T.A. Osman

Bio: T.A. Osman is an academic researcher from Cairo University. The author has contributed to research in topics: Bearing (mechanical) & Reynolds equation. The author has an hindex of 11, co-authored 14 publications receiving 436 citations.

Microstructure and Mechanical Properties of MWCNTs Reinforced A356 Aluminum Alloys Cast Nanocomposites Fabricated by Using a Combination of Rheocasting and Squeeze Casting Techniques

Abstract: A356 hypoeutectic aluminum-silicon alloys matrix composites reinforced by different contents of multiwalled carbon nanotubes (MWCNTs) were fabricated using a combination of rheocasting and squeeze casting techniques. A novel approach by adding MWCNTs into A356 aluminum alloy matrix with CNTs has been performed. This method is significant in debundling and preventing flotation of the CNTs within the molten alloy. The microstructures of nanocomposites and the interface between the aluminum alloy matrix and the MWCNTs were examined by using an optical microscopy (OM) and scanning electron microscopy (SEM) equipped with an energy dispersive X-ray analysis (EDX). This method remarkably facilitated a uniform dispersion of nanotubes within A356 aluminum alloy matrix as well as a refinement of grain size. In addition, the effects of weight fraction (0.5, 1.0, 1.5, 2.0, and 2.5 wt%) of the CNT-blended matrix on mechanical properties were evaluated. The results have indicated that a significant improvement in ultimate tensile strength and elongation percentage of nanocomposite occurred at the optimal amount of 1.5 wt% MWCNTs which represents an increase in their values by a ratio of about 50% and 280%, respectively, compared to their corresponding values of monolithic alloy. Hardness of the samples was also significantly increased by the addition of CNTs.

Static Performance of Finite Hydrodynamic Journal Bearings Lubricated by Magnetic Fluids with Couple Stresses

Abstract: Based upon the Stokes micro-continuum theory, the problem of lubrication of finite hydrodynamic journal bearing lubricated by magnetic fluids with couple stresses is investigated. By taking into account the couple stresses due to the microstructure additives and the magnetic effects due to the magnetization of the magnetic fluid, modified Reynolds equation is obtained. The effects of couple stresses are studied by defining the couple stress parameter L that can be considered as a measure of the chain length of the additive molecule. The magnetic effects of the magnetic fluid are investigated by the magnetic coefficient γ. Using the finite-difference technique and for different values of couple stress parameter and magnetic coefficient, the Reynolds equation is solved, and pressure distributions are obtained. The bearing static characteristics namely load carrying capacity, attitude angle, friction coefficient, and side leakage flow are determined. The results indicate that the influence of couple stresses and magnetic effects on the bearing characteristics are significantly apparent. It is concluded that fluids with couple stresses are better than Newtonian fluids. The improvement of the bearing characteristics is enhanced if the magnetic effects are present.

Influence of friction stir welding parameters on metallurgical and mechanical properties of dissimilar AA5454–AA7075 aluminum alloys

Abstract: Friction Stir Welding (FSW) is a solid-state welding process used for welding similar and dissimilar materials. FSW is especially suitable to join Al alloys sheets, and this technique allows different material couples to be welded continuously. In this study dissimilar joints between aluminum alloy (AA5454) and aluminum alloy (AA7075) produced by friction stir welding, to optimize these parameters and determine which of them is significant by using Taguchi L16 optimization method. Seven parameters at two levels were selected in this study. The selected parameters are tool rotational speed, traverse speed, pin profile (based on taper angle), the ratio between shoulder diameter (D) and pin diameter (d) (D/d ratio), tool tilt angle, plunge depth, and base metal location (weld location)). The ultimate tensile strength (UTS) and ductility are considered as the mechanical properties of the dissimilar joints. Then, mathematical models are built for ultimate tensile strength and ductility as a function of significant parameters/interactions using response surface methodology. In addition, the microstructures of the optimum joint and the weakest joint are studied using optical microscopy. The results of this work showed that the rotational speed, traverse speed, D/d ratio and plunge depth are significant parameters in determining UTS (mean, signal to noise ratio (S/N)) at different confidence levels, but pin profile, location of base metal and tool tilt angle are insignificant parameters at any confidence levels. The traverse speed has the highest contribution to the process for UTS about 18.5% and 16.9% for S/N ratio and mean, respectively. The accuracy of the models according to the UTS is 97.6% and 99.5% for mean and S/N ratio, respectively. The maximum joint efficiency, compared to the strength of the AA5454, is 85.3%.

Different magnetic models in the design of hydrodynamic journal bearings lubricated with non-Newtonian ferrofluid

Abstract: This work is concerned with theoretical study of hydrodynamic journal bearings lubricated with ferrofluids exhibiting non-Newtonian behavior Based on the momentum and continuity equations for ferrofluid under an applied magnetic field, a modified Reynolds equation has been obtained Assuming linear behavior for the magnetic material of the ferrofluid, the magnetic force was calculated The Reynolds equation has been derived to be able to apply to any magnetic field distribution model Using different magnetic field models, the equation has been solved numerically by the finite-difference technique with appropriate iterative technique and pressure distributions have been obtained The boundary shape of the load-carrying active regions (positive-pressure regions) and cavitation regions (zero-pressure regions) could be then determined The solution renders the bearing performance characteristics, namely: load-carrying capacity, attitude angle of the journal center, frictional force at the journal surface, friction coefficient and bearing side leakage The results indicated that the flow-behavior index has a large effect on the bearing performance When the bearing operates at high eccentricity ratios, the increase of flow-behavior index gives higher load capacity, lower attitude angle, higher frictional force, lower friction coefficient and higher side leakage At low eccentricity ratios where the magnetic effects are significant, the effect of the flow-behavior index depends mainly on the magnetic field distribution model used

Magnetic fluid rheology and flows

Abstract: Major recent advances Magnetic fluid rheology and flow advances in the past year include: (1) generalization of the magnetization relaxation equation by Shliomis and Felderhof and generalization of the governing ferrohydrodynamic equations by Rosensweig and Felderhof; (2) advances in such biomedical applications as drug delivery, hyperthermia, and magnetic resonance imaging; (3) use of the antisymmetric part of the viscous stress tensor due to spin velocity to lower the effective magnetoviscosity to zero and negative values; (4) and ultrasound velocity profile measurements of spin-up flow showing counter-rotating surface and co-rotating volume flows in a uniform rotating magnetic field. Recent advances in magnetic fluid rheology and flows are reviewed including extensions of the governing magnetization relaxation and ferrohydrodynamic equations with a viscous stress tensor that has an antisymmetric part due to spin velocity; derivation of the magnetic susceptibility tensor in a ferrofluid with spin velocity and its relationship to magnetically controlled heating; magnetic force and torque analysis, measurements, resulting flow phenomena, with device and biomedical applications; effective magnetoviscosity analysis and measurements including zero and negative values, not just reduced viscosity; ultrasound velocity profile measurements of spin-up flow showing counter-rotating surface and co-rotating volume flows in a uniform rotating magnetic field; theory and optical measurements of ferrofluid meniscus shape for tangential and perpendicular magnetic fields; new theory and measurements of ferrohydrodynamic flows and instabilities and of thermodiffusion (Soret effect) phenomena.

Application of Computational Fluid Dynamics and Fluid–Structure Interaction Method to the Lubrication Study of a Rotor–Bearing System

Abstract: Computational methods were used to analyse the elasto-hydrodynamic lubrication of a complex rotor–bearing system. The methodology employed computational fluid dynamics (CFD), based on the Navier–Stokes equation and a fluid–structure interaction (FSI) technique. A series of models representing the system were built using the CFD–FSI methodology to investigate the interaction between the lubrication of the fluid film, and elastic dynamics of the rotor and journal bearing. All models followed an assumption of isothermal behaviour. The FSI methodology was implemented by setting nodal forces and displacements to equilibrium at the fluid–structure interface, therefore allowing the lubrication of the fluid and the elastic deformation of structures to be solved simultaneously. This is significantly different to the more common techniques—such as the Reynolds equation method—that use an iterative solution to balance the imposed load and the force resulting from the pressure of the fluid film to within a set tolerance. Predictions using the CFD–FSI method were compared with the results of an experimental study and the predictions from an ‘in-house’ lubrication code based on the Reynolds equation. The dynamic response of the system was investigated with both rigid and flexible bodies for a range of different bearing materials and dynamic unbalanced loads. Cavitation within the fluid film was represented in the CFD–FSI method using a simplified phase change boundary condition. This allowed the transition between the liquid and vapour phases to be derived from the lubricant’s properties as a function of pressure. The combination of CFD and FSI was shown to be a useful tool for the investigation of the hydrodynamic and elasto-hydrodynamic lubrications of a rotor–bearing system. The elastic deformation of the bearing and dynamic unbalanced loading of the rotor had significant effects on the position of its locus.