Indirect damage in composite materials due to raindrop impact

The unique confinement of shock waves inside isolated liquid volumes amplifies the density of shock-liquid interactions. We investigate this universal principle through an interdisciplinary study of shock-induced cavitation inside liquid volumes, isolated in 2 and 3 dimensions. By combining high-speed visualizations of ideal water drops realized in microgravity with smoothed particle simulations, we evidence strong shock-induced cavitation at the focus of the confined shocks. We extend this analysis to ground-observations of jets and drops using an analytic model and argue that cavitation caused by trapped shocks offers a distinct mechanism of erosion in high-speed impacts ð100 ms À1 Þ.Shock waves in liquids are a common cause of cavitation, 1-5 in particular when shocks are reflected and focussed. 6 Famous examples include shock-induced cavitation in lithotripsy 7,8 and the "white crown" on the sea surface following an underwater detonation. 9-11 However, little is known about shock-induced cavitation inside "isolated" liquid volumes, 12 which are completely bounded by a free surface. The crucial feature of such systems is their unique confinement: the closed surface acts as a mirror trapping the shock and amplifying its local interaction with the fluid. Experimental hints for the importance of this mechanism for generating cavitation were provided by some of the earliest high-speed visualizations of shocked drops [Ref. 13, see reprint in Fig. 1(a)].In this letter, we study the amplification of shockinduced cavitation in liquid volumes isolated in three dimensions (3D), such as drops, and in two dimensions (2D), such as jets. Illustrations of shock-driven cavitation in both cases are provided in Fig. 1. To understand this cavitation, we perform a systematic experimental study of shock-induced cavitation inside large, spherical water drops. These drops are realized in microgravity conditions aboard parabolic flights (European Space Agency, 42nd Parabolic Flight Campaign). In parallel to those experiments, we provide a quantitative explanation of the observed cavitation patterns through numerical simulations of dissipative shocks inside spheres and derive an analytic model to predict the location of shock-induced cavitation. Thereby, we demonstrate that shock-induced cavitation in isolated volumes is a universal phenomenon. Finally, we discuss a potential implication of cavitation caused by trapped shocks for drop erosion of solid surfaces.Our microgravity experiment can be seen as an ideal laboratory for the study of shock dynamics inside stable drops. The setup [details in Ref. 15] can produce a spherical drop of demineralized water (diameter D ¼ 16-26 mm). This drop is smoothly expelled through an injector tube, which also serves as a permanent attach-point for the drop (see Fig. 2, left). A movable pair of electrodes penetrating the drop from the top in Fig. 2 releases a fast (10 ns) discharge at a specific location within the drop. This discharge forms a supersonically expanding point-plasma [physics in Ref....

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“…by rain [23,24] and Pelton turbine blades eroded by high-speed (up to 200 m s −1 ) droplets and jets [25].…”

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confidence: 99%

Confined shocks inside isolated liquid volumes: A new path of erosion?

et al. 2011

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“…Moreover, non-uniform distribution of drops across the body and wings may adversely influence control and manoeuverability. Potential damage to wing microstructures caused by high impact pressures of falling drops, as observed for airfoils [4], might also be expected. Surprisingly, some birds [5], bats [6] and insects [7] have been observed flying even during heavy rain, although the various aforementioned mechanical penalties while doing so have not yet been characterized.…”

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confidence: 99%

Flying in the rain: hovering performance of Anna's hummingbirds under varied precipitation

Ortega-Jimenez

Dudley

2012

Proc. R. Soc. B.

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Flight in rain represents a greater challenge for smaller animals because the relative effects of water loading and drop impact are greater at reduced scales given the increased ratios of surface area to mass. Nevertheless, it is well known that small volant taxa such as hummingbirds can continue foraging even in extreme precipitation. Here, we evaluated the effect of four rain intensities (i.e. zero, light, moderate and heavy) on the hovering performance of Anna's hummingbirds (Calypte anna) under laboratory conditions. Light-to-moderate rain had only a marginal effect on flight kinematics; wingbeat frequency of individuals in moderate rain was reduced by 7 per cent relative to control conditions. By contrast, birds hovering in heavy rain adopted more horizontal body and tail positions, and also increased wingbeat frequency substantially, while reducing stroke amplitude when compared with control conditions. The ratio between peak forces produced by single drops on a wing and on a solid surface suggests that feathers can absorb associated impact forces by up to approximately 50 per cent. Remarkably, hummingbirds hovered well even under heavy precipitation (i.e. 270 mm h 21 ) with no apparent loss of control, although mechanical power output assuming perfect and zero storage of elastic energy was estimated to be about 9 and 57 per cent higher, respectively, compared with normal hovering.

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“…Damage created on the upper surface of the blade may not be the exclusive damage process for blade leading edges, as it has been shown in aerospace studies that in composite materials sub-surface damage can occur with little-to-no indication of damage on the surface [9], as shown in Fig. 4.…”

Section: Figure 2 Leading Edge Erosion (Presented Without Scale) Somentioning

confidence: 98%

Modelling Rain Drop Impact of Offshore Wind Turbine Blades

Keegan

Nash

Stack

2012

Volume 6: Oil and Gas Applications; Concentrating Solar Power Plants; Steam Turbines; Wind Energy

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The effects of rain and hail erosion and impact damage on the leading edge of offshore wind turbine blades have been investigated.A literature review was conducted to establish the effects of exposure to these conditions and also to investigate the liquid impact phenomena and their implications for leading edge materials. The role of Explicit Dynamics software modelling in simulating impact events was then also established.Initial rain impact modelling is then discussed with the results showing good agreement with theoretical predictions both numerically and with respect to the temporal and spatial development of the impact event. Future development of the rain model and a proposed hail model are then detailed.Planned rain impact and erosion testing work is addressed which will be used to validate, inform and compliment the ongoing modelling efforts.

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Indirect damage in composite materials due to raindrop impact

Cited by 25 publications

References 2 publications

Confined shocks inside isolated liquid volumes: A new path of erosion?

Confined shocks inside isolated liquid volumes: A new path of erosion?

Flying in the rain: hovering performance of Anna's hummingbirds under varied precipitation

Modelling Rain Drop Impact of Offshore Wind Turbine Blades

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