Setting tweaks to adjust blade size and strength by station
From the Aerodynamics main screen shown below, press the "Parameters"
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 "Tweaks" tab is selected as shown
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 "Tweaks" input tab allows you to make small adjustments at
particular stations to fix problems of low blade strength or hydrofoil
cavitation or low Reynolds number at a particular station radius. Such
problems will have been noticed when looking at detailed caclulation
results for a particular station radius (using the "Reports/Show
Working" menu item).
The fluid flow thrust on the blade tries to push the blade downwind.
This is resisted by the blade being fixed at the hub with the hub
pushing back on the blade in the upwind direction but that fixed point
is a long way from the the point at which the thrust is acting so there
is a "couple", or moment or turning force or beam bending force. Since
the blade does not actually rotate in this direction (the plane of the
airflow stream lines) the original "couple" must be balanced by an
equal couple set up by tension in the upwind surface of the blade beam
and compression in the downwind surface of the blade beam. A size
of a couple is determined by a force multiplied by a distance. If the
bending couple must be resisted by tension and compression operating
over the very small thickness of the blade then the forces for a given
couple are very great. If the distances can be made larger then the
forces for a given couple become proportionally smaller. There are two
ways to increase the distance over which the couple is resisted. One
way is to make the blade thicker. Another way is to attach a tension
member external to the blade on the upwind face at some reasonable
angle to provide an extra resisting couple. Such a tension member can
reduce the bending force at the blade roots to zero and much reduce the
bending moments elsewhere.
If your blade bends back under the thrust load too much, or if the
tension and compression forces are too great for the thin blade to
support, then the stiffness and strength requirement can be reduced by
a large factor of more 3 by providing a tension member (like a
wire) attached between a point near the blade tip and point on the axis
well upwind of the blades (e.g. on a greatly extended and pointed
central hub nose cone). Such a tension member converts much of
bending stress on the blade into a small compression stress in the
blade and a pure tension stress in the tension member (which can thus
be a wire or rope). Compression stresses may be preferable in materials
such as concrete or ceramic. Also compression stresses subtract from
the centrifugal tension stresses that dominate small turbines. The
problems of streamlining this guy wire and
avoiding oscillation remain.
- Tension member radius factor
- This is the station radius at which the tension member is
attached, expressed as a fraction of the tip radius (i.e. 0 means hub
and 1.0 means at the blade tip). The thrust forces are larger
nearer the tip since the blade tip sweeps out the largest area of fluid
flow, yet the bending force in the blade near the tip are near zero
since the distance of the forces from the station radius are small. An
optimum point of attachment of the tension member will be between 0.7
and 0.8 of the tip radius.
- Tension member attachment angle
- This is the angle that the tension member makes with the plane of
the rotor expressed in degrees. The larger the angle, the less tension
is required for a given couple. 90 degrees would mean the tension
member is directed directly upwind. This would be difficult to arrange
as there is nothing to which the other end of the tension member could
be attached. An angle of zero would mean the tension member lies
parallel to and internal to the blade. A practical angle might be
between 20 and 45 degrees.
- Lift coefficient additional to nominal
- This is an addition (or for negative numbers a subtraction) to
the nominal design lift coefficient that was specified on the "Airfoil"
tab. Since this tweak can be specified at 5 different stations (and is
smoothly interpolated for all stations in between) then the effective
design lift coefficient can be made fully variable over the stations.
So if the nominal lift coefficient was 0.85 and the tweak at station
radius factor 0.8 was -0.2 then the effective design lift coefficient
for station radius factor 0.8 would be 0.85-0.2 = 0.65. Two side
effects follow from this tweak of negative 0.2. One side effect
is that the blade chord width will be automatically designed larger to
get the optimum required absolute lift force from the reduced lift
coefficient. This increase in size then means a stronger blade and an
increased Reynolds number. It also means a reduced negative pressure on
the back of the blade (which can be important for hydrofoils). The
other side effect is that, since the airfoil is unchanged, the reduced
lift coefficient will be achieved by reducing the design angle of
attack for this station from the nominal value (by about 1 degree for
each 0.1 of lift coefficient). This in turn will change the shape
of the negative pressure curve which can again be important for
hydrofoils. You the user need make no changes to the nominal values for
lift coefficient or angle of attack on the "Airfoil" tab since the
tweaks are applied automatically during calculations and reports.
- Thickening factors
- This is a multiplicative factor to
the normal thickness of the airfoil shape that was specified on the
"Airfoil" tab. For example a "Wortmann FX 63-137" airfoil is
13.7% thick relative to a unit chord width. At weak stations of the
blade (e.g. near the hub) we may want to gain geometric strength by
thickening the airfoil section while retaining its normal lift
characteristics. So a thickening factor of 2.0 would give a Wortman
airfoil which was 27.4% thick relative to unit chord and be twice as
strong to resist bending for the same weight. The airfoil might not
give such good aerodynamic performance as the original but this may not
matter for station radius factors less than 0.5 since high lift to drag
ratios are much less important here than they are at the tip.
The other tabs have many more parameters that you can set. You
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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