PROJECT SUMMARY REPORT
Flow Separation Control on Low-Pressure Turbine Blades
Using Plasma Actuators
Submitted To
The 2013-20014 Academic Year NSF AY-REU Program
Part of
NSF Type 1 STEP Grant
Sponsored By
The National Science Foundation
Grant ID No.: DUE-0756921
College of Engineering and Applied Science
University of Cincinnati
Cincinnati, Ohio
Prepared By
Josh Combs, Junior, Aerospace Engineering
Devon Riddle, Senior, Aerospace Engineering
Report Reviewed By:
Dr. Kirti Ghia
REU Faculty Mentor
Professor of Aerospace Engineering
School of Aerospace Systems
University of Cincinnati
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Flow Separation Control on Low-Pressure Turbine
Blades Using Plasma Actuators
Josh Combs1 and Devon Riddle2
Advisor: Dr. K. N. Ghia3
University of Cincinnati, Cincinnati, OH 45221
Abstract
At high altitudes, low-pressure turbines (LPT) experience flow separation in the gas turbine
engine. Air flow inside the turbine blade passage separates on the suction side of the blades due
to a loss of momentum within the boundary layer. As the size of this wake increases, the drag on
the LPT increases and the overall efficiency decreases. The phenomenon of flow separation on
LPT blades is investigated, including the multitude of flow control methodology, both passive and
active. Through the use of these flow control methods, it was discovered that the wake resulting
from the flow separation could be reduced or prevented. Plasma actuators ionize the air within
the boundary layer, resulting in a body force that increases the flow momentum. This will either
delay the point of separation, or eliminate it all together.
Three types of plasma actuators
investigated were Single Dielectric Barrier Discharge Plasma Actuators (SDBD), Glow Discharge
Actuators, and Synthetic Discharge Actuators. The next phase of this research project will use
our knowledge of flow separation and flow control methods to generate models using
computational fluid dynamics (CFD) software.
1
University of Cincinnati Junior, Aerospace Engineering
University of Cincinnati Senior, Aerospace Engineering
3 Professor of Aerospace Engineering, School of Aerospace Systems, University of Cincinnati
2
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Introduction
The need for unmanned aerial vehicles (UAVs) to operate at high altitudes and with greater
engine efficiency is growing.
Modern engine design aims to reduce manufacturing costs and fuel
consumption by reducing the overall engine weight. As P. Gonzalez et al. discussed, the low-pressure
turbine (LPT) accounts for 20-30% of the total weight in most engines, making it a prime choice for weight
reduction. This is done by reducing the blade count, which increases the aerodynamic loading on each
remaining blade. As the air flows inside the blade passage along the suction surface of the blade, the
pressure decreases first up to the shoulder and then gradually increases in the flow direction, which is
known as an adverse pressure gradient. If the flow does not have enough momentum to overcome this
pressure gradient, then it will separate from the surface, generating a wake that is proportional to drag.
The adverse pressure gradient is not the only issue with the aerodynamic efficiency of LPT
blades. High altitude long endurance (HALE) UAVs operate at around 60,000 feet so the Reynolds
number (Re) is significantly lower than that of a typical aircraft at lower altitude.
The value of the
Reynolds number also indicates whether streamlines along a body are smooth and regular, or random
and erratic. Intuitively, it seems that the latter flow is undesirable for any streamlined body, but actually
this turbulent flow is preferred because it delays and reduces the effects of flow separation. When the
flow is laminar, that is, steady and regular, it does not have enough momentum to overcome the adverse
pressure gradient. As discussed by Anderson (2011), 𝑅𝑒 ≈ 105 or less for external flow exhibits laminar
behavior, while 𝑅𝑒 ≈ 106 or greater, the turbulent behavior. However, Re is not only dependent on air
density (altitude) but also the characteristic length of the body. Also, the Re for which flow transitions
from laminar to turbulent will depend on the body shape.
Flow Control Methods
In this specific application, modern LPT blades are susceptible to flow separation due to an
adverse pressure gradient and at relatively low Reynolds number. There are several methods to delay the
point of separation and reduce the negative effects. These methods are generally divided into two main
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categories, passive and active techniques. Passive techniques are permanent devices that are fixed on
the body surface, and although they are beneficial for high altitude conditions, they create unnecessary
drag in other modes of operation. Active techniques are devices that may be “turned off” and are likewise
beneficial at high altitudes, but they usually add a considerable amount of weight to the aircraft and will
require an energy source. The focus of this research project will be on a relatively new active device
known as a plasma actuator.
One of the simplest passive devices was shown by Kwangmin Son et al. (2010). They fixed a trip
wire onto the surface of a sphere in order to induce a turbulent flow and study the change in drag. This
was done by varying the streamwise location of the trip wire, the size of the trip wire, and Reynolds
number of the flow.
They found that the drag coefficient decreased as each parameter increased.
However, once the azimuth location (referenced to the horizontal plane through the sphere center) was
greater than 70⁰, it had hardly any effect on reducing the drag. Ultimately, they achieved 60% drag
reduction because the turbulent flow carries higher momentum to delay the separation to the aft of the
sphere. Results of the drag reduction are shown in Figure 1.
Figure 1: Variations of the drag coefficient with the Reynolds number. □, Smooth sphere. ● = 20⁰;
x = 30⁰; ▲ = 40⁰; ■ = 50⁰; * = 60⁰; + = 70⁰, Trip wire locations.
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Synthetic jets (SJ) are an active device investigated by David Lengani et al. (2010). In this
experiment, a cavity is created inside of a flat plate. Air is sucked from the surface boundary layer into
the cavity, and then blown back into the boundary layer. This results in two counter-rotating vortices that
are high in momentum and thereby delay the flow separation. The plate was placed between two
contoured walls inside of a wind tunnel to simulate an adverse pressure gradient similar to what LPT
blades are subject to. A piston system generates the oscillating flow which comes through a slot on the
surface of the plate. The parameters include actuator frequency, jet to main flow velocity ratio and the jet
momentum coefficient. By plotting the velocity profiles along the plate surface, they found inflection
points when the SJ was turned off, indicating that the flow changed direction. In other words, flow
separation did occur. When the SJ was turned on, the inflection points are either delayed or non-existent,
as shown in Figure 2.
Figure 2: Velocity profiles along the plate surface
Plasma Actuators
Glancing further into active flow separation control methods, plasma actuators became the main
focus of the research partially due to the fact they were designed for aerodynamics flow control. Three
types of plasma actuators that were analyzed include single dielectric barrier discharge (SDBD) plasma
actuators, glow discharge plasma actuators and plasma synthetic jet actuators.
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SDBD plasma actuators are made of two separated layers of electrodes that are placed on the
opposite side of the dielectric material as shown in Fig. 4. There is a slight overlap between the dielectric
barrier materials, where the dielectric material is sandwiched between two electrodes. A voltage source
is used to power the electrodes and has the capability of ionizing the air surrounding the electrodes. This
means that the actuator pulses at a varied frequency which is what creates the plasma downstream of the
actuator. As the plasma forms and builds, it creates a body force on the fluid flow, helping it to move
downstream. The force built up behind the fluid flow is what accelerates the reattachment and has little
effect on the airflow once the reattachment occurs. It is noted that SDBD plasma actuators have a
plasma discharge containing a unique property where it can sustain a large volume discharge at
atmosphere pressure without arcing. The plasma discharge is self-limiting by preventing this arc and
maintaining its connection with the airfoil.
Figure 3: SDBD plasma actuator with the flow going from bottom to top
Figure 4: Basic schematic of the SDBD plasma actuator
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Figure 5: Airflow with plasma actuators on and off
The results of an experiment performed by W. Shyy et al. (2002) are shown in Fig. 5. The
cylinder was investigated using SDBD plasma actuators for landing gear noise reduction. Through this
experiment it was found that the plasma will only stay on the airfoil if the voltage travelling through the
actuator is continuously increasing. Figure 6 portrays what we are trying to develop in the future through
ANSYS in the fact that we want to use plasma actuators to control the flow between the airfoil and keep it
from creating a wake and therefore creating drag.
Glow Discharge plasma actuators are similar to SDBD plasma actuators, but unlike SDBD
plasma actuators, glow discharge plasma actuators can be placed directly behind the propeller
immediately attaching the flow to the airfoil. The glow discharge plasma actuator is placed upstream from
the flow separation developing plasma that forces the fluid through similar to how the SDBD plasma
actuators develop plasma before forcing the fluid flow downstream. Glow discharge actuators create
pulses that are sent to the electrodes with opposite polarities with different periods. This creates a beat
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frequency of the glow discharge plasma. This allows for a wide frequency range and promotes a swifter
transition into the shear layer of the separation bubble leading to an earlier reattachment.
In plasma synthetic jet actuators, the flow is described as quiescent flow where a circular plasma
region is shown to generate a vertical zero-net mass flux jet. This is where the name plasma synthetic jet
actuator developed from. As the actuator pulses, it creates a vortex ring ahead of the jet while another is
created near the actuator surface. With a varied frequency, multiple vortex rings are created close to the
airfoil in the fluid flow which increases the velocity and the force acting on the fluid flow.
Explained in the Shyy Trial, the buildup of plasma in glow discharge plasma actuators results
from the amount of energy added to a molecular gas. The gas will then split resulting from the collisions
between the particles that have enough kinetic energy to exceed the molecular binding energy creating
the buildup plasma behind the fluid flow.
Below are four figures that we are striving to develop in ANSYS to demonstrate the development
of flow over the chosen NACA airfoil at varied angles of attack. It is clear that as the angle of attack
increases, the point of flow separation occurs closer to the leading edge. Similarly, the size of the trailing
wake increases with angle of attack. A separation bubble is developed at the maximum angle of attack,
as shown in Fig. 4.
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Figure 4: Streamlines plotted from the inlet showing separation at a 10 degree angle of attack
Figure 5: Streamlines (NACA0012) at a 5 degree angle of attack
Figure 6: Streamlines (NACA0012) at a 0 degree angle of attack
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Figure 7: NACA 0012 Pathlines at a 10 degree angle of attack
Conclusions
This research project focused on using plasma actuators as an active technique to reduce flow
separation. Flow separation over a body is difficult to eliminate, but there are proven methods, in addition
to plasma actuators, that will reduce the negative effects.
The three plasma actuators that were
discussed operate in a similar fashion, in that they apply a body force to the flow to give it enough
momentum to overcome the adverse pressure gradient. It was shown using a cylinder that the resultant
wake of using plasma actuators is significantly smaller than that of not using plasma actuators. The
ANSYS models show baseline conditions at various angles of attack where no plasma actuators are
used. For future work, we will generate our own baseline models and incorporate plasma actuators to
show flow separation control. Also, we would like to explore thermal effects on flow separation, and
modeling using energy equations rather than momentum equations.
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Acknowledgements
Funding for this research was provided by the NSF CEAS AY REU Program, Part of NSF Type 1
STEP Grant ID No.: DUE-0756921.
References
[1]
Anderson, J.D. (2011). Fundamentals of Aerodynamics, 5th. McGraw-Hill
[2]
Gonzalez, P., Ulizar, I., Hodson, H.P., (2001). “Improved Blade Profiles for High Lift Low Pressure
Turbine Applications”, Wittle Laboratory, University of Cambridge, Cambridge, CB3 ODY, UK.
[3]
Son, K., Choi, J., Jeon, W.P., Choi, H., (2011). “Mechanism of Drag Reduction by a Surface Trip
Wire on a Sphere”, J. Fluid Mech., vol. 672, pp. 411-427.
[4]
Lengani, D., Simoni, D., Ubaldi, M., Zunino, P., Bertini, F., (2011). “Application of a Synthetic Jet
to Control Boundary Layer Separation under Ultra-High-Lift Turbine Pressure Distribution”, Flow
Turbulence Combust, 87:597-616.
[5]
Shyy, W., Jayaraman, B., Andersson, A., (2002) “Modeling of Glow Discharge-Induced Fluid
Dynamics”, Department of Mechanical and Aerospace Engineering, University of Florida,
Gainesville, Florida
[6]
Santhanakrishnan, A., Jacob, J., (2007). “Flow Control with Plasma Synthetic Jet Actuators”,
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Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, Oklahoma
[7]
Newcamp, J., (2005). “Effects of Boundary Layer Flow Control Using Plasma Actuator
Discharges”, Department of the Air Force Air University, Air Force Institute of Technology; Wright
Patterson Air Force Base, Ohio
[8]
Cline, M., Mullen, B., (2012). “Study of Separated Flow Over Low-Pressure Turbine Blades and
Automobiles Using Active Flow Control Strategies”, College of Engineering and Applied Science,
University of Cincinnati, Cincinnati, Ohio
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