Coherence of temperature and velocity superstructures in turbulent Rayleigh-Bénard flow

Krug, D.; Lohse, D.; Stevens, R.J.A.M.

doi:10.1017/jfm.2019.1054

Publication 71 · Thermal convection

Coherence of temperature and velocity superstructures in turbulent Rayleigh-Bénard flow

D. Krug, D. Lohse, R.J.A.M. Stevens, J. Fluid Mech. 887, A2 (2020).

Main finding

Power-spectrum peak, integral scale, co-spectrum peak, and coherence scale are non-equivalent measures. The apparent smaller velocity superstructure arises because velocity variance is dominated by intermediate scales. Temperature-variance production is concentrated near the wall and at small scales; buoyancy produces vertical-velocity variance across intermediate scales and through the bulk, explaining different spectral weighting.

Coherence spectra and inferred structure size against Rayleigh number for three domain widths — **How to read the figure.** (a) Coherence spectrum at mid-height across Rayleigh numbers. (b) The inferred superstructure size against Rayleigh number for three domain widths and for several different definitions of that size. (c, d) Premultiplied spectra of temperature and vertical velocity. Different definitions give different sizes for the same flow. Open the full-resolution figure. Figure 8. D. Krug et al. (2020). No separate licence is stated here; consult the original publication and credited source before reuse.

Why this matters

The work shows why a power-spectrum peak alone can give a misleading measure of coherent structure size.

Research context

This reanalysis corrects the earlier spectral calculation and shows why peak scale, integral scale, co-spectrum, and coherence are not interchangeable diagnostics. Temperature and vertical velocity can remain coherent at large scales even when their variance spectra peak at different wavelengths.

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