Pantelis G. Nikolakopoulos

CFD simulation of magnetorheological fluid journal bearings

Abstract Magnetorheological fluid journal bearing can be controlled by a steady magnetic field doing that very effective for attenuating and controlling the performance of the rotor bearing systems. An integrated simulation study, of a magnetorheological (MRF) fluid journal bearing, via computational fluid dynamics (CFD) and finite element method (FEM) is presented in this paper. The journal bearing characteristics such as, eccentricity, attitude angle, oil flow and friction coefficients are calculated and presented as functions of the magnetic field, and L/D bearing ratios. A specific procedure in order to simulate an MRF bearing operated in high eccentricity ratios is also presented and the meshing requirements are discussed.

Design of an active hydromagnetic journal bearing

An active hydromagnetic journal bearing is designed and presented in this study, which is a new and innovative type of hybrid journal bearing. The proposed new mechatronic smart device has a common bearing shell that contains the two parts of the bearing, which is an attempt to combine the advantages of both types of bearings to overcome their drawbacks and give rise to new possibilities for instability control, increased load-carrying capacity, diagnostics of the rotor system, etc. The active hydromagnetic journal bearing should operate as either a hydrodynamic, active magnetic or hybrid journal bearing, depending on the needs of the rotating machinery. The hydrodynamic lubrication is developed in the inner bearing surface, and the electromagnets of the magnetic bearing are placed outside its outer surface. The active hydromagnetic journal bearing is designed with the appropriate clearance, so that both the hydrodynamic and magnetic bearings can operate. The suitable operational regions for the active hydromagnetic journal bearing are calculated and presented as a combination of the optimum ratio between the air gap and the clearance. Additionally, a simulation code using the two-dimensional finite element method (ANSYS) is developed to simulate the performance of the magnetic component of the active hydromagnetic journal bearing, and the ANSYS software is used to obtain a solution for the hydrodynamic pressure field. The dynamic characteristics of the hybrid journal bearing in terms of the stiffness and damping coefficients versus the Sommerfeld number are calculated and presented. In the hybrid bearing operation, the corresponding stiffness and damping coefficients are used to simulate the hydromagnetic bearing dynamic behaviour. Each hybrid coefficient consists of two parts; the hydrodynamic and the magnetic part. When the operation is purely hydrodynamic, the magnetic parts of the dynamic coefficients are zero and vice versa, whereas both parts are present in the hybrid operation mode.

Rotordynamic Analysis of a Shaft Using Magnetorheological and Nanomagnetorheological Fluid Journal Bearings

ABSTRACT The rotordynamic behavior of a system supported by journal bearings is critical to its reliability. A suitable method of control of the orbital motion of a shaft in a journal bearing is the use of smart lubricants, in effect fluids with controllable physical properties. There are various categories of smart lubricants. One class of smart lubricants, magnetorheological fluids, are produced as a dispersion of magnetic particles in a carrier fluid, which is usually a conventional lubricant. These particles form chains under the influence of a magnetic field, which hinder the lubricant flow, thus changing its apparent viscosity. Magnetorheological fluids (MRFs) exhibit high yield stress, low delay of response, and relatively low friction while not in their active state. A subcategory of MRFs, nanomagnetorheological fluids (NMRFs) with particle size on the nanometer scale, exhibits lower yield stress than MRFs but display high viscosity. The effect of the MRFs and NMRFs on the rotordynamic behavior of a shaft is calculated through a combined finite element and computational fluid dynamics analysis. Though the MRF with the specific geometrical configuration of the bearing is not sufficiently activated and therefore does not improve the performance of the magnetorheological journal bearing, the NMRF has the ability of limiting up to 82% the amplitude of the vibrations of the shaft.

Experimental and Analytical Investigations of Dynamic Characteristics of Magnetorheological and Nano Magnetorheological Fluid Film Journal Bearing

The integrity and reliability of a rotor depends significantly on the dynamic characteristics of its bearings. Bearing design has been altered in many ways in order to achieve improvement in terms of damping and stiffness. A promising field in terms of vibrations control and overall performance improvement for the journal bearings is the use of smart lubricants. Smart lubricants are fluids with controllable properties. A suitable excitation, such as an electric or a magnetic field, is used as a means of smart fluid properties control. Magnetorheological (MR) fluids consist one category of lubricants with controllable properties thanks to magnetic particles inside the fluid volume. In this case of material, a magnetostatic field affects the apparent viscosity of the fluid by aligning the magnetic particles into chains. In this work a magnetorheological fluid is produced. A magnetorheological fluid film bearing was constructed, capable of exciting the magnetorheological fluid. These bearing performances are examined experimentally and its dynamic properties are evaluated using an impact excitation method for a SAE-10W lubricant as well as with the produced magnetorheological fluid both in its active and in its inactive state.Copyright

Temperature Influence on the Behavior of a Magnetorheological Fluid Journal Bearing

Active control of vibrations is an important capability of bearings using magnetorheological fluids as lubricants. Magnetorheological fluids are a suspension of micron sized iron particles in a carrier fluid, usually a mineral oil. These particles, polarized under the influence of a magnetic field, form chains inside the lubricant volume, they hinder the flow of the fluid, change its apparent viscosity and offer active control on the available damping of the bearing. Magnetorheological fluid’s apparent viscosity relies heavily on the viscosity of the base fluid. The base fluid’s viscosity depends on temperature. On the other hand the existence of particles is a factor that may influence the lubricant’s temperature. In this paper the dynamic characteristics of a journal bearing lubricated with magnetorheological fluids are investigated for a range of temperature and load conditions.

On the damages detection in aluminium beam using Hilbert-Huang transformation

Purpose – Continuous on-line monitoring of structural integrity are in priority in many engineering fields such as aerospace, automotive, civilian structures, and industrial applications. Of all these possible applications, the aerospace industry has one of the highest payoffs. Possible damage can lead to catastrophic failures and costly inspections. On the other hand, processing a signal consists of important feature from sensors measurements to reach the considered target. Typically, the sensors translate a physical phenomenon from one or many sources in temporal variations or in spatial variations. The purpose of this paper is to investigate damages, in terms of suddenly screw removal or in a small cut, detection in vibrating (clamped-free) aluminum beam structures using the empirical mode decomposition (EMD) method along with the Hilbert-Huang transformation (HHT). The perspective is to identify very small defects in real aircraft structures. Design/methodology/approach – The proposed method deals wit...

Simulation of Static Performance of Air Foil Bearings Using Coupled FEM and CFD Techniques

The main objective of the current work is to determine a relationship between the top and bump foil’s geometry and load carrying capacity in a journal compliant air foil bearing. Static and steady state operation is assumed throughout the analysis. A finite element model is adopted in order to investigate the operational characteristics of the specific bearing. Bump foil’s elastic behavior is modeled using two node linear spring elements. During fluid analysis, compressible viscous steady state Navier-Stokes equations are numerically solved. The material used during the structural analysis is Inconel X750 and it is assumed that it has linear and elastic behavior. Thermal effects are not taken under consideration. Constant ambient pressure is applied at the free faces of the fluid as well as no slip condition at the surface of the fluid that faces the top foil. CFD and structural models are solved separately. At the beginning of the analysis CFD problem is solved with the assumption that the top foil has not yet been deformed. After the solution of the CFD problem, the pressure distribution at the surface of the fluid that faces the top foil is applied at the top foil and then the structural problem is solved. Consequently the deflections of the top foil are applied on the corresponding surface of the CFD model and the algorithm continues until convergence is obtained. As soon as the converged solution for the pressure distribution is obtained, numerical integration is performed along the surface of the bearing in order to calculate its load carrying capacity. The same procedure is repeated for different values of bump foil thickness, height and pitch in order to define a pattern that describes the bearing’s load carrying capacity as a function of the geometric parameters of the structural problem. Static bearing performance characteristics, such as pressure distribution, bump foil deflection and load capacity are calculated and presented. Furthermore fluid film thickness, top foil deflection and fluid pressure are investigated as functions of the bearing angle as well as load carrying capacity as a function of the bump and top foil stiffness. Finally, a simple thermal analysis is incorporated in order to estimate the temperature rise in the CFD domain due to viscous heat.© 2013 ASME

A Tribological Study of Partial-Arc Bearings With Egg-Shaped Surface Texture for Microturbine Applications

Recent research has demonstrated that proper use of texture geometries can improve the performance of journal and thrust bearings. In particular, for journal bearings, elliptical and egg-shaped texture patterns appear as promising candidates for improving bearing performance, in terms of load carrying capacity and friction coefficient. The expected advantages should also hold for partial-arc bearings, which, for small scale – high speed applications, are characterized by significant advantages, in comparison to full bearings. In the present paper, a tribological study of partial-arc journal bearings with periodic egg-shaped texture applied on the stator surface is presented. Computational Fluid Dynamics (CFD) simulations are performed and processed to yield the bearing performance indices. Here, the bearing geometry and the texture characteristics are defined parametrically; a wide range of bearing designs is thus accounted for. Flow simulations are based on the numerical solution of the Navier-Stokes equations for incompressible isothermal flow. The effects of dimple shape, bearing eccentricity, bearing arc angle, and slenderness ratio on the bearing performance are investigated. The present results demonstrate that a substantial improvement of journal bearing performance, especially in terms of the friction coefficient, in comparison to that of smooth bearings, is feasible.Copyright