Eric Matlis

Sensing and control of flow separation using plasma actuators

Single dielectric barrier discharge plasma actuators have been used to control flow separation in a large number of applications. An often used configuration involves spanwise-oriented asymmetric electrodes that are arranged to induce a tangential wall jet in the mean flow direction. For the best effect, the plasma actuator is placed just upstream of where the flow separation will occur. This approach is generally more effective when the plasma actuator is periodically pulsed at a frequency that scales with the streamwise length of the separation zone and the free-stream velocity. The optimum frequency produces two coherent spanwise vortices within the separation zone. It has been recently shown that this periodic pulsing of the plasma actuator could be sensed by a surface pressure sensor only when the boundary layer was about to separate, and therefore could provide a flow separation indicator that could be used for feedback control. The paper demonstrates this approach on an aerofoil that is slowly increasing its angle of attack, and on a sinusoidally pitching aerofoil undergoing dynamic stall. Short-time spectral analysis of time series from a static pressure sensor on the aerofoil is used to determine the separation state that ranges from attached, to imminent separation, to fully separated. A feedback control approach is then proposed, and demonstrated on the aerofoil with the slow angle of attack motion.

Control of Stationary Cross-flow Modes Using Patterned Roughness at Mach 3.5

Spanwise-periodic roughness designed to excite selected wave lengths of stationary cross-flow modes was investigated in a 3-D boundary layer at Mach 3.5. The test model was a sharp-tipped 14° right-circular cone. The model and integrated sensor traversing system were placed in the Mach 3.5 Supersonic Low Disturbance Tunnel (SLDT) equipped with a “quiet design” nozzle at NASA Langley RC. The model was oriented at a 4.2° angle of attack to produce a mean cross-flow velocity component in the boundary layer over the cone. Three removable cone tips have been investigated. One has a smooth surface that is used to document the baseline (“natural”) conditions. The other two have minute “dimples” that are equally spaced around the circumference, at a streamwise location that is just upstream of the linear stability neutral growth branch for cross flow modes. The azimuthal mode numbers of the dimpled tips were selected to either enhance the most amplified wave numbers or to suppress the growth of the most amplified wave numbers. The results indicate that the stationary cross-flow modes were highly receptive to the patterned roughness.

Plasma Lens for Optical Path Difference Control

This research investigated the feasibility of a plasma lens for wave front control of coherent light sources. The approach is based on the relation between a plasma electron density and its index of refraction. The plasma was encapsulated in a hollow glass cylinder with flat optical glass at its ends. Air in the glass cylinder was ionized using a dielectric barrier discharge. The wave front distortion produced by the ionized air was characterized by placing the plasma lens in one arm of a Michelson interferometer setup. The effect of gas pressure and plasma power were investigated. The results were compared with a derived analytic model that related the electron density and optical path difference to the plasma power. The agreement between the experiment and analytic model was very good, especially at the higher plasma power levels. The maximum optical path difference increased with the gas pressure inside the lens. A maximum optical path difference of approximately 1.5 μm was achieved in the experiments. This brackets optical path difference levels that are typical of aero-optic applications, and otherwise corrected using electromechanical deformable mirrors. Although air was used as the gas in the plasma lens in these feasibility experiments, the use of Penning mixtures would further increase possible optical path difference levels and provide greater dynamic range.

AC plasma anemometer—characteristics and design

The characteristics and design of a high-bandwidth flow sensor that uses an AC glow discharge (plasma) as the sensing element is presented. The plasma forms in the air gap between two protruding low profile electrodes attached to a probe body. The output from the anemometer is an amplitude modulated version of the AC voltage input that contains information about the mean and fluctuating velocity components. The anemometer circuitry includes resistance and capacitance elements that simulate a dielectric-barrier to maintain a diffuse plasma, and a constant-current feedback control that maintains operation within the desired glow discharge regime over an extended range of air velocities. Mean velocity calibrations are demonstrated over a range from 0 to 140 m s?1. Over this velocity range, the mean output voltage varied linearly with air velocity, providing a constant static sensitivity. The effect of the electrode gap and input AC carrier frequency on the anemometer static sensitivity and dynamic response are investigated. Experiments are performed to compare measurements obtained with a plasma sensor operating at two AC carrier frequencies against that of a constant-temperature hot-wire. All three sensors were calibrated against the same known velocity reference. An uncertainty based on the standard deviation of the velocity calibration fit was applied to the mean and fluctuating velocity measurements of the three sensors. The motivation is not to replace hot-wires as a general measurement tool, but rather as an alternative to hot-wires in harsh environments or at high Mach numbers where they either have difficulty in surviving or lack the necessary frequency response.

Geometric Optimization of a Cylindrical Plasma Adaptive Optics Lens

Plasma is a dynamic optical medium with potential applications in the field of aero-optics for wavefront control. The objective is to develop “plasma adaptive optic” devices which rely upon the relationship between plasma electron density and index of refraction. The advantages of plasma adaptive optic devices are that they have no moving parts and can have temporal responses two orders of magnitude higher than the fastest deformable mirror. Therefore, plasma adaptive optic devices have the potential to be more robust, less subject to fatigue, and faster than conventional technology. Experimental results and a theoretical model are presented which investigate the spatial distribution of the plasma inside a cylindrical plasma adaptive optic lens. The experiment reveals two distinct plasma formation regimes that occur depending on the lens geometry. The theoretical model is used to help identify the geometric parameters required to produce each plasma regime. The excellent agreement between the experiment a...

Quantitative Hot-Wire Measurements in Supersonic Boundary Layers

Mean and time-resolved measurements in a supersonic boundary layer were performed in the Mach 3.5 quiet tunnel facility at the NASA Langley Research Center. This facility uses an annular bleed suction system to remove the turbulent boundary layer, thus reducing the disturbance intensities in the measurement region. A frequency-compensatedconstant current hot-wire anemometer was used to obtain fluctuation data in the boundary layer of a sharp cone at zero angle of attack. The hotwire was calibrated against the mean mass-flux profiles provided by solutions of the similarity profiles for compressible Blasius flow. A stability analysis code provided by Langley was used to solve parabolized stability equations to provide predictions of the most amplified wave-numbers, frequencies, and N-factors for the Tollmien-Schlicting instability. The results from these computations are compared to the experimental measurements performed with the anemometer. In addition, these measurements are compared to spectra obtained in high-disturbance conditions with the bleed system turned off.Copyright

Plasma Adaptive Optics Evaluation Using Two-Wavelength Heterodyne Interferometry

The third part of a research program that investigates the possibility of using low-temperature ionized air (plasma) for adaptive optics in an airborne laser directed-energy system is presented. It...

Electromagnetic wave transmittance control using self-organized plasma lattice metamaterial

A reconfigurable glow discharge plasma lattice structure is examined for its ability to interact with and suppress electromagnetic (EM) wave energy with wavelengths on the order of centimeters. The plasma lattice is formed in the air gap between a double dielectric electrode arrangement that formed a rectangular cross-section channel. The lattice consists of columns that span the gap between the electrodes. The spacing between the plasma columns in the lattice results from a surface charge instability that is controllable by a combination of channel height, AC voltage, and gas pressure. The lattice number is highly repeatable and predictable following packing theory. The effect of the plasma lattice spacing on the transmittance of O(cm) wavelength EM waves was investigated. Excellent agreement was found between the experiments and simulations, with S21 transmittance reduced by up to 75%. In addition, experiments in which the EM waves were oriented at an oblique angle to the plasma lattice incident axis were performed. This documented a narrow-band absorption that was predicted from an anisotropic medium permittivity tensor analysis. These experiments also indicated a negative index of refraction of the oblique EM waves for the plasma lattice that provided further evidence of its anisotropic behavior.A reconfigurable glow discharge plasma lattice structure is examined for its ability to interact with and suppress electromagnetic (EM) wave energy with wavelengths on the order of centimeters. The plasma lattice is formed in the air gap between a double dielectric electrode arrangement that formed a rectangular cross-section channel. The lattice consists of columns that span the gap between the electrodes. The spacing between the plasma columns in the lattice results from a surface charge instability that is controllable by a combination of channel height, AC voltage, and gas pressure. The lattice number is highly repeatable and predictable following packing theory. The effect of the plasma lattice spacing on the transmittance of O(cm) wavelength EM waves was investigated. Excellent agreement was found between the experiments and simulations, with S21 transmittance reduced by up to 75%. In addition, experiments in which the EM waves were oriented at an oblique angle to the plasma lattice incident axis ...

Plasma-Actuated Flow Control of Hypersonic Crossflow-Induced Boundary-Layer Transition in a Mach-6 Quiet Tunnel

by Harrison B. Yates The purpose of this research was to control crossflow-induced boundary-layer transition on a cone at angle of attack in hypersonic quiet flow. A 7° half-angle cone model with interchangeable nosetips was designed and fabricated from stainless steel, polyether ether ketone (PEEK), and Macor. Transition was characterized using infrared thermography and Kulite pressure transducers in the Boeing/AFOSR Mach-6 Quiet Tunnel at Purdue University. A plasma-based active flow-control system was used to control the transition location of the stationary crossflow waves, which manifested themselves as hot streaks on the cone. The transition location was accelerated by critical forcing (where the actuator wavenumber equals the wavenumber of naturally largest amplitude waves) and delayed by subcritical forcing (where the actuator wavenumber is larger than the natural waves). The disturbance wavenumber input of the plasma actuators was observed downstream on the model for many of the plasma-on runs, demonstrating that the plasma actuators introduced discrete forcing into the flow. The precise locations of the hot streaks varied for different nosetips, presumably due to differences in their microscale roughness. The experimental data were used to inform an improved stability analysis. Stationary crossflow vortex N-factors were calculated over the surface of a yawed circular

Experimental Investigation of the Effect of Wall Suction on Cross-Flow Absolute Instability in a Rotating Disk Boundary Layer

The research investigates the effect of uniform suction on the absolute instability of Type I cross-flow modes in the boundary layer on a rotating disk. Specifically, it is designed to investigate if wall suction will transform the absolute instability into a global mode as postulated in the numerical simulations of Davies and Carpenter. The disk is designed so that with a suction parameter a = −vz0/ √ νω, where vz0 is the dimensional wall-normal velocity (due to suction or blowing) at the surface of the disk, the radial location of the absolute instability critical Reynolds number, RecA = 650, occurs on the disk. Uniform wall suction is applied from Re = 317 to 696.5. The design for wall suction follows that of Gregory and Walker where an array of holes through the disk communicate between the measurement side of the disk and the underside of the disk that is inside of an enclosure that is maintained at a slight vacuum. The enclosure pressure is adjustable so that a range of suction or blowing parameters can be investigated. The holes in the measurement surface are covered by a compressed wire porous mesh to aid in uniformizing the suction on the measurement surface of the disk. The mesh is covered by a thin, porous, high density Polyethylene sheet featuring a 20 micron pore size that provides a smooth, finely porous surface. A companion numerical simulation was performed to investigate the effect that the size and vacuum pressure of the underside enclosure had on the uniformity of the measurement surface suction. Temporal disturbances are introduced using the method of Othman and Corke. The results document the evolution of disturbance wave packets in space and time for different suction parameter values.