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The coronavirus disease 2019 (covid-19) pandemic caused by the novel severe acute respiratory syndrome coronavirus 2 has necessitated changes to the way patients with chronic diseases are managed. Given that patients with multiple myeloma are at increased risk of covid-19 infection and related complications, national bodies and experts around the globe have made recommendations for risk mitigation strategies for those vulnerable patients. Understandably, because of the novelty of the virus, many of the proposed risk mitigation strategies have thus far been reactionary and cannot be supported by strong evidence. In this editorial, we highlight some of the risk mitigation strategies implemented at our institutions across Canada during the first wave of covid-19, and we discuss the considerations that should be made when managing patients during the second wave and beyond. [ABSTRACT FROM AUTHOR]

In this paper the subject of friction prediction is revisited, with the aim of obtaining a general formula predicting the coefficient of friction over a wide range of operating conditions. By means of full numerical simulations of the smooth isothermal elliptic contact, and assuming an Eyring non-Newtonian behavior, the coefficient of friction is computed for a wide range of operating conditions. It is shown that with respect to sliding friction, all results can be presented on a single generalized friction curve relating a reduced coefficient of friction to a characteristic nondimensional shear stress. Finally, it is shown that some measured data presented in the literature when presented in terms of the derived parameters closely follow the derived behavior, which provides a validation of the theoretical results. [DOI: 10.1115/1.1308021]

In view of increasingly severe operating conditions and the use of composite (strongly) heterogeneous materials, detailed modelling and optimisation methods are needed to predict the effects of subsurface material topology, either by design or resulting from inclusions and material anisotropy, on rolling-sliding contact fatigue life. In this paper, a method is proposed showing that such predictions can at present be obtained for realistic configurations on small-scale computers by integrating efficient numerical solution of the 3D displacement and stress field in the (heterogeneous) material with fast rough surface-lubricated contact models.Contact pressures, subsurface stresses and surface fatigue effects are shown for cases of bonded individual or multiple (clusters) of statistically distributed inhomogeneities close to the surface in realistic actual rolling–sliding rough contact geometries. The model contributes to the development of optimised failure criteria for composite/heterogeneous materials close to the surface. [ABSTRACT FROM AUTHOR]

A high-order polynomial gas distribution cam mechanism is investigated theoretically from the viewpoint of thermal elastohydrodynamic lubrication (EHL). First, a cam with a larger base circle radius is employed, which results in slide-roll ratio 2.0 < S <9.0 when the two surfaces move oppositely. The pressure, film thickness, and temperature profiles at a number of angular positions of the cam are presented, together with the isothermal results. The comparison between thermal and isothermal oil characteristics is also shown. It is revealed that the isothermal analysis partly overestimates the actual film thickness and it also misses some essential local phenomena. Second, a cam with a smaller base circle radius is studied, which leads to drastic variations in the slide-roll ratio which encounters four times' occurrences of infinity in one working period. The pressure, film thickness, and temperature profiles at some angular cam positions together with the oil characteristics are given, showing much dramatic variations. A very small film thickness is observed at the contact of the tappet with the start of the cam basic segment, which suggests a possible risk of direct contact of both surfaces. [ABSTRACT FROM AUTHOR]

The effect of single-sided and double-sided harmonic surface waviness on the film thickness, pressure, and temperature oscillations in an elastohydrodynamically lubricated eccentric-tappet pair has been investigated in relation to the eccentricity and the waviness wavelength. The results show that, during one working cycle, the waviness causes significant fluctuations of the oil film, pressure, and temperature, as well as a reduction in minimum film thickness. Smaller wavelength causes more dramatic variations in oil film. The fluctuations of the pressure, film thickness, temperature, and traction coefficient caused by double-sided waviness are nearly the same compared with the single-sided waviness, but the variations are less intense. [ABSTRACT FROM AUTHOR]

Somatic mosaicism is present in slightly more than 50% of small supernumerary marker chromosome (sSMC) carriers. Interestingly, non-acrocentric derived sSMC show mosaicism much more frequently than acrocentric ones. sSMC can be present in different mosaic rates, which may go below 5% of the studied cells. Also cryptic mosaicism can be present and mosaics may be differently expressed in different tissues of the body. Even though in the overwhelming majority of the cases somatic sSMC mosaicism has no direct clinical effect, there are also cases with altered clinical outcomes due to mosaicism. Also clinically important is the fact that a de novo sSMC, even present in mosaic, may be a hint of uniparental disomy (UPD). As it is under discussion to possibly replace standard karyotyping by methods like array-CGH, the impracticality of the latter to detect low-level sSMC mosaics and/or UPD has to be considered as well. Overall, sSMC mosaicism has to be studied carefully in each individual case, as it can be extremely informative and of importance, especially for prenatal genetic counseling. Copyright © 2012 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

The prediction of bearing life and performance requires detailed knowledge of the behaviour of elasto-hydrodynamically lubricated (EHL) contacts under varying conditions of load and lubricant supply during many load cycles. Considering starved lubrication, a model is developed that predicts the change of the supply layer to the EHL contact. As the film thickness inside a starved EHL contact is determined by the thickness and distribution of the supply layer, this model can be used to predict the film thickness decay. A formula for the central film thickness decay is obtained for the long-term operation of circular or elliptical contacts under starved conditions. The model is validated experimentally. [ABSTRACT FROM AUTHOR]

This article traces the contribution of the Dowson and Higginson work to numerical line contact elasto-hydrodynamic lubricated film thickness prediction and the Hamrock and Dowson contribution to the film thickness prediction in elliptical contacts. Considering the line contact work, this article shows that both the numerical pressure and film thickness results and the curve-fitted film thickness predictions are very accurate, even by today's standards. Concerning the elliptical results, the authors show that the original predictions remain surprisingly accurate but that the issue of the minimum to central film thicknesss ration Hm/Hc is not yet completely settled. The article then continues to discuss some limitations of the current models that require additional work, mainly in the area of realistic non-Newtonian lubricant rheology for film thickness predictions and pressure spike analysis. [ABSTRACT FROM AUTHOR]

Many elastohydrodynamically lubricated contacts in practical applications, e.g., in bearings, operate in the starved lubrication regime. As a result their performance is sensitive to variations of the lubricant layers present on the surfaces, which form the supply to the contact. Their shape is often determined by previous overrollings of the track and also by replenishment mechanisms and various migration effects. Variations of the layers induced in the direction Of rolling lead to a time-varying lubricant supply to the contact. In this paper, by means of numerical simulations using a starved lubrication model, the film thickness modulations in the center of the contact induced by a harmonically varying inlet supply have been investigated. First, for a given load condition and layer wavelength, the effect of the nominal layer thickness (degree of starvation) and the layer variation amplitude is illustrated. Subsequently, using results for different load conditions, wavelengths, and degrees of starvation, it is shown that the response of the contact to such variations is determined by a nondimensional parameter, which represents the ratio of the entrainment length of the contact to the wavelength of the induced variation, and by the degree of starvation. A simple formula is presented for use in engineering predicting the ratio of the amplitude of the film modulations in the center of the contact to the amplitude of the layer variations in the inlet. [ABSTRACT FROM AUTHOR]

In this paper, the effects of an oscillatory entrainment speed on the film thickness in an elasto-hydrodynamically lubricated circular contact operating in the piezo-viscous regime as predicted by a conventional elasto-hydrodynamic lubrication model are described in detail. The relative variation of the film thickness on the centreline of the contact mainly depends on two dimensionless parameters: relative entrainment velocity amplitude and dimensionless entrainment velocity frequency. For small to moderate values of dimensionless frequency, the results can be accurately predicted through a simplified ‘engineering approach’: propagation of inlet modulations along oscillatory characteristics. For values of the dimensionless frequency of O(1), film thickness modulations occur on a sub-contact scale. This work was initiated by the experimental results of Glovnea and Spikes obtained for two lubricants PAO and 5P4E, which have the same ambient viscosity but a different viscosity–pressure coefficient. During the experiments significant difference in behaviour was observed. In this paper, it is shown that a conventional model assuming Newtonian rheological behaviour predicts the same relative film thickness results for both the lubricants. This agrees well with the behaviour observed for PAO, which can be considered as a standard oil that is well characterized by its bulk viscosity, density, and their pressure dependence alone. Nevertheless, further experiments need to be carried out to validate the predictions for larger values of the dimensionless frequency. The behaviour observed in the experiments for 5P4E is significantly different. For accurate prediction of friction, the need to include non-Newtonian lubricant behaviour in models is well known. Film thickness predictions are often less sensitive to the specifics of the lubricant behaviour. However, the results presented here indicate that for film thickness calculations, under rapidly varying conditions in time, more advanced rheological models, including visco-elastic behaviour, may have to be considered as well. [ABSTRACT FROM AUTHOR]

In this paper, the effects of rotation on the distribution of a layer of oil on a bearing raceway are analyzed in relation to the geometry of the raceway. The research is motivated by the need to understand the behavior of grease lubricated bearings. Some specific aspects of grease lubrication can be understood by approximating the contact as a starved lubricated elastohydrodynamic lubrication contact. In such a contact, the shape and thickness of the inlet layer of oil, supplied to the contact on the running track, are of crucial importance to the film formation and contact performance. Small changes in the distribution of lubricant on the rolling track, as a result of reflow or redistribution, may have a large (local) effect on the film thickness inside the contact. Starting from the Navier-Stokes equations, the free surface thin layer flow equation for axisymmetric rotating surfaces is derived. For the case of bearing applications, it is shown that a simple quasilinear equation can be derived for the layer thickness, as a function of location and time, which can be solved analytically. Experiments have been carried out, measuring the changes of a layer of oil on rotating raceways in time in relation to the rotational speed and the raceway geometry. It is shown that the simplified model accurately predicts the thin layer flow, except in a region near the outflow boundary, where the effect of the boundary condition on the layer shape is crucial. This is further illustrated by results of numerical simulations. [ABSTRACT FROM AUTHOR]

The performance prediction of elastohydrodynamic lubricated contacts with rough surfaces is currently an important issue, because in many engineering applications, the ratio of film thickness to surface roughness is small and decreasing. The classical way of introducing roughness in the lubrication equations is through the approach pioneered by Patir and Cheng. This approach treats the roughness in a statistical manner and it cannot be used to predict the behaviour of a particular surface texture. Furthermore, the main hypotheses stem from hydrodynamic lubrication and neglect the viscosity change with pressure and the elastic surface deformation. The opposite approach uses the full transient EHL equations, introduces a real, measured roughness, and studies the lubrication of this surface. This approach has several drawbacks. First, it is difficult to analyse the precision of the results obtained. Furthermore, this direct approach of surface roughness is difficult to generalize as each roughness, or set of operating conditions is unique, and general trends are difficult to extract. A third approach has emerged over the last decade, in which the amplitude reduction of a sinusoidal waviness is studied as a function of the amplitude, the wavelength, and the operating conditions. The simplification of the roughness to a sinusoidal waviness can be justified in several ways. The first and simplest justification points out that many engineering surface finishes have one dominant wavelength and amplitude. Even if the geometry is removed from the sinusoidal one, a first approximation of the roughness behaviour can be obtained through the study of the waviness with the same wavelength and amplitude. A second argument is slightly more sophisticated; it argues that if the amplitude reduction is approximately linear, Fourier decomposition of the roughness into sinusoidal components is permitted. Next, the amplitude reduction of each Fourier component is computed, and finally, the deformed waviness signals are assembled to yield the deformed roughness signal. The major results of this amplitude reduction work are reviewed in this article. [ABSTRACT FROM AUTHOR]

In this paper, it is investigated how accurately current models predict the response of an elasto-hydrodynamically lubricated contact to time varying load conditions. For two patterns of time varying load, under conditions of pure rolling, the results obtained experimentally on a ball on disk interferometry apparatus by Sakamoto et al. (Behaviour of point contact EHL films under pulsating loads. Proceedings of the 30th Leeds–Lyon Symposium on Tribology, Elseviers Tribology Series, Vol. 43, pp. 391–399) are compared with the results of numerical simulations using the dynamic contact model of Wijnant (Contact dynamics in the field of elastohydrodynamic lubrication. PhD Thesis, University of Twente, Enschede, The Netherlands, 1998 ISBN 90-36512239), Wijnant and Venner (Contact dynamics in starved elastohydrodynamic lubrication. Proceedings of the 25th Leeds-Lyon Symposium on Tribology, Elsevier Tribology Series, Vol. 36, pp. 705–716). The observed film changes are explained and related to the piezoviscous behaviour of the lubricant. Results for two different viscosity pressure relations are presented. It is shown that the details of the viscosity pressure relation have little effect on the local phenomena observed. The agreement between experimental and theoretical results is excellent in terms of the location and width of induced film thickness changes. The accuracy of the amplitude of the predicted changes is less. A more accurate amplitude prediction requires more precise knowledge of the dynamic parameters, which describe the load system used in the experiments. [ABSTRACT FROM AUTHOR]

When numerical and experimental results are compared to validate elasto-hydrodynamic lubrication (EHL) models, it is of utmost importance that grid-converged results are used. In particular at low speeds and high loads, solutions obtained using grids that are not sufficiently dense will exhibit an artificial trend that does not represent the behaviour of the continuous modelling equations. As it coincides with a trend observed in experiments this may lead to the erroneous conclusion that the theoretical model on which the numerical simulations are based is accurate. This risk is illustrated in detail. It is further shown that EHL models based on the Reynolds equation in a steady state circular contact predicts a positive film thickness as long as the grid used in the calculations is sufficiently dense. This has significant implications for the validity of results obtained using mixed lubrication models based on a Reynolds model and a film thickness threshold. [ABSTRACT FROM AUTHOR]

A model is presented that facilitates prediction of the transient solution for the pressure and film thickness in an elastohydrodynamically lubricated line contact for the case of arbitrary surface microgeometry (roughness) under the assumption of Newtonian lubricant behaviour and small roughness amplitude. In the model the two parts of the transient solution (particular integral and complementary function) are calculated for each Fourier component of a surface real roughness and combined to obtain the complete transient solution. As no time-consuming numerical simulations are required, the computational scheme forms a simple and efficient tool for use in a practical engineering environment. To validate the approach, first simple cases such as single dents and harmonic waviness were considered. The predictions obtained for these cases are compared with the results of full numerical simulations, showing good agreement. Subsequently the results obtained for a realistic roughness profile are presented. [ABSTRACT FROM AUTHOR]

Surface roughness effects in line and point elastohydrodynamically lubricated contacts are compared and it is shown that the underlying behaviours in both types of contact are identical. Long wavelength components of roughness are attenuated; short wavelength components pass through the conjunction unaltered. It is also shown that the roughness attenuation, plotted against a non-dimensional wavelength, follows almost identical curves in both cases. [ABSTRACT FROM AUTHOR]

This paper presents a first extension of the amplitude reduction approach, where the deformation of harmonic components of the roughness is considered, to non-Newtonian elastohydrodynamic lubrication (EHL) line contact. The amplitude reduction of one-sided waviness is computed as a function of the slide--roll ratio. Non-Newtonian effects result in an increase in the deformation for positive slide--roll ratios. This contrasts with published results using a Newtonian fluid where the deformation decreased for such conditions. Moreover, the amplitude reduction varies significantly throughout the contact, requiring an additional parameter for the characterization of the amplitude reduction. An explanation of these differences based on the velocity profile of the lubricant inside the contact concludes the paper. [ABSTRACT FROM AUTHOR]

The introduction and further development of algorithms based on multigrid (multilevel) techniques has resulted in fast and efficient solvers for elastohydrodynamically lubricated (EHL) point contact problems. As a result the capability to simulate steady state and transient point contact problems numerically has greatly increased in the past decade. This has led to an increased understanding of the mechanisms controlling film formation and pressure in the contact in relation to the operating conditions. In this article an overview is given of the essential ingredients of an efficient multigrid algorithm for an EHL point contact problem. Results are presented for some characteristic problems that illustrate today's numerical simulation capability. It is concluded that, with respect to EHL, one of the main questions of the last part of the century has been answered, i.e. how to solve efficiently the modelling equations such that realistic and highly loaded situations can be simulated. The challenge that researchers and engineers now face is how to translate the new insights into simple rules for design and engineering, and to establish clearly the limits of validity of the models that are used in simulations both on the scale of the contact as a whole as well as on a subcontact scale. [ABSTRACT FROM AUTHOR]

By means of numerical simulations the amplitude reduction of waviness in a transient elastohydrodynamically lubricated line contact is investigated as a function of slip, under conditions representative of a highly loaded (piezoviscous) situation Firstly, the case of waviness on one of the surfaces is investigated. It is shown that the behaviour of the deformed amplitude as a function of the sliding velocity can be traced back to the variations in the mass flow induced by the waviness (which are forced by the squeeze term oscillations). The results are compared with recent theoretical work, showing excellent agreement. In particular, the peculiar effect of (significant) amplification of the amplitude (instead of reduction) for short wavelengths under conditions of sliding predicted in this work is confirmed by the presented results. However, for similar conditions it is almost absent if lubricant compressibility is taken into account Secondly, waviness was assumed on both surfaces. The magnitude of the film oscillations in the centre of the contact has been monitored as a function of the sliding velocity and amplitude ratio. Again the behaviour can be traced back to the squeeze term and mass flow variations. Furthermore, it is shown that, for the conditions considered here, accurate predictions can be obtained on the basis of the results obtained for single-sided waviness. A simple formula is presented. [ABSTRACT FROM AUTHOR]