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You are here : eLibrary : IAHR World Congress Proceedings : 36th Congress - The Hague (2015) ALL CONTENT : Hydro-environment : Evaluation of integral length scale from experimental from experimental data in a flow through subme...
Evaluation of integral length scale from experimental from experimental data in a flow through submerged vegetation
The interaction between flow and vegetation is a complex topic. Effects of vegetation on turbulent characteristics can be
stressed through experimental measurements.
One of the main concerns in turbulence studies is the estimate of temporal and, if possible, spatial characteristics of the
existing eddies in the flow. Possibly most easy is the evaluation of the integral timescale of turbulence defined with the
use of the autocorrelation function R(¦Ó) derived by the analysis of time series collected in fixed positions.
The transformation from temporal characteristics of the signal to spatial characteristics can be made by means of the
Taylor hypothesis of the ¡°frozen turbulence¡±, which is applicable if the intensity of the turbulence of the flow is small.
Generally, the autocorrelation function of the longitudinal velocities assumes the shape of curve rapidly decaying to its
first zero-crossing, after which an alternation of positive and negative values may be evident. In these cases this
characteristic point determines the time lag.
In other cases the determination of the integration domain could be not straight-forward. In particular, the domain of the
autocorrelation function from experimental data is finite, and there is some uncertainty on how best to define the
integration domain. Sometimes the integral timescale is assumed corresponding to the time lags at which R(¦Ó) drops to
In this paper the results of experimental studies aimed to the recognition of Eulerian integral length scale in channel with
simulated woody vegetation, are discussed. The model of vegetation consisted of regular arrays of stiff vertical cylinders
with variation in cylinders density. Two measurements locations were considered. In order to measure instantaneous
velocities an LDA system was used. A software was developed to evaluate autocorrelation functions.
The integral timescales were assumed corresponding to the time lags at which R(¦Ó) drops to 1/en, testing different values
of the exponent n. Eulerian integral length scales were evaluated through Taylor¡¯s hypothesis. Their distributions suggest
that, increasing n, the time lag 1/en corresponds to larger integral length scales and measuring location and cylinders
density affect Eulerian integral length scales.
File Size : 531,376 bytes
File Type : Adobe Acrobat Document
Chapter : IAHR World Congress Proceedings
Category : 36th Congress - The Hague (2015) ALL CONTENT
Article : Hydro-environment
Date Published : 20/08/2015
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