# BPCPPDYSOE21 – scientific programme

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# DY: Fachverband Dynamik und Statistische Physik

## DY 2: Fluid Physics 1 - organized by Stephan Weiss and Michael Wilczek (Göttingen)

### DY 2.1: Talk

### Monday, March 22, 2021, 09:00–09:20, DYa

Transition to the ultimate regime in a stochastic model for radiatively driven turbulent convection — •Marten Klein^{1}, Heiko Schmidt^{1}, and Alan R. Kerstein^{2} — ^{1}Lehrstuhl Numerische Strömungs- und Gasdynamik, Brandenburgische Technische Universität Cottbus-Senftenberg, Germany — ^{2}Consultant, Danville, California, USA

Heat transfer in thermal convection is investigated using the stochastic one-dimensional-turbulence model (ODT).
A Boussinesq fluid of Prandtl number 1 is confined between two horizontal adiabatic no-slip walls (located at z=0 and H) and exposed to constant gravity that points in vertical (−z) direction.
A flow is driven by radiative heating from below yielding the local heating rate Q(z)= (P/ℓ) exp(−z/ℓ), where P is the prescribed heat flux and ℓ the absorption length.
ODT resolves all relevant scales of the flow, including molecular-diffusive scales, along a vertical one-dimensional domain, whereas stochastically sampled eddy events represent the effects of turbulent advection.
ODT results reproduce and extrapolate available reference experiments direct numerical simulations of Lepot et al. (Proc. Natl. Acad. Sci. USA, 115, 2018,
pp. 8937–8941) and Bouillaut et al. (J. Fluid Mech., 861, 2019, R5) in particular capturing the turbulent transition from the classical to the ‘ultimate’ regime.
For these regimes, the exponent values in Nu∼ Ra^{p} scaling are found to be p≈0.3 and p≈0.55, respectively, in agreement with measured values.
Joint probabilities of eddy size and location indicate that the regime transition is accompanied by a relative increase of bulk turbulence.