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| Titel |
Numerical Simulation of Turbulence Collapse and Intermittency in an SBL |
| VerfasserIn |
Cedrick Ansorge, Juan Pedro Mellado |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250079835
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| Zusammenfassung |
| Direct numerical simulation of a turbulent Ekman layer is used to investigate some aspects of
a planetary boundary layer under the influence of stable stratification. In particular, we study
bulk properties of the flow under atmospheric conditions in which the flow relaminarizes
partly, i.e. it is globally intermittent, or completely. The physical model depends on two
non-dimensional parameters: (1) a Reynolds number determining the bulk shear
and (2) a Froude- or Richardson number characterizing the stratification. In our
study, we fix the Reynolds number and vary the stratification over an interval that
covers from neutral conditions to very stable conditions, the latter mimicking the
turbulence collapses sometimes observed in arctic wintertime or nocturnal boundary
layers.
The weakly, intermediately and strongly stably stratified regimes of turbulence known
from observations, modelling as well as theoretical considerations are reproduced in the
simulations. In the very stable regime, spatio-temporal intermittency emerges on rather large
spatial but small temporal scales if a sufficiently large domain is used. Qualitatively, the
spatio-temporal patterns of intermittency in the wall-region resemble those observed in
turbulent channel flows over a wide range of stratifications and Reynolds numbers.
Quantitatively, however, we show that, with the rotational effects considered here, the
arrangement of those patterns is linked to the external intermittency in the outer layer that is
also present in a neutrally stratified reference case. These results suggest that – through the
establishment of large-scale modes – the rotation of the wind in Ekman flow plays
an important role for the structure of global intermittency. Hence, findings from
non-rotating geometries should be complemented with rotating configurations. |
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