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<p>We present measurements of temperature fields and flow structures in a liquid metal Rayleigh–Bénard convection at a low Prandtl number, which were carried out for the first time using embedded fiber Bragg grating sensors (FBG) in combination with ultrasonic Doppler velocimetry (UDV). The FBG sensors enable minimally invasive, spatially resolved temperature measurements in optically opaque and electrically conductive liquids, thereby overcoming significant limitations of conventional thermocouples and optical techniques. This approach was applied in a cuboid Rayleigh–Bénard cell with an aspect ratio <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\Gamma = 5\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="normal">Γ</mi> <mo>=</mo> <mn>5</mn> </mrow> </math></EquationSource> </InlineEquation> filled with GaInSn. In this paper, we present measurements at two Rayleigh numbers, <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\textrm{Ra} = 6.8 \times 10^4\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mtext>Ra</mtext> <mo>=</mo> <mn>6.8</mn> <mo>×</mo> <msup> <mn>10</mn> <mn>4</mn> </msup> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(2.1 \times 10^5\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mn>2.1</mn> <mo>×</mo> <msup> <mn>10</mn> <mn>5</mn> </msup> </mrow> </math></EquationSource> </InlineEquation>. At the lower Rayleigh number, a coherent three-roll structure is observed with low-frequency modulation of thermal fluctuations. At the higher Rayleigh number, a cellular convection regime emerges, featuring checkerboard-like temperature patterns in the mid-plane and periodic plume emissions. Spectral analysis reveals a dominant oscillation frequency near <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(f = 0.029\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>f</mi> <mo>=</mo> <mn>0.029</mn> </mrow> </math></EquationSource> </InlineEquation> Hz, while autocorrelation and extremum tracking highlight strong temporal coherence near the center and more volatile plume behavior near the sidewalls. The results of the temperature measurements and the UDV velocity measurements are consistent, thus confirming the capability of FBG sensors as a robust tool for investigating the spatio-temporal dynamics in convective systems.</p>

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Spatiotemporal temperature measurements in low-Prandtl-number Rayleigh–Bénard convection at \(\Gamma =5\)

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  • 发表日期：

    2025年11月14日

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  • Nayoung Kim,
  • Sylvie Su,
  • Mahmoud M. Abdelkader,
  • Felix Schindler,
  • Sven Eckert,
  • Tobias Vogt

摘要

We present measurements of temperature fields and flow structures in a liquid metal Rayleigh–Bénard convection at a low Prandtl number, which were carried out for the first time using embedded fiber Bragg grating sensors (FBG) in combination with ultrasonic Doppler velocimetry (UDV). The FBG sensors enable minimally invasive, spatially resolved temperature measurements in optically opaque and electrically conductive liquids, thereby overcoming significant limitations of conventional thermocouples and optical techniques. This approach was applied in a cuboid Rayleigh–Bénard cell with an aspect ratio \(\Gamma = 5\) Γ = 5 filled with GaInSn. In this paper, we present measurements at two Rayleigh numbers, \(\textrm{Ra} = 6.8 \times 10^4\) Ra = 6.8 × 10 4 and \(2.1 \times 10^5\) 2.1 × 10 5 . At the lower Rayleigh number, a coherent three-roll structure is observed with low-frequency modulation of thermal fluctuations. At the higher Rayleigh number, a cellular convection regime emerges, featuring checkerboard-like temperature patterns in the mid-plane and periodic plume emissions. Spectral analysis reveals a dominant oscillation frequency near \(f = 0.029\) f = 0.029 Hz, while autocorrelation and extremum tracking highlight strong temporal coherence near the center and more volatile plume behavior near the sidewalls. The results of the temperature measurements and the UDV velocity measurements are consistent, thus confirming the capability of FBG sensors as a robust tool for investigating the spatio-temporal dynamics in convective systems.