Investigation of turbulent superstructures in Rayleigh–Bénard convection by Lagrangian particle tracking of fluorescent microspheres

Abstract

Abstract We present spatially and temporally resolved velocity and acceleration measurements of turbulent Rayleigh–Bénard convection (RBC) in the entire fluid sample of square horizontal cross section with length $$L=320$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>L</mml:mi> <mml:mo>=</mml:mo> <mml:mn>320</mml:mn> </mml:mrow> </mml:math> mm and height $$H=20$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>H</mml:mi> <mml:mo>=</mml:mo> <mml:mn>20</mml:mn> </mml:mrow> </mml:math> mm, resulting in an aspect ratio of $$\Gamma =H/L=16$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Γ</mml:mi> <mml:mo>=</mml:mo> <mml:mi>H</mml:mi> <mml:mo>/</mml:mo> <mml:mi>L</mml:mi> <mml:mo>=</mml:mo> <mml:mn>16</mml:mn> </mml:mrow> </mml:math> . The working fluid was water with a Prandtl number of Pr =7 and the Rayleigh number was set to $${\text{Ra}}= 1.1\times 10^6$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mtext>Ra</mml:mtext> <mml:mo>=</mml:mo> <mml:mn>1.1</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>6</mml:mn> </mml:msup> </mml:mrow> </mml:math> . In order to minimize surface reflections, we used fluorescent polyethylene microspheres as tracer particles that were imaged at a rate of 19 Hz by six cameras. From the images, the particle positions, velocities, and accelerations were determined via the ‘Shake-The-Box’ (STB) Lagrangian particle tracking algorithm. With this approach, we could simultaneously track more than 300,000 particles and hence study the resulting turbulent structures in the Eulerian and Lagrangian frames while resolving the smallest spatial and temporal scales of the flow.

Topics & Concepts