GertJan van Heijst

Numerical simulation of tripolar vortices in 2D flow

The formation of a tripolar vortex in a two-dimensional flow is simulated numerically for two different cases, viz. the tripole arising from a collision of two Lamb dipoles, and the emergence of a tripole from an initially axisymmetric, unstable vortex. This latter situation was also considered in a laboratory experiment by van Heijst, Kloosterziel and Williams, and the numerical results show very good agreement with their observations, both qualitatively and quantitatively. Under certain conditions a higher wavenumber instability is found, resulting in a triangular vortex which itself turns out to be unstable. The results of the numerical simulation agree fairly well with laboratory observations of this higher-order instability scenario.

Vortex interactions with flapping wings and fins can be unpredictable

As they fly or swim, many animals generate a wake of vortices with their flapping fins and wings that reveals the dynamics of their locomotion. Previous studies have shown that the dynamic interaction of vortices in the wake with fins and wings can increase propulsive force. Here, we explore whether the dynamics of the vortex interactions could affect the predictability of propulsive forces. We studied the dynamics of the interactions between a symmetrically and periodically pitching and heaving foil and the vortices in its wake, in a soap-film tunnel. The phase-locked movie sequences reveal that abundant chaotic vortex-wake interactions occur at high Strouhal numbers. These high numbers are representative for the fins and wings of near-hovering animals. The chaotic wake limits the forecast horizon of the corresponding force and moment integrals. By contrast, we find periodic vortex wakes with an unlimited forecast horizon for the lower Strouhal numbers (0.2–0.4) at which many animals cruise. These findings suggest that swimming and flying animals could control the predictability of vortex-wake interactions, and the corresponding propulsive forces with their fins and wings.

The role of Stewartson and Ekman layers in turbulent rotating Rayleigh-Bénard convection

When the classical Rayleigh–Benard (RB) system is rotated about its vertical axis roughly three regimes can be identified. In regime I (weak rotation) the large-scale circulation (LSC) is the dominant feature of the flow. In regime II (moderate rotation) the LSC is replaced by vertically aligned vortices. Regime III (strong rotation) is characterized by suppression of the vertical velocity fluctuations. Using results from experiments and direct numerical simulations of RB convection for a cell with a diameter-to-height aspect ratio equal to one at Ra~10 8-10 9 (Pr=4-6) and 0<~1/Ro<~25 we identified the characteristics of the azimuthal temperature profiles at the sidewall in the different regimes. In regime I the azimuthal wall temperature profile shows a cosine shape and a vertical temperature gradient due to plumes that travel with the LSC close to the sidewall. In regimes II and III this cosine profile disappears, but the vertical wall temperature gradient is still observed. It turns out that the vertical wall temperature gradient in regimes II and III has a different origin than that observed in regime I. It is caused by boundary layer dynamics characteristic for rotating flows, which drives a secondary flow that transports hot fluid up the sidewall in the lower part of the container and cold fluid downwards along the sidewall in the top part.

Modeling of oceanic vortices

A colloquium was held in Amsterdam, from May 11 to 13, 1993, to discuss research on oceanic vortices and coherent structures in fluid dynamics. The colloquium was organized under the auspices of the Royal Netherlands Academy of Arts and Sciences with additional support from several Dutch educational institutions. The interest in vortices and coherent structures stems from their ubiquity in geophysical flows, their relative longevity, and their crucial role in turbulence.

Effects of Rotation and Stratification: An Introduction

Large-scale flows in the natural environment can be influenced by the planetary rotation and also by density differences. This chapter aims to provide an informal introduction into the effects of background rotation and stratification.

Mixing and Dispersion in Flows Dominated by Rotation and Buoyancy

The book presents a state-of-the-art overview of current developments in the field in a way accessible to attendees coming from a variety of fields. Relevant examples are turbulence research, (environmental) fluid mechanics, lake hydrodynamics and atmospheric physics. Topics discussed range from the fundamentals of rotating and stratified flows, mixing and transport in stratified or rotating turbulence, transport in the atmospheric boundary layer, the dynamics of gravity and turbidity currents eventually with effects of background rotation or stratification, mixing in (stratified) lakes, and the Lagrangian approach in the analysis of transport processes in geophysical and environmental flows. The topics are discussed from fundamental, experimental and numerical points of view. Some contributions cover fundamental aspects including a number of the basic dynamical properties of rotating and or stratified (turbulent) flows, the mathematical description of these flows, some applications in the natural environment, and the Lagrangian statistical analysis of turbulent transport processes and turbulent transport of material particles (including, for example, inertial and finite-size effects). Four papers are dedicated to specific topics such as transport in (stratified) lakes, transport and mixing in the atmospheric boundary layer, mixing in stratified fluids and dynamics of turbidity currents. The book is addressed to doctoral students and postdoctoral researchers, but also to academic and industrial researchers and practicing engineers, with a background in mechanical engineering, applied physics, civil engineering, applied mathematics, meteorology, physical oceanography or physical limnology.

The effect of finger spreading on drag of the hand in human swimming

The effect of finger spread on overall drag on a swimmers hand is relatively small, but could be relevant for elite swimmers. There are many sensitivities in measuring this effect. A comparison between numerical simulations, experiments and theory is urgently required to observe whether the effect is significant. In this study, the beneficial effect of a small finger spread in swimming is confirmed using three different but complementary methods. For the first time numerical simulations and laboratory experiments are conducted on the exact same 3D model of the hand with attached forearm. The virtual version of the hand with forearm was implemented in a numerical code by means of an immersed boundary method and the 3D printed physical version was studied in a wind tunnel experiment. An enhancement of the drag coefficient of 2% and 5% compared to the case with closed fingers was found for the numerical simulation and experiment, respectively. A 5% and 8% favorable effect on the (dimensionless) force moment at an optimal finger spreading of 10° was found, which indicates that the difference is more outspoken in the force moment. Moreover, an analytical model is proposed, using scaling arguments similar to the Betz actuator disk model, to explain the drag coefficient as a function of finger spacing.

Spin-up of a source-sink flow over a model continental shelf

Abstract Results of analytical and experimental models are presented in which the role of various forms of bottom topography on externally driven continental shelf currents has been investigated. The shelf currents are generated in a rotating cylindrical geometry by means of a source-sink technique. A linear analytical model for a homogeneous fluid in this configuration predicts that the azimuthal (swirl) velocity above a flat bottom is inversely proportional to the radial distance from the origin. This velocity profile is shown to be altered if the bottom boundary consists of a model continental shelf and slope. Then a geometrical function has to be included to describe the azimuthal velocity profile above the sloping bottom. This function depends only on the slope angle a and differs only significantly from unity for large values of α (α > 30°). As a result, a free Stewartson layer is generated above the shelf break to account for the azknuthal velocity shear between the two interior regions. The net ve...