Lars Sætran

• PhD
• Professor (Full) at Norwegian University of Science and Technology

With increasing subsea oil and gas activities, the safety is challenged by accidental gas release. This can be caused by leakage from gas transport pipelines or blowouts from oil and gas wells. Risk assessment of such events is associated to the correct prediction of gas flux and gas distribution through the ocean surface and the resulting surface...

The vortex interaction in the wake behind a two- and three-bladed model scale wind turbine is investigated. The two rotors have equal solidity, and produce similar power and thrust at the design tip speed ratio. Phase-averaged quantities of the wake flow from one to four rotor diameters behind the turbines are measured in a wind tunnel. It is found...

Proper turbulence modelling of bubble plumes is critical for accurate simulation of liquid phase and bubble dynamics. The k-epsilon turbulence model is widely used. However, the model is unsatisfactory, due to not accounting for the following physics: 1) turbulence damping in the vicinity of a free surface; 2) turbulence modifications created by no...

Abstract. Many coastal regions in Norway, Spain, Portugal, Japan or the United States are comprised of large water depths (> 50 m) making the installation of typical bottom-fixed off-shore wind turbines very difficult and expensive. This is the reason why the floating wind turbines (FOWT) are a promising solution able to exploit the high energy pot...

The cover image is based on the Original Article An experimental study on the effects of winglets on the tip vortex interaction in the near wake of a model wind turbine by Jan Bartl et al., https://doi.org/10.1002/we.2486.

Most icing research focuses on the high Reynolds number regime and manned aviation. Information on icing at low Reynolds numbers, as it is encountered by wind turbines and unmanned aerial vehicles, is less available, and few experimental datasets exist that can be used for validation of numerical tools. This study investigated the aerodynamic perfo...

Understanding the vertical wind profile is paramount for design and operation of wind turbines. It is needed not only for extrapolation of the wind velocity to hub height but also for structural load calculations, to name the most obvious issues. As wind turbines grow in size and development transitions offshore, issues such as shallow surface laye...

An experimental study of the near wake up to four rotor diameters behind a model wind turbine rotor with two different wing tip configurations is performed. A straight‐cut wing tip and a downstream‐facing winglet shape are compared on the same two‐bladed rotor operated at its design tip speed ratio. Phase‐averaged measurements of the velocity vecto...

Abstract. Phenomena such as internal boundary layers and low-level jets can cause short-term wind fluctuations resulting in the vertical wind profile deviating from its expected logarithmic shape. Analysis of several years of data from the 6 measurement heights on the 100m high coastal Skipheia mast reveals that the 10-minute averaged vertical wind...

Understanding the vertical wind profile is paramount for design & operation of wind turbines. It is needed not only for extrapolation of the wind velocity to hub height but also for structural load calculations, to name the most obvious issues. As wind turbines grow in size and development transitions offshore, issues such as shallow surface layers...

Nine research teams organized a round-robin measurement campaign of the wake of two porous discs in a homogeneous and “low-turbulent" flow. Mean streamwise velocity and turbulence intensity profiles at four diameters downstream of the discs were measured and compared through such metrics as the maximum velocity deficit, the maximum turbulence inten...

This article summarizes the results of the “Blind test 5” workshop, which was held in Visby, Sweden, in May 2017. This study compares the numerical predictions of the wake flow behind a model wind turbine operated in yaw to experimental wind tunnel results. Prior to the workshop, research groups were invited to predict the turbine performance and w...

This paper presents an analysis of stratification-dependent mean velocity profiles measured in a Norwegian coastal wind climate, and its comparison with models available in the literature. For this purpose, we use 3 years of observations from a 100 m meteorological mast located at the Frøya island (150 km west of Trondheim, Norway), equipped with a...

Lift, drag and surface pressure measurements are performed on a wing section of the NREL S826 wind turbine airfoil at eight Reynolds numbers ranging from Re=0.5x10^5 to 6.0x10^5. Alongside with the measurements two types of Reynolds averaged Navier–Stokes (RANS) simulations are performed, one of which includes a laminar to turbulent transition mode...

Wake effects in wind farms can cause significant power losses. In order to reduce these losses layout and control optimization can be applied. For this purpose, simple and fast prediction tools for the wake flow are needed. In the first part of this work, five analytical wind turbine wake models are compared to small-scale turbine wind tunnel measu...

In times of intense renewable energy development, the planning of wind farms and the improvement of their total efficiency has become a major field of research. A precise analysis of the velocity deficit, fluctuation load and the wake properties behind a turbine is essential to identify the optimal positioning and control of a wind farm cluster. Du...

For an accurate prediction of the complex flow conditions in wind farms, model scale wake flow measurements represent important references in well-defined boundary conditions. State-of-the art flow measurement techniques are often time-expensive and require elaborate post-processing to assess the data quality. In this wind tunnel study we demonstra...

In this experimental wind tunnel study the effects of intentional yaw misalignment on the power production and loads of a downstream turbine are investigated for full and partial wake overlap. Power, thrust force and yaw moment are measured on both the upstream and downstream turbine. The influence of inflow turbulence level and streamwise turbine...

Understanding the atmospheric stability conditions is important in order to obtain accurate estimates of the vertical wind speed profile. This work compares and evaluates common methods for estimation of atmospheric stability using standard meteorological mast observations. Atmospheric stability distributions from three different met-masts located...

The wake characteristics behind a yawed model wind turbine exposed to different customized inflow conditions are investigated. Laser Doppler anemometry is used to measure the wake flow in two planes at x/D = 3 and x/D = 6, while the turbine yaw angle is varied from γ=-30∘ to 0∘ to +30∘. The objective is to assess the influence of grid-generated inf...

This paper presents an investigation of wakes behind model wind turbines, including cases of yaw misalignment. Two different turbines were used and their wakes are compared, isolating effects of boundary conditions and turbine specifications. Laser Doppler anemometry was used to scan full planes of wakes normal to the main flow direction, six rotor...

This article summarizes the results of a fifth Blind test workshop, which was held in Visby, Sweden, in May 2017. This study compares the numerical predictions of the wake flow behind a model wind turbine operated in yaw to experimental wind tunnel results. Prior to the work shop, research groups were invited to predict the turbines' performances a...

In this experimental wind tunnel study the effects of intentional yaw misalignment on the power production and loads of a downstream turbine are investigated for full and partial wake overlap situations. Power, thrust force and yaw moment are measured on both the upstream and downstream turbine. The influence of inflow turbulence level and streamwi...

This paper presents an investigation of wakes behind model wind turbines, including cases of yaw misalignment. Two different turbines were used and their wakes are compared, isolating effects of boundary conditions and turbine specifications. Laser Doppler Anemometry was used to scan a full plane of a wake normal to the main fow direction, 6 rotor...

The wake characteristics behind a yawed model wind turbine exposed to different customized inflow conditions are investigated. Laser Doppler Anemometry is used to measure the wake flow in two planes at x/D = 3 and x/D = 6 while the turbine yaw angle is varied from −30° and 0° to +30°. The objective is to assess the influence of grid-generated inflo...

Computational Fluid Dynamics (CFD) is widely used to predict wind conditions for wind energy production purposes. However, as wind power development expands into areas of even more complex terrain and challenging flow conditions, more research is needed to investigate the ability of such models to describe turbulent flow features. In this study, th...

The demand for wind power is rapidly increasing, creating opportunities for wind farm installations in more challenging climates. Cold climate areas, where ice accretion can be an issue, are often sparsely populated and have high wind energy potential. Icing may lead to severely reduced aerodynamic performance and thereby reduced power output. To r...

In this study the effect of turbulence on wind turbine performance measurements has been investigated, and the turbulence normalization method introduced in a draft for the second edition of the IEC 61400-12-1 standard has been evaluated. Experimental data has been collected from a wind turbine test site at the coast of Norway over a 10 month perio...

A dataset of wind tunnel power and wake flow measurements on a setup of three aligned model wind turbines is presented. The power outputs of the three turbines are in good agreement with measurements from a full-scale wind farm of similar inter-turbine spacing. A comparison of the wake flow behind the first row and the second row shows a significan...

Various concepts have been investigated in wind turbine research to improve the productivity of wind farms. However, there are some strategies, which offer further potential for improvement. One of these concepts is the rotational direction of a wind turbine, which has influence on its wake characteristics and development. In this study the effect...

Measurement campaigns with continuous-wave Doppler Lidars (Light detection and ranging) developed at DTU Wind Energy in Denmark were performed in two very different wind tunnels. Firstly, a measurement campaign in a small icing wind tunnel chamber at VTT in Finland was performed with high frequency measurements for increasing the understanding of t...

Turbulent flow at Reynolds numbers 5 · 10⁴ to 10⁶ around the NREL S826 airfoil used for wind turbine blades is simulated using delayed detached eddy simulation (DDES). The 3D domain is built as a replica of the low speed wind tunnel at the Norwegian University of Science and Technology (NTNU) with the wind tunnel walls considered as slip walls. The...

In wind farms, the wake of the upstream turbines becomes the inflow for the downstream machines. Ideally, the turbine wake is a stable vortex system. In reality, because of factors like background turbulence, mean flow shear, and tower-wake interaction, the wake velocity deficit is not symmetric and is displaced away from its mean position. The irr...

Nowadays, not only the size of single wind turbines but also the size of wind farms is increasing. Understanding the interaction between the turbines and especially the wakes formed behind them are getting more important to further improve such wind turbine arrays. Consequently, new issues in wind energy research arise. An experimental wind tunnel...

In this wind tunnel campaign, detailed wake measurements behind two different model wind turbines in yawed conditions were performed. The wake deflections were quantified by estimating the rotor-averaged available power within the wake. By using two different model wind turbines, the influence of the rotor design and turbine geometry on the wake de...

This is an invitation to a 5 th blind test workshop jointly organized by the Norwegian University of Science and Technology (NTNU), the Norwegian University of Life Sciences (NMBU) and the Carl-von-Ossietzky University of Oldenburg (ForWind). Modelers are invited to submit predictions for the wake flow behind a yawed turbine for the described test...

This is a summary of the results of the fourth blind test workshop that was held in Trondheim in October 2015. Herein, computational predictions on the performance of two in-line model wind turbines as well as the mean and turbulent wake flow are compared to experimental data measured at the wind tunnel of the Norwegian University of Science and Te...

Due to cost benefit and weight reduction, 2-bladed wind turbines have the potential to become more important for offshore wind applications. In order to optimize the arrangement of wind turbines in wind farms and for accurate forecasts of the power production, a detailed knowledge of the wake flow is needed. In this study, three different rotors wi...

This is a summary of the results of the fourth Blind test workshop which was held in Trondheim in October 2015. Herein, computational predictions on the performance of two in-line model wind turbines as well as the mean and turbulent wake flow are compared to experimental data measured at NTNU's wind tunnel. A detailed description of the model geom...

2D and 3D steady state simulations were done using the commercial CFD package Star-CCM+ with three different RANS turbulence models. Lift and drag coefficients were simulated at different angles of attack for the NREL S826 airfoil at a Reynolds number of 100 000, and compared to experimental data obtained at NTNU and at DTU. The Spalart-Allmaras an...

In state-of-the-art wind farms each turbine is controlled individually aiming for optimum turbine power not considering wake effects on downstream turbines. Wind farm control concepts aim for optimizing the overall power output of the farm taking wake interactions between the individual turbines into account. This experimental wind tunnel study inv...

Wind measurements a short distance upstream of a wind turbine can provide input for a feedforward wind turbine controller. Since the turbulent wind field will be different at the point/plane of measurement and the rotor plane the degree of correlation between wind speed at two points in space both in the longitudinal and lateral direction should be...

In this study, a performance test has been performed on a 200W marine wind turbine, both in a wind tunnel, and mounted on a Wavescan ocean buoy in a coastal location near Trondheim. Long term wind data satisfying the DNV-RP-C205 recommended practice for describing environmental conditions and environmental loads have been extracted from the Eklima...

A wake study and combined power output analysis of an array of two model wind turbines is presented. In a wind farm arrangement wakes behind the upstream turbines directly affect the performance and structural loads of the downstream turbines. In this analysis the characteristics of the mean and turbulent wake flow behind an upstream model turbine...

Gust wind speeds in high wind speed conditions are important for load estimation on large structures in exposed locations, like a multi-megawatt wind turbine. Modern wind turbines operate at wind speeds up to 25 m/s so it is important to know the expected amplitude, acceleration and relevant scale of typical wind gusts under operating conditions.

This Site Assessment predicts which environmental conditions an object at the Hywind Demo site will be exposed to. The work is based on 2 years of data from a Seawatch buoy located at the site. By use of Gumbel distributions the 50 year extreme values of wind gust at 3.5 m height (30.5 m/s), ocean current speed at 20 m depth (1.4 m/s) and significa...

The present work intends to validate computational fluid dynamics (CFD) simulations to subsonic wind tunnel experiments. The models tested in the wind tunnel at NTNU (a mannequin and real cyclist in static position) were scanned using a 3D scanner consisting 48 single-lens reflex cameras surrounding the object in three heights (low/ground-midi-abov...

In this study a full wind turbine performance test has been performed on a 3MW onshore wind turbine in a coastal location in Norway utilizing a Windcube v2 lidar. The test was performed following the current edition (1. ed) of the IEC 61400-12-1 wind turbine power performance measurement standard with some necessary adaptations for remote sensing a...

Results from three "Blind test" Workshops on wind turbine wake modeling are presented. While the first "Blind test" (BT1, 2011) consisted of a single model turbine located in a large wind tunnel, the complexity was increased for each new test in order to see how various models performed. Thus the next "Blind test" (BT2, 2012) had two turbines mount...

This note describes a 4 th blind test case organized by NOWITECH and NORCOWE. We invite you to submit predictions for the described test cases and participate in a two-day workshop to be held in Trondheim, scheduled for October 2015. Here, the results from the predictions will be discussed and a comparison with measurements will be presented.

The present work shows a comparison between computational fluid dynamics (CFD) simulations obtained using the Unsteady Reynolds Averaged Navier-Stokes solver STARCCM+ from CD-Adapco and experiments carried out in the subsonic wind tunnel at NTNU. The models tested in the wind tunnel (a mannequin and real cyclist in static position) were 3D scanned...

This is a report on data presented at the "Blind test 3" Workshop organized jointly by Nowitech and Norcowe in Bergen, December 2013. A number of research groups were invited to predict the performances and the wake development behind two model wind turbines that have been extensively tested at the Department of Energy and Process Engineering, NTNU...

With increasing subsea oil and gas activities, the safety is challenged by accidental gas release. This can be caused by leakage from gas transport pipelines or blowouts from oil and gas wells. Risk assessment of such events is associated to the correct prediction of gas flux and gas distribution through the ocean surface and the resulting surface...

Pneumatic oil barriers (so called “bubble oil boom (BOB)”) are based on the rise of air bubbles which are injected from the submerged parallel line spargers (McClimans et al., 2012). Local outward flows are formed when the bubbles and entrained water reach the sea surface, and thereby could counterbalance the opposing sea current to retain the spil...

The present work focuses on individual posture optimization with the aim to individually reduce the drag and increase the power output on six elite cylists. In order to be able to quantify the changes in drag, power output and VO2max, wind tunnel tests combined with power output and oxygen intake measurements were carried out on each of the athlete...

The present work describes and compares previous results in the field of garment aerodynamics combined with not published results coming from recent experiments conducted at the Norwegian University of Science and Technology (NTNU). A number of factors resulting from the combination of the athlete's motion and posture, garment and aerodynamics are...

Detailed wake measurements with two similar model wind turbines, with a diameter of 0.9m, were performed in the wind tunnel. A single turbine arrangement and a tandem setup, where a wind turbine was operated in the wake of an upstream wind turbine, were tested. Measurements of the cross section in the near and far wake as well as measurements in ax...

The advantage of tripping boundary layer turbulence around bluff bodies in order to reduce the aerodynamic drag and stabilize the flow has been known for many decades and applied in a number of engineering fields. However, its use in the sport technology area is quite recent. The first practical application of modeled surfaces used to reduce drag h...

In bubbly flow, bubbles and the surrounding fluid interact through both force and turbulence coupling. The effects of flow turbulence on bubble trajectory are reflected in turbulent dispersion. Bubbles will introduce extra turbulence into the fluid through wake effects (so-called pseudo turbulence). The single phase k-epsilon model does not incorpo...

Proper modelling of turbulence in bubble plumes is critical for accurate prediction of the mean flow velocity field and also the dynamics of bubbles. One of the deficiencies of the standard k-epsilon model lies in that it does not account for turbulence damping in the vicinity of a free surface, which may lead to an underestimation of the outward m...

In the design process of a wind farm the aerodynamic interactions between the single turbines have become a field of major interest. The upwind turbines in a wind farm will affect the energy potential and inflow conditions for the downwind turbines. The flow field in the wake behind the first row turbines is characterized by a significant deficit i...

The present work aims to investigate possible improvements in cross country ski suits from an aerodynamic perspective with the main goal of evaluating if air permeability plays a significant role in the aerodynamic performances of the textiles. Three different fabrics with different surface pattern and different permeability were tested in a wind t...

This investigation was designed to (a) develop an individualized mechanical model for measuring aerodynamic drag (F(d) ) while ski racing through multiple gates, (b) estimate energy dissipation (E(d) ) caused by F(d) and compare this to the total energy loss (E(t) ), and (c) investigate the relative contribution of E(d) /E(t) to performance during...

The aerodynamic drag force acting on a circular cylinder clad with knitted wool and polyester textiles has been investigated in wind tunnel experiments in this study. Particle image velocimetry (PIV) was utilized to determine the flow field, both around the separation point and a wake profile in the close wake. The drag forces and the characteristi...

In sports where high speed is reached and drag is the main force acting on the athlete's body, the posture highly influences the total drag. In speed skating, due to the fact that the total drag is proportional to the frontal area, the athletes minimize their frontal area keeping the trunk parallel to the air flow. In this study it is shown that th...

In a ski cross competition, a group of four skiers take simultaneously part of the race and attempt to reach the end of the course. The first two to cross the finish line will advance to the next round. It is then not a timed racing event like the other ski competitions. Since it has been proved that low drag apparels have a major role in sports wh...

In this paper, the double layer concept of knitted fabrics suitable for performance sportswear, where each layer is separate from the other, was studied. Aerodynamic properties of this assembly, where the base layer is made of 100% wool and an external layer made of 100% filament polyester were determined. Fabric samples were placed over a single d...

The scope of this article is to find out what are the differences between four modern soccer balls and find out how these differences influences the flight path of the ball in a free kick. Experiments have been carried out in static and spinning conditions and drag and side forces have been acquired. The results from the wind tunnel experiments hav...

A method for calibrating the hot-film anemometer is presented. The turbulent boundary layer in a recirculating water tunnel was investigated. Both mean velocity and turbulence intensivity were measured. The results are compared with the theory and the agreement is satisfactory.

The flow behind two circular grids of equal diameter but different mesh geometry was investigated via PIV measurements, CFD simulations and wind tunnel flow visualizations, at 8 • 10 . It was found out that the grid made of biplane mesh induced a non-axisymmetric flow characterized by double-peaked non-dimensional velocity profiles, and that the wa...

Performances analysis in sports is complex since many factors linked together contribute to the final result thus a quantification of the effects of different suits on speed skating performances just with field measurements is a difficult task. In fact, field measurements and competition results do not clearly show the effect of using different sui...

Running economy is important, but can it be altered? It seems that persons beginning exercise definitely become more efficient with training, as do persons who are already trained but who continue heavy training. (1981) followed a single runner during 6 months of interval training and found that the subject’s running efficiency improved by between...

A magnetic resonance velocimetry (MRV) experimental technique based on magnetic resonance imaging and capable of measuring the turbulent Reynolds stresses in a 3D flow domain is described. Results are presented in backward facing step flow in a square channel with a Reynolds number of 48,000 based on step height and freestream velocity at the step....

Based on the results of several different experiments, it has been concluded that the weight of a ski jumper is crucial in performing a long ski jump. In response to this conclusion, many of the best ski jumpers in the world began dieting to reduce their weight, resulting in many underweight athletes and some incidents of anorexia. In order to deal...

Introduction: Drag in cycling counts for as much as 90% of total resistance opposing motion in a normal time-trial course. A small gain in term of drag reduction can, over a longer time-span (30–60 minutes) give a large advantage to the cyclists in terms of power output saved or velocity gained. The aim of present study was to aerodynamically optim...

This paper presents the characteristics of hydrodynamically focused streams to be used in Microscale Particle Image Velocimetry (micro-PIV). The investigation was done by means of Confocal Laser Scanning Microscopy (CLSM) in channels of cross-sections 260 × 200 μ m and 1040 × 800 μ m, within the downstream velocities range from 0.1 to 2.5 cm/s. The...

This paper presents an experimental investigation into the effect of surface roughness on athletes legs and arms. Because of their cylindrical shape, arms and legs of an athlete can be approximately studied as flow over circular bodies. The variation of roughness has been obtained using three different textiles and changing the diameter of the cyli...

In this study, we examined the effects of body position (from deep to high position) in slide boarding by well-trained cross-country skiers. The main hypothesis was that a deeper, more "crouched" position would lead to reduced air resistance and enhanced power production during explosive extension of the lower limbs, and thereby to an increased per...

A novel seeding method for microscale particle image velocimetry (micro-PIV) is presented. The method relies on selective seeding of a thin fluid layer within an otherwise particle-free flow. In analogy to the laser sheet in macroscale PIV, the generated particle sheet defines both the depth and the position of the measurement plane, independent of...

A novel seeding method, permitting high out-of-plane resolution and instantaneous (time-resolved) velocity field measurements using a standard Microscale Particle Image Velocimetry (micro-PIV) setup, is presented. The method relies on selective seeding of a thin fluid layer within an otherwise particle-free flow. The generated particle sheet define...

We present an analysis of the flowfield inside a novel crossflow microfiltration device. The filter performance relies on shear focusing by means of a corrugated channel. The flow and shear stress characteristics inside the filter are studied by means of both micro Particle Image Velocimetry (micro-PIV) measurements and Computational Fluid Dynamics...

We present an overview of Micro Particle Image Velocimetry (µPIV); an optical, non- intrusive measurement technique permitting detailed flow-field measurements in microflu- idic devices. Both theoretical and practical issues related to µPIV are discussed, such as optical set-up, microscope objective properties, optical filters, seeding particles, p...

A micro-PIV system is presented in detail, pointing out important aspects of micro-PIV system design cruicial for its operation. The micro-PIV system is then applied on a sinusoidal microchannel, and the fluid motion inside the device is presented and discussed. The wall shear stress at the waist of the channel is measured to be up to 60% higher th...