Dynamic instability of thermal-flying-height-control sliders at touchdown

With the wide application of thermal flying-height control (TFC) technology in the hard disk drive industry, the head-disk clearance can be controlled to as low as ~1?nm. At this ultra-low clearance, the air bearing slider is subject to relatively large interfacial forces, and it experiences more complicated dynamics, compared with the flying case. In this study we conduct a numerical analysis to investigate the dynamics of TFC sliders during touchdown. The general trend of the slider’s motion predicted by the numerical simulation qualitatively agrees with experimental findings. The touchdown process begins with a slight intermittent contact between the slider’s trailing edge and the disk, followed by a partial slider-disk contact at the trailing edge accompanied by a large pitch motion at the 1st air bearing mode; this pitch motion gets suppressed and the slider comes into stable sliding on the disk as the protrusion is further increased.     

Tribological testing of sliders on discrete track media and verification with numerical predictions

“Flyability” tests were conducted with sliders designed for discrete track recording disks. Laser Doppler vibrometry and acoustic emission were used to characterize the dynamics of the sliders as a function of discrete track parameters. Lubricant depletion was observed depending on the slider nominal flying height. Comparison of experimental results with numerical predictions showed good qualitative agreement.     

A numerical investigation of central binary collision of droplets

The paper presents a numerical investigation of the central collision of two equal-sized droplets in a gaseous phase. The investigation is based on the numerical solution of the Navier-Stokes equations in their axi-symmetric form using the finite volume technique. The Volume of Fluid Method (V.O.F) is employed for tracking the liquid-gas interface. An adaptive local grid refinement technique developed recently is used in order to increase the resolution around the interface. By using two V.O.F indicator functions the identity of each droplet is preserved and can be detected after droplet contact until coalescence. The results are compared with available experimental data and provide a very detailed picture of the collision process, the ligament formation and dimensions, the pinch off mechanism and the creation of the satellite droplet. The conversion of the droplet’s kinetic energy to the surface energy and vise versa, the energy viscous dissipation as well as the maximum deformation of the droplets are also evaluated.     

A numerical investigation on AC electrowetting of a droplet

We numerically analyze the AC electric field around a droplet placed on an insulator-covered electrode. The time-averaged effective electrical wetting tension, which is a function of AC frequency, is computed by integrating the Maxwell stress. The computed wetting tension is compared with the experimental result converted from the separately obtained contact-angle data. There is a good agreement between the two results at a low-frequency range and a qualitative agreement at a high-frequency range. Interestingly, the numerical results show that the electric-field strength decreases remarkably in the insulating layer near the TCL as the AC frequency increases. This decrease may account for the delay of the dielectric breakdown of an insulating layer in the AC case, which could be related to the contact-angle saturation phenomenon.     

A numerical investigation of Benard convection using finite elements

The results of numerical calculations based on the finite element for the two dimensional Benard convection are presented. The fluid is confined between two horizonal solid walls and is subjected to a vertical temperature gradient. Aspect ratios of 10.0 and 10.5 are used. The effect of the hydrodynamic boundary conditions on the vertical walls on the number and the size of cells is demonstrated. Values of average Nusselt number are presented for various Rayleigh numbers ranging from 1710 to 10,000. The conduction mode of heat transfer is replaced by the convection mode at a Rayleigh number of 1710, with the latter mode becoming more prominent at higher Rayleigh numbers. The calculation of the length of each cell and the heat flux compare very well with the values reported in the literature.     

A parameter identification method for thermal flying-height control sliders

This paper employs higher order frequency response functions to describe the closed-form solution of nonlinear dynamics of MEMS thermal actuated flying-height control (TFC) slider, and employs the measured acoustic emission signal of a TFC slider during touch down as its vibration approximation to correlate with the derived closed-form solution for parameter identification. It is shown that during slider touchdown when heater power is increased gradually, the slider performs weak nonlinear vibrations in early transition phase, and exhibits linear vibration in the following steady sliding phase. These results are useful for the clarification of complicated dynamics and the designs of active sliders in sub-nanometer clearance regime.     

A numerical investigation of turbulent flows in a spanwise rotating channel

A numerical investigation of fully developed turbulent flow in a spanwise rotating channel is performed to study turbulence characteristics subject to system rotation. The work provides insight into several salient features of the spanwise rotating turbulent channel flows, including the near-wall vortical structures, turbulence energy cascade and redistribution, and vortex stretching. The influence of system rotation on the near-wall vortical structures is investigated based on the vorticity fluctuations and their probability density functions (PDF). The properties of the Lamb vector fluctuation and the corresponding PDF are examined to reveal the effect of rotation on the turbulence energy cascade and production in the rotating channel. The budgets of Reynolds stresses and fluctuating enstrophy are analyzed to elucidate the role of the Coriolis force on turbulence energy redistribution between the streamwise and wall-normal directions and the mechanisms of vortex stretching for the generation of the vorticity fluctuations near the pressure and suction walls.     

A numerical and experimental investigation of the buckling behavior of composite panels

The experimental buckling behavior of axially compressed, fiber-reinforced, circular cylindrical panels is compared with numerical predictions obtained from an energy-based, finite difference, computer program. Test specimens were clamped along the curved edges and either unsupported or simply supported along the straight edges.Numerical predictions were obtained for a perfect cylindrical geometry and for small axially symmetric initial imperfections. Agreement between experimental and numerical buckling behaviors are in good agreement for both types of boundary conditions.     

A numerical investigation of direct and indirect closed-loop architectures for estimating nonminimum-phase zeros

ABSTRACT

This paper presents a numerical investigation of three direct architectures and three indirect architectures for identifying a plant operating in closed loop. Motivated by adaptive control of systems with nonminimum-phase (NMP) zeros and taking advantage of the fact that zeros are not moved by feedback, the performance metric is the accuracy of the estimates of the NMP zeros of the plant. Assuming known plant order, single-input, single-output, infinite-impulse-response models are constructed in the presence of process and sensor noise. Least squares provides the baseline estimation technique, and prediction error methods are used to account for correlation between the model input and noise. The goal is to compare the accuracy of the NMP-zero estimates obtained from each method and for each architecture.     

Nano-texturing of magnetic recording sliders via laser ablation

To increase the storage density of hard disk drives, the flying height of the slider needs to be reduced to <10 nm. This requires super-smooth surfaces of the disk and slider. As the roughness decreases, stiction and adhesion are found to increase substantially leading to failures of the head/disk interface. Texturing the slider surface is a well-known approach to this issue. In this study we investigated laser ablation as a potential process for texturing magnetic recording sliders. It was found that straight laser machining caused unwanted re-deposition of material. These deposits could be significantly reduced by using a chemical etching enhanced laser process.     

A numerical analysis method for layered plate with different material constants

In this paper, a new numerical analysis method for a three layered plate in which the middle layer has a smaller elasticity coefficient compared with upper and lower layers is presented. The method can investigate the Brazier effect in plate structures. One of numerical models is a three layered rectangular plate due to the pure bending. For the model, the Brazier effect is explained from the following view points: (a) the reason for the occurrence of the Brazier effect in a rectangular plate; (b) factors which cause the Brazier effect; and (c) quantitative estimate of the Brazier effect by approximate method. The another one is a three layered square plate subjected to uniformly distributed load. For the model, the finite element method is introduced. Analysis flow and various analysis results are presented.     

A numerical investigation of fluid flow over a rotating cylinder with cross flow oscillation

This paper studies a two-dimensional incompressible viscous flow past a rotating cylinder with cross flow oscillation using a finite element method based on the characteristic based split (CBS) algorithm to solve governing equations including full Navier–Stokes and continuity equations. Dynamic unstructured triangular grid is used employing lineal and torsional spring analogy which is coupled with the solver by an Arbitrary Lagrangian–Eulerian (ALE) formulation. After verifying the accuracy of the numerical code, simulations are conducted for the flow past a rotating cylinder with cross flow oscillation at moderate Reynolds numbers of 50, 100, and 200 considering different non-dimensional rotational speeds based on the free-stream velocity in the range 0–2.5, and various oscillating amplitudes and frequencies. Effects of the oscillation and rotation of the cylinder on the vortex shedding both in lock-on and non-lock-on regions, the mean drag and lift coefficients, and the Strouhal number are investigated in detail. It is found that similar to the fixed cylinder beyond a critical non-dimensional rotational speed the vortex shedding is highly suppressed. In addition, by increasing the rotational speed of the cylinder, the lift coefficient increases while decreasing the drag coefficient. However, in the vortex lock-on region both the lift and the drag coefficients increase significantly.     

A numerical investigation of the stability of expanding liquid sheets and the influence of boundary conditions

Incompressible flow solutions are found numerically for a radially expanding liquid sheet in order to confirm analytical results for inviscid flow and to investigate viscous and non-linear effects. An hp-finite element method is used to perform the numerical simulations. In our unsteady simulations, we observe that forced sinuous pulses cause two different speed waves to travel downstream for Weber numbers greater than one. We also witness wave deceleration for Weber numbers approaching one, confirming the predictions of inviscid linear stability analysis. Comparisons are also made to theoretical predictions of the radius where the sheet becomes unstable. To determine the critical radius, the inlet Weber number is reduced until the theoretical critical radius is within the simulated domain. Surprisingly, instead of leading to breakup, this causes the sheet to change from a stable symmetric shape to a stable asymmetric shape. The transition between these shapes occurs by both supercritical and subcritical bifurcations when the Weber number based on the sheet thickness approaches one, in agreement with the theoretical work of Taylor. The absence of breakup in our simulations appears to be a direct result of allowing the interface to span the entire domain. To verify this, we examine the dependence of the solutions on domain aspect ratio, shape, and exit boundary conditions. No boundary conditions are found that allows the sheet to break-up.     

A numerical methodology to support the development of liquid adhesive spreading patterns on display bonding process

A numerical methodology was implemented to support the development of adhesive spreading patterns for the Optical Bonding (OB) material on display bonding process, in order to reduce the experimental trial and error effort, being this iterative process done mainly through numerical simulations. These simulations were performed using the commercial software Abaqus, more specifically the Abaqus/Explicit module, which includes the Coupled Eulerian-Lagrangian method (CEL), used in cases where extreme deformations of the material are observed. This way, it was possible to obtain a methodology capable of predicting the spreading behaviour of a predefined OB material pattern. These results were initially compared to experimental results for a simple validation case and later for two more complex real cases. The numerical approach developed provided results very close to what was observed in experimental tests. This methodology can be a useful tool to optimize OB material patterns for display bonding process, but the simulation time needed is still a concern to have in mind.     

Six-DOF vibrations of partial contact sliders in hard disk drives

A six-degree-of-freedom slider dynamic simulator is developed to analyze the slider’s motion in the vertical, pitch, roll, yaw, length and width directions. The modified time-dependent Reynolds equation is used to model the air bearing and a new second order slip model is used for a bounded contact air bearing pressure. The simulator considers the air bearing shear acting on the air bearing surface and the slider–disk contact and adhesion. Simulation results are analyzed for the effects of the disk surface micro-waviness and roughness, skew angle, slider–disk friction and micro-trailing pad width on the vertical bouncing, down-track and off-track vibrations of a micro-trailing pad partial contact slider.     

Integration of different numerical techniques in shape optimization

Shape optimal design is the synthesis of structural analysis and mathematical programming (MP) combined with methods of computer aided geometric design (CAGD) and behaviour sensitivity analysis (SA). The main aspect in optimal shape design is the integration of these disciplines. At the same time an efficient and well-posed model must be formulated for the overall optimization problem. The present contribution deals with the hierarchy of the different modelling levels and numerical techniques of the design process. It will be shown for the frequency tuning of a bell how the individual disciplines can be identified step by step in the optimization procedure of the program CARAT (Computer Aided Research and Analysis Tool).Presented at NATO ASI Optimization of Large Structural Systems, Berchtesgarden, Sept. 23 – Oct. 4, 1991     

An investigation of the fault-proneness of clone evolutionary patterns

Two identical or similar code fragments form a clone pair. Previous studies have identified cloning as a risky practice. Therefore, a developer needs to be aware of any clone pairs in order to properly propagate any changes between clones. A clone pair may experience many changes during the creation and maintenance of a software system. A change can either maintain or remove the similarity between clones in a clone pair. If a change maintains the similarity between clones, the clone pair is left in a consistent state. When a change makes the clones no longer similar, the clone pair is left in an inconsistent state. The set of states and changes experienced by clone pairs over time form an evolution history known as a clone genealogy. In this paper, we examine clone genealogies to identify fault-prone “patterns” of states and changes. We explore the use of clone genealogy information in fault prediction. We conduct a quasi-experiment with four long-lived software systems (i.e., Apache Ant, ArgoUML, JEdit, Maven) and identify clones using the NiCad and iClones clone detection tools. Overall, we find that the size of the clone can impact the fault-proneness of a clone pair. However, there is no clear impact of the time interval between changes to a clone pair on the fault-proneness of the clone pair. We also discover that adding clone genealogy information can increase the explanatory power of fault prediction models.     

Tribological impact of touchdown detection on bit patterned media robustness

Touchdown detection by thermal-flying height control (TFC) has been implemented to calibrate flying height (FH) of magnetic sliders for all hard disk drives. For bit patterned media (BPM) to be successful as a revolutionary technology to further increase recording areal density, it must experience the touchdown process with similar robustness as conventional continuous media. Here we numerically study the tribological impact of TFC touchdown detection on the continuous media and unplanarized BPM by three-dimensional (3D) transient finite-element models with the frictional heating and thermal-elastic-plastic materials included. Our results demonstrate that the continuous media exhibits no plastic deformation due to the TFC touchdown with an over-push as large as 2 nm, whereas the plastic strain of the BPM may reach 3 % at higher sliding velocities, and it exists over a wide range of bulge radius and disk velocity. Such plastic deformation can lead to permanent media damage and data loss. Besides, the temperature rise of the BPM (~27 K) is approximately 1.3 times of that of the continuous media (~21 K), and may have to be considered when designing a robust head-disk interface for BPM. Although planarization may improve slider’s flyability performance, our analysis shows that planarizing BPM with SiO₂ deteriorates the tribological robustness of the media in particular at a high disk velocity probably due to the inhomogeneous composition and mismatch of material properties between the filling material and recording material. Hence extreme caution must be exercised when choosing a filling material.