Numerical simulation of turbulence modulation in Two-Phase flows

With the increase of computational power, computational modelling of two-phase flow problems using computational fluid dynamics (CFD) techniques is gradually becoming attractive in the engineering field. The major aim of this thesis is to investigate the Turbulence Modulation (TM) of dilute two phase flows. In order to carry out this approach, an in house research code employing Two-Fluid model, with additional source terms to account for the presence of the dispersed phase in the turbulence equations has been employed. Various density regimes of the two-phase flows have been investigated in this thesis, namely the dilute gas-particle flow, liquid-particle flow and also the liquid-air flows. While the density is quite high for the dispersed phase flow for the gas-particle flow, the density ratio is almost the same for the liquid particle flow, while for the air-liquid flow the density is quite high for the carrier phase flow. The study of all these density regimes gives a clear picture of how the carrier phase behaves in the presence of the dispersed phases, which ultimately leads to better design and safety of many two-phase flow equipments. For the dilute gas-particle flows, particle-turbulence interaction over a backward-facing step geometry was numerically investigated. Two different particle classes with same Stokes number and varied particle Reynolds number are considered in this study. The turbulence modulation of the carrier phase in the presence of the dispersed particulate phase is simulated and compared against the experimental data. Despite the fact that the two particles used in this study share the same Stokes number their behaviour is found to be considerably different in the turbulent flow field, which basically underlines the fact that the Stokes number alone is not enough to fully describe the behaviour of particles, there by, herein particle Reynolds number is also investigated to fully understand their behaviour. A detailed study into the turbulent behaviour of dilute particulate flow under the influence of two carrier phases namely gas and liquid was also been carried out behind a sudden expansion geometry. The major endeavour of the study is to ascertain the response of the particles within the carrier (gas or liquid) phase. The main aim prompting the current study is the density difference between the carrier and the dispersed phase. While the ratio is quite high in terms of the dispersed phase for the gas-particle flows, the ratio is far more less in terms of the liquid-particle flows. For the Liquid-Air flows the phenomenon of drag reduction by the injection of micro-bubbles into turbulent boundary layer has been investigated using an Eulerian-Eulerian two-fluid model. Two variants namely the Inhomogeneous and MUSIG (MUltiple-SIze-Group) based on Population balance models are investigated. The simulated results were benchmarked against the experimental findings and also against other numerical studies explaining the various aspects of drag reduction. The under predictions of the MUSIG model at low rates was investigated and reported, their predictions seem to fair better with the decrease of the break-up tendency among the micro-bubbles

Numerical study of particle interaction in gas-particle and liquid particle flows: part I analysis and valdation

A detailed study into the turbulent behaviour of dilute particulate flow under the influence of two carrier phases namely gas and liquid has been carried out behind a sudden expansion geometry. The major endeavour of the study is to ascertain the response of the particles within the carrier (gas or liquid) phase. The main aim prompting the current study is the density difference between the carrier and the dispersed phases. While the ratio is quite high in terms of the dispersed phase for the gas-particle flows, the ratio is far more less in terms of the liquid-particle flows. Numerical simulations were carried out for both these classes of flows using an Eulerian two-fluid model with RNG based k-e model as the turbulent closure. An additional kinetic energy equation to better represent the combined fluid-particle behaviour is also employed in the current set of simulations. In the first part of this two part series, experimental results of Fessler and Eaton (1995) for Gas-Particle (GP) flow and that of Founti and Klipfel (1998) for Liquid-Particle (LP) flow have been compared and analysed. This forms the basis of the current study which aims to look at the particulate behaviour under the influence of two carrier phases. Further numerical simulations were carried out to test whether the current numerical formulation can used to simulate these varied type of flows and the same were validated against the experimental data of both GP as well LP flow. Qualitative results have been obtained for both these classes of flows with their respective experimental data both at the mean as well as at the turbulence level for carrier as well as the dispersed phase

Skin friction Reduction by Introduction of Micro-bubbles into Turbulent Boundary Layer

The phenomenon of drag reduction by the injection of microbubbles into turbulent boundary layer has been investigated using an Eulerian-Eulerian two-fluid model. Two variants namely the Inhomogeneous and MUSIG (MUltiple SIze Group) based on Population balance models are investigated. The simulated results are compared against the experimental findings of Madavan et al [1]. The model employed in the investigation comprises of a twodimensional micro-bubble laden flow wherein the Reynolds averaged Navier-Stokes (RANS) transport equations were used to describe both the phases of the flow. A SST (Shear Stress Transport) turbulence model is used as the turbulent closure for the primary phase and a zero equation turbulence model is used for the micro-bubbles

A WIRELESS COMMUNICATIONS LINK FOR AN UNMANNED SURFACE VEHICLE

"Life was simple before World War II.After that, we had systems." - Admiral Grace Hopper An Unmanned Surface Vehicle (USV) is a remotely controlled or autonomous craft that operates on the surface of the water' .The USV is a small, rugged, remotely or autonomous controlled marine vehicle that has demonstrated the capability to conduct a wide variety of missions. The USVs inherent low observable characteristics make it difficult to detect with active or passive sensor systems. The U S V is a common platform capable of accepting a variety of task specific payloads; it includes undertaking environmental and hydrographic surveys in coastal and inlands waters. Through recent developing and operations, the U SV has clearly shown the utility and potential growth of various areas. Task force user response is positive and supportive, while the technique the USV has performed well, areas were improving such as to enhance system robustness to meet operational demands. These areas include platform performance of base station, telemetry interface, control and reliability. A family of USVs is needed with common telemetry interface device, and control systems to meet user requirements .Task specifically includes the interface and board control systems will employ distributed devices and integrated systems and platforms to provide a networked-force with the ability to share "extremely rapid, high -volume transmission of digitized data, "as well as the capability for "precision strike and a common operational picture ^ Many of the capabilities needed to implement and still under development, but researchers foresee unmanned vehicles as primary the communication that will compose the network.School of Engineerin

On the numerical simulation of turbulence modulation in two-phase flow

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Two-fluid model for particle-turbulence interaction in a backward-facing step

Particle-turbulence interaction for dilute gas-particle flows over a backward-facing step geometry is numerically investigated. An Eulerian two-fluid model with additional turbulence transport equations for particles is employed in this investigation. RNG based k- model is used as the turbulent closure with additional transport equations solved, to better represent the combined gas-particle interactions. Two different particle classes with same Stokes number and varied particle Reynolds number are considered in this study. The turbulence modulation of the carrier phase in the presence of the dispersed particulate phase is simulated and compared against the experimental data. However prior to this endeavour, the simulated flow field is validated for mean streamwise velocities and fluctuations for both the phases. Despite the fact that the two particles used in this study share the same Stokes number their behavior is found to be considerably different in the turbulent flow field, which basically underlines the fact that the Stokes number alone is not enough to fully describe the behavior of particles, thereby, herein particle Reynolds number is also investigated to fully understand their behavior. Two other turbulence modulation models were also tested against our own formulation, and our model was found to compare better with the experimental findings

Numerical simulation of gas-particle and liquid particle flows over a sudden expansion geometry

A detailed study into both the mean flow and the turbulent behaviour of dilute particulate flow under the influence of two carrier phases namely gas and liquid has been carried out behind a sudden expansion geometry. The major endeavour of the study is to ascertain the response of the particles within the carrier (gas or liquid) phase. The main aim prompting the current study is the density difference between the carrier and the dispersed phase. While the ratio is quite high in terms of the dispersed phase for the gas particle flows, the ratio is far more less in terms of the liquid-particle flows. Numerical simulations were carried out for both these classes of flows using an Eulerian two-fluid model with RNG based k-e model as the turbulent closure. An additional kinetic energy equation to better represent the combined fluid-particle behaviour is also employed in the current set of simulations. The numerical simulations are validated against the experimental data of Fessler and Eaton (1997) for Gas-Particle (GP) flows while experimental data from Founti and Klipfel (1998) was used to validate the Liquid-Particle (LP) flows. Qualitative results have been obtained for both these classes of flows with their respective experimental data, furthermore their response to their carrier phase has been investigated both at the mean and turbulence level for a range of Stokes number. While the particulate velocity seems to increase with the corresponding increase in Stokes number amidst both the carrier phases, the particulate turbulence shows entirely a different pattern

Evaluation of Specific Absorption Rate in Three-Layered Tissue Model at 13.56 MHz and 40.68 MHz for Inductively Powered Biomedical Implants

This paper presents an optimized 3-coil inductive wireless power transfer (WPT) system at 13.56 MHz and 40.68 MHz to show and compare the specific absorption rate (SAR) effects on human tissue. This work also substantiates the effects of perfect alignment, lateral and/or angular misalignments on the power transfer efficiency (PTE) of the proposed WPT system. Additionally, the impacts of different tissue composition, input power and coil shape on the SAR are analyzed. The distance between the external and implantable coils is 10 mm. The results have been verified through simulations and measurements. The simulated results show that the SAR of the system at 40.68 MHz had crossed the limit designated by the Federal Communications Commission and hence, it is unsafe and causes tissue damage. Measurement results of the system in air medium show that the optimized printed circuit board coils at 13.56 MHz achieved a PTE of 41.7% whereas PTE waned to 18.2% and 15.4% at 10 mm of lateral misalignment and 60° of angular misalignment respectively. The PTE of a combination of 10 mm lateral misalignment and 60° angular misalignment is 21%. To analyze in a real-environment, a boneless pork sample with 10 mm of thickness is placed as a medium between the external and implantable coils. At perfect alignment, the PTE through pork sample is 30.8%. A RF power generator operating at 13.56 MHz provides 1 W input power to the external coil and the power delivered to load through the air and tissue mediums are 347 mW and 266 mW respectively

Numerical study of particle dispersion behind a sudden expansion geometry and its effect on step heights

Numerical simulations were performed for dilute gas-particle flows over two-dimensional turbulent backward-facing step geometry to examine the effects of step heights on turbulent separated flow with particles and their inherent dispersion behaviour. Eulerian two-fluid model along with RNG based k-E model is used as the turbulent closure to study this mechanism. However, additional turbulence transport equations are solved to better represent the combined gas-particle turbulence interactions. Two different particle classes with different Stokes number are considered in this study in order to gain a better understanding of the particle behaviour/response to the mean flow and also their effective dispersion. This study helps to better understand the effective particulate viscosity used by two-fluid practioners in order to better capture dispersed phase distribution. The mean flow of the carrier phase along with the dispersed particulate phase is simulated and compared against the experimental data for the step height with maximum expansion ratio (ER). The main objective of this process is to streamline the code to replicate the experimental results and use it further to simulate various other step heights and their particle distribution. This is carried out by keeping the inlet velocity and the flow exit width the same throughout all the different step heights. On the general standing, it is observed that the modelled particulate viscosity works in tandem to the particle number density (PND) of the dispersed phase particles