Session E29: Experimental Techniques: Multiphase Flow

Three-Dimensional Feature Extraction from Multiphase Flows

Light field imaging (LFI) and synthetic aperture (SA) refocusing techniques have been combined in an emerging method to resolve three-dimensional (3D) flow fields over time. Image volumes of a scene are captured using an array of multiple cameras. SA refocusing yields a stack of post-processed images at different focal depths, each with a narrow depth of field. Although this technique has previously been used to reconstruct flow features that are small relative to the field of view, blur artifacts are more clearly visible when this method is applied to relatively larger features. The presence of these artifacts prevents 3D scene reconstruction. To eliminate the artifacts, circles are detected in the raw camera images, and their rims are converted to white pixels, while the rest of each raw image is made black. This simplifies 3D feature detection in the stack of refocused images and allows the scene to be reconstructed in 3D. Simulations and experiments using the aforementioned modified SA method show that it is possible to extract the center coordinates in 3D and radii of spheres found in a scene being recorded with simple back illumination. This technique has been applied to various types of multiphase flows, including bubble flow fields in air and sneezes.   

Phase-locked measurements of gas-liquid horizontal flows

A flow of gas and liquid in a horizontal pipe can be described in terms of various flow regimes, e.g. wavy stratified, annular or slug flow. These flow regimes appear at characteristic gas and liquid Reynolds numbers and feature unique wave phenomena. Wavy stratified flow is populated by low amplitude waves whereas annular flow contains high amplitude and long lived waves, so called disturbance waves, that play a key role in a liquid entrainment into the gas phase (droplets). In a slug flow regime, liquid-continuous regions travel at high speeds through a pipe separated by regions of stratified flow. We use a refractive index matched dynamic shadowgraphy technique using a high-speed camera mounted on a moving robotic linear rail to track the formation and development of features characteristic for the aforementioned flow regimes. We show that the wave dynamics become progressively more complex with increasing liquid and gas Reynolds numbers. Based on the shadowgraphy measurements we present, over a range of conditions: (i) phenomenological observations of the formation, and (ii) statistical data on the downstream velocity distribution of different classes of waves.   

Hard X-ray nanotomography of colloidal suspensions

Colloidal suspensions are complex fluids that include colloidal nanoparticles or microparticles suspended in a liquid medium. In-situ characterizations of colloidal suspensions are necessary in many topics: for instance, wetting properties for colloidal particles on a fluid-fluid interface are essential but hard to be directly taken with conventional imaging techniques. Here we show that hard X-ray nanotomography clearly visualizes individual colloidal particles inside fluids in three dimensions (3D). In particularly, we demonstrate 3D images for colloidal particles adsorbed on water-oil emulsions: contact angle and configuration of colloids could be measured. We believe that hard X-ray nanotomography would be a powerful tool to identify the nature of colloidal particles inside or on fluids.   

High-speed imaging of the transient ice accretion process on a NACA 0012 airfoil

Ice accretion on aircraft wings poses a performance and safety threat as aircraft encounter supercooled droplets suspended in the cloud layer. The details of the ice accretion depend on the atmospheric conditions and the fight parameters. We present the measurement results of the experiments conducted in the Iowa State icing wind tunnel on a NACA 0012 airfoil to study the transient ice accretion process under varying icing conditions. The icing process on the wing consists of a complex interaction of water deposition, surface water transport, and freezing. The aerodynamics affects the water deposition, the heat and mass transport, and ice accumulation; meanwhile, the accumulating ice also affects the aerodynamics. High-speed video of the unsteady icing accretion process was acquired under controlled environmental conditions to quantitatively measure the transient water run back, rivulet formation, and accumulated ice growth, and the experiments show how varying the environmental conditions modifies the ice accretion process.   

An Experimental Investigation on the Wind-Driven Water Film Flows over Rough Arrays by using a Digital Image Projection (DIP) Technique

In the present study, an experimental investigation was conducted to quantify the transient behavior of the wind-driven surface water film flows over a rouged surface in order to examine the water mass trapped effect due to the presence of roughness arrays pertinent to aircraft icing phenomena. A novel digital image projection (DIP) technique was developed and applied to achieve time-resolved measurements of the thickness distributions of the unsteady surface water film flows over the roughness arrays, in comparison with those over a flat plate as the comparison baselines. The measurement results reveal clearly that, at relatively low wind speed, the roughness arrays would perform as a dam to block the wind-driven water film flow at the front side of the roughness arrays. For the cases with relatively high wind speeds, the trapped water mass was found to stagnate mainly at the backside of the roughness arrays. The time-averaged mass trapping ratio was found to be very sensitive to the wind speed, but less sensitive to the flow rate of the surface water film flows over the test plate.   

Velocity and thickness measurement of a thin-liquid film via a single-tip optical fiber probe micro-fabricated by femtosecond pulse laser

Optical fiber probing is a simple and compact measurement system for a gas-liquid two phase flow. This probe detects a gas-liquid interface responsively. We have developed a new measurement technique for a thin-liquid-film that utilizes a single-tip optical fiber probe (Fs-TOP) micro-fabricated through femtosecond laser pulses. The Fs-TOP is installed horizontally along the channel base, and vertically traversed to the other side of the base. The signal from the Fs-TOP is sufficiently understood by using the originally developed 3D ray-tracing-numerical simulation. The maximum liquid film thickness was measured as well as the average liquid film thickness. Based on the simulation, it was found out that when a fraction of liquid phase is 52{\%}, the installed position of the Fs-TOP is equal to the average liquid film thickness. We measured velocity of the liquid film by the Fs-TOP and visualization. The results accorded with each other, and implied that the Fs-TOP measurement was superior to the visualization, against a high-velocity flow.