The paper presents a numerical and analytical analysis on transient effects in Thermal Elasto-Hydrodynamic (TEHD) contacts subjected to fluctuations of their operating conditions. Firstly, the system of equations, boundary conditions and numerical scheme used in the model are described. In addition, developments to solve transient TEHD problems are detailed, on the basis of a previous work [1] dedicated to the study of steady state cases. Then, the phenomena at the origin of transient effects in TEHD contacts are reviewed along with their characteristic time. In this context, a particular focus is placed on the thermal contribution. The complexity in finding a relevant thermal characteristic time is illustrated by the influence of the slide-to-roll ratio (SRR) on the contact performance (film thickness and friction). The dependency of the dominant heat transfer mode on the SRR is showed. A distinction between low sliding, pure sliding and high sliding conditions is made. Each configuration is thoroughly analyzed, leading to the formulation of new thermal characteristic times. Finally, time-dependent TEHD computations are performed in order to investigate the onset of transient effects. A direct relationship with the dominant thermal time is established and validated through a parametric study on the contact operating conditions and on the solid material properties. Numerical simulation of transient Thermal Elasto-Hydrodynamic (TEHD) systems has several interests: understanding local behavior to prevent failure of a designated system, working conditions, fluid properties, overall design, etc. For practical applications, taking into account the thermal effects is mandatory, and since time-dependent operating conditions often apply, the decision to use a quasisteady or a transient model is also required. In this article, we derive, a priori, characteristic time scales for a generic line contact TEHD problem, and discuss the discrepancies between the two models depending on the dynamical character of the problem. A decision criteria is proposed and tested on various conditions to assess its validity.

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