by Fayaz Rasheed (2005 - 2009 batch)
This document contains excerpts from “Flow
Separation Control on Wind Turbine Blades Using an Array of Fluidic
Oscillators,” MPhil Thesis, The University of Manchester, 2012
Scope: The article offers insights into the
importance of flow control technology and the associated research involved.
Students currently enrolled in engineering programmes can obtain ideas for
future projects, engineers in industry can implement this technology for their
product and research.
The world today is dependent highly
dependent on non-renewable energy resources to meet the everyday needs. The
ever growing fuel prices and the depleting resources have put strain on the
dependence of the non-renewable resources. In addition to the depletion, these
sources of energy cause pollution and damage the environment.
The future holds for renewable sources of
energy such as wind, solar, tidal energy etc. Wind energy is becoming a popular
form of usable energy source and uses efficient wind turbines to draw energy.
It is predicted that 80% of U.K’s energy will be drawn from wins by 2050 and
during the same period countries such as Denmark will be fully dependent on
wind energy and totally independent of non- renewable form of energy. It is
also predicted that during the same period UK will be able to export energy
using these renewable resources.
Increase in the efficiency of wind turbines
is critical for the efficient use of wind energy. Researchers around the globe
have been studying different methods of increasing efficiency of existing wind
turbines including the design and development of wind turbines blades which
would increase the overall efficiency of the wind turbines. One of the methods
popularly used in wind turbines is the use development of blade for increasing
the overall efficiency of the aerofoil.
Separation of flow over the wind turbine blades
can cause loss of lift and hence low efficiency. The efficiency of wind
turbines can also be increased using flow control technologies. Flow control
works in such a way that the separation of flow is delayed, hence increasing
the lift and hence the efficiency of the aerofoil.
Figure
1: Separation of Flow from the surface [1]
The achievement of the control of
separation by manipulation of the flow field is done with the help of flow control
devices. These flow control devices assist in bringing changes in the flow field,
which help in the bringing about effective flow control. The flow control methods can be classified into
active and passive flow control methods. Active flow control methods employ the
use of moving devices such as fluids or plasma to achieve flow separation
control. Passive flow control devices, on
the other hand, has non-moving parts such as turbulators or LEX (Leading Edge
eXtension) in order to control flow separation.
Figure
2: Flow control Technologies [2]
Active flow control technologies involve the
addition of energy to the flow field. This addition of energy can take the form
of addition of steady or unsteady energy inputs without considering the state
of the flow field. This type of active flow control is known as predetermined
method. The use of piezoelectric actuators, plasma actuators, jet vectoring,
post-stall lift enhancement and form drag reduction using oscillatory blowing
come under this category.
On the other hand, the use of an input
system such as sensor to sense the flow field and provide input to the actuator
continuously would be an effective method.
This type of flow separation control method is termed as interactive
methods of flow separation control. The
flow in this case can be effectively controlled based on the input from the
sensors. The interactive control can be
made possible by a feedback or a feed-forward loop. Feedback (closed) loop involves sensing the
actuated flow behind the actuator and the feed-forward (open) involves sensing
the flow ahead of the actuator. The
feedback system can enable the control of energy introduced into the flow. Flow field variations downstream and upstream
can be induced as a result of flow interaction with the sensors and actuators.
The active flow control technologies posses
a few advantages as compared to the passive flow control technologies. The addition of small and localised energy
input enables better flow control, which controls the natural stability of the
flow. Effective control is made when the control is introduced at a high
receptivity region. These high receptive points are the regions where boundary
layer transition or flow separation occurs. Another advantage of the control is
the ability to control complex and dynamical processes like turbulence
production in turbulent boundary layers.
This enables the reduction in skin friction drag and hence the viscous
drag where the reduction is proportional to the area covered by the drag.
Fluidic oscillator is a device that can be
used as active flow control devices for flow control. Fluidic oscillator works with the help of
coanda effect (the sticking to the wall phenomenon) and produced pulsated flow
which turbulises the boundary layer and in turn increases efficiency. The
increase Since the fluidic oscillators have no moving parts they can be used
for the long term operation.
Figure
3: A Fluidic Oscillator prototype designed by F. Rasheed at the University of
Manchester [2]
A typical fluidic oscillator comprise of an
input port, two control ports and two exit ports. A feedback loop is connected
to the control ports in order to control the frequency of the exit fluid flow.
The testing of these systems involves
experimental tests and to a lesser extent computational tests. The experimental
test involves two parts:
1) Characteristic analysis of output flow
from the fluidic oscillator using Hotwire anemometry. Here, the exit flow is
measured to determine the flow velocity and frequency using a hotwire probe
connected to a data acquisition system.
2) The second part involves the wind tunnel
tests and the flow is studied with the help of Tuft flow visualisation and Oil
flow visualisation.
The development of an array of fluidic
oscillators place beneath the wind turbine blades will ensure proper control of
flow ensuring high efficiency wind turbines with low maintenance. This
technology hence has a promising future.
Figure 4:
Initial wind tunnel test model [3]
A cool simulation
of the work involved can be found in the link (or QR code) given below.
References:
[1] F. Rasheed, “Flow Separation Control on Wind Turbine Blades Using an
Array of Fluidic Oscillators,” 2012, MPhil Thesis, The University of Manchester
[2] Kral L.D.,
1998, “Active Flow
Control Technology,” Washington
University, ASME Fluids Engineering Division Technical Brief
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