We first study the impact of a liquid drop of low viscosity on a super-hydrophobic surface. Denoting the drop size and speed as $D_{0}$ and $U_{0}$, we find that the maximal spreading $D_{\hbox{\scriptsize\it max}}$ scales as $D_{0}\hbox{\it We}^{1/4}$ where We is the Weber number associated with the shock ($\hbox{\it We}\,{\equiv}\,\rho U_{0}^2 D_{0}/\sigma$, where $\rho$ and $\sigma$ are the liquid density and surface tension). This law is also observed to hold on partially wettable surfaces, provided that liquids of low viscosity (such as water) are used. The law is interpreted as resulting from the effective acceleration experienced by the drop during its impact. Viscous drops are also analysed, allowing us to propose a criterion for predicting if the spreading is limited by capillarity, or by viscosity
Galloping is a critical type of flow-induced vibration (FIV) arising on power transmission lines, high rise buildings, pipe and cables bundles in the oil and gas industry. In this paper, we present a purely nonlinear energy sink (NES) that mitigates the galloping of a square prism. The NES is composed of a ball rotating freely in a circular track attached to the prism. The ball’s dynamics is coupled to that of the prism in a purely nonlinear way by inertia. We experimentally assess how this simple NES reduces the prism vibration by comparing the prism amplitude responses with and without the NES. A supplementary video presents these experiments, during which the NES ball exhibits different dynamics in three regimes; oscillatory, intermittent, and rotational. We characterize the ball behaviour and its effect on the prism response in each regime. The oscillatory regime appears at low flow speeds at which both the prism and the ball oscillate with small amplitude. The intermittent regime represents a transition mode within a small range of flow speeds and corresponds to a small jump in the vibration amplitude of the prism. The rotational regime appears at higher flow speeds, where the ball oscillates with relatively high angular speeds resulting in a strong modulated response of the prism. The design of the NES allows to easily vary its track dimensions to use a ball of different sizes and masses. Accordingly, we demonstrate the influence of the main NES parameters, which are the ball mass, NES track radius, ball friction, and radial clearance between NES track walls and the rotating ball, on both the prism response and the ball behaviour. The NES we present is directly amenable to mitigate other types of FIV.
This paper deals with the numerical and experimental determination of stability derivative inside a parallel triangular tube bundle for pitch Reynolds number Rep ∈ [60 3.104]. The present work focuses on the derivative of the lift coefficient, in the direction transverse to the flow, of the central cylinder for Rep ∈ [60 1.2.103]. We consider a viscous and incompressible flow for both approaches. First, experiments were done in a loop containing an adjustable central cylinder set with strain gauges to indirectly measure the lift derivative, via the moment of lift. Reynolds number is controlled by using a few glycerin solutions with different viscosities. In parallel, same flow conditions were simulated within 2D simulations. Comparisons were performed between experimental and numerical results. A critical Reynolds was found where the stability derivative seems to cross zero. This fact raises a question about applicability of quasi-steady model for fluidelastic instability.
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