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| Titel |
A scaling analysis of the turbulent boundary-layer in a shallow urban lake |
| VerfasserIn |
Yacine Mezemate, George Fitton, Ioulia Tchiguirinskaia, Daniel Schertzer, Celine Bonhomme, Frédéric Soulignac, Bruno Lemaire, Brigitte Vinçon Leite |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250099289
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| Publikation (Nr.) |
 EGU/EGU2014-15052.pdf |
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| Zusammenfassung |
| The turbulent boundary-layer (TBL) has been the focus of countless experimental and numerical studies. Due to its complex nature the dynamics of the TBL are still far from being understood. Thus, to study, in particular the scaling properties of a TBL, we use a three-dimensional velocity time-series measured from an Acoustic Doppler Current Profiler(ADCP). The ADCP is particularly useful for analysing the TBL as it is able to measure the 3D velocity in the vertical, 127 cells over 3 meters.
The ADCP is positioned next to a storm water discharge point at the bottom of a shallow urban lake
in Créteil, a region in Paris. The positioning of the ADCP, in a stable, stratified lake, with a strong
turbulent flow occurring close to the surface has given us a unique situation in which a turbulent
bounded-layer can be analysed. Vertical profiles measured in the atmospheric boundary-layer are
typically intrusive due to the requirement of masts and other complex measuring structures.
Moreover atmospheric profilers are normally coarsely spaced in the vertical.
In order to analyse the scaling properties of the velocity we compute its energy spectrum. In a log-
log plot, if the velocity is scaling, the spectral exponent is its slope. It frequently that in the
presence of a boundary-layer, a -1 spectral exponent is observed. Dimensional arguments suggest a
-1 spectral exponent when the energy flux becomes dependent on the friction velocity instead of the
length-scale.
Due to the fine vertical spacing of the measurements we are not only able to observe a -1 spectral exponent, but observe a smooth transition from a free-stream turbulent regime (spectral exponent close to -5/3) to a boundary-layer -1 exponent. Because the transition shows such a strong a depth dependence we are able to propose a general model based on dynamical equations for the scaling exponent as a function of height. This generalised scaling boundary-layer model allows one to easily reproduce the turbulent statistics in the boundary-layer. |
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