20 research outputs found
Reducing cross-flow vibrations of underflow gates: experiments and numerical studies
An experimental study is combined with numerical modelling to investigate new
ways to reduce cross-flow vibrations of hydraulic gates with underflow. A
rectangular gate section placed in a flume was given freedom to vibrate in the
vertical direction. Holes in the gate bottom enabled leakage flow through the
gate to enter the area directly under the gate which is known to play a key
role in most excitation mechanisms. For submerged discharge conditions with
small gate openings the vertical dynamic support force was measured in the
reduced velocity range 1.5 < Vr < 10.5 for a gate with and without holes. The
leakage flow through the holes significantly reduced vibrations. This
attenuation was most profound in the high stiffness region at 2 < Vr < 3.5.
Two-dimensional numerical simulations were performed with the Finite Element
Method to assess local velocities and pressures for both gate types. A moving
mesh covering both solid and fluid domain allowed free gate movement and
two-way fluid-structure interactions. Modelling assumptions and observed
numerical effects are discussed and quantified. The simulated added mass in
still water is shown to be close to experimental values. The spring stiffness
and mass factor were varied to achieve similar response frequencies at the same
dry natural frequencies as in the experiment. Although it was not possible to
reproduce the vibrations dominated by impinging leading edge vortices (ILEV) at
relatively low Vr, the simulations at high Vr showed strong vibrations with
movement-induced excitation (MIE). For the latter case, the simulated response
reduction of the ventilated gate agrees with the experimental results. The
numerical modelling results suggest that the leakage flow diminishes the
whipping effect of fluctuations at the trailing edge associated with the
streamwise pressure drop across the gate and the body's vertical oscillatory
motion.Comment: 27 pages, 15 figures, 2 table
Evolutionary Design of Numerical Methods: Generating Finite Difference and Integration Schemes by Differential Evolution
Classical and new numerical schemes are generated using evolutionary
computing. Differential Evolution is used to find the coefficients of finite
difference approximations of function derivatives, and of single and multi-step
integration methods. The coefficients are reverse engineered based on samples
from a target function and its derivative used for training. The Runge-Kutta
schemes are trained using the order condition equations. An appealing feature
of the evolutionary method is the low number of model parameters. The
population size, termination criterion and number of training points are
determined in a sensitivity analysis. Computational results show good agreement
between evolved and analytical coefficients. In particular, a new fifth-order
Runge-Kutta scheme is computed which adheres to the order conditions with a sum
of absolute errors of order 10^-14. Execution of the evolved schemes proved the
intended orders of accuracy. The outcome of this study is valuable for future
developments in the design of complex numerical methods that are out of reach
by conventional means.Comment: 19 pages, 7 figures, 10 tables, 4 appendice
Mio-Pliocene Faunal Exchanges and African Biogeography: The Record of Fossil Bovids
The development of the Ethiopian biogeographic realm since the late Miocene is here explored with the presentation and review of fossil evidence from eastern Africa. Prostrepsiceros cf. vinayaki and an unknown species of possible caprin affinity are described from the hominid-bearing Asa Koma and Kuseralee Members (∼5.7 and ∼5.2 Ma) of the Middle Awash, Ethiopia. The Middle Awash Prostrepsiceros cf. vinayaki constitutes the first record of this taxon from Africa, previously known from the Siwaliks and Arabia. The possible caprin joins a number of isolated records of caprin or caprin-like taxa recorded, but poorly understood, from the late Neogene of Africa. The identification of these two taxa from the Middle Awash prompts an overdue review of fossil bovids from the sub-Saharan African record that demonstrate Eurasian affinities, including the reduncin Kobus porrecticornis, and species of Tragoportax. The fossil bovid record provides evidence for greater biological continuity between Africa and Eurasia in the late Miocene and earliest Pliocene than is found later in time. In contrast, the early Pliocene (after 5 Ma) saw the loss of any significant proportions of Eurasian-related taxa, and the continental dominance of African-endemic taxa and lineages, a pattern that continues today
Free-surface flow simulations for discharge-based operation of hydraulic structure gates
We combine non-hydrostatic flow simulations of the free surface with a discharge model based on elementary gate flow equations for decision support in the operation of hydraulic structure gates. A water level-based gate control used in most of today's general practice does not take into account the fact that gate operation scenarios producing similar total discharged volumes and similar water levels may have different local flow characteristics. Accurate and timely prediction of local flow conditions around hydraulic gates is important for several aspects of structure management: ecology, scour, flow-induced gate vibrations and waterway navigation. The modelling approach is described and tested for a multi-gate sluice structure regulating discharge from a river to the sea. The number of opened gates is varied and the discharge is stabilized with automated control by varying gate openings. The free-surface model was validated for discharge showing a correlation coefficient of 0.994 compared to experimental data. Additionally, we show the analysis of CFD results for evaluating bed stability and gate vibrations
UvA-DARE (Digital Academic Repository) How to Speed up Optimization? Opposite-Center Learning and Its Application to Differential Evolution How to Speed up Optimization? Opposite-Center Learning and Its Application to Differential Evolution
Abstract This paper introduces a new sampling technique called Opposite-Center Learning (OCL) intended for convergence speed-up of meta-heuristic optimization algorithms. It comprises an extension of Opposition-Based Learning (OBL), a simple scheme that manages to boost numerous optimization methods by considering the opposite points of candidate solutions. In contrast to OBL, OCL has a theoretical foundation -the opposite center point is defined as the optimal choice in pair-wise sampling of the search space given a random starting point. A concise analytical background is provided. Computationally the opposite center point is approximated by a lightweight Monte Carlo scheme for arbitrary dimension. Empirical results up to dimension 20 confirm that OCL outperforms OBL and random sampling: the points generated by OCL have shorter expected distances to a uniformly distributed global optimum. To further test its practical performance, OCL is applied to differential evolution (DE). This novel scheme for continuous optimization named Opposite-Center DE (OCDE) employs OCL for population initialization and generation jumping. Numerical experiments on a set of benchmark functions for dimensions 10 and 30 reveal that OCDE on average improves the convergence rates by 38% and 27% compared to the original DE and the Oppositionbased DE (ODE), respectively, while remaining fully robust. Most promising are the observations that the accelerations shown by OCDE and OCL increase with problem dimensionality