Session J21: Experimental Techniques: Pressure/Temperature Scalar Surface Visualization

Imaging configurations and partial occlusion mapping for multi-camera volumetric velocimetry around bodies

Moving bodies present imaging challenges for multi-camera volumetric velocimetry techniques. In addition to measurement volume regions which are fully obstructed by the body, partially-occluded regions arise, where some viewpoints are obstructed but a reduced set of cameras still view the particle field. These partially-occluded regions may have reduced resolution and accuracy compared to the unoccluded regions of the measurement volume. To characterize these limits, this work evaluates the effects of varying camera configurations and occlusion parameters (e.g., size, shape) on particle reconstruction and velocity measurement in partially-occluded regions. Of particular interest are trade-offs between optical access, depth resolution, and the fraction of the measurement volume which is partially- or fully- occluded. Using synthetic images, comparisons are made to a ground truth reference flow field and the same particle volume without any occlusions present. Results suggest that adding even 1-2 additional cameras to a typical four camera experiment can enhance measurement capabilities in partially-occluded regions, even when the additional camera does not extend the viewing angle.   

Novel optical method for reconstruction of axisymmetric capillary wave surface topography: theory, simulation, and experimentation

Mathematical modeling of waves is integral for gaining insight into real-world phenomena. Despite their omnipresence, water waves were considered unsuitable for elementary courses by Feynman due to their inherent complexities and non-linear behavior. However, capillary waves on liquid-gas interface are described quite accurately by the linear wave theory. Resulting equations relate the fundamental liquid properties like surface tension and viscosity to the wave properties like wavelength and attenuation.

A novel optical method is presented to reconstruct wave surface topography of parametrically excited finite-amplitude axisymmetric capillary waves. Leveraging refraction of light to magnify amplitude, the method solves the inverse (geometric) optics problem while exploiting the underlying symmetric nature to recreate the entire wave surface uniquely and accurately. Its novelty lies in the simplicity and affordability (laser pointer and smartphone) making it ideal for classroom demonstrations. A MATLAB script is developed to serve as a proof-of-concept for the proposed method. Additionally, multiphase flow simulations are conducted to assess the agreement between the theoretical predictions and the real-world behavior observed in the experiments. This analysis yields valuable insights for determining the optimum tuning parameter values. Subsequently, an experimental investigation is performed to rigorously assess the accuracy and feasibility of the proposed method.   

Demonstration of Quantum Dot Thermometry for the NASA ZBOT Experiment

We have previously discussed the development and performance estimates of our quantum dot thermometry (QDT) as a whole-field planar optical technique for thermometry in the NASA Zero Boil-off Tank experiment. The technique is based on ratiometric Laser Induced Fluorescence (LIF) using nanocrystal quantum dots that are modified to dissolve/disperse into the working fluid (Perfluoropentane) of the ZBOT experiment. In this talk, we will discuss experiments using a single-color camera demonstrating the QDT technique. The thermal fields observed highlight the importance of using a ratiometric approach. We will discuss the specific challenges in processing QDT images obtained using a single-color camera, and also provide an update on the final hardware arrangement that will be used on the upcoming experiment onboard the ISS.   

Combined schlieren and imaging spectroscopy of a helium-iodine plume

Schlieren imaging is useful for imaging refractive disturbances, but it does not provide information about the chemical composition of a flow.  Here a method for simultaneous schlieren and imaging spectroscopy is developed to provide the ability to visualize and interpret a chemically complex environment with multiple gas species. Tests were conducted utilizing laminar helium gas plumes infused with iodine. Using the schlieren and spectroscopy techniques simultaneously allowed for direct comparison between the refractive index changes seen in the schlieren images and the spectral changes associated with local gas species observed through the imaging spectrometer. The presence of iodine was tracked through the helium gas plume over time using absorption spectroscopy.  The intensity changes in the absorption spectroscopy images allow calculation of a local iodine concentration. These iodine containing locations were directly connected to locations of refractive index changes measured through schlieren imaging. Implementation of quantitative schlieren imaging allows measurement of the local refractive field which is used with the local iodine concentration to develop a measurement of the density field through the plume.   

A 3D PTV System for Measuring Small-Scale Ocean Turbulence in the Coastal Environment

Small-scale stirring of the density field by turbulent fluctuations is the fundamental process driving mixing in the ocean interior. Turbulence observations in the ocean have historically been limited by a lack of high-resolution, 3D-resolved velocity data. To bridge this gap, we developed a field-deployable 3D PTV system capable of measuring small-scale velocities in a 3D volume. We will discuss the design of the system and present both laboratory validation data and results from field deployments in Monterey Bay, CA.   

Fast estimation of pressure from PTV measurements using smooth particle hydrodynamics

Estimation of pressure fields from particle tracking velocimetry (PTV) techniques such Shake-The-Box (STB) remains challenging. Traditional methods such VIC, FLOWFIT, etc. involve the estimation of Eulerian flows fields from measured Lagrangian particle tracks subject to physical constraints. These methods are memory intensive and potentially introduce averaging errors. In this work, we propose a purely Lagrangian approach to compute instantaneous pressure from PTV using discretization based on smooth particle hydrodynamics (SPH). Pressure is obtained via a linear system that enforces the full Navier-Stokes equations. We use Voronoi tessellation on the particle field to obtain local volumes and efficiently identify nearest neighbors. A comparison with other neighbor detection approach is also reported. This framework does not require special treatment at measurement boundaries and is verified and validated using a model problem of a Taylor--Green vortex and analysed on a canonical 3D flow past the cylinder at moderate Reynolds numbers. Computational cost, convergence, and challenges are reported.   

A technique for fluid-structure interaction diagnostics using a single plenoptic camera.

This work presents a new methodology for experimentally studying fluid-structure interaction (FSI) using a plenoptic camera. Current approaches to 3D measurements in FSI problems use 4-camera tomographic particle image velocimetry to capture flow motion and 2-camera digital image correlation to capture surface motion, but face limitations due to the optical access required for a 6-camera arrangement. Thus, we propose using a plenoptic camera that encodes the 4D light field information, to capture both 3D flow field and surface motion using a single camera. We utilize proper orthogonal decomposition to separate the particle and surface information in a plenoptic image. Furthermore, we introduce a novel correlation-based plenoptic depth estimation technique for robust surface reconstruction. 3D vector fields are generated using algebraic reconstruction followed by cross-correlation. Validation of the methodology is performed with synthetic analysis of a moving flat plate immersed in a uniformly moving particle field. Results show an uncertainty of less than 1% of full depth for the reconstructed surface and an uncertainty of less than 3.5% of maximum velocity for the flow field. Finally, we demonstrate this method experimentally by capturing the flapping motion of a flexible flag in a water tunnel.   

Minimizing the Distortions Induced by Mean Shear within 3D Reconstructions of Turbulent Flows from Time-Resolved sPIV Measurements

The efficacy of reconstructing a 3D volume of time-evolving 3 component velocity from planar experimental measurements is explored within strongly shear-distorting turbulent flows. A common approach to convert the temporal dimension into the streamwise spatial dimension is Taylor’s frozen turbulence hypothesis where the mean velocity is imposed as the convective velocity. In flows with a strong mean shear-rate the instantaneous turbulence structure is distorted when a traditional Taylor’s hypothesis method is used to reconstruct 3D volumes. In the current study, we compare existing methods that extend the classical Taylor’s hypothesis approach to retain time-locality in the convective velocity in order to accurately reconstruct a 4D (time-resolved) velocity field for accurate analysis of turbulence structure. Specifically, we analyze a local mean convective velocity approach (Pinton & Labbe 1994) as well as an instantaneous convective velocity approach (Fratantonio et al 2021) using time-resolved sPIV measurements in transverse and longitudinal planes within the near-wall surface layer of a canonical flat-plate turbulent boundary layer at Re_θ=7,700. The reconstruction methods are evaluated based on their ability to preserve both the statistical properties of the flow and the instantaneous structure of the turbulence eddies as well as the streamwise extent to which these methods can be applied.   

Go with the Flow: Micro Aerial Vehicles as Lagrangian Particles in the Atmospheric Boundary Layer

Conventional particle velocimetry techniques are restricted to measurement volumes on the centimeter scale; recent tracking algorithm and tracer particle advances increase domain sizes into the meter range, but to reach hundreds or thousands of meters alternative methods are required. The authors propose LagranDrone, a particle velocimetry technique based on 30 gram Micro Aerial Vehicles (MAVs). Tasked only to oppose the force of gravity, LagranDrones move freely due to the wind force in three dimensions. A "swarm" of such drones is released simultaneously and tracked by GPS, collecting atmospheric boundary layer data as they are swept by the wind. The data can be used to analyze the atmosphere at the kilometer scale.