Michael H. Wahl

An Efficient Finite Element-Based Air Bearing Simulator for Pivoted Slider Bearings using Bi-Conjugate Gradient Algorithms

A computationally efficient air bearing simulator-for pivoted slider bearings has been developed that is based on finite element theory and uses bi-conjugate gradient algorithms in conjunction with a sparse matrix storage scheme. The air bearing simulator involves the simultaneous solution of the Reynolds equation and the slider equilibrium equations. The highly nonlinear problem requires the repeated solution of a nonsymmetric system of equations, typically with a large number of unknowns depending on the complexity of the slider geometry. Iterative solvers, such, as the bi-conjugate gradient algorithms used for this study, require significantly less core memory as compared to direct solvers and reduce the solution time if combined with a suitable preconditions. Of the bi-conjugate gradient algorithm/preconditioner combinations implemented, the Bi-CGSTAB algorithm combined with an ILU preconditioner provided the best performance in terms of smooth convergence and computational efficiency. Presented as a ...

Thermodynamic Benefits of Opposed-Piston Two- Stroke Engines

A detailed thermodynamic analysis was performed to demonstrate the fundamental efficiency advantage of an opposed-piston two-stroke engine over a standard four-stroke engine. Three engine configurations were considered: a baseline six-cylinder four-stroke engine, a hypothetical threecylinder opposed-piston four-stroke engine, and a threecylinder opposed-piston two-stroke engine. The bore and stroke per piston were held constant for all engine configurations to minimize any potential differences in friction. The closed-cycle performance of the engine configurations were compared using a custom analysis tool that allowed the sources of thermal efficiency differences to be identified and quantified. The simulation results showed that combining the opposed-piston architecture with the twostroke cycle increased the indicated thermal efficiency through a combination of three effects: reduced heat transfer because the opposed-piston architecture creates a more favorable combustion chamber area/volume ratio, increased ratio of specific heats because of leaner operating conditions made possible by the two-stroke cycle, and decreased combustion duration achievable at the fixed maximum pressure rise rate because of the lower energy release density of the two-stroke engine. When averaged over a representative engine cycle, the opposed-piston two-stroke engine had 10.4% lower indicated-specific fuel consumption than the four-stroke engine.

Simulation of Asperity Contacts at the Head/Disk Interface of Tri-Pad Sliders During Steady-State Flying©

A model for asperity contacts at the head/disk interface has been developed that allows for elastic asperity deformations based on an experimentally derived function for the contact pressure. The model predicts the distributed contact pressure, the contact force and the apparent area of contact on the air bearing surface of the slider. A second approach that assumes entirely rigid asperities is employed to verify the asymptotic behavior of the contact force as the asperity stiffness increases. Further results are reported for the elastic asperity method and the dependence of the contact force, contact area and flying height on various operating conditions is investigated. The comparison between numerical calculations and experimental results shows excellent qualitative agreement. Presented as a Society of Tribologlsts and Lubrication Engineers paper at the ASME/STLE Tribology Conference in San Francisco, California, October 13–17, 1996

Multi-Wavelength Intensity-Based Interferometry for Flexible Head/Medium Interfaces

An intensity-based interferometer has been developed for measuring the steady-stale separation at the flexible head/medium interface using multiple wavelength interferometry. The approach differs from previous methods in that it employs a calibration technique that does not require a mechanical load/unload procedure nor a numerical fit of the calibration envelopes. In addition, the calibration technique improves upon previous methods in its ease of use, making the measurements both more repeatable and more accurate. The measurement range of the multi-wavelength interferometer is on the order of 25–125 nm which covers the minimal head/medium separation of magnetic storage devices currently of interest. For illustration, spacing measurements at the Bernoulli head/flexible disk interface and a linear head/tape interface using the present method have been obtained and compared with previously reported monochromatic light interferometry measurements. Presented as a Society of Tribologists and Lubrication Engin...

Cylinder Cooling for Improved Durability on an Opposed-Piston Engine

The cooling system design for a two-stroke, opposed-piston (OP) engine is substantially different from that of a conventional four-stroke engine as the opposed-piston engine requires efficient cooling at the center of the cylinder where the heat load is highly concentrated. A thermally efficient design ensures engine durability by preserving the oil film at the top ring reversal zone. This is achieved by limiting the surface temperature of the liner to below 270° C at this location. Various water jacket designs have been analyzed with computational fluid dynamics (CFD) using a “discretized” Nusselt number approach for the gas side heat flux prediction. With this method, heat transfer coefficients are computed locally given the flow field of the combustion gases near the liner surface and then multiplied by the local gas/liner temperature difference to generate the heat flux distribution into the cylinder liner. The heat flux is then averaged over the cycle before being applied as a boundary condition to the CFD simulation. The baseline design consists of a simple water jacket with coolant flowing axially from the inlet near the intake port to the outlet near the exhaust port. This approach yields uneven cooling both longitudinally and circumferentially about the cylinder liner. A greatly improved thermal response has been achieved by introducing the coolant at the hot center section of the liner with roughly half of the coolant flowing toward either end of the cylinder. A detailed analysis shows that liner surface temperatures well below 270° C can be achieved for an engine with a power density of 50kW/liter by carefully optimizing the coolant velocities in the center section of the liner.

Numerical simulation of the steady state flying characteristics of a 50% slider with surface texture

The steady state flying characteristics of a 50% taper-flat two-rail slider with discrete surface texture features have been numerically simulated using finite elements. Various shapes and patterns of surface texture features are considered. This investigation analyzes the dependence of flying height and pitch angle on the height and number of the surface texture features and on the linear velocity. The results suggest that the surface texture does not significantly change the flying behavior at nominal flying heights but the influence of surface texture becomes more important at low spacings. >

Accelerated Natural Convergence for Pivoted Slider Bearings

A solution technique is discussed which accelerates the convergence of the pivoted slider bearing problem. The technique is based on a fully populated, three-degrees-of-freedom stiffness matrix for the hydrodynamic lubricating film. This matrix is developed using a unique perturbation method which, when used in conjunction with a finite element formulation of the Reynolds equation, yields the bearing stiffnesses in a computationally efficient manner. The resulting natural convergence of this hydrodynamic stiffness approach is examined for a typical rigid disk magnetic recording slider. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Lahaina, Hawaii, October 16–20, 1994

Finite element simulation of a helical scanner with head/tape contacts

The head/tape interface for a helical scanner is simulated using finite element methods in conjunction with a two-stage zooming technique. The head/tape spacing is investigated as a function of velocity and tension as well as surface roughness of the tape. Contact pressure is calculated using experimentally determined parabolic asperity compliance curves. The results show that head/tape spacing is limited by surface roughness.

Experimental Investigation of the Bernoulli Head/Disk Interface

The Bernoulli head/disk interface is investigated using interferometry as well as the measurement of acoustic emission, normal force, and disk deflection. The dependence of head-to-disk spacing on radial head position and velocity for upper and lower heads is measured by monochromatic light interferometry. Acoustic emission measurements are used to study intermittent contacts between head and flexible disk. The normal force between the head and the disk h measured and is found to correlate with the acoustic emission signal. The disk deflection characteristics show that the normal force at the head/disk interface is related to the vertical displacement of the head. The normal force at the head/disk interface is determined from the relative disk deflection using the disk stiffness in the normal direction and compared with the measured force. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in San Diego, California, October 19–21, 1992

Concepts and Limitations of Using Optical Profilometry for Air Bearing Simulations of Hard Disk Sliders

The topography of hard disk sliders is commonly measured with optical profilometers that produce a regularly spaced matrix of topography data corresponding to the pixel array of the CCD camera. This paper explores a new concept that combines optical profilometry and air bearing simulations to achieve an accurate method for predicting fly height early in the manufacturing process. Various sources of error in the topography measurement are analyzed with regards to their impact on the accuracy of air bearing simulations. The findings are illustrated by a feasibility study based on numerical simulations of a slider with a negative pressure air bearing design. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Toronto, Ontario, Canada, October 26–28, 1998