scholarly journals Turbulence modulation in buoyancy-driven bubbly flows

2021 ◽  
Vol 932 ◽  
Author(s):  
Vikash Pandey ◽  
Dhrubaditya Mitra ◽  
Prasad Perlekar
Keyword(s):  
Numerical Simulation ◽  
Large Scale ◽  
Bubble Diameter ◽  
Bubbly Flows ◽  
Air Bubbles ◽  
Rise Velocity ◽  
Turbulence Modulation ◽  
Budget Analysis ◽  
Average Rise ◽  
Turbulence Scaling

We present a direct numerical simulation (DNS) study of buoyancy-driven bubbly flows in the presence of large-scale driving that generates turbulence. On increasing the turbulence intensity: (a) the bubble trajectories become more curved and (b) the average rise velocity of the bubbles decreases. We find that the energy spectrum of the flow shows a pseudo-turbulence scaling for length scales smaller than the bubble diameter and a Kolmogorov scaling for scales larger than the bubble diameter. We conduct a scale-by-scale energy budget analysis to understand the scaling behaviour observed in the spectrum. Although our bubbles are weakly buoyant, the statistical properties of our DNS are consistent with the experiments that investigate turbulence modulation by air bubbles in water.

Dynamics of homogeneous bubbly flows Part 2. Velocity fluctuations

2002 ◽  
Vol 466 ◽  
pp. 53-84 ◽  
Author(s):  
BERNARD BUNNER ◽  
GRÉTAR TRYGGVASON
Keyword(s):  
Kinetic Energy ◽  
Reynolds Stress ◽  
Void Fraction ◽  
Three Dimensional ◽  
Bubbly Flows ◽  
Rise Velocity ◽  
Velocity Fluctuations ◽  
Kinetic Energy Spectrum ◽  
Average Rise ◽  
High Wavenumbers

Direct numerical simulations of the motion of up to 216 three-dimensional buoyant bubbles in periodic domains are presented. The bubbles are nearly spherical and have a rise Reynolds number of about 20. The void fraction ranges from 2% to 24%. Part 1 analysed the rise velocity and the microstructure of the bubbles. This paper examines the fluctuation velocities and the dispersion of the bubbles and the ‘pseudo-turbulence’ of the liquid phase induced by the motion of the bubbles. It is found that the turbulent kinetic energy increases with void fraction and scales with the void fraction multiplied by the square of the average rise velocity of the bubbles. The vertical Reynolds stress is greater than the horizontal Reynolds stress, but the anisotropy decreases when the void fraction increases. The kinetic energy spectrum follows a power law with a slope of approximately −3.6 at high wavenumbers.

Phase Distribution in Buoyancy-Driven Bubbly Flows

2002 ◽  
Author(s):  
Asghar Esmaeeli ◽  
Chan Ching ◽  
Mamdouh Shoukri
Keyword(s):  
Pair Distribution Function ◽  
Phase Distribution ◽  
Liquid Velocity ◽  
Momentum Equation ◽  
Bubbly Flows ◽  
Bubble Coalescence ◽  
Rise Velocity ◽  
Topology Change ◽  
Moderate Reynolds Number ◽  
Average Rise

This study aims to investigate the effect of topology change on the rise velocity of bubbly flows and the phase distribution in a channel at a moderate Reynolds number. A front tracking/finite difference method is used to solve the momentum equation inside and outside deformable bubbles. It is found that bubble/bubble coalescence enhances the average rise velocity of the bubbles dramatically and also increases the fluctuations of the liquid velocity. Examination of the pair distribution function shows that the flow becomes more non-homogeneous as a result of topology change.

Experiments and Modeling in Bubbly Flows at Elevated Pressures

2003 ◽  
Author(s):  
R. Kumar ◽  
T. A. Trabold ◽  
C. C. Maneri
Keyword(s):  
Void Fraction ◽  
Bubble Size ◽  
Bubbly Flow ◽  
Bubble Diameter ◽  
Bubbly Flows ◽  
Rise Velocity ◽  
Two Phase ◽  
Flow Models ◽  
Gamma Densitometer ◽  
Visualization Techniques

Measurements of local void fraction, rise velocity and bubble diameter have been obtained for cocurrent, wall-heated, upward bubbly flows in a pressurized refrigerant. The instrumentation used was the gamma densitometer and the hot-film anemometer. Departure bubble size and bulk size measurements were also made and correlated with appropriate parameters. Flow visualization techniques have also been used to understand the two-phase flow structure and the behavior of the bubbly flow for different bubble shapes and sizes, and to obtain the rise velocity. Such insight, coupled with quantitative local and averaged data on void fraction and bubble size at different pressures, has aided in developing bubbly flow models applicable to heated two-phase flows at high pressure.

Experiments and Modeling in Bubbly Flows at Elevated Pressures

2003 ◽  
Vol 125 (3) ◽  
pp. 469-478 ◽  
Author(s):  
Ranganathan Kumar ◽  
Thomas A. Trabold ◽  
Charles C. Maneri
Keyword(s):  
Void Fraction ◽  
Bubble Size ◽  
Bubbly Flow ◽  
Bubble Diameter ◽  
Bubbly Flows ◽  
Rise Velocity ◽  
Two Phase ◽  
Flow Models ◽  
System Pressure ◽  
Eotvos Number

Measurements of local void fraction, rise velocity, and bubble diameter have been obtained for cocurrent, wall-heated, upward bubbly flows in a pressurized refrigerant. The instrumentation used are the gamma densitometer and the hot-film anemometer. Departure bubble size is correlated in terms of liquid subcooling and bulk bubble size in terms of void fraction. Flow visualization techniques have also been used to understand the two-phase flow structure and the behavior of the bubbly flow for different bubble shapes and sizes, and to obtain the bubble diameter and rise velocity. The lift model is provided explicitly in terms of Eotvos number which is changed by changing the system pressure. In general, Eotvos number plays a strong role in determining both bubbly lift and drag. Such insight coupled with quantitative local and averaged data on void fraction and bubble size at different pressures has aided in developing bubbly flow models applicable to heated two-phase flows at high pressure.

scholarly journals Characteristics of Large-Scale Circulation Affecting the Inter-Annual Precipitation Variability in Northern Sumatra Island during Boreal Summer

Atmosphere ◽  
2021 ◽  
Vol 12 (2) ◽  
pp. 136
Author(s):  
Yahya Darmawan ◽  
Huang-Hsiung Hsu ◽  
Jia-Yuh Yu
Keyword(s):  
Large Scale ◽  
Precipitation Variability ◽  
Specific Humidity ◽  
Boreal Summer ◽  
Moisture Budget ◽  
Large Scale Circulation ◽  
Budget Analysis ◽  
Sumatra Island ◽  
Precipitation Anomalies ◽  
The Impact

This study aims to explore the contrasting characteristics of large-scale circulation that led to the precipitation anomalies over the northern parts of Sumatra Island. Further, the impact of varying the Asian–Australian Monsoon (AAM) was investigated for triggering the precipitation variability over the study area. The moisture budget analysis was applied to quantify the most dominant component that induces precipitation variability during the JJA (June, July, and August) period. Then, the composite analysis and statistical approach were applied to confirm the result of the moisture budget. Using the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Anaysis Interim (ERA-Interim) from 1981 to 2016, we identified 9 (nine) dry and 6 (six) wet years based on precipitation anomalies, respectively. The dry years (wet years) anomalies over the study area were mostly supported by downward (upward) vertical velocity anomaly instead of other variables such as specific humidity, horizontal velocity, and evaporation. In the dry years (wet years), there is a strengthening (weakening) of the descent motion, which triggers a reduction (increase) of convection over the study area. The overall downward (upward) motion of westerly (easterly) winds appears to suppress (support) the convection and lead to negative (positive) precipitation anomaly in the whole region but with the largest anomaly over northern parts of Sumatra. The AAM variability proven has a significant role in the precipitation variability over the study area. A teleconnection between the AAM and other global circulations implies the precipitation variability over the northern part of Sumatra Island as a regional phenomenon. The large-scale tropical circulation is possibly related to the PWC modulation (Pacific Walker Circulation).

scholarly journals Numerical Simulation of EPB Shield Tunnelling with TBM Operational Condition Control Using Coupled DEM–FDM

2021 ◽  
Vol 11 (6) ◽  
pp. 2551
Author(s):  
Hyobum Lee ◽  
Hangseok Choi ◽  
Soon-Wook Choi ◽  
Soo-Ho Chang ◽  
Tae-Ho Kang ◽  
...  
Keyword(s):  
Numerical Simulation ◽  
Large Scale ◽  
Tunnel Boring Machine ◽  
Chamber Pressure ◽  
Screw Conveyor ◽  
Shield Tunnel ◽  
Pressure Balance ◽  
Operational Conditions ◽  
Shield Tunnelling ◽  
Epb Shield

This study demonstrates a three-dimensional numerical simulation of earth pressure balance (EPB) shield tunnelling using a coupled discrete element method (DEM) and a finite difference method (FDM). The analysis adopted the actual size of a spoke-type EPB shield tunnel boring machine (TBM) consisting of a cutter head with cutting tools, working chamber, screw conveyor, and shield. For the coupled model to reproduce the in situ ground condition, the ground formation was generated partially using the DEM (for the limited domain influenced by excavation), with the rest of the domain being composed of FDM grids. In the DEM domain, contact parameters of particles were calibrated via a series of large-scale triaxial test analyses. The model simulated tunnelling as the TBM operational conditions were controlled. The penetration rate and the rotational speed of the screw conveyor were automatically adjusted as the TBM advanced to prevent the generation of excessive or insufficient torque, thrust force, or chamber pressure. Accordingly, these parameters were maintained consistently around their set operational ranges during excavation. The simulation results show that the proposed numerical model based on DEM–FDM coupling could reasonably simulate EPB driving while considering the TBM operational conditions.

scholarly journals Optimal Design of Slit Impeller for Low Specific Speed Centrifugal Pump Based on Orthogonal Test

2021 ◽  
Vol 9 (2) ◽  
pp. 121
Author(s):  
Yang Yang ◽  
Ling Zhou ◽  
Hongtao Zhou ◽  
Wanning Lv ◽  
Jian Wang ◽  
...  
Keyword(s):  
Numerical Simulation ◽  
Centrifugal Pump ◽  
Large Scale ◽  
Internal Flow ◽  
Orthogonal Test ◽  
Centrifugal Pumps ◽  
Initial Model ◽  
Low Specific Speed ◽  
Specific Speed ◽  
Four Levels

Marine centrifugal pumps are mostly used on board ship, for transferring liquid from one point to another. Based on the combination of orthogonal testing and numerical simulation, this paper optimizes the structure of a drainage trough for a typical low-specific speed centrifugal pump, determines the priority of the various geometric factors of the drainage trough on the pump performance, and obtains the optimal impeller drainage trough scheme. The influence of drainage tank structure on the internal flow of a low-specific speed centrifugal pump is also analyzed. First, based on the experimental validation of the initial model, it is determined that the numerical simulation method used in this paper is highly accurate in predicting the performance of low-specific speed centrifugal pumps. Secondly, based on the three factors and four levels of the impeller drainage trough in the orthogonal test, the orthogonal test plan is determined and the orthogonal test results are analyzed. This work found that slit diameter and slit width have a large impact on the performance of low-specific speed centrifugal pumps, while long and short vane lap lengths have less impact. Finally, we compared the internal flow distribution between the initial model and the optimized model, and found that the slit structure could effectively reduce the pressure difference between the suction side and the pressure side of the blade. By weakening the large-scale vortex in the flow path and reducing the hydraulic losses, the drainage trough impellers obtained based on orthogonal tests can significantly improve the hydraulic efficiency of low-specific speed centrifugal pumps.

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