Imperial College London

Professor Omar K. Matar, FREng

Faculty of EngineeringDepartment of Chemical Engineering

Vice-Dean (Education), Faculty of Engineering
 
 
 
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Contact

 

+44 (0)20 7594 9618o.matar Website

 
 
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Assistant

 

Miss Nazma Mojid +44 (0)20 7594 3918

 
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Location

 

506ACE ExtensionSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

275 results found

Angeli P, Azzopardi BJ, Hewakandamby B, Hewitt GF, Pain CC, Simmons MJH, Matar OKet al., 2014, The next-generation predictive tools for multiphase flows, Pages: 221-228

In this paper, we outline the framework that we are developing as part of the Multi-scale Exploration of Multiphase PHysIcs in flowS (MEMPHIS) programme to create the next generation modelling tools for complex multiphase flows. These flows are of central importance to microfluidics, oil-and-gas, nuclear, and biomedical applications, and every processing and manufacturing technology. This framework involves the establishment of a transparent linkage between input and prediction to allow systematic error-source identification, and, optimal, modeldriven experimentation, to maximise prediction accuracy. The framework also involves massivelyparallelisable numerical methods, capable of running efficiently on 105-106 core supercomputers, with optimally-adaptive, three-dimensional resolution, and sophisticated multi-scale physical models. The overall aim of this framework is to provide unprecedented resolution of multi-scale, multiphase phenomena, thereby minimising the reliance on correlations and empiricism.

Conference paper

Purvis JA, Mistry RD, Markides CN, Matar OKet al., 2013, An experimental investigation of fingering instabilities and growth dynamics in inclined counter-current gas-liquid channel flow, PHYSICS OF FLUIDS, Vol: 25, ISSN: 1070-6631

Journal article

Wray AW, Papageorgiou DT, Matar OK, 2013, Electrostatically controlled large-amplitude, non-axisymmetric waves in thin film flows down a cylinder, Journal of Fluid Mechanics, Vol: 736, ISSN: 1469-7645

We examine the dynamics of a thin film flowing under gravity down the exterior of a vertically aligned inner cylinder, with a co-aligned, concentric cylinder acting as an outer electrode; the space between the outer cylinder and the film is occupied by an inviscid gas. The stability of the interface is studied when it is subjected to an electric field, applied by imposing a potential difference between the two cylinders. Leaky-dielectric theory is used in conjunction with asymptotic reduction, in the large-conductivity limit, to derive a single, two-dimensional evolution equation for the interfacial location, which accounts for gravity, capillarity, and electrostatic effects. A linear stability analysis is carried out which shows that non-axisymmetric modes become more dominant with increasing electric field strength. Our fully two-dimensional numerical solutions of the evolution equation demonstrate qualitative agreement between the trends observed in the nonlinear regime and those predicted by linear theory. These numerical solutions also show that, depending on the electric field strength and the relative proximity of the outer electrode, the interface either remains spatially uniform, or exhibits either axisymmetric or, importantly, non-axisymmetric travelling waves. The effect of wave formation on the interfacial area is investigated in connection with the use of electric fields to control thin film flows to enhance heat and mass transfer rates.

Journal article

Zadrazil I, Matar OK, Markides CN, 2013, Slug front gas entrainment in gas-liquid two-phase horizontal flow using hi-speed slug-tracking, American Physical Society - Division of Fluid Dynamics

A gas-liquid flow regime where liquid-continuous regions travel at high speeds (i.e. slugs) through a pipe separated by regions of stratified flow (i.e. elongated bubbles) is referred to as a ``slug flow.'' This regime is characterised by the turbulent entrainment of gas into the slug front body. We use a high-speed camera mounted on a moving robotic linear rail to track the formation of naturally occurring slugs over 150 pipe diameters. We show that the dynamics of the slugs become progressively more complex with increasing liquid and gas Reynolds numbers. Based on the slug- tracking visualization we present, over a range of conditions: (i) phenomenological observations of the formation and development of slugs, and (ii) statistical data on the slug velocity and gas entrainment rate into the slug body.

Conference paper

Heiles B, Zadrazil I, Matar OK, 2013, The effect of surfactant on stratified and stratifying gas-liquid flows, American Physical Society - Division of Fluid Dynamics

We consider the dynamics of a stratified/stratifying gas-liquid flow in horizontal tubes. This flow regime is characterised by the thin liquid films that drain under gravity along the pipe interior, forming a pool at the bottom of the tube, and the formation of large-amplitude waves at the gas-liquid interface. This regime is also accompanied by the detachment of droplets from the interface and their entrainment into the gas phase. We carry out an experimental study involving axial- and radial-view photography of the flow, in the presence and absence of surfactant. We show that the effect of surfactant is to reduce significantly the average diameter of the entrained droplets, through a tip-streaming mechanism. We also highlight the influence of surfactant on the characteristics of the interfacial waves, and the pressure gradient that drives the flow.

Conference paper

Zadrazil I, Matar OK, Markides CN, 2013, On the Frequency of Large Waves in Vertical Gas-Liquid Annular Flow, American Institute of Chemical Engineers

Conference paper

Wray AW, Papageorgiou DT, Matar OK, 2013, Electrified coating flows on vertical fibres: enhancement or suppression of interfacial dynamics, JOURNAL OF FLUID MECHANICS, Vol: 735, Pages: 427-456, ISSN: 0022-1120

Journal article

Zhao Y, Markides CN, Matar OK, Hewitt GFet al., 2013, Disturbance wave development in two-phase gas-liquid upwards vertical annular flow, International Journal of Multiphase Flow, Vol: 55, Pages: 111-129, ISSN: 0301-9322

Disturbance waves are of central importance in annular flows. Such waves are characterised by their large amplitudes relative to the mean film thickness, their high translation velocities relative to the mean film speed, and their circumferential coherence. The present paper is concerned with the existence, development and translation of disturbance waves in upwards, gas–liquid annular flows. Experiments are described, which featured simultaneous high-frequency film thickness measurements from multiple conductance probes positioned circumferentially and axially along a vertical pipe, these measurements were aimed at studying the three-dimensional development of these interfacial structures as a function of distance from the tube inlet. From the results, it is found that disturbance waves begin to appear and to achieve their circumferential coherence from lengths as short as 5–10 pipe diameters downstream of the liquid injection location; this coherence gradually strengthens with increasing distance from the inlet. It is further shown that the spectral content of the entire interfacial wave activity shifts to lower frequencies with increasing axial distance from the inlet, with the peak frequency levelling off after approximately 20 pipe diameters. Interestingly, on the other hand, the frequency of occurrence of the disturbance waves first increases away from the inlet as these waves form, reaches a maximum at a length between 7.5 and 15 pipe diameters that depends on the flow conditions, and then decreases again. This trend becomes increasingly evident at higher gas and/or liquid flow-rates. Both wave frequency measures increase monotonically at higher gas and/or liquid flow-rates.

Journal article

Saenz PJ, Valluri P, Sefiane K, Karapetsas G, Matar OKet al., 2013, Linear and nonlinear stability of hydrothermal waves in planar liquid layers driven by thermocapillarity, PHYSICS OF FLUIDS, Vol: 25, ISSN: 1070-6631

Journal article

Karapetsas G, Sahu KC, Matar OK, 2013, Effect of Contact Line Dynamics on the Thermocapillary Motion of a Droplet on an Inclined Plate, LANGMUIR, Vol: 29, Pages: 8892-8906, ISSN: 0743-7463

Journal article

Zadrazil I, Markides CN, Hewitt GF, Matar OKet al., 2013, Wave Structure and Velocity Profiles in Downwards Gas-Liquid Annular Flow, 8th International Conference on Multiphase Flow

The downwards co-current gas-liquid annular flows inside a vertically oriented pipe have been experimentally investigated.The measurements and characterisation were performed using advanced optical non-intrusive laser-based techniques, namelyLaser Induced Fluorescence, and Particle Image/Tracking Velocimetry. The investigated conditions were in the range of ReL =306 – 1,532 and ReG = 0 – 84,600. Temporal film thickness time traces were constructed using the Laser Induced Fluorescenceimages. Based on these, the wave frequency was evaluated using direct wave counting approach and power spectral densityanalysis. Additionally, qualitative PIV observations revealed the presence of recirculation zones within a wave front ofdisturbance waves.

Conference paper

Zadrazil I, Matar OK, Markides CN, 2013, On the frequency of large waves in vertical gas-liquid annular flows

Conference paper

Zadrazil I, Hewitt GF, Matar OK, Markides CNet al., 2012, Wave Structure and Velocity Profiles in Downwards Gas-Liquid Annular Flows, American Physical Society, American Physical Society - Division of Fluid Dynamics

A downwards flow of gas in the core of a vertical pipe, and of liquid in the annulus between the pipe wall and the gas phase is referred to as a ``downwards annular flow'' (DAF). DAFs are conventionally described in terms of short-lived, small-amplitude ``ripples,'' and large-amplitude, high-speed ``disturbances.'' We use a combination of Laser Induced Fluorescence (LIF), Particle Image and Tracking Velocimetry (PIV, PTV) to study DAFs. We demonstrate through these techniques that the liquid films become progressively more complex with increasing liquid Reynolds number (ReL), while a similar increase of complexity is observed for increasing gas Reynolds number (ReG). Disturbance waves are observed for low and high ReL, and ripples for intermediate ReL. Additionally, a high degree of rolling breakdown of disturbance waves is observed in falling films at the highest ReL, which is a source of bubble entrainment into the film body. Our results will comprise: (i) statistical data on film thickness, and (ii) wave frequency, velocity, wavelength. In addition, a qualitative (e.g. re-circulation zones) and quantitative (e.g. mean/rms velocity profiles) velocity characterisation of the film flows will be presented.

Conference paper

Zadrazil I, Markides CN, Naraigh LO, Matar OKet al., 2012, Dynamics of Turbulent Falling Films, American Physical Society - Division of Fluid Dynamics

The dynamics of laminar falling films have received considerable attention over the past several decades. In contrast, turbulent falling films have been the subject of far fewer studies. We seek to redress this balance by studying the stability of falling films which have already undergone a transition from a laminar to a turbulent flow regime. We derive a uniform-film base-state for this flow by assuming the averaged turbulent velocity field to be steady and fully-developed, and by employing a modified version of mixing-length theory. The latter features an interpolation function for the eddy viscosity, and van Driest-type functions for turbulence-damping near the wall and interface regions. The predicted base-state streamwise velocity component is in good agreement with experimental data. A linear stability analysis of this base-state is then carried out by solving a modified version of the Orr-Sommerfeld equation. Our results suggest that the unstable mode is a long-wave one. This provides motivation for the derivation of long-wave equations for the nonlinear evolution of the film.

Conference paper

Zhao Y, Zadrazil I, Markides CN, Matar OK, Hewitt GFet al., 2012, Wave structure in Upwards Gas-Liquid Annular Flows, American Physical Society - Division of Fluid Dynamics

A two-phase flow system in a vertical pipe in which the liquid around the pipe periphery is lifted by the gas core is referred to as an ``upwards annular flow'' (UAF). UAFs have a complex interfacial structure, which consists of short-lived, small-amplitude ``ripple'' waves, and large amplitude, high-speed ``disturbances'' waves. Two sets of flush-mounted electrically conducting probes together with axial view photography were used to study UAFs. The overall wave frequency decreased with increasing distance from the inlet until saturation. Disturbance waves were observed over a wide range (both low and high) of liquid Reynolds numbers, ReL, while ripples were observed at lower ReL. Disturbance ``bursts,'' which are a source of liquid entrainment into the gas core, were also observed, with increasing frequency at progressively higher ReL. The waves appeared more chaotic near the inlet, which hindered the formation of the correlated waves. As the small (ripple) waves coalesced into bigger waves with increasing distance from the inlet, the waves became more coherent around the pipe periphery. The results that will be presented comprise: (i) statistical film thickness data, and (ii) wave, frequency, velocity, and wavelength.

Conference paper

Conroy DT, Craster RV, Matar OK, Cheng L-J, Chang H-Cet al., 2012, Nonequilibrium hysteresis and Wien effect water dissociation at a bipolar membrane, PHYSICAL REVIEW E, Vol: 86, ISSN: 1539-3755

Journal article

Zadrazil I, Markides CN, Matar OK, Naraigh LO, Hewitt GFet al., 2012, Characterisation of Downwards Co-Current Gas-Liquid Annular Flows, Turbulence, Heat and Mass Transfer 7, Publisher: Begell House

The hydrodynamic characteristics of downwards co-current two-phase (gas-liquid) flows inside avertical tube (ID = 32 mm) have been investigated experimentally. Advanced optical techniques, namely LaserInduced Fluorescence and Particle Tracking Velocimetry, were utilised for the characterisation of these flowsover a wide range of gas and liquid superficial velocities (U_G = 0 – 34 m/s and U_L = 0.034 – 0.182 m/s),corresponding to Reynolds numbers Re_G = 0 – 84,600 and Re_L = 1,230 – 6,130. A flow regime map, whichcontains a previously unreported flow regime, is constructed based on the flow observations. The quantitativeanalysis of the liquid films allows the generation of film thickness, wave frequency, bubble size, bubblefrequency and velocity profile data. It was found that the different observed flow regimes posses a characteristiccombination of the investigated quantitative parameters. A model, based on modified mixing-length theory, wasused to predict the liquid film velocity profiles and good agreement was found with the experimental results.

Conference paper

Karapetsas G, Matar OK, Valluri P, Sefiane Ket al., 2012, Convective Rolls and Hydrothermal Waves in Evaporating Sessile Drops, LANGMUIR, Vol: 28, Pages: 11433-11439, ISSN: 0743-7463

Journal article

Conroy DT, Matar OK, Craster RV, Papageorgiou DTet al., 2012, Compound viscous thread with electrostatic and electrokinetic effects, JOURNAL OF FLUID MECHANICS, Vol: 701, Pages: 171-200, ISSN: 0022-1120

Journal article

Wray AW, Matar O, Papageorgiou DT, 2012, Non-linear waves in electrified viscous film flow down a vertical cylinder, IMA JOURNAL OF APPLIED MATHEMATICS, Vol: 77, Pages: 430-440, ISSN: 0272-4960

Journal article

Spandagos C, Goudoulas TB, Luckham PF, Matar OKet al., 2012, Surface Tension-Induced Gel Fracture. Part 2. Fracture of Gelatin Gels, LANGMUIR, Vol: 28, Pages: 8017-8025, ISSN: 0743-7463

Journal article

Spandagos C, Goudoulas TB, Luckham PF, Matar OKet al., 2012, Surface Tension-Induced Gel Fracture. Part 1. Fracture of Agar Gels, LANGMUIR, Vol: 28, Pages: 7197-7211, ISSN: 0743-7463

Journal article

Mavromoustaki A, Matar OK, Craster RV, 2012, Dynamics of a climbing surfactant-laden film II: Stability, JOURNAL OF COLLOID AND INTERFACE SCIENCE, Vol: 371, Pages: 121-135, ISSN: 0021-9797

Journal article

Mavromoustaki A, Matar OK, Craster RV, 2012, Dynamics of a climbing surfactant-laden film - I: Base-state flow, JOURNAL OF COLLOID AND INTERFACE SCIENCE, Vol: 371, Pages: 107-120, ISSN: 0021-9797

Journal article

Wray AW, Matar OK, Papageorgiou, Non-linear waves in electrified viscous film flow down a vertical cylinder, Ima Journal of Applied Mathematics, ISSN: 1464-3634

Journal article

Matar OK, 2012, PATTERN FORMATION IN EVAPORATING DROPS WITH AND WITHOUT NANOPARTICLES, 9th International Conference on Nanochannels, Microchannels and Minichannels, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 599-606

Conference paper

Karapetsas G, Craster RV, Matar OK, 2011, Surfactant-driven dynamics of liquid lenses, PHYSICS OF FLUIDS, Vol: 23, ISSN: 1070-6631

Journal article

Mavromoustaki A, Matar OK, Craster RV, 2011, Shock-wave solutions in two-layer channel flow. II. Linear and nonlinear stability, PHYSICS OF FLUIDS, Vol: 23, ISSN: 1070-6631

Journal article

Sahu KC, Matar OK, 2011, Three-dimensional convective and absolute instabilities in pressure-driven two-layer channel flow, INTERNATIONAL JOURNAL OF MULTIPHASE FLOW, Vol: 37, Pages: 987-993, ISSN: 0301-9322

Journal article

Naraigh LO, Spelt PDM, Matar OK, Zaki TAet al., 2011, Interfacial instability in turbulent flow over a liquid film in a channel, INTERNATIONAL JOURNAL OF MULTIPHASE FLOW, Vol: 37, Pages: 812-830, ISSN: 0301-9322

Journal article

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