Entering more 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 "Empirical experience" 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.

The "Empirical experience" input tab allows you to estimate the average real electrical power delivery of your turbine over the long term. The input parameters are both intended to be long term averages measured by empirical methods over many different historical turbine installations. The outputs of the computation will be displayed via the "Reports/Show working" menu item at the top of the screen and will be the line labeled "Empirical mean power"

Power coefficient (efficiency factor)

This is the overall efficiency factor, combining the Betz limit (0.59), rotor losses, gearbox losses, running off-design TSR losses, generator and transmission losses, battery losses. For small turbine installations this is historically 0.15. Larger turbines may manage 0.2.

Mean windspeed

This is the mean windspeed for the site, as read from published windspeed maps, or measured over the year. Since power varies as the cube of the instantaneous windspeed, and mean windspeed is composed of a large range of windspeeds and durations, then mean power for a mean windspeed is not the same as the instantaneous power for an instantaneous windspeed having the mean value.

Click to ensure the "Fluid acceleration" tab is selected as shown below:-

Fluid total acceleration factor

This is  Vfour/V, the ratio of the windspeed far downwind of the turbine (Vfour) and the windspeed (V) far upwind of the turbine (assuming no boundary mixing between air passing through turbine and air passing around turbine). For a Betz ideal turbine, the downwind speed is 1/3 of the upwind speed, i.e. the factor is 0.3333. For a propellor or a compressor, the speed downwind is greater that the speed upwind and the factor may be greater than 1.0, say 1.3 or 2.0. For a windbreak intended to stop the wind altogether, you might  choose a factor of 0.0. This flexibility in specifying the total acceleration allows both turbines and propellors to be designed and analysed. This factor is the overall factor  and is independent of the number of stages contributing to the acceleration. For a 2 stage contra-rotating turbine or propellor this factor would be the same as for a single stage device. The acceleration factor is used to compute the flow velocity at the turbine rotor. It is assumed that half of the acceleration occurs before the rotor and half occurs after the rotor and so the flow velocity at the rotor is the mean of the far upwind and far downwind values.

Pressure factor this stage

The overall acceleration of the fluid is achieved by creating a pressure between the front face and back face of the rotor disk. The program computes an appropriate pressure to do this. This pressure can be achieved in 1 stage in which case the pressure factor would be 1.0. Alternatively if there were say 2 stages of rotor blades, then the pressures of each stage would combine in series (be summed) and  the pressures could be shared equally between stages (pressure factors of 0.5 + 0.5 thus summing to +1.0) or  perhaps  the first stage could be a propellor with  a pressure factor of -2.0 and the second stage could be a turbine with a pressure factor of +3.0 (still summing to +1.0).  This flexibility to assign different  pressure factors (including negative factors) to particular stages of  a multistage turbine might be useful since it could be used as  a rotational speed changing device instead of mechanical gears. Any first stage propellor would consume power but this  power might be supplied by a  directly attached larger diameter single stage turbine.

This stage rotates clockwise viewed from upwind

This boolean input determines the direction in which the rotor of this stage will rotate. If ticked (true) , then direction is clockwise as viewed by an observer standing upwind. If unticked (false) then the direction will be anti-clockwise. This  flexibility will be required for a 2 stage contra-rotating turbine. Contra-rotation of a multistage turbine is  desirable because torque moment on the tower is thus cancelled as is the exactly equal torque moment on the air flow (the torque which causes residual swirl  downwind of the turbine). Your generator or gearbox may prefer a particular rotational direction.

Inflow swirl angle

Swirl is a component of flow velocity that is tangential to the circle described by a station of the blade at a particular radius. This swirl velocity component added to the  other components changes the angle at which the flow passes through the rotor plane. A single stage turbine will assume that there is no net swirl on the incoming airflow and so the flow will be exactly normal to the plane of the rotor. The inflow swirl angle would then be set to 0.0 for all station radii. In the case of a multistage turbine, the later stages will see some swirl on the incoming flow caused by the torque reaction of the preceeding turbine stage. Since swirl from an preceeding turbine stage will vary depending on the station radius, then the swirl profile is specified by specifying the swirl at 5 station radii. The swirl for all other radii will be found by smooth interpolation from these 5 station reference points.

Induced velocity factor

The turbine blades are not often enclosed in a duct. This means that the pressure difference between the front and back faces of the rotor disk will not only decelerate the flow in an axial direction but will also accelerate the flow in an outward radial direction in front of the turbine and accelerate the flow in a inward radial direction behind the turbine. This is a circulation ring vortex caused by the lack of flow constraints at the tips of the blades and one component of this circulation is modelled as a reverse axial flow velocity component at the rotor plane. This circulation is sometimes called "downwash" by aircraft designers and they often try to make it constant over the wing span by using elliptical planforms. Once constant, aircraft designers treat it as a change of angle of attack of the wing ("induced angle of attack") and as an extra drag ("induced drag") but it is really that the air is moving downwards from the aircraft point of view in level flight and so effectively the plane must fly "uphill" to maintain height (hence the "drag") and angle of attack must take the downward velocity into account. These aircraft designer fictional drags and angles are not appropriate models for the turbine since the blades are rotating and not in linear motion. It is best to directly model the reverse flow axial velocity component and this is expressed here as a factor of the normal forward axial velocity component. If the normal axial velocity is (2/3)V and an induced velocity factor of 0.1 is specified then the actual axial flow velocity will be (2/3)(V-0.1V)=(2/3)(0.9)V.  The axial velocity at the rotor plane is thus modified by the downwash and thus forces, angles etc are modified and this can be both used in computing performance (effectively covering induced drag and tip loss type of effects) and the changed forces and angles  can be used to affect the blade setting angles and chord widths to maintain the desired overall deceleration pressure in the face of a reduced axial velocity at the rotor plane. Since induced velocity will vary depending on the station radius, then the induced velocity factor profile is specified by specifying the induced velocity factor at 5 station radii. The induced velocity factor for all other radii will be found by smooth interpolation from these 5 station reference points. The induced velocity factors should all be zero if the turbine is enclosed in an effective duct.

Click to ensure the "Model options" tab is showing as shown below:-

Vortex induced drag exists

This boolean option should normally be unticked (false) since vortex induced drag does not really exist and is a fiction invented by aircraft designers to save them from considering the velocity of the downwash at the wing itself due to circulation over the finite tips of the wings. If this input "Vortex induced drag exists" is ticked (true) then you should ensure that the later "Vortex induced velocity affects analysis" is unticked since these two options are different ways of looking at the same losses and if both are ticked then the losses will be included twice.

Vortex induced velocity affects design

This boolean option should normally be ticked (true). The vortex induced velocities specified via the "Induced Velocity factors" detailed on the "Fluid acceleration" tab affect the forces and angles of forces of the blade and so the blade width and setting angle needs to be adjusted to compensate for the change of local airflow velocity from that which would occur if there were no vortex induced velocity (e.g a ducted rotor). This option takes account of effects that are traditionally called "induced angle of attack" and "induced drag" by aircraft designers. Hugh Piggot's formulae do not compensate for induced velocity but this results in a lower wind retarding force which then results in a higher windspeed at that station which then results in the angles and forces being altered in a way that the original errors are self corrected. The slight variation from the Betz ideal makes little difference to efficiency since the efficiency function is flat topped near the optimum Betz design point.

Vortex induced velocity affects analysis

This boolean option should normally be ticked (true). The vortex induced velocities specified via the "Induced Velocity factors" detailed on the "Fluid acceleration" tab affect the forces and angles of forces of the blade and so the power generated is not as great as if there were no vortex induced velocity. This effect takes account of losses that are traditionally called "induced drag" by aircraft designers. This option should not be ticked if the "Vortex induced drag exists" input is ticked since they model the same losses but with a different viewpoint. If both are ticked then the same loss would counted twice.

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

This screen allows you to input the material properties and structural member skin thickness for the materials intended to be used to construct your blade or at least the strong parts of your blades.  In turbulent air, stresses will fluctuate and this can lead to fatigue after many thousands or millions of stress cycles, so you may want to reduce the stresses for some materials prone to fatigue such as steel and especially welded steel (high cycle fatigue strength < 35 MPa). Composite materials such as wood or fibreglass/resin have better fatigue life but are less stiff. This screen assumes a single square box section  provides the strength with one side being in tension and the other side being in compression, the other sides of the box not providing strength but preventing buckling, maintaining the spacing and preventing shear. The program will later report the side dimensions of the square needed. It is your job to vary the skin thicknesses or materials until the side length fits inside your chosen airfoil. In a typical airfoil section, you could use 2 such square box sections side by side in which case the effective strength is doubled and so you could half the skin thickness during construction from the value for a single box section. For a given station, the beam deflection is also computed and this should be checked to ensure it does not cause so much deflection that the blade hits the tower. Big turbines often tilt the whole rotor plane by 5 degrees and/or mount the blades at a forward angle from the rotation plane to increase clearances.

Material yield stress (MPa)

This is the material yield stress measured in MegaPascals. The yield stress is a stress below which the material will have not have been permanently deformed by the stress. It is less than the tensile strength stress. It can be looked up in data books for materials. You need to derate this if there are stress concentration points such as weld cracks, bolt holes, sharp corners etc, or if the material is prone to fatigue after high numbers of cycles (fatigue is the slow growth of a crack with every stress cycle)

Beam section skin thickness (m)

This is the thickness of the sheet or plate (in metres) used to construct the single square beam.

Elastic modulus (GPa)

This is the Young modulus of the material (it's stiffness) measured in GigaPascals. It is relevant to estimating the amount by which the beam is deflected by the force.

The other tabs have many more parameters that you can set. 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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