M.M. Khonsari

On the Prediction of Cavitation in Dimples Using a Mass-Conservative Algorithm

The Floberg―Jakobsson―Olsson cavitation theory is implemented using a mass-conservative algorithm to accurately predict the behavior of cavitation in flat surfaces enhanced with dimples. The multigrid method is used to accelerate the convergence speed. Comparison is made on different cavitation theories. The results reveal that the load-carrying capacity of dimple-enhanced surfaces is limited under the simulated conditions.

Stability Boundary of Non-Linear Orbits Within Clearance Circle of Journal Bearings

The transient, nonlinear study of journal bearing stability requires tracking the locus of the shaft center as a function of time. It is shown that there exists a boundary within the bearing clearance circle outside of which any initial condition would yield an unstable orbit. This is shown to be the case for operating speeds that are well below the threshold of instability according to the linearized stability analysis.

An EHD Inlet Zone Analysis Incorporating the Second Newtonian

An apparently new approach to the solution of EHD film thickness is presented for line contact. This approach can readily accept non-Newtonian response as it is not based upon the Reynolds equation. Thermal effects and sliding may also be incorporated, although for large departure from Newtonian the pressure criterion should be reexamined

Effect of Dimple’s Internal Structure on Hydrodynamic Lubrication

An experimental and numerical study of the lubrication effectiveness of textured surfaces with dimples possessing an internal structure is presented. A fabrication method to create dimples with different profiles is described. Thrust bearing-like specimens are textured with dimples of three different internal structural shapes: rectangle (R), oblique triangle (T1) and isosceles triangle (T2). The load-carrying capacity test results show that the dimple internal structure has a profound influence on the load-carrying capacity. Specifically, cylindrical dimples with rectangular cross-sectional shape produce more load-carrying capacity than those with triangular profile. Also, presented are the results of a mass-conservative algorithm that predicts the performances of dimples with different internal structural shapes. The simulation results indicate that the higher load-carrying capacity produced by the cylindrical dimple is due to the better pressure building capacity as a result of the converging step shape.

A Modification of the Switch Function in the Elrod Cavitation Algorithm

The implementation of the mass-conservative cavitation algorithm, as derived by Elrod, requires an automatic setting of a binary switch function that delineates the full-film and cavitation zones. Abrupt changes associated with this function often causes convergence and instability issues. A method is that which improves numerical instability and overcomes the convergence issues caused by the conventional binary switch function. Two test cases are presented, which show that by using the new switching algorithm together with a successive over-relaxation solver, the Elrod cavitation algorithm converges up to 61% faster while it is less prone to numerical instabilities.

On the Prediction of Running-In Behavior in Mixed-Lubrication Line Contact

A model is presented, which enables one to predict the running-in performance of the rolling/sliding surfaces subjected to mixed-lubrication line contact. The load-sharing concept was used, in which it is assumed that both the fluid film and the asperities contribute in carrying the imposed load. The plastic deformation of asperities during the running-in is taken into consideration. In the application of the load-sharing method, it is often assumed that asperity heights have a Gaussian distribution. This assumption has been relaxed in this model. Prediction results for the variation in the arithmetic average of asperity heights (R a ) during the running-in period for contact of two rollers are compared with published experimental data. Also presented are the results for the variation in wear volume, wear rate, and friction coefficient during the running-in period. The effect of surface pattern, speed, and load on the running-in behavior is studied. The steady-state wear rate for different surface patterns calculated from this model is compared with the wear rate predicted by the thermal desorption model, and the results are in agreement both in trend and magnitude. The effect of running-in on the Stribeck curve for different surface pattern is discussed.

Bifurcation Analysis of a Flexible Rotor Supported by Two Fluid-Film Journal Bearings

The effects of rotor stiffness on the bifurcation regions of a flexible rotor supported by two identical fluid-film journal bearings are presented. It is shown that the rotor stiffness has a pronounced influence on the bifurcation characteristics at the instability threshold speed. For short bearings, two bifurcation regions exist if the dimensionless rotor stiffness K¯⩾4.3. On the other hand, three bifurcation regions exist if the dimensionless rotor stiffness K¯<4.3. Information is presented that allows one to easily predict both the instability threshold speed and its bifurcation type of a rotor-bearing system with any specific set of operating parameters. The results presented have been verified by laboratory experiments as well as several published results in the open literature. Several examples are presented to illustrate the application of the results for design purposes.

Traction in EHL Line Contacts Using Free-Volume Pressure-Viscosity Relationship With Thermal and Shear-Thinning Effects

This paper investigates the traction behavior in heavily loaded thermo-elastohydro-dynamic lubrication (EHL) line contacts using the Doolittle free-volume equation, which closely represents the experimental viscosity-pressure-temperature relationship and has recently gained attention in the field of EHL, along with Tails equation of state for compressibility. The well-established Carreau viscosity model has been used to describe the simple shear-thinning encountered in EHL. The simulation results have been used to develop an approximate equation for traction coefficient as a function of operating conditions and material properties. This equation successfully captures the decreasing trend with increasing slide to roll ratio caused by the thermal effect. The traction-slip characteristics are expected to be influenced by the limiting shear stress and pressure dependence of lubricant thermal conductivity, which need to be incorporated in the future.

Combined Effects of Shear Thinning and Viscous Heating on EHL Characteristics of Rolling/Sliding Line Contacts

The combined influence of shear thinning and viscous heating on the behavior of film thickness and friction in elastohydrodynamic lubrication (EHL) rolling/sliding line contacts is investigated numerically. The constitutive equation put forward by Carreau is incorporated into the model to describe shear thinning. An extensive set of numerical simulations is presented. Comparison of the film thickness predictions with published experiments reveals good agreement, and it is shown that thermal effect plays an important role in the precise estimation of EHL film thickness and friction coefficient. Parametric simulations show that thermal effect in shear-thinning fluids is strongly affected by the power-law index used in the Carreau equation. Comparisons of prediction of the Newtonian fluid model are presented to quantify the degree to which it overestimates the film thickness.

Three-Dimensional Thermohydrodynamic Analysis of a Wet Clutch With Consideration of Grooved Friction Surfaces

A model is developed to investigate the effect of radial grooves and waffle-shape grooves on the performance of a wet clutch. Three-dimensional formulation of the governing equations, boundary conditions, and numerical solution scheme are presented for modeling the thermal aspects of the engagement process in a wet clutch. The thermal model includes full consideration of the viscous heat dissipation in the fluid as well as heat transfer into the separator, the friction material, and the core disk. The convective terms in the energy equations for the oil as well as the heat conduction equations in the bounding solids are properly formulated to determine the temperature fields corresponding to the domains between grooves. Roughness, centrifugal force, deformability, and permeability of the friction material with grooves are taken into account. The effects of groove geometry such as groove depth, grooved area, and number of grooves on the engagement characteristic of a wet clutch are investigated. It is also shown that the thermal effects in a wet clutch influence the engagement time and the torque response and should be included in the analytical studies.