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Dit eindwerk is opgebouwd uit drie delen. Het eerste deel is een algemeen gedeelte met daarin de geschiedenis, de verschillende skateboarddisciplines, een korte analyse van het skateboard en de verschillende posities op het board. Het tweede deel bestaat uit bewegingsanalysen van enkele basistricks, slidetricks, grindtricks en fliptricks. Bij elke trick staat telkens nog een korte opbouw uitgeschreven hoe je die trick kan aanleren. In het derde en laatste deel wordt nog omschreven hoe je skateboarden aan bod kan laten komen op school. Hier worden geschikte leerinhouden, enkele veiligheidsgrepen en didactische en praktische tips uitgelegd. Het tweede en derde deel kunnen ook bekeken worden op de toegevoegde cd-rom. Hier staan nog verschillende videofragmenten op die de verschillende 'tricks' extra verduidelijken. Het eerste deel is bedoeld om een beeld te schetsen wat skateboarden juist inhoud en hoe het ontstaan is. Het tweede deel is interessant voor beginnende skateboarders om een nieuwe trick aan te leren en het derde deel is bedoeld om leerkrachten een idee te geven hoe je skateboarden op school aan bod kan laten komen. (Bron: auteur)

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In this thesis the dynamic behavior of a scroll compressor with piping system is studied using numerical and experimental tools in order to improve the knowledge in numerical modeling. A multi-body dynamics analysis is firstly performed by completing an existing finite element model to obtain a complete compressor. The model inputs are chosen to best represent the real conditions applied to the compressor. Then, the results of this first model are used to create excitations in a second model which studies the harmonic response of the system. A modal analysis must be done to obtain the different natural frequencies of the system and then determine how it will react to these excitations. A second harmonic analysis is realized using the same excitations but calculated analytically. The comparison of the two harmonic responses leads to similar results which allows to affirm that the analytical calculations are sufficient to have a consistent harmonic response and that a multi-body analysis is not essential in this case. Finally, experimental modal analysis and vibration tests are conducted on an experimental structure in order to measure eigenfrequencies, accelerations and strains at several points of the real system and compare them with numerical results. It appears that the experimental accelerations are up to three times lower than the numerical ones because the multi-body model does not start from a static equilibrium and therefore, under the application of gravity, it is dropped on the grommets and bounces on them. The simulation is too short to dampen this phenomenon which means that the model is not yet in a steady state. These excessive accelerations imply too high displacements and thus lead to high strains which explains the factor 10 between the numerical and experimental strains. Lastly, some points of possible improvement are detailed for future works.

Ingénierie, informatique & technologie > Ingénierie mécanique

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Friction is the main mechanical loss taking place inside scroll compressors. It can lead to wear and
reduces the reliability and service life of the compressors. The friction phenomenon needs to be understood and modelled so that the friction can ultimately be reduced and correctly taken into account during the design of the scroll compressors. This thesis researches and develops a friction model for the tip-base interface contact of the scroll, in order to obtain a friction coefficient depending on the operating conditions of the compressor.
This model is implemented in Python. It corresponds to a mixed lubrication model able to model
friction for all operating conditions and lubrication regimes. The model is discrete, and the tip-base
interface is thus discretized using a regular mesh composed of quadrangle elements. The model is fed with different inputs. MBD simulations are used to generate the tip-base gap geometry. The boundary and axial load are derived from analytical computations, while the fluid and material parameters are sourced from the literature. It is composed of a fluid and a dry model that are combined to give the most general model possible. The fluid model is based on the Reynolds equation that is averaged in order to take into account the influence of the rough surface on the lubricant flow when the lubricant film is very small. The Reynolds equation is solved by an iterative process using finite difference method. The dry model is based on the stochastic model of Greenwood-Tripp that makes assumptions on the statistical distribution of the surface roughness. The two models are then assembled, and the tip-base gap is adjusted through the offset film thickness determined using a root-finding algorithm on the load balance condition.
The fluid model is verified using known academic results, and the numerical solution is optimized
through a convergence study. The complete implemented model is then used to understand the frictional behaviour of the tip-base contact, first, for operating conditions -5 to 60°C at 8000 RPM. The average friction coefficient has been found to be 0.023 with a fluid friction significantly lower than
the dry one. The influence of the rotational speed is then explored by studying the results at 2900
and 6000 RPM. It is shown that the friction coefficient decreases when the speed increases, as the
speed increase tends to separate the surfaces. The influence of operating conditions is finally studied by changing the inlet and outlet temperature to 8 to 45°C and keeping the speed at 2900 RPM. It is shown that the friction coefficient decreases when the temperature ratio decreases as the inlet boundary pressure is higher for these conditions. Overall, it is shown that the average friction coefficient for a complete revolution of the scroll varies between 0.02 and 0.03, suggesting that the 0.04 value used in Copeland computations is slightly overestimated. Stribeck curve for this contact is finally derived.
This curve shows that at low speeds, the coefficient decreases when speed increases, as the surfaces are separated. At high speeds, the coefficient increases with the rotational speed, as the viscous shear stresses are increased.

Compressor --- Friction --- Tribology --- Lubrication --- Scroll Compressors --- Ingénierie, informatique & technologie > Ingénierie mécanique