1 

Eco Flowform
Flowforms zijn relatief nieuwe waterlopen en zijn niet heel bekend. In flowforms kan er een bijzondere stroming opgewekt worden. Ze zijn puur experimenteel ontworpen en er zit geen theorie achter het ontwerp. Het doel is om deze bijzondere stroming toe te passen bij vistrappen. Het probleem is dat de flowforms kleinschalig zijn in vergelijking tot vistrappen. Daarnaast is er geen theorie bekend over de stroming. Daarom wordt voor het onderzoek de flowform als eerst gemodelleerd en de daar bijbehorend theorie wordt opgesteld. Het model met theorie wordt getoetst aan de hand van experimenten die gedaan worden aan een flowformcascade die in Rotterdam saan. Hieruit blijkt dat het gekozen model klopt en met deze theorie kan verder gewerkt worden. De bijzondere stroming in de flowform kan plaatsvinden door de vorm en afmetingen van de flowform, en ontstaat door een verstoring waardoor het systeem in eigentrilling raakt. Voor het opschalen is het daarom van belang om elke geometrisch eigenschap met een lineaire verhouding op te schalen. Uit een extra experiment dat door middel van plexiglazen bakken is uitgevoerd volgt, zoals eerder theoretisch is verondersteld, dat de schaling van de hydrodynamische eigenschappen plaatsvindt volgens Froude. Verder is er gekeken wanneer de gezochte beweging in resonantie raakt en wanneer deze beweging nog wel kan bestaan ondanks da de stroming niet zo aansprekend is wanneer het in resonantie is.
Voor c=1 geldt dat resonantie optreedt. Voor c>1 treedt er geen resonantie op maar blijft de beweging in minder mooie vorm bestaan. En voor c=0,5 vindt er demping van het systeem plaats. De toepasbaarheid kan onderzocht worden nu het duidelijk is hoe de stroming werkt en hoe het geheel is op te schalen. Volgens bovenstaande verhouding is de diepte uitgezet tegen de snelheid. Dit is in een Excel‐document gedaan. Uit deze data volgt dat de bijzondere stroming alleen kan plaatsvinden in het minder diepe water in de orde van enkele decimetrs. Hieruit blijkt dat het opschalen voor de toepassing van vistrappen vooral geldt voor de kleinere sterkere vis.

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2 

Turbulence in shallow jet flows
The general flow pattern of an open channel flow, downstream of a width restriction by two artificial dams, is analysed. A physical Froudescaled model, under hydraulic rough conditions, with a significant large Reynolds number is used to ensure turbulent flow.
Upstream of the dams the flow is uniform in transverse direction, in between and downstream of the narrow part a jet is formed. On both sides of the jet large eddies are formed bounded by the wall, the jet and the dams. Due to the large velocity gradient in transverse direction a mixing layer develops at both sides of the jet. The width of the mixing layer, as expected, grows with the downstream distance and exceeds the water depth. 2D structures are clearly visible by injecting dye. In the mixing layer besides the macro time and spatial scales, the small Taylor and Kolmogorov scales are present. Whereas the macro scales are well represented in the measured data, the small scales are impossible to mark due to limitations of the Doppler device.
When there is initial no net momentum in transverse direction present the jet is expected to appear symmetrical. However the jet is aligned to one of the sides every time the model starts to run. The preference for one or the other side seems to be random and cannot be related to momentum in transverse direction in between the dams. During measurements the position of the jet is stationary. The fixed position of the jet during measurements can be related to the Coandă effect.
When the flow is disturbed and transverse momentum is added to the upstream flow, the jet can be deflected. The position of the jet and the evolving mixing layers can be related very well to the measured velocities upstream.
Due to the limitations of the used momentum balance equation and use of the mean velocity in the bottom friction calculation the measured head loss is large compared to the calculated dissipative terms (bottom friction and Carnot loss).

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3 

Hydrodynamics of partially vegetated channels: stem drag forces and application to an instream wetland concept for tropical, urban drainage systems
Introduction:
The addition of vegetation to the banks urban drainage channels is an increasingly common measure for improving water quality, enhancing ecological health, and improving aesthetic appeal. An instream wetland concept is proposed for channels in Singapore to just that, but faces a major challenge in that high flow rates during storm events will increase the risk of damage to the vegetation.
Problem definition:
In highflow situations, flow around the vegetation causes a region of turbulent shear to develop along the interface between the vegetation and main, openchannel flow. This is expected to impact the hydraulic resistance in the channel and the drag forces experience by individual plants, especially those nearest to the interface. For design it is important to understand the effects of this turbulent shear on drag to optimize design to limit upstream flooding risk and assess the risks of damage to plants.
Research:
The aim of the research was to increase understanding of the effects of lateral, turbulent shear on the channel resistance and forces experienced by individual stems in a uniform patch of vegetation. A patch of uniform vegetation was modelled by an array of rigid cylinders in an experimental flume. Measurements of the flow field and the drag forces on individual cylinders were recorded. Using this data, answers to the following questions were found:
1) What is the effect of lateral turbulent motion on channel resistance?
2) How do drag forces on individual stems vary spatially over the patch and in time, with special attention to local maximums?
3) What is the effect of lateral turbulent motion on fluctuations in the stem drag force?
4) What are the implications for estimation of the mean and maximum stem drag forces?
Results:
Analysis of the experimental results revealed the following:
1) The presence of a lateral shear layer significantly increased channel resistance, by 175% when compared to similar conditions when the shear layer was not allowed to develop.
2) Drag forces on stems mirrored the velocity distributions in both time and space, showing both higher mean and maximum values near the interface between the stem array and open channel.
3) Similar periodicity in the velocity and force signals gave evidence of coherent, turbulent structures as the primary means of momentum transport across the vegetation interface. This motion causes a sweepejection pattern in the flow at the interface with a net flux of momentum towards the vegetation, resulting in a skewed distribution of stem forces towards higher extreme values.
Conclusions and recommendations:
Lateral turbulent shear is an important factor in both the channel resistance and stem drag forces in a partially vegetated channel. Coherent structures at the vegetation interface were determined to be the main factor in stem force distribution within the region of shear. The mean stem force can be derived directly from the mean velocity given adequate assumptions of the vegetative drag coefficients. A conceptual model was developed to describe the maximum force in the vegetation patch as a function of the mean velocity at the vegetative interface and a fraction of the difference in velocities between the vegetated and open channel sections.

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4 

A hydromechanically coupled flowline model for simulating glacier surge behaviour of landterminating, hardbedded, temperate glaciers
Glacier surge behaviour is characterized by multiyear, quasiperiodic changes in glacier geometry and ice flow. Various attempts have been done on modelling glacier surge behaviour. The focus of this research will be on landterminating, hardbedded, temperate glaciers (LHTglaciers). Whereas the drainage system at the base of the glacier is thought to be the key element in the surge behaviour of those glaciers, developed models do not include a basal drainage model.
The surges of LHTglaciers are characterized by a sudden initiation and a sudden termination of the surge. This sudden on and offset is assigned to switches in basal drainage system. Therefore, a description of the transformation of the basal drainage system during a surge cycle of LHTglaciers is proposed. The proposed transformation is based on observations of the 19821983 surge of Variegated Glacier, Alaska. Despite the fact that models that are capable of simulating quasiperiodic behaviour related to basal frictional lubrication do not include a basal drainage model, building blocks of a basal drainage model for surging glaciers have been proposed for a long time. By means of translating the proposed transformation of the basal drainage system into those building blocks, a basal drainage model is constructed.
The constructed basal drainage model is coupled to the ice mass continuity equation. By means of implementing the coupled model, it is shown that the model simulates the characteristic features of the surge cycle of LHTglaciers.

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5 

Golvende watersprong door verschil in bodemruwheid
Tijdens experimenten in een stroomgoot, waarvan de bodem grotendeels bel<leed was met kiezeis, werd iets opmerkelijks opgemerkt. Op de plek waar de kiezels ophielden, een groot abrupt verschil in bodemruwheid dus, ontstond bij bepaalde combinaties van debieten en schuifhoogtes een verschijnsel wat leek op een golvende watersprong. In de literatuur is er niets te vinden over watersprongen die ontstaan door een verschil in bodemruwheid, dus dat is de aanleiding voor dit onderzoek. Kennis over dit soort verschijnsels kan belangrijk zijn voor ontwerp van (waterbouwkundige constructies in) waterwegen, tevens zou het een toepassing kunnen vinden in de recreatie voor bijvoorbeeld kanoën, surfen of raften.
De watersprong is een overgang tussen een superkritisch en een subkritisch stromingsregime. De golvende watersprong is een speciale vorm van de watersprong waarbij een staande golf zich manifesteerd, samengaand met een grotere waterdiepte. De vraag is hoe de ruwheidsovergang in de goot ervoor zorgt dat er (iets wat lijkt op) een golvende watersprong ontstaat en of de eigenschappen ervan, zoals amplitude en waterdieptes, te voorspellen zijn. Daarnaast is het interessant te kijken naar de randvoorwaarden waarbij het verschijnsel zich laat zien en de toepasbaarheid op bestaande situaties en nieuwe ontwerpen.
De eerste hypothese is dat het verschijnsel voldoet aan het theoretische verband (dat voor watersprongen geldt) dat bestaat tussen het bovenstroomse Froudegetal en de ratio tussen benedenstroomse en bovenstroomse waterdiepte. Omdat er sprake is van golven die niet meer als lange golven bestempeld kunnen worden is er een uitdrukking voor het golfgetal k afgeleid van een Boussinesqvergelijking voor middellange golven, zodat de niethydrostatische drukverdeling wordt meegenomen. De tweede hypothese is dat met die uitdrukking de ontstane golflengte kan worden voorspeld. De metingen zijn vrij eenvoudig van opzet en bestaan uit enkele dieptemetingen met de peilnaald op verschillende plekken, het meten van het debiet en het meten van de golflengte. Tevens wordt er een foto van het verschijnsel gemaakt zodat deze digitaal bewerkt kunnen worden tot een golfprofiel. Deze metingen worden een scala aan debieten herhaald om zo genoeg data te hebben om de hypotheses te kunnen toetsen.
De resultaten van het experiment ondersteunen beide hypotheses en het is dus aannemelijk dat het hier daadwerkelijk een golvende watersprong betreft en dat middellange golftheorie die watersprong goed kan voorspellen. De randvoorwaarden nodig voor het ontstaan van de golvende watersprong kunnen nog niet goed in kaart gebracht worden vanwege het beperkte debiet dat, vanwege angst voor beschadiging van de opstelling, gehanteerd kan worden. De toepassing in bestaande situaties en nieuwe ontwerpen moet vooral gezocht worden in het feit dat met deze kennis kan worden bepaald of er golvende watersprongen zouden kunnen optreden in bepaalde situaties.
Eventueel vervolgonderzoek zou zich vooral moeten richten op het kijken van de toepasbaarheid van de middellange golftheorie op model om het gehele golfprofiel te voorspellen, verdere analyse van de voorwaarde voor het ontstaan van de watersprong, het gedrag bij hogere debieten en een voorspelling van de amplitude van de staande golf.

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6 

Input for a 1D sandgravel morphodynamic computation including a bedload layer
In the mass balance of a riverbed, some terms are usually assumed constant for reasons of simplicity. One of the terms assumed to be constant is the change in bedload layer, which is defined as the sediment transport divided by the particle velocity. To study the effect of this assumption, a numerical morphodynamic model has to be computed. For such a morphodynamic model to work, some input parameters and models have to be determined.
The most important input models are those of the predicted sediment transport and particle velocity, which are studied in this research.
The Dutch upper Rhine is used as basis for this study, with particle diameters and sediment composition as stated in earlier studies. To compare the predicted sediment transport by sediment transport models, sediment transport data of three measurement campaigns is used. Before the sediment transport models are compared with the measured data, a small literature study is used to determine which sediment transport models might be applicable for predicting the sediment transport. With the available specifications of the sediment transport models, two fractional transport models were chosen as applicable: the model of Wu et al. (2000) and the model of Wilcock & Crowe (2003). Both original models approximate the measured data quite well, so no calibration of the models is required. The model of Wu however, shows some unusual behavior for increasing sand/gravel ratios and is more sensitive for changes in its calibration parameter. Therefore, the model of Wilcock & Crowe is chosen as most applicable for this part of the Rhine.
To determine the propagation velocity of the particles, two models of Van Rijn (1984) and one model of EngelundFredsoe (1976) were used. One of the Van Rijn models predicted unrealistic particle velocities, but the remaining models approximated the particle velocity in a similar way, approaching measurements from Francis (1973) and Luque (1974). Due to the similar behavior of both models, no choice was made between them. Eventually, the thickness of the bedload layer was determined, providing a realistic input for a 1D sand/gravel morphodynamic model.

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7 

Largescale turbulence structures in shallow separating flows
The Ph.D. thesis “Largescale turbulence structures in shallow separating flows” by Harmen Talstra is the result of a Ph.D. research project on largescale shallowflow turbulence, which has been performed in the Environmental Fluid Mechanics Laboratory at Delft University of Technology. The dynamics of quasi twodimensional turbulence structures in shallow separating flows have been studied both experimentally and numerically. The research work contained three parts: respectively laboratory experiments, threedimensional simulations and twodimensional simulations. A number of schematized flow cases have been investigated in a largescale shallow laboratory flume, using the freesurface measurement technique of Particle Image Velocimetry (PIV). Subsequently, detailed threedimensional Large Eddy Simulations (LES) have been performed on a parallel cluster, providing useful 3D data on the flow cases studied experimentally as well as on additional flow geometries. The conclusions drawn are useful for e.g. design purposes in engineering practice. Finally, the flow cases studied before have been revisited by means of 2D depthaveraged computations, testing a new approach to accurately resolve largescale shallowflow turbulence in a 2D schematization. The new approach has been coined DepthAveraged NavierStokes with Large Eddy Stimulation (DANSLES). The thesis offers a rather complete picture of the turbulent flow cases that have been studied, both in terms of physical behavior and numerical modeling aspects.

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8 

Curved openchannel flows  a numerical study
The flow of water through a river bend can be characterized as a turbulent openchannel flow that is dominated by streamline curvature. In order to adequately manage the planimetry of meandering rivers and the according safety aspects, the features of the flow and the associated turbulence have to be understood well. This thesis presents results of detailed numerical simulations of curved openchannel flows on a laboratory scale. These flows can basically be considered as downscaled river bend flows. The presented results provide a broad image of the behavior of curved openchannel flows in general and their characteristics with respect to the secondary flow, the bed shear stresses and turbulence in particular. Hereby, these results also facilitate further development of parameterizations of these key bend flow features for lowerdimensional modeling tools that are used in the engineering practice.

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9 

Transport of suspended particles in turbulent open channel flows
Two experiments are performed in order to investigate suspended sediment transport in a turbulent open channel flow. The first experiment used particle image velocimetry (PIV) to measure the fluid velocity with a high spatial resolution, while particle tracking velocimetry (PTV) was used to measure the velocity of individual sediment particles. The sediment particles were injected in the flume close to the free surface at different distances from the measurement section. In this way, the development of a sediment plume towards an equilibrium situation could be studied. The results were compared with direct numerical simulations, in which the particle equation of motion was used to calculate the movement of individual sediment particles.
The second experiment used refractive index matching, in order to make the sediment particle invisible. In this way, a PIV experiment could be performed in order to determine changes in the flow and turbulence structure due to high sediment concentrations.

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10 

Flow over Weirlike Obstacles

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11 

Breach Flow Modeled as Flow over a Weir
The main objective of the study is to understand the effects of breach properties (e.g. the top width, the depth and the bottom width of the breach) on the breach flow. The breach flow was modeled as flow over a compound broadcrested weir. A laboratory experiment was carried out with a fixed weir model in a flume. Five cases with different breach properties were tested.
Firstly, the discharge coefficients for all cases were quantitatively determined. The calibrations were done under two flow conditions, namely emerged flow conditions (flow only through the breach) and overtopping flow conditions (flow through the compound crosssection of weir). In perfect weir situations, it turned out that the values of the discharge coefficient were below 1 and rather different in the two flow conditions. Particularly in the case of overtopping flow, the linear combination of traditional discharge equations was verified for predicting the discharge and the discharge distribution over the weir. In imperfect weir situations, submergence coefficient was introduced in emerged condition. Secondly, the dependence of the energy head loss caused by the weir on the upstream discharge and the downstream water depth was discussed in imperfect weir situations. The Form Drag model for estimating the energy head loss was proved to be applicable for the modeled breach flow. Thirdly, the local hydraulic characteristics of breach flow were described by means of velocity distribution, water level elevation and flow patterns appearing behind the weir. At last, the numerical model (Delft3D) was verified on the modeling of the present experiment by comparing with the experimental data.
The report provides the information for the breach flow which can be of use in the development of breach models, inundation models and compound weir design.

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12 

Resistance of submerged groynes
Groynes are commonly used in the major rivers in the Netherlands. They confine the flow of the river to a main channel but also act as resistance element once submerged and in that way influence the stage discharge relationship of the river. Several formulas exist describing groynes as a drag resistance. Another possible way of determining their resistance is by using a weir formula and fitting the up and downstream water levels to the water level slope of the river.
In this thesis a schematized model of the river Waal is set up and progressively expanded. At first seven different drag and weir formulas are compared. There is no consensus for resistance is between them. They are therefor compared to a 2DV computer model of flow over a highly submerged weir in the SWASH software package. Drag resistance, expressed as a function of water depth to groyne height ratio has the same scaling as two weir formulas though they do not match in absolute terms.
A 2DH computer model is then used to determine the magnitude of the lateral turbulent momentum exchange between the main channel, groyne fields and flood plain.
Finally a 3D computer model is used to determine groyne resistance and the distribution of discharge and momentum around the groyne. Treating groynes as weirs is found to be an acceptable assumption.
The schematized model is used to simulate a high discharge of 13.550 m3 s1. Groynes, when seen as a weir, would be responsible for a 36 cm water level increase. Lateral turbulent momentum exchange increases this by another 34 cm, while using the groyne resistance found in the 3D model added only another 7 cm.

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13 

Modelling the anisotropy of turbulence with the SWASH model: Heterogeneous roughness conditions in open channel flows
In this study the focus is on modelling turbulence anisotropy in open channel flows with the SWASH model. Turbulence anisotropy significantly influences the flow features of: channel flows with heterogeneous roughness conditions, curved open channel flows, compound channel flows with different floodplain depths, etc.
The SWASH model is a nonhydrostatic waveflow model, mainly used to predict the transformation of surface waves from offshore to the beach. For this study, adaptations were made to this SWASH model, in order to model turbulence anisotropy. Two different modelling approaches were used: RANS modelling and Large Eddy Simulation (LES). The SWASH model is extended with a nonlinear kε closure to the RANS equations, since the standard linear closure does not take turbulence anisotropy into account. A 3D subgrid model is implemented to perform LES.
The performance of the LES code and the RANS model with the nonlinear kε closure is tested on two flow geometries: an open channel flow with homogeneous bottom roughness conditions and an open channel flow with parallel smooth to rough bed sections.
Results of the RANS computations, for both horizontal homogeneous and nonhomogeneous open channel flow, show good agreement with laboratory measurements of Muller and Studerus [13], Nezu and Rodi [17] and Wang and Cheng [32]. Although there is a number of closure constants involved with the nonlinear kε model, additional tuning of these coefficients was not necessary for this study: both the homogeneous and nonhomogenous test case were simulated successfully using the standard values proposed by Speziale [25]. With its low computational costs and robustness, the nonlinear kε model appears to be a useful extension to the SWASH waveflow model.
LES results for horizontal uniform flow are validated with DNS data of Moser, Kim and Mansour [12]. Especially near the bed the LES results deviate from the DNS data. The mean velocity, as well as the transverse and vertical turbulence intensities, is seriously underestimated. The deviation from the DNS data is related to the use of non periodic boundary conditions, the coarse grid resolution, the size of the computational domain and the amount of numerical dissipation that is involved.
Since it is the bottom region where secondary currents are generated, the use of the present LES code for problems involving heterogeneous roughness is not appropriate.

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14 

Mixing layers in open channel flow with abrupt bed roughness changes
Hydraulic roughness is a key factor in modeling open channel flow. The frictional effects of roughness elements are generally parameterized by a roughness coefficient, representative for the roughness of a grid cell in a model. Bed roughness can be very heterogeneous in practical situations. Especially in floodplains, the roughness height can differ an order of magnitude over a small distance. This roughness heterogeneity impacts the shear stress distribution and the effective friction exerted on the flow. Previous research showed that the effective friction was 20% more than the theoretically weighted average value (Jarquín, 2007) in a flume with a parallel smoothtorough bed. Another calculation showed even 80% additional effective friction (Jarquín, 2007; Vermaas et al., 2007). New measurements and a detailed Large Eddy Simulation model described in this report were used to investigate the underlying mixing layer processes and the corresponding development length scales. This may provide the basis to parameterize roughness heterogeneity.
Measurements in a developed flow over a parallel smoothtorough bottom show a secondary circulation in vertical planes across the flow. This circulation causes a transverse momentum transport from the smooth to the rough side. The momentum transport by this mechanism has nearly the same order of magnitude as the transverse momentum exchange by turbulent mixing. The transverse momentum exchange enhances the effective friction. An example with a 2D model shows that this can not explain the entire increase in effective friction; additional friction is probably also caused by extra turbulence production near the smoothtorough interface, and bed shear stress in the spanwise direction.
In the transition from a uniform flow to a compound flow over parallel roughness lanes, transverse volume transport occurs mainly in the first 4 meter (twice the width of the flume), with a maximum velocity at the start of the parallel roughness section. The development length of the velocity profiles can be scaled to the depth of flow. The vertical profiles outside the mixing layer develop in about 25 times the water depth; the mixing layer at mid depth in about 50 water depths. The secondary circulation was estimated to be fully developed after 80 water depths, but has already a significant momentum transport at half of this distance. Furthermore, the depth averaged transverse mass transport causes a gradient in the advected longitudinal momentum and therefore the water level slope is even more increased above the start of a parallel rough bottom.
As a typical example of repetitive changing roughness, the flow over a roughness pattern resembling an elongated checkerboard pattern was tested. The flow appeared to develop much slower in each section than over a single parallel (infinitely long) roughness. The maximum velocity remains close to the smoothtorough interface and no secondary flow is observed in this configuration. Turbulent mixing is neither very effective since the vortices are changing direction not before 1 meter after a roughness change. Nevertheless, the effective friction is seriously increased by this configuration; about 30% additional friction is observed in comparison with a developed parallel flow without transverse interaction. This can be explained by the large adaptation length of the flow relative to the size of the checkerboard fields. The flow velocity is relatively large over the rough fields, and slow over the smooth fields, causing the additional drag.

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15 

Drag forces on vegetation due to waves and currents
Vegetation in coastal areas plays an important role in its environment. In addition, vegetation can also be utilized for coastal protection. Incoming wave energy could be effectively attenuated by the upstanding vegetation plants. The reduced wave energy results in stabilized seabed and harmonious environment in coastal zones. Nowadays, an increasing number of projects have been carried out to apply vegetation as a soft measure for coastal protection.
Wave energy dissipation by vegetation is primarily induced by the work done by drag force acting on the vegetation. A drag coefficient (CD) is introduced to characterize the flow resistance from the plant stems. Knowledge of CD is of great importance for understanding and predicting the wave dissipation process. In previous studies, relations between CD and Reynolds number (Re) have been proposed in pure current or pure wave. In addition, relations between CD and the KeuleganCarpenter number (abbreviated as KC and KC=Uw*Tw/d, Tw is the wave period and d is the plant stem diameter) have also been suggested. In KeuleganCarptenter number, the wave period Tw is also considered. Since waves are oscillatory flow, it would be preferable to use the KC number to describe the behaviour of CD in wavepresent conditions.
However, contradictory conclusions are found in the literature on the CDKC relation in pure wave condition. Monotonous decreasing of CD with KC has been reported for multiple vegetation mimics. On the contrary, the riseandfall variation trend has been observed in pure wave, but only for single cylinder. It is noted that the transition point (from rise to fall) occurs when KC value is small, which is often left out in the experiment with multiple vegetation mimics in previous studies. Hence, it is necessary to investigate the variation trend of CDKC for multiple vegetation mimics in pure wave with a wider KC range. Moreover, background tidal currents may also play a role in the wave dissipation process. It is often the case that when the tide penetrates the coastal wetlands during flooding phase, waves propagate in the same direction as the tidal currents. The underlying current may affect the behaviour of oscillatory wave flow during the energydamping process and the CDKC relation. Yet, the CDKC relation in combined currentwave flow has not been reported in previous studies.
In order to fill the knowledge gap in the CDKC relation, an experimental approach was adopted by using the laboratory flume to replicate such complicated hydrodynamics. The flume is 40m long and 0.8m wide, with a patch of rigid wooden cylinders as vegetation mimics installed over the entire channel width over a 6m long test section. Pure wave can be generated by the wave generator. The underlying current can be made by using a water circulation system in the flume. After the generation of underlying current, waves could be generated afterwards and propagate together with the underlying current in the flume. The velocity was measured using EMS within the vegetation patch. Direct measurement data of the force on individual rods within the array were collected by attchaching the rods to the force sensors and embedding them in the false bottom of the flume. Three densities of the vegetation mimics were investigated for two water depths in this study.
The results of the experiments reveal a riseandfall variation trend of CDKC for multiple vegetation mimics in pure wave. The risepart occurs when KC is small, around KC=3 to 10 and this phenomenon could be phycisally explained based on the changes in vortexes shedding directions. In this range of KC, the vortexes motions would change its propagation direction from lateral to oblique and longitudally parallel with the incoming flow. It is the changes in vortexes directions that lead to the increase of flow resistance experienced by the cylinder. Beyond this range of KC, the vortexes motion would keep moving longitudinally parallel and behave much the same way as in steady current. Natuarally, similar to the behaviour found in steady current conditions, the values of CD would decrease gradually and converge to 1.
In the combined currentwave flow conditions, it is necessary to make a distinction between oscillatorydominated flow and unidirectionaldominated flow. For small underlying current applied in this study, the flow could be regarded as oscillatorydominated. It is found that the similar riseandfall pattern of CDKC relation occurs in this kind of combined flow. And the transition point of KC locates at around KC=10, which is also the case found in pure wave conditions. But the peak value of CD would decrease a little bit.
However, for larger underlying current conditions, the combined flow is similar to pure current. Consequently, the peak values of CD would collapse. Thus, the values of CD obtained in these conditions are stable and close to 1.
The experimental results suggest the vegetation density as well as the water depths has limited effects on the values of CD and its variation trend with KC. It is recommended to carry further investigation concerning the influence on CD caused by vegetation density (N) and relative vegetation height (α) in future studies.
The product of this thesis is a general description and explaination for the variation trend of CDKC in pure wave and combined currentwave flow conditions. Physically, more insights have been gained about the evolution of vortex shedding in different flow conditions, say from pure wave to combined currentwave flow conditions. Moreover, both the calibration approach (used by Mendez and Losada, 2004, etc.) and direct measurement method have been utilised for data processing. The direct measurement method is recommended to apply in all the complicated flow conditions. As to the calibration approach, it should not be applied to obtain CD values in combined currentwave flow.

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16 

OuterBank Shear Stress in River Bends: Numerical Modeling of Curved Flow
In the dimensional averaged numerical models, that are used for practicle purposes, there need to be accounted for threedimensional processes. Amongst others a bank shear stress parameterization need to be incorperated. This master thesis focusses on the outerbank shear stress in order to obtain a parameterization for the outerbank shear stress for naturally curved flows.
The outerbank shear stress is obtained from threedimensional numerical computations. The numerical modeling is based on the Large Eddy Simulator, available at the Fluid Mechanics Department of the Technical University of Delft. Despite the considerable progress achieved, the software is still under development and a thorough review of its code is therefore necessary. The aim of this master thesis is to assess the effect of the outerbank roughness, the outerbank angle and the transverse bed slope on the outerbank shear stress and to test and improve the performance of the boundary method of the computational algorithm.
The magnitude of the outerbank shear stress and the outerbank cell increase for increasing roughness of the outerbank, leading to a less uniform distribution of the outerbank shear stress. The magnitude of the outerbank shear stress decreases for increasing inclination of the outerbank. For a more inclined outerbank, the magnitude of the outerbank shear stress is more dependent on the outerbank roughness. No significant dependency of the distribution of the outerbank shear stress on the outerbank inclination can be found from the results. Inclusion of the point bar related transverse bed slope does not lead to a significant change of the magnitude and distribution of the outerbank shear stress. Clearly, the helical motion outscores the effect of topographic steering.
The boundary method adds the frictional effects of solid boundaries to the computational algorithm. The implemented boundary method, the Immersed Boundary Method, allows for the implementation of complex boundaries on a structured grid. Although in some cases the boundary method has a good performance, it is not very robust and prone to errors. It is recognized that the near wall velocity profile using the Immersed Boundary Method does not often coincide with the actual momentum `loss' at the wall. This problem, not recognized in earlier studies, can be attributed to inaccuracy in the description of the turbulent viscosity. The latter is understood and partly solved. The accuracy can be further improved by coarsening the grid or reconsidering the implemented turbulence closure model, the Smagorinsky Model. Different improved Smagorinsky Models are available.

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17 

Development of the Generalised Hybrid Turbulence Model for RANS simulations
Turbulence plays an important role in a broad range of engineering applications. In the industry RANS simulations are a common method for predicting turbulent flow. A broad range of RANS turbulence models have been developed over the past decades. For the assessment of complex threedimensional flow fields Reynolds Stress Transport Models are a better choice than eddy viscosity models, even though their computational cost is higher. A hybrid model that combines the accuracy of the Reynolds Stress Transport Models with the computational speed of the eddy viscosity models could be a valuable tool in the design of structures subjected to complex threedimensional flows. Therefore the development of such a hybrid model was the objective of this thesis.
A literature study on existing RANS turbulence models showed that the Reynolds Stress Transport Models (RSTM) are the most physical but also the computationally most expensive models. The linear eddy viscosity models have a reduced computational cost, but are not capable of predicting flow features that are caused by the Reynolds stress anisotropy, since these models can not account for this anisotropy. The nonlinear eddy viscosity models include extra anisotropy by means of higher order terms, but the coefficients in these models are calibrated using simple benchmark test cases, making their applicability to more complex flows uncertain. The Hybrid Turbulence Model (HTM) of Basara combines a RSTM with the formulation of the linear eddy viscosity models to reduce the computation time of the simulation. This hybrid model is also unable to account for Reynolds stress anisotropy just like the linear eddy viscosity models.
The development of the Generalised Hybrid Turbulence Model (GHTM) in this thesis, was motivated by this deficiency of the HTM. This novel hybrid model combines a RSTM with the general formulation of the nonlinear eddy viscosity models. By using multiple base tensors additional Reynolds stress anisotropy is included. The HTM and the Improved kepsilon model
The GHTM was implemented in OpenFOAM and three test cases were investigated with this new model. The simulations with the GHTM do not reach convergence, except when the full tensor base is considered. The simulations with the Improved kepsilon model converge and yield more physical results for the Ubend test case, showing a recirculation zone, where the standard kepsilon model does not predict this flow feature.
A mesh refinement study showed that the grid size has no influence on the performance of the GHTM. Also the use of different gradient schemes or underrelaxation did not affect the convergence of the GHTM simulations. To improve the performance of the GHTM different smoothing techniques have been tested, since peaks in the model coefficients seemed to cause the instability of the simulations. The proposed smoothing methods are modifications of the original GHTM and are therefore not useful to improve the performance of the GHTM.
A closer look at the properties of the tensors used in the GHTM for statistically twodimensional flows showed that an error in trace of the mean rate of strain tensor cause the resulting Reynolds stress anisotropy tensor to be incorrect. This problem was solved by constructing more accurate cell face velocities which correspond to the known cell face fluxes. With these new face velocities the GHTM with N=2 converges for the twodimensional cases, but the linear GHTM still does not converge.
A further investigation of the performance of the GHTM for statistically twodimensional flows showed that in that case the GHTM with N=2 is identical to the background RSTM. This shows that the GHTM with N>1 could only lead to a reduction in computation time for threedimensional flows.

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18 

Performance of RANS turbulence models for the numerical simulation of the flow affected by micro vortex generators
Additional thesis with a size of 10 European Credits.
The increasing demand for wind energy has led to the development of more efficient wind turbines. A common way to increase the performance of wind turbines is the use of vortex generators. Vortex generators are small devices that are mounted on the turbine blade and they create a stream wise vortex. The vortex transports high momentum fluid towards the surface of the blade, delaying the flow separation.
Although vortex generators improve the efficiency of the blade by preventing flow separation, they also create additional drag. This drag can be reduced by using micro vortex generators, which have a height that is smaller than the boundary layer thickness. The positive effect of these micro vortex generators is more local, due to the weaker vortex they create. This means that their effects on the flow should be predicted with more precision to place them in the correct location upstream of the separation.
The flow affected by a micro vortex generator can be computed by means of a RANS simulation. A threedimensional RANS simulation has been performed on a micro vortex pair, using different RANS turbulence models: the standard kepsilon model, the komegaSST model, the Launder Gibson (LG) Reynolds Stress Transport Model and the Speziale Sarkar Gatski (SSG) Reynolds Stress Transport Model. The first two models are eddy viscosity models, which are computationally less expensive than the Reynolds Stress Transport Models (RSTM).
The results from the numerical simulations are compared to available experimental data, which include the mean velocity components and the Reynolds stresses. In terms of computational time simulation with the komegaSST model was the fastest, whereas the two Reynolds Stress Transport Model simulations were 40% slower.
The mean flow field and the decay of the vortex as predicted by the komegaSST model and the two Reynolds Stress Transport Models were in agreement with the experimental data. The standard kepsilon model predicts the wrong shape for the vortex. When the turbulent kinetic energy is compared to the experimental data, all models fail to predict the correct profiles close to the leading edge of the vortex generators. Further downstream the prediction for the turbulent kinetic energy made by the RSTM correspond better with the experimental data than the eddy viscosity models.
The degree of Reynolds stress anisotropy is also considered, by looking at the two invariants of the Reynolds stress anisotropy tensor. The eddy viscosity models underpredict the Reynolds stress anisotropy, whereas the RSTM slightly overpredict it, compared to the experimental results. The distribution of Reynolds stress in the flow domain is predicted best by the RSTM.
It is concluded that when the mean flow features are of interest the best choice is the komegaSST model, because the computation time of this model is less than the RSTM, but the mean flow is predicted with almost the same accuracy. The RSTM predicts the Reynolds stresses the most precise, but there are still some differences with the experimental results that could be improved. The use of the kepsilon model is not advised.

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19 

Wave dissipation over vegetation fields
It has been widely recognized that ongoing climate change, most likely due to human interference with nature, may accelerate sea level rise and increase storm intensity. It is therefore urgent to design countermeasures to alleviate the impact of climate change on coastal regions. Apart from the view point of coastal protection, it is also very important for coastal engineers to keep an eye on environmental issues in the coastal region. In this context, vegetation fields such as salt marshes, sea grasses and mangrove forests in coastal regions have started to attract the attention of coastal engineers due to their function as wave attenuator.
However, the wave attenuation function of a vegetated field is not well understood yet. To utilize coastal vegetation fields as a part of coastal management in practice, it is crucial to accumulate more knowledge about the physical processes, especially the hydraulic processes, and these need to be modeled in a practical sense. Hence, this thesis is intended as an investigation of the process of wave dissipation over vegetation fields through various approaches, specifically theoretical, physical and numerical studies.

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20 

CFD in drinking water treatment
Hydrodynamic processes largely determine the efficacy of drinking water treatment systems, in particular disinfection systems. A lack of understanding of the hydrodynamics has resulted in suboptimal designs of these systems. The formation of unwanted disinfectionbyproducts and the energy consumption or use of chemicals is therefore higher than necessary.
In drinking water engineering, computational fluid dynamics (CFD) is therefore increasingly applied to predict the performance of treatment installations and to optimise these installations. CFD uses advanced numerical models to predict flow, mixing and (bio)chemical reactions. In this thesis, the hydrodynamics and (bio)chemistry in ozone and UV systems are studied by means of CFD models combined with experimental techniques. This combination leads to further development of CFD modelling as a tool to evaluate the performance of drinking water treatment installations. If the CFD model is applied properly, accounting for the complex turbulent motions and validated by experiments, this tool leads to a better design of UV reactors, ozone systems and other systems dictated by hydrodynamics. This work resulted in new insights in the applicability of models in ozone and UV installations, and new insights in design aspects of these installations.

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