Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session B18: Turbulence Measurements II |
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Chair: Daniela Tordella, Politecnico di Torino Room: 400 |
Saturday, November 23, 2019 4:40PM - 4:53PM |
B18.00001: The alignment of vortical structures in turbulent flow through a contraction Vivek Mugundhan, Pugazenthi Ramakrishnan, Nathan Speirs, Ravi Samtaney, Sigurdur Thoroddsen Contracting turbulent streams occur in many industrial applications, such as flows in ducts and nozzles. Herein we use Tomo-PIV to study the alignment of turbulent vortical structures advecting through a 2-D contraction, with a contraction ratio of 2.5:1. The turbulence is generated with an active grid inside a water tunnel, attaining Re$_{\mathrm{\lambda }}$ of 250. Volumetric and time-resolved Tomo-PIV and Shake-The-Box velocity measurements are used to characterize the streamwise evolution of coherent vortical structures at three locations, upstream of and two within the contraction. We confirm the conceptual picture of coherent large-scale vortical structures being stretched and aligned with the mean strain rate. We also find that the local vorticity vector aligns with the mean strain, while remaining preferentially aligned with the intermediate eigenvector of the local instantaneous strain-rate tensor. The observed behavior of alignments are unaffected by the grid-rotation protocols and the modest transverse inhomogeneity. [Preview Abstract] |
Saturday, November 23, 2019 4:53PM - 5:06PM |
B18.00002: Measurement of flow fields and local pressures on a circular cylinder at the Reynolds numbers up to 10$^{\mathrm{6}}$ Jae Hwan Jung, Seokkyu Cho, Hyunwoo Jung, Jun-Hee Lee, Yong-Guk Lee, Hong Gun Sung, Bu-Geun Paik The present study is aimed to measure the turbulent flow fields and local pressures on a circular cylinder at the Reynolds numbers of up to 10$^{\mathrm{6}}$. Unlike the previous studies mainly focused on the subcritical flow regime, the supercritical flow regime, which is more realistic to the full-scale condition of engineering, is considered in the present study. The particle image velocimetry (PIV) and local pressure scanner systems are employed to measure local mean pressures and flow fields on the circular cylinder. The results show that the flow characteristics of the circular cylinder strongly depends on the Reynolds number. As the Re increases, this size and shape of wake structures become smaller and narrower with the dramatic reduction of the drag. This is due to the one and two bubble transition leading to the reattachment and delay of the separation point. Although the general flow and load characteristics of the circular cylinder are well documented in the literature, it is still hard to find the detailed information of flow fields and local pressure distributions on the circular cylinder, especially at the supercritical Reynolds number. Therefore, this study will be able to provide meaningful information to improve our understanding of the physical insight into the relationship between the loads and wake structures. Moreover, this will contribute to the validation of CFD codes. [Preview Abstract] |
Saturday, November 23, 2019 5:06PM - 5:19PM |
B18.00003: Turbulent characterization generated by an active grid in a circular geometry. Loïc Duffo, R. Jason Hearst, Thierry Schuller, James R. Dawson Experimentally inducing turbulence is of importance in understanding many practical flows.Recently active grids have been used to control the turbulence intensity and the integral length scale. However, these studies are almost exclusively based on diamond grids inserted in a square section. The aim of the current study is to design and test an active grid suitable for small diameter circular sections, with the ultimate goal of controlling inlet conditionsfor reacting flows. The characterization of the turbulence created by the circular sector paddles includevarious parameters such aspaddle rotationspeeds, number and shapes of paddles andnumber of active gridstages. These parameterswere tested and compared in order to find the most efficient way to obtain a high level of turbulence intensity whileretaininghomogeneity. High turbulent intensities in the range of 10 to 20{\%} were achieved using the circular geometry, which was comparable to that of previous square geometry studies. These experiments and results will enable the study of high intensity turbulence in circular geometry. [Preview Abstract] |
Saturday, November 23, 2019 5:19PM - 5:32PM |
B18.00004: Influence of Atmospheric Flow Conditions on Scalar-mediated Mosquito Behavior Yi-Chun Huang, Neil Vickers, Marcus Hultmark Mosquito-vectored diseases are increasingly problematic around the globe where they pose a current and emerging threat to public health. Female mosquitoes locate potential hosts by tracking CO2, volatile skin emanations, humidity, and thermal cues, each of which act as passive scalars that are distributed by local flow conditions. In a preliminary study, 3D field measurements of wind conditions and simultaneous mosquito captures in CO2-baited traps have shown that flight activity occurs during turbulent conditions. However, using field data, it is impossible to uncouple turbulent statistics from each other and from the mean wind speed. To fully explore the parameter space, a unique active flow modulation grid of independently operated paddles was developed. Unlike static grids that generate turbulence within a predefined range of scales, an active grid imposes variable and controllable turbulent structures onto the moving air by synchronized rotation of the paddles at specified frequencies. In the long-term, by leveraging such technology, host-seeking orientation strategies of these insects can be studied and the efficacy of traditional or new approaches that target the behavioral responses of mosquitoes can be rigorously assessed. [Preview Abstract] |
Saturday, November 23, 2019 5:32PM - 5:45PM |
B18.00005: Turbulence scaling in the heliosheath and in the local interstellar medium from Voyager observations Federico Fraternale, Nikolai V. Pogorelov, John D. Richardson, Daniela Tordella This study considers the recent observations of magnetic field fluctuations in the inner heliosheath (IHS) and in the Local Interstellar Medium (LISM). We analyze in-situ data provided by the Voyager In- terstellar Mission (V1 and V2 spacecraft) for heliocentric distances up to 115 AU (V2) and 135 AU (V1). Turbulence, instabilities and recon- nection near the heliopause affect the particle transport and magnetic energy conversion into kinetic energy and into heat. A relevant medi- ation effect on plasma fluctuations is expected from charge exchange processes, which extensively generate pickup ions and energetic neu- tral atoms. We discuss the multi-scale features of magnetic fluctuations for a six-decades range of spacecraft-frame frequencies, which includes the energy-injection regime, the inertial cascade, and the fluid-to-kinetic transition. In the IHS, our study reveals the presence of two relevant scales within the magnetohydrodynamic regime, and highlights a cor- relation between the intermittency and compressibility of fluctuations. In the LISM, we show that the dominant Alfvnic character observed in 2015/2016 is not retained during 2017, when the large-scale compress- ibility reaches the value of 0.6. [Preview Abstract] |
Saturday, November 23, 2019 5:45PM - 5:58PM |
B18.00006: Innovative, mini, biodegradable radiosondes to track small-scale fluctuations in warm cloud and clear air environments Shahbozbek Abdunabiev, Tessa C. Basso, Daniela Tordella, Miryam Paredes, Eros Pasero, Flavio Canavero Modelling clouds depends on a variety of chemical and physical processes. These processes range from the nanometer scale, where nucleation, coalescence and rain formation take place, to airflow dynamics which can range up to kilometres. Turbulence plays an important role in the behaviour of these processes and can be noted on different scales. The innovative, ultralight green radiosondes presented here are designed to passively track fluctu- ations of temperature, humidity, pressure and velocity for several hours on isopycnic surfaces in cloud and clear air environments at altitudes between 1-3 km. To equalize the buoyancy force to the weight of the system, the balloon is filled with a mixture of helium gas and air. The sonde incorporates various set of sensors and a long range/low power ra- dio module, which makes it possible to track fluctuations. A Lagrangian set of data can then be obtained by fusion of GPS (Global Positioning System) and IMU (Inertial Measurement Unit) sensor output. GPS and IMU data can be integrated using the Kalman filter, where GPS pro- vides the periodic updates for removing drifts in the IMU output. This data set can be of help in developing stochastic models to account for turbulence effects on cloud formation. [Preview Abstract] |
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