Modelling of a single drop impact onto liquid film using particle method

The process of single liquid drop impact on thin liquid surface is numerically simulated with moving particle semi‐implicit method. The mathematical model involves gravity, viscosity and surface tension. The model is validated by the simulation of the experimental cases. It is found that the dynamic processes after impact are sensitive to the liquid pool depth and the initial drop velocity. In the cases that the initial drop velocity is low, the drop will be merged with the liquid pool and no big splash is seen. If the initial drop velocity is high enough, the dynamic process depends on the liquid depth. If the liquid film is very thin, a bowl‐shaped thin crown is formed immediately after the impact. The total crown subsequently expands outward and breaks into many tiny droplets. When the thickness of the liquid film increases, the direction of the liquid crown becomes normal to the surface and the crown propagates outward. It is also found that the radius of the crown is described by a square function of time: r_C = [c(t ? t₀)]^0.5. When the liquid film is thick enough, a crown and a deep cavity inside it are formed shortly after the impact. The bottom of the cavity is initially oblate and then the base grows downward to form a sharp corner and subsequently the corner moves downward. Copyright © 2004 John Wiley & Sons, Ltd.     

Experimental verification of drop/impact simulation for a cellular phone

Conducting drop tests to investigate impact behavior and identify failure mechanisms of small-size electronic products is generally expensive and time-consuming. Nevertheless, strict drop/impact performance criteria for hand-held electronic products such as cellular phones play a decisive role in the design because they must withstand unexpected shocks. The design of product durability on impact has heavily relied on the designer's intuition and experience. In this study, a reliable drop/impact simulation for a cellular phone is carried out using the explicit code LS-DYNA. Subsequently globallocal experimental verification is accomplished by means of high-speed photography and impact response measurement. Using this methodology, we predict potential damage locations in a cellular phone and compare them with real statistical data. It is envisaged that development of a reliable methodology of drop/impact simulation will provide us with a powerful and efficient vehicle for improvement of the design quality and reduction of the product development cycle.     

Influence of liquid layers on energy absorption during particle impact

The influence of the thickness of a covering liquid layer and its viscosity as well as the impact velocity on energy loss during the normal impact on a flat steel wall of spherical granules with a liquid layer was studied. Free-fall experiments were performed to obtain the restitution coefficient of elastic–plastic Al2O3 granules by impact on the liquid layer, using aqueous solutions of hydroxypropyl methylcellulose with different concentrations for variation of viscosity (1–300 mPa s). In the presence of a...     

Spray impact onto flat and rigid walls: Empirical characterization and modelling

An experimental study of spray impact onto horizontal flat and rigid surfaces is presented and used as input data for a new empirical model. A phase Doppler instrument has been used to measure drop size and two components of velocity directly above the target. The average film thickness formed due to spray impact has been measured using a high-speed CCD camera. The spray–wall interaction has been characterized in terms of correlations for the velocity and trajectory of secondary droplets and the mass and number ratio of the secondary spray. The novel aspect of the model is that the correlations are based on mean statistics over many events and not on the outcome of single drop impact experiments. Furthermore a rather large range of oblique impact angles have been studied and incorporated into the empirical models as an influencing factor.     

Experimental investigation of splash and crown formation during single drop impact on wetted surfaces

Experimental results concerning crown formation during liquid drop impact on wetted surfaces are reported. Different liquids and numerous impact conditions are investigated. In particular, crown-splash (C-S) and deposition-crown (D-C) limits are determined on the basis of the experimental observations. These limits converge for dimensionless film thickness thinner than 0.03, leaving the outcome of crown formation unobserved. The sole Weber number and dimensionless film thickness cannot explain the phenomenon. It appears that all these data can be described using a combination of Weber and Ohnesorge numbers versus dimensionless film thickness.     

Influence of liquid layers on energy absorption during particle impact

The influence of the thickness of a covering liquid layer and its viscosity as well as the impact velocity on energy loss during the normal impact on a flat steel wall of spherical granules with a liquid layer was studied. Free-fall experiments were performed to obtain the restitution coefficient of elastic-plastic γ- Al2O3 granules by impact on the liquid layer, using aqueous solutions of hydroxypropyl methylcellulose with different concentrations for variation of viscosity （1-300 mPa s）, In the presence of a liquid layer, increase of liquid viscosity decreases the restitution coefficient and the minimum thickness of the liquid layer at which the granule sticks to the wall. The measured restitution coefficients were compared with experiments performed without liquid layer. In contrast to the dry restitution coefficient, due to viscous losses at lower impact velocity, higher energy dissipation was obtained, A rational explanation for the effects obtained was given by results of numerically solved force and energy balances for a granule impact on a liquid layer on the wall. The model takes into account forces acting on the granule including viscous, surface tension, capillary, contact, drag, buoyancy and gravitational forces. Good agreement between simulations and experiments has been achieved.     

Inertia dominated flow and heat transfer in liquid drop spreading on a hot substrate

The present work deals with computational modeling of the fluid flow and heat transfer taking place in the process of impact of a cold liquid drop (T_d = 20-25 °C) onto a dry heated substrate characterized by different thermophysical properties. The computational model, based on the volume-of-fluid method for the free-surface capturing, is validated by simulating the configurations accounting for the conjugate heat transfer. The simulations were performed in a range of impact Reynolds numbers (Re = 2000-4500), Weber numbers (We = 27-110) and substrate temperatures (T_s = 100-120 °C). The considered temperature range of the drop-surface, i.e. liquid-solid system does not account for the phase change, that is boiling and evaporation. The model performances are assessed by contrasting the results to the reference database originating from the experimental and complementary numerical investigations by Pasandideh-Fard et al. [Pasandideh-Fard, M., Aziz, S., Chandra, S., Mostaghimi, J., 2001. Cooling effectiveness of a water drop impinging on a hot surface. International Journal of Heat and Fluid Flow, 22, 201-210] and Healy et al. [Healy, W., Hartley, J., Abdel-Khalik, S., 2001. On the validity of the adiabatic spreading assumption in droplet impact cooling. International Journal of Heat and Mass Transfer, 44, 3869-3881]. In addition, the thermal field obtained is analyzed along with the corresponding asymptotic analytical solution proposed by Roisman [Roisman, I.V., 2010. Fast forced liquid film spreading on a substrate: flow, heat transfer and phase transition. Journal of Fluid Mechanics, 656, 189-204]. Contrary to some previous numerical studies, the present computational model accounts for the air flow surrounding the liquid drop. This model feature enables a small air bubble to be resolved in the region of the impact point. The reported results agree reasonably well with experimental and theoretical findings with respect to the drop spreading pattern and associated heat flux and temperature distribution.     

Three-dimensional water entry of a solid body: A particle image velocimetry study

Understanding and predicting the hydrodynamic loading experienced by a solid body during water impact is critical for researchers and practitioners in naval engineering. While two-dimensional (2D) water entry problems have been extensively investigated, experimental data on 3D fluid–structure interactions during water impact are rather limited. Here, particle image velocimetry (PIV) is utilized to study the free fall vertical impact of a solid body, modeling a ship hull, on an otherwise quiescent fluid. Planar PIV is used to measure the velocity field on multiple cross-sections along the length and width of the model. These data are combined to infer the 3D velocity field in the entire fluid. The 3D velocity field is then utilized to reconstruct the pressure field by integrating the incompressible 3D Navier–Stokes equations in a time-varying domain, where both the free surface and the fluid–solid interface evolve in time. By evaluating the pressure field on the wetted surface of the model, we estimate the hydrodynamic loading during water entry. Experimental results demonstrate the central role of 3D effects on both the flow physics and the hydrodynamic loading. As the cross-sectional velocity decreases away from the mid-span, we observe a robust increase in the axial velocity component. This translates into a complex spatio-temporal dependence of the hydrodynamic loading, which is initially maximized in the vicinity of the pile-up and later increases toward the keel. Due to the deceleration of the model during the impact and the increase in the wetted surface, the hydrodynamic loading close to the mid-span in the early stage of the impact is considerably larger than the ends. The 3D flow physics is used to study the energy imparted to the fluid during the impact, which we find to be mostly transferred to the risen water, consisting of the pile-up region and the spray jet. Our methodology can be implemented for the analysis of other solid bodies with multiple geometric curvatures, and our experimental results can be utilized for the validation of 3D mathematical models of water entry.     

基于SPH方法粒子射流破岩数值模拟与实验研究

钻井液中加入体积分数为1%~3%的钢质粒子在钻头喷嘴处高速喷出冲击岩石,实现了粒子射流冲击和钻头机械联合破岩,有效提高了破岩效率。利用瞬态非线性动力学有限元模拟软件,基于光滑粒子流体动力学(smoothed particle hydrodynamics,SPH)方法,考虑流体对粒子射流冲击的影响,建立了粒子射流冲击破岩的物理模型,获得了粒子射流参数对破岩体积的影响规律,进行了室内实验验证,验证了SPH方法的有效性。结果表明:粒子射流冲击岩石表面形成规则的V型冲击坑;同条件下粒子射流破岩体积是水射流破岩体积的2~4倍;随着粒子射流冲蚀时间的增加,粒子射流破岩体积不断增加,但破岩效率降低;粒子射流压力大于10 MPa后,粒子射流破岩效率迅速增大;喷射角度大于6°后,破岩效率迅速减小。     

An experimental study on the static and impact torsional buckling of cylindrical shells

In this paper, experimental studies are carried out on the buckling of circular cylindrical thin shells under impact torque. Experiments of impact buckling are made on a Hopkinson torsional bar. The torsional bar gives a step torque on the shells. Through an analysis of the strain-time curve obtained in experiment, the dynamic buckling critical torqueM _er and buckling waves numbern of the shell with different geometric data and some qualitative results are obtained. The buckling behavior of circular cylindrical thin shells under static and impact torque is compared.     

An experimental method for studying the high speed impact of a liquid drop on a liquid surface

Summary An experimental method for studying the high speed impact of a liquid drop on a liquid surface is described in which the liquid surface is moved against the stationary drop. Photographs are presented of the impact of a water surface at 47,7 m/sec on a stationary water drop 0,175 cm diameter. Proposals are made for further work to resolve certain discrepancies between results obtained using this method and those derived from earlier work in which drops fall freely onto a standing surface.Übersicht Es wird eine experimentelle Methode zur Untersuchung des Aufprallens von Flüssigkeitstropfen auf eine Flüssigkeitsoberfläche beschrieben, bei der die flüssige Fläche mit hoher Geschwindigkeit gegen einen stationären Tropfen bewegt wird. Fotos des Aufprallvorgangs bei Geschwindigkeiten von 47,7 m/s mit Tropfen von 0,175 cm Durchmesser werden gezeigt. Weiterhin werden Vorschläge gemacht, wie bei künftigen Untersuchungen ] gewisse Unterschiede in den Ergebnissen nach der hier beschriebenen Methode und bei frei fallenden Tropfen aufgeklärt werden könnten.Acknowledgment is made to Mr. M. J. B. Nash for his assistance in doing the experimental work. This work was carried out at the Royal Aircraft Establishment, Farnborough, Hampshire, England.     

大药片落锤撞击感度研究

设计建立了一种炸药大药片撞击感度试验方法,落锤质量为20 kg、落高度为0~15 m,对规格20 mm5 mm、重约2.8 g的大药片进行撞击感度测试。试验测试了两种典型炸药Tetryl和JOB-9003炸药的落锤撞击感度,落锤撞击Tetryl炸药和JOB-9003炸药的爆炸阈值落高分别约3.5 m和6.5 m。对落锤撞击JOB-9003炸药样品的过程进行了数值计算,计算结果与试验值相符。试验结果表明,该试验方法可以测量炸药的落锤撞击感度。     

Numerical study of particle motion near a charged collector

The behavior of particles impacting the surface of a charged droplet involves adhesion, rebound, and submersion. In the present study, a numerical model for simulating particle impacts on charged droplets is presented that takes into account the various impact modes. With the droplet considered as a solid boundary, the criterion for rebounding is that the particle’s impact angle is <85°. The simulated trajectories of the particles are verified by comparing with experimental data for low-velocity particles to assess the reliability of the model. For impact angles >85°, particles undergo three distinct modes depending on normal impact velocities. The critical velocity of adhesion/rebound and rebound/submersion is used to identify the mode that the particles are undergoing. The criteria are also verified by comparing with analytical data. The results show that the impact angle of particles increases with increasing Coulomb number and decreases dramatically with increasing Stokes number, both of which lead to a high probability for particle rebound.     

Numerical study of a single drop impact onto a liquid film up to the consequent formation of a crown

In this paper, the whole dynamic process of a single drop impact onto a thin liquid surface up to the consequent formation of a thin crown is numerically studied using the smoothed particle hydrodynamics (SPH) method. Especially, the gravity, artificial viscosity, and surface tension are introduced into the model. The obtained SPH numerical results are compared with experimental results. The numerical model of the SPH method is valid for simulating the dynamic process of a single drop impact onto a liquid surface. Meanwhile, it is found that the whole dynamic process mainly depends on the depth of the liquid pool and the initial velocity of the droplet.     

A predictive model for impact response of viscoelastic polymers in drop tests

We show for the first time that a classical Hookean viscoelastic constitutive law for rubbery materials can predict the impact forces and deflections measured with a commercial drop tester when a mass, or tup with a flat impacting surface is dropped onto a flat pad of commercial impact-absorbing rubber. The viscoelastic properties of the rubber, namely the relaxation times and strengths, are obtained by a standard rheological linear-oscillatory test, and the equation of momentum transfer is then solved, using these measured parameters, assuming a uniaxial deflection of the pad during the impact. Good agreement between measured and predicted forces and deflections is obtained for a series of various drop heights, tup masses, impact areas, and pad thicknesses, as long as the deflection of the pad relative to its thickness is small or modest (<50% or so), and as long as the area of the pad is less than or equal to that of the tup. When the pad area is greater than the tup, forces are higher than predicted, unless an empirical factor is introduced to account for the nonuniaxial stretching of the ring of material that extends outside of the impact area. These results imply that the impact-absorbing properties of a rubbery polymeric material can be assessed by simply examining the material's linear viscoelastic spectrum.     

储液容器跌落冲击动力学神经网络建模

运用LS-DYNA程序中的ALE算法模拟储液容器在不同的跌落角度、跌落高度、壳体厚度下的跌落冲击过程,获取神经网络预测模型的训练样本集;利用BP神经网络建立储液容器结构参数、跌落冲击参数与接触点最大应力之间的映射关系预测模型,并将各种参数下的接触点最大应力网络预测值与仿真值比较,两者差异较小,表明该方法是有效的,可以为实际生产过程中参数选择提供理论依据.     

An experimental study of drop migration in shear flow between concentric cylinders

The phenomenon of migration of liquid drops in Couette flow between concentric cylinders due to non-Newtonian fluid properties and shape deformation has been studied experimentally. The results agree very well with the theory of Chan and Leal, which included the effect of hydrodynamic interaction with the bounding walls, and that of velocity profile curvature in a Couette device. Significant observations that were not reported in previous studies include the migration of a deformable Newtonian drop to an equilibrium position between the centerline and the inner rotor, and the competition between normal stresses and shape deformation effects for the case of a Newtonian drop in a non-Newtonian fluid.     

An experimental study of bending impact waves in beams

An experimental study of the propagation of flexural waves in an elastic beam of circular cross section subjected to an approximate-step-function bending moment is given. The test beam was a low-carbon-steel bar 2 in. diam × 30-ft long and was suspended in a vertical position by a pin located near the upper end. The step moment was applied at the upper pinned end of the bar by an arrangement of two high-pressure, nitrogen-operated cylinders. The strains were measured with strain gages located at eight stations along the bar. The experimental results are compared with results obtained from a solution of the “Timoshenko” beam by W. Flügge and E. E. Zajac for a semi-infinite pinned-end beam subjected to a suddenly applied bending moment at the pinned end. The experimental results are correlated with the predictions of the theoretical solution.     

An efficient algorithm for detecting particle contact in non-uniform size particulate system

Algorithms for detecting particle collision play an important role in the discrete element method (DEM) for the simulation of granular flow systems since the time taken to detect the contact pairs usually occupies a considerable proportion of the total CPU time for the simulation. In this study, we developed a new octree algorithm called multi-octree algorithm, for detecting candidate contact pairs. The so-called multioctree algorithm adopts the topology of octree for detecting possible contacts, in which t...