Coherent structures over a smooth and a triangular riblet drag reducing surface

Manen, A. D. Schwarz-Van; Hoogsteen, R.; Stouthart, J. C.; Prasad, K. Krishna; Nieuwstadt, F. T. M.

doi:10.1007/978-94-011-3526-9_5

Cited by 2 publications

(1 citation statement)

References 11 publications

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“…Comparison with similar statistics from earlier investigations (e.g. Willmarth & Lu 1972;Antonia 1977;Schwarz-van Manen 1993) is presented in table 2. Schwarzvan Manen's data come from the 200 m Cabauw tower (mentioned in §1).…”

Section: Momentum Flux Events Under Nearly Neutral Conditionssupporting

confidence: 65%

Turbulent flux events in a nearly neutral atmospheric boundary layer

Narasimha¹,

Kumar

Prabhu

et al. 2007

Phil. Trans. R. Soc. A.

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We propose here a novel method of analysing turbulent momentum flux signals. The data for the analysis come from a nearly neutral atmospheric boundary layer and are taken at a height of 4m above ground corresponding to 1.1 x 10(5) wall units, within the log layer for the mean velocity. The method of analysis involves examining the instantaneous flux profiles that exceed a given threshold, for which an optimum value is found to be 1 s.d. of the flux signal. It is found feasible to identify normalized flux variation signatures separately for positive and negative 'flux events'-the sign being determined by that of the flux itself. Using these signatures, the flux signal is transformed to one of events characterized by the time of occurrence, duration and intensity. It is also found that both the average duration and the average time-interval between successive events are of order 1s, about four orders of magnitude higher than a wall unit in time. This episodic description of the turbulence flux in the time domain enables us to identify separately productive, counter-productive and idle periods (accounting, respectively, for 36, 15 and 49% of the time), taking as criterion the generation of the momentum flux. A 'burstiness' index of 0.72 is found for the data. Comparison with laboratory data indicates higher (/lower) ejection (/sweep) quadrant occupancy but lower (/higher) contributions to flux from the ejection (/sweep) quadrant at the high Reynolds numbers of the atmospheric boundary layer. Possible connections with the concept of active and passive motion in a turbulent boundary layer are briefly discussed.

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Section: Momentum Flux Events Under Nearly Neutral Conditionssupporting

confidence: 65%

Turbulent flux events in a nearly neutral atmospheric boundary layer

Narasimha¹,

Kumar

Prabhu

et al. 2007

Phil. Trans. R. Soc. A.

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Effect of acoustic excitation on flow over a partially grooved circular cylinder

1995

Experiments in Fluids

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An experimental investigation was carried out on the flow over a partially grooved circular cylinder over a Reynolds number range of 3 x 104 to 1.22 x 105 with and without acoustic excitation. Without excitation the flow over the smooth half of the cylinder was observed to shift to higher subcritical regime. The flow over the groove half, however, is shifted to supercritical or transcritical flow regime. With excitation, on the smooth half it is the separated laminar shear layer which locks in with the excitation frequency, resulting in the shift from subcritical to supercritical or transcritical regimes. On the groove half excitation is not effective for the flow within the transcritical regime. With excitation, the lift is found to reverse its direction while the drag is nearly the same.

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Coherent structures over a smooth and a triangular riblet drag reducing surface

Cited by 2 publications

References 11 publications

Turbulent flux events in a nearly neutral atmospheric boundary layer

Turbulent flux events in a nearly neutral atmospheric boundary layer

Effect of acoustic excitation on flow over a partially grooved circular cylinder

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