Liquid-gas Density Ratio Research Articles

Controlling bubble coalescence in metallic foams: A simple phase field-based approach

The phase-field method is used as a basis to develop a strictly mass conserving, yet simple, model for simulation of two-phase flow. The model is aimed to be applied for the study of structure evolution in metallic foams. In this regard, the critical issue is to control the rate of bubble coalescence compared to concurrent processes such as their rearrangement due to fluid motion. In the present model, this is achieved by tuning the interface energy as a free parameter. The model is validated by a number of benchmark tests. First, stability of a two dimensional bubble is investigated by the Young-Laplace law for different values of the interface energy. Then, the coalescence of two bubbles is simulated until the system reaches equilibrium with a circular shape. To address the major capability of the present model for the formation of foam structure, the bubble coalescence is simulated for various values of interface energy in order to slow down the merging process. These simulations are repeated in the presence of a rotational flow to highlight the fact that the model allows to suppress the coalescence process compared to the motion of bubbles relative to each other. Moreover, since density is treated as an auxiliary variable “slaved” to the volume occupied by a given phase, the present model allows realization of arbitrarily large liquid-gas density ratios. This property is demonstrated by simulation of a system with ρl/ρg=10,000.

Unstable breakup of a power-law liquid fuel jet in the presence of a gas crossflow

A theoretical model is presented for the effect of a slight gas crossflow on the instability of the power-law fuel jet in this paper. Based on a linear approximation, the dispersion equation was deduced theoretically with axisymmetric disturbance. The influence of various dimensionless factors for jet instability is studied. Theoretical calculation results of temporal mode show that the transverse motion of gas phase could effectively accelerate the power-law fuel breakup. With the increase of transverse gaseous velocity, power-law fuel jets would become more susceptible to be broken up. Increasing gas-liquid density ratio could enhance the aerodynamic forces in the axial and vertical directions and promote fuel fragmentation. Simultaneously, a huge gas density could weaken the influence of transverse airflow. With the increase of the transvers airflow, Weber number has a weakening effect on the fuel breakup, but it has a significant positive effect on the primary breakup scale. However, with gas crossflow, the liquid viscous force also takes strong disadvantage of development of surface wave. Besides, the effects of crossflow and co-current airflow environments on the instability of power-law liquid jet are studied. For the generalized Reynolds number and power law exponent, the fuel jet under cross-flow conditions is more unstable, but the jet under the co-current flow condition is smaller on the fracture scale. These issues are beneficial to understand the disintegration of the power-law fuel jet with gas crossflow, and supply reference for the application of power-law liquids in real engineering technologies.

Primary breakup of power-law biofuel sprays in pressurized gaseous crossflow

The spray characteristics of biofuels have attracted broad interests, due to its bright prospect as one of the alternative fuels. The rheology of some biofuels was found to be power-law behavior. The spray performance of these fuels still remains unclear. Besides, biofuel atomization in pressurized gaseous crossflow can be of interest for combustion engines. The influential factors, namely the momentum flux ratios, Weber number and liquid-gas density ratios, are investigated employing controlling variable approach in the present. Power-law behavior fuel has shown great advantages in bringing a better atomization compared to the Newtonian one with case-control method. Jet column surface disturbance and the deformation magnitude during the injection process is emphasized. The penetration core lengths and breakup points are identified to illustrate the column deformation and breakup times. Besides, the column furcation processes which form finger-like features, as well as surface wavelengths are detected and compared along the studies of the influential factors. A larger injection momentum brings a smaller column deflection while the deflection is independent of Weber number. Besides, the locations of the furcation points increase with the raise of the momentum flux ratios while reduce with the increase of Weber number. The significant effect for increasing the density ratios is on promoting jet atomization. Column breakup processes tend to take place when keep increasing the gas densities. The column surface fluctuations are enhanced while the quantity of the ligaments raises and the shapes become richer.

Experimental Study and Method of Sloshing Model Test Considering Gas-Liquid Density Ratio

Experimental Study and Method of Sloshing Model Test Considering Gas-Liquid Density Ratio

Comparison of simulation and experiments for multimode aerodynamic breakup of a liquid metal column in a shock-induced cross-flow

While the mechanisms that drive breakup and aerodynamic dispersion of traditional liquids such as water have been extensively studied, it is not yet clear if models for traditional liquids can be used to accurately describe the behavior of molten metals. In this paper, multiphase simulations with the interface-capturing combined level-set volume-of-fluid approach are used to provide time-resolved morphology and breakup data for a liquid column subject to a shock-induced cross-flow. For the first time, numerical simulation of the behavior of a liquid metal (Galinstan alloy composed of gallium, indium, and tin) is compared to the well-documented behavior of water. Simulations consider a gas cross-flow Weber number between 10 and 12, which produces a multimode breakup morphology consisting of multiple baglike structures. Up to bag breakup, we confirm that the deformation rate of Galinstan follows the same dependence on the gas cross-flow Weber number as ordinary liquids when time is nondimensionalized by including the liquid-gas density ratio. Moreover, we determine that the appearance of a central stem along the column upstream surface in multimode bag breakup is consistent with the occurrence of Rayleigh-Taylor instability. We also resolve bag stretching and fragmentation, to the full extent allowed by our computational resources, and carry out a direct comparison with the measurements of size and velocity of secondary droplets from high-speed digital inline holography. For Galinstan, we illustrate the differences between simulation and experiment that emerge because of the modification of the surface properties of the metal exposed to air.

Investigation on Asymmetric Instability of Cylindrical Power-Law Liquid Jets

An investigation has been performed to reveal the breakup mechanism of three-dimensional power-law cylindrical jets with different mode disturbances. It is observed experimentally that the asymmetric mode disturbances could prevail over the counterpart of symmetric mode under special conditions. The dispersion equation characterizing the instability of three-dimensional cylindrical jets of power-law fluids is deduced. The effects of the Weber number, generalized Reynolds number, power-law exponent, and gas–liquid density ratio on the jet instability are studied in detail. It is found that the maximum growth rates of asymmetric mode disturbances are usually larger than those of symmetric mode disturbances under high Weber numbers and low generalized Reynolds numbers, which implies that the former are more likely to be responsible for the breakup of power-law fluids. Meanwhile, the large gas–liquid interaction could trigger more short, unstable waves. Interestingly, with the increase of jet velocity, the interaction between liquid and gas phases plays an increasingly leading role on the breakup of power-law cylindrical jets, whereas the viscous force and the power-law exponent have less significant impacts. Theoretical analysis results give a better comprehensive understanding for the power-law jets.

Instability of an annular liquid sheet exposed to compressible gas flows

A temporal linear stability analysis was performed for a viscous annular liquid sheet exposed to compressible gas flows. An analytical dispersion relation illustrating the disturbance growth characteristics at the gas–liquid interfaces was formulated and validated, and a parametric study was performed to explore the mechanism that affects the sheet instability. The results reflect that para-sinuous disturbances on the liquid sheet always predominate over para-varicose disturbances under the considered flow conditions. The aerodynamic interaction at the gas–liquid interfaces of the liquid sheet is the decisive factor on sheet instability. As for the main factors affecting the aerodynamic effects, the contribution of the gas–liquid velocity difference to aerodynamic interactions is more significant than that of the gas–liquid density ratio. Neglecting the gas compressibility markedly underestimates the aerodynamic effects at the gas–liquid interfaces for flow conditions of high gas Mach number, which results in decreased disturbance growth rate and dominant wave number. Moreover, both the liquid viscous force and the surface tension force hold back the disturbance growing and stabilize the liquid sheet under the considered conditions. Note that the destabilizing effect of the viscous force is limited for high liquid Reynolds number cases. In general, it is very necessary to consider the compressibility of the gas flows in the instability study of practical twin-fluid atomization.

Study on the similarity of wet gas pressure drop in long-throat Venturi

This study investigates the effect of pipe diameter on pressure drop with the same diameter ratio, similar pressure-sampling position and throat length of long-throat Venturi. Considering the factors including the void fraction, the friction between the two phases and the entrainment in the gas core, the one-dimensional momentum equation for gas has been solved in the axial direction of long-throat Venturi. A novel void fraction model is established, by considering the effects of dryness and gas-liquid density ratio, then predicting the distribution of wet gas static pressure between the two pressure tapings of the long-throat Venturi. The comparison between the values predicted by the model and those measured experimentally reveals that all the relative deviations of the predicted points by the modified model were within ±15%. In the same entrance conditions, the effect of pipe diameter on pressure drop in long-throat Venturi is similar.

Experimental and numerical study on the effect of dimensionless parameters on the characteristics of droplet atomization caused by periodic inertial force

Fuel atomization will seriously affect the combustion and emission characteristics of IC engine. It is important to study the atomization characteristics of single droplet under dynamic inertial force for improving the high precision secondary atomization model in cylinder. Firstly, the process of droplet atomization under sinusoidal inertial force was studied experimentally. Then, the effects of dimensionless parameters, including Bond number (Bo), gas-liquid density ratio (ρG/L), gas-liquid viscous ratio (μG/L), Weber number (We) and Reynolds number (Re), on the average wavelength and atomization time of droplet surface wave were numerically investigated. The results show that with the development of time, the droplet surface appears zonal standing wave, radial standing wave and volcanic standing wave in turn. When the droplet atomizes, the sub droplet sprays first from the top of the droplet, and the atomization intensity at the top is stronger than that at both sides. Bo, ρG/L and Re have little influence on the average wavelength of droplet surface wave, μG/L has no effect, while We has a great influence. As Bo, We and Re increase, the atomization time decreases rapidly at first, and then converges. When We > 104 or Re > 104, the effect of surface tension and viscous force on atomization time can be neglected. As ρG/L increases, the atomization time first remains unchanged and then increases rapidly. When ρG/L < 0.1, its influence can be neglected. μG/L has no effect on the atomization time. Furthermore, according to the variation of the average wavelength and We number, the dimensionless form of the empirical formula for the average diameter of droplets is obtained: dm∗=(1.0±0.1)∙We-1/3. Finally, the critical conditions of droplet atomization are determined based on the actual atomization time of IC engine.

Simulation of liquid jet primary breakup in a supersonic crossflow under Adaptive Mesh Refinement framework

Compressible two-phase flows were simulated based on the five-equation model under the Adaptive Mesh Refinement (AMR) framework to balance the requirements between space resolution and computational cost. And the simulation system was established in an open source software AMROC (Adaptive Mesh Refinement Object-oriented C++). A combination of Godunov method and wave propagation method was introduced to integrate numerical methods with the AMR algorithm. High speed and high liquid-gas density ratio are two main challenges in the simulation of liquid jet in a supersonic crossflow. To enhance the robustness of the simulation system, a MOON-type positivity preserving method was adopted in the development of the codes. Based on the system mentioned above, a liquid jet in a Mach 1.5 supersonic crossflow was simulated as the standard case to study the primary breakup process in the near field. The simulation captured the column and surface breakup which were the results of the development of the unstable waves in two directions respectively. The instabilities causing the surface breakup were found to be generated in the transonic region initially. Crossflow of a higher Mach number (Ma 1.8) was found being able to augment the instable waves along the injection direction and increase the number of instabilities responsible for the surface breakup. While there was no obvious enhancement of the penetration in the condition of periodic injection, extra unstable waves were imposed on both of windward and leeward liquid surface. The introduced unstable waves had an improvement on the column and surface breakup.

Lattice Boltzmann Simulation of Fluid Flow Characteristics in a Rock Micro-Fracture Based on the Pseudo-Potential Model

Slip boundary has an important influence on fluid flow, which is non-negligible in rock micro-fractures. In this paper, an improved pseudo-potential multi-relaxation-time (MRT) lattice Boltzmann method (LBM), which can achieve a large density ratio, is introduced to simulate the fluid flow in a micro-fracture. The model is tested to satisfy thermodynamic consistency and simulate Poiseuille flow in the case of large liquid-gas density ratio. The slip length is used as an index for evaluating the flow characteristics, and the effects of wall wettability, micro-fracture width, driving pressure and liquid-gas density ratio on the slip length are discussed. The results demonstrate that the slip length increases significantly with the increase of the wall contact angle in rock micro-fracture. And the liquid-gas density ratio has an important impact on the slip length, especially for the hydrophobic wall. Moreover, under the laminar flow regime the driving pressure and the micro-fracture width has little effect on the slip length.

Detailed numerical simulation of liquid jet atomization in crossflow of increasing density

Atomization of liquid fuel jets by cross-flowing airstream is critical to the performance of many aerospace combustors. Recent advances in numerical methods and increases in computational power have enabled the first principle, high fidelity simulation of this phenomenon. In the recent past we presented a detailed validation and analysis of such simulations against experimental data at ambient conditions. At combustor operating conditions, however, both temperature and pressure are significantly elevated. In this work we extend the same computational approach to the study of the impact of reduced liquid–gas density ratio due to increased air density associated with operating pressure elevation on the atomization physics. The previously validated ambient condition case is used as the baseline for comparison with three cases with decreasing density ratios. The density ratio is independently varied by adjusting the gas density and velocity together so that the momentum flux ratio and Weber number are maintained constant. The global-scale jet penetration and trajectory are slightly modified as the density ratio varies. The most significant changes are observed for the local processes of liquid breakup and atomization. As the density ratio is independently reduced, a transition of dominance from the Rayleigh–Taylor to the Kelvin–Helmholtz instability mechanisms on the liquid column surface is detected by analyzing the vorticity fields. Such a transition is verified using linear stability analyses and is also shown to be consistent with the simulation results of a non-monotonic variation of surface wavelength with density ratio. Downstream, a counter-rotating vortex pair (CVP) arises as the density ratio is reduced, the physical origins of which are similar to those documented for gaseous jet in cross-flow studies. Atomization effectiveness is reduced downstream with reduced density ratio and this is in agreement with experimental observation. This result is explained in terms of the density-ratio-related velocity-ratio changes that allow for a switch in behavior from a faster to a slower crossflow speed with respect to the liquid. As a result, at low density ratio recoiling of stretched structures and possibility of increased collision result in increasing Sauter Mean Diameter (SMD).

Direct numerical simulation of near nozzle diesel jet evolution with full temporal-spatial turbulence inlet profile

Fuel atomization which begins with turbulence perturbation has a significant influence on engine performance. The effect of turbulence on near nozzle field characteristics under various liquid-gas density ratios is performed in present models. First, using three-dimensional DNS (3-D Direct Numerical Simulation) method, a time-varying single-phase fully developed turbulent pipe flow is generated to store time-varying outlet velocity databases which are then mapped as the two-phase jet inlet velocities. After that, the characteristics of near nozzle diesel jet evolution are studied by solving the two-phase turbulent flows and tracking the two-phase interfaces based on DNS and VOF (Volume of Fluid) methods. It shows that with fully developed turbulent inject velocities, the topology of the jet evolves gradually from violent wavy surface, ligaments, to droplet parcels with different dimensions and shapes, which are considered more realistic and different from uniform/sinusoidal perturbation inject velocities, especially in the near nozzle field. Jets surface distortion, stretched and sheared interface are captured. Three different separation processes in different downstream positions are detected. In the initial stage of the jet, long and narrow ligaments act as primary breakup. In the later stage, the whole breakup process evolves more disorderly and more intensively, so that flat and curly ligaments are observed. Also, higher gas densities and narrower nozzle sizes can accelerate the jet break-up process. Furthermore, the present work is reasonably compared with experimental and numerical modeling results.

A multicomponent multiphase lattice Boltzmann model with large liquid–gas density ratios for simulations of wetting phenomena**Project supported by the National Natural Science Foundation of China (Grant Nos. 51371051 and 51306037) and the Scientific Research Foundation of Graduate School of Southeast University, China (Grant No. YBJJ1627).

A multicomponent multiphase (MCMP) pseudopotential lattice Boltzmann (LB) model with large liquid–gas density ratios is proposed for simulating the wetting phenomena. In the proposed model, two layers of neighboring nodes are adopted to calculate the fluid–fluid cohesion force with higher isotropy order. In addition, the different-time-step method is employed to calculate the processes of particle propagation and collision for the two fluid components with a large pseudo-particle mass contrast. It is found that the spurious current is remarkably reduced by employing the higher isotropy order calculation of the fluid–fluid cohesion force. The maximum spurious current appearing at the phase interfaces is evidently influenced by the magnitudes of fluid–fluid and fluid–solid interaction strengths, but weakly affected by the time step ratio. The density ratio analyses show that the liquid–gas density ratio is dependent on both the fluid–fluid interaction strength and the time step ratio. For the liquid–gas flow simulations without solid phase, the maximum liquid–gas density ratio achieved by the present model is higher than 1000:1. However, the obtainable maximum liquid–gas density ratio in the solid–liquid–gas system is lower. Wetting phenomena of droplets contacting smooth/rough solid surfaces and the dynamic process of liquid movement in a capillary tube are simulated to validate the proposed model in different solid–liquid–gas coexisting systems. It is shown that the simulated intrinsic contact angles of droplets on smooth surfaces are in good agreement with those predicted by the constructed LB formula that is related to Young’s equation. The apparent contact angles of droplets on rough surfaces compare reasonably well with the predictions of Cassie’s law. For the simulation of liquid movement in a capillary tube, the linear relation between the liquid–gas interface position and simulation time is observed, which is identical to the analytical prediction. The simulation results regarding the wetting phenomena of droplets on smooth/rough surfaces and the dynamic process of liquid movement in the capillary tube demonstrate the quantitative capability of the proposed model.

Lattice Boltzmann modeling of pool boiling with large liquid-gas density ratio

In the present paper, a 2D multiple-relaxation-time pseudopotential lattice Boltzmann model combined with the modified thermal lattice Boltzmann method is adopted to simulate the bubble nucleation, growth and departures process on a heated plate. It is a direct numerical simulation of boiling heat transfer determined by the local temperature and thermodynamic relation given by the equation of state. By using a smaller value of a in the P-R equation of state, a thicker liquid-vapor interface is formed and a better numerical stability at a large liquid/vapor density ratio is obtained. Furthermore, the conjugated boundary of heated plate and fluids is specially dealt with to avoid the rapid change of heat flux at the interface. The boiling heat transfer at a density ratio around 200 can be simulated. The results show that: the boiling heat flux decreases during the bubble expansion process while increases during the rewetting process; the average heat flux of boiling at Ts = 0.68Tc is much larger than that at Ts = 0.86Tc; bubble occurs earlier on a hydrophobic surface than a hydrophilic one; there exists a remained vapor on a hydrophobic surface after bubble departure, while it is not observed for hydrophilic surface; for the simulated boiling curve, the maximum (critical) heat flux decreases with the decreasing wettability of surfaces; there exists an optimal width of the rectangular cavity making the best heat transfer performance of surfaces; in this study, the roughness surface with a circle cavity has the best heat transfer performance.

Effects of ullage gas and scale on sloshing loads

Gas–liquid density ratio (DR) is a key dimensionless number in sloshing assessment methodologies of membrane containment systems for LNG tanks of floating structures. Earlier studies on the effect of DR were mainly statistical and effects of DR were usually mixed with those of gas compressibility and ullage gas pressure but attributed only to DR. In an attempt to separately study such effects, Karimi et al. (2015) [11] studied the effects of DR far from impact zones (global effects of gas–liquid density ratio) which proved to be small in the studied range of DR (0.0002 to 0.0060). The effects of DR near impact zones and before detection of any compressibility effects are referred to as local effects and correspond to modifications of wave shape before impact. They were treated in Karimi et al. (2016). This paper studies the influence of ullage gas at the same scale as well as scaling of sloshing loads at different scales.The test setups were similar to those presented in Karimi et al. (2015) [11] and Karimi et al. (2016) and consisted of three 2D model tanks as transverse slices of tank 2 (out of 4) of a membrane LNG carrier with total capacity of 152000 m3 at scales 1:10, 1:20 and 1:40. All model tests were performed at a fill level corresponding to 20% of the tank heights. Water as liquid and different ullage gases of helium (He), air, two mixtures of sulfur hexafluoride (SF6) and nitrogen (N2), and pure SF6, all at atmospheric pressure with a range of DRs from 0.0002 to 0.0060 were used. Synchronized High-speed video cameras (@4000 fps) and arrays of piezo-electric PCB pressure sensors (@40 kHz) monitored and measured impacts on the tank walls. The study was mainly based on the definition of Impact ID based on impact coincidence.The results are presented at 4 main stages. First, in the same way that sloshing loads measured in irregular model tests are treated in the current methodologies, the measured pressure peaks are studied as statistical samples. Next by the notion of impact ID, the effect of change of ullage gas at the same scale is verified. Thirdly with the same notion of impact ID, impacts are tracked down through three scales to verify scaling. At last dominant impact IDs are introduced. It is shown that the most severe impacts are generated by only a few dominant IDs.

High fidelity simulation and analysis of liquid jet atomization in a gaseous crossflow at intermediate Weber numbers

Recent advances in numerical methods coupled with the substantial enhancements in computing power and the advent of high performance computing have presented first principle, high fidelity simulation as a viable tool in the prediction and analysis of spray atomization processes. The credibility and potential impact of such simulations, however, has been hampered by the relative absence of detailed validation against experimental evidence. The numerical stability and accuracy challenges arising from the need to simulate the high liquid-gas density ratio across the sharp interfaces encountered in these flows are key reasons for this. In this work we challenge this status quo by presenting a numerical model able to deal with these challenges, employing it in simulations of liquid jet in crossflow atomization and performing extensive validation of its results against a carefully executed experiment with detailed measurements in the atomization region. We then proceed to the detailed analysis of the flow physics. The computational model employs the coupled level set and volume of fluid approach to directly capture the spatiotemporal evolution of the liquid-gas interface and the sharp-interface ghost fluid method to stably handle high liquid-air density ratio. Adaptive mesh refinement and Lagrangian droplet models are shown to be viable options for computational cost reduction. Moreover, high performance computing is leveraged to manage the computational cost. The experiment selected for validation eliminates the impact of inlet liquid and gas turbulence and focuses on the impact of the crossflow aerodynamic forces on the atomization physics. Validation is demonstrated by comparing column surface wavelengths, deformation, breakup locations, column trajectories and droplet sizes, velocities, and mass rates for a range of intermediate Weber numbers. Analysis of the physics is performed in terms of the instability and breakup characteristics and the features of downstream flow recirculation, and vortex shedding. Formation of “Λ” shape windward column waves is observed and explained by the combined upward and lateral surface motion. The existence of Rayleigh-Taylor instability as the primary mechanism for the windward column waves is verified for this case by comparing wavelengths from the simulations to those predicted by linear stability analyses. Physical arguments are employed to postulate that the type of instability manifested may be related to conditions such as the gas Weber number and the inlet turbulence level. The decreased column wavelength with increasing Weber number is found to cause enhanced surface stripping and early depletion of liquid core at higher Weber number. A peculiar “three-streak-two-membrane” liquid structure is identified at the lowest Weber number and explained as the consequence of the symmetric recirculation zones behind the jet column. It is found that the vortical flow downstream of the liquid column resembles a von Karman vortex street and that the coupling between the gas flow and droplet transport is weak for the conditions explored.

Axisymmetric multiphase lattice Boltzmann method for generic equations of state

We present an axisymmetric lattice Boltzmann model based on the Kupershtokh et al. multiphase model that is capable of solving liquid–gas density ratios up to 103. Appropriate source terms are added to the lattice Boltzmann evolution equation to fully recover the axisymmetric multiphase conservation equations. We validate the model by showing that a stationary droplet obeys the Young–Laplace law, comparing the second oscillation mode of a droplet with respect to an analytical solution and showing correct mass conservation of a propagating density wave.

An investigation of wet gas over-reading in orifice plates under ultra-low liquid fraction conditions using dimensional analysis

Wet gas represents a special gas-liquid two-phase flow case which is often encountered in the oil and gas industry, such as wet natural gas extraction from a condensate field. The on-line, non-separation measurement of wet gas under ultra-low liquid fraction conditions is an important research area that presents significant technical challenges. In this paper, a method based on dimensional analysis is investigated that may enable predictions of the over-reading parameter in wet gas with liquid fractions less than 0.5% using non-separation measurement. A standard orifice plate with a diameter of 50 mm and a beta ratio of 0.55 was used as the flow measurement device. The dimensional analysis method was used to investigate the flow characteristics of a wet gas flowing through an orifice plate for Lockhart-Martinelli parameters ranging from 0 to 0.08, gas Froude numbers ranging from 0.5 to 2.4 and liquid gas density ratios ranging from 75 to 170. A measurement-based correlation of the wet gas over-reading parameter in an orifice plate is derived using dimensionless parameters combined with physical analysis. The experimental data shows that the relative deviation of this correlation is less than ±2% in experimental verification. The results of this study show that this method can be applied to the non-separation measurement of wet natural gas from the ultra-low liquid fraction of an oil and gas field in the future.

Effect of ullage gas on sloshing, Part II: Local effects of gas–liquid density ratio

Gas–liquid density ratio (DR) is a key dimensionless number in sloshing assessment methodologies of membrane containment systems for LNG tanks of floating structures. Earlier studies on the effect of DR were mainly statistical and effects of DR were usually mixed with those of gas compressibility and ullage gas pressure but attributed only to DR. In an attempt to separately study such effects, part I of this work studied the effects of DR far from impact zones (global effects of gas–liquid density ratio) which proved to be small in the studied range of DR (0.0002 to 0.0060). The effects of DR near impact zones and in the instants prior to the detection of any compressibility effects are referred to as local effects and are treated in the current paper (part II).The test setup was identical to the one presented in Part I and consisted of two 2D model tanks representing transverse slices of tank 2 (out of 4) of a membrane LNG carrier with total capacity of 152000m3 at scales 1:20 and 1:40. Both model tests were performed at 20% fill level of the tank heights. Water was the main liquid that was used. In some tests at scale 1:20 a solution of sodium polytungstate (SPT) was also used which had a higher density compared to water. Different ullage gases of helium (He), air, two mixtures of sulfur hexafluoride (SF6) and nitrogen (N2), and pure SF6, all at atmospheric pressure with a range of DRs from 0.0002 to 0.0060 were utilized. Synchronized high-speed video cameras (@4000 fps) and arrays of piezo-electric PCB (112A21 and 112M361) pressure sensors (@40 kHz) monitored and measured impacts on the tank walls. In Part II of the study short and more regular tank motions which generated highly repeatable single impact waves (SIW) were used instead of long irregular tank motions which were considered in part I.By comparing the single impact waves (SIW) generated by identical tank motions but with different DRs, it was observed that DR clearly modifies wave shapes prior to the moment of wave breaking. Larger DRs tend to slow down the wave front and delay breaking. It was also observed that larger DRs slightly slow down wave trough runup as well. Those effects would also lead to a mild shift of impact types by changing the DR (for example Flip-through to slosh or large gas-pocket to small gas-pocket impacts). By comparing single impact waves (SIW) generated by identical tank motions and the same DR but with different gas and liquid densities it was shown that keeping the same DR is essentially needed to keep the same impact geometry as recommended by the existing sloshing assessment methodologies. Free surface instabilities were also very similar for those waves generated with the same tank motions and similar DR but with different gases and liquids. Considering the reduction of wave kinetic energy by heavier11Throughout the paper the term heavy gas would refer to gases with higher density and the term light gas would refer to gases with lower density. This was thought to make the text easier to read (and write) compared to the terms dense and less dense although dense and less dense are more scientific. Nonetheless wherever needed, the value of DR is mentioned explicitly. ullage gases as a relevant source of the statistical reduction of impact pressures and having in mind the mild shift of wave impact types caused by the change of DR it is still to be studied further why the heavier gas leads to smaller statistical pressures.