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| Titel |
Estimation of turbulence scales in gravel-bed river flows |
| VerfasserIn |
Mário J. Franca, Rui M. L. Ferreira, Ulrich Lemmin |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250036816
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| Zusammenfassung |
| Four scales are important in the study of turbulent flows: Kolmogorov dissipative scale;
Taylor micro-scale or simply Taylor scale; integral or Taylor macro-scale; and the scale
corresponding to the higher energy content, herein called energetic scale. All of them
correspond to the size of different repetitive patterns present in the flow, each one related
either to production, transport or dissipation processes. In the present research, turbulence
scales are estimated throughout the vertical using instantaneous velocity profile
measurements made in the gravel-bed Swiss river Venoge, using one 3D Acoustic Doppler
Velocity Profiler. Fifteen velocity profiles, equally spaced in the spanwise direction with a
distance of 10 cm and in the streamwise direction with a distance of 15 cm, were measured
on a 3x5 rectangular horizontal grid. With a vertical resolution of around 0.5 cm, a 3D
measuring grid covering was defined. Instantaneous velocities were sampled during 3.5 min
at a frequency of 26 Hz. With a water depth of 0.20 m and the bed material composed
of coarse round gravel with a D50 of 68 mm, the riverbed is hydraulically rough
and the flow has relative submergence of 2.94 (ratio of the water depth by D50).
Kolmogorov scale is estimated from the dissipative dynamic scales; micro-scale
estimation considers this as a measure of the amount of dissipation; the integral scale is
defined from the 1D velocity auto-correlation function; and, the energetic scale is
estimated from the energy spectrum based on a multilevel wavelet decomposition of
the fluctuating velocity. Production is estimated directly from the measured shear
stresses and mean velocity gradients whereas dissipation is estimated indirectly using
the second Kolmogorov hypothesis by relating it with the energy spectral density
function for the streamwise velocity fluctuations. The analyzed turbulence scales
distribution may be described fairly well by power laws as indicated by previous authors;
best-fit coefficients and exponents are presented as well as the ratio between both
productive scales and between both dissipative scales. From these ratios, productive and
dissipative ranges are inferred. These results contribute with empirical information
essential for mixing and transport processes modelling in turbulent gravel-bed river
flows.
Research supported by Portuguese Foundation for Science and Technology
(PTDC/ECM/65442/2006) and by the Swiss National Science Foundation (2000-063818). |
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