28-07-2012, 11:22 AM
THE WIND TURBINE
BonusTurbine.pdf (Size: 677.99 KB / Downloads: 45)
THE AERODYNAMICS OF
THE WIND TURBINE
important and most visible part of the
wind turbine. It is through the rotor
that the energy of the wind is transformed
into mechanical energy that turns
the main shaft of the wind turbine.
We will start by describing why the
blades are shaped the way that they are
and what really happens, when the
blades rotate.
BASIC THEORY
Aerodynamics is the science and study
of the physical laws of the behavior of
objects in an air flow and the forces that
are produced by air flows.
The front and rear sides of a wind
turbine rotor blade have a shape roughly
similar to that of a long rectangle,
with the edges bounded by the leading
edge, the trailing edge, the blade tip and
the blade root. The blade root is bolted to
the hub.
The radius of the blade is the distance
from the rotor shaft to the outer edge of
the blade tip. Some wind turbine blades
have moveable blade tips as air brakes,
and one can often see the distinct line
separating the blade tip component from
the blade itself.
If a blade were sawn in half, one
would see that the cross section has a
streamlined asymmetrical shape, with the
flattest side facing the oncoming air flow
or wind. This shape is called the bladeÕs
aerodynamic profile.
THE AERODYNAMIC PROFILE
The shape of the aerodynamic profile is
decisive for blade performance. Even
minor alterations in the shape of the
profile can greatly alter the power curve
and noise level. Therefore a blade designer
does not merely sit down and outline
the shape when designing a new blade.
The shape must be chosen with great care
on the basis of past experience. For this
reason blade profiles were previously
chosen from a widely used catalogue of
airfoil profiles developed in wind tunnel
research by NACA (The United States
National Advisory Committee for Aeronautics)
around the time of the Second
World War.
BEHAVE IN THE SAME WAY
Returning to the wind turbine blade, just
as in the situation for the cyclist, we can
observe the aerodynamic and force
diagrams in two different situations,
when the wind turbine is stationary and
when it is running at a normal operational
speed. We will use as an example the
cross section near the blade tip of a
Bonus 450 kW Mk III operating in a
wind speed ÒvÒ of 10 m/s.
WHAT HAPPENS WHEN
THE WIND SPEED CHANGES?
The description so far was made with
reference to a couple of examples where
wind speed was at a constant 10 m/s.
We will now examine what happens
during alterations in the wind speed.
In order to understand blade behavior
at different wind speeds, it is necessary
to understand a little about how lift and
drag change with a different angle of
attack. This is the angle between the
resulting wind ÒwÓ and the profile chord.
In the drawing below the angle of
attack is called ÒaÓ and the setting
angle is called ÒbÓ.
The setting angle has a fixed value at
any one given place on the blade,
but the angle of attack will grow as the
wind speed increases.
BonusTurbine.pdf (Size: 677.99 KB / Downloads: 45)
THE AERODYNAMICS OF
THE WIND TURBINE
important and most visible part of the
wind turbine. It is through the rotor
that the energy of the wind is transformed
into mechanical energy that turns
the main shaft of the wind turbine.
We will start by describing why the
blades are shaped the way that they are
and what really happens, when the
blades rotate.
BASIC THEORY
Aerodynamics is the science and study
of the physical laws of the behavior of
objects in an air flow and the forces that
are produced by air flows.
The front and rear sides of a wind
turbine rotor blade have a shape roughly
similar to that of a long rectangle,
with the edges bounded by the leading
edge, the trailing edge, the blade tip and
the blade root. The blade root is bolted to
the hub.
The radius of the blade is the distance
from the rotor shaft to the outer edge of
the blade tip. Some wind turbine blades
have moveable blade tips as air brakes,
and one can often see the distinct line
separating the blade tip component from
the blade itself.
If a blade were sawn in half, one
would see that the cross section has a
streamlined asymmetrical shape, with the
flattest side facing the oncoming air flow
or wind. This shape is called the bladeÕs
aerodynamic profile.
THE AERODYNAMIC PROFILE
The shape of the aerodynamic profile is
decisive for blade performance. Even
minor alterations in the shape of the
profile can greatly alter the power curve
and noise level. Therefore a blade designer
does not merely sit down and outline
the shape when designing a new blade.
The shape must be chosen with great care
on the basis of past experience. For this
reason blade profiles were previously
chosen from a widely used catalogue of
airfoil profiles developed in wind tunnel
research by NACA (The United States
National Advisory Committee for Aeronautics)
around the time of the Second
World War.
BEHAVE IN THE SAME WAY
Returning to the wind turbine blade, just
as in the situation for the cyclist, we can
observe the aerodynamic and force
diagrams in two different situations,
when the wind turbine is stationary and
when it is running at a normal operational
speed. We will use as an example the
cross section near the blade tip of a
Bonus 450 kW Mk III operating in a
wind speed ÒvÒ of 10 m/s.
WHAT HAPPENS WHEN
THE WIND SPEED CHANGES?
The description so far was made with
reference to a couple of examples where
wind speed was at a constant 10 m/s.
We will now examine what happens
during alterations in the wind speed.
In order to understand blade behavior
at different wind speeds, it is necessary
to understand a little about how lift and
drag change with a different angle of
attack. This is the angle between the
resulting wind ÒwÓ and the profile chord.
In the drawing below the angle of
attack is called ÒaÓ and the setting
angle is called ÒbÓ.
The setting angle has a fixed value at
any one given place on the blade,
but the angle of attack will grow as the
wind speed increases.