† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant Nos. 11274092 and 1140040119) and the Natural Science Foundation of Jiangsu Province, China (Grant No. SBK2014043338).
The dynamics of the cavitation bubble collapse is a fundamental issue for the bubble collapse application and prevention. In the present work, the modified forcing scheme for the pseudopotential multi-relaxation-time lattice Boltzmann model developed by Li Q et al. [Li Q, Luo K H and Li X J 2013 Phys. Rev. E
Cavitation is a unique phenomenon in liquid, and has made extensive research efforts in science and engineering. Due to the various dynamic effects during the cavitation generation and cavitation bubble collapse, such as bubble wall oscillation and the formations of shockwave and micro-jet, the potential applications of cavitation have been explored and are being extensively studied in the fields of medicine, biology, lab on chip, sewage treatment, and material surface cleaning treatment.[1–5] However, as too many phenomena are involved, a theoretical model of cavitation bubble collapse is difficult to be established, and for particular boundary conditions, the analytical solution is even impossible. Therefore, the numerical simulation becomes a powerful way to gain a better understanding. The conventional numerical simulation methods of cavitation bubble mainly include the finite volume method (FVM),[6–8] the finite element method (FEM),[9] and the boundary element method (BEM).[10] In these partial differential equation based numerical simulations, the methods to track or capture the interfaces (such as the volume of fluid (VOF)[11] or level set method (LSM)[12]) are required, which are often demanding computationally. In addition, the Poisson equation needs to be solved to satisfy the continuity equation, which drastically reduces the computational efficiency.[13]
In recent years, the lattice Boltzmann method (LBM), which is a mesoscopic approach based on the kinetic Boltzmann equation, has emerged as a powerful tool for simulating multiphase flow problems.[14–18] As an alternative tool for the numerical simulations and investigations of multiphase flows, the LBM provides many advantages including clear physical pictures, easy implementation of boundary conditions, and fully parallel algorithms.[19] Generally, the existing LBM multiphase models can be mostly classified as being in four categories: the color-gradient method,[20] the pseudopotential method (or Shan-Chen model),[21,22] the free-energy method,[23,24] and the phase-field method.[25] The pseudopotential method is widely and successfully used in the LBM multiphase community due to its conceptual simplicity and computation efficiency. In the pseudopotential method, the fluid interactions are mimicked by an interparticle potential, from which a non-monotomic equation of state (EOS) can be obtained. As a result, the separation of fluid phases or components can be achieved automatically in this method, and the methods to track or capture the interfaces are not required yet. Moreover, the pressure can be calculated from EOS efficiently instead of the Poisson equation.
Recently, the pseudopotential LBM was firstly introduced into the issue of cavitation by Sukop and Or.[26] In the following several years, research efforts have been made to investigate the mechanism of cavitation. Chen et al.[27] simulated the cavitating bubble growth by using the modified pseudopotential LB model with the EDM force scheme. The results in quiescent flows agree fairly well with the solution of the Rayleigh–Plesset equation. Mishra et al.[28] introduced a model of cavitation based on the pseudopotential LB model that allows for coupling between the hydrodynamics of a collapsing cavity and supported solute chemical species. Using a pseudopotential LB model, Daemi et al.[29] modeled the bubble cluster in an acoustic field, and verified that the deformation and coalescence phenomena in bubble cluster dynamics can be simulated by LBM. However, the above-mentioned research on cavitation did not include the cavitation bubble collapse process, which is essential for the study of the cavitation phenomenon, and the density ratio between liquid phase and gas phase was limited below 100.
For the pseudopotential LB model, the density ratio is a synthetic problem relating to the lattice model, forcing scheme, thermodynamic consistency, numerical stability etc., and great efforts have been devoted to this issue.[30–35] In the present work, the modified forcing scheme for pseudopotential Multi-Relaxation-Time (MRT) LB developed by Li Q et al.[33] is adopted to develop a cavitation bubble collapse model. Essentially, Li et al.’s scheme is an approximate approach to thermodynamic consistency by adjusting forcing scheme. This approach retains the concise and high efficiency of the original LBM, and achieves the stable LB multiphase model with a large density ratio.
The rest of the present paper is organized as follows. Section 2 will briefly introduce the pseudopotential MRT-LB model and the forcing scheme modified by Li et al. The numerical analysis of the modified pseudopotential MRT-LB model for a bubble will be given in Section 3. In Section 4, the validity and feasibility of the method for cavitation and bubble collapse will be verified. Finally, a brief conclusion will be made in Section 5.
The pseudopotential LB model, also known as the Shan–Chen model, was developed by Shan and Chen in 1993.[21] In the pseudopotential method, the fluid interactions are mimicked by an interparticle potential, which is now widely called peseudopotential. In the original pseudopotential LB model, the single-relaxation-time (SRT) collision operator was employed. In recent years, the MRT collision operator has been verified, showing that it is superior to the SRT operator in terms of numerical stability. The MRT-LB evolution equation can be given as follows:[33]
For the pseudopotential LB model, the
For the pseudopotential multiphase LB model, the vapor density ρg, liquid density ρl and the pressure p0 in an equilibrium coexistence fluid, should satisfy the mechanical stability condition expressed as[33,36]
To address the thermodynamic inconsistency, the integrands in Eqs. (
The second strategy is to guarantee that the bracketed parts in Eqs. (
In the second strategy framework, Li et al.[33] proposed an MRT version forcing scheme to achieve thermodynamic consistency. For the D2Q9 lattice, Li et al.’s forcing scheme can be given by
In this section, numerical investigations for the bubble will be conducted with Li et al.’s improved pseudopotential LB model. Firstly, the improved model will be investigated for bubble simulations from three aspects: the optimal ɛ for bubble simulations, the effect on coexistence curves from relaxation time τυ, and the validation of the Laplace law. Then, the homogeneous and heterogeneous cavitation will be simulated to investigate the feasibility of the presented LB model for the cavitation phenomenon.
Three numerical simulations of bubbles are considered to investigate Li et al.’s improved pseudopotential MRT-LB model. The first one is to simulate the stationary bubble, which can be used to obtain a numerical coexistence curve approximate to that given by the Maxwell construction by tuning ɛ. The second numerical simulation is to investigate the effect of relaxation time. The last simulation is conducted to validate the law of Laplace. In the present work, the Carnahan–Starling (CS) EOS is adopted, which can be given by[38]
Li et al. have estimated that the optimal value of ɛ should be between 1 and 2.[32] For the stationary liquid droplet with radius r0 = 50, they considered that ɛ = 1.76 is an optimal value. With this value, the coexistence curves obtained from LBM simulations are in good agreement with those given by the Maxwell equal-area construction. For a stationary vapor bubble, the coexistence curves of the case τυ = 0.6 with different ɛ values are shown in Fig.
Furthermore, the effect of relaxation time τυ, which is associated with kinematic viscosity, is investigated. The coexistence curves of the cases τυ = 0.51, τυ = 0.6, and τυ = 0.8 as ɛ = 1.86 are shown in Fig.
Satisfying the Laplace law is an important benchmark test for multiphase flows. The surface tension force, meanwhile, can be calculated when the Laplace law is verified. For the bubble case, the Laplace law can be given as
The inception of a cavitation bubble is essentially a phase transition from liquid to vapor. It happens when the pure liquid is exposed to such a high negative pressure that makes the density of liquid in the range of coexistence density.
In the present work, the cavitation inception phenomenon is investigated by the proposed MRT pseduopotential LB model. A (lx,ly) = (201,201) lattice is adopted. The whole density field is initialized as ρinit = 0.2609 which is between the critical density ρc = 0.1305 and the liquid density ρl = 0.4541 at 0.5Tc with τυ = 0.6 and ɛ = 1.86. Then a density perturbation of ρinit/108 is set at (ly − 1)/2. The periodical boundary conditions are applied to x and y directions. The densities at ((lx − 1)/2,ly − 1) and ((lx − 1)/2, (ly − 1)/2) are detected. The density evolutions with time are shown in Fig.
If there is a cavitation nucleus in the static fluid, the later development of the cavitation bubble is determined by the critical radius of cavitation nucleus, which can be given as follows:[39]
A set of investigations for the proposed MRT pseduopotential LB model demonstrates that the LB model can establish cavitation numerical models in line with the law of physics. And with the stability including such a high density ratio close to that between water and vapor in reality, the LB model is promising to establish an efficient cavitation numerical model. In this section, we will establish a numerical model to investigate the mechanism of the bubble collapse between two parallel solid walls.
The dynamics of cavitation bubble near a solid wall is an important issue for understanding the mechanism of surface damages in fluid machinery. It has been widely studied and most of these studies are based on the semi-infinite fluid domain.[40–42] However, in the case of the bubble between two parallel solid walls, the collapse of the bubble will be significantly different from in the semi-infinite assumption.[3,43] Although several investigations have been conducted based on macroscopic methods, the LB modeling of cavitation bubble collapse between two solid walls is still in its infancy. In this section, using the proposed MRT pseduopotential LB model with high density ratio, we make an attempt to simulate bubble collapse between two parallel solid walls in the 2D case.
The computational domain for the bubble collapse between two parallel solid walls is shown in Fig.
For describing the initial simulation states quantitatively, two dimensionless parameters are introduced as follows:
In order to simulate the bubble collapse process, a positive pressure difference ΔP = P∞ − Pv is achieved by artificially tuning the initial liquid density based on the equilibrium state. The periodical boundary condition is applied to the x direction, and a no-slip bounce-back boundary condition is implemented in the y direction. The parameters of the LB model are set to be T/Tc = 0.5, τυ = 0.51 and ɛ = 1.86.
The simulation results of bubble collapse between two parallel walls by the proposed MRT pseduopotential LB model are shown in Fig.
To investigate the dynamic characteristics in more details when the bubble collapses and the jet forms, the pressure distribution, the regional density and velocity distributions at these two key moments are shown in Figs.
Due to the regional higher pressure at the tips of sub-bubbles, the tips are flatted and then sag. Accordingly, there emerges the high flow velocity towards the insides of two sub-bubbles, respectively. Then the jets of the collapsing bubble form, as shown in Fig.
For the modeling of a collapsing bubble, an improved MRT pseduopotential LB model is investigated in some respects such as thermodynamic consistency, Laplace law, homogeneous and heterogeneous cavitation. Then the bubble collapse between two parallel walls is simulated. The dynamic process of the collapsing bubble is consistent with the results from experiments and simulations obtained by other numerical methods.
The improved forcing scheme developed by Li et al. provides a convenient and efficient approach to achieving thermodynamic consistency. Adopting Li et al.’s forcing scheme, we find that the thermodynamic consistency and surface tension are independent of kinematic viscosity, which makes it superior to investigate the physical mechanism of multiphase flows. Even at 0.5Tc with τυ = 0.51, the improved MRT pseduopotential LB model also has enough stability to describe the collapsing bubble with a high density ratio beyond 700. It is demonstrated that the present MRT pseduopotential LB model is applicable and efficient, and the LBM is an alternative tool for collapsing bubble modeling.
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