Session ET: Experimental Techniques III

Plasma Sensor Suite

Progress has been made towards the development of a new class of sensors which have the potential to overcome the temperature limitations found in conventional sensors, thus addressing an important measurement challenge faced in the measurement of high speed flows. The new approach is based on the a.c.-driven mass-flow laboratory plasma anemometer developed by Matlis et al. and uses a weakly ionized glow discharge encapsulated between two electrodes as the sensing element. These sensors will feature proven elements of the technology used in the plasma anemometer, but will be extended for high-temperature, multiparameter operation. The sensitivity to different parameters can be provided by the design and orientation of the electrodes. The objective is to replace conventional sensors which provide diagnostics in the laboratory but are known to fail in real-world applications with a suite of rugged sensors optimized to measure wall shear-stress, pressure, temperature, heat flux, mass-flow, strain, and gas species. The advantages of the plasma sensor are that it has no mechanical parts (like a pressure transducer diaphragm) to fatigue or break, its operation is insensitive to temperature, it has a very high frequency response (2MHz +), and its output can be received wirelessly. These advantages over other sensors makes it ideal for use in high speed flows.   

Falling Spheres in Sharply Stratified Fluids

We explore the motion of heavy spheres falling through a sharp salt stratified fluid layer in which an intriguing levitation phenomena is observed: the heavy sphere experiences a transient levitation in which the sphere descends through the sharp transition, stops, and rises back into the layer before ultimately returning to descent. Careful new measurements will be presented showing how the bounce amplitude depends upon layer thickness. The hydrodynamics, which involves a strong coupling between variable density fluid, and moving solid boundary, entrained, turbulently mixed fluid, and strong internal waves will be discussed. We will discuss exact and asymptotic calculations in potential flows yielding the potential energy associated with the sharp interface which may provide an arrestment criteria for the falling sphere.   

Rotational-Vibrational Raman Spectroscopy for the Measurement of Thermochemistry in Nonisobaric Flames

The present work examines the feasibility of Raman line imaging spectroscopy for multiscalar measurements of thermochemistry in reacting flows under varying pressure. Line imaging of the rotational and vibrational Raman scattering was combined onto a single detector, thus allowing for a single-shot measurement of major species, pressure, and temperature in turbulent nonisobaric conditions. The diagnostic technique also allows for the calculation of two important derived quantities of interest, namely a conserved scalar and its dissipation rate. Additionally the present work introduces ``canonical'' flows that are optically accessible and involve high-speed, supersonic combustion with pressure variation. Small-scale, nonreacting supersonic underexpanded jets have been studied experimentally, using both a Schlieren system and the Raman line imaging technique, and computationally, using a method of characteristics approach.   

A Two-Color Fluorescent Thermometry Technique for Microfluidic Systems

The feasibility of implementing a two-color laser-induced fluorescence (LIF) technique to study thermal transport at the microscale is investigated. A temperature-sensitive fluorescent dye (Rhodamine B) and a temperature-insensitive fluorescent dye (Sulforhodamine-101) are used in tandem to determine fluid temperature with high accuracy and low noise using a pulsed Nd:YAG laser as an illumination source. While the fluorescence intensity of the temperature-sensitive dye is proportional to temperature, it is also biased by variations in the illuminating intensity. Therefore, a second temperature-insensitive dye is utilized in order to compensate for such biases. Calibration of the two-color LIF system reveals that the two-dye mixture in water yields a temperature sensitivity of 2.7\%/K with volumetric illumination from the pulsed Nd:YAG laser. Additionally, the feasibility of this methodology for conducting temperature measurements is explored by measuring a steady-state temperature gradient generated across a microfluidic channel array by two large hot and cold reservoirs. These measurements yielded mean steady-state temperatures in the microchannels within $\pm0.3\,^{\circ}$C of the predicted temperatures, with experimental uncertainties in the range $\pm0.48\,^{\circ}$C to $\pm0.56\,^{\circ}$C. Finally, this technique is applied to study the thermal transport characteristics of laminar and transitional flow within a heated rectangular copper microchannel.   

Orthogonal Double View Digital Holographic Diagnostics for Random Motion of Micro Polymer Jet by Electrospinning

An experimental investigation of three-dimensional random behavior of polymer micro jet generated by electrospinning is described. Two frequency doubled Nd:YAG lasers were used as the light source and a commercial grade CCD sensor (Nikon D-70) was used for holograms recording. The two lasers could be fired with a pulse separation as small as 100 ns, and the two laser beams were aligned with three polarized beam splitter cubes. Orthogonal double-view and double-pulses were recorded on the same camera frame. The camera frame was split into two, and both of the halves of the frame were used for each view. Two objective lenses (M 5x) and two spatial filters (Pinhole $\sim $ 5$\mu $m) were used to generate expanding laser beams in the digital microscopic holography (DMH) optical setup. As the electric field ($\sim $20 kV) was intensified, the polymer solution formed a charged filament (or multiple filaments) from the tip of the Taylor cone. As the filament was accelerated toward the collector, its diameter was shrunk and axisymmetric disturbances grew further away from the exit. The polymer was randomly deposited on the collector as non woven microfiber.   

An Experimental Study of Turbulent Vortex Rings

Vortex rings in this laboratorial study is generated by pushing a piston through a tube with an orifice opening in water environment. In this paper, turbulent vortex rings upon formation was studied. Turbulence is produced by increasing the Reynolds number (based on slug model) and the piston stroke length over critical values, the vortex ring is then highly excited. Up to date, the only systematic study of turbulent vortex rings is by means of Laser Doppler Velocimeter (LDV), which can only give velocities at one point. The entire ring structure has to be visualized by some statistical treatments which maybe smear out some important physics inside a single turbulent vortex ring and can include errors because of dispersion. In this paper, turbulent vortex rings are studied by means of Stereoscopic Particle Image Velocimetry (PIV), which is able to give three-dimensional velocity field on the entire plan of interest and to overcome the disadvantages of LDV mentioned above.   

Facility documentation measurements in ``new'' 7-inch high-speed water tunnel

A small high-speed, variable-pressure water tunnel was relocated from St.\ Anthony Falls Laboratory (SAFL), U.\ of MN, to the University of New Hampshire (UNH). The water tunnel was originally designed and constructed at SAFL in the late 1940s for a series of physical model studies for the design of a 60-inch high-speed water tunnel at the Navy's David Taylor Model Basin (now Carderock Division, NSWC). The 1:10 scale model water tunnel initially had a circular cross section with 6 inch I.D. It was tested in many configurations through the 1950s, with different test sections (incl.\ a free jet) and features such as a gas absorption dome and a two-story tall resorber in the return leg. In the 1980s it was retrofitted with a new test section of 7 inch width/height with fillets, for an octagonal cross section of 47 sq.in. The water tunnel is fitted with an axial flow propeller pump, which at 1500 rpm is capable of producing flow rates of 280 l/s. Based on the original model study data (15 m/s in 6 inch TS), a maximum velocity greater than 9 m/s will be achievable in the current square/octagonal 7-inch test section. The water tunnel has been restored and connected to a compressor and vacuum pump. Preliminary velocity distribution and pressure measurements are presented and compared to the original model study results. Head losses are measured for the various tunnel parts and compared to the original configuration.   

Characteristics of Single Dielectric Barrier Discharge Plasma Actuators at Sub-atmospheric Pressures

Experiments were performed to determine the effect of static air pressure on the performance of single dielectric barrier discharge (SDBD) plasma actuators. This was motivated by the need for expanding the validity of numerical models for plasma actuators to include changes in air properties. The performance metric chosen for the actuator was the amount of the thrust force generated by the actuator. The experimental setup consisted of a plasma actuator mounted on a flat surface that was standing vertically on a digital scale placed in a vacuum vessel. The dependence of the generated thrust on the air pressure was then documented, for a range of input ac voltages and frequencies, the area of the covered electrode, and the dielectric characteristics. The pressures ranged from atmospheric to 10in-Hg absolute. As the pressure was lowered, the threshold voltage to ionize the air decreased, thereby generating thrust at lower voltages. Up to the maximum thrust limit, the thrust was proportional to the applied voltage to the 7/2 power previously observed at atmospheric pressure. The maximum thrust could be limited by having too small of a covered electrode area, or by a transition from a diffuse plasma to concentrated filaments. As the pressure was lowered, filaments occur at lower input voltages. A summary map of operation is presented.   

A newly developed optical fiber probe processed by femtosecond pulses for a measurement of micro bubbles and droplets

Optical fiber probes can be applied to an efficient and reliable measurement for gas-liquid two-phase flows. This measurement technique enables a real-time and high-accuracy measurement of characterization of bubbles/droplets. In this study, we newly developed an optical fiber probe processed by femtosecond pulses (fs-Probe) in order to measure tens-of-micrometers bubbles/droplets. The new fs-Probe whose detecting point is formed at tens micrometer from its tip using fs pulses can detect gas-liquid interface velocity. We demonstrated a measurement of millimeters size bubbles/droplets and discussed its capability for measurement of velocities and diameters of them using fs-Probe. Furthermore, we visualized contact processes between fs-Probe and bubbles/droplets on the measurement. We discussed the relation between output signal characteristics and contact processes depending on a geometry of fs-Probe, wettability, and so on.