Entering parameters into the wind turbine design tool

The Parameters entry screen will appear as shown below :-

You can select any tab in any order by clicking on the tab label. Click to ensure the "Rotor geometry" tab is selected as shown above.

The fields and their meaning are now described. Note that the values that can be entered are constrained to reasonable values and that out-of-range or bad values will be ignored without warning.

Turbine rotor diameter (m)

This is the diameter of the disc area swept by the rotor assembly (i.e. all blades mounted on the hub). The units are metres. The wind power captured will be proportional to the area or proportional to the diameter squared. However larger diameter rotors will rotate more slowly thus making directly attached generators less powerful for a given size and weight.

Number of blades

This is the number of blades per rotor. A three blade rotor with blades at 120 degrees is normal for smooth running. A two blade rotor can be made as a single rigid piece since the two blades will be at 180 degrees but pulsating forces cased by wind shear may be more of a problem in large turbines. Such large two blade rotors are often mounted on a teetering hub to avoid stresses near the roots. A single blade rotor is possible but this would need a counter weight on a short stub to balance weight. A single blade rotor will never be aerodynamically balanced. The smaller the number of blades, the wider the blades will need to be and hence the larger will be the reynolds number and hence better lift to drag ratio at low wind speeds.

Practical chord width limit

This constrains the width of the chord (the distance between leading and trailing edges of the blade) to be less than the value specified here in metres. This is because the ideal chord width gets larger for those parts of the blade at a radius that approaches the hub or axis of rotation and we may wish to sacrifice some ideal performance in order to make construction easier or use less materials or avoid collisions with the tower. The area swept by the blade at small radius is small anyway so the loss of performance by limiting the chord width is not great.

Tip speed ratio

The ratio of the speed at which the tip of the blade would move relative to the undisturbed free wind speed far upstream of the turbine. This ratio is usually 5 or 6 for a 3 bladed turbine and 7 or 8 for a 2 bladed turbine. The higher the ratio, the faster the rotation and the smaller the blades (less wide). The swirl energy imparted to the flow will also be less for higher ratios. However a high tip speed also means higher noise level.

Click to ensure the "Airfoil" tab is selected as shown below.

Airfoil

This drop down choice list gives you  up to 20 airfoils and hydrofoils that  may  suit your performance requirements. The program knows the shape (coordinates) of the airfoil but not details of how it performs aerodynamically under all conditions of speed and angle. You should therefore consult books and databases (such as http://www.aae.uiuc.edu/m-selig/ads/coord_database.html ) to check the best setting conditions for your particular size and site. However, in the name of the airfoil in the chooser is some hint of what might be a good design cooefficient of lift and angle of attack. Learn about airfoil polar diagrams. You should ensure that the angle of attack and cooefficient of lift input fields match these hints or are consistent with it in a way described below. They will not be changed automatically when you change airfoil!

Angle of attack

The angle of attack is the angle between the chord line (the line joining leading edge or nose and the trailing edge) and the local relative airflow direction.  The coefficient of lift increases by 0.11 from the reference value for every extra degree of angle of attack up to some maximum lift limit when the airflow separates from the wing and lift suddenly drops and drag increases. The wing is said to be stalled in this condition. For a wind turbine a high lift is desirable but other considerations such the desire to be able to operate at TSRs far removed from the design point, or the need for a larger reynolds number (larger blade) or the choice of a low camber airfoil may dictate lower lifts or angles of attack. Typical design angles of attack range from 4 to 7 degrees.

Lift coefficient

The lift coefficient relates to the amount of lifting force perpendicular to the relative airflow direction for a normalised chord and velocity. It is dependant on the angle of attack and the shape of the airfoil and the reynolds number. The coefficient of lift increases by 0.11 from the reference value for every extra degree of angle of attack within certain limits to maintain attached flow. The maximum lift of an airfoil is greater for highly cambered airfoils (e.g Cl=1.5) and can be achieved at lower angles of attack. Less cambered or symetric airfoils may have a maximum lift Cl=1.0.  As lift increases so does drag and there is usually a point at which the penalty of the increase in drag outways the benefit of the increase in lift. Different airfoils are optimised to give best performance at different lift coefficients.

Drag coefficient

The drag coefficient relates to the amount of drag force parallel to the relative airflow direction for a normalised chord and velocity. It is dependent on the angle of attack and the shape of the airfoil and the reynolds number. For good airfoils the drag coefficient is very much less than the lift coefficient (e.g Cd=0.02) making the lift to drag ratio Cl/Cd better than 50! This explains how it is possible for a heavy airplane to support its weight using the much lower thrust from its engine and propellors. The secret of a low drag is to keep the airflow over the wing smooth and attached to the surface. Any turbulance or wake eddies will contain energy which, by conservation of energy principles, must have come from the drag force. If the airfoil attempts to change the direction of flow too dramatically while the flow is decelerating then the  wing may stall and the flow may  separate from the wing before reaching the trailing edge  and  a large region of turbulent or stagnant air may  form which both increases drag and reduces lift. For this reason airfoils have their characteristic very gradual reduction in thickness and very thin trailing edge. Note that when the flow is accelerating from the leading edge to the fattest point of the airfoil then flow will still remain attached even though the changes of direction may be quite large. For this reason even quite blunt and fat airfoils can have low drag.

Moment coefficient

The moment coefficient relates to the pitching moment (which is the torque attempting to rotate the airfoil such as to reduce the angle of attack and operating about a nominal axis situated at the point one quarter of  a chord length measured from the leading edge). This moment is usually zero for symetric airfoils,  -0.1 for slightly cambered airfoils and  -0.27 for a strongly cambered airfoil. Like lift and drag it can be found from charts or tables for the airfoil. Provided your blade is torsionally stiff and does not twist under the force, then moment does not affect your design, however you can arrange to substantially cancel out the force by combining it with the lift force (also acting at 1/4 chord point) to give an effective "centre of pressure" point which will usually then be about at the 1/3 chord point. If your blade is supported at this point then there is no net twisting moment.

Draw X as cylinder surface distance (not tangent plane)

This boolean input should normally be false and unticked and the image of the airfoil on screen and printer will then be correct for a flat plane section through the blade at the particular station radius from the hub axis. However should you wish to view the airfoil section from the point of view of the relative airflow (which is curved by the rotation of the blade especially nearer the hub), then you can make the input true by ticking the checkbox and the  shape will then  be  as seen by the airflow.  If you were to use this new plot for constructing a  blade then you would have to wrap the plot onto the surface of a cylinder of station radius before using as a template. An advantage of this method is that any discontinuities at the joints caused by assembling the blade from  several sections will not cross an airflow path.

Behaviour near hub

Because the geometry screen allows you to practically limit the maximum chord length, nearer the hub, the ideal chord length may want to be greater than the limit and the question arises as to what to do with the airfoil profile in such circumstances. If you choose the "do nothing" option then  the airfoil profile remains  true to the  original shape but is of constant chord length which clearly makes it weaker than it otherwise would be since the forces are increasing towards the hub. If you choose the "thicken" option then  the airfoil profile gets fatter but is of constant chord length which  makes it stronger than the previous case. The lift and drag performance should stay as designed even though the profile is fatter since the camber is the same. This is the recommended option. Other options are not properly implemented and may change the lift characteristics.

Radius of station (blade section of interest)(m)

A station means a particular point of interest somewhere between the root of the blade near the hub and the tip of the blade. The station is identified by means of the radius or radial distance in metres that a particular reference point of the station has in respect to the axis of rotation of the rotor. Each possible station can have a station radius between 0.0 and the tip radius (which will be half of the rotor diameter). Each station experiences a different relative airflow velocity and direction since the station speed (following a circular path) gets larger (it is proportional to the radius) while the absolute windspeed is constant. The relative windspeed is the sum of these 2 vectors. The 2D graphics display of a section through the blade shown on the top level screen of the wind turbine design tool is the section at the particular station radius set here on this parameters tab. If you wish to view sections at another station radius, then you adjust the station radius value here and then press the "Apply parameters" button. This will return you to the top level screen and draw the wanted 2D airfoil with the correct absolute size shape and angle suitable for the relative airflow at the wanted station radius. To construct a 1.8 m blade for attachment to a 0.4m diameter hub for a 4 m diameter turbine rotor,  you might choose 10 stations and for each station you would change the station radius to each of the values (in hub to tip order, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0 ). For each station you would print the 2D section to paper/card and cut out to make a template. For each station other than the root and tip stations you would also print a second 2D section but having changed the "View from viewpoint" choice list from "View from blade tip" to "View from hub". You only need one template for the tip (radius=2.0) that is "View from tip". You only need one template for the hub (radius=0.2) that is "View from hub". At all intermediate stations you have 2 templates, one is "View from hub" and the other is "View from tip". Any segment of the blade can now made by sandwiching some material such as a sheet expanded polystyrene foam of thickness 0.2 m  between 2 templates from 2 station radii differing by 0.2m (one "view from tip" and one "view from hub") and cutting with a hot wire. The 2 templates will each have a reference cross-hair that you will use to ensure the centres are aligned and the cross-hairs are parallel. In this way the setting angles will be assured to be correct.

Center X (fraction chord width

Center X is the x coordinate of the alignment cross-hair intersection point expressed as a fraction of the chord width measured from the leading edge in the direction of the chord line. A good value to minimise net pitching torque is the estimated "center of pressure" point which is often around 1/3 of chord or 0.33.

Center Y (fraction of chord width)

Center Y is the y coordinate of the alignment cross-hair intersection point expressed as a fraction of the chord width and measured from the chord line in a direction perpendicular to the chord line. A good value might be one that puts the intersection point near the centre of gravity of your blade section where the resultant centrifugal forces will least tend to bend the blade (e.g 0.045).

Extra blades share chord width

This boolean input, should normally be false (unticked). If true (ticked), then any loss of ideal thrust from the main rotor blades caused by  the practical limitation of chord width near the hub will be assumed to be compensated for and corrected by additional short blades added to the hub. The contribution that the extra short blades make will be reflected in the integrated power and thrust computations and the section through the extra short blade at the current station radius will be drawn on the main 2D top level screen at the same time as the section through the main blade. This feature will be particularly useful for a single bladed turbine where a short balancing stub is needed anyway and may as well be aerodynamic and contributing to thrust.

Save also to CAD file when saving to scenario

This boolean input, should normally be false (unticked) to save time when saving. If true (ticked), then , in addition to the scenario being saved to a file such as "mydesign4m.zip" when the button "Save Scenario" is pressed, 2 additional files will be created and written. These additional files will be named "mydesign4m_R_RR.iges" and "mydesign4m_R_RR.dxf" and will contain CAD format data for the blade section at the current station radius of "R.RR" metres. For example if the Radius of station is 0.5 m then the files will be called "mydesign4m_0_50.iges" etc. These CAD files can be imported into a CAD system for further work or for plotting on large plotters. The IGES format will be preferred since this has the smoothest curves. The DXF file is an approximation of many straight line segments.

View from viewpoint

The blade section can be drawn from the point of view of an observer viewing from the blade tip towards the hub, or can be drawn from the point of view of an observer viewing from the hub towards the tip. The section will be identical in shape but will be a mirror image between the 2 viewpoints. This is so that when the sections are plotted, they can be used as templates for cutting a segment of the blade while the plotted cross-hairs used for alignment will be visible at both ends of the segment.

Integration lower limit and Integration upper limit

When computing the total forces on the rotor or the total powers, the  contributions  of all possible stations from a radius of the lower limit to a radius of the upper limit are summed or integrated. The limits are expressed as fractions of the rotor radius, so that 0.0 represents the axis and 1.0 represents the tip radius. The last 2% of a blade near the tip probably contributes less power because air can leak over the tip in  a radial direction rather than following the airfoil intended path. The radii between 0.0 and the hub radius (say 0.1 of tip radius) contain no blades and so also contribute no power. The best limits for integration might thus be a lower limit of 0.1 and an upper limit of 0.98.

Click  to ensure that the "Wind properties" tab is showing as below :-

Note that the turbine is designed to operate at a particular tip speed ratio (TSR) and it does not matter what the actual wind speed is, regarding the shape or twist angles of the blades, provided that the tip speed ratio is maintained. In other words, your rotor design will capture energy efficiently at most windspeeds if you control the RPM so as to allow the TSR to remain at the design point. The wind parameters entered here will affect the stresses and powers and optimum RPM of your turbine and so will affect your generator operating point, tower loads, material strengths required and the expected amount of energy captured.

Windspeed (m/s)

This is the speed of the wind in metres per second at a point far upwind of the turbine (or what the windspeed would be at the location of the turbine if the turbine did not affect the airflow in any way). Since an effective operating turbine slows down the wind, the windspeed at the rotor during operation will always be slower than this value.

Air temperature (Celcius)

This is the temperature of the air in units of Celcius (same as degrees Centigrade). To convert fahrenheit F to celcius  C= (5/9)*(F-32). The air density decreases as temperature increases and so this temperature is used as an input to compute air density when the "Compute air density" button is pressed.

Altitude (m asl)

This is the altitude measured in metres above mean sea level of the turbine rotor. You would look on a map to find the height of the land above sea level and then add the height of the tower. Air density decreases with increasing height (because there is less air above you and hence pressure is less) and so this altitude is used as an input to compute air density when the "Compute air density" button is pressed.

Actual air density (Kg/m^3)

This is the air or other fluid density measured in Kilograms per cubic metre. Air density is around 1.2 Kg/m^3 and is mainly affected by temperature and altitude which is why a "Compute air density" button is provided to assist in estimating density should you not have a precise figure. If you are designing a turbine which uses water as the fluid (say a marine current flow  or river flow or tidal flow turbine) then you would directly enter the density of the water (1000 Kg/m^3 for freshwater or up to 1035 Kg/m^3 for saline). Water density is also affected by temperature, salinity and pressure but the changes are not very significant compared to gases like air. Interestingly, the shape, sizes and angles of the turbine blades are not affected by flow speed or density and so wind turbines and water turbines will look the same if designed for the same tip speed ratio (TSR). The material strengths to support the forces will however be very different.

Absolute air viscosity (Kg/m/s/10^6 or Poise/10^5)

This is one type of viscosity figure for the fluid medium (the other type is called kinematic viscosity which we can compute from the absolute viscosity). The absolute viscosity is also called the dynamic viscosity. The figure for air is about 17.894 x 10^-6 Kg/m/s or 17.894 x 10^-5 Poise (0.00017894 Poise). The figure for water is about 1000 x 10^-6 Kg/m/s or 1000 x 10^-5 Poise (0.01 Poise). There is a small temperature effect but it is not significant for our purposes in the range of temperatures expected in a natural flow. The viscosity affects a figure called the "Reynolds number" which is used to determine at what fluid speeds the flow is more likely to change from a laminar to a turbulent flow.

The other tabs have many more parameters that you can set, but those explained above are enough to know the dimensions and performance of your standard turbine. You can save your scenario to a file such as "mydesign.zip" (zip filename extension is recommended) by pressing the "Save scenario" button. You can return to the main aerodynamics top level screen by pressing the "Apply parameters" button.

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