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Frankhouser, Matthew WilliamNanosecond Dielectric Barrier Discharge Plasma Actuator Flow Control of Compressible Dynamic Stall
Master of Science, The Ohio State University, 2015, Aero/Astro Engineering
Dynamic stall is a performance-limiting phenomenon experienced by rotorcraft in directional and maneuvering flight. Dynamic stall occurs on the retreating blade due to the high angles of attack that are experienced by the blades. Increasing the angle of attack is required to overcome the asymmetry of lift across the rotor disk that is a result from the velocity disparities between the advancing and retreating blade. This works sets out to study and improve the performance of a dynamically pitching NACA 0015 airfoil. The airfoil is subjected to both an incompressible and compressible flow field to simulate the dynamics of a rotor blade with cyclic pitching. In this experimental investigation of dynamic stall flow control, the effectiveness of nanosecond dielectric barrier discharge (NS-DBD) plasma actuation will be evaluated as a means to exert control authority. The NS-DBD plasma actuation is generated by a high-voltage magnetic compression pulsed power supply that was designed and built at The Ohio State University. To measure the influence of plasma actuation on the flow, surface pressures on the airfoil were measured through discrete pressure taps located on both the suction and pressure surfaces. The surface pressures are used to calculate the lift and moment during the dynamic pitching cycle. To visualize the compressibility effects in the outer flow, shadowgraph imagery was used to capture features in the flow around the leading edge of the test article. Tests were conducted at static and oscillating angles of attack at both Mach 0.2 and 0.4, and Reynolds numbers of 1.2 million and 2.2 million respectively. Pitch oscillations were conducted at reduced frequencies of k = 0.05. Actuation frequencies varied from non-dimensional frequencies (F + ) of 0.78 to 6.09. Surface pressures acquired at Mach 0.2 without actuation applied agreed with historical data at static angles of attack, validating that the application of the actuator had limited intrusiveness to the flow. When subjected to pitch oscillations, plasma actuation reduced the severity of lift and moment stall by altering the development of the dynamic stall vortex at Mach 0.2. At Mach 0.4, marginal improvements were gained through actuation. Excitation resulted in a strong dynamic stall vortex that convected more slowly in comparison to the baseline case. Shadowgraph imagery revealed lambda shock waves forming over the first 15 percent of the airfoil chord in the same proximity of th

Committee:

James Gregory, PhD (Advisor); Jeffrey Bons, PhD (Committee Member); Mo Samimy, PhD (Committee Member)

Subjects:

Aerospace Engineering

Keywords:

dynamic stall; nanosecond dielectric barrier discharge plasma actuation; flow control

Sheehe, Suzanne Marie LanierHeat Release Studies by pure Rotational Coherent Anti-Stokes Raman Scattering Spectroscopy in Plasma Assisted Combustion Systems excited by nanosecond Discharges
Doctor of Philosophy, The Ohio State University, 2014, Chemistry
Heat release studies of plasma assisted combustion have been performed in fuel-air mixtures excited by nanosecond dielectric barrier discharges initially at room temperature and maintained at low pressure (~40 – 50 torr). The following topics have been extensively investigated: (i) the applicability of pure O2 broadband Rotational Coherent Anti-Stokes Raman Scattering spectroscopy at very low O2 pressures of ~8 torr or less to obtain rotational temperature, (ii) validation of a proposed low temperature fuel-oxidation kinetics mechanism fully decoupled from NOx chemistry, (iii) characterization of nanosecond pulse discharges in a dielectric barrier discharge cell and a pin-to-pin discharge geometry, and (iv) effect of fuel addition on heat release in a pin-to-pin discharge geometry at low pressure. For the first topic, the applicability of pure O2 broadband Rotational Coherent Anti-Stokes Raman Scattering (RCARS) Spectroscopy at very low O2 partial pressure of ~ 8 torr or less to obtain rotational temperature has been demonstrated. Very good experimental precisions of ~ ± 1 to 2 K has been demonstrated for diffuse and volumetric plasmas excited by a repetitively pulsed nanosecond discharge. It is shown that the electron-multiplication feature of an EMCCD camera increases the signal to noise ratio significantly. For the second topic, the pure O2 RCARS system was applied to the dielectric barrier discharge cell to obtain time-resolved temperature measurements in nanosecond pulse discharges in 20% O2-Ar, H2-O2-Ar and C2-H2-O2-Ar mixtures, initially at room temperature, operated at a high pulse repetition rate of 40 kHz, in plane-to-plane double dielectric barrier geometry at a pressure of 40 Torr. Nitrogen was deliberately excluded from the system so as to decouple NOx chemistry from the plasma fuel-oxidation processes. It was found that a 0-D model predictions for temperature are in very good agreement in the baseline mixture without fuel and the hydrogen containing mixtures. However, the model predicts that the heat release in hydrogen containing mixtures is only weakly dependent on equivalence ratio, which is inconsistent with experimental results. Furthermore, In C2H2 containing mixtures, the model consistently under-predicts the temperature, further delineating the need for more accurate low-temperature plasma/combustion chemistry decoupled from NOx processes for both hydrogen and ethylene fuels. For the third topic, plasma characterization has been carried out for the mixtures in the aforementioned dielectric barrier discharge cell in addition to air-fuel mixtures in a pin-to-pin discharge geometry. The pin-to-pin discharge was excited by a nanosecond discharge at 60 Hz. Broadband plasma emission images were obtained for both types of discharges using an (ICCD) camera. In the dielectric barrier discharge, the 20% O2-Ar and O2-Ar-H2 mixtures were both shown to be diffuse and volumetric. The O2-Ar-C2H4 mixtures, on the other hand, showed significant striation and plasma constriction. In the pin-to-pin discharge, the plasma filament in both air and air-hydrogen was fairly homogeneous along the discharge gap, but radially decreases outward from the filament centerline. The energy coupling to the plasma for both types of discharges was determined using current-voltage waveforms. The dielectric barrier discharge couples a very small amount of the energy, ~0.1 mJ/pulse, that is stored in the capacitive load formed during breakdown to the plasma. Good agreement between these energy coupling results and a prediction from a 0-D analytical model was found. On the other hand, the pin-to-pin discharge has a higher energy loading of ~3 mJ/pulse and a model is currently in development. On the fourth topic, in the pin-to-pin discharge geometry it is demonstrated that a fast heating and a slow heating regime exist in air and air-fuel mixtures and are clearly distinct from each other after the onset of the discharge pulse. It is indicated that air-ethylene mixtures do not exhibit a clear distinction between slow and fast heating. In all cases, with increasing fuel addition, the rate of the heat release increases. Radial temperature profiles were taken for air at three different time points relative to the onset of the pulse. The radius was found to be the same in all three cases, strongly indicating that there is no contraction or expansion of the plasma filament. Preliminary results with a 1-D model still in development show very good agreement, which is promising. It is expected that the model will attribute fast heating primarily to collisional quenching of N2 excited states in air and air-hydrogen containing mixtures. In ethylene mixtures, ethylene oxidation processes are expected to have a larger contribution as experimental results indicate a strong dependence on equivalence ratio. Slow heating is expected to be dominated by V-T transfer from vibrationally excited N2 by collisional quenching of O-atoms, with additional release by fuel-oxidation.

Committee:

Walter Lempert, PhD (Advisor); Anne McCoy, PhD (Committee Member); Terry Gustafson, PhD (Committee Member)

Subjects:

Chemistry; Engineering; Physical Chemistry

Keywords:

CARS; pure Rotational Coherent Anti-Stokes Raman Scattering Discharge; dielectric barrier discharge; plasma instability; Plasma Assisted Combustion; Gas heating; third-order non-linear susceptibility; pin-to-pin discharge; nanosecond discharge

Stanfield, Scott AlanA SPECTROSCOPIC INVESTIGATION OF A SURFACE-DISCHARGE-MODE, DIELECTRIC BARRIER DISCHARGE
Doctor of Philosophy (PhD), Wright State University, 2009, Engineering PhD

The use of aerodynamic actuators, such as leading edge slats, trailing edge flaps, roughing strips and ailerons interact with the air during flight, providing maneuverability for air vehicles. These mechanical devices have many inherent, detrimental attributes, such as space requirements on the wing, added wing weight, second response times, increased drag, and increased airframe vibration, resulting in the production of noise. The potential to eliminate or improve upon these detrimental attributes may be realizable by replacing the current mechanical actuators with plasma actuators. Specifically, the surface-discharge-mode, dielectric barrier discharge (SDBD), plasma actuator has a response time on the order of microseconds to milliseconds, does not increase vibration by mounting flush to the wing surface, does not increase drag, and adds negligible weight to the wing. Unfortunately, these devices are not yet powerful enough to perform many of the tasks required for aerodynamic applications; however, they have demonstrated the potential to do so, providing motivation for the current study. Currently, the approach of the research community has focused on coordinating studies designed to determine the physics of the device and parametric studies to determine optimal configurations required for immediate application.

In this work, an experimentally based study utilizing optical emission spectroscopy, current-voltage measurements, and a force balance have been successfully applied, contributing new, specific detail to the morphology and characterization of the SDBD. The results of this study were tailored to aid the development of the appropriate, essential physics required for computational modeling of the SDBD. Initially, force measurements of the induced thrust were obtained to demonstrate how week the induced thrust is, justifying the need for a fundamental study. These results are also important in understanding an apparent discrepancy in the reported dependence of the induced thrust upon applied voltage amplitude.

Electrical properties of the device such as breakdown voltage, surface charge voltage, effective capacitance with and without a discharge, electrical current, dissipated power, and the details of breakdown are measured as a function of applied voltage. The measured surface potential is of particular interest because it provides information about one of the boundary conditions needed to solve Maxwell equation’s of electromagnetics. Measurements showed that the surface charge potential along the dielectric surface is around 4000 and 4200 volts for the positive and negative voltage half-cycle, respectively, at an applied potential of 6000 volts.

Properties determined from emission, including the relative concentrations of N2(C3Πu) and N2+(B2Πg), and rotational and vibrational temperatures, as a function of position, voltage amplitude and phase of the driving voltage, have been measured. The spatially resolved relative concentrations of N2(C3Πu) and N2+(B2Πg) are useful in demonstrating the difference in structure between the discharge occurring during the positive voltage half-cycle versus the discharge occurring during the negative voltage half-cycle. The rotational temperature obtained from the 1st negative band system of N2+ was shown to be significantly greater than the rotational temperature obtained from the 2nd positive band system of N2 and was shown to be a direct consequence of the local electric field. This is shown to be important when calculating the rate constants for reactions involving ions and neutrals. For example, neglecting this deviation in temperature results in an order-of-magnitude difference in rate constants. Therefore in modeling the plasma, measurements show it is important to calculate the ion temperature via the Wannier relationship and then calculate the rate constants.

The details of these experiments including set-up, results, significance and discussion, along with an exhaustive summary of the current understanding of the surface-discharge-mode, dielectric barrier discharge, constitutes the bulk of this dissertation.

Committee:

James Menart, PhD (Advisor); William Bailey, PhD (Committee Member); Jerry Clark, PhD (Committee Member); Roger Kimmel, PhD (Committee Member); Joseph Shang, PhD (Committee Member); Henry Young, PhD (Committee Member)

Subjects:

Electrical Engineering; Fluid Dynamics; Mechanical Engineering; Physics

Keywords:

DBD; optical emission spectroscopy; rotational temperature; vibrational temperature; dielectric barrier discharge; aerospace

Marks, Christopher R.Surface Stress Sensors for Closed Loop Low Reynolds Number Separation Control
Doctor of Philosophy (PhD), Wright State University, 2011, Engineering PhD
Low Reynolds number boundary layer separation causes reduced aerodynamic performance in a variety of applications such as MAVs, UAVs, and turbomachinery. The inclusion of a boundary layer separation control system offers a way to improve efficiency in conditions that would otherwise result in poor performance. Many effective passive and active boundary layer control methods exist. Active methods offer the ability to turn on, off, or adjust parameters of the flow control system with either an open loop or closed loop control strategy using sensors. This research investigates the use of a unique sensor called Surface Stress Sensitive Film (S3F) in a closed loop, low Reynolds number separation control system. S3F is an elastic film that responds to flow pressure gradients and shear stress along its wetted surface, allowing optical measurement of wall pressure and skin friction. A new method for installing the S3F sensor to assure a smooth interface between the wall and wetted S3F surface was investigated using Particle Image Velocimetry techniques (PIV). A Dielectric Barrier Discharge (DBD) plasma actuator is used to control laminar boundary layer separation on an Eppler 387 airfoil over a range of low Reynolds numbers. Several different DBD plasma actuator electrode configurations were fabricated and characterized in an open loop configuration to verify separation control of the Eppler 387 boundary layer. The open loop study led to the choice of a spanwise array of steady linear vertical jets generated by DBD plasma as the control system flow effecter. Operation of the plasma actuator resulted in a 33% reduction in section drag coefficient and reattachment of an otherwise separated boundary layer. The dissertation culminates with an experimental demonstration of S3F technology integrated with a control system and flow effecter for closed loop, low Reynolds number separation control. A simple On/Off controller and Proportional Integral (PI) controller were used to close the control loop.

Committee:

Mitch Wolff, PhD (Advisor); Rolf Sondergaard, PhD (Committee Member); James Menart, PhD (Committee Member); Mark Reeder, PhD (Committee Member); Joseph Shang, PhD (Committee Member)

Subjects:

Aerospace Engineering; Engineering; Fluid Dynamics; Mechanical Engineering

Keywords:

low Reynolds number; fluid dynamics; surface stress sensitive film; flow control; separation control; S3F; plasma actuator; dielectric barrier discharge;