line 12:		Line 12:
	It is typically preferable to use differing numbers of stator & rotor blades, especially prime numbers, in order to reduce resonant vibrations that could otherwise decrease service life of parts.		It is typically preferable to use differing numbers of stator & rotor blades, especially prime numbers, in order to reduce resonant vibrations that could otherwise decrease service life of parts.

	The development of static aerofoils, for use in a shroud and stator blades, lends itself well to the use of rapid-prototyping or additive manufacturing methods such as [[3D-Printing]]. One design exploiting this fact is the [http://www.thingiverse.com/thing:27791 ~~Storm Turbine~~].		The development of static aerofoils, for use in a shroud and stator blades, lends itself well to the use of rapid-prototyping or additive manufacturing methods such as [[3D Printer\|3D-Printing]]. One design exploiting this fact is the [http://www.thingiverse.com/thing:27791 Open Wind Lens].

	High-rotational-velocity HAWTs with their rotor blades mounted in upwind of their post/tower run a risk of the blade tips colliding with their tower at high wind-speeds, and so are typically constructed with a moving tail vane that furls upwards in high winds, causing such a turbine to yaw to one side of the approaching wind, which limits the danger that blades pose to the tower and decreases their top speed<ref>''A Wind Turbine Recipe Book - The Axial Flux Windmill Plans'' - Hugh Piggott, January 2009, Metric Edition, page 9, where he says "''In very turbulent and wild conditions the gyroscopic forces on the blades have been known to push them back into contact with the tower so that they break. This is a very rare but persistent problem. In this 2008 book I have changed the direetion of furliag of the turbines so that the gyro forces push the blade tips out from the tower as the machine moves into furl. This is the yaw movement where the blades tend to be racing fastest. This change should reduce or even prevent the contact of blades with tower from now on''"		High-rotational-velocity HAWTs with their rotor blades mounted in upwind of their post/tower run a risk of the blade tips colliding with their tower at high wind-speeds, and so are typically constructed with a moving tail vane that furls upwards in high winds, causing such a turbine to yaw to one side of the approaching wind, which limits the danger that blades pose to the tower and decreases their top speed<ref>''A Wind Turbine Recipe Book - The Axial Flux Windmill Plans'' - Hugh Piggott, January 2009, Metric Edition, page 9, where he says "''In very turbulent and wild conditions the gyroscopic forces on the blades have been known to push them back into contact with the tower so that they break. This is a very rare but persistent problem. In this 2008 book I have changed the direetion of furliag of the turbines so that the gyro forces push the blade tips out from the tower as the machine moves into furl. This is the yaw movement where the blades tend to be racing fastest. This change should reduce or even prevent the contact of blades with tower from now on''"

4ndy: Found and listed FloDesign's US patent on particulars of their ducted-wind-turbine design, also added detail to a reference to one of Hugh Piggott's books.

2012-08-21T04:26:35Z

Found and listed FloDesign's US patent on particulars of their ducted-wind-turbine design, also added detail to a reference to one of Hugh Piggott's books.

← Older revision		Revision as of 04:26, 21 August 2012
Line 1:		Line 1:
	A 'shrouded', '[http://en.wikipedia.org/wiki/Cowling cowled]', '[http://en.wikipedia.org/wiki/Ducted_fan ducted]' or '[http://en.wikipedia.org/wiki/Wind_lens wind lens]' turbine is one that is housed in a ring-shaped aerofoil that increases airflow through the turbine blades' swept area by generating a localised [http://en.wikipedia.org/wiki/Vortex_ring ring vortex].		A 'shrouded', '[http://en.wikipedia.org/wiki/Cowling cowled]', '[http://en.wikipedia.org/wiki/Ducted_fan ducted]' or '[http://en.wikipedia.org/wiki/Wind_lens wind lens]' turbine is one that is housed in a ring-shaped aerofoil that increases airflow through the turbine blades' swept area by generating a localised [http://en.wikipedia.org/wiki/Vortex_ring ring vortex].

	Such systems can increase the power of a [[Horizontal-Axis Wind Turbine]] by 2-5 times<ref name="wind lens energies study">[http://www.mdpi.com/1996-1073/3/4/634 A Shrouded Wind Turbine Generating High Output Power with Wind-lens Technology - Yuji Ohya and Takashi Karasudani, Energies, volume 3 issue 4] (open access, PDF, has diagrams of duct profiles tested)</ref>, for a given blade diameter and windspeed. While the use of an engine duct in aircraft design is considered a trade-off of additional weight for efficiency, the weight that it adds to ground-based wind turbine systems is of negligible importance, and a carefully designed diffuser can not only increase power output, but also cut out downstream turbulence and noise by inhibiting or removing [http://en.wikipedia.org/wiki/Wingtip_vortices blade-tip vortices] and by inducing rapid mixing of the high and low-speed air flows behind the turbine<ref>Elements of Gas Turbine Propulsion - Jack D. Mattingly, McGraw Hill International Editions, 1996, pp.804, Figure 10-56.</ref><ref>[http://www.youtube.com/watch?v=WB5CawKfE2M FloDesign wind turbine] - might as well mute the video, since the narration is full of 'marketing'.</ref>		Such systems can increase the power of a [[Horizontal-Axis Wind Turbine]] by 2-5 times<ref name="wind lens energies study">[http://www.mdpi.com/1996-1073/3/4/634 A Shrouded Wind Turbine Generating High Output Power with Wind-lens Technology - Yuji Ohya and Takashi Karasudani, Energies, volume 3 issue 4] (open access, PDF, has diagrams of duct profiles tested)</ref>, for a given blade diameter and windspeed. While the use of an engine duct in aircraft design is considered a trade-off of additional weight for efficiency, the weight that it adds to ground-based wind turbine systems is of negligible importance, and a carefully designed diffuser can not only increase power output, but also cut out downstream turbulence and noise by inhibiting or removing [http://en.wikipedia.org/wiki/Wingtip_vortices blade-tip vortices] and by inducing rapid mixing of the high and low-speed air flows behind the turbine<ref>Elements of Gas Turbine Propulsion - Jack D. Mattingly, McGraw Hill International Editions, 1996, pp.804, Figure 10-56.</ref><ref>[http://www.youtube.com/watch?v=WB5CawKfE2M FloDesign wind turbine] - might as well mute the video, since the narration is full of 'marketing'.</ref><ref name="patent US8021100B2">[http://www.google.com/patents?hl=en&lr=&vid=USPAT8021100&id=IAPwAQAAEBAJ&oi=fnd&dq=US+6877960+B1&printsec=abstract ''Wind turbine with mixers and ejectors''] -
			Walter M. Presz, Jr. et al, US Patent number: 8021100, Filing date: Mar 24, 2008, Issue date: Sep 20, 2011, Application number: 12/054,050</ref>

	==Design Considerations==		==Design Considerations==
Line 13:		Line 14:
	The development of static aerofoils, for use in a shroud and stator blades, lends itself well to the use of rapid-prototyping or additive manufacturing methods such as [[3D-Printing]]. One design exploiting this fact is the [http://www.thingiverse.com/thing:27791 Storm Turbine].		The development of static aerofoils, for use in a shroud and stator blades, lends itself well to the use of rapid-prototyping or additive manufacturing methods such as [[3D-Printing]]. One design exploiting this fact is the [http://www.thingiverse.com/thing:27791 Storm Turbine].

	High-rotational-velocity HAWTs with their rotor blades mounted in upwind of their post/tower run a risk of the blade tips colliding with their tower at high wind-speeds, and so are typically constructed with a moving tail vane that furls upwards in high winds, causing such a turbine to yaw to one side of the approaching wind, which limits the danger that blades pose to the tower and decreases their top speed<ref>A Wind Turbine Recipe Book - The Axial Flux Windmill Plans - Hugh Piggott, January 2009, Metric Edition</ref>, while increasing wear on the blades as some of them run backwards as they turn away from the approaching wind.		High-rotational-velocity HAWTs with their rotor blades mounted in upwind of their post/tower run a risk of the blade tips colliding with their tower at high wind-speeds, and so are typically constructed with a moving tail vane that furls upwards in high winds, causing such a turbine to yaw to one side of the approaching wind, which limits the danger that blades pose to the tower and decreases their top speed<ref>''A Wind Turbine Recipe Book - The Axial Flux Windmill Plans'' - Hugh Piggott, January 2009, Metric Edition, page 9, where he says "''In very turbulent and wild conditions the gyroscopic forces on the blades have been known to push them back into contact with the tower so that they break. This is a very rare but persistent problem. In this 2008 book I have changed the direetion of furliag of the turbines so that the gyro forces push the blade tips out from the tower as the machine moves into furl. This is the yaw movement where the blades tend to be racing fastest. This change should reduce or even prevent the contact of blades with tower from now on''"
			Incidentally, on the very same page he makes a sweeping statement dismissing ducted turbines "''It simply isn't worth all the extra material involved in building a duct like that. The wind tends to divert around it so you don't gain as much as you would think. It is actually more effective to build a conventional blade rotor with larger diameter, than to make a duct. Some big companies have spent a lot of their investors' money finding this out''"; statement 1 is his conclusion, 2 is a mix of incorrect and meaningless depending on what "you would think", 3 depends on your specification, and 4 reflects some failings prior to japanese research.</ref>, while increasing wear on the blades as some of them run backwards as they turn away from the approaching wind.
	Downwind turbines do not run any risk of colliding with their tower, but due to being pulled into line with the wind (especially with a duct), they are more subject to extremely high rotational velocities during storms, and so their blades must be constructed thick enough to withstand the resulting stresses, and alternators must have a way of keeping cool enough so that they are not damaged by overheating. While powering a dump load in order to slow blades down can help, some form of air-cooled heat sink next to stator coils may be needed to achieve enough cooling.		Downwind turbines do not run any risk of colliding with their tower, but due to being pulled into line with the wind (especially with a duct), they are more subject to extremely high rotational velocities during storms, and so their blades must be constructed thick enough to withstand the resulting stresses, and alternators must have a way of keeping cool enough so that they are not damaged by overheating. While powering a dump load in order to slow blades down can help, some form of air-cooled heat sink next to stator coils may be needed to achieve enough cooling.

4ndy: Added detail about furling, high wind speeds and duct shape to design considerations

2012-08-17T23:04:30Z

Added detail about furling, high wind speeds and duct shape to design considerations

← Older revision		Revision as of 23:04, 17 August 2012
Line 1:		Line 1:
	A 'shrouded', '[http://en.wikipedia.org/wiki/Cowling cowled]', '[http://en.wikipedia.org/wiki/Ducted_fan ducted]' or '[http://en.wikipedia.org/wiki/Wind_lens wind lens]' turbine is one that is housed in a ring-shaped aerofoil that increases airflow through the turbine blades' swept area by generating a localised [http://en.wikipedia.org/wiki/Vortex_ring ring vortex].		A 'shrouded', '[http://en.wikipedia.org/wiki/Cowling cowled]', '[http://en.wikipedia.org/wiki/Ducted_fan ducted]' or '[http://en.wikipedia.org/wiki/Wind_lens wind lens]' turbine is one that is housed in a ring-shaped aerofoil that increases airflow through the turbine blades' swept area by generating a localised [http://en.wikipedia.org/wiki/Vortex_ring ring vortex].

	Such systems can increase the power of a [[Horizontal-Axis Wind Turbine]] by 2-5 times<ref>[http://www.mdpi.com/1996-1073/3/4/634 A Shrouded Wind Turbine Generating High Output Power with Wind-lens Technology - Yuji Ohya and Takashi Karasudani, Energies, volume 3 issue 4] (open access, PDF)</ref>, for a given blade diameter and windspeed. While the use of an engine duct in aircraft design is considered a trade-off of additional weight for efficiency, the weight that it adds to ground-based wind turbine systems is of negligible importance, and a carefully designed diffuser can not only increase power output, but also cut out downstream turbulence and noise by inhibiting or removing [http://en.wikipedia.org/wiki/Wingtip_vortices blade-tip vortices] and by inducing rapid mixing of the high and low-speed air flows behind the turbine<ref>Elements of Gas Turbine Propulsion - Jack D. Mattingly, McGraw Hill International Editions, 1996, pp.804, Figure 10-56~~. I have seen a much clearer example of this, but can't remember what book it was in just now~~.</ref><ref>[http://www.youtube.com/watch?v=WB5CawKfE2M FloDesign wind turbine] - might as well mute the video, since the narration is full of 'marketing'.</ref>		Such systems can increase the power of a [[Horizontal-Axis Wind Turbine]] by 2-5 times<ref name="wind lens energies study">[http://www.mdpi.com/1996-1073/3/4/634 A Shrouded Wind Turbine Generating High Output Power with Wind-lens Technology - Yuji Ohya and Takashi Karasudani, Energies, volume 3 issue 4] (open access, PDF, has diagrams of duct profiles tested)</ref>, for a given blade diameter and windspeed. While the use of an engine duct in aircraft design is considered a trade-off of additional weight for efficiency, the weight that it adds to ground-based wind turbine systems is of negligible importance, and a carefully designed diffuser can not only increase power output, but also cut out downstream turbulence and noise by inhibiting or removing [http://en.wikipedia.org/wiki/Wingtip_vortices blade-tip vortices] and by inducing rapid mixing of the high and low-speed air flows behind the turbine<ref>Elements of Gas Turbine Propulsion - Jack D. Mattingly, McGraw Hill International Editions, 1996, pp.804, Figure 10-56.</ref><ref>[http://www.youtube.com/watch?v=WB5CawKfE2M FloDesign wind turbine] - might as well mute the video, since the narration is full of 'marketing'.</ref>

	==Design Considerations==		==Design Considerations==
Line 12:		Line 12:

	The development of static aerofoils, for use in a shroud and stator blades, lends itself well to the use of rapid-prototyping or additive manufacturing methods such as [[3D-Printing]]. One design exploiting this fact is the [http://www.thingiverse.com/thing:27791 Storm Turbine].		The development of static aerofoils, for use in a shroud and stator blades, lends itself well to the use of rapid-prototyping or additive manufacturing methods such as [[3D-Printing]]. One design exploiting this fact is the [http://www.thingiverse.com/thing:27791 Storm Turbine].

			High-rotational-velocity HAWTs with their rotor blades mounted in upwind of their post/tower run a risk of the blade tips colliding with their tower at high wind-speeds, and so are typically constructed with a moving tail vane that furls upwards in high winds, causing such a turbine to yaw to one side of the approaching wind, which limits the danger that blades pose to the tower and decreases their top speed<ref>A Wind Turbine Recipe Book - The Axial Flux Windmill Plans - Hugh Piggott, January 2009, Metric Edition</ref>, while increasing wear on the blades as some of them run backwards as they turn away from the approaching wind.
			Downwind turbines do not run any risk of colliding with their tower, but due to being pulled into line with the wind (especially with a duct), they are more subject to extremely high rotational velocities during storms, and so their blades must be constructed thick enough to withstand the resulting stresses, and alternators must have a way of keeping cool enough so that they are not damaged by overheating. While powering a dump load in order to slow blades down can help, some form of air-cooled heat sink next to stator coils may be needed to achieve enough cooling.

			Longer ducts have shown to induce a stronger vortex effect, and so increase power, but are more difficult to construct and mount.<ref name="wind lens energies study"/>

	==References==		==References==
	{{reflist}}		{{reflist}}

4ndy: Added a link to my turbine design, please discuss

2012-08-17T22:04:40Z

Added a link to my turbine design, please discuss

← Older revision		Revision as of 22:04, 17 August 2012
Line 1:		Line 1:
	A 'shrouded', '[http://en.wikipedia.org/wiki/Cowling cowled]', '[http://en.wikipedia.org/wiki/Ducted_fan ducted]' or '[http://en.wikipedia.org/wiki/Wind_lens wind lens]' turbine is one that is housed in a ring-shaped aerofoil that increases airflow through the turbine blades' swept area by generating a localised [http://en.wikipedia.org/wiki/Vortex_ring ring vortex].		A 'shrouded', '[http://en.wikipedia.org/wiki/Cowling cowled]', '[http://en.wikipedia.org/wiki/Ducted_fan ducted]' or '[http://en.wikipedia.org/wiki/Wind_lens wind lens]' turbine is one that is housed in a ring-shaped aerofoil that increases airflow through the turbine blades' swept area by generating a localised [http://en.wikipedia.org/wiki/Vortex_ring ring vortex].

	Such systems can increase the power of a [[Horizontal-Axis Wind Turbine]] by 2-5 times<ref>[http://www.mdpi.com/1996-1073/3/4/634 A Shrouded Wind Turbine Generating High Output Power with Wind-lens Technology - Yuji Ohya and Takashi Karasudani, Energies, volume 3 issue 4] (open access, PDF)</ref>, for a given blade diameter and windspeed. While the use of an engine duct in aircraft design is considered a trade-off of additional weight for efficiency, the weight that it adds to ground-based wind turbine systems is of negligible importance, and a carefully designed diffuser can not only increase power output, but also cut out downstream turbulence and noise by inhibiting or removing [http://en.wikipedia.org/wiki/Wingtip_vortices blade-tip vortices] and by inducing rapid mixing of the high and low-speed air flows behind the turbine<ref>Elements of Gas Turbine Propulsion - Jack D. Mattingly, McGraw Hill International Editions, 1996, pp.804, Figure 10-56. I have seen a much clearer example of this, but can't remember what book it was in just now. ~~--~~~~~~</ref><ref>[http://www.youtube.com/watch?v=WB5CawKfE2M FloDesign wind turbine] - might as well mute the video, since the narration is full of 'marketing'. ~~--~~~~~~</ref>		Such systems can increase the power of a [[Horizontal-Axis Wind Turbine]] by 2-5 times<ref>[http://www.mdpi.com/1996-1073/3/4/634 A Shrouded Wind Turbine Generating High Output Power with Wind-lens Technology - Yuji Ohya and Takashi Karasudani, Energies, volume 3 issue 4] (open access, PDF)</ref>, for a given blade diameter and windspeed. While the use of an engine duct in aircraft design is considered a trade-off of additional weight for efficiency, the weight that it adds to ground-based wind turbine systems is of negligible importance, and a carefully designed diffuser can not only increase power output, but also cut out downstream turbulence and noise by inhibiting or removing [http://en.wikipedia.org/wiki/Wingtip_vortices blade-tip vortices] and by inducing rapid mixing of the high and low-speed air flows behind the turbine<ref>Elements of Gas Turbine Propulsion - Jack D. Mattingly, McGraw Hill International Editions, 1996, pp.804, Figure 10-56. I have seen a much clearer example of this, but can't remember what book it was in just now.</ref><ref>[http://www.youtube.com/watch?v=WB5CawKfE2M FloDesign wind turbine] - might as well mute the video, since the narration is full of 'marketing'.</ref>

	==Design Considerations==		==Design Considerations==
Line 10:		Line 10:

	It is typically preferable to use differing numbers of stator & rotor blades, especially prime numbers, in order to reduce resonant vibrations that could otherwise decrease service life of parts.		It is typically preferable to use differing numbers of stator & rotor blades, especially prime numbers, in order to reduce resonant vibrations that could otherwise decrease service life of parts.

			The development of static aerofoils, for use in a shroud and stator blades, lends itself well to the use of rapid-prototyping or additive manufacturing methods such as [[3D-Printing]]. One design exploiting this fact is the [http://www.thingiverse.com/thing:27791 Storm Turbine].

	==References==		==References==
	{{reflist}}		{{reflist}}

4ndy: Initial description of function and design considerations

2012-08-17T21:57:58Z

Initial description of function and design considerations

New page

A 'shrouded', '[http://en.wikipedia.org/wiki/Cowling cowled]', '[http://en.wikipedia.org/wiki/Ducted_fan ducted]' or '[http://en.wikipedia.org/wiki/Wind_lens wind lens]' turbine is one that is housed in a ring-shaped aerofoil that increases airflow through the turbine blades' swept area by generating a localised [http://en.wikipedia.org/wiki/Vortex_ring ring vortex].

Such systems can increase the power of a [[Horizontal-Axis Wind Turbine]] by 2-5 times<ref>[http://www.mdpi.com/1996-1073/3/4/634 A Shrouded Wind Turbine Generating High Output Power with Wind-lens Technology - Yuji Ohya and Takashi Karasudani, Energies, volume 3 issue 4] (open access, PDF)</ref>, for a given blade diameter and windspeed. While the use of an engine duct in aircraft design is considered a trade-off of additional weight for efficiency, the weight that it adds to ground-based wind turbine systems is of negligible importance, and a carefully designed diffuser can not only increase power output, but also cut out downstream turbulence and noise by inhibiting or removing [http://en.wikipedia.org/wiki/Wingtip_vortices blade-tip vortices] and by inducing rapid mixing of the high and low-speed air flows behind the turbine<ref>Elements of Gas Turbine Propulsion - Jack D. Mattingly, McGraw Hill International Editions, 1996, pp.804, Figure 10-56. I have seen a much clearer example of this, but can't remember what book it was in just now. --~~~~</ref><ref>[http://www.youtube.com/watch?v=WB5CawKfE2M FloDesign wind turbine] - might as well mute the video, since the narration is full of 'marketing'. --~~~~</ref>

==Design Considerations==
A wind lens must be supported around the turbine somehow, which implies a highly rigid (in order to prevent blade-tip collision) framework within the shroud, held from below, or radial members spanning from the turbine's axis to the inner surface of the shroud.
Using fixed radial support members necessitates that they be streamlined with aerofoils/fairings in order to reduce drag and turbulence that could otherwise interfere with the turbine's operation.

Once a system is installed with radial aerofoils around the rotor shaft, it becomes possible to use them to induce an initial rotation in the airflow, [http://en.wikipedia.org/wiki/Axial_compressor#Description as do stator blades] [http://en.wikipedia.org/wiki/Turbine#Theory_of_operation in gas turbine engines], which can improve turbine blade performance characteristics, however as the turbine blades cut through regions of higher & lower pressure, created by stator camber and/or angle of attack, at high speeds this can result in generation of additional noise, and so inclining stator blades may not always be desirable.

It is typically preferable to use differing numbers of stator & rotor blades, especially prime numbers, in order to reduce resonant vibrations that could otherwise decrease service life of parts.

==References==
{{reflist}}