2342 resultaten

  • How to speed up optimization? : opposite-center learning and its application to differential evolution

    Auteurs: Xu, H.; Erdbrink, C.D.; Krzhizhanovskaya, V.V. (2015)
    Gepubliceerd in: Procedia computer science, volume 51 (2015), pagina 805-814

    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.

  • Spectral wave-driven sediment transport across a fringing reef

    Auteurs: Pomeroy, A.W.M.; Lowe, R.J.; Dongeren, A.R. van; Ghisalberti, M.; Bodde, W.; Roelvink, J.A. (2015)
    Gepubliceerd in: Coastal engineering : an international journal for coastal, harbour and offshore engineers, volume 98 (2015), pagina 78-94

    A laboratory experiment was conducted to investigate the dynamics of cross-shore sediment transport across a fringing coral reef. The aim was to quantify how a highly bimodal spectrum of high-frequency (sea-swell) and low-frequency (infragravity and seiching) waves that is typically present on coral reef flats, influences the various sediment transport mechanisms. The experiments were conducted in a 55 m wave flume, using a 1:15 scale fringing reef model that had a 1:5 forereef slope, a 14 m long reef flat, and a 1:12 sloping beach. The initial 7 m of reef flat had a fixed bed, whereas the back 7 m of the reef and the beach had a moveable sandy bed. Four seven-hour irregular wave cases were conducted both with and without bottom roughness elements (schematically representing bottom friction by coral roughness), as well as for both low and high still water levels. We observed that the wave energy on the reef flat was partitioned between two primary frequency bands (high and low), and the proportion of energy within each band varied substantially across the reef flat, with the low-frequency waves becoming increasingly important near the shore. The offshore transport of suspended sediment by the Eulerian mean flow was the dominant transport mechanism near the reef crest, but a wide region of onshore transport prevailed on the reef flat where low-frequency waves were very important to the overall transport. Ripples developed over the movable bed and their properties were consistent with the local high-frequency wave orbital excursion lengths despite substantial low-frequency wave motions also present on the reef flat. This study demonstrated that while a proportion of the sediment was transported by bedload and mean flow, the greatest contributions to cross-shore transport were due to the skewness and asymmetry of the high and low-frequency waves.

  • Application of a three-dimensional hydrodynamic model for San Quintin Bay, B.C., Mexico. Validation and calibration using OpenDA

    Auteurs: Garcia, M.; Ramirez, I.; Verlaan, M.; Castillo, J.E. (2015)
    Gepubliceerd in: Journal of computational and applied mathematics, volume 273 (2015), pagina 428-437

    A 3D hydrodynamic model (Delf3D) was applied for San Quintin Bay (SQB), B.C., Mexico. Calibration and validation were conducted using measured bathymetry, wind, water surface elevation and velocities fields. The calibration period took place during the winter of 2010. Model predictions were evaluated graphically and statistically against field observations in order to quantify the accuracy of the predictions and evaluate the success of the model calibration. A 2D Calibration of tide constituents at the boundaries, seabed roughness and depths was done to evaluate the improvement in the velocities for the 3D Model. Comparisons between the model predictions and the field observations of the water surface elevation at interior stations indicated that the model had been successfully calibrated. It also indicated that the model predictions were highly correlated with the observed water surface elevations and velocities. This connection between the observed and simulated values was detected through graphical comparisons and root-mean-square errors and correlation coefficient. The objective of this study was to demonstrate that OpenDA could be used effectively to rapidly calibrate a hydrodynamic model for smaller sub-meso scales like the size of SQ Bay.

  • 10th International symposium on ultrasonic Doppler methods for fluid mechanics and fluid engineering - ISUD 10 (Tokyo, Japan, 28-30 September 2016)

    Originally published in 2016

  • Late Pleistocene to Holocene evolution of the Emba Delta, Kazakhstan, and coastline of the north-eastern Caspian Sea : sediment, ostracods, pollen and dinoflagellate cyst records

    Auteurs: Richards, K.; Hoogendoorn, R.M. (2016)
    Gepubliceerd in: Palaeogeography, palaeoclimatology, palaeoecology : an international journal for the geo-sciences (2016), pagina 1-90

    Six cores, each approximately 10 m long, of late Pleistocene to Holocene age were studied from the Emba Delta region in the north-eastern Caspian Sea. Radiocarbon dates provide ages within the range of 47,820 to 12,020 cal BP for the middle sections, and for post-1950 close to surface. The ages fall within Marine Isotope Stage (MIS) 3, MIS 2 and MIS 1 (with MIS 4 also inferred). Four lithological units are present, each separated by an erosional contact. Unit 4 is equated with MIS 4 and consists of over-consolidated, east-west trending aeolian sands deposited during the late Pleistocene Atelian lowstand. Unit 3 is equated with MIS 3 and is a low-energy, shallow open water or lagoonal deposit based on ostracod faunas. Pollen from mesophilic trees is common, confirming warm climatic conditions. Floristic elements such as Engelhardia and Carya were shared with East Asia. Frequent Taxodiaceae pollen occurs, derived from Glyptostrobus pensilis, a seasonal freshwater swamp tree, now found naturally only in isolated relict stands in East Asia. This suggests that the north-eastern Caspian region was a ‘refugium’ supporting Glyptostrobus swamp vegetation during MIS 3. There is no evidence to indicate that these are reworked occurrences. Unit 2 consists of early Khvalynian transgressive barrier sands and Unit 1 of shoreface sands and muds of Holocene age. The late Khvalynian highstand and the Last Glacial Maximum (LGM) record are eroded by the Mangyshlak unconformity after ca. 12,500 cal BP. The Holocene interval contains frequent foraminifera and dinoflagellate cysts (dinocysts) of restricted ‘marine’ affinity (e.g. Lingulodinium machaerophorum). The dinocysts Pterocysta cruciformis and Impagidinium inaequalis are found commonly in the Caspian Sea for the first time. Results are compared with palynological and ostracod assemblages in surface samples from the eastern Caspian Sea coastal region.

  • A technology for sewer pipe inspection (part 1) : design, calibration, corrections and potential application of a laser profiler

    Auteurs: Stanic, N.; Lepot, M.; Catieau, M.; Langeveld, J.; Clemens, F.H.L.R. (2017)
    Gepubliceerd in: Automation in construction : an international research journal, volume 75 (2017), pagina 91-107

    In order to ensure and extend the serviceability of sewage infrastructure, sewer managers need more accurate data to estimate its actual state. For that purpose, various inspection techniques can be implemented. A first prototype of an unbiased laser profiler was developed to improve the accuracy of collected data (2015). However, a higher accuracy needs to be achieved. This article presents the results of an improved prototype that delivers an increase in the measuring accuracy of 5–10 times, which was achieved by using more accurate equipment (lasers and cameras) and by improvements in the methodologies applied (e.g. removing the last bias image distortion by camera calibration). The new design presented hereafter provides accurate measurements of cross section and accurate 3D image of a pipe. The potential applications of the improved laser profiling technique are comprehensive: e.g. deposit measurements, roughness measurements.

  • Designing monitoring arrangements for collaborative learning about adaptation pathways

    Auteurs: Hermans, L.M.; Haasnoot, M.; Maat, G.J. ter; Kwakkel, J.H. (2017)
    Gepubliceerd in: Environmental science and policy, volume 69 (2017), pagina 29-38

    Adaptation pathways approaches support long-term planning under uncertainty. The use of adaptation pathways implies a systematic monitoring effort to inform future adaptation decisions. Such monitoring should feed into a long-term collaborative learning process between multiple actors at various levels. This raises questions about who should monitor what, when and for whom. We formulate an approach that helps to address these questions, developed around the conceptual core offered by adaptive policy pathways methods and their notion of signposts and triggers. This is embedded in a wider approach that revisits the critical assumptions in underlying basic policies, looks forward to future adaptation decisions, and incorporates reciprocity in the organization of monitoring and evaluation. The usefulness and practical feasibility of the approach is studied for a case of the Delta Programme in the Netherlands, which incorporated adaptation pathways in its planning approach called adaptive delta management. The case results suggest that our approach adds value to existing monitoring practices. They further show that different types of signposts exist. Technical signposts, in particular, need to be distinguished from political ones, and require different learning processes with different types of actors.

  • The influence of grain size on the performance of conductivity concentration meters

    Auteurs: Wijk, J.M. van; Blok, B.W.G. (2015)
    Gepubliceerd in: Flow measurement and instrumentation, volume 45 (2015), pagina 384-390

    The use of conductivity as a means to determine the volume fraction of solids in a suspension of poorly conductive solids in water combines the benefits of high sampling rates and ease of use at the cost of a high sensitivity to salinity and fluid temperature. In this paper we investigate a custom built CCM, to which a third parameter was found to be of much influence: the grain size of the suspended phase. This paper describes a calibration experiment with which the influence of grain size on the CCM output has been investigated. The data give rise to an alternative calibration curve which is different from the regular effective media theories.

  • Analyzing the physics of non-tidal barotropic sea level anomaly events using multi-scale numerical modelling in Singapore regional waters

    Auteurs: Kurniawan, A.; Babovic, V.; Gerritsen, H. (2015)
    Gepubliceerd in: Journal of hydro-environment research, volume 9 (2015) nummer 3, pagina 404-419

    The hydrodynamic flows in the Singapore regional waters (SRW) are the result of a complex mix of tide, seasonal and meteorological effects. The study of non-tidal effects or sea level anomalies (SLA) in this region has shown that it is possible to model some of these anomalies. The present study addresses the non-tidal barotropic water levels and currents in the Singapore region in detail. This analysis includes a multi-scale approach, and addresses amongst others hydrodynamic model grid resolution and the importance of resolving non-linear tide–surge interaction. The results show that the water level and current anomalies phenomena in a complex region like SRW can be effectively modelled using an approach combining non-tidal barotropic and multi-scale numerical modelling. A detailed investigation of the levels of non-linear tide–surge interaction is carried out by simulating SLA events in the Singapore regional waters during North-East (positive SLA) and South-West (negative SLA) monsoons based on ECMWF numerical weather forcing conditions. The results of combining both approaches suggest that the finer grid resolution improves the accuracy of water level and current anomalies simulations. Furthermore, the results also indicate that for the simulations of non-tidal barotropic flows in this area, non-linear tide–surge interaction is important and should be taken into account. Finally, the behaviour of the non-tidal barotropic flow in the region and its changes with tidal variation in the shallow region of the Singapore Strait is now much better understood. Therefore, for reliable operational forecast of sea level, the inclusion of non-linear tide–surge interaction should be required in numerical models to reproduce both tides and surges with improved accuracy in this region.

  • Advies voor algemeen functioneel ontwerp voor de 6e-generatie modellen van RWS

    Auteurs: Spruyt, A.; Minns, T.; Yossef, M.F.M.; Kerkhoven, D.; Zijl, F.; Genseberger, M. (2016)

    Op 15 september 2015 is een mijlpaal gerealiseerd door de release van versie 1.0 van het softwarepakket D-HYDRO Suite. D-HYDRO is het modelleringsplatform van de toekomst voor hydrodynamica, morfodynamica, waterkwaliteit en golven. Door Rijkswaterstaat is aan Deltares gevraagd om de mogelijkheden te onderzoeken die de nieuwe software biedt en een uitgebreid functioneel ontwerp te leveren dat er voor zorgt dat met de bouw van de zesde-generatie modellen kan worden gestart. Dit functioneel ontwerp is bedoeld om de ontwikkeling (opzet, kalibratie en gebruik) van de eerste set modellen in goede banen te leiden. Deze eerste modellen zullen bestaan uit een model van een rivier, zee en meer. Nadat de eerste modellen zijn opgezet, zullen de richtlijnen opnieuw worden bekeken en indien nodig aangepast.

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