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University of Twente Publications (58.244 recursos)

University of Twente Repository provides publications of UT scientific staff, in many cases full text and publicly available.

Group = KEUR

Mostrando recursos 1 - 20 de 7.218

  1. Zandsuppletie Oesterdam. De ontwikkeling van een zandplaat in de Oosterschelde

    Pezij, M.; Werf, J.J. van der; Vet, L. de; Hulscher, S.J.M.H.

  2. Tide-scale flow routing and sedimentation in mangrove forests: combining field data and numerical modelling, doi: 10.1016/j.geomorph.2014.08.011

    Horstman, E.M.; Dohmen-Janssen, C.M.; Bouma, T.J.; Hulscher, S.J.M.H.
    Tidal-scale biophysical interactions establish particular flow routing and sedimentation patterns in coastal mangroves. Sluggish water flows through the mangrove vegetation and enhanced sediment deposition are essential to maintain these valuable ecosystems, thereby enabling their contribution to coastal protection and stabilization. Spatially explicit field observations of tidal-scale flow routing and sediment deposition were obtained in an elevated mangrove stand dissected by tidal creeks, located in the Trang river estuary at the Thai Andaman coast. An accurate and efficient depth-averaged process-based numerical model of this field site was developed in Delft3D to study the contributions of various biogeophysical mangrove settings to the...

  3. Vertrouwen in een vitale democratie of democratische vernieuwing?

    Thomassen, Jacques

  4. Een nieuwe dimensie in de Nederlandse politiek

    Aarts, Kees; Thomassen, Jacques

  5. Exploring the phase space of multiple states in highly turbulent Taylor-Couette flow

    Veen, R.C.A. van der; Huisman, S.G.; Dung, O.Y.; Tang, H.L.; Sun, C.; Lohse, D.
    We investigate the existence of multiple turbulent states in highly turbulent Taylor-Couette flow in the range of Ta=10 11 to 9×10 12 by measuring the global torques and the local velocities while probing the phase space spanned by the rotation rates of the inner and outer cylinders. The multiple states are found to be very robust and are expected to persist beyond Ta=10 13 . The rotation ratio is the parameter that most strongly controls the transitions between the flow states; the transitional values only weakly depend on the Taylor number. However, complex paths in the phase space are necessary...

  6. Line Tension and Wettability of Nanodrops on Curved Surfaces

    Maheshwari, S.; Hoef, M.A. van der; Lohse, D.
    In this work we study the formation of nanodrops on curved surfaces (both convex and concave) by means of molecular dynamics simulations, where the particles interact via a Lennard-Jones potential. We find that the contact angle is not affected by the curvature of the substrate, in agreement with previous experimental findings. This means that the change in curvature of the drop in response to the change in curvature of the substrate can be predicted from simple geometrical considerations, under the assumption that the drop’s shape is a spherical cap, and that the volume remains unchanged through the curvature. The resulting...

  7. How a Surface Nanodroplet Sits on the Rim of a Microcap

    Peng, S.; Devic, I.; Tan, H.; Lohse, D.; Zhang, X.
    The location and morphology of femtoliter nanodroplets that nucleate and grow on a microcap-decorated substrate in contact with a liquid phase were investigated. We experimentally examined four different wetting combinations of the flat area and the microcaps. The results show that depending on the relative wettability, the droplets sit either on the plain surface or on the top of the microcap or on the rim of the microcap. The contact angle and, for the last case, the radial positions of the nanodroplets relative to the microcap center were characterized, in reasonable agreement with our theoretical analysis, which is based on...

  8. Impact of a high-speed train of microdrops on a liquid pool

    Bouwhuis, W.; Huang, X.; Chan, C.U.; Frommhold, P.E.; Ohl, C.D.; Lohse, D.; Snoeijer, J.H.; Meer, D. van der
    A train of high-speed microdrops impacting on a liquid pool can create a very deep and narrow cavity, reaching depths more than 1000 times the size of the individual drops. The impact of such a droplet train is studied numerically using boundary integral simulations. In these simulations, we solve the potential flow in the pool and in the impacting drops, taking into account the influence of liquid inertia, gravity and surface tension. We show that for microdrops the cavity shape and maximum depth primarily depend on the balance of inertia and surface tension and discuss how these are influenced by...

  9. Effect of Disjoining Pressure on Surface Nanobubbles

    Svetovoy, V.; Devic, I.; Snoeijer, J.H.; Lohse, D.
    In gas-oversaturated solutions, stable surface nanobubbles can exist thanks to a balance between the Laplace pressure and the gas overpressure, provided the contact line of the bubble is pinned. In this article, we analyze how the disjoining pressure originating from the van der Waals interactions of the liquid and the gas with the surface affects the properties of the surface nanobubbles. From a functional minimization of the Gibbs free energy in the sharp-interface approximation, we find the bubble shape that takes into account the attracting van der Waals potential and gas compressibility effects. Although the bubble shape slightly deviates from...

  10. Dynamic Leidenfrost Effect: Relevant Time and Length Scales

    Shirota, M.; Limbeek, M.A.J. van; Sun, C.; Prosperetti, A.; Lohse, D.
    When a liquid droplet impacts a hot solid surface, enough vapor may be generated under it to prevent its contact with the solid. The minimum solid temperature for this so-called Leidenfrost effect to occur is termed the Leidenfrost temperature, or the dynamic Leidenfrost temperature when the droplet velocity is non-negligible. We observe the wetting or drying and the levitation dynamics of the droplet impacting on an (isothermal) smooth sapphire surface using high-speed total internal reflection imaging, which enables us to observe the droplet base up to about 100 nm above the substrate surface. By this method we are able to...

  11. Microbubbles and Microparticles are Not Faithful Tracers of Turbulent Acceleration

    Mathai, V.; Calzavarini, E.; Brons, J.; Sun, C.; Lohse, D.
    We report on the Lagrangian statistics of acceleration of small (sub-Kolmogorov) bubbles and tracer particles with Stokes number St≪1 in turbulent flow. At a decreasing Reynolds number, the bubble accelerations show deviations from that of tracer particles; i.e., they deviate from the Heisenberg-Yaglom prediction and show a quicker decorrelation despite their small size and minute St. Using direct numerical simulations, we show that these effects arise due the drift of these particles through the turbulent flow. We theoretically predict this gravity-driven effect for developed isotropic turbulence, with the ratio of Stokes to Froude number or equivalently the particle drift velocity...

  12. Drop impact of shear thickening liquids

    Boyer, F.C.; Sandoval Nava, E.; Snoeijer, J.H.; Dijksman, J.F.; Lohse, D.
    The impact of drops of concentrated non-Brownian suspensions (cornstarch and polystyrene spheres) onto a solid surface is investigated experimentally. The spreading dynamics and maximal deformation of the droplet of such shear thickening liquids are found to be markedly different from the impact of Newtonian drops. A particularly striking observation is that the maximal deformation is independent of the drop velocity and that the deformation suddenly stops during the impact phase. Both observations are due to the shear thickening rheology of the suspensions, as is explained theoretically from a balance between the kinetic energy and the viscously dissipated energy, from which...

  13. Drop deformation by laser-pulse impact

    Gelderblom, H.; Lhuissier, H.E.; Klein, A.L.; Bouwhuis, W.; Lohse, D.; Villermaux, E.; Snoeijer, J.H.
    A free falling, absorbing liquid drop hit by a nanosecond laser pulse experiences a strong recoil pressure kick. As a consequence, the drop propels forward and deforms into a thin sheet which eventually fragments. We study how the drop deformation depends on the pulse shape and drop properties. We first derive the velocity field inside the drop on the time scale of the pressure pulse, when the drop is still spherical. This yields the kinetic energy partition inside the drop, which precisely measures the deformation rate with respect to the propulsion rate, before surface tension comes into play. On the time...

  14. Direct numerical simulation of Taylor–Couette flow with grooved walls: torque scaling and flow structure

    Zhu, X.; Ostilla-Monico, Rodolfo; Verzicco, R.; Lohse, D.
    We present direct numerical simulations of Taylor–Couette flow with grooved walls at a fixed radius ratio ${\it\eta}=r_{i}/r_{o}=0.714$η=ri/ro=0.714 with inner cylinder Reynolds number up to $Re_{i}=3.76\times 10^{4}$Rei=3.76×104, corresponding to Taylor number up to $Ta=2.15\times 10^{9}$Ta=2.15×109. The grooves are axisymmetric V-shaped obstacles attached to the wall with a tip angle of 90°. Results are compared to the smooth wall case in order to investigate the effects of grooves on Taylor–Couette flow. We focus on the effective scaling laws for the torque, flow structures, and boundary layers. It is found that, when the groove height is smaller than the boundary layer thickness, the...

  15. Drag reduction in numerical two-phase Taylor–Couette turbulence using an Euler–Lagrange approach

    Arza, V.S.; Ostilla-Monico, Rodolfo; Verzicco, R.; Lohse, D.
    Two-phase turbulent Taylor–Couette (TC) flow is simulated using an Euler–Lagrange approach to study the effects of a secondary phase dispersed into a turbulent carrier phase (here bubbles dispersed into water). The dynamics of the carrier phase is computed using direct numerical simulations (DNS) in an Eulerian framework, while the bubbles are tracked in a Lagrangian manner by modelling the effective drag, lift, added mass and buoyancy force acting on them. Two-way coupling is implemented between the dispersed phase and the carrier phase which allows for momentum exchange among both phases and to study the effect of the dispersed phase on...

  16. Collective and convective effects compete in patterns of dissolving surface droplets

    Laghezaa, G.; Dietrich, E.; Yeomans, J.M.; Ledesma-Aguilar, R.A.; Kooij, E.S.; Zandvliet, H.J.W.; Lohse, D.
    The effects of neighboring droplets on the dissolution of a sessile droplet, i.e. collective effects, are investigated both experimentally and numerically. On the experimental side small approximately 20 nL mono-disperse surface droplets arranged in an ordered pattern were dissolved and their size evolution is studied optically. The droplet dissolution time was studied for various droplet patterns. On the numerical side, lattice-Boltzmann simulations were performed. Both simulations and experiments show that the dissolution time of a droplet placed in the center of a pattern can increase by as much as 60% as compared to a single, isolated droplet, due to the...

  17. Effects of varying degrees of surface strain anisotropies on endothelial cells

    Sinha, R.; Gac, S. le; Verdonschot, N.J.J.; Berg, A. van den; Koopman, H.F.J.M.; Rouwkema, J.
    Introduction: Cyclic strain is well known to affect cell behavior. It is also known that isotropic and anisotropic strain can affect cells differently[1]. While in-vivo cells experience varying degrees of anisotropy (d.o.a.), in-vitro anisotropic strain studies have mostly focused on uniaxial strains. In order to create a better understanding of cellular behaviour under physiological strain conditions, the response of cells to strains with varying d.o.a. should be investigated. In this study, we determined the effects of varying d.o.a. on human umbilical vein endothelial cells (HUVECs) using a newly developed device. Studies like this can be used to determine and optimize the...

  18. The effectiveness of a math game: The impact of integrating conceptual clarification as support

    Vandercruysse, Sylke; Vrugte, Judith ter; Jong, Ton de; Wouters, Pieter; Oostendorp, Herre van; Verschaffel, Lieven; Moeyaert, Mariola; Elen, Jan
    This study investigates the impact of integrating conceptual clarifications as support in an educational math game, and explores the impact of adding this (internal vs. external) support on students’ game and mathematical performance, intrinsic motivation, and game perception. Three conditions are established: a condition in which internal support is offered, a condition in which (identical) external support is offered, and a control condition in which no support is added to the game. One hundred twenty-two vocational secondary education students participated in this study. The results of this study indicate that students benefit from playing with an educational game in order...

  19. Analysis of the driving force of hydrodynamics in Lake Naivasha, Kenya

    Ndungu, J.N.; Chen, W.; Augustijn, D.C.M.; Hulscher, S.J.M.H.
    Initial fixation of press-fit implants depends on interference fit, surface morphology, and bone material properties. To understand the biomechanical effect of each factor and their interactions, the pull-out strength of seven types of CoCrMo tapered implants, with four different interference fits, three different surface morphologies (low, medium and high roughness), and at two time points (0 and 30 min) were tested in trabecular bone with varying density. The effect of interference fit on pull-out strength depended on the surface morphology and time. In contrast with our expectations, samples with a higher roughness had a lower pull-out strength. We found a...

  20. CO Adsorption on Pt Nanoparticles in Low E-Fields Studied by ATR-IR Spectroscopy in a Microreactor

    Susarrey Arce, A.; Tiggelaar, R.M.; Gardeniers, J.G.E.; Houselt, A. van; Lefferts, L.
    The adsorption of carbon monoxide (CO) on platinum (Pt) nanoparticles under the influence of a low electrical field (E-field) is studied. The adsorption of CO is monitored by attenuated total reflection infrared (ATR-IR) spectroscopy, while an external electrical field is applied over the microreactor. Linearly (on top; COL), bridged (two-fold hollow; COB), and multiply (three-fold hollow; CO) adsorbed CO species were found on the Pt nanoparticles. In a sequence of adsorption experiments the integrated ATR-IR intensity of the absorption band of the linearly adsorbed COL was found to reach steady-state values after an initial decrease, which is attributed to a...

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