High-speed Droplet Impact Research Articles

Pressure and wall shear stress from high-speed droplet impact

We report on experimental and numerical results of the impact of millimeter sized droplet with high-speed projectile. The impact velocity Vimp ranges between 70m/s and 245m/s and is sufficiently high that the compressibility of the liquid becomes important. High-speed images reveal non-axisymmetric lamella spreading, hydrodynamic and secondary cavitation and the ejection of a thin jet from the distal side of the droplet. The spreading velocity is supersonic for the impact velocities tested. Secondary cavitation at the distal side of the droplet is found for Vimp≳ 120m/s. The experiments are compared and analyzed further with a CFD model based on a compressible volume of fluid (VoF) method. Excellent agreement of the early droplet and lamella dynamics is obtained with the experimental data. Additionally, we provide quantitative data for the pressure loading and the shear stress on the projectile using spatio-temporal maps.

Simulation and analytical modeling of high-speed droplet impact onto a surface

The fluid dynamics of liquid droplet impact on surfaces hold significant relevance to various industrial applications. However, high impact velocities introduce compressible effects, leading to material erosion. A gap in understanding and modeling these effects has motivated this study. We simulated droplet impacts on solid surfaces and proposed a new analytical model for impact pressure and droplet turning line, targeting at predictions for enhanced cavitation. The highly compressed liquid behind the droplet expands sideways, causing lateral jetting. As the droplet encounters a shock wave, it reflects as a rarefaction wave, leading to low-pressure zones within the droplet. These zones converge at the droplet's center, causing cavitation, which, upon collapse, induces another shock wave, contributing to erosion. Using the well-established model for the low-velocity impact shows a significant discrepancy. Hence, an analytical model for the turning line radius is introduced, incorporating the lateral jetting's characteristic length scale. Comparing our model with existing ones, our new model exhibits superior predictive accuracy.

Superhydrophobic coating induced anti-icing and deicing characteristics of an airfoil

Aircraft icing seriously threatens flight safety. The widely used electrothermal anti/de-icing technology has high energy consumption and poor performance. The bionic superhydrophobic surface has anti-icing potential, which can be combined with electric heater technology to achieve energy savings and efficiency improvement. In the previous research, we developed a superhydrophobic coating with mechanochemical robustness for aircraft anti-icing and proved its excellent anti-icing performance. This research combines this coating with the graphene electric heater to obtain a coupling system and explores its anti-icing effect under aviation conditions through the ice wind tunnel test. The results showed that the coating significantly improves the anti-icing properties, delaying the icing process for 10 s, saving the anti-icing energy consumption by 21% compared with graphene heating alone, and increasing the deicing efficiency by 250%. When the coating protection area reaches 30% chord, it can achieve a 'dry' anti-icing state, and this coupling system saves energy by 60% compared with wire heating. Furthermore, The ‘honeycomb structure’ coating is better in anti-icing effect, and the ‘island structure’ coating has stronger anti-icing stability. The hydrophobicity of the coating is less affected by the high-speed droplet impact, so it has a long life expectancy in the active anti-icing mode. This study can provide new ideas for solving the problem of aircraft icing.

Study of stress state changes in steel with Ti-TiC-DLC coating under high speed droplet impact

This study investigates the effects of droplet impact on the change in stress state of 08kp steel with Ti-TiC-DLC type ion plasma diamond-like carbon coating deposited by high-speed pulsed magnetron sputtering (HiPIMS) technology. To this end, a method for determining the stress state in the coating was developed based on measuring the substrate deformation (curvature). The conclusion is that the initial application of coating results in compressive stresses and then the stresses begin to gradually change their sign and become tensile due to the applied specific droplet impact force. Further growth of stress state and increase in tensile stresses in the coating gradually leads to the end of erosion incubation stage and the start of destruction of the "coating-substrate" system.

Consideration of Airflow Aided Single High-Speed Droplet Generation Method

The water hammer pressure at the high-speed droplet impact has not yet been elucidated, and a reproducible droplet generator is needed to investigate it. We developed a single droplet generation device using airflow with an abrupt contraction pipe. We obtained higher velocity droplets than the smooth convergence tube of the early study. To clarify the reasons, we investigated the details of the droplet acceleration using the fluid analysis software OpenFOAM, especially focusing on the pressure gradient force in the abrupt contraction pipe. Comparing the experimental and simulation results, we found that the pressure gradient force has a small contribution to the droplet acceleration and the drag force is dominant for the motion. After examining various shapes, such as a combination of a smooth convergence with a straight pipe, we found that applying as large a drag force as possible at the beginning of acceleration leads to efficient acceleration.

Superhydrophobic surface with excellent mechanical robustness, water impact resistance and hydrostatic pressure resistance by two-step spray-coating technique

Superhydrophobic surfaces (SHSs) exhibit outstanding liquid-repellent properties, attracting numerous attentions from both academia and industry. However, their weak mechanical robustness and poor water impact resistance still limit their practical applications. Herein, SHSs were fabricated by a simple and scalable two-step spray-coating technique, i.e., spray-coating micro-particles as bottom layer, followed by spray-coating nanoparticles onto the micro-particles as top layer; both micro- and nano-features were firmly affixed by low-surface-energy polymer binder. By carefully tuning the micro-particle ratio in bottom layer and mixed solvent ratio in top layer, we obtained the optimized micro/nano- hierarchical structures, which offered the SHSs super-repellence to various liquids (e.g., water, blood and honey) and excellent mechanical robustness to cloth wipe, eraser rub, multi-cycled sandpaper abrasion and tape peel-off. The SHSs withstood high-speed droplet impact and water jet impact with water impact speed of up to ~9.5 m/s (corresponding to ~3760 of Weber value). In addition, they maintained the superhydrophobicity under a hydrostatic pressure of 0.1 MPa for more than one month, or 3 MPa for more than 12 h, which is the highest record ever reported.

High-speed droplet impingement on dry and wetted substrates

High-speed droplet impact is of great interest to power generation and aerospace industries due to the accrued cost of maintenance in steam and gas turbines. The repetitive impacts of liquid droplets onto rotor blades, at high relative velocities, result in blade erosion, which is known as liquid impingement erosion (LIE). Experimental and analytical studies in this field are limited due to the complexity of the droplet impact at such conditions. Hence, numerical analysis is a very powerful and affordable tool to investigate the LIE phenomenon. In this regard, it is crucial to understand the hydrodynamics of the impact in order to identify the consequent solid response before addressing the LIE problem. The numerical study of the droplet impingement provides the transient pressure history generated in the liquid. Determining the transient behavior of the substrate, in response to the pressure force exerted due to the droplet impact, would facilitate engineering new types of surface coatings that are more resistant to LIE. To that end, quantifying the impact pressure of compressible liquid droplets impinged at very high velocities, up to 500 m/s, on rigid solid substrates and liquid films is the main objective of the present work. A wide range of scenarios that commonly arise in the LIE problem are considered, i.e., droplet sizes between 200 µm and 1000 μm, impact velocities ranging from 100 m/s to 500 m/s, and liquid film thicknesses of 0 µm–200 μm. The maximum pressure exerted on the solid surface due to the droplet impact is calculated for both dry and wetted substrates. The results obtained from compressible fluid modeling are compared to those of other numerical studies and analytical correlations, available in the open literature. New correlations are developed for maximum impact pressure on rigid solids and liquid films that can be used to characterize the solid stress and estimate the lifetime of the material by carrying out the fatigue analysis.

A Σ-ϒ two-fluid model with dynamic local topology detection: Application to high-speed droplet impact

A numerical methodology resolving flow complexities arising from the coexistence of both multiscale processes and flow regimes is presented. The methodology employs the compressible Navier-Stokes equations of two interpenetrating fluid media using the two-fluid formulation; this allows for compressibility and slip velocity effects to be considered. On-the-fly criteria switching between a sharp and a diffuse interface within the Eulerian-Eulerian framework along with dynamic interface sharpening is developed, based on an advanced local flow topology detection algorithm. The sharp interface regimes with dimensions larger than the grid size are resolved using the VOF method. For the dispersed flow regime, the methodology incorporates an additional transport equation for the surface-mass fraction (Σ-ϒ) for estimating the interface surface area between the two phases. To depict the advantages of the proposed multiscale two-fluid approach, a high-speed water droplet impact case has been examined and evaluated against new experimental data; these refer to a millimetre size droplet impacting a solid dry smooth surface at a velocity as high as 150 m/s, which corresponds to a Weber number of 7.6×105. Droplet splashing is followed by the formation of highly dispersed secondary cloud of droplets, with sizes ranging from 10 μm close to the wall to less than 1 μm forming at the later stages of droplet fragmentation. Additionally, under the investigated impact conditions, compressibility effects dominate the early stages of droplet splashing. A strong shock wave forms and propagates inside the droplet, where transonic Mach numbers occur; local Mach numbers up to 2.5 are observed for the expelled surrounding gas outside the droplet. Relative velocities between the two fluids are also significant; local values on the tip of the injected water film up to 5 times higher than the initial impact velocity are observed. The proposed numerical approach is found to capture relatively accurately the flow phenomena and provide additional information regarding the produced flow structure dimensions, which is not available from the experiment.

Simulation of high-speed droplet impact against a dry/wet rigid wall for understanding the mechanism of liquid jet cleaning

Physical cleaning techniques are of great concern to remove particulate contamination because of their low environmental impact. One of the promising candidates is based on water jets that often involve fission into droplet fragments. Particle removal is believed to be achieved by droplet-impact-induced wall shear flow. Here, we simulate a high-speed droplet impact on a dry/wet rigid wall to investigate the wall shear flow as well as water hammer after the impact. The problem is modeled by the axisymmetric compressible Navier–Stokes equations and solved by a finite volume method that can capture both shocks and material interface. As an example, we consider the impact of a spherical water droplet (200 µm in diameter) at velocity from 30 to 50 m/s against a dry/wet rigid wall. In our simulation, we can reproduce both acoustic and hydrodynamic events. In the dry wall case, the strong wall shear appears near the moving contact line at the wetted surface. On the other hand, once the wall is covered with the liquid film, the wall shear stress gets weaker as the film thickness increases—a similar trend holds for the water-hammer shock loading at the wall. According to the simulated base flow, we compute hydrodynamic force acting on small particles that are assumed to be attached at the wall, in a one-way-coupling manner. The hydrodynamic force acting on the particles is estimated under Stokes’ assumption and compared to particle adhesion of van der Waals type, enabling us to derive a simple criterion of the particle removal.

Numerical study on splashing of high-speed microdroplet impact on dry microstructured surfaces

The high-speed impact of µm-sized droplets on solid surfaces is a key phenomenon in many industrial applications. In this work, we aim to investigate splashing of high-speed microdroplet impact on micropatterned surfaces through full three-dimensional numerical simulations. An adaptive mesh refinement method is employed to address the challenging issues facing simulations of high-speed impact of microdroplets on the rough surface, such as thin spreading lamella, secondary droplet breakup, and small features of rough surfaces. Validation examples are first presented to evaluate the accuracy of the simulation code for modeling high-speed droplet impact on the textured rough surface. Then, we carried out 3D simulations of a 10 µm diameter water droplet impact on textured surfaces with different impact velocities. We find that during spreading a large portion of the thin lamella actually surfs over the top of pillars with only center area of impact saturated with liquid. When splashing does not occur, the maximum spreading factor for µm-sized droplet impact on smooth surface can be correctly predicted with existing simplified models, whereas these models need to be adjusted for predictions for textured surfaces. Both impact velocity and surface morphology play an important role in the splashing phenomenon. Increasing pillar spacing or reducing pillar height makes droplet impact more prone to splashing. Splashing on surfaces of larger pillar spacing is characterized by the breakup of high-speed jets. Larger impact velocity results in more intensified splashing. For a given impact velocity, densely packed pillars (i.e., smaller pillar spacing) or higher pillars can reduce or even suppress the splashing due to viscous drag effect from pillars in wetted region. The existing splashing threshold models that depend only on surface roughness fail in the prediction of the critical speed for splashing on textured surfaces.

One-way-coupling simulation of cavitation accompanied by high-speed droplet impact

Erosion due to high-speed droplet impact is a crucial issue in industrial applications. The erosion is caused by the water-hammer loading on material surfaces and possibly by the reloading from collapsing cavitation bubbles that appear within the droplet. Here, we simulate the dynamics of cavitation bubbles accompanied by high-speed droplet impact against a deformable wall in order to see whether the bubble collapse is violent enough to give rise to cavitation erosion on the wall. The evolution of pressure waves in a single water (or gelatin) droplet to collide with a deformable wall at speed up to 110 m/s is inferred from simulations of multicomponent Euler flow where phase changes are not permitted. Then, we examine the dynamics of cavitation bubbles nucleated from micron/submicron-sized gas bubble nuclei that are supposed to exist inside the droplet. For simplicity, we perform Rayleigh–Plesset-type calculations in a one-way-coupling manner, namely, the bubble dynamics are determined according to the pressure variation obtained from the Euler flow simulation. In the simulation, the preexisting bubble nuclei whose size is either micron or submicron show large growth to submillimeters because tension inside the droplet is obtained through interaction of the pressure waves and the droplet interface; this supports the possibility of having cavitation due to the droplet impact. It is also found, in particular, for the case of cavitation arising from very small nuclei such as nanobubbles, that radiated pressure from the cavitation bubble collapse can overwhelm the water-hammer pressure directly created by the impact. Hence, cavitation may need to be accounted for when it comes to discussing erosion in the droplet impact problem.

Computations of two-fluid models based on a simple and robust hybrid primitive variable Riemann solver with AUSMD

This paper is to continue our previous work in 2008 on solving a two-fluid model for compressible liquid-gas flows. We proposed a pressure-velocity based diffusion term original derived from AUSMD scheme of Wada and Liou in 1997 to enhance its robustness. The proposed AUSMD schemes have been applied to gas and liquid fluids universally to capture fluid discontinuities, such as the fluid interfaces and shock waves, accurately for the Ransom's faucet problem, air-water shock tube problems and 2D shock-water liquid interaction problems. However, the proposed scheme failed at computing liquid-gas interfaces in problems under large ratios of pressure, density and volume of fraction. The numerical instability has been remedied by Chang and Liou in 2007 using the exact Riemann solver to enhance the accuracy and stability of numerical flux across the liquid-gas interface. Here, instead of the exact Riemann solver, we propose a simple AUSMD type primitive variable Riemann solver (PVRS) which can successfully solve 1D stiffened water-air shock tube and 2D shock-gas interaction problems under large ratios of pressure, density and volume of fraction without the expensive cost of tedious computer time. In addition, the proposed approach is shown to deliver a good resolution of the shock-front, rarefaction and cavitation inside the evolution of high-speed droplet impact on the wall.

低圧力環境下での高速液滴衝突により発生する液膜流れ

低圧力環境下での高速液滴衝突により発生する液膜流れ

0317 固体壁面の濡れ性が液滴衝突により発生する流れの進展挙動に与える影響の観察

Droplet impact on a solid surface is essential phenomenon for industry applications such as semiconductor cleaning. We present experimental observation of droplet impact on a solid surface. Using the curvature of droplet surface as a lens to intensify the illumination, we can observe the development of lamella at early stage after the impact. We apply plasma treatment to change wettability of the solid surface. In case of high-speed droplet impact on non-plasma-treated surface, fingers are formed from the edge of lamella, and then splash is ejected at the tip of finger. On the other hand, lamella spreads with a circle shape and no finger and splash are observed when droplet collides with plasma-treated surface in high-speed impact.

Effects of Target Compliance on a High-Speed Droplet Impact

High speed spray cleaning which utilize droplets impact has been used for removing contaminants from wafer surface. When a droplet impacts a solid surface at high speed, the contact periphery expands very quickly and liquid compressibility plays an important role in the initial dynamics and the formation of lateral jets. Impact results in high pressures that can clean or damage the surface. In this study, we numerically investigated a high speed droplet impacts on a solid wall. In order to compare the available theory and experiments, 1D, 2D and axisymmetric solutions are obtained. The generated pressures, shock speeds, and the lateral jetting mechanism are investigated; especially the effect of target compliance is focused.

High-Speed Droplet Impact as an Elementary Process of Physical Cleaning

In the steam-water mixed spray-cleaning technique, droplet impact on the solid surface is the one of the primarily important physical processes. We experimentally studied a droplet impact on a solid plate that moves with a speed ranging from 1 to 50 m/s using high-speed video camera. We observed that high-speed liquid film flows radially outward along the surface just after the impact. In addition, we also observed that liquid turns into corona splash in the cases of higher impact velocities. These flows with large velocities can generate huge shear stress on the solid surface, which may contribute to cleaning. Furthermore, we numerically studied the velocity field development and found that gas flow also spreads obliquely upward. The obliquely upward flow field may also contribute in advecting removed particles or stripped resists away from the original place.

Observation of Removal Process of Thin Metal Film on Glass Surface by Steam-Water Mixed Spray: Application to Au Film Removal in LED Manufacturing

In the LED manufacturing process, a thin Au film is deposited on a wafer, and then most of it is removed with small pieces of Au film left for electrodes. For this removal, we apply physical cleaning method, which can remove the metal film without chemicals. In this study, we investigate a removal process of a thin aluminum film deposited on a glass surface by steam-water mixed spray. At the initial stage of this process, tiny spots, whose sizes are the order of droplet diameter, are observed. And then very characteristic removal profiles, which are typically lines with sub-millimeter width, are observed. We discuss the removal mechanism due to high-speed droplet impact.

High-Speed Droplet Impact as an Elementary Process of Physical Cleaning

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Numerial Analysis on High-Speed Droplet Impact on a Solid Surface (Study on Cavitation Inception)

Our experimental results [T. Sanada, et. al., ASME-FEDSM 2007-37129] showed that spraying relatively low pressure stream (0.1MPa-0.2MPa), mixed with super-purified water in the nozzle, on a solid surface, caused harsh erosion. We turned our attention to the occurrence of a strong focused rarefaction wave in the middle of the droplet, possibly cavitation. Experimental results that the degrees of the erosion are heavily dependent on temperature also may support the existence of cavitation. We numerically studied the dynamics of high speed liquid droplet impacts on a solid surface by solving the Euler equation of gas-liquid two phase compressible flow. We discuss the possibility of cavitation as the primary cause of the experimentally observed harsh erosion on the solid surface.

Wave structure in the contact line region during high speed droplet impact on a surface: Solution of the Riemann problem for the stiffened gas equation of state

Upon impact of a liquid droplet on a rigid substrate at sufficiently high velocity, a wave front adjacent to the impacted surface is created, separating the compressed area from the undisturbed bulk of the liquid. At a certain point in the evolution of the phenomenon, the free surface at the vicinity of the contact line yields and an intense lateral jetting phenomenon occurs. The often employed assumption that the wave front is composed of a single shock wave across its entire surface leads to the emergence of an anomaly, prior to the jetting eruption. This anomaly can be removed by the proposition of the existence of a multiple wave structure at the contact line region. In this article, the wave structure at the contact line region is explored analytically. The exact one-dimensional Riemann problem solution for the stiffened gas equation of state is presented, under the assumption of an isentropic flow in the smooth flow region (i.e., in the region with no contact discontinuities). For the case of impact velocity V=500 m/s and a water droplet with radius R=0.1 mm, we show that in addition to the single shock structure, solutions consisting of an expansion fan and shock wave are physically admissible. In this scenario, the pressure created at the contact line decreases compared to the pressure of the single shock structure. It is demonstrated that the liquid particle velocity in the intermediate region between the waves is lower than the velocity in the bulk of compressed liquid. An algorithm for estimating the intermediate region state, when the left and right states are known, is also presented.