Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userDirect numerical simulation of the dynamics of sliding rough surfaces
444
0
0.0
(
0
)
Authors
Viet Hung Dang
Ask ChatGPT about the research
No Arabic abstract
The noise generated by the friction of two rough surfaces under weak contact pressure is usually called roughness noise. The underlying vibration which produces the noise stems from numerous instantaneous shocks (in the microsecond range) between surface micro-asperities. The numerical simulation of this problem using classical mechanics requires a fine discretization in both space and time. This is why the finite element method takes much CPU time. In this study, we propose an alternative numerical approach which is based on a truncated modal decomposition of the vibration, a central difference integration scheme and two algorithms for contact: The penalty algorithm and the Lagrange multiplier algorithm. Not only does it reproduce the empirical laws of vibration level versus roughness and sliding speed found experimentally but it also provides the statistical properties of local events which are not accessible by experiment. The CPU time reduction is typically a factor of 10.
rate research
Read More
At separations below 100 nm, Casimir-Lifshitz forces strongly influence the actuation dynamics of micro-electromechanical systems (MEMS) in dry vacuum conditions. For a micron size plate oscillating near a surface, which mimics a frequently used setup in experiments with MEMS, we show that the roughness of the surfaces significantly influences the qualitative dynamics of the oscillator. Via a combination of analytical and numerical methods, it is shown that surface roughness leads to a clear increase of initial conditions associated with chaotic motion, that eventually lead to stiction between the surfaces. Since stiction leads to malfunction of MEMS oscillators, our results are of central interest for the design of microdevices. Moreover, they are of significance for fundamentally motivated experiments performed with MEMS.
Rod bundle flows are prevalent in nuclear engineering for both light water reactors (LWR) and advanced reactor concepts. Unlike canonical channel flow, the flow in rod bundles presents some unique characteristics, notably due to the inhomogeneous cross section which can present different local conditions of turbulence as well as localized effects characteristic of external flows. Despite the ubiquity of rod bundle flows and the decades of experimental and numerical knowledge acquired in this field, there are no publicly available direct numerical simulations (DNS) of the flow in multiple pin rod bundles with heat transfer. A multiple pin DNS study is of great value as it would allow for assessment of the reliability of various turbulence models in the presence of heat transfer, as well as allow for a deeper understanding of the flow physics. We present work towards DNS of the flow in a square 5x5 rod bundle representative of LWR fuel. We consider standard configurations as well as configurations where the central pin is replaced with a guide thimble. We perform simulations in STAR-CCM+ to design the numerical DNS, which is to be conducted using the open source spectral element code Nek5000. Large Eddy Simulations are also performed in Nek5000 to confirm that the resolution requirements are adequate. We compare results from STAR-CCM+ and Nek5000, which show very good agreement in the wide gaps with larger discrepancies in the narrow gaps. In particular, evidence of a gap vortex street is seen in the edge subchannels in LES but is not predicted by STAR-CCM+.
Light scattering from self-affine homogeneous isotropic random rough surfaces is studied using the ray-optics approximation. Numerical methods are developed to accelerate the first-order scattering simulations from surfaces represented as single-connected single-valued random fields, and to store the results of the simulations into a numerical reflectance model. Horizon mapping and marching methods are developed to accelerate the simulation. Emphasis is given to the geometric shadowing and masking effects as a function of surface roughness, especially, to the azimuthal rough-surface shadowing effect.
In the study of ocean wave impact on structures, one often uses Froude scaling since the dominant force is gravity. However the presence of trapped or entrained air in the water can significantly modify wave impacts. When air is entrained in water in the form of small bubbles, the acoustic properties in the water change dramatically and for example the speed of sound in the mixture is much smaller than in pure water, and even smaller than in pure air. While some work has been done to study small-amplitude disturbances in such mixtures, little work has been done on large disturbances in air-water mixtures. We propose a basic two-fluid model in which both fluids share the same velocities. It is shown that this model can successfully mimic water wave impacts on coastal structures. Even though this is a model without interface, waves can occur. Their dispersion relation is discussed and the formal limit of pure phases (interfacial waves) is considered. The governing equations are discretized by a second-order finite volume method. Numerical results are presented. It is shown that this basic model can be used to study violent aerated flows, especially by providing fast qualitative estimates.
When implementing a non-continuous controller for a cyber-physical system, it may happen that the evolution of the closed-loop system is not anymore piecewise differentiable along the trajectory, mainly due to conditional statements inside the controller. This may lead to some unwanted chattering effects than may damage the system. This behavior is difficult to observe even in simulation. In this paper, we propose an interval approach to characterize the sliding surface which corresponds to the set of all states such that the state trajectory may jump indefinitely between two distinct behaviors. We show that the recent notion of thick sets will allows us to compute efficiently an outer approximation of the sliding surface of a given class of hybrid system taking into account all set-membership uncertainties. An application to the verification of the controller of a child swing is considered to illustrate the principle of the approach.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
