University of Twente Student Theses


Numerical analysis of flow characteristics near neighbouring vegetation patches of different densities

Geurts, Maartje (2022) Numerical analysis of flow characteristics near neighbouring vegetation patches of different densities.

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Abstract:River vegetation provides a lot of benefits to its environment, however there are also some negative effects related to this vegetation. Some of these benefits are improving the water quality and providing a habitat, a negative effect is the addition of drag which could lead to a rise in water levels. In order to manage river vegetation it is important to gain a better understanding of the effect it has on the flow. A lot of research has focused on either a single vegetation patch or two vegetation patches in order to investigate the flow processes generated by the presence of a vegetation patch and the interaction between multiple patches. Not much research has gone into understanding the processes surrounding two patches with different densities, and when this is considered, the location between the patches is not varied to study the additional influence of that aspect. In order to expand the knowledge on river vegetation, it is necessary to gain a better understanding of the behaviour around vegetation patches of different densities with varying interpatch distances. This led to the objective of this research: to study the effect of vegetation patches with different densities on the flow velocity distribution and sediment deposition. In order to do this, the first step was to construct a numerical model with the CFD code FLUENT®. Several different model settings and turbulence closure models were tested and compared against validation data of a single vegetation patch with a solid volume fraction of 3%. Two aspects were of main interest: the length of the steady wake region and the simulated flow velocities on the centerline behind the patch. A circular patch with 37 stems and a RNG k − ϵ turbulence closure model with standard wall functions corresponded best with the validation data. The difference between the simulated steady wake length and the steady wake length of the validation patch was 13%. The maximum difference between the simulated centerline velocities and the measured centerline velocities of the validation data was 11%. It was found that changing the density of an upstream patch does not have a significant effect on the wake velocity of the downstream patch. The turbulence levels in the wake of the downstream patch were affected and increased for denser upstream patches. Different densities for the downstream patch had a much larger effect on the wake of the upstream patch compared to the effect of an upstream patch with different densities. A denser downstream patch resulted in a shorter steady wake length and higher minimum velocities behind the upstream patch. Turbulence levels were also higher and a Turbulent Kinetic Energy (TKE) peak was seen further upstream for a denser downstream patch compared to a sparser patch. When the patches were placed further apart in the longitudinal direction (6D), the steady wake length behind the upstream patch became longer and the TKE at the end of the steady wake region was higher than for patches placed closer together. When the patches were placed further apart in the transverse direction (2.5D), it was found that the upstream wake recovered quicker to the undisturbed upstream velocity. The last step was to evaluate the sediment deposition. Two methods were used to analyse this. The first method was the velocity and TKE method, in which a threshold velocity and threshold TKE value were selected. If the measured values are below the threshold values, sediment deposition takes place.With this method it was found that the location of the patches does not matter, only the amount of flow blockage they represent combined. A patch combination with sparser patches resulted in a larger area of enhanced sediment deposition. Another method was the velocity threshold method. For this method only a threshold velocity was chosen, below which sediment deposition takes place. This method concluded that patch combinations which presented a larger flow blockage will result in more area of enhanced sediment deposition and thus an increase in potential new vegetation growth. Both methods did find that the most significant location of enhanced sediment deposition occurred behind the patch, extending from the steady wake region. Both methods also did not report the presence of a secondary deposition zone. In conclusion, the density of a neighbouring patch placed downstream of another patch had a significant effect on the flow velocities and turbulence levels in the wake of the upstream patch. Depending on the density and location, these effects were increased or reduced, but always present for the chosen interpatch distances. The density of a neighbouring patch placed upstream of another patch had no significant eff on the wake of the other patch. In terms of sediment deposition and vegetation growth, the most significant changes in the areas of enhanced sediment deposition were related to different patch densities and not as much to the location of the patches.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Programme:Civil Engineering and Management MSc (60026)
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