Topic
Strouhal number
About: Strouhal number is a research topic. Over the lifetime, 5203 publications have been published within this topic receiving 139840 citations. The topic is also known as: Thomson number.
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TL;DR: In this article, the conditions most favorable to vortex parting were determined as a function of the excitation Strouhal number, the Reynolds number, and the initial shear-layer state.
Abstract: Hot-wire and flow visualization studies were performed in three air jets subjected to pure-tone excitation The instability, vortex roll-up, and transition to the controlled excitation were investigated The conditions most favorable to vortex parting were determined as a function of the excitation Strouhal number, the Reynolds number, and the initial shear-layer state; it was shown that the rolled-up vortex rings undergo pairing under 'the shear layer mode', and the 'jet-column mode' when the Strouhal numbers based on the initial shear-layer momentum thickness are 0012 and 085, respectively Coherent ring-like vortical structures could be educed to the end of the potential core; however, the paired vortex becomes weaker with increasing downstream distances The transverse transport of 'u' momentum by the coherent structures was much larger during the pairing process than in regions where a single vortex is studied
469 citations
TL;DR: In this paper, it was shown through experimental data from flapping foils and data from fish observation that thrust develops through the formation of a reverse von Karman street whose preferred Strouhal number is between 0.25 and 0.35.
Abstract: Foils oscillating transversely to an oncoming uniform flow produce, under certain conditions, thrust. It is shown through experimental data from flapping foils and data from fish observation that thrust develops through the formation of a reverse von Karman street whose preferred Strouhal number is between 0.25 and 0.35, and that optimal foil efficiency is achieved within this Strouhal range.
452 citations
TL;DR: In this article, the Strouhal number is defined as the appearance of a time-averaged outward velocity along the jet axis and corresponds to the generation and subsequent convection or escape of a vortex ring.
Abstract: Af ormation criterion for synthetic jets is proposed and validated. A synthetic jet actuator is a zero-net mass-flux device that imparts momentum to its surroundings. Je tf ormation is defined as the appearance of a time-averaged outward velocity along the jet axis and corresponds to the generation and subsequent convection or escape of a vortex ring. It is shown that over a wide range of operating conditions synthetic jet formation is governed by the jet Strouhal number Sr (or Reynolds number Re and Stokes number S). Both numerical simulations and experiments are performed to supplement available two-dimensional and axisymmetric synthetic jet formation data in the literature. The data support the jet formation criterion 1/Sr = Re/S 2 > K, where the constant K is approximately 1 and 0.16 for two-dimensional and axisymmetric synthetic jets, respectively. In addition, the dependence of the constant K on the normalized radius of curvature of a rounded orifice or slot is addressed. The criterion is expected to serve as a useful design guide for synthetic jet formation in flow control, heat transfer, and acoustic liner applications, in which a stronger jet is synonymous with increased momentum transfer, vorticity generation, and acoustic nonlinearities.
441 citations
TL;DR: In this paper, a harmonically heaving and pitching foil with a superposed pitch bias was evaluated to determine its propulsive efficiency under conditions of significant thrust production, as function of the principal parameters: the heave amplitude, Strouhal number, angle of attack, and phase angle between heave and pitch.
436 citations
TL;DR: In this paper, coherent axisymmetric structures in a turbulent jet are modelled as linear instability modes of the mean velocity profile, regarded as the profile of a fictitious laminar inviscid flow.
Abstract: Coherent axisymmetric structures in a turbulent jet are modelled as linear instability modes of the mean velocity profile, regarded as the profile of a, fictitious laminar inviscid flow. The usual multiple-scales expansion method is used in conjunction with a family of profiles consistent with similarity laws for the initial mixing region and approximating the profiles measured by Crow & Champagne (1971), Moore (1977) and other investigators, to deal with the effects of flow divergence. The downstream growth and approach to peak amplitude of axisymmetric wave modes with prescribed real frequency is calculated numerically, and comparisons are made with various sets of experimental data. Excellent agreement is found with the wavelength measurements of Crow & Champagne. Quantities such as the amplitude gain which depend on cumulative effects are less well predicted, though the agreement is still quite tolerable in view of the facts that this simple linear model of slowly diverging flow is being applied far outside its range of strict validity and that many of the published measurements are significantly contaminated by nonlinear effects. The predictions show that substantial variations are to be expected in such quantities as the phase speed and growth rate, according to the flow signal (velocity, pressure, etc.) measured, and that these variations depend not only on the axial measurement location but also on the cross-stream position. Trends of this kind help to explain differences in, for example, the preferred Strouhal number found by investigators using hot wires or pressure probes on the centre-line, in the mixing layer or in the near field.
430 citations