Harmen Talstra’s scientific contributions

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Publications (4)


Modelling and analysis of the horizontal configuration of tidal fences in barrages
  • Article

February 2024

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31 Reads

Renewable Energy

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H. Talstra

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Emergence of large-scale coherent structures in a shallow separating flow

August 2006

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20 Reads

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7 Citations

In rivers, the phenomenon of flow separation past obstacles often gives rise to large-scale coherent structures. This study focuses on quasi-2d coherent structures associated with shallow flow separation (vortex shedding). New insights about the physical mechanism governing vortex shedding have been obtained from largescale Particle Image Velocimetry (PIV) experiments on a Shallow Lateral Expansion (SLE) with a variable inflow width. Analysis of the acquired data stresses the crucial role of the secondary circulation cell, which is often present in shallow separation geometries. A sufficiently developed secondary circulation cell is significantly contributing to vortex shedding. Moreover, the interaction between primary and secondary circulation cells causes a "scale jump" in the horizontal length scale of the shed vortices. An analysis of Reynolds' stresses and the downstream development of conditional averaged eddies does clearly show this jump. Since the scale jump is essentially due to interaction of discrete horizontal eddies, 3D Large Eddy Simulations (LES) are performed in analogy with the measured PIV geometries. The obtained LES data will be used to develop a depth-averaged flow model that will reproduce the discrete horizontal eddy interaction in an accurate way, thus enabling researchers and engineers to predict vortex shedding in river geometries.


Figure 4 Action of body forces at impermeable walls in a computational grid. Blue: location of pressure points and velocity vectors. Red: location of normal-wall body forces ensuring wall impermeability
Figure 6 Detection of large-scale eddies by means of a vector potential function. White vectors visualize the instantaneous velocity field at 3 m downstream of separation point. The upper part shows the main flow (from left to right); the lower part shows the primary recirculation backflow and a secondary separation point
Figure 7a-f Comparison of experimental and computational streamwise velocities at the surface. Black solid lines are zero velocity contours; the black circles are locating the secondary separation points; 1 st = location primary gyre; 2 nd = location secondary gyre.
3D LES computations of a shallow lateral expansion using an immersed boundary method
  • Article
  • Full-text available

64 Reads

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2 Citations

Paper presented at The Seventh International Conference on HydroScience and Engineering (ICHE) hosted by the College of Engineering at Drexel Univeristy on September 10-13, 2006 in Philadelphia, Pennsylvania. The conference theme was IT in the Field of HydroSciences. It included several mini-symposia that emphasized IT topics in HydroSciences and the yearly meeting of the metadata group of the International Oceanographic Data and Information Exchange organization. In environmental shallow flows, the phenomenon of flow separation often gives rise to large-scale turbulent structures (vortex shedding). In this study, 3D LES computations of three Shallow Lateral Expansion geometries are performed. The resolved large-scale turbulent structures are studied in detail in order to allow a comparison with laboratory experiments, carried out using the Particle Image Velocimetry (PIV) technique. When LES is applied for practical cases involving flow separation, immersed boundaries are often an essential part of the geometry. These boundaries can cause problems with respect to the Navier Stokes solver used, especially regarding the pressure correction module. A solution to this problem, known as Immersed Boundary Method (IBM), is found by using body forces to ensure the impermeability of internal boundaries. The classical IBM formulation, however, makes a systematic error regarding momentum transfer in the vicinity of solid walls. In this study an adjusted IBM is proposed, based on momentum fluxes instead of body forces. The adjusted model is applied to Shallow Lateral Expansion geometries of various aspect ratios. In order to analyze the real-time large-scale turbulent structures, the vector potential function of the velocity field is computed. This is a very suitable tool to detect large-scale flow structures. The turbulence features observed in the 3D LES computation are compared with the PIV data, especially regarding the vortex shedding behaviour. An analysis of Reynolds stresses and the downstream development of eddy length scales reveals the existence of two different regimes in the vortex shedding behaviour. The difference can be explained by the interaction of shed vortices with the primary and secondary recirculation cells that are present.

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Citations (1)


... Many previous works have investigated the spatio-temporal dynamics of Q2CS created by plane shear instabilities e.g. ( Chu and Babarutsi, 1988;Rhoads and Sukhodolov, 2004;Uijttewaal and Tukker, 1998 ), and have shown that due to the effects of bedfriction the spread/growth rate of a shallow mixing layer is modified. In an experimental study to investigate the effect of topographical forcing on a shallow flow, Talstra et al. (2006) found that unlike in a deep flow, ( Armaly et al., 1983 ), the shallow flow mixing layer bound a second counter rotating recirculation cell. They also found at the downstream edge of the first recirculation cell there was a sudden expansion in the mixing layer. ...

Reference:

Using modal decompositions to explain the sudden expansion of the mixing layer in the wake of a groyne in a shallow flow
Emergence of large-scale coherent structures in a shallow separating flow
  • Citing Article
  • August 2006