Open Access Research Article Issue
On the experimental characterization of the fluid volume influence on the friction between rough surfaces
Friction 2023, 11 (7): 1334-1348
Published: 18 October 2022

The load-bearing behaviour of lubricated contacts depends primarily on the normal force, the relative velocity, and the geometry. Thus, with the aid of the Stribeck curve, it is usually well possible to characterize whether hydrodynamics, mixed friction, or boundary friction is more likely to be present. The fact that the load regime can also depend on the fluid quantity is obvious, but has hardly been systematically investigated so far. Especially for contacts with microscopic roughness, the defined application of a very small amount of fluid is a very challenging requirement. In this paper, a very fundamental study shows how a pin-on-disc tribometer can be used to achieve the transition from dry friction via mixed friction to predominant hydrodynamics by the amount of supplied fluid. The experiments are carried out on samples filed with different coarseness. In addition, the simultaneous influence of partial filling and normal force as well as relative velocity is also shown. Very good reproducibility has been practically reached over the entire range of the tests. Regarding the quantities for the coefficient of friction (COF), it was concluded that close to full filling, a reduction of the fluid quantity has a similar effect on the COF as the reduction of the velocity. This property goes along with the common theory of starved lubricated systems. Such behaviour was not observed to the same extent for the normal force. In the vicinity of smaller fluid quantities, the COF increases very rapidly with further reduction in fluid quantity, far more disproportionately than that with reduction in velocity. With a deeper understanding of this problem, various practical issues such as idling or the run-up process in bearings can also be studied in a more focused manner.

Open Access Research Article Issue
Towards the efficient modelling of trapped air pockets during squeeze flow
Experimental and Computational Multiphase Flow 2023, 5 (1): 29-52
Published: 19 January 2022

In most bonding processes, an adhesive is applied to a substrate in a specific pattern before the second substrate is subsequently pressed against it. During this, the adhesive flows in such a way that, ideally, it completely fills the joint. In practice, however, areas with entrapped air frequently remain in the bonded adhesive layer. Within the scope of a research project, these flows are systematically analyzed in order to identify optimal initial application patterns for the adhesive and substrate geometry to minimise such risks. For this purpose, the authors use an efficient flow model, the partially filled gaps model (PFGM), extended in this study to include the functionality of trapped air pockets. Depending on the volume fractions of air and adhesive, the flow of both phases is computed. Therefore, the model is introduced and fully described, benchmarked with respect to its plausibility and functionality, and results obtained are compared with a CFD calculation. Thereafter, the functionality of openings and closings of the pockets are analyzed. Lastly, the model is then applied to a real scenario created with a Hele–Shaw cell measurement. The benchmark as well as the comparison with the measurement results show the high potential of this technique.

total 2