Session A14: General Experiments I

Multi-variable calibration of temperature estimation in individual non-encapsulated thermo liquid crystal micro particles

An experimental method to track the temperature of individual non-encapsulated thermo-liquid crystal (TLC) particles is presented. TLC thermography has been investigated for several years but the low quality of individual TLC particles, as well as the methods used to relate their color to temperature, has prevented the development of a reliable approach to track their temperature individually. In order to overcome these challenges, a Shirasu Porous Glass (SPG) membrane approach was used to produce an emulsion of stable non-encapsulated TLC micro particles, with a narrower size distribution than that of encapsulated TLC solutions which are commercially available (Segura et al, Microfluid Nanofluid, 2012). On the other hand, a multi-variable calibration approach was used, as opposed to the well known temperature-hue relationship, using the three-components of the HSI color space measured in each particle image. A third degree three-dimensional polynomial was fitted to the color data of thousands of particles to estimate their temperature individually. The method is able to measure individual temperatures over a range exceeding the nominal range of the TLC material, with lower uncertainty than any method used for individual particle thermography reported in the literature.   

Laboratory investigation of the erosion of cohesive sediments under oscillatory flows using a synchronized imaging technique

A synchronized dual stereo particle image velocimetry (PIV) measurement technique is used to examine the erosion process of a cohesive sediment core in the Small Oscillatory Flow Tunnel (S-OFT) in the Sediment Dynamics Laboratory at the Naval Research Laboratory, Stennis Space Center, MS. The dual stereo PIV windows were positioned on either side of a sediment core inserted along the centerline of the S-OFT allowing for a total measurement window of about 20 cm long by 10 cm high with sub-millimeter spacing on resolved velocity vectors. The period of oscillation ranged from 2.86 to 6.12 seconds with constant semi-excursion amplitude in the test section of 9 cm. During the erosion process, Kelvin--Helmholtz instabilities were observed as the flow accelerated in each direction and eventually were broken down when the flow reversed. The relative concentration of suspended sediments under different flow conditions was estimated using the intensity of light scattered from the sediment particles in suspension. By subtracting the initial light scattered from the core, the residual light intensity was assumed to be scattered from suspended sediments eroded from the core. Results from two different sediment core samples of mud and sand mixtures will be presented.   

A new method to determine the yield stress of diluted polymeric solutions

A new method to measure the yield stress for diluted polymeric solutions is presented. The tested solutions exhibit shear thinning behavior a once the critical yield stress is overcame. In rheology, these fluids are known as Herschel-Buckley. The yield stress phenomenon and its relation with bubble motion is an important issue for different industries, for example, personal care, paints and some others. As a result of the yield stress, small bubbles remain trapped in the fluid bulk, but above a critical volume, which is related with the characteristic yield stress, the bubbles flow in the liquid. In order to change the bubble volume, the liquid is placed in a cylindrical container whose pressure is decreased by a vacuum pump. The bubble growths as the pressure decreases and keeps its position until it reaches the critical volume. The bubble shape changes with volume and velocity, and a competition among surface, gravitational, inertial and viscous forces is discussed. The yield stress determined value is higher than the obtained from simple shear measurements due to the complex flow around the bubble.   

Plasma Adaptive Optics Characterization using Dispersive FTIR Interferometry

Previous work by the authors has investigated the implementation of plasma as an adaptive optic element. Plasma has no moving parts, high frequency response, and an index of refraction that is dependent on the applied voltage potential. The refractive index is a function of the electron density and heavy particle density, and the accurate measurement of these values is critical to the development of plasma optical devices. Traditionally, such measurements are achieved by probing plasma with multiple wavelengths or by imposing specific assumptions on the heavy particle density. This work uses dispersive Fourier transform infrared (FTIR) interferometry to probe a plasma optic element and measure its optical properties across a wide range of wavelengths. This approach is based on a least-squares reconstruction of the plasma electron density and heavy particle density values. Both theoretical modeling and experimental validation are presented.   

Visualization of Capsule Reentry Vehicle Heat Shield Ablation using Naphthalene Planar Laser-Induced Fluorescence Imaging

NASA has continued interest in the study of ablation owing to the need to develop suitable thermal protection systems for spacecraft that undergo planetary entry. Ablation is a complex multi-physics process, and codes that predict it require a number of coupled submodels, each of which requires validation. For example, Reynolds-averaged Navier Stokes (RANS) and large-eddy simulation (LES) codes require models of the turbulent transport of ablation products under variable compressibility and pressure gradient conditions. A new technique has been developed at The University of Texas at Austin that uses planar laser-induced fluorescence (PLIF) of a low-temperature sublimating ablator (naphthalene) to enable visualization of the ablation products as they are transported in a boundary layer. While high temperature ablation is extremely difficult to recreate in a laboratory environment, low temperature ablation creates a limited physics problem that can be used to simulate the ablation process. In the current work a subscale capsule reentry vehicle model with a solid naphthalene heat shield is tested in a Mach 5 wind tunnel. PLIF imaging reveals the distribution of the ablation products as they are transported into the boundary layer and over the capsule shoulders.   

Development of a Digital Fringe Projection Technique to Quantify the Transient Behavior of Wind-Driven Surface Droplet/Rivulet Flows

A novel digital fringe projection (DFP) technique is developed to achieve non-intrusive thickness measurements of wind-driven water droplet/rivulet flows. The DFP technique is based on the principle of structured light triangulation in a similar manner as a stereo vision system but replacing one of the cameras for stereo imaging with a digital projector. The digital projector is used to project a fringe pattern of known characteristics onto a test object (i.e., the water droplet/rivulet on the test plate). Due to the 3D shape profile of the test object, the fringe pattern is deformed seen from a perspective different from the projection axis. By comparing the distorted fringe pattern over the test object and a reference fringe pattern on a reference plane, the 3D profile of the test object with respect to the reference plane (i.e., the thickness distribution of the water droplet/rivulet flow) can be retrieved quantitatively and instantaneously. The DFP system is used to achieve time-resolved thickness distribution measurements of a droplet/rivulet flow driven by a boundary layer wind. The dynamic shape change and stumbling runback motion of the wind-driven water droplet/rivulet flow over the test plate are revealed clearly and quantitatively from the DFP measurement results. Such information is highly desirable to elucidate underlying physics to improve our understanding about the surface water transport process pertinent to ice formation and accretion over aircraft wings in atmospheric icing conditions.   

Surface tension profiles in vertical soap films: an intrusive measurement

Soap films are known to be the basic constituent of complex fluid objects such as bubbles and foams. In order for their weight to be counterbalanced, those objects need to exhibit vertical surface tension profiles, which can only exist due to the presence of surfactant molecules at their interfaces. We here present a method that aims to probe vertical soap films surface tension profiles by use of elasto-capillary deformations. Both cases of free-draining and entertained soap films are investigated, leading the spatio-temporal behavior of the surface tension. We show that this behavior is highly dependent on the nature of the surfactant used to create soap films, emphasizing the influence of the physico-chemistry on the global behavior of the system. We propose order of magnitude calculations, which are in good agreement with experimental data.   

Measuring bubbles in a bubbly wake flow

This paper presents measurements of the velocity and size distribution of bubbles in a bubbly wake. This was carried out by utilizing particle shadow velocimetry (PSV). This technique is a non-scattering approach that relies on direct in-line volume illumination by a pulsed source such as a light-emitting diode (LED). A narrow depth-of-field (DoF) is required for imaging a 2-dimensional plane within a flow volume. Shadows of the bubbles were collected by a high-speed camera. Once a reference image, taken when no bubbles were present in the flow, was subtracted from the images, the image was segmented using an edge detection technique. The Canny algorithm was determined to be best suited for this application. A curvature profile method was employed to distinguish individual bubbles within a cluster of highly overlapping bubbles. The utilized algorithm was made to detect partly overlapping bubbles and reconstruct the missing parts. The movement of recognized individual bubbles was tracked on a two dimensional plane within a flow volume. In order to obtain quantitative results, the wake of a ventilated hydrofoil was investigated by applying the shadowgraphy technique and the described bubble detection algorithm. These experiments were carried out in the high speed cavitation tunnel at Saint Anthony Falls Laboratory (SAFL) of the University of Minnesota.