The subject of the present work is the investigation of thermal convection
in terms of the Rayleigh dependency of the different characteristic
features of the flow field like the boundary layers, the thermal plumes and
the core region by means of DNS and LES.Analysis of the probability density
function (PDF) of the thermal dissipation rates shows that they reflect
three distinct regions that can be assigned the turbulent background, the
plumes / mixing layers and the conductive sublayer. With increasing
Rayleigh number (Ra) the dynamics of the core volume and the boundary
layers are increasingly decoupled, which among other things is reflected by
a decreasing correlation between the turbulent fluctuations of the
temperature and the velocity field. This is considered to confirm the
behaviour of soft and hard turbulence sketched by Castaing et al. [J. Fluid
Mech., 204, 1989].
Furthermore it is shown that thermal dissipation rates within the
conductive sublayer are increasingly dominated by turbulent fluctuations,
and therefore show an entirely different behaviour than the plumes / mixing
layers. Consequently, it is proposed to extend the ansatz by Grossmann &
Lohse [Phys. Fluids, 16(12), 2004] to separate the thermal dissipation
rates to account for this behaviour.The comparison of three different
geometries (periodic, rectangular and cubic cell) shows that the flow
fields exhibit significant differences at low Rayleigh numbers, but the
differences narrow with increasing Rayleigh number. This behaviour is
reflected by the contributions of the characteristic features of the flow
field as well as the scaling of the Nusselt-Rayleigh relation.
Finally, the turbulent Rayleigh-Benard convection (Ra = 3.5e5) is compared
with thermal mixed convection (Ar = 1). It is observed qualitatively that
the structure formation in mixed convection differs significantly: for the
same Rayleigh number smaller and a larger number of thermal plumes are
observed. It is found that the location of separation of the wall jet is
strongly affected by the interaction with the thermal plumes and the
convective heat flux through the outflow boundary is oscillating. This
behaviour is in qualitative agreement with experimental results. A
comparison of the thermal dissipation rates of Rayleigh-Benard and mixed
convection shows that the influence of the incoming jet predominantly
affects the large-scale structures rather than the small-scale mixing
processes.