The development of new wind farm control strategies can benefit from combined analysis of flow dynamics in the farm and the behavior of individual turbines within one simulation environment. In this work, we present such an environment by developing a new coupling between the large-eddy simulation (LES) code GRASP and the multiphysics wind turbine simulation tool OpenFAST via an actuator line model (ALM). In addition, the implementation of the recently proposed filtered actuator line model (FALM) within the coupling is described. The new ALM implementation is cross-verified with results from four other commonly used research LES codes. The results for the blade loads and the near wake obtained with the new coupling are consistent with the other codes. Deviations are observed in the far wake. The results further indicate that the FALM is able to reduce the lift and power overprediction from which the traditional ALM suffers on coarse LES grids. This new simulation environment paves the way for future wind farm simulations under realistic weather conditions by leveraging GRASP's ability to impose data from large-scale meteorological models as boundary conditions.

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The climate actions defined by United Nations require a rapid transition to low environmental footprint technologies. The energy sector is the major emitter of carbon dioxide emissions and a significant contributor to extracting resources for fuel and power plant construction materials. Wind energy is projected to produce a significant share of electricity and energy in the following decades. The wind turbines have a small footprint during the operation, but the turbine with its foundation is a massive structure with a significant material footprint. Airborne wind energy uses tethered devices to harness high-altitude wind energy, substantially reducing bulk material use. However, better models are required to make the systems reliable and efficient. This thesis focuses on membrane traction kites that harness wind energy by flying fast crosswind maneuvers. A high-fidelity aeroelastic model for the kites is developed to predict the aerodynamic loads and the structural deformations of real systems. The aeroelastic model assumes that the membrane kite flight can be modeled as multiple steady-states without memory from the past. The steady-state aerodynamics are simulated by solving the incompressible Reynolds-averaged Navier-Stokes equations numerically. High-quality numerical grid generation strategies are developed for the unconventional wing shape of the membrane kites. The membrane kites are tensile structures, and therefore a finite element model with cable and membrane elements without rotational degrees of freedom is used to calculate the deformed shape. The solver calculates the average surface without wrinkles and applies an additional model when an element is under compression. The steady-state response of the structure is calculated with a dynamic relaxation technique. The two solvers are coupled in a partitioned manner, and during each iteration, both solvers compute a steady state. The staggered approach requires several coupling iterations to converge. The fluid mesh needs to be altered to the deformed geometry during each iteration, and therefore, the mesh is deformed with radial basis function with greedy point selection. This thesis presents three computational studies with the framework. The first two studies focus on the aerodynamics of rigidized LEI kite airfoil and wing. The aerodynamic model is validated with an already existing wind tunnel experiment on a similar airfoil. Generally, the largest model uncertainty in CFD is the mesh and therefore, the uncertainty is assessed by mesh refinement studies. A range of flight conditions is simulated by varying the inflow angle of attack, sideslip angle and Reynolds number. The flow around the wing is characterized by a recirculation zone behind the leading edge tube due to the lack of second skin. The zone is highly influenced by the inflow conditions. The effect of the chordwise inflatable tubes on aerodynamics is assessed by creating a model with and without them. The results show that the chordwise tubes have an almost negligible impact on the aerodynamic forces, which suggests they could be left out of the aerodynamic model in future work, simplifying the mesh generation and mesh deformation. The third study shows the aeroelasticity of a ram-air kite for several power configurations by changing the trim of the bridle lines. The kite forms a typical ram-air shape with ballooning in between ribs, and the nose of the wing is flattened at the stagnation region. The aerodynamics of the flexible kite is compared to a rigidized version of it. The wing is fixed at the symmetry plane and fixed to the pre-inflated shape with stagnation pressure. The results show that the flexible kite is aerodynamically more efficient than the rigidized version. The morphing wing adapts itself to the incoming flow in a way that extends the range of feasible flight conditions and improves efficiency. The aeroelastic framework converges satisfactorily with all the power setups, and it is computationally relatively inexpensive for fidelity. Consequently, the framework could be integrated into a membrane kite design process and could be a valuable asset in evaluating kite designs. @en

In the past, fin-and tube heat exchanger (FTHE) tube pattern ratios have been largely based on ad-hoc design principles. Here, we investigate the optimal tube arrangements for a FTHE with plain fins in marine environments represented by two different air types; one for unfiltered air with high condensation rate and one for clean dry filtered air conditions. The thermal-hydraulic efficiency of the FTHE design is measured by comparing a modified ratio of Colburn j-factor and Fanning friction factor. The regression model generated from the CFD data is then used to identify the maximum efficiency for two design specific fin pitches separately. We identified two optimal tube patterns: one for a large fin pitch for unfiltered air, and another for a small fin pitch for filtered air. Manufacturing restrictions were found to significantly limit the maximum achievable efficiency of a tube pattern. By neglecting the related manufacturing restrictions, 4% higher efficiency for a fin pitch of 1.5 mm and 23% higher efficiency for a fin pitch of 3.5 mm is achieved. Without any application specific limitations or manufacturing restrictions the fin pitch 1.5 mm can have a 36% increased efficiency than fin pitch 3.5 mm. These novel results show that development in manufacturing have potential for significant improvements in thermal-hydraulic efficiency.

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Leading-edge inflatable (LEI) kites use a pressurized tubular frame to structurally support a single skin membrane canopy. The presence of the tubes on the pressure side of the wing leads to characteristic flow phenomena for this type of kite. In this paper, we present steady-state Reynolds-Averaged Navier-Stokes (RANS) simulations for a LEI wing for airborne wind energy applications. Expanding on previous work where only the leading-edge tube was considered, eight additional strut tubes that support the wing canopy are now included. The shape of the wing is considered to be constant. The influence of the strut tubes on the aerodynamic performance of the wing and the local flow field is assessed, considering flow configurations with and without side-slip. The simulations show that the aerodynamic performance of the wing decreases with increasing side-slip component of the inflow. On the other hand, the chordwise struts have little influence on the integral lift and drag of the wing, irrespective of the side-slip component. The overall flow characteristics are in good agreement with previous studies. In particular, it is confirmed that at a low Reynolds number of Re=105, a laminar separation bubble exists on the suction side of this hypothetical rigid wing shape with perfectly smooth surface. The destruction of this bubble at low angles of attack impacts negatively on the aerodynamic performance. @en

Vortex-induced vibrations (VIVs) of wind turbine towers can be critical during the installation phase, when the rotor-nacelle assembly is not yet mounted on the tower. The present work uses numerical simulations to study VIVs of a two-dimensional cylinder in the transverse direction under flow conditions that are representative of wind turbine towers both from a fluid dynamics and structural dynamics perspective. First, the numerical tools and fluid-structure interaction algorithm are validated by considering a cylinder vibrating freely in a laminar flow. In that case, both the motion amplitude and frequency are shown to agree well with previous results from the literature. Second, VIVs are modelled in the turbulent supercritical regime using unsteady Reynolds-averaged Navier-Stokes equations. In this context, the turbulence model is first validated against flow past a stationary cylinder with a high Reynolds number. Then, the results from forced vibrations are validated against experimental results for a range of reduced frequencies and velocities. It is shown that the behaviour of the aerodynamic damping changes with the frequency ratio and can therefore lead to either self-limiting or self-exciting VIVs when the cylinder is left to freely vibrate. Finally, results are shown for a freely vibrating cylinder under realistic flow and structural conditions. While a clear lock-in map is identified and shows good agreement with published numerical and experimental data, the work also highlights the unsteady nature of the aerodynamic forces and motion under certain operating conditions..

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In this paper we present a computational approach to simulate the steady-state aeroelastic deformation of a ram-air kite for airborne wind energy applications. The approach is based on a computational fluid dynamics (CFD) solver that is two-way coupled with a finite element (FE) solver. All components of the framework, including the meshing tools and the coupling library, are available in open source. The flow around the wing is described by the steady-state Reynolds-averaged Navier-Stokes (RANS) equations closed by an SST turbulence model. The FE model of the cellular membrane structure includes a wrinkling model and uses dynamic relaxation to find the deformed steady-state shape. Each simulation comprises four distinct steps: (1) generating the FE mesh of the design geometry, (2) pre-inflation of the wing, applying a uniform pressure on the inside, (3) generating the CFD mesh around the pre-inflated wing, and (4) activating the exterior flow and two-way coupling iterations. We first present results for the aerodynamics of the pre-inflated rigid ram-air wing and compare these to similar results for a leading edge inflatable (LEI) tube kite. Both wings are characterized by a high anhedral angle and low aspect ratio which induce spanwise flows that reduce the aerodynamic performance. The comparison shows a better performance for the LEI wing which can be attributed to its higher aspect ratio. The aeroelastic deformation of the ram-air wing further improves the aerodynamic performance, primarily because of the increasing camber which in turn increases the lift force. A competing aeroelastic phenomenon is the formation of bumps near the leading edge which increase the drag.

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In this work we present Reynolds-averaged Navier-Stokes (RANS) simulations of the flow past the constant design shape of a leading-edge inflatable (LEI) wing. The simulations are performed with a steady-state solver using a k-ω SST turbulence model, covering a range of Reynolds numbers between 10⁵ ≤ and ≤ 15 × 10⁶ and angles of attack varying between-5° and 24°, which are representative for operating conditions in airborne wind energy applications. The resulting force distributions are used to characterize the aerodynamic performance of the wing. We found that a γ-Re_θ transition model is required to accurately predict the occurrence of stall up to at least Re= 3 × 10⁶. The work highlights similarities with the flow past a two-dimensional LEI airfoil, in particular, with respect to flow transition and its influence on the aerodynamic properties. The computed values of the lift and drag coefficients agree well with in-flight measurements acquired during the traction phase of the LEI wing operation. The simulations show that the three-dimensional flow field exhibits a significant cross flow along the span of the wing.

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We present a computational fluid dynamic analysis of boundary layer transition on leading edge inflatable kite airfoils used for airborne wind energy generation. Because of the operation in pumping cycles, the airfoil is generally subject to a wide range of Reynolds numbers. The analysis is based on the combination of the shear stress transport turbulence model with the (Formula presented.) transition model, which can handle the laminar boundary layer and its transition to turbulence. The implementation of both models in OpenFOAM is described. We show a validation of the method for a sailwing (ie, a wing with a membrane) airfoil and an application to a leading edge inflatable kite airfoil. For the sailwing airfoil, the results computed with transition model agree well with the existing low Reynolds number experiment over the whole range of angles of attack. For the leading edge inflatable kite airfoil, the transition modeling has both favorable and unfavorable effects on the aerodynamics. On the one hand, the aerodynamics suffer from the laminar separation. But, on the other hand, the laminar boundary layer thickens slower than the turbulent counterpart, which, in combination with transition, delays the separation. The results also indicate that the aerodynamics of the kite airfoil could be improved by delaying the boundary layer transition during the traction phase and tripping the transition in the retraction phase.@en

We investigate inflatable kites made of membranes such as ram-air [1] and leading edge inflatable [2] kites. The kites are very flexible and therefore exhibit a strong coupling between fluid and structure. An accurate aerodynamic model is essential to design kites which are aerodynamically efficient and of high steering capability@en

In the kite power group at TU Delft we are currently investigating leading edge inflatable (LEI) kites. The kite consists of a membrane canopy which is supported by an inflatable tubular frame. The frame transfers the wind loads from the canopy to the bridle line system which is further connected to the main tether. This tensile structure is highly flexible and exhibits large displacements as a result of the aerodynamic loads. Consequently, the loads and displacements are strongly coupled and form a complex fluid-structure interaction (FSI) problem. The flow separates both on the suction side due to the high angle of attack to maximize the lift force, and behind the tubular leading edge, where a constant separation bubble is formed. Moreover, the low aspect ratio and high anhedral angle of the kite make the flow highly three dimensional and the effect of wingtip vortices and crossflow cannot be neglected. Currently, at TU Delft we use a simulation method which was initiatedby Breukels [1]. His aerodynamic model uses two dimensional finite strip-approximation which divides the wing into two dimensional sections (airfoils) and neglects the three dimensionality of the flow. The section forces are calculated from two dimensional simulations with computational fluid dynamics (CFD). In this project, we continue the work by Breukels and use a full three dimensional CFD model. The model is coupled with a nonlinear structural finite element method (FEM). At this stage we are focusing on two dimensional LEI kite airfoils. The two dimensional simulations are used to verify and validate the framework and also to study the effect of flow transition. The CFD simulations are carried out by using OpenFOAM which is coupled with a TU Delft inhouse structural solver by an efficient preCICE coupling environment. The results are compared to already existing experimental data from sailwing airfoils [2, 3]. @en