Mac Gaunaa

• Technical University of Denmark

This work explores the effect of streamwise flow inhomogeneities on power performance and whether this can be described by the same engineering model for both the flow in wind farms and over complex terrain. For this purpose, data from previous computational studies are reanalyzed. Results suggest that in both cases the engineering model is capable...

Leading-edge inflatable (LEI) kites are morphing aerodynamic surfaces that are actuated by the bridle line system. Their design as tensile membrane structures has several implications for the aerodynamic performance. Because of the pronounced C-shape of the wings, a considerable part of the aerodynamic forces is redirected sideways and used for ste...

Advancements in wind turbine technology have led to larger, more flexible blades and an increasing interest in aerodynamic load calculations and design optimization of blades featuring significant sweep, prebend or coning. High-fidelity blade-resolved computational fluid dynamics (CFD) simulations provide precise rotor performance predictions but a...

In the present work, a simple model is derived for the situation of an actuator disc (AD) operating in a background flow field featuring a constant streamwise velocity gradient. Reynolds-averaged Navier-Stokes (RANS) simulations of this scenario are performed, showing that a positive acceleration yields a reduction of induction and vice versa, a ne...

Kevlar walled wind tunnels have become widely used for aeroacoustic testing. Our research documents that a pressure difference across the Kevlar membrane influences the transmission of sound through it. Such effects have been ignored up to the present day which might lead to inaccurate results when testing objects that generate large aerodynamic fo...

Wind tunnel test of the NACA63-018 airfoil is performed investigating high Reynolds number dynamic stall behaviour at PLCT [1]. The foil is tested within the reduced frequency range of k ≃0.03 and Reynolds number range Re =1.5-6M in the linear attached flow range and at stalled mean AoA at moderate harmonic pitch amplitudes. The foil is fitted with...

The blade element momentum (BEM) method is among the mostly used engineering aerodynamic models for wind turbine rotors. Even though the BEM method is strictly only applicable to straight blades forming a planar rotor, it is in practice used for load calculation and design optimization of blades with relatively large sweep, prebend or coning. The p...

This article discusses results from an international contest, open for university student teams (bachelor and master), involving the design, construction, and testing of small wind turbines in a large wind tunnel. The wind tunnel has an outlet of 2.85 x 2.85 m allowing a maximum rotor swept area of 2 m ² without significant tunnel effects. Both hor...

The performance of a wind turbine is usually characterized by its power curve, which relates the wind speed at hub height with its energy production. This relation does not take streamwise inhomogeneities of the inflow – as they might be present in complex terrain – into account. In this work, the consequences of this simplification for performance...

Design optimization and aeroelastic load evaluation for wind turbines are still infeasible for CFD due to computational cost. These tasks are carried out using 3D engineering aerodynamic methods, where the local aerodynamic loads are obtained from tabulated 2D aerodynamic polars, which makes the models fast. With recent developments such as highly...

Although technologically challenging, airborne wind energy systems have several advantages over conventional wind turbines that make them an interesting option for deployment on Mars. However, the environmental conditions on the red planet are quite different from those on Earth. The atmosphere’s density is about 100 times lower, and gravity is abo...

Renewable energy for Mars habitats is of key interest for future crewed missions. However, the low solar irradiation and low atmospheric pressure on Mars pose serious challenges to exploiting these resources reliably. In this chapter, we investigate the technical feasibility of a soft-kite-based airborne wind energy system and its potential to powe...

Rotary Airborne Wind Energy Systems (RAWES) are networks of wings that act as flying wind turbines. The power extracted from the wind can be transmitted to ground-based generators through open tensegrity structures. In recent years, there have been different approaches to the topology of the Tensile Rotary Power Transmission (TRPT) and prototype de...

A simple analytical vortex model is presented and used to study an idealized wind turbine rotor in uniform and sheared inflow, respectively. Our model predicts that 1D momentum theory should be applied locally when modelling a non-uniformly loaded rotor in a sheared inflow. Hence the maximum local power coefficient (expressed with respect to the lo...

Soft-wing kites for airborne wind-energy harvesting function as flying tensile membrane structures, each of whose shape depends on the aerodynamic load distribution and vice versa. The strong two-way coupling between shape and loading poses a complex fluid-structure interaction problem. Since computational models for such problems do not yet meet t...

A simple analytical vortex model is presented and used to study an idealized wind turbine rotor in uniform and sheared inflow, respectively. The work reveals that locally 1D momentum theory is valid for a non-uniformly loaded rotor in a sheared inflow. Hence the maximum local power coefficient (expressed with respect to the local, upstream velocity...

In the present work, a consistent method for calculating the lift and drag forces from the 2-D airfoil data for the dihedral or coned horizontal-axis wind turbines when using generalized lifting-line methods is described. The generalized lifting-line methods refer to the models that discretize the blade radially into sections and use 2-D airfoil da...

Blade leading edge erosion (LEE) is a major cost driver in the wind energy sector. LEE is caused by the environmental conditions under which the blades operate. The impact energy of the airborne particles striking the leading edge determines the speed of erosion, thus LEE severity grows towards the blade tip and with a turbine’s tip speed. Currentl...

The present work compares non-planar rotors designed using the blade element momentum (BEM) method and a vortex cylinder model. In a previous work, it is shown that blade element theory coupled with the superposition of the vortex cylinder model (BEVC) is able to model the loads of non-planar rotors. The result predicted by the BEVC model is in sig...

Different Design Driving Load constraints (DDLs), are explored in this work to determine under which constraints and conditions a winglet can have an added value to the wind turbine blade design. Multi-objective Bayesian optimization is used to maximize the rotor’s power production while minimizing the flapwise DDLs. Surrogate models, created using...

This paper describes a benchmark of four airfoils in the Poul la Cour Tunnel (PLCT). The wind tunnel, the corrections used and the method of making adapters for the airfoils are also described. Very good agreement was in general observed between the measurements in PLCT and in other high quality wind tunnels. Some deviations were seen, but they wer...

In this work, a computationally efficient engineering model for the aerodynamics of swept wind turbine blades is proposed for the extended blade element momentum (BEM) formulation. The model is modified based on a coupled near- and far-wake model, in which the near wake is assumed to be the first quarter revolution of the non-expanding helical wake...

In the present work, a computationally efficient engineering model for the aerodynamic load calculation of non-planar wind turbine rotors is proposed. The method is based on the vortex cylinder model and can be used in two ways: either used as a correction to the currently widely used blade element momentum (BEM) method or used as the main model, r...

In the present work, a consistent method for calculating the lift and drag forces from the 2-D airfoil data for the dihedral or coned horizontal-axis wind turbines when using generalized lifting-line methods is described. The generalized lifting-line methods include, for example, lifting-line (LL), actuator line (AL), blade element momentum (BEM) a...

One promising design solution for increasing the efficiency of modern horizontal axis wind turbines is the installation of curved tip extensions. However, introducing such complex geometries may move traditional aerodynamic models based on blade element momentum (BEM) theory out of their range of applicability. This motivated the present work, wher...

In the present work, a computationally efficient engineering model for the aerodynamic load calculation of non-planar wind turbine rotors is proposed. The method is based on the vortex cylinder model, and can be used in two ways: either as a correction to the currently widely used blade element momentum (BEM) method, or used as the main model, repl...

In this work, a computationally efficient engineering model for the aerodynamics of swept wind turbine blades is proposed for the extended blade element momentum (BEM) formulation. The model is modified based on a coupled near- and far-wake model, in which the near-wake is assumed to be the first quarter revolution of the non-expanding helical wake...

This paper presents an analysis and optimization of Airborne Wind Energy Systems (AWESs), designed to maximize the Annual Energy Production (AEP) and, in the second part, the economic profit. A gradient-based optimization algorithm is used to perform the preliminary design of the main AWES sub-systems. A global sensitivity analysis is carried out t...

One promising design solution for increasing the efficiency of modern horizontal axis wind turbines is the installation of curved tip extensions. However, introducing such complex geometries may move traditional aerodynamic models based on Blade Element Momentum (BEM) theory out of their range of applicability. This motivated the present work, wher...

The design and computational model of a representative multi-megawatt airborne wind energy (AWE) system operated in pumping cycles is presented [1], together with a simulation framework that accounts for the flight dynamics of the fixed wing aircraft and the sagging of the tether, combining this with flight control and optimisation strategies to de...

In this paper, we present the design and computational model of a representative multi-megawatt airborne wind energy (AWE) system, together with a simulation framework that accounts for the flight dynamics of the fixed-wing aircraft and the sagging of the tether, combining this with flight control and optimisation strategies to derive the power cur...

The aim of this work is to propose engineering models to include the external forces acting on the tether into the power estimation of an Airborne Wind Energy System flying crosswind. The influence of aerodynamic drag, centrifugal force and gravitational force on power production is investigated. These three types of external loading acting on the...

Passive load alleviation on wind turbine blades can be achieved through geometric bend-twist coupling, for example by sweeping the blade backwards. In order to obtain the correct load distribution of a curved blade with in-plane sweep and/or out-of-plane dihedral, the influence of the blade shape on the aerodynamics must be modelled correctly. This...

The present study investigates the differences in the predicted lift for a wind tunnel model from either surface pressures or side wall pressures. The investigated model is the NACA63-418 tested in the Poul la Cour tunnel. The measurements are compared with previous conducted wind tunnel campaigns for a Reynolds number of 3E6. The 3D CFD model of t...

The present paper introduces a new, physically consistent definition of effective induction that should be used in engineering models for power kite performance that use aerodynamic coefficients for the wing. It is argued that in such cases it is physically inconsistent to use disc-based induction models – like momentum models – and thus a new, phy...

We analyze the new Danish Poul la Cour Tunnel (PLCT) facility using 2D and 3D Computation Fluid Dynamics simulations, with the aim to address the need for wind tunnel corrections. The assessment of tunnel corrections is based on 2D free simulations representing undisturbed airfoil flow, 2D tunnel configurations and a full 3D tunnel setup using lami...

This paper presents analytic equations modelling the main physics and cost of a generic Airborne Wind Energy System flying crosswind. The power equation of a system that can handle ground and on-board generation is presented, under the assumption of steady-state flight. A structural and a take-off model are used to derive the key equations for the...

The aim of the current work was to analyze the effect of retrofitting vortex generators (VGs) on the blades of a constant RPM, pitch‐regulated, megawatt‐sized turbine suffering from surface roughness. Engineering modelling and experimental work were utilized, indicating that the degradation of energy production may be mitigated by the VGs. The mode...

This paper presents tests carried out in the Virginia Tech Stability Wind Tunnel with Kevlar walls to allow for aeroacoustic measurements. Two-dimensional Computational Fluid Dynamics (CFD) calculations using the EllipSys2D solver were carried out to investigate the impact of no-slip condition, porosity, and flexibility of the simulated walls on th...

Erosion of the leading edges of wind turbine blades due to the repeated impact of rain droplets at high speed over time can wear down the blade surfaces to the extent that power production is significantly reduced for the wind turbines. Therefore a rain erosion tester, which is a test bench for accelerated test of leading erosion due to rain impact...

Passive load alleviation can be achieved through geometric bend-twist coupling, for example, by sweeping the blade backwards. The influence of the blade sweep on the trailing vorticity and bound vorticity is not modelled in the current fast aeroelastic wind turbine codes suitable for certification. A near wake trailed vorticity model which was coup...

Vortex generators have in recent years been used extensively on pitch regulated wind turbines. A new trend has been to use vortex generators on thinner airfoils on the outer part of the blades. However, not much data is available for thin airfoils with vortex generators. That is the reason to carry out wind tunnel tests on a NACA 633-418 airfoil wi...

The Beddoes-Leishman type dynamic stall model was originally implemented in HAWC2 with a focus on horizontal axis wind turbines. In case of HAWTs, some terms in the unsteady airfoil lift and drag are very small and can be neglected, which are very important for VAWTs. Furthermore, the angle of attack variations during normal operation of VAWTs are...

Vortex-induced integral loading fluctuations on a single suspended blade at various inflow angles were modeled in the presents work by means of stochastic modelling methods. The reference time series were obtained by 3D DES CFD computations carried out on the DTU 10MW reference wind turbine blade. In the reference time series, the flapwise force co...

In this paper the critical leading edge roughness height is analyzed in two cases: 1) leading edge roughness influencing the lift-drag ratio and 2) leading edge roughness influencing the maximum lift. The analysis was based on wind tunnel measurements on the airfoils NACA0015, Risoe-B1-18 and Risoe-C2-18 and at three different Reynolds numbers with...

Measurements on a heavily instrumented pitch regulated variable speed Vestas V52 850 kW wind turbine situated at the DTU Risø Campus are carried out, where the effect of vortex generators mounted on almost the entire blade is tested with and without leading edge roughness. The measurements are compared to the predictions carried out by a developed...

The crossflow principle is one of the key elements used in engineering models for prediction of the aerodynamic loads on wind turbine blades in standstill or blade installation situations, where the flow direction relative to the wind turbine blade has a component in the direction of the blade span direction. In the present work, the performance of...

A vortex particle representation of turbulent fields is devised in order to address the following questions: Does a wind turbine affect the statistics of the incoming turbulence? Should this imply a change in the way turbulence boxes are used in wind turbine aero-elastic simulations? Is it acceptable to neglect the influence of the wake and the win...

A row of wind turbine rotors with a mutual spacing of three diameters is simulated using both Reynolds averaged Navier-Stokes (RANS) simulations and a simple inviscid vortex model. The angle between the incoming wind and the line connecting the turbines is varied between 45 and 90 degrees. The simulations show that the power production of the turbi...

Joukowski introduced in 1912 a helical vortex model to represent the vorticity of a rotor and its wake. For an infinite number of blades but finite tip-speed ratio, the model consists of a vortex cylinder of longitudinal and tangential vorticity, a root vortex and a bound vortex disk. A superposition of cylinders is used in this paper to model roto...

Vortex particles methods are applied to the aeroelastic simulation of a wind turbine in sheared and turbulent inflow. The possibility to perform large-eddy simulations of turbulence with the effect of the shear vorticity is demonstrated for the first time in vortex methods simulations. Most vortex methods formulation of shear, including segment for...

An extension of the spectrum of applicability of rotors with active aerodynamic devices is presented in this paper. Besides the classical purpose of load alleviation, a secondary objective is established: optimization of power capture. As a first step, wind speed regions that contribute little to fatigue damage have been identified. In these regio...

An experimental campaign using Particle Image Velocimetry (2C-PIV) technique has been conducted on a H-type Vertical Axis Wind Turbine (VAWT) to create a benchmark for validating and comparing numerical models. The turbine is operated at tip speed ratios (TSR) of 4.5 and 2, at an average chord-based Reynolds number of 1.6e5 and 0.8e5. At both TSR,...

A vortex system consisting of a bound vortex disk, a root vortex and a vortex cylinder is presented and applied for skewed wake situations. Both the longitudinal and tangential components of vorticity of the cylinder are considered. A subset of this system leads to a model, which is commonly used in Blade Element Momentum method codes for yawed con...

An experimental campaign using a phase-locked Particle Image Velocimetry (2C-PIV) technique has been conducted on a H-type Vertical Axis Wind Turbine (VAWT). The turbine is operated at tip speed ratios (TSR) 4.5 and 2, at accordingly average chord-based Reynolds number of 1.6e 5 and 0.8e 5 . At both TSR, the velocity fields are presented in the mid...

The effect of wake rotation is modeled based on a vortex cylinder model. The model is implemented in a BEM code and results are compared to actuator disk (AD) simulations. Significant improvements are obtained compared to the standard stream tube theory BEM implementation.

The paper presents an analysis of the aeroelastic loads on a wind turbine in normal operation. The characteristic of the loads causing the highest fatigue damage are identified, so to provide indications to the development of active load alleviation systems for smart- rotor applications. Fatigue analysis is performed using rain-flow counting and Pa...

A new analytical formulation of the tip-loss factor is established based on helical vortex filament solutions. The derived tip-loss factor can be applied to wind-turbines, propellers or other rotary wings. Similar numerical formulations are used to assess the influence of wake expansion on tip-losses. Theodorsen's theory is successfully applied for...

This work presents an analysis of vortex-induced vibrations of a DU96-W-180 airfoil in deep stall at a 90° angle of attack, based on 2D and 3D Reynolds Averaged Navier Stokes and 3D Detached Eddy Simulation unsteady Computational Fluid Dynamics computations with non-moving, prescribed motion and elastically mounted airfoil suspensions. Stationary v...

In the present paper, single-wake dynamics have been studied both experimentally and numerically. The use of pulsed lidar measurements allows for validation of basic dynamic wake meandering modeling assumptions. Wake center tracking is used to estimate the wake advection velocity experimentally and to obtain an estimate of the wake expansion in a f...

The vortex system consisting of a bound vortex disk, a root vortex and a vortex cylinder as introduced by Joukowski in 1912 is further studied in this paper. This system can be used for simple modeling of rotors (e.g. wind turbines) with infinite number of blades and finite tip-speed ratios. For each vortex element, the velocity components in all d...

Limitations on the wind speed at which blade installation can be performed bears important financial consequences. The installation cost of a wind farm could be significantly reduced by increasing the wind speed at which blade mounting operations can be carried out. This work characterizes the first-order aerodynamic and aeroelastic behavior of a s...

The main results from a recently developed vortex model are implemented into a Blade Element Momentum(BEM) code. This implementation accounts for the effect of finite tip-speed ratio, an effect which was not considered in standard BEM yaw-models. The model and its implementation are presented. Data from the MEXICO experiment are used as a basis for...

The aim of the current work is to analyze possible advantages of mounting Vortex Generators (VG's) on a wind turbine blade. Specifically, the project aims at investigating at which radial sections of the DTU 10 MW Reference Wind Turbine blade it is most beneficial to mount the VG's in order to increase the Annual Energy Production (AEP) under reali...

The study presents and compares aerodynamic simulations for an airfoil section with an adaptive trailing edge flap, which deflects following a smooth deformation shape. The simulations are carried out with three substantially different methods: a Reynolds-averaged Navier–Stokes solver, a viscous–inviscid interaction method and an engineering dynami...

This work presents an analysis of two-dimensional (2D) and three-dimensional (3D) non-moving, prescribed motion and elastically mounted airfoil computational fluid dynamics (CFD) computations. The elastically mounted airfoil computations were performed by means of a 2D structural model with two degrees of freedom. The computations aimed at investig...

The newly implemented vortex code Omnivor coupled to the aero-servo-elastic tool hawc2 is described in this paper. Vortex wake improvements by the implementation of viscous effects are considered. Different viscous models are implemented and compared with each other. Turbulent flow fields with sheared inflow are used to compare the vortex code perf...

The present study investigated physical phenomena related to stall-induced vibrations potentially existing on wind turbine blades at standstill conditions. The study considered two-dimensional airfoil sections while it omitted three-dimensional effects. In the study, a new engineering-type computational model for the aeroelastic response of an elas...

An extension of the spectrum of applicability of rotors with active aerodynamic devices is presented in this paper. Besides the classical purpose of load alleviation, a secondary objective is established: optimization of power capture. As a first step, wind speed regions that contribute little to fatigue damage have been identified.In these regions...

The aeroelastic response of wind turbines is often simulated in the time domain by using indicial response techniques. Unsteady aerodynamics in attached flow are usually based on Jones's approximation of the flat plate indicial response, although the response for finite-thickness airfoils differs from the flat plate one. The indicial lift response...

Standard blade element momentum (BEM) codes use Prandtl??s tip-loss correction which relies on simplified vortex theory under the assumption of optimal operating condition and no wake expansion. The various tip-loss functions found in the literature are listed. A simple comparison between them shows important differences in Annual Energy Production...

A deformable trailing edge section (3) of a wind turbine blade (1), at least part of said section (3) being formed in a deformable material. The blade section (3) comprises one or more cavities (5) being in connection with or connectable to a fluid source in a way that allows fluid to stream from the fluid source to the cavity or cavities (5), so t...

The present work continues the investigations of using slats on the inner parts of wind turbine rotors by using an updated version of the 2D CFD based airfoil/slat design tool earlier used by the authors in combination with the rotor design methods from [1] to design slats for 0:1 > r=R > 0:3 for the DTU 10 MW reference rotor [2]. For the slatted c...

This paper describes the design of a rotor and a wind turbine for an artificial 10-MW wind turbine carried out in the Light Rotor project. The turbine called the Light Rotor 10-MW Reference Wind Turbine (LR10- MW RWT), is designed with existing methods and techniques and serves as a reference to future advanced rotor designs in the project. The res...

Various research projects have focused on active aerodynamic load control of wind turbines using control devices on the blades, for example flaps. The aerodynamic load predictions of utilized aeroelastic codes have not yet been fully validated with full rotor CFD or experimental results. In this study, a comparison between aerodynamic predictions o...

The present work attempts at quantifying the effects of using slats on the inner part of a modern multi-MW rotor as a post-fix to increase power production. In the process of doing this, the main behavior of the underlying physics is illuminated using a simplified BEM approach, and the effect of the lift and drag forces on local and integral power...

A simple analytical rotor model based on vortex theory is briefly presented and used to investigate the main mechanisms for wind turbine rotors operating at yaw misalignment. The overall findings of the model is verified by comparing with an existing model as well as with results obtained using the actuator disc technique combined with full Navier-...

It is a known fact that the power produced by wind turbines operating inside an array decreases due to the wake effects of the upstream turbines. It has been proposed previously to use the yaw mechanism as a potential means to steer the upstream wake away from downstream turbines, however such a mechanism introduces control complications due to cha...

In this work a 2D CFD solver has been used to optimize the shape of a leading edge slat with a chord length of 30% of the main airfoil which was 40% thick. The airfoil configuration was subsequently tested in a wind tunnel and compared to numerical predictions. The multi-element airfoil was predicted to achieve a Cl-max of 3.1 based on the main air...

In the present work, CFD simulations of the DU96-W-180 airfoil at 26 and 24 deg. angles of attack were performed. 2D RANS and 3D DES computations with non-moving and prescribed motion airfoil suspensions were carried out. The openings of the lift coefficient loops predicted by CFD were different than those predicted by engineering models. The avera...

Standard blade element momentum(BEM) codes use Prandtl's tip-loss correction which relies on simplied vortex theory under the assumption of optimal operating condition and no wake expansion. A new tip-loss correction for implementation in BEM codes has been developed using a lifting-line code to account for the eect of wake expansion, roll-up and d...

The present work includes analysis of the results obtained with a free wake code to determine two simple, non-iterative prescribed wake models aimed for use in steady load and design calculations for rotors with winglets. The results show that results from a prescribed wake model using only the integral axial load on the rotor and the main geometry...

A new computationally efficient method for de- termination of the aerodynamic performance of kites is proposed in this paper. The model is based on an iterative coupling between a Vor- tex Element Method (VLM) and 2D sectional airfoil coefficients to introduce the effect of air- foil thickness and effects of viscosity while re- taining the strength...

This report describes the DAN-AERO MW experiments carried out within a collaborative, three years research project between Risø DTU and the industrial partners LM Glasfiber, Siemens Wind Power, Vestas Wind Systems A/S and the utility company DONG Energy. The main objective of the project was to establish an experimental data base which can provide...

The present work contains a deformable trailing edge flap controller integrated in a numerically simulated modern, variablespeed, pitch-regulated megawatt (MW)-size wind turbine. The aeroservoelastic multi-body code HAWC2 acts as a component in the control loop design. At the core of the proposed controller, all unsteady loads are divided by freque...

This work presents a method to determine flutter and divergence instability limits for a two-dimensional (2-D) airfoil section fitted with an actively controlled trailing edge flap. This flap consists of a deformable trailing edge, which deformation is governed by control algorithms based on measurements of either heave displacement, local angle of...

In the present work, analytical expressions for distributed and integral unsteady two-dimensional forces on a variable geometry airfoil undergoing arbitrary motion are derived under the assumption of incompressible, irrotational, inviscid flow. The airfoil is represented by its camber line as in classic thin-airfoil theory, and the deflection of th...

A wind tunnel test of the wind turbine airfoil Risø-B1-18 equipped with an Active Trailing Edge Flap (ATEF) was carried out. The ATEF was 9% of the total chord, made of piezo electric actuators attached to the trailing edge of a non-deformable airfoil and actuated using an (electric) amplifier. The airfoil was tested at Re = 1.66 × 106. Steady stat...