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True pinch mode of magnetorheological fluids
(IOP publishing, 2024-10-29) Sapinski, Bogdan; Macháček, Ondřej; Kubík, Michal; Goldasz, Janusz; Žáček, Jiří; Bańkosz, Wojciech
Magnetorheological (MR) fluids are representatives of smart materials. They react to magnetic fields by developing a yield stress. The effect has been employed in real-world applications such as automotive chassis systems or optical finishing. By convention, MR devices can be operated in at least one of the fundamental modes: flow, shear, squeeze, gradient pinch of which the former has been the least studied and understood. In pinch mode, the material in the flow channel is exposed to non-uniform magnetic fields in the direction parallel to fluid flow. As a result, only the volume of MR fluid near the channel walls are energized to modify the particular material property (yield stress). The result is the channel's effective diameter change. The behavior of the material in pinch mode is unique and unseen in the other controllable fluids. To study the material's characteristics in the specific mode, the authors developed a novel circuit concept for energizing the material in an effort to achieve the 'true-zero' pinch mode magnetic behavior. Contrary to the existing pinch mode valve concepts, the concept valve allows to achieve zero magnetic flux density in the center of the flow channel regardless of the current level. To test the hypothesis a prototype valve was modeled, manufactured and tested across a range of external (flow rate, current/magnetic flux) stimuli. The obtained results yield sufficient evidence proven by results of magnetic simulations to support the underlying hypothesis. The experimental results illustrate the pinch mode type behaviour, i.e. the slope change in the pressure vs flow rate characteristics.
Wheel Squeal Mitigation Under Water Lubrication
(Faculty of Engineering, University of Kragujevac, Serbia, 2024-09-15) Navrátil, Václav; Galas, Radovan; Klapka, Milan; Kvarda, Daniel; Omasta, Milan
This study investigates the potential of applying water to the wheel-rail contact to reduce squealing noise. For this purpose, a twin-disc device with a single tram wheel and real wheel suspension stiffnesses was devel-oped. Three types of tests were performed. During the tests, adhesion coefficient, sound pressure level and wheel axial vibration were meas-ured. The tests under dry conditions were carried out to describe the frequency spectrum of wheel vibration and to establish reference values for further measurements. The tests under wet conditions were carried out to investigate the ability of water to reduce adhesion and noise. Finally, tests with varying amounts of water in contact were carried out because of the low adhesion risk. The experimental results showed that the twin-disc device was able to reproduce both the adhesion and noise properties of the contact. Tests with different amounts of water showed that the application of water can be a promising way to reduce squealing noise from wheel-rail contact.
Current state-of-the art review of footwear-ground friction
(Springer, 2024-08-14) Rebenda, David; Sáha, Tomáš
The most important role of footwear is to ensure safe, functional walking, and foot protection. For the proper functionality of not only the work shoes, the anti-slip behavior of the shoe under various conditions and environments plays an important role in the prevention of slips, trips, falls, and consequent injuries. This article is intended to review the current understanding of the frictional mechanisms between shoe outsoles and various counterfaces that impact the evaluation of outsole slipperiness. Current research focuses on the mechanisms driving outsole friction on different ground surfaces or the definition and description of parameters that influence outsole friction. Subsequently, the review discusses the effect of various surface contaminants on footwear friction. Lastly, challenges and outlooks in the field of footwear outsoles are briefly mentioned.
Impact of magnetorheological fluid composition on their behaviour in gradient pinch mode
(Spriger Nature, 2024-12-28) Žáček, Jiří; Strecker, Zbyněk; Jeniš, Filip; Macháček, Ondřej; Goldasz, Janusz; Sapinski, Bogdan; Vrbka, Martin; Kubík, Michal
Magnetorheological (MR) fluids can be utilized in one of the fundamental operating modes of which the gradient pinch mode has been the least explored. In this unique mode non-uniform magnetic field distributions are taken advantage of to develop a so-called Venturi-like contraction in MR fluids. By adequately directing magnetic flux the material can be made solidified in the regions near the flow channel wall, thus creating a passage in the middle of the channel for the fluid to pass through. This leads to unique variations of the slope in the pressure-flow rate characteristics. It can be stated that the effect of the MR fluid composition on the behaviour of the MR fluid in gradient pinch mode has not been thoroughly investigated yet. In this study, the behaviour of MR fluids was assessed with a dedicated pinch mode MR valve that provided a valuable insight into the contraction mechanism using fluorescence microscopy. Briefly, seven MR fluid samples were prepared with different particle concentrations (10%, 22% and 32 vol%) and mean particle sizes (2, 4.5 and 8.2 m). It was found that the MR fluid sample with the larger particle size exhibits a significantly larger slope change observed in the pressure-flow rate characteristics. Increasing the particle size from 2 m (base) to 8 m resulted in the slope increase by a factor of 2.6 compared to the base sample. Increasing the particle concentration has a negligible effect on the pinch mode effect. Finally, these results were analysed using the modified Wuest equation, which is commonly used for characterizing sharp-edged orifices in low Reynolds number flow regimes. The simple equation was determined to describe the behaviour of MR fluids in gradient pinch mode with adequate accuracy.
Labyrinth seal design for space applications
(Elsevier, 2025-02-14) Pouzar, Josef; Košťál, David; Westerberg, Lars-Göran; Nyberg, Erik; Křupka, Ivan
Labyrinth seals, extensively used in space applications, serve to prevent the loss of liquid lubricants and shield satellite subsystems from contamination. These seals are essential for the reliable functioning of bearings and for protecting satellite subsystems from contamination. This study compares analytical predictions of lubricant loss against experimental measurements and computer simulations to optimize labyrinth seal configurations. Analytical models tend to overestimate mass loss by 5-8 times compared to experimental data, indicating limited reliability for complex seal geometries. Simulations using MolFlow+ and COMSOL Multiphysics align closely with experimental results, providing accurate mass loss predictions. Key findings highlight that labyrinth length, width, and surface roughness are critical factors in minimizing evaporative mass loss. Notably, stepped labyrinth seals with relief grooves and optimized step positioning effectively reduce molecular beaming effects and improve sealing performance compared to straight geometries. Effective sealing not only reduces mission failures but also helps to minimize space debris, thereby promoting safer satellite missions.

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