Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session ZC24: Experimental Techniques: Laser-Based Diagnostics |
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Chair: Matthew Fu, Caltech Room: 251 B |
Tuesday, November 26, 2024 12:50PM - 1:03PM |
ZC24.00001: Ultra High-Resolution 3-D flow velocimetry using Cross-Scanning Laser-Induced-Fluorescence (XLIF) Andres A Aguirre Pablo, Krishna Raja Dharmarajan, Vivek Mugundhan, Sigurdur T Thoroddsen This work leverages the use of two identical LIF scanning setups placed orthogonally to each other. Each laser sheet scanning system consists of a galvanometer and a set of optics that create a collimated laser sheet, which scans through the measured volume. These scanning sheets are perpendicular to a high-speed video camera. The use of orthogonal dual scanning XLIF allows us to 3D map the dye scalar structures of the measured flow with high spatial and temporal resolution in all three axes at nearly the same instant. Therefore, the recorded images are processed to reconstruct a series of instantaneous 3D scalar fields in real-world coordinates. By directly correlating the scalar volumes in 3D, we can obtain high density 3D-3C velocity fields for analyzing the turbulence characteristics of the flow. We apply this technique to a counter-swirling coaxial jet setup to discretize the volume with nearly 573 million voxels of the full scalar volume (45 x 37 x 43 mm3) at every time instant. To demonstrate the velocimetry capabilities of the method, we perform direct correlation of the volumes in time with high overlap to compute nearly one vector per voxel, achieving a final vector pitch of 57.6 µm/vector in all directions, over most of the volume. Future effort will be focused on inverting the advection-diffusion equation to further refine the velocity measurements. |
Tuesday, November 26, 2024 1:03PM - 1:16PM |
ZC24.00002: Designing and optimizing multi-tracer PLIF for application in variable-density turbulent mixing environment Samuel Petter, Quinton Dzurny, Prasoon Suchandra, Devesh Ranjan Measuring the volume fraction of multiple components simultaneously is a challenge for mixing studies across a variety of different fields. In multilayer studies on the Rayleigh-Taylor and Richtmyer-Meshkov instabilities, experiments have been limited by single-tracer planar laser induced fluorescence (PLIF) to volume fraction measurements of only one fluid layer at a time. To overcome this challenge, a multi-tracer PLIF approach has been developed. With multiple PLIF tracers, it is possible to resolve the volume fraction of multiple components in a mixture simultaneously. This presentation focuses on the application and challenges of using two PLIF tracers in gases. Specifically, acetone and anisole are used as fluorescent tracers. Acetone excited by 266 nm UV light will emit fluorescence across a broad band peaking around 410 nm, and anisole excited by the same 266 nm will fluoresce at a narrowband 295 nm. Such is demonstrated through a three-layer Rayleigh-Taylor driven mixing study. The mixing is being generated in a blown-down three-layer gas tunnel, by blowing a lighter air-helium mixture in between two heavier air layers. In this presentation, a comparison of tracers is made, and the conflicts that arise from their simultaneous use are discussed in detail. This includes spectral-conflicts such as tracer crosstalk, fluorescence reabsorption, and tracer-tracer collisional quenching. This work contributes to the advancement of diagnostics in gas mixing and opens new avenues of research for multi-component/variable density mixing. |
Tuesday, November 26, 2024 1:16PM - 1:29PM |
ZC24.00003: Optical ray tracing simulations to image water microdroplets and negative pressure modulations inside them Armin Kalita, Bryan Oller, Sebastian Marte, Thomas Paula, Stefan Adami, Nikolaus A Adams, Claudiu Andrei Stan The ablation of water microdroplets with X-ray lasers generates large negative pressure waves through shock reflection on the droplet surfaces. These negative pressures might be the largest produced by any method but were measured with low accuracy. Due to the droplets' small sizes direct pressure measurements were impossible, and the accuracy of computational fluid dynamics (CFD) simulations was limited by uncertainties in initial conditions, equation of state at negative pressures as well as phase-transition modeling. During the experiments, the droplets were imaged optically and the pressure waves were visible as darker regions due to pressure-induced changes in the refractive index. To understand and quantify wave images, we ran ray-tracing image simulations of droplets containing refractive index profiles predicted by CFD simulations. Previously, optical ray tracing was used to simulate images of cavitation bubbles, but more accurate simulations are needed to see the effect of refractive index gradients in a liquid drop. We found that improving the accuracy of image simulations requires modeling accurately complicated imaging optics, and modeling the illumination based on experimental calibrations. With these improvements we achieved a good qualitative agreement between simulated and experimental images. These ray-tracing simulations can in turn be used to refine and validate the CFD simulations, leading to an iterative procedure that improves the accuracy of pressure measurements. |
Tuesday, November 26, 2024 1:29PM - 1:42PM |
ZC24.00004: Neural Optical Flow Velocimetry Andrew I Masker, Ke Zhou, Joseph P. Molnar, Samuel J Grauer Optical flow (OF) is a computer vision framework for estimating dense displacement fields between images. It can be used for particle image velocimetry (PIV) processing, and has been shown to enhance the accuracy and resolution of PIV relative to cross-correlation. The discrete nature of current OF methods limits the accuracy of velocity gradients, hampering data assimilation efforts and analysis of turbulence. We report Neural Optical Flow (NOF), which uses a coordinate neural network and differentiable image-warping operator for OF. A continuous 2D or 3D neural velocity field is used to advect the discrete intensity field from ti to ti+1. Residuals between the advected image at ti+1 and its real counterpart are squared and summed to form a data loss. Exact, continuous regularization functionals may be included (Navier–Stokes, Euler, div–curl, etc.) to improve accuracy and potentially infer pressure. We also show how the continuity equation can be included as a hard constraint in both 2D and 3D. Velocity fields are reconstructed by minimizing the aggregate loss. Our method is demonstrated using synthetic planar and stereo PIV measurements of turbulent flows. We compare NOF to a state-of-the-art wavelet OF technique, and discuss the application of NOF to other diagnostics. |
Tuesday, November 26, 2024 1:42PM - 1:55PM |
ZC24.00005: New measurement method via optical waveguide film for droplet and liquid film flow Kosuke Nakano, Hajime Furuichi, Yuki Mizushima We developed a gas-liquid two-phase flow sensor with an optical waveguide film (OWF) to measure droplet impingement and wavy thin-film flow directly. The OWF is a flexible polyimide film about 0.1 mm thick with multiple built-in micro-optical sensors. The sensor with propagating laser light detects the reflected light from the interface, enabling the distinction between gas and liquid phases and the measurement of the thickness of the liquid phase. Adjusting the arrangement and function of adjacent sensors also enables the estimation of wave velocity and detection of interfacial waves' local shape. This study conducted application tests of droplet impingement and liquid film flow measurements using the OWF. In the droplet measurement test, we measured the impact behavior of water and ethanol droplets. In the liquid film measurement test, we measured the time-series thickness of shear-driven film flows in a vertical pipe and a horizontal rectangular duct. These results were compared with visualization images obtained from a high-speed camera to demonstrate the validity of droplet detection and measurements of local film thickness and wave velocity with the OWF. We expect this technique to significantly contribute to two-phase flow sensing in narrow channels, where the measurement is generally difficult. |
Tuesday, November 26, 2024 1:55PM - 2:08PM |
ZC24.00006: Krypton-Tagging Velocimetry in a Low-Speed Atmospheric-Pressure Inductively-Coupled Plasma Plume Dillon Ellender, Noel T Clemens As part of an ongoing project to develop an exascale, multi-physics simulation of an inductively-coupled plasma (ICP) torch, measurements of the plasma plume velocity are needed for validation purposes. The 50 kW ICP torch studied, operates on argon and exhausts to ambient air at low-speed and with a temperature of about 6000 K. Pitot-static probe measurements have been made in the plasma using a water-cooled probe, however the cooling necessitates a relatively large diameter probe that introduces an unquantified blockage effect and converting to velocity requires assumptions about the flow that may not be valid. The high temperatures also make many common non-intrusive techniques such as PIV or MTV difficult to implement. Krypton Tagging Velocimetry (KTV) was employed and successfully used to measure velocity in an atmospheric pressure argon plasma plume. KTV is a two-step technique in which a metastable electronic state of krypton is first populated with one laser pulse. Once populated, this metastable state is probed with a separate laser pulse after a known delay to produce a line of fluoresence that has convected with the flow. Fluoresence was detected out to 20 µs of delay. The various challenges in making such measurements will be discussed. |
Tuesday, November 26, 2024 2:08PM - 2:21PM |
ZC24.00007: Vibrationally Excited Micro-Hydroxyl Tagging Velocimetry In Wall-Bounded Turbulent Flows Mir Muhammad Tareq, Charles Fort, Philippe M Bardet In an attempt to improve the signal-to-noise ratio (SNR) and effective resolution in the Hydroxyl Tagging Velocimetry (HTV) technique previously deployed in a low-speed anechoic wind tunnel, several upgrades have been implemented. The scope of the current report is to discuss the relevant progress. A micro-lens array (MLA) is introduced into the tracer-creating beam path to create multiple beamlets, which provides several benefits: it helps achieve narrower beamlets, which is especially useful in the viscous sublayer; reduces the energy density in the beam transmitting through the delivery window; and, provides more flow information through multi-line tagging. A vibrationally mediated photodissociation (VMP) step, powered by an infrared laser, is introduced to increase the tracer number density, significantly increasing the signal strength. Finally, a low-noise qCMOS camera is added to the setup, replacing the existing intensified imaging system and thus removing the noise introduced by the image intensifier. The combined effect of the aforementioned upgrades results in high-resolution and high-SNR data within the viscous sublayer region. |
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