Turbulent bursts in a near-neutral atmospheric boundary13; layer - part I

Narasimha, R and Kailas, SV and Kaimal, JC (1990) Turbulent bursts in a near-neutral atmospheric boundary13; layer - part I. Technical Report. National Aeronautical Laboratory, Bangalore, India.

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Abstract

The high Reynolds number boundary layer data obtained over the lowest 300m of a nearly neutrally stable atmospheric boundary layer at the Boulder Atmospheric Observatory13; are analysed here to detect and study turbulent events by the Variable Interval Time Averaging (VITA) technique. Both momentum events (detected from the horizontal speed, u') and flux events (detected by a modification of VITA, applied directly to the momentum flux signal, u'w') are found to have conditionally averaged signatures similar to those observed in the laboratory at much lower Reynolds numbers by Johansson amp; Alfredsson (1982) and Thomas (1977), showing that turbulent bursts of the familiar kind occur also at vastly higher Reynolds numbers. However the atmospheric events last longer and occur less frequently, in a manner not inconsistent with outer scaling, which is particularly significant as wall scales are of the same13; order in both atmospheric and wind tunnel flows. It is found that each flux event has an identifiable core where the stress generation rate is an order of magnitude higher than in the wings. Based on an analysis of 'chronicles' of events, we also identify a class of'super-events', which contribute twice as much to the flux as ordinary events and at twice the rates. Further more, defining a hierarchy of flux events in order of size as measured by their contribution to the total momentum flux, a novel measure of burstiness is introduced, through an index which varies from zero when the contribution is evenly distributed in time to unity when it occurs in instantaneous events. It is found that the burstiness in the momentum flux has consistent values around 0.3 over most of the atmospheric boundary layer considered here. A study of the variation13; of various event parameters with height, including event frequency, distribution of time intervals between events, stress generation rates and burstiness, shows a remarkable homogeneity of the flow over the height of the tower, in particular over the range 20-300m above the surface, thus providing strong indications of large-scale coherent motion at extremely high Reynolds number.

Item Type:	Monograph (Technical Report)
Uncontrolled Keywords:	Turbulent bursts;Near-neutral;Atmospheric boundary layer
Subjects:	ENGINEERING > Fluid Mechanics and Thermodynamics
Depositing User:	Mr. Ravikumar R
Date Deposited:	19 Sep 2006
Last Modified:	13 Aug 2015 09:28
URI:	http://nal-ir.nal.res.in/id/eprint/2743

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