Session G29: Experimental Techniques: Flow Visualization and Quantitative Imaging

High Spatial Resolution Femtosecond Laser Measurements of Turbulence

Experimental study of turbulence at length scales smaller than the Taylor microscale can provide unique information about isotropy, homogeneity, and intermittency. We use femtosecond laser electronic excitation tagging (FLEET), an experimentally simple and unseeded molecular tagging method, to probe small-scale turbulent structures in air and nitrogen, measuring velocity with better than 100-micron spatial resolution. At these scales, the density perturbation from the tagging process may influence measured turbulence parameters. Here, we quantify the effect of small perturbations during measurement on the observed statistics of turbulence and explore the spatial limits at which FLEET can be employed.   

Multi-photon Molecular Tagging Velocimetry with Femtosecond Excitation (FemtoMTV)

We report results from first MTV measurements in water under nonlinear resonant femtosecond excitation of a phosphorescent supramolecule. Both two-photon and three-photon absorption processes are examined and the feasibility of measurements is demonstrated by single component velocimetry in a simple jet flow. The new capabilities enabled by FemtoMTV include elimination of the need for short wavelength UV excitation source and UV optical access in flow facilities, and potential for high rep-rate flow imaging.   

Spatiotemporal evolution of a laser-induced shock wave measured by the background-oriented schlieren technique

We investigate the spatiotemporal evolution of a laser-induced shock wave in a liquid filled thin tube. In order to measure pressure distribution at shock front, we adopt the background-oriented schlieren (BOS) technique. This technique provides two- or three-dimensional pressure field in a small region with a simple setup. With an ultra high-speed video camera and a laser stroboscope, we successfully capture the spatial evolution of the shock every 0.2 $\mu$s. We find an angular variation of the pressure at the shock front. The maximum pressure is in the direction of the laser shot while the minimum value is in the perpendicular direction. We compare the temporal evolution of the pressure measured by BOS technique with those obtained by another method, i.e. pressure estimation from the shock front position. Overall trend from both methods show a good agreement. The pressure from the shock front position exists between the maximum and minimum values from BOS technique. It indicates that our quantification method can measure more detailed pressure field in two- or three-dimensions. Our results might be used for the efficient generation systems for the microjet, which can be applicable for needle free injection devices.   

New applications of focusing schlieren imaging

Focusing schlieren is a refractive imaging technique that visualizes refractive disturbances in a limited depth of field. Whereas traditional schlieren visualizes refractive disturbances along an entire optical path, focusing schlieren can be used to see inside of a refractive flow or to eliminate disturbances outside of a defined test section. The basic optical layout and design of a focusing schlieren system are reviewed. Comparisons between traditional schlieren and focusing schlieren images are presented to highlight the ability to selectively image refractive disturbances. The imaging technique is applied to measuring quantitative density fields with low- and high-speed applications. Additional applications to refractive feature tracking and schlieren image velocimetry are presented.   

Resolving gas-liquid interface geometry using light field imaging

We present a novel approach for reconstructing the geometry of a three-dimensional specular gas-liquid interface from an image captured by a light-field camera. Whereas the scanning of a diffuse surface can be accomplished with a simple projector-camera system, the local reconstruction of a specular surface is non-unique and requires a more constrained sampling method. In our set-up, a known array of laser points is reflected by the unknown specular surface onto the image plane of a light-field camera. For each illuminated pixel, possible surfaces are generated that are defined by a depth location and local surface normal vector. We show that when the aperture is sufficiently small we can find the exact location and orientation of the local surface. Further, we present an algorithm that allows us to reconstruct a reflective surface from images that are taken with wider apertures. The algorithm searches the possible surfaces for points and normal vectors that are most consistent with each other based on input parameters. We present our simulated results with experimental validation.   

Fabrication of temperature sensitive hollow micro capsule for the flow visualization

Temperature and oxygen sensitive hollow micro capsules were fabricated using the bubble template method. The micro bubbles were nucleated in droplets of a dichloromethan solution of polymer. The polymer covered micro bubbles were suspended in aqueous solution. The dichloromethan solution of temperature and oxygen sensitive dye was dissolved into the polymer solution and the temperature and oxygen sensitive dye was incorporated into the capsule shell. Using the bubble template method, large amount of hollow micro capsules could be formed with very high number of density. The diameter of capsules was 1 $\sim$ 3 micro meter and the specific gravity of capsules was 1.01 g/cm$^{3}$. They seemed to be suitable as tracer particles for the PIV measurement. The temperature sensitivity and the oxygen sensitivity of fluorescence intensity from the functional capsules were measured using a spectrometer. The effect of excitation wavelength on these sensitivities and the quenching due to large excitation intensity were also evaluated. The temperature sensitivity was about -2{\%}/$^{\circ}$C and the fluorescence intensity was stable and no quenching was detected in 20 minutes, even under the intense excitation of 1W.   

Characterization of Scalar Mixing in Dense Gaseous Jets Using X-Ray Computed Tomography

An experimental technique based on X-Ray Computed Tomography (XCT) is used to characterize scalar mixing of gaseous jets at Reynolds numbers up to 20,000. In this study, the mixing of a krypton jet with ambient air is considered. The high radiodensity of the krypton gas enables non-intrusive volumetric measurements of gas density and mixture composition based on spatial variations in x-ray attenuation. Comparisons to theoretical and computational results are presented, and the viability of this diagnostic technique is assessed. Important aspects of x-ray attenuation theory and practice are considered in data processing and their impacts on future development of this technique are discussed.   

Measurement of density gradient across wind turbine interface

The wake of a field installed model turbine was visualized using a large-scale shadowgraph apparatus. To enable a large field of view a focused shadowgraph apparatus was used where the camera lens and the light source axis were aligned. A retroreflective screen is used as a back plane to reflect the image back to the camera. Sonic anemometer measurements of velocity and temperature were obtained at points overlapping the field of view. As much as 2{\%} change in temperature has been observed within wake, enough to cause measurable index of refraction fluctuations. Schlieren method will be used to directly measure the density gradient across the wake interface. These measurements will be used to explain the dynamics at the wake interface for different atmospheric boundary layer stability (stratification) conditions.   

Water turbulence effects on vapor blanket present in a flat-ended cylindrical probe at high temperature and, cooled by forced convection

In this work, the effect of turbulent flow on the vapor blanket, which originates in cooling by with water of a flat-end cylindrical probe was studied to observe the flow patterns around the vapor layer and its effect on heat extraction. The experiments to visualize the vapor blanket were carried out in an experimental device using flat-end cylindrical probes made with AISI 304 stainless steel. The probe was heated up to 915 $^{\circ}$C and plunged into a tube of plexiglass in which water was flowing. For helping to visualize the streamlines within the fluid, polyamide particles were added to the flow and illuminated with a sheet of laser light to visualize a slice of fluid flow pattern near the probe surface. Water velocities were considered: 0.2 m/s, 0.4 m/s and 0.6 m/s. During each experiment, thermal response data was acquired at 10 Hz. Results show that the vapor layer around the cylinder is stable at water 0.2 m/s, however, when the water velocity is increased, the flow becomes more turbulent, and the vapor layer becomes unstable, and vapor is entrained by eddies originate in the water. All this is reflected in the thermal histories.   

Development of Naphthalene PLIF for Making Quantitative Measurements of Ablation Products Transport in Supersonic Flows

Ablation is a multi-physics process involving heat and mass transfer and codes aiming to predict ablation are in need of experimental data pertaining to the turbulent transport of ablation products for validation. Low-temperature sublimating ablators such as naphthalene can be used to create a limited physics problem and simulate ablation at relatively low temperature conditions. At The University of Texas at Austin, a technique is being developed that uses planar laser-induced fluorescence (PLIF) of naphthalene to visualize the transport of ablation products in a supersonic flow. In the current work, naphthalene PLIF will be used to make quantitative measurements of the concentration of ablation products in a Mach 5 turbulent boundary layer. For this technique to be used for quantitative research in supersonic wind tunnel facilities, the fluorescence properties of naphthalene must first be investigated over a wide range of state conditions and excitation wavelengths. The resulting calibration of naphthalene fluorescence will be applied to the PLIF images of ablation from a boundary layer plug, yielding 2-D fields of naphthalene mole fraction. These images may help provide data necessary to validate computational models of ablative thermal protection systems for reentry vehicles.