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J. van Asselt
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Master thesis
(2025)
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J. van Asselt, M.A. Khan, S.P. Porchetta, S.J. Watson, J.O. (Oriol) Colomes Gene, Bas Gradussen
This study investigates the aerodynamic interaction between two neighbouring offshore wind farms operating in a Conventionally Neutral Boundary Layer (CNBL), where the atmospheric boundary layer (ABL) is neutrally stable, capped by a stable inversion layer and a less stable free atmosphere above. While previous studies have investigated: 1) wake effects between two neighbouring wind farms in a Truly Neutral Boundary Layer (TNBL) with neutral conditions throughout the ABL and above, and 2) the impact of a single wind farm on the flow field in a CNBL, there is a lack of research on the interaction between neighbouring wind farms in a CNBL. Improving our understanding of wind farm interactions in a realistic atmosphere is important for offshore wind farm planning and operation. In this study the performance of the two wind farms under two atmospheric conditions, a CNBL and a TNBL. While the TNBL assumes neutral conditions throughout the ABL and above, the CNBL represents a more complex and realistic modelling approach.
When the wind approaches a wind farm in a TNBL, the combined induction of the turbines creates a zone with increased pressure just before the start of the wind farm, redirecting the flow laterally and vertically. In a CNBL, the vertical component of the redirected wind interacts with a stable inversion layer aloft, which creates an enhanced high pressure area at the start of the wind farm, a phenomena commonly known as global blockage. At the end of the wind farm, the wind is directed down towards the low pressure region in the wake of the wind farm, lowering the inversion layer and creating a zone with even lower pressure. The combination of the two processes described, reduces the wind speed at the start of the wind farm and accelerates it towards the end.
In this study, we look at the interaction of two neighbouring wind farms at varying distances in both a TNBL and CNBL using the relatively fast Multi-Scale Coupled model (Stipa, Ajay, Allaerts, & Brinkerhoff, 2024) which uses a simplified mesoscale model to determine a background flow field which is then used to drive an engineering wake model. Each wind farm consists of a regularly aligned array of five turbines in the spanwise and ten turbines in the streamwise direction with a five rotor diameter (5D) spacing in both dimensions. The turbines are based on the 5 MW NREL reference (Jonkman et al., 2009) and simulations are run at a wind speed of 9 m/s at hub height, on the plateau of the thrust curve. For the CNBL simulations, we choose a Froude number of 1.0 for the inversion layer, resulting in maximum inversion layer displacement due to a phenomena called choking, and a Froude number of 0.10 for the free atmosphere.
Key findings:
1. For infinitely spaced wind farms in a CNBL, lower power output is seen compared to a TNBL due to stronger global blockage effects at the start of each wind farm despite a small speed-up towards the end of the farm.
2. For small wind farm separations (up to ±72D), the speed-up effect at the downstream end of the first wind farm enhances the power output of the second wind farm compared to a TNBL, whilst the blockage effect of the second wind farm negatively impacts the first wind farm.
3. The combined output of the two wind farms at small wind farm spacings is higher than when the farms are infinitely spaced, though lower than in a TNBL. ...
When the wind approaches a wind farm in a TNBL, the combined induction of the turbines creates a zone with increased pressure just before the start of the wind farm, redirecting the flow laterally and vertically. In a CNBL, the vertical component of the redirected wind interacts with a stable inversion layer aloft, which creates an enhanced high pressure area at the start of the wind farm, a phenomena commonly known as global blockage. At the end of the wind farm, the wind is directed down towards the low pressure region in the wake of the wind farm, lowering the inversion layer and creating a zone with even lower pressure. The combination of the two processes described, reduces the wind speed at the start of the wind farm and accelerates it towards the end.
In this study, we look at the interaction of two neighbouring wind farms at varying distances in both a TNBL and CNBL using the relatively fast Multi-Scale Coupled model (Stipa, Ajay, Allaerts, & Brinkerhoff, 2024) which uses a simplified mesoscale model to determine a background flow field which is then used to drive an engineering wake model. Each wind farm consists of a regularly aligned array of five turbines in the spanwise and ten turbines in the streamwise direction with a five rotor diameter (5D) spacing in both dimensions. The turbines are based on the 5 MW NREL reference (Jonkman et al., 2009) and simulations are run at a wind speed of 9 m/s at hub height, on the plateau of the thrust curve. For the CNBL simulations, we choose a Froude number of 1.0 for the inversion layer, resulting in maximum inversion layer displacement due to a phenomena called choking, and a Froude number of 0.10 for the free atmosphere.
Key findings:
1. For infinitely spaced wind farms in a CNBL, lower power output is seen compared to a TNBL due to stronger global blockage effects at the start of each wind farm despite a small speed-up towards the end of the farm.
2. For small wind farm separations (up to ±72D), the speed-up effect at the downstream end of the first wind farm enhances the power output of the second wind farm compared to a TNBL, whilst the blockage effect of the second wind farm negatively impacts the first wind farm.
3. The combined output of the two wind farms at small wind farm spacings is higher than when the farms are infinitely spaced, though lower than in a TNBL. ...
This study investigates the aerodynamic interaction between two neighbouring offshore wind farms operating in a Conventionally Neutral Boundary Layer (CNBL), where the atmospheric boundary layer (ABL) is neutrally stable, capped by a stable inversion layer and a less stable free atmosphere above. While previous studies have investigated: 1) wake effects between two neighbouring wind farms in a Truly Neutral Boundary Layer (TNBL) with neutral conditions throughout the ABL and above, and 2) the impact of a single wind farm on the flow field in a CNBL, there is a lack of research on the interaction between neighbouring wind farms in a CNBL. Improving our understanding of wind farm interactions in a realistic atmosphere is important for offshore wind farm planning and operation. In this study the performance of the two wind farms under two atmospheric conditions, a CNBL and a TNBL. While the TNBL assumes neutral conditions throughout the ABL and above, the CNBL represents a more complex and realistic modelling approach.
When the wind approaches a wind farm in a TNBL, the combined induction of the turbines creates a zone with increased pressure just before the start of the wind farm, redirecting the flow laterally and vertically. In a CNBL, the vertical component of the redirected wind interacts with a stable inversion layer aloft, which creates an enhanced high pressure area at the start of the wind farm, a phenomena commonly known as global blockage. At the end of the wind farm, the wind is directed down towards the low pressure region in the wake of the wind farm, lowering the inversion layer and creating a zone with even lower pressure. The combination of the two processes described, reduces the wind speed at the start of the wind farm and accelerates it towards the end.
In this study, we look at the interaction of two neighbouring wind farms at varying distances in both a TNBL and CNBL using the relatively fast Multi-Scale Coupled model (Stipa, Ajay, Allaerts, & Brinkerhoff, 2024) which uses a simplified mesoscale model to determine a background flow field which is then used to drive an engineering wake model. Each wind farm consists of a regularly aligned array of five turbines in the spanwise and ten turbines in the streamwise direction with a five rotor diameter (5D) spacing in both dimensions. The turbines are based on the 5 MW NREL reference (Jonkman et al., 2009) and simulations are run at a wind speed of 9 m/s at hub height, on the plateau of the thrust curve. For the CNBL simulations, we choose a Froude number of 1.0 for the inversion layer, resulting in maximum inversion layer displacement due to a phenomena called choking, and a Froude number of 0.10 for the free atmosphere.
Key findings:
1. For infinitely spaced wind farms in a CNBL, lower power output is seen compared to a TNBL due to stronger global blockage effects at the start of each wind farm despite a small speed-up towards the end of the farm.
2. For small wind farm separations (up to ±72D), the speed-up effect at the downstream end of the first wind farm enhances the power output of the second wind farm compared to a TNBL, whilst the blockage effect of the second wind farm negatively impacts the first wind farm.
3. The combined output of the two wind farms at small wind farm spacings is higher than when the farms are infinitely spaced, though lower than in a TNBL.
When the wind approaches a wind farm in a TNBL, the combined induction of the turbines creates a zone with increased pressure just before the start of the wind farm, redirecting the flow laterally and vertically. In a CNBL, the vertical component of the redirected wind interacts with a stable inversion layer aloft, which creates an enhanced high pressure area at the start of the wind farm, a phenomena commonly known as global blockage. At the end of the wind farm, the wind is directed down towards the low pressure region in the wake of the wind farm, lowering the inversion layer and creating a zone with even lower pressure. The combination of the two processes described, reduces the wind speed at the start of the wind farm and accelerates it towards the end.
In this study, we look at the interaction of two neighbouring wind farms at varying distances in both a TNBL and CNBL using the relatively fast Multi-Scale Coupled model (Stipa, Ajay, Allaerts, & Brinkerhoff, 2024) which uses a simplified mesoscale model to determine a background flow field which is then used to drive an engineering wake model. Each wind farm consists of a regularly aligned array of five turbines in the spanwise and ten turbines in the streamwise direction with a five rotor diameter (5D) spacing in both dimensions. The turbines are based on the 5 MW NREL reference (Jonkman et al., 2009) and simulations are run at a wind speed of 9 m/s at hub height, on the plateau of the thrust curve. For the CNBL simulations, we choose a Froude number of 1.0 for the inversion layer, resulting in maximum inversion layer displacement due to a phenomena called choking, and a Froude number of 0.10 for the free atmosphere.
Key findings:
1. For infinitely spaced wind farms in a CNBL, lower power output is seen compared to a TNBL due to stronger global blockage effects at the start of each wind farm despite a small speed-up towards the end of the farm.
2. For small wind farm separations (up to ±72D), the speed-up effect at the downstream end of the first wind farm enhances the power output of the second wind farm compared to a TNBL, whilst the blockage effect of the second wind farm negatively impacts the first wind farm.
3. The combined output of the two wind farms at small wind farm spacings is higher than when the farms are infinitely spaced, though lower than in a TNBL.
Student report
(2023)
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J. van Asselt, A.C.C. Bervoets, C.M.A. Bloem, V.R.J. Kindermann, R.P. Koopman, L.J. Zwaan, C. Mai Van, H. Alkisaei, S. Truong Hong
The Mekong Delta in Vietnam is facing several challenges as a result of climate change. Among others, the effects include an increase in river discharge during the wet season, leading to river floods, and a decrease in river discharge during the dry season. The decrease in discharge results in a shortage of fresh water required for irrigation and drinking water. Besides that, the combination of sea-level rise, land subsidence, and decreased river discharge during the dry season results in saltwater intrusion. This threatens freshwater supply even more. Furthermore, there is an increasing risk of floods from the sea due to low land elevation and the rising sea level in combination with the occurrence of storm surges. The scope of this research is the area around the Ham Luong estuary, which is a branch of the Mekong River. The partial closure of this river branch is considered by the Vietnamese government as a measure to reduce the effect of the above-mentioned effects of climate change. However, not enough research has been conducted yet on the impact of a partial closure on the Ham Luong estuary. This has lead to the following research question: “What is the impact of various closure scenarios on the hydraulic characteristics and social activities in the Ham Luong estuary, considering a 75-year forecast?” The region of the Ham Luong estuary is characterised by its intensive agri- and aquaculture. More than 60% of the inhabitants is directly active within the agri- or aquaculture. As these activities are strongly dependent on the salinity of the estuary, they are highly affected by the effects of climate change. The region is densely populated with more than 125,000 inhabitants living near the Ham Luong estuary. It is clear that the effects of climate change are threatening the region in hydraulic aspects, as well as socio-economic aspects. A partial closure could reduce these effects, but will influence the region in several ways. In order to estimate the impact, a combination of hydraulic and socio-economic aspects is assessed based on a criteria set. This criteria set contains the criteria of freshwater supply, agricultural and aquaculture adaptation, biodiversity, stable riverbanks, and navigability. These criteria will be tested on a total of four alternative interventions in the Ham Luong estuary. Three alternatives with a storm surge barrier and one alternative without a storm surge barrier. All alternatives include heightening of the existing dyke system, as this seems to be inevitable when aiming for long-term development in the region. The extend of dyke heightening is subject to the choice of alternative. As a part of the impact analysis, a Delft3D model was built to analyse the hydrodynamic and morphodynamic processes in the Ham Luong estuary. The model was restricted to the chosen spatial scope, which only covers the Ham Luong estuary, without any upstream bifurcations. The model gave insights in processes like salt intrusion, sedimentation rates, and water levels. However, due to model simplifications and assumptions, the outcomes of the model where not useful for quantitative assessments. Still, the results are used to compare the impact of the different alternatives to each other. As expected, the alternatives that include a storm surge barrier will provide more possibilities to retain fresh water than the alternative without a barrier. From the results, it followed that the limited spatial scope excludes the redistribution of upstream discharge. It is recommended to look at a larger scale of the Mekong Delta when assessing hydro- and morphodynamic processes. Forming a flood protection system, the structural design of such a storm surge barrier, together with a quick estimation of a dyke system. The dyke system is different for each alternative, depending on the presence and the location of a barrier. The barrier design includes a thorough analysis on feasibility of gate types, technical requirements, load combinations, design of dimensions, and the operation. The load combinations take hydrostatic, hydrodynamic, wind, and soil loads into account. The design of the dimensions is done for the gates, sill, lifting structure, pier, foundation, and the bed protection. By assessing the above-mentioned criteria, a preferred solution is identified. This preference is based on a Multi-Criteria Analysis, which includes weighted scores for all alternatives. The outcome of the Multi-Criteria Analysis appears to be very sensitive to the rating and weights of the criteria, which makes it difficult to identify one of the alternatives as the preferred solution based on only the score on the different criteria. For this reason more research is needed. However, when including a cost estimation of the four alternatives, it can be stated that the alternative of no storm surge barrier and only the corresponding extensive dyke heightening could be considered as most cost-beneficial alternative and therefore as the preferred solution. It is expected that with or without closure of the Ham Luong estuary the system will change. The availability of fresh water will be improved by the presence of a closure, although more research is needed to specify this further. The increasing salt intrusion, as a result of Relative Sea-Level Rise (RSLR) will lead to agricultural and aquaculture adaptation in all alternatives. Either due to the construction of the barrier, or due to the gradual RSLR. A closure also has effect on the biodiversity, stability of the river banks, and navigability in the river. When implementing a closure these effects should be further investigated to assess the effect quantitatively.
...
The Mekong Delta in Vietnam is facing several challenges as a result of climate change. Among others, the effects include an increase in river discharge during the wet season, leading to river floods, and a decrease in river discharge during the dry season. The decrease in discharge results in a shortage of fresh water required for irrigation and drinking water. Besides that, the combination of sea-level rise, land subsidence, and decreased river discharge during the dry season results in saltwater intrusion. This threatens freshwater supply even more. Furthermore, there is an increasing risk of floods from the sea due to low land elevation and the rising sea level in combination with the occurrence of storm surges. The scope of this research is the area around the Ham Luong estuary, which is a branch of the Mekong River. The partial closure of this river branch is considered by the Vietnamese government as a measure to reduce the effect of the above-mentioned effects of climate change. However, not enough research has been conducted yet on the impact of a partial closure on the Ham Luong estuary. This has lead to the following research question: “What is the impact of various closure scenarios on the hydraulic characteristics and social activities in the Ham Luong estuary, considering a 75-year forecast?” The region of the Ham Luong estuary is characterised by its intensive agri- and aquaculture. More than 60% of the inhabitants is directly active within the agri- or aquaculture. As these activities are strongly dependent on the salinity of the estuary, they are highly affected by the effects of climate change. The region is densely populated with more than 125,000 inhabitants living near the Ham Luong estuary. It is clear that the effects of climate change are threatening the region in hydraulic aspects, as well as socio-economic aspects. A partial closure could reduce these effects, but will influence the region in several ways. In order to estimate the impact, a combination of hydraulic and socio-economic aspects is assessed based on a criteria set. This criteria set contains the criteria of freshwater supply, agricultural and aquaculture adaptation, biodiversity, stable riverbanks, and navigability. These criteria will be tested on a total of four alternative interventions in the Ham Luong estuary. Three alternatives with a storm surge barrier and one alternative without a storm surge barrier. All alternatives include heightening of the existing dyke system, as this seems to be inevitable when aiming for long-term development in the region. The extend of dyke heightening is subject to the choice of alternative. As a part of the impact analysis, a Delft3D model was built to analyse the hydrodynamic and morphodynamic processes in the Ham Luong estuary. The model was restricted to the chosen spatial scope, which only covers the Ham Luong estuary, without any upstream bifurcations. The model gave insights in processes like salt intrusion, sedimentation rates, and water levels. However, due to model simplifications and assumptions, the outcomes of the model where not useful for quantitative assessments. Still, the results are used to compare the impact of the different alternatives to each other. As expected, the alternatives that include a storm surge barrier will provide more possibilities to retain fresh water than the alternative without a barrier. From the results, it followed that the limited spatial scope excludes the redistribution of upstream discharge. It is recommended to look at a larger scale of the Mekong Delta when assessing hydro- and morphodynamic processes. Forming a flood protection system, the structural design of such a storm surge barrier, together with a quick estimation of a dyke system. The dyke system is different for each alternative, depending on the presence and the location of a barrier. The barrier design includes a thorough analysis on feasibility of gate types, technical requirements, load combinations, design of dimensions, and the operation. The load combinations take hydrostatic, hydrodynamic, wind, and soil loads into account. The design of the dimensions is done for the gates, sill, lifting structure, pier, foundation, and the bed protection. By assessing the above-mentioned criteria, a preferred solution is identified. This preference is based on a Multi-Criteria Analysis, which includes weighted scores for all alternatives. The outcome of the Multi-Criteria Analysis appears to be very sensitive to the rating and weights of the criteria, which makes it difficult to identify one of the alternatives as the preferred solution based on only the score on the different criteria. For this reason more research is needed. However, when including a cost estimation of the four alternatives, it can be stated that the alternative of no storm surge barrier and only the corresponding extensive dyke heightening could be considered as most cost-beneficial alternative and therefore as the preferred solution. It is expected that with or without closure of the Ham Luong estuary the system will change. The availability of fresh water will be improved by the presence of a closure, although more research is needed to specify this further. The increasing salt intrusion, as a result of Relative Sea-Level Rise (RSLR) will lead to agricultural and aquaculture adaptation in all alternatives. Either due to the construction of the barrier, or due to the gradual RSLR. A closure also has effect on the biodiversity, stability of the river banks, and navigability in the river. When implementing a closure these effects should be further investigated to assess the effect quantitatively.