Abstract
A new rational procedure is proposed for determining the intermittency in the streamwise direction. One of the key parameters for the intermittency determination is the selection of a threshold value, which often involves a certain level of subjectivity. Here, a reliable way of choosing the threshold value in a more objective manner is proposed. The proposed approach involves a single threshold value, equal to the magnitude of the maximum laminar perturbation in the transitional flow. The results obtained are validated with the widely used dual-slope method. In this paper, the measurements are carried out on an experimental arrangement, involving the interaction of an upstream aerofoil wake with a downstream flat plate boundary layer. As a by-product of the study, a scaling parameter has been identified which captures the length of the transition zone as the proximity of the aerofoil in the wall-normal direction is varied.Graphic abstract
Highlights
In the study of turbulence, the intermittency factor ( ) at a given point in the flow field is defined as the fraction of time the flow remains turbulent
The sensitised signal is smoothed within the sampling time interval of 7Δ t(= 0.0007 s), which here is approximately 230 times the Kolmogorov time scale, Tk . (By following Turns et al 1996, Tk is defined in Eq (1), where TI refers to the integral time scale, which was obtained using the autocorrelation of the fluctuating velocity signals in the fully turbulent regime.) Thereafter, the chosen threshold value is applied to the smoothed signal, which in turn yield the indicator function, I(t) given in Eqs. (2) and (3)
Most of the intermittency estimation methods presented in the literature are subjective, with some level of arbitrariness in choosing the threshold value
Summary
In the study of turbulence, the intermittency factor ( ) at a given point in the flow field is defined as the fraction of time the flow remains turbulent. The value of the intermittency factor varies from 0 to 1, where a zero value represents a fully laminar region and the value of one represents a fully turbulent regime.
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