Browsing by Author "Gourma, Mustapha"

Now showing 1 - 13 of 13
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    Analysis of De-Laval nozzle designs employed for plasma figuring of surfaces
    (Springer, 2016-02-27) Yu, Nan; Jourdain, Renaud; Gourma, Mustapha; Shore, Paul
    Plasma figuring is a dwell time fabrication process that uses a locally delivered chemical reaction through means of an inductively coupled plasma (ICP) torch to correct surface figure errors. This paper presents two investigations for a high temperature jet (5000 K) that is used in the context of the plasma figuring process. Firstly, an investigation focuses on the aerodynamic properties of this jet that streamed through the plasma torch De-Laval nozzle and impinged optical surfaces. Secondly, the work highlights quantitatively the effects of changing the distance between the processed surface and nozzle outlet. In both investigations, results of numerical models and experiments were correlated. The authors’ modelling approach is based on computational fluid dynamics (CFD). The model is specifically created for this harsh environment. Designated areas of interests in the model domain are the nozzle convergent-divergent and the impinged substrate regions. Strong correlations are highlighted between the gas flow velocity near the surface and material removal footprint profiles. In conclusion, the CFD model supports the optimization of an ICP torch design to fulfil the demand for the correction of ultra-precision surfaces.
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    Analysis of nozzle design used for the creation of advanced energy beam
    (American Society for Precision Engineering (ASPE), 2014-11-14) Yu, Nan; Jourdain, Renaud; Gourma, Mustapha; Shore, Paul
    A variety of scientific and industrial projects, such as segmented ground based telescopes, compact space based observers, short wavelength microlithography and high power laser systems, demand metre scale ultra-precise surfaces [1]. Cranfield University and Loxham Precision have been engaged in developing effective fabrication of medium to large optical surfaces for the aforementioned applications. A process chain of three sequential machining steps has been proposed (Figure 1). These steps are ultra-precision grinding, robot based polishing and plasma figuring. The fabrication target is to reach a 20 hours cycle time for each stage of surface generation for 1.5m size optics: equating to 1ft2 per hour [2-3].
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    CFD analysis of an enhanced nozzle designed for plasma figuring of large optical surfaces
    (European Society for Precision Engineering and Nanotechnology, 2016-06-30) Yu, Nan; Jourdain, Renaud; Gourma, Mustapha; Shore, Paul
    For addressing the correction of Mid Spatial Frequency (MSF) errors on metre scale optical surfaces induced by sub aperture figuring process, a new generation of non-contact plasma based surface figuring tools has been created at Cranfield University. In this context, this paper presents an investigation that focuses on novel enhanced nozzles that were created for a Radio Frequency (RF) Inductively Coupled Plasma (ICP) torch. The characteristics of plasma jet delivered by prototype nozzle and a selected enhanced nozzle are compared using an in-house created CFD model. The enhanced nozzle design is based on the results previously obtained throughout a numerical analysis that enabled to identify the key design aspects of these nozzles. This enhanced nozzle is predicted to provide 12.5% smaller footprint and 15.5% higher temperature.
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    CFD simulation of horizontal oil-water flow with matched density and medium viscosity ratio in different flow regimes
    (Elsevier, 2017-01-10) Shi, Jing; Gourma, Mustapha; Yeung, Hoi
    Simulation of horizontal oil-water flow with matched density and medium viscosity ratio (μo/μw=18.8) in several different flow regimes (core annular flow, oil plugs/bubbles in water and dispersed flow) was performed with the CFD package FLUENT in this study. The volume of fluid (VOF) multiphase flow modeling method in conjunction with the SST k-ω scheme was applied to simulate the oil-water flow. The influences of the turbulence schemes and wall contact angles on the simulation results were investigated for a core annular flow (CAF) case. The SST k-ω turbulence scheme with turbulence damping at the interface gives better predictions than the standard k-ε and RNG k-ε models for the case under consideration. The flow regime of density-matched oil-water flow with medium viscosity ratio, or more generally speaking, the flow regime of fluids where the surface tension is playing a prevailing role is sensitive to the wall contact angle. Simulation results were compared with experimental counterparts. Satisfactory agreement in the prediction of flow patterns were obtained for CAF and oil plugs/bubbles in water. The simulation results also demonstrated some detailed flow characteristics of CAF with relatively low-viscosity oil (oil viscosity one order higher than the water viscosity in the present study compared to the extensively studied CAF with oil viscosity being two to three orders higher than the water viscosity). Different from the velocity profiles of high-viscosity oil CAF where there is sharp change in the velocity gradient at the phase interface with velocity across the oil core being roughly flat, there is no sharp change in the velocity gradient at the phase interface for CAF with relatively low-viscosity oil.
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    Estimation of the power absorbed by the surface of optical components processed by an inductively coupled plasma torch
    (Elsevier, 2016-08-06) Jourdain, Renaud; Castelli, Marco; Yu, Nan; Gourma, Mustapha; Shore, Paul
    The focus of this work is the determination of the heat flux function -thermal footprint- of a plasma jet generated by an inductively coupled plasma (ICP) torch. The parameters of the heat flux function were determined through the correlation of modelling and experimental results. One surface of substrates was exposed to an impinging jet while the temperature changes of the unexposed surface was recorded, analysed and used to derive the parameters of the heat flux function. From a modelling viewpoint, a series of finite element analyses (FEA) were carried out to predict temperatures of substrate surfaces. From an experimental viewpoint, the plasma torch was powered by a 1 kW radio frequency signal generator tuned at 39 MHz. The ICP torch equipped with a De-Laval nozzle impinged the surfaces of selected substrates at atmospheric pressure. Three sets of experiments -static, single pass and multi passes- were carried out to determine and validate the numerical description of the plasma jet. Also this work enabled to determine the maximum intensity of the heat flux distribution and the total power absorbed by substrate surfaces. Finally, the most advanced numerical model was used to assess the effect of a bi-directional raster scanning strategy that was used for the processing of large optical components.
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    Investigation of power dissipation in a collimated energy beam
    (Trans Tech Publications, 2015-08-20) Yu, Nan; Jourdain, Renaud; Gourma, Mustapha; Shore, Paul
    To satisfy the worldwide demand for large ultra-precision optical surfaces, a fast process chain - grinding, polishing and plasma figuring- has been established by the Precision Engineering Institute at Cranfield University. The focus of Cranfield Plasma Figuring team is the creation of next generation of highly collimated energy beam for plasma figuring. Currently, plasma figuring has the capability to shorten processing duration for the correction of metre-scale optical surfaces. High form accuracy can be achieved (e.g. 2.5 hours and 31 nm RMS for 400mm diameter surface). However, it is known that Mid Spatial Frequency (MSF) surface errors are induced when the plasma figuring process is carried out. The work discussed in this paper deals with the characterisation of highly collimated plasma jets delivered by the Inductively Coupled Plasma (ICP) torches. Also a computational fluid dynamics (CFD) model is introduced. This model is used to assess the behaviour of the plasma jet within the best known processing condition. Finally temperature measurement experiments were performed to determine the energy dissipated values that characterise best the ICP torch coil and its De-Laval nozzle.
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    Nature and magnitude of operating forces in a horizontal bend conveying gas-liquid slug flows
    (Elsevier, 2020-02-18) Gourma, Mustapha; Verdin, Patrick G.
    Operating forces and magnitude of loads from gas-liquid slug flows exerted on a horizontally orientated 90o bend are investigated. The distributed forces are either Newtonian, associated with the fluids motion or Configurational, inherent to the internal distributions of the phases. The forces are derived through the conventional balances of mass and linear momentum arising from the volume of fluid (VOF) description of gas-liquid flows. The study uses the integral form of the momentum balance to estimate the operating forces budget. Invoking dynamical time scales separation discloses the connection of the Lamb vector (vortex-force) to the local time rate of momentum. An interesting outcome being an explicit expression for Favre-Reynolds stress that reveals the contribution of void fraction fluctuations in the redistribution of the stress across the interface. Numerical simulations are performed to determine the magnitude of Newtonian loads on bend using a segmented domain technique to represent the fully established slug flow regime. The time-dependent traces of the relevant flow variables such as liquid hold-up, flow rates and resultant forces on the bend are recorded and analysed. Compared to the isotropic component, the deviatoric stresses are shown to have a marginal contribution to the total forces. It is also shown that loading cycles on bends are much higher than slugging cycles; this is an important feature for the structural integrity assessment of pipelines with bends.
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    Numerical simulation of two-phase gas and non-Newtonian shear-thinning fluid flows in pipelines
    (Cranfield University, 2011-08) Jia, Ninghong; Thompson, Christopher P.; Gourma, Mustapha
    The one goal of this research is to present the adaptive mesh refinement (AMR) technique for one dimensional two-phase slug flows. Uniform fine meshes for these long devices are costly and, in general situations, the optimum space discretisation could not be determined a priori. The adaptive mesh refinement (AMR) procedure permits this problem to be remedied by refining the mesh locally, within regions where sharp discontinuities and steep gradients are present. With the appropriate algorithm and data organisation, it helps to reduce CPU time and speed up simulations of flows in long pipes, while preserving accuracy and acceptable execution times. The main objective of this research is to investigate the behaviour of the gas and non-Newtonian shear-thinning fluids in horizontal pipes. Predictions of drag reduction ratio and holdup are presented for the stratified flow of gas and non-Newtonian Ostwald-deWaele liquid. For slug flow regimes, the mechanistic slug unit model is adopted in order to estimate the pressure gradients along the slug unit. The slug unit model is rearranged and reinterpreted as inviscid Burgers’s equation for incompressible phases. For both stratified and slug flow regimes, three dimensional CFD (computational fluid dynamics) simulations were performed in order to compare the drag reduction ratio and pressure gradients. In stratified flows, CFD is also used in an attempt to evaluate the liquid wall friction factor and to compare the obtained values with those given by empirical standard correlations.The estimated pressure gradient and drag reductions are compared with experimental data. Calculations showed an excellent agreement between the simulation and experimental data. Shear thinning effects are also correctly modelled in this work.
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    Power dissipation of an inductively coupled plasma torch under E mode dominated regime
    (MDPI, 2021-07-18) Yu, Nan; Jourdain, Renaud; Gourma, Mustapha; Xu, Fangda; Bennett, Adam; Fang, Fengzhou
    This paper focuses on the power dissipation of a plasma torch used for an optical surface fabrication process. The process utilizes an inductively coupled plasma (ICP) torch that is equipped with a De-Laval nozzle for the delivery of a highly collimated plasma jet. The plasma torch makes use of a self-igniting coil and an intermediate co-axial tube made of alumina. The torch has a distinctive thermal and electrical response compared to regular ICP torches. In this study, the results of the power dissipation investigation reveal the true efficiency of the torch and discern its electrical response. By systematically measuring the coolant parameters (temperature change and flow rate), the power dissipation is extrapolated. The radio frequency power supply is set to 800 W, E mode, throughout the research presented in this study. The analytical results of power dissipation, derived from the experiments, show that 15.4% and 33.3% are dissipated by the nozzle and coil coolant channels, respectively. The experiments also enable the determination of the thermal time constant of the plasma torch for the entire range of RF power.
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    Slug initiation and prediction using high accuracy methods - applications with field data
    (Cranfield University, 2011) Kalogerakos, Stamatis; Thompson, Chris; Gourma, Mustapha
    The sponsoring company of the project is BP. The framework within which the research is placed is that of the Transient Multiphase Flow Programme (TMF-4), a consortium of companies that are interested in phenomena related to flow of liquids and gases, in particular with relevance to oil, water and air. The deliverables agreed for the project were: • validating EMAPS through simulations of known problems and experimental and field data concerning slug flow • introducing numerical enhancements to EMAPS • decreasing computation times in EMAPS • using multi-dimensional methods to investigate slug flow The outcome of the current project has been a combination of new product development (1D multiphase code EMAPS) and a methodological innovation (use of 2D CFD for channel simulations of slugs). These are: • New computing framework composed of: – Upgraded version of 1D code EMAPS – Numerical enhancements with velocity profile coefficients – Validation with wave growth problem – Parallelisation of all models and sources in EMAPS – Testing suite for all sequential and parallel cases – Versioning control (SVN) and automatic testing upon code submission. • Use of 2D CFD VOF for channel simulation with: – Special initialisation techniques to allow transient simulations – Validation with wave growth problem – Mathematical perturbation analysis – Simulations of 92 experimental slug flow cases The cost of uptake of the above tools is relatively small compared to the benefits that are expected to follow, regarding predictions of hydrodynamic slugging. Depending on the timescales involved, it is also possible to use external consultancies in order to implement the solutions proposed, as these are software based and their uptake could be carried out in a small time-frame. Moreover it may not be necessary to build a parallel hardware infrastructure as it is now possible to have easy access to large parallel clusters and pay rates depending on use.
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    Thermal analysis of energy beam using de-laval nozzle in plasma figuring process
    (Cranfield University, 2016-10) Yu, Nan; Jourdain, Renaud; Gourma, Mustapha; Shore, Paul
    In 2012, plasma figuring was proven to be an alternative solution for the fabrication of large scale ultra-precise optical surfaces. Indeed, plasma figuring was successfully demonstrated on a metre class glass surface. The process was exceptionally rapid but residual errors were observed. This thesis addresses this issue by proposing an enhanced tool that provides a highly collimated plasma jet. The enhanced tool is characterized by a targeted material removal footprint in the range 1 to 5 mm FWHM. The energy beam is provided by an Inductively Coupled Plasma (ICP) torch equipped with a De-Laval nozzle. This thesis focuses on characterization and optimisation of the bespoke plasma torch and its plasma jet. Two research investigations were carried out using both numerical and experimental approaches. A novel CFD model was created to analyse and understand the behaviour of high temperature gas in the De-Laval nozzle. The numerical approach, that was based on appropriate profiles of temperature and velocity applied to the nozzle inlet, led to a significant reduction of computational resources. This model enabled to investigate the aerodynamic phenomena observed from the nozzle inlet up to the processed surface. Design rules and the effect of changing nozzle parameters were identified. Sensitivity analysis highlighted that the throat diameter is the most critical parameter. A challenging power dissipation analysis of the plasma torch was carried out. Temperature and flow rate in key components of the torch were measured. Experimental results enabled to calculate the power dissipation values for RF power up to 800 W and for the entire series of designed nozzles. This work enabled to scientifically understand the power dissipation mechanism in the bespoke ICP torches. In addition, by comparing the intensity of the power dissipation values, one nozzle was clearly identified as being more capable to provide a highly efficient plasma jet.
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    Towards better understanding of the Smoothed Particle Hydrodynamic Method
    (Cranfield University, 2003-09) Gourma, Mustapha; Vignjevic, Rade
    Numerous approaches have been proposed for solving partial differential equations; all these methods have their own advantages and disadvantages depending on the problems being treated. In recent years there has been much development of particle methods for mechanical problems. Among these are the Smoothed Particle Hydrodynamics (SPH), Reproducing Kernel Particle Method (RKPM), Element Free Galerkin (EFG) and Moving Least Squares (MLS) methods. This development is motivated by the extension of their applications to mechanical and engineering problems. Since numerical experiments are one of the basic tools used in computational mechanics, in physics, in biology etc, a robust spatial discretization would be a significant contribution towards solutions of a number of problems. Even a well-defined stable and convergent formulation of a continuous model does not guarantee a perfect numerical solution to the problem under investigation. Particle methods especially SPH and RKPM have advantages over meshed methods for problems, in which large distortions and high discontinuities occur, such as high velocity impact, fragmentation, hydrodynamic ram. These methods are also convenient for open problems. Recently, SPH and its family have grown into a successful simulation tools and the extension of these methods to initial boundary value problems requires further research in numerical fields. In this thesis, several problem areas of the SPH formulation were examined. Firstly, a new approach based on ‘Hamilton’s variational principle’ is used to derive the equations of motion in the SPH form. Secondly, the application of a complex Von Neumann analysis to SPH method reveals the existence of a number of physical mechanisms accountable for the stability of the method. Finally, the notion of the amplification matrix is used to detect how numerical errors propagate permits the identification of the mechanisms responsible for the delimitation of the domain of numerical stability. By doing so, we were able to erect a link between the physics and the numerics that govern the SPH formulation.
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    Two-phase slug flows in helical pipes: Slug frequency alterations and helicity fluctuations
    (Elsevier, 2016-07-29) Gourma, Mustapha; Verdin, Patrick G.
    Air-water numerical simulations in the slug flow regime have been performed in horizontal helical pipes and the effects of geometries on the flow regime have been investigated. Depending on the length of the helix, outlet slug frequencies have been reduced with various degrees of efficiency. Correlations between mean tangential velocity and helicity density fluctuations have been identified and investigated qualitatively. These flow fields show smaller time scales than those obtained in volume fractions fluctuations. Shifts observed in the tangential velocity and mean helicity fluctuations to smaller time scales (high frequencies) are associated with regime transitions. For a slug flow undergoing a continuous transition to the annular flow regime, it is shown that the presence of slower (low frequencies) helicity fluctuations is attributed to the variations in the axial velocity. Finally, the analysis of the helicity at gas-liquid interface confirms the presence of the mixing zone at the slug front.