Boundary Layer Flow Control Using Plasma Induced Velocity

An examination of the effects of plasma induced velocity on boundary layer flow was conducted. A pair of thin copper film electrodes spanned the test section, oriented at thirty degrees from normal to the free stream flow. An adverse pressure gradient was imposed over the electrode configuration using a pressure coefficient profile similar to that associated with suction side of a Pac-B low pressure turbine blade. In addition, suction was applied to keep flow attached on the upper wall, inducing separation over the electrode. The electrode is supplied by an AC source at three different power levels with the free stream flow at three separate chord Reynolds numbers. The chord length was based on the geometry of the simulated airfoil profile used for the upper wall of the test section. The flow turbulence intensity was varied by means of a passive grid in the upstream flow. Velocity data were collected using particle imaging velocimetry as well as with a boundary layer pitot probe. The power levels applied to the plasma were between 20 and 40 watts. The flow regimes studied were between chord Reynolds numbers of 50,000 to 100,000. It was found that the use of plasma to control the boundary layer enabled the flow to remain attached in the presence of an adverse pressure gradient. However, at the studied Reynolds numbers and electrode configuration the plasma was unable to affect an already separated flow regardless of the power input to the electrode. It was finally ascertained that two types of turbulent structures could be resolved, one being a counter-rotating vorticity pair and the other being a counter-rotating vorticity sheet

Effects of Boundary Layer Flow Control Using Plasma Actuator Discharges

This study addresses the usage and effects of atmospheric plasma discharges on the near wall flow conditions for a Pak-B low-pressure turbine blade. A plasma actuator was built normal to the freestream flow in a low-speed wind tunnel. The test section of the wind tunnel had a contoured upper wall geometry designed to mimic the suction side of a Pak-B turbine blade. A high frequency ac voltage source supplied three voltages in the kilovolt range at four Reynolds numbers in the experiment, between 10,000 and 103,000. The effect of the plasma on the near-wall boundary layer conditions was evaluated at each of the Reynolds numbers and each of the three voltage levels. The corresponding power levels were between 15 and 25 W. Particle image velocimetry (PIV) was used to determine the 2D boundary layer characteristics of the flow. This research showed that the plasma discharges were able to dramatically increase the flow velocity near the wall; however, the plasma was unable to reattach an already detached boundary layer. Boundary layer traces were taken to validate the PIV results. Additionally, multiple manufacturing techniques were evaluated in an effort to make the electrodes more usable in turbine blade applications

Experimental Investigation of Active Control of Bluff Body Vortex Shedding

Mean and fluctuating forces acting on a body are strongly related to vortex shedding generated behind it. Therefore, it is possible to obtain substantial reductions of at least the unsteady forces if vortex shedding is controlled or its regularity is reduced. While conventional active flow control methods are mainly concerned with direct interaction with, and alteration of, the mean flow about a body, modern techniques involve altering existing flow instabilities using relatively small inputs to obtain large-scale changes of mean flows. Aerodynamic flow control may be intended to delay or suppress boundary layer separation through creation of a boundary layer downstream from the control input that is able to withstand adverse pressure gradients imposed by the outer (global) flow. In the present work, aerodynamic characteristics of a circular cylinder at Re=156,000 and an axisymmetric body (ogive cylinder) at Re=170,000 are first analyzed using a proposed phase averaging technique for the Particle Image Velocimetry (PIV) data. Later, the effect of plasma actuators on the aerodynamic characteristics of these bodies is investigated. When plasma actuators were placed 10° upstream of the separation point on the circular cylinder, momentum addition, and maybe the effect of local heating, modified the streamwise pressure gradient, leading to the establishment of a thinner boundary layer downstream. Phase synchronization of vortex shedding was also obtained for Re=156,000 for a narrow frequency band of the carrier signal of the actuators when they operated with a 90° phase shift To the knowledge of the author no other method has been shown to achieve vortex shedding control up to this high a Reynolds number. Effects of the different configurations of plasma actuators on the circumference, on the base, and in a streamwise direction were investigated for the ogive cylinder. It was observed that direct alteration of the mean flow about a body was not as effective as the boundary layer flow control where the flow instabilities are exploited. Also, the three dimensionalities in this flow made it significantly more complex to analyze

Charge Mitigation Technologies for Aircraft Platforms

Research into ion-based advanced propulsion systems, such as air-breathing Hall effect thrusters on high-velocity aircraft and ion-propelled thrusters on spacecraft, necessitates addressing accompanying residual electric charge accumulation on the ungrounded flight platform. An experimental testbed was constructed to assess charge mitigation technologies and their effectiveness on aircraft. A Van de Graaff generator provided static charge accumulation levels exceeding a megavolt when combined with a high voltage direct current source generator. This research attached an isolated airfoil structure to the Van de Graaff generator\u27s lower terminal to measure the induced leakage current under various applied environmental conditions, including up to three static wicks along the structure\u27s trailing edge, airflow across the structure of up to 10 m/s, and an insulative painted coating. The airfoil was a symmetric teardrop shape; air flowed over the rounded edge first to the tapered edge. Statistical tests indicated airflow improved a conductive airfoil\u27s leakage current at α = 0.0739. The average increase was -0.1256 µA. No statistically significant improvements were observed with an insulative airfoil

Turbulent drag reduction using surface plasma

An experimental investigation has been undertaken in a wind tunnel to study the induced airflow and drag reduction capability of AC glow discharge plasma actuators. Plasma is the fourth state of matter whereby a medium, such as air, is ionized creating a system of electrons, ions and neutral particles. Surface glow discharge plasma actuators have recently become a topic for flow control due to their ability to exert a body force near the wall of an aerodynamic object which can create or alter a flow. The exact nature of this force is not well understood, although the current state of knowledge is that the phenomenon results from the presence of charged plasma particles in a highly non-uniform electric field. Such actuators are lightweight, fully electronic (needing no moving parts or complicated ducting), have high bandwidth and high energy density. The manufacture of plasma actuators is relatively cheap and they can be easily retrofitted to existing surfaces. The first part of this study aims at characterising the airflow induced by surface plasma actuators in initially static air. Ambient air temperature and velocity profiles are presented around a variety of actuators in order to understand the nature of the induced flow for various parameters such as applied voltage, frequency, actuator geometry and material. It is found that the plasma actuator creates a laminar wall jet along the surface of the material on which it is placed. The second part of the study aims at using plasma actuators to reduce skin-friction drag in a fully developed turbulent boundary layer. Actuators are designed to induce spanwise forcing near the wall, oscillating in time. Thermal anemometry measurements within the boundary layer are presented. These show that the surface plasma can cause a skin-friction drag reduction of up to 45% due to the creation of streamwise vortices which interact with, and disrupt the near-wall turbulence production cycle

Boundary Layer Flow Acceleration by Paraelectric and Peristaltic EHD Effects of Aerodynamic Plasma Actuators

Plasma actuators based on the OAUGDP TM (One Atmosphere Uniform Glow Discharge Plasma) promise to be a convenient method to modify aerodynamic boundary layers. The development of the OAUGD Plasma has made it possible to locate enough plasma actuators on airfoils and the wings and fuselage of aircraft to have significant effects on flow control and re-attachment at relatively low power cost. In this study OAUGD Plasma actuators have been used to effect flow re-attachment and to manipulate aerodynamic flows, possibly leading to improved methods of flight control. The OAUGDPTM actuators have been tested in the 7 x 11 Inch Low Speed Wind Tunnel at the NASA Langley Research Center during several experimental campaigns that began in the mid 1990’s. The 7 x 11 Inch Low Speed Wind Tunnel is instrumented to conduct drag measurements, smoke flow visualization tests, Pitot tube velocity profile measurements and airfoil flow re-attachment visualization studies. This thesis is concerned with two EHD (electrohydrodynamic) flow control methods that utilize only RF displacement currents to produce the body forces that accelerate the plasma; paraelectric and peristaltic (traveling wave) flow acceleration. Paraelectric flow acceleration is achieved when the applied electric field acts on the net charge density of the plasma, to provide a body force capable of accelerating the neutral gas to velocities as high as 10 m/sec. During the acceleration process, the plasma moves paraelectrically towards increasing electric field gradients, and drags the neutral gas along with it as the result of frequent ion-neutral Lorentzian collisions. Peristaltic flow acceleration results from a traveling electrostatic wave, analogous to the apparent motion of light in a phased array of bulbs on a theatre marquee. To produce a traveling electrostatic wave, adjacent plasma actuators are energized at progressively larger phase angles. The resulting horizontal electric field produces a body force that accelerates the plasma. A OAUGDPTM panel or a plasma actuator intended for aerodynamic flow acceleration consists of linear strip electrodes adhering to either side of a dielectric panel. The actuators are energized using RF power at voltages between 0 and 10 kV, and frequencies between 0.5 kHz and 8 kHz. A major contribution described in this thesis was the development of flexible and ceramic panels, the polyphase signal generator based on LabVIEW, and accessories for the polyphase power supply. During development of the plasma actuators, many electrode geometries were tested to achieve the best operating conditions i.e. highest flow velocity. This thesis presents experimental results from several plasma actuator configurations, and performance data from both paraelectric and peristaltic flow acceleration

IMECE2010-37324 TURBULENT BOUNDARY LAYER SEPARATION CONTROL BY USING DBD PLASMA ACTUATORS: PART Ⅰ-EXPERIMENTAL INVESTIGATION

ABSTRACT Turbulent boundary layer separation is an important issue for a variety of applications, one of which is S-shaped aircraft engine intakes. The turbulent separation at the engine intake causes inlet flow distortion, which can deteriorate engine performance, cause fatigue and reduce engine component life. Various flow control techniques have been applied for turbulent boundary layer separation control, such as vortex generators, vortex generator jets and synthetic jets. The recent advent of dielectric barrier discharge (DBD) plasma actuators can potentially provide a robust method for the control of turbulent boundary layer separation. Compared to other flow control techniques, these new actuators are simple, robust and devoid of moving mechanical parts, which make them ideal for aerodynamic applications. The present work studies the effects of DBD plasma actuators on the suppression of 2-D turbulent boundary layer separation induced by an imposed adverse pressure gradient. First, the flow field with and without actuation in a low-speed wind tunnel is investigated experimentally by Particle Image Velocimetry (PIV) measurements. The results show that plasma actuation can suppress turbulent boundary layer separation in both continuous and pulsed modes. In the pulsed mode, the actuation with an optimal actuation frequency, corresponding to a dimensionless frequency of order one, is found to most effectively suppress the turbulent separation. Moreover, the effects of plasma actuation on the flow is demonstrated and analyzed by using Proper Orthogonal Decomposition (POD). The effect of the actuation is found to be correlated to the second POD mode which corresponds to large flow fluctuations

Advanced Ignition Strategies for Future Internal Combustion Engines with Lean and Diluted Fuel-Air Mixtures

The main objective of this research was to study the mechanisms of the spark ignition process of lean or diluted fuel-air mixtures under enhanced gas flow conditions for applications in future internal combustion engines. Various spark ignition strategies were deployed by controlling the spark discharge process via different spark ignition hardware configurations. Modulated spark discharge parameters, such as enhanced discharge power, prolonged discharge duration, and boosted discharge current were facilitated in the research. The impact of gas flow on the spark discharge process in air was investigated under varying air flow conditions with a range of flow velocities from 0 m/s to 60 m/s. The ignition performance of the spark strategies was investigated with lean or diluted fuel-air mixtures under controlled gas flow conditions in an optical constant volume combustion chamber test platform. The mixture flow velocity across the spark gap ranged from 0 m/s to 35 m/s during the combustion tests.Experiments were carried out with air as the background media. Short circuits and restrikes were observed under air flow conditions. The frequency of these occurrences increased with increased air flow velocity. The length of the spark plasma increased, due to the stretch of the plasma channel by the air flow. The plasma was stretched at a speed similar to the air flow velocity across the spark gap. The maximum length of the spark plasma was affected by the air flow velocity and the spark gap size. The spark discharge duration reduced with increased air flow velocity. To enhance the ignition of a lean or diluted fuel-air mixture under quiescent conditions, high spark discharge power or high spark discharge current were applied. With equivalent spark discharge energy, a larger flame kernel was achieved by the high-power spark whereas the impacts of spark discharge current level and discharge duration during the arc and glow phases were insignificant on the flame kernel growth. A transient high-current spark also generated a larger flame kernel, although with much higher spark energy as compared with that from a conventional spark. Under gas flow conditions, both the spark discharge current magnitude and discharge duration were critical for the flame kernel growth. It is postulated that this kernel growth was the result of a prolonged spark discharge duration effectively increasing the interaction volume between the plasma channel and the combustible gas engulfed by the mixture flow. Consequently, a longer spark discharge duration proved beneficial in establishing a larger flame kernel, probably because the spark discharge current was sufficient to support the flame kernel growth. Indeed, it was observed that boosted spark current was advantageous for the flame kernel growth, especially at higher flow velocities. However, the high-power spark and transient high-current spark proved to be less effective with higher flow velocities, probably because of the short discharge duration

Documentation and Control of Flow Separation on a Low Pressure Turbine Linear Cascade of Pak-B Blades Using Plasma Actuators

This work involved the documentation and control of flow separation that occurs over low pressure turbine (LPT) blades at low Reynolds numbers. A specially constructed linear cascade was utilized to study the flow field over a generic LPT cascade consisting of Pratt & Whitney "Pak-B" shaped blades. Flow visualization, surface pressure measurements, LDV measurements, and hot-wire anemometry were conducted to examine the flow fields with and without separation control. Experimental conditions were chosen to give a range of chord Reynolds numbers (based on axial chord and inlet velocity) from 10,000 to 100,000, and a range of freestream turbulence intensities from u'/U(infinity) = 0.08 to 2.85 percent. The blade pressure distributions were measured and used to identify the region of separation that depends on Reynolds number and the turbulence intensity. Separation control was performed using dielectric barrier discharge (DBD) plasma actuators. Both steady and unsteady actuation were implemented and found to work well. The comparison between the steady and unsteady actuators showed that the unsteady actuators worked better than the steady ones. For the steady actuators, it was found that the separated region is significantly reduced. For the unsteady actuators, where the signal was pulsed, the separation was eliminated. The total pressure losses (a low Reynolds number) was reduced by approximately a factor of two. It was also found that lowest plasma duty cycle (10 percent in this work) was as effective as the highest plasma duty cycle (50 percent in this work). The mechanisms of the steady and unsteady plasma actuators were studied. It was suggested by the experimental results that the mechanism for the steady actuators is turbulence tripping, while the mechanism for the unsteady actuators is to generate a train of spanwise structures that promote mixing