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Measurements of intracrater flow dynamics utilizing a mound-bearing crater in a refractive index matched environment
Get PDFThe processes controlling crater mound formation are the subject of ongoing research (Bennett and Bell III, 2016). Several theories exist on the formation of a central mound, with those pointing to wind processes as the predominant driving mechanisms being among the most compelling (Kite et al., 2013; Day et al., 2016; Anderson and Day, 2017). Few experimental studies have been conducted to uncover impact crater driven flow dynamics. As such, direct and experimental flow measurements that could be used to validate previously developed fluid-topography interaction theories are not yet available. The objective of this study is to elucidate the intra- and extra-crater circulation induced by unidirectional winds using experimentation on scaled models coupled with high spatial and temporal resolution flow measurements
Stereo PIV measurements of oscillatory plasma forcing in the cross-plane of a channel flow
Get PDFThe present work describes an experimental investigation that applies stereo particle image velocimetry in a cross-plane of a turbulent channel flow that is additionally perturbed by spanwise oscillatory body forces, induced by a plasma actuator and designed to mimic the effect of spanwise wall oscillations. The experiment is aimed at retrieving the forcing-correlated scales and the turbulent flow stochastic fluctuations for the measured cross-plane. The first are macroscopic scales and require a larger investigation domain while the latter benefit of a higher resolution. Furthermore, the extended flow-field dynamic range posed a challenge on the experiment design, finally leading to an optimal tradeoff. The results of the unactuated flow compare well to the direct numerical simulations of Hoyas and Jimenez ́ (2008), while the actuated case demonstrates strong near-wall momentum addition and spanwise modulation of the streamwise flow component
On the determination of 3D position and orientation of spheroidal particles using defocusing and deep learning
Get PDFTracking the 3D position of tracer particles or small objects like cells or unicellular organisms in miniaturized lab-on-a-chip or biomedical devices is complicated since it is often not possible in these setups to use multi-camera approaches. Most successful single-camera approaches for these applications are based on holography or defocusing. Holographic methods have been used to track complex objects such has bacteria (Bianchi et al. (2019)) and even to estimate their orientation (Wang et al. (2016)). However, these methods require a complex and expensive experimental setup which is not always available in research laboratories. On the other hand, defocusing methods work with conventional microscopic optics, are easy to implement, and have shown excellent results in 3D PTV experiments (Qiu et al. (2019)). One main drawback is that they normally work only with spherical and mono-dispersed tracer particles. A defocusing method that has potential to measure non-spherical particles is the General Defocusing Particle Tracking (Barnkob and Rossi (2020)) which is based on pattern recognition and can be conceptually extended to more complex tasks by extending the reference library of particle images, including not only spherical particles at different depth positions, but also non-spherical particles at different orientations. However, whether this approach could work in practice is still unknown. First, is the information contained in simple defocused images sufficient to reconstruct depth and orientation of non-spherical particles, and eventually under which circumstances? Second, how to practically collect the labelled reference images
Mounting and support for pseudo biaxial Scheimpflug focusing for unity-magnification, high-speed particle velocimetry
Get PDFA camera mount that can support both heavy cameras and heavy optics allowing a total of seven degrees of freedom shared between them has been designed. This allows for Scheimpflug focusing along one or two axes. A paper proposing a solution to two- axes Scheimpflug focusing has been examined and a new nomer is proposed for two-axes Scheimpflug focusing. The newly designed mounts allow for a broader range of solutions for combinations of positioning and alignment than traditional Scheimpflug mounts
A novel laboratory pushing the limits for optics-based basic turbulence investigations
Get PDFDevelopments in theoretical investigations and experimental techniques are reaching a level of maturity for which it is finally becoming possible to answer some of the most pressing questions in turbulence. The prevailing classical theories all have their strengths and drawbacks based on their respective principal assumptions. To better understand the implications of these assumptions, we have developed a theoryintensive experimental strategy. For these purposes, a laboratory has been established at the Department of Mechanical Engineering, Technical University of Denmark. The objective being to provide the data necessary to test the (bounds of) validity of the existing theories; Most prominently the classical Richardson-Kolmogorov-Batchelor paradigm, but also other generally adopted views such as Rapid Distortion Theory and Equilibrium Similarity. The measurements will be analyzed within a novel theoretical framework that enables not only quantification of the degree to which the small and intermediate scale turbulence behaves according to the existing theories (and their central assumptions), but also unveiling the underlying processes that create the respective state of turbulent flow. The present work will describe the current state of the developments of building up the laboratory
Three-Dimensional Particle Tracking Velocimetry using a Single Time-of-Flight Camera
Get PDFTime of Flight (ToF) cameras are a type of range-imaging camera that provides three-dimensional scene information from a single camera. This paper assesses the ability of ToF technology to be used for threedimensional particle tracking velocimetry (3D-PTV). Using a commercially available ToF camera various aspects of 3D-PTV are considered, including: minimum resolvable particle size, environmental factors (reflections and refractive index changes) and time resolution. Although it is found that an off-the-shelf ToF camera is not a viable alternative to traditional 3D-PTV measurement systems, basic 3D-PTV measurements are shown with large (6mm) particles in both air and water to demonstrate future potential use as this technology develops. A summary of necessary technological advances is also discussed
Soap bubbles seeding for quantitative time resolved velocity measurements of a turbulent wake flow behind a cylinder
No full textThe wake flow behind a cylinder of 100mm diameter is investigated using time resolved 2D PIV technique applied to an air flow generated in a closed loop open test section wind tunnel. The flow is seeded using a micro soap bubble generator (BG-1000, TSI Inc.). The bubbles in the air flow were illuminated with a CW laser source and imaged using a high-speed camera. The main purpose of this study is to show features and advantages of using soap bubbles as seeding for a relatively large-scale PIV investigation under low power illumination conditions
PIV and deformation measurements on the rotor blade of a rotating, scaled model wind turbine with flexible blades under tailored inflow conditions
Get PDFWind turbines face harsh inflow conditions when operating in the atmospheric boundary layer or in the wake of other wind turbines. The incoming velocity field can change within seconds due to the turbulent structures it contains, resulting in a rapid change of several degrees in the angle of attack for the rotating blade. Aerodynamics are hence rapidly altered, leading to changes of the occurring forces on the rotor. Such dynamic forces cause the blades to twist and bend, accelerating fatigue and reducing the lifetime of a turbine Spinato et al. (2009)
Three-dimensional temperature and velocity measurements in fluids using thermographic phosphor tracer particles
Get PDFMany flows of technical and scientific interest are intrinsically three-dimensional. Extracting slices using planar measurement techniques allows only a limited view into the flow physics and can introduce ambiguities while investigating the extent of 3D regions. Nowadays, thanks to tremendous progress in the field of volumetric velocimetry, full 3D-3C velocity information can be gathered using tomographic PIV or PTV hence eliminating many of these ambiguities (Discetti and Coletti, 2018; Westerweel et al., 2013). However, for scalar quantities like temperature, 3D measurements remain challenging. Previous approaches for coupled 3D thermometry and velocimetry combined astigmatism PTV with encapsulated europium chelates particles (Massing et al., 2018) or tomographic PIV with thermochromic liquid crystals particles (Schiepel et al., 2021). Here we present a new technique based on solid thermographic phosphor tracer particles, which have been extensively used for planar fluid temperature and velocity measurements (Abram et al., 2018) and are applicable in a wide range of temperatures. The particles are seeded into a gas flow where their 3D positions are retrieved by triangulation from multiple views and their temperatures are derived from two-colour luminescence ratio imaging. In the following, the experimental setup and key processing steps are described before a demonstration of the concept in a turbulent heated jet is shown
Robust approach to monitoring Lagrangian transport in very large volume
Get PDFState-of-the-art flow measurements utilize four or more high-speed cameras to perform highly-accurate Lagrangian particle tracking (LPT) in small to medium-sized measurement volumes (Schanz et al., 2016). Hou et al. (2021) suggested a novel approach to allow measurements in significantly larger measurement volumes (O(10m3 )) while reducing the experimental effort. A single camera is used to track centimeter-sized soap bubbles in three dimensions by not only evaluating the bubble-center location but also the bubbleimage size. Possible applications of the suggested approach include - but are not limited to - measurements in industrial wind tunnels (Hou et al., 2021), full-scale measurements in the atmospheric boundary layer (Rosi et al., 2014; Toloui et al., 2014), and the characterization of airflow in indoor spaces, such as offices or classrooms (Kahler et al., 2020). In the context of the recent pandemic, the latter application could ¨help to reduce infection risk by designing appropriate air circulation. Hereby, frequent air exchange is recommended, while direct airflow from individual to individual should be avoided (WHO, 2020). The present study strives to optimize and simplify the experimental set-up as well as to characterize the accuracy of the novel single-camera approach. Figure 1(a) shows the set-up used to characterize the novel approach