Splitting Of Bubble Research Articles

The degradation mechanisms of a nanobainitic steel under cavitation erosion conditions

The nanobainitic steel was investigated using a chamber with the cavitating jet at two distances from a nozzle. Fe-0.6C-1.7Mn-1.6Si-1.35Cr-0.4Mo steel subjected to austenitising at 950 °C followed by controlled air cooling and the isothermal heat treatment at 215 °C shows high strength and good plasticity. The microstructure of steel consists of packets of laths of carbide-free bainite and 10 % in volume of retained austenite in the form of laths, plates, and grains (blocky). Due to its very good combination of strength and plasticity, nanobainitic steel is promising for applications under extreme loading conditions. For this reason, it was decided to analyse the behaviour of the steel under dynamic loads with high-pressure amplitude. Such conditions are obtained during cavitation erosion tests. The distance from the nozzle affected the degradation process (type of damage, maximum and final erosion rates). Three types of damage were observed: A) caused by simple collapse of the cavitation bubble, B) caused by splitting of the cavitation bubble into two smaller bubbles, and C) caused by fragmentation of the cavitation bubble into many tiny bubbles. Some similarities in the appearance of the damage were observed: dark rings with a changed structure and a recess in the middle, a dark surface layer, a quasi-etched structure with numerous small recesses, and a structure with fine needles. The “needle” structure was formed only after reaching the maximum rate of erosion. As the distance from the nozzle increased, more and more cavitation bubbles fragmented, which resulted in an increase in erosion intensity and the maximum rate of erosion. Higher erosion intensity affected the higher transformation of austenite to martensite and finally rounding/convexity of the entire surface and a decrease in erosion rate.

Study on the Law of Pseudo-Cavitation on Superhydrophobic Surface in Turbulent Flow Field of Backward-Facing Step

Superhydrophobic surface is regarded as important topic in the field of thermal fluids today due to its unique features on flow drag reduction and heat transfer enhancement. In this study, the pseudo-cavitation phenomenon on the superhydrophobic surface in the backward-facing step turbulent flow field is observed through experiments. The underlying reason for this phenomenon is studied with experimental observation and analysis, and the time variant mechanisms of this phenomenon with various Reynolds number is summarized. The research results indicate that the superhydrophobic surface and the backward-facing step provide the material basis and dynamic condition for the generation of pseudo-cavitation. The pseudo-cavitation induces a large bubble on the superhydrophobic surface below the backward-facing step. The size, position, shape, oscillation amplitude, detachment, and splitting of the large bubble show regularity with the changes of Reynolds number. Meanwhile, the bubble growth, oscillation, detachment, split, and regeneration over time also show regularity. The study of bubble generation and development laws can be used to better control the perturbation of the flow field. Importantly, the present study has meaning in better understanding the flow mechanisms and gas coverage of superhydrophobic surface under condition of backward-facing step, paving the way for studying the flow drag reduction effect of superhydrophobic surface.

A three-dimensional modeling for coalescence of multiple cavitation bubbles near a rigid wall

The Boundary Integral Method (BIM) has been widely and successfully applied to cavitation bubble dynamics; however, the physical complexities involved in the coalescence of multiple bubbles are still challenging for numerical modeling. In this study, an improved three-dimensional (3D) BIM model is developed to simulate the coalescence of multiple cavitation bubbles near a rigid wall, including an extreme situation when cavitation bubbles are in contact with the rigid wall. As the first highlight of the present model, a universal topological treatment for arbitrary coalescence is proposed for 3D cases, combined with a density potential method and an adaptive remesh scheme to maintain a stable and high-accuracy calculation. Modeling for the multiple bubbles attached to the rigid boundary is the second challenging task of the present study. The effects of the rigid wall are modeled using the method of image; thus, the boundary value problem is transformed to the coalescence of real bubbles and their images across the boundary. Additionally, the numerical difficulties associated with the splitting of a toroidal bubble and self-coalescence due to the self-film-thinning process of a coalesced bubble are successfully overcome. The present 3D model is verified through convergence studies and further validated by the purposely conducted experiments. Finally, representative simulations are carried out to elucidate the main features of a coalesced bubble near a rigid boundary and the flow fields are provided to reveal the underlying physical mechanisms.

Effect of near-wake jet on the lock-in of a freely vibrating square cylinder

We present a numerical study of a freely vibrating square cylinder with steady bleeding at its base side. In particular, we focus on the suppression of vortex-induced vibration (VIV) and the reduction of drag force for the elastically mounted square cylinder at a laminar flow condition via near-wake jet. We examine the base bleeding mechanism in the near-wake region of a square cylinder and its influence over the flow dynamics and the wake characteristics for both stationary (nonlock-in) and freely vibrating (lock-in) conditions. We consider the near-wake jet parameter as a function of the bleed coefficient (Cq), which is the ratio of near-wake jet flow velocity to the freestream velocity and depends on the Reynolds number (Re) based on the diameter of the cylinder. Investigations of the hydrodynamic coefficients and the flow features are carried out for the laminar Re range, namely, Re = 40, 60, 100, and 150. A single dominant frequency peak is observed in the lift coefficient spectrum plot for all the Reynolds numbers considered, but two peaks are observed for Re = 150 at Cq = 0.175 and 0.2. Higher Cq values behave like a splitter plate thereby preventing the interaction of alternating shear layers. The variation of the mean drag is associated with the pressure distribution around the cylinder surface and along the streamwise locations. This leads to a thinner wake width, weaker vortices, and higher vortex shedding frequency as observed earlier in the literature. The sharp spikes of pressure coefficient at the base side of the cylinder are observed for Re ∈ [40, 150] due to the near-wake jet, accounting for the fluctuations of drag force coefficient. We demonstrate the formation of multiple vortices at the wake region due to the near-wake jet from our detailed qualitative analysis. We observe counter-rotating pair of recirculating fluids flanking at the near-wake jet location and examine the recovery of base pressure due to the jet flow. We demonstrate the splitting of big circulation fluid bubble into many smaller counter-rotating fluids due to high-velocity jet flow, resulting into the stabilization of flow profiles in the wake region. We extend this investigation to quantify the effect of the near-wake jet on the two-degree-of-freedom cylinder system at the representative Reynolds number Re = 100 and three mass ratios m* = 1, 2, and 3. We demonstrate the reduction of peak transverse VIV amplitude by 90% in comparison to the plain cylinder counterpart.

Experimental characterization of gas-liquid flows in splitting distributor for parallel micro-channels

The present work is focused on experimental investigations of gas-liquid flows in T-junction splitting distributor for parallel micro-channels. The tree type T-junction splitting distributor was designed with four successive T-junctions (T1, T2, T3 and T4) that are linked in series to split a single main channel into four parallel channels. The design strategy was adopted from Adamson et al. [1], Hoang et al. [2], and modified with a combination of micro and milli channel dimensions and reduction in width of the successive blocks by a factor of 20.5 to ensure the flow uniformity in terms of constant relative lengths (Lbubble/slug/Wchannel) of bubbles/slugs in all the blocks. Experiments were performed to study the effect of Qoil (=3, 6, 8, 10, 12 and 16 ml/min) at fixed Qair (=3 ml/min) on formation dynamics of bubbles/slugs at 1st T-junction and splitting mechanisms of bubbles/slugs at the successive T-junctions, the corresponding relative lengths of bubbles/slugs and flow regimes. The results obtained from the present work showed that, followed by formation of parent gas slug at T1 the splitting of gas slug occurred at the successive T-junctions (T2, T3 and T4). Three types of splitting mechanisms were observed such as obstructed (Qoil = 3, 6, 8, 10 ml/min; 0.0023 ⩽CaBlock-I⩽0.0050), partially obstructed (Qoil = 12 ml/min; CaBlock-I = 0.00585) and non-obstructed (Qoil = 16 ml/min; CaBlock-I = 0.0074). It was observed that asymmetry of the splitting at the successive T-junctions was reduced for Qoil⩾10 ml/min (CaBlock-I⩾0.0050). No splitting of bubble was observed for Qoil > 16 ml/min. However, it was also found that the proposed design strategy of splitting distributor for parallel micro-channels yields an almost constant relative lengths of bubbles/slugs (flow uniformity) in all the blocks for Qoil⩾8 ml/min.

Numerical simulation of single bubble dynamics under acoustic travelling waves

The objective of this paper is to apply CLSVOF method to investigate the single bubble dynamics in acoustic travelling waves. The Naiver-Stokes equation considering the acoustic radiation force is proposed and validated to capture the bubble behaviors. And the CLSVOF method, which can capture the continuous geometric properties and satisfies mass conservation, is applied in present work. Firstly, the regime map, depending on the dimensionless acoustic pressure amplitude and acoustic wave number, is constructed to present different bubble behaviors. Then, the time evolution of the bubble oscillation is investigated and analyzed. Finally, the effect of the direction and the damping coefficient of acoustic wave propagation on the bubble behavior are also considered. The numerical results show that the bubble presents distinct oscillation types in acoustic travelling waves, namely, volume oscillation, shape oscillation, and splitting oscillation. For the splitting oscillation, the formation of jet, splitting of bubble, and the rebound of sub-bubbles may lead to substantial increase in pressure fluctuations on the boundary. For the shape oscillation, the nodes and antinodes of the acoustic pressure wave contribute to the formation of the “cross shape” of the bubble. It should be noted that the direction of the bubble translation and bubble jet are always towards the direction of wave propagation. In addition, the damping coefficient causes bubble in shape oscillation to be of asymmetry in shape and inequality in size, and delays the splitting process.

Microstructural characterization of 5083 aluminum alloy thick plates welded with GMAW and twin wire GMAW processes

Thick plates of 5083 aluminum alloy are welded with GMAW and GMAW-TW processes; the influence of weld pool convection on porosity formation, microstructure, and mechanical property of the welding joints is investigated. The results showed that due to the appearance of outward flow and “push-pull” patterns, and intense mixing and stirring effects in GMAW-TW process, a sound weld seam with less porosity can be obtained. The less probability of bubble formation, the shorter escaping distance of the bubble, the easy splitting of bubble, the higher bubble escape velocity, and lower solidification rate are responsible for porosity suppression. Even though the grain size in GMAW-TW process is larger than that in GMAW process, the ultimate tensile strength of the joints is larger, which is caused by less porosity formation, larger number, and uniform distribution of second-phase particles in the welds. Compared with GMAW process, the GMAW-TW process possesses the advantages of higher efficiency, less weld defects, and higher mechanical property in the welding of 5083 aluminum alloy thick plates.

Hydrodynamic Study of Bubbles in a Bubble Column Reactor Part II – Numerical Study

The present work deals with the use of CFD analysis and the validation of the experimental work carried out on the artificial splitting of an air bubble in a bubble column reactor. In Part I of this work, artificial splitting of bubble in a bubble column rector is experimentally studied by using a high speed camera. Image processing technique was used to identify bubble size and bubble velocity. In present work CFD simulations are carried out using ANSYS FLUENT software using Volume of Fluids (VOF) method. VOF is based on a surface tracking technique applied to a fixed Eulerian space. The phase fraction in physical quantities that can be used to distinguish the distribution of gas hold up in a bubble Column reactor. The numerical study of splitting of bubble into two bubbles of nearly equal size is considered. In the bubble column reactor, the liquid phase is stationary and gas flow rate in it is varied. The superficial gas flow rates are 10 lph, 15 lph, 20 lph and 25 lph. The characteristics of bubble after splitting which include its shape, size and velocity for various gas flow rates mentioned above are studied numerically and are compared with experimental results. These hydrodynamic characteristics play a pivotal role in the reactions occurring between the liquid and gas phases in the bubble column reactor.

Dynamics of an air bubble induced by an adjacent oscillating bubble

This study is concerned with the collapse of an air bubble induced by its adjacent oscillating bubble, including the splitting of the air bubble and the subsequent transitions of the two split sub-bubbles from singly-connected to doubly-connected. The numerical modeling is based on the potential flow theory coupled with the boundary integral method. A two-vortex-rings model is put forward to further simulate the interaction between two toroidal bubbles and a singly-connected one, which is rarely seen in previous studies. To validate the numerical model, experiments are carried out for the dynamics of an air bubble induced by a spark generated bubble captured by a high speed camera. Our numerical results agree qualitatively with the experimental data. It is found that the strength parameter ε of the oscillating bubble greatly affects the jet velocity of air bubble and the length ratio l′ of air bubble determines its collapsing pattern.

Study on splitting of a toroidal bubble near a rigid boundary

The splitting of a toroidal bubble near a rigid boundary is commonly observed in experiments, which is a quite complex phenomenon in bubble dynamics and still not yet well understood. In present study, the bubble splitting phenomenon is studied using the boundary integral method. The vortex ring model is extended to multiple vortex rings to simulate the interaction between two toroidal bubbles after splitting. Buoyancy and non-buoyancy cases are investigated numerically in this study. Numerical results with buoyancy effects show favorable agreement with the experimental observations, which validates the present model. Generally, the first split of the toroidal bubble occurs when an annular “sideways jet” collides with the other side of the bubble. After the toroidal bubble splitting, some new phenomena are found as follows: (i) An annular high pressure region is generated at the splitting location, and the maximum pressure is associated with the velocity differences between the two sides therein just before splitting. (ii) The total volume varies continuously, while the two sub-bubbles vary differently in volume after splitting. (iii) The sideways jet continues propagating on a sub-bubble surface, which would cause more splits or partial breakup of the splash film into droplets. This may be an important reason for the formation of bubble cloud and the rough bubble surface in the rebounding process.

Numerical study on the splitting of a vapor bubble in the ultrasonic assisted EDM process with the curved tool and workpiece

Electrical discharge machining (EDM) is a powerful and modern method of machining. In the EDM process, a vapor bubble is generated between the tool and the workpiece in the dielectric liquid due to an electrical discharge. In this process dynamic behavior of the vapor bubble affects machining process. Vibration of the tool surface affects bubble behavior and consequently affects material removal rate (MRR). In this paper, dynamic behavior of the vapor bubble in an ultrasonic assisted EDM process after the appearance of the necking phenomenon is investigated. It is noteworthy that necking phenomenon occurs when the bubble takes the shape of an hour-glass. After the appearance of the necking phenomenon, the vapor bubble splits into two parts and two liquid jets are developed on the boundaries of the upper and lower parts of the vapor bubble. The liquid jet developed on the upper part of the bubble impinges to the tool and the liquid jet developed on the lower part of the bubble impinges to the workpiece. These liquid jets cause evacuation of debris from the gap between the tool and the workpiece and also cause erosion of the workpiece and the tool. Curved tool and workpiece affect the shape and the velocity of the liquid jets during splitting of the vapor bubble. In this paper dynamics of the vapor bubble after its splitting near the curved tool and workpiece is investigated in three cases. In the first case surfaces of the tool and the workpiece are flat, in the second case surfaces of the tool and the workpiece are convex and in the third case surfaces of the tool and workpiece are concave. Numerical results show that in the third case, the velocity of liquid jets which are developed on the boundaries of the upper and lower parts of the vapor bubble after its splitting have the highest magnitude and their shape are broader than the other cases.

Numerical study on the splitting of a vapour bubble in the process of EDM

Electrical discharge machining (EDM) is a powerful technique for machining of hard and brittle materials. In this process, because of electrical discharge, a vapour bubble is generated in the dielectric liquid between the tool and the workpiece. The growth and collapse phases of the vapour bubble have significant effect on the hydrodynamic behaviour of the dielectric liquid domain between the tool and the workpiece and cause molten material to escape from the crater. Previous numerical studies on the dynamics of an electrical discharge-generated vapour bubble have simulated the growth and collapse of the bubble until it has taken the shape of an hour-glass. This is necking phenomenon which is followed by splitting of the bubble into two parts. In this paper dynamics of an electrical discharge-generated vapour bubble between the tool and the workpiece after its splitting are investigated by using the boundary integral equation method. Development of a liquid jet on the boundary of the each of the upper and lower parts of the bubble and the impingement of the liquid jets on the nearby rigid surfaces are sought. This paper consists of two parts. In part one, the vapour bubble is initially located between the tool and the workpiece. Consequently the dynamic behaviour of the two parts of the bubble in the absence of the buoyancy forces are symmetric with respect to a horizontal axis through the mid-point between the tool and the workpiece. In part two, the elrecrical discharge-generated vapour bubble is initially located in the vicinity of the workpiece. Therefore during the necking phenomenon the upper part of the bubble is smaller than its lower part. Consequently the dynamic behaviour of the two parts of the bubble after its splitting are significantly different.

Electric phenomena in multi-bubble cavitation fields

The processes of formation and accumulation of electric charges in the splitting and deformation of cavitation bubbles in an ultrasonic wave field are considered in terms of the local electrification theory. The influence of different factors on the electrification of the bubble-liquid interface is discussed. It is established that, in the splitting of a cavitation bubble and, possibly, in its deformation, the local field strength near the bubble surface dramatically depends on the radius of the neck formed in the bubble. It is shown that, although the stationary concentration of cavitation bubbles may be very high (∼104–105 cm−3), the probability for several deformed cavitation bubbles of “required size” to emit luminescence at a given instant of time depends on the ultrasound intensity and other test conditions, a conclusion supported by experimental data.