PROJECT SUMMARY REPORT

Flow Separation Control on Low-Pressure Turbine Blades

Using Plasma Actuators

Submitted To

The 2013-20014 Academic Year NSF AY-REU ProgramPart of

NSF Type 1 STEP Grant

Sponsored ByThe National Science FoundationGrant ID No.: DUE-0756921

College of Engineering and Applied Science University of Cincinnati

Cincinnati, Ohio

Prepared By

Josh Combs, Junior, Aerospace EngineeringDevon Riddle, Senior, Aerospace Engineering

Report Reviewed By:

Dr. Kirti GhiaREU Faculty Mentor

Professor of Aerospace EngineeringSchool 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.

Introduction

1 University of Cincinnati Junior, Aerospace Engineering2 University of Cincinnati Senior, Aerospace Engineering

3 Professor of Aerospace Engineering, School of Aerospace Systems, University of Cincinnati

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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), ℜ≈105 or less for external flow exhibits laminar

behavior, while ℜ≈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

categories, passive and active techniques. Passive techniques are permanent devices that are fixed on

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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, 5 th . 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”,

Department of Mechanical Engineering, University of Kentucky, Lexingtong, Kentucky: School of

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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