Simultaneous Measurement of Turbulence and Particle Kinematics Using Flow Imaging Techniques

Summary

The technique described herein offers a low cost and relatively simple method to simultaneously measure particle kinematics and turbulence in flows with low particle concentrations. The turbulence is measured using particle image velocimetry (PIV), and particle kinematics are calculated from images obtained with a high-speed camera in an overlapping field-of-view.

Abstract

Numerous problems in scientific and engineering fields involve understanding the kinematics of particles in turbulent flows, such as contaminants, marine micro-organisms, and/or sediments in the ocean, or fluidized bed reactors and combustion processes in engineered systems. In order to study the effect of turbulence on the kinematics of particles in such flows, simultaneous measurements of both the flow and particle kinematics are required. Non-intrusive, optical flow measurement techniques for measuring turbulence, or for tracking particles, exist but measuring both simultaneously can be challenging due to interference between the techniques. The method presented herein provides a low cost and relatively simple method to make simultaneous measurements of the flow and particle kinematics. A cross section of the flow is measured using a particle image velocimetry (PIV) technique, which provides two components of velocity in the measurement plane. This technique utilizes a pulsed-laser for illumination of the seeded flow field that is imaged by a digital camera. The particle kinematics are simultaneously imaged using a light emitting diode (LED) line light that illuminates a planar cross section of the flow that overlaps with the PIV field-of-view (FOV). The line light is of low enough power that it does not affect the PIV measurements, but powerful enough to illuminate the larger particles of interest imaged using the high-speed camera. High-speed images that contain the laser pulses from the PIV technique are easily filtered by examining the summed intensity level of each high-speed image. By making the frame rate of the high-speed camera incommensurate with that of the PIV camera frame rate, the number of contaminated frames in the high-speed time series can be minimized. The technique is suitable for mean flows that are predominantly two-dimensional, contain particles that are at least 5 times the mean diameter of the PIV seeding tracers, and are low in concentration.

Introduction

There exist a large number of applications in both scientific and engineering fields that involve the behavior of particles in turbulent flows, for example, aerosols in the atmosphere, contaminants and/or sediments in engineered systems, and marine micro-organisms or sediment in the ocean^1,2,3. In such applications, it is often of interest to understand how the particles respond to turbulence, which requires simultaneous measurement of the particle kinematics and the fluid dynamics.

Existing technologies to measure particle motions, called particle....

Protocol

Note: All personnel should be trained in the safe use and operation of Class IV lasers as well as in the safe use and operation of hand and power tools.

1. Experimental Set-up

1. PIV setup
   1. Set up the dual-head laser and optics.
      1. Place the laser on an optical plate. Level the laser with respect to the bottom of the facility (or with respect to the ground if it is desired to have vertical velocities aligned with gravitational acceleration) and vertically align the laser beam with the center of the 2D plane to be imaged.
      2. Place the cylindrical lens in the path of the laser beam by s....

Results

A schematic of the experimental setup is shown in Figure 1. The figure shows the arrangement of light sheets (LED and laser), the overlap in the FOVs, and the position of the FOVs relative to the oscillating grid and tank walls. The turbulence and particles are measured simultaneously as described in the protocol section. Figure 2 shows example results of the measurements of instantaneous velocity and vorticity along with sample .......

Discussion

The method described herein is relatively inexpensive and provides a simple way to simultaneously measure particle trajectories and turbulence in order to examine the influence of flow on particle kinematics. It is noteworthy to mention that flows or particle motions that are strongly three-dimensional are not well-suited for this technique. The out-of-plane motion will result in errors¹⁷ in both the 2D tracking and the PIV analysis and should be minimized. In addition, the method requires the con.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Optical lenses	CVI LASER OPTICS	Y2-1025-45, RCC-25.0-15.0-12.7-C, PLCC-25.4-515.1-UV	Other optics companies are acceptable. Spherical and cyclindrical lenses for generating PIV light sheet.
Camera lens for PIV	Nikon	Nikkor 105mm f/2D	Other camera lens companies are acceptable. Camera lens for PIV imaging.
Camera lens for high-speed	Nikon	Nikkor 50mm f/1.8D	Other camera lens companies are acceptable. Camera lens for high-speed imaging.
Dual-head pulsed laser	Quantel	EverGreen: 532nm, 70mJ@15Hz	Other laser companies are acceptable. Dual-head Pulsed-laser for PIV: Nd:YAG
LED line light	Gardasoft Vision, Ltd.	VLX2 LED Line Lighting - Green - GAR-VLX2-250-LWD-G-T04	Other companies are acceptable. Line light for LED.
PIV seeding particles/tracers	Potters Industries	SPHERICAL Hollow Glass Spheres: 11 mm average diameter	Other companies are acceptable. PIV seeding particles
CCD cross-correlation camera	TSI, Inc.	POWERVIEW 11M: CCD, Double-exposure, 4008x2672 pixels @ 4.2 Hz with 12bit dynmic range	Other companies are acceptable. Double-exposurem, CCD camera for PIV imaging.
High-speed camera	Photron	FASTCAM SA3; Model 60K: 1024x1024 pixels @ 1kHz	Other companies are acceptable. CMOS camera for high speed imaging.
Synchronizer	TSI, Inc.	LASERPULSE SYNCHRONIZER 610036	Other companies are acceptable. Synchronize the acquisition of the PIV camera and laser.
Calibration target	TSI, Inc.		Other companies are acceptable. Precision target for image calibration.