Development and validation of a three-dimensional morphological model

Coastal morphodynamic evolution techniques

The predictability of cross-shore bed evolution of sandy beaches at the time scale of storms and seasons using process-based Profile models

When Segre and Silberberg in 1961 witnessed particles in a laminar pipe flow congregating at an annulus in the pipe, scientists were perplexed and spent decades learning why such behavior occurred, finally understanding that it was caused by previously unknown forces on particles in an inertial flow. The advent of microfluidics opened a new realm of possibilities for inertial focusing in the processing of biological fluids and cellular suspensions and created a field that is now rapidly expanding. Over the past five years, inertial focusing has enabled high-throughput, simple, and precise manipulation of bodily fluids for a myriad of applications in point-of-care and clinical diagnostics. This review describes the theoretical developments that have made the field of inertial focusing what it is today and presents the key applications that will make inertial focusing a mainstream technology in the future.

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Inertial Focusing in Microfluidics

In a channel with bed topography that does not vary in the downstream direction, a secondary circulation composed of outward flow at the surface and inward flow near the bottom extends across the entire width. If the curvature is constant, the cross-stream velocity component near the bed and the pattern of boundary shear stress can be estimated by assuming fluid accelerations to be small. Unfortunately, this procedure cannot be used in analyzing the flow through natural river meanders, or through channels with downstream constant bottom topography but with rapidly changing curvature. In these latter cases, effects arising from bed- and bank-induced momentum changes must be accounted for. Evidence for a substantial topographically induced alteration in the cross-stream flow pattern relative to that for the analogous constant bottom topography case is provided through new analyses of several sets of laboratory and field data. Shoaling over the point bar in the upstream part of the bend is shown to force the high-velocity core of the flow toward the pool. This is accomplished by a convective acceleration-caused decrease in the cross-stream water surface slope and a resulting dominance of the vertically averaged centrifugal force. The primary effect is a velocity component toward the outside or concave bank throughout the flow depth over the upstream, shallow part of the point bar and an outward component of boundary shear stress in this region. The channel curvature-induced inward component of boundary shear stress consequently is confined to 20 or 30% of the channel width at the pool. Outward transfer of momentum over the point bar, as manifested by a rapid crossing of the high-velocity core from the inside bank to the outside one, contributes to an enhanced decrease in boundary shear stress along the convex side of the stream as the top of the bar is approached. Forces arising from topographically induced spatial accelerations are of the same order of magnitude as the downstream boundary shear stress and water surface slope force components, so they must be modeled as zero-order, not first- or second-order, effects.

Influence of the point bar on flow through curved channels

Recent research activities at the Delft Hydraulics Laboratory and the Delft University of Technology have increased the understanding of the large-scale bed deformation in alluvial rivers with relatively stable banks. In this paper the most important results of these activities are given. First bed deformation in river bends is explained in the light of the results of some laboratory experiments in curved flumes with fixed banks. The deformation is then explained in terms of wave length and damping with a linear analysis of the water and sediment motion for the steady state. From this analysis the conclusion is drawn that the point bar height and pool depth in bends cannot be predicted solely from local conditions. A significant part of the lateral bed slope is due to an overshoot effect induced by the redistribution of the water and sediment motion in the first part of the bend. Finally the results of computations with a non-linear two-dimensional model, simulating three curved flume experiments, are dis...

Bed deformation in curved alluvial channels

https://repository.tudelft.nl/islandora/object/uuid%3Aa45c14e5-05c0-48b3-8fed-2bea3740e69c/datastream/OBJ/download

Approaches to long-term modelling of coastal morphology : A review

Secondary currents are a characteristic feature of flow in open-channel bends. Besides the classical helical motion (centre-region cell), a weaker and smaller counter-rotating circulation cell (outer-bank cell) is often observed near the outer bank, which is believed to play an important role in bank erosion processes. The mechanisms underlying the circulation cells, especially the outer-bank cell, are still poorly understood, and their numerical simulation still poses problems, not least due to lack of detailed experimental data. The research reported herein provides detailed experimental data on both circulation cells in an open-channel bend such as found in nature. Furthermore, the underlying dynamics are investigated by simultaneously analysing the vorticity equation and the kinetic energy transfer between the mean flow and the turbulence. This shows that turbulence plays a minor role in the generation of the centre-region cell, which is mainly due to the centrifugal force. By accounting for the feedback between the downstream velocity profile and the centre-region cell, a strongly simplified vorticity balance is shown to yield accurate predictions of the velocities in the centre region. For strong curvatures, however, a fully threedimensional flow description is required. Due to the non-monotonic velocity profiles, the centrifugal force favours the outer-bank cell. Moreover, terms related to the anisotropy of the cross-stream turbulence, induced by boundary proximity, are of the same order of magnitude and mainly enhance the outer-bank cell. Both mechanisms strengthen each other. The occurrence of the outer-bank cell is shown to be not just due to flow instability, like in the case of curved laminar flow, but also to kinetic energy input from turbulence.

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Secondary Flow in Sharp Open-channel Bends

https://ris.utwente.nl/ws/files/68838082/DeVriend1993medium.pdf

Medium-term 2DH coastal area modelling

Starting from a logarithmic main velocity profile, the vertical distribution of the secondary flow in a shallow curved channel is derived. The secondary circulation turns out to have one vertical and two horizontal components. The horizontal component in the main flow direction arises from the longitudinal acceleration of the main flow; the transverse horizontal component is due to the curvature of the main flow. In addition, a numerical method for the computation of the depth-averaged flow field and the bed shear stress (magnitude and direction) is developed. This model has great flexibility of allowable channel geometry. Its results agree better with experimental data if the bed is not flat than in case of a flat bed.

/pdf/a-mathematical-model-of-steady-flow-in-curved-shallow-13dx0p1ujh.pdf

H.J. De Vriend

Papers

Bed deformation in curved alluvial channels

Approaches to long-term modelling of coastal morphology : A review

Secondary Flow in Sharp Open-channel Bends

Medium-term 2DH coastal area modelling

A Mathematical Model Of Steady Flow In Curved Shallow Channels