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

Abstract 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 $$\Gamma = 5$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Γ</mml:mi> <mml:mo>=</mml:mo> <mml:mn>5</mml:mn> </mml:mrow> </mml:math> filled with GaInSn. In this paper, we present measurements at two Rayleigh numbers, $$\textrm{Ra} = 6.8 \times 10^4$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mtext>Ra</mml:mtext> <mml:mo>=</mml:mo> <mml:mn>6.8</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>4</mml:mn> </mml:msup> </mml:mrow> </mml:math> and $$2.1 \times 10^5$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>2.1</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>5</mml:mn> </mml:msup> </mml:mrow> </mml:math> . 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 $$f = 0.029$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>f</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.029</mml:mn> </mml:mrow> </mml:math> 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.