# Håkan NilssonChalmers University of Technology · Department of Applied Mechanics

Håkan Nilsson

Professor

## About

108

Publications

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1,068

Citations

Citations since 2017

Introduction

Additional affiliations

September 2015 - present

## Publications

Publications (108)

A larger part of the electricity is today from intermittent renewable sources of energy. However, the energy production from such sources varies in time. Energy storage is one solution to compensate for this variation. Today pumped hydro storage (PHS) is the most common form of energy storage. Usually, it requires a large head, which limits where i...

To tackle the growing demand for grid-scale energy storage, the ALPHEUS project proposes a novel low-head pumped hydro storage system aimed for coastal application in countries where the topography does not allow for traditional high-head storage. This system consists of a reversible pump-turbine technology with two contra-rotating runners coupled...

The present work concerns the electromagnetic force models in computational fluid dynamics simulations of melt pools produced with electric arcs. These are commonly applied to gas metal arcs with metal transfer, in welding and additive manufacturing. Metal drop impact on the melt pool is thus included in this study. The electromagnetic force models...

The use of Computational Fluid Dynamics (CFD) for the design of modern hydraulic turbines has increased and matured significantly the last decades. More recently, CFD is also used to understand how to safely widen the hydraulic turbine operating ranges, and to avoid hazardous conditions during transient operation. The accuracy of such CFD results r...

To counteract a potential reduction in grid stability caused by a rapidly growing share of intermittent renewable energy sources within our electrical grids, large scale deployment of energy storage will become indispensable. Pumped hydro storage is widely regarded as the most cost-effective option for this. However, its application is traditionall...

Hydraulic turbines are increasingly responsible for regulating the electric grid, due to the rapid growth of the intermittent renewable energy resources. This involves a large increase in the number of starts and stops, which cause severe flow-induced pulsations and fluctuating forces that deteriorate the machines. Better knowledge of the evolution...

Despite the increase in computational power of HPC clusters, it is in most cases not possible to include the entire hydraulic system when doing detailed numerical studies of the flow in one of the components in the system. The numerical models are still most often constrained to a small part of the system and the boundary conditions may in many cas...

The flexibility and fast responsiveness of hydropower systems make them a reliable solution to overcome the intermittency of renewable energy resources and balance the electrical grid. Therefore, investigating the complex flow fields during such operation is essential to increase the reliability and lifetime of future hydropower systems. The curren...

The flexibility and fast responsiveness of hydropower systems make them a reliable solution to overcome the intermittency of renewable energy resources and balance the electrical grid. Therefore, investigating the complex flow fields during such operation is essential to increase the reliability and lifetime of future hydropower systems. The curren...

The Deep Green technique for tidal power generation is suitable for moderate flows which is attractive since larger areas for tidal energy generation hereby can be used. It operates typically at mid-depth and can be seen as a “flying” kite with a turbine and generator attached underneath. It moves in a lying figure-eight path almost perpendicular t...

Power production from intermittent renewable energy resources, such as solar and wind, has increased in the past few decades, leading researchers and engineers to establish techniques to preserve a stable electrical grid. Consequently, hydraulic turbines are being used more frequently in transient operating modes to regulate the grid. The present w...

Pumped Hydropower Storage (PHS) is the maturest and most economically viable technology for storing energy and regulating the electrical grid on a large scale. Due to the growing amount of intermittent renewable energy sources, the necessity of maintaining grid stability increases. Most PHS facilities today require a geographical topology with larg...

Renewable sources of energy are on the rise and will continue to increase the coming decades [1]. A common problem with the renewable energy sources is that they rely on effects which cannot be controlled, for instance the strength of the wind or the intensity of the sunlight. The ALPHEUS Horizon 2020 EU project has the aim to develop a low-head hy...

A novel numerical framework in OpenFOAM is proposed in this work, to simulate transient operation of Kaplan hydraulic turbines. Such transient operations involve a variation of both runner blade and guide vane angles, which also gives rise to a flow rate variation. A numerical simulation of such a process is very challenging, since it requires a de...

Power generation from intermittent renewable energy resources (e.g. wind, solar) requires regulation of the electric grid. Although most hydraulic turbines are designed to work in their best efficiency points, nowadays they are being used more often under varying operating conditions to stabilize the electric grid. Unstable and varying conditions o...

The pan-European power grid is experiencing an increasing penetration of Variable Renewable Energy (VRE). The fluctuating and non-dispatchable nature of VRE hinders them in providing the Ancillary Service (AS) needed for the reliability and stability of the grid. Today’s grid is reliant on synchronous generators. In case of sudden frequency deviati...

The naphthalene sublimation technique is an experimental method for indirectly determining convective heat transfer. The technique is here assessed for two different configurations: the local heat transfer distribution for a circular air jet impinging normal to a flat surface, and the heat transfer occurring in the stator core of an electric genera...

Due to the new intermittent electric energy sources, hydropower is forced to run more and more at off-design conditions and to regulate the operating conditions. This causes flow instabilities with pressure fluctuations, and load variations that may deteriorate the machine. Previous numerical research on the flow in water turbines has mainly focuse...

The dorsal root ganglion (DRG) that is embedded in the foramen of the cervical vertebra can be injured during a whiplash motion. A potential cause is that whilst the neck bends in the whiplash motion, the changes of spinal canal volume induce impulsive pressure transients in the venous blood outside the dura mater (DM) and in the cerebrospinal flui...

Tidal energy has the potential to provide a substantial part of the sustainable electric power generation. The tidal power plant developed by Minesto, called Deep Green, is a novel technology using a 'flying' kite with an attached turbine, moving at a speed several times higher than the mean flow. Multiple Deep Green power plants will eventually fo...

The boundary coupling high-intensity electric arc and refractory cathode is characterized by three sub-layers: the cathode sheath, the Knudsen layer and the pre-sheath. A self-consistent coupling boundary condition accounting for these three sub-layers is presented; its novel property is to take into account a non-uniform distribution of electron e...

A parametric investigation of radiative heat transfer is carried out, including the effects of conjugate heat transfer between fluid and solid media. The thermal radiation is simulated using the -model. The numerical model and the thermal coupling strategy, suitable for a transient solver, is described. Such numerical coupling requires that the rad...

Various models coupling the refractory cathode, the cathode sheath and the arc at atmospheric pressure exist. They assume a homogeneous cathode with a uniform physical state, and differ by the cathode layer and the plasma arc model. However even the most advanced of these models still fail in predicting the extent of the arc attachment when applied...

Purpose
Embolic stroke is a dreaded complication of thoracic endovascular aortic repair. The prevailing theory about its cause is that particulate debris from atherosclerotic lesions in the aortic wall are dislodged by endovascular instruments and embolize to the brain. An alternative source of embolism might be air trapped in the endograft deliver...

Whiplash Associated Disorder (WAD) is still one of the most poorly understood traffic injuries. Cervical dorsal root ganglion lesion is a potential cause of WAD. This short communication covers factors that potentially contribute to the difference in WAD risk between female and male car occupants in rear impacts. A prototype female rear impact dumm...

The U9 Kaplan turbine is simulated at two operating conditions using conventional and
non-conventional URANS models and a hybrid URANS-LES method. All clearances are
included in the computational domain, such as the guide vane clearances, the runner blade
tip-clearances, and the runner blade hub-clearances at both the leading and trailing edges. A...

This work describes a coupling between the 1D method of characteristics (MOC) and the 3D finite volume method of computational fluid dynamics (CFD). The coupling method is applied to compressible flow in hydraulic systems. The MOC code is implemented as a set of boundary conditions in the OpenFOAM open source CFD software. The coupling is realized...

he vortex rope and pressure pulsations caused by radial pressure gradient in a conical diffuser of a swirl generator is controlled using continuous slot jets with different momentum fluxes and angles injected from the runner crown. The swirl apparatus is designed to generator the flows similar to those different operating conditions of a Francis tu...

Air-water gas-exchange is studied in direct numerical simulations (DNS) of free-surface flows driven by natural convection and weak winds. The wind is modeled as a constant surface-shear-stress and the gas-transfer is modeled via a passive scalar. The simulations are characterized via a Richardson number where B, v, and u* are the buoyancy flux, ki...

The results and the main contents presented in this document, have been reported in the journal paper: Carlo Cintolesi, Hakan Nilsson, Andrea Petronio, Vincenzo Armenio. ”Numerical a simulation of conjugate heat transfer and surface radiative heat transfer using the P1 thermal radiation model: parametric study in benchmark cases”. Submitted to Int....

Thermal radiation rules, or strongly influences, many different environmental and industrial systems. A reliable numerical simulation cannot neglect radiative heat transfer within the fluid medium, along with the thermal interaction between fluid and the surrounding solids, due to surface heat transfer by conduction and radiation. Indeed, the tempe...

Long-term durability after endovascular aortic repair is influenced by stent graft migration causing types I and III endoleaks. Flow induced displacement forces have been shown to have the potential to cause migration. In this study, the influence of the distal diameter of iliac limb stent grafts and the shape of graft curvature on flow induced dis...

Arc attachment radius imposed a priori when modelling the coupling between cathode, cathode layer and thermal plasma still hinders models from being predictive, as underlined in a recent review1. The aim of this work was to find a physical element, still lacking in the models, which could contribute in governing the arc attachment. In this study th...

The gas flux at the water surface is affected by physical processes including turbulence from wind shear, microscale wave breaking, large-scale breaking, and convection due to heat loss at the surface. The main route in the parameterizations of the gas flux has been to use the wind speed as a proxy for the gas flux velocity, indirectly taking into...

We investigate the motion of inertial, rod-like fibres in the decelerating flow of a wedge-shaped channel with non-creeping fibre–flow interactions. We consider the trajectories of isolated fibres to identify the conditions for which these trajectories deflect from the streamlines of the flow as well as a rectilinear path. We carry out analytical a...

The flow unsteadiness generated in a swirl apparatus is investigated experimentally and numerically. The swirl apparatus has two parts: a swirl generator and a test section. The swirl generator which includes two blade rows, one stationary and one rotating, is designed such that the emanating flow at free runner rotational speed resembles that of a...

Direct numerical simulations (DNS) of free surface flows driven by natural convection are used to evaluate different methods of estimating air-water gas exchange at no-wind conditions. These methods estimate the transfer velocity as a function of either the horizontal flow divergence at the surface, the turbulent kinetic energy dissipation beneath...

In vehicle collisions, the occupant's torso is accelerated in a given direction while the unsupported head tends to lag behind. This mechanism results in whiplash motion to the neck. In whiplash experiments conducted for animals, pressure transients have been recorded in the spinal canal. It was hypothesized that the transients caused dorsal root g...

We investigate flocculation in dilute suspensions of rigid, straight fibers in a decelerating flow field of a diffuser. We carry out numerical studies using a particle-level simulation technique that takes into account the fiber inertia and the non-creeping fiber-flow interactions. The fluid flow is governed by the Reynolds-averaged Navier-Stokes e...

The vortex rope in a flow similar to 70% of Francis turbine part-load is controlled using the active jet flows injected from the runner crown. The study is undertaken with numerical modeling using hybrid RANS-LES method. The comprehensive study of Javadi and Nilsson (J. Flow, Turbulence and combustion, 2015) is considered as the base case and the e...

A rotation/curvature correction for the shear stress transport model (SST-CC) is implemented and the effects of curvature on turbulence quantities and the performance and added functionality of the model are explored. Unsteady simulations are run using complex geometries that are similar in streamline curvature at high Reynolds number as in hydropo...

The role of hydropower to provide regulated power is important to the Swedish power system. This becomes even more accentuated with the expansion of intermittent renewable electricity sources, such as wind power. The variation of hydropower operation ranges over a large spectrum of time scales, from seconds to years. For scales larger than a minute...

A model for simulating vertical water level fluctuations with coupled liquid and gas phases is presented. The Preissmann implicit scheme is used to linearize the governing equations for one-dimensional transient flow for both liquid and gas phases, and the linear system is solved using the chasing method. Some classical cases for single liquid and...

A series of numerical simulations is undertaken to study a highly swirling turbulent flow generated by rotor-stator interaction in a swirl generator. The purpose is to assess the applicability of different turbulence models in swirling flow with a high level of unsteadiness and a significant production and dissipation of turbulence in the flow away...

The efficiency of the ventilation system is a key point for durable and reliable electric generators. The design of such system requires a detailed understanding of the air flow in the generator. Computational fluid dynamics (CFD) has the potential to resolve the lack of information in this field. The present work analyses the air flow inside a gen...

The present work compares the results from a Large Eddy Simulation (LES) and the result of two Laser Doppler Velocimetry (LDV) measurements. The work focuses on a model of a vortex flow meter, a common flow meter type in process industry. Its measuring principle is that the frequency of a vortex street, generated by a shedder bar, is acquired by pr...

A detailed numerical study is undertaken to investigate the physical processes that are engendered in the strongly swirling turbulent flows through a suddenly expanding circular pipe. The delayed DES Spalart-Allmaras (DDES-SA), improved DDES-SA (IDDES-SA), a dynamic k-equation LES (oneEqLES), dynamic Smagorinsky LES (dynSmagLES) and implicit LES wi...

Temporally and spatially resolved simulations of turbulent flow need realistic inlet velocity fluctuations. The vortex method, described and implemented in this work, adds vortices to the inlet mean flow profile. The vortices are randomly distributed over the inlet and they move across the extent of the inlet with a finite life-time. The initial vo...

This work investigates the flow in the scale model of the high-head Tokke Francis turbine at part load, best efficiency point and high load, as a contribution to the first Francis-99 workshop. The work is based on the FOAM-extend CFD software, which is a recent fork of the OpenFOAM CFD software that contains new features for simulations in rotating...

A fully predictive computational fluid dynamics approach is assessed for the flow of cooling air in an axially cooled electric generator. The flow is driven solely by the rotation of the rotor, as in the real application. A part of the space outside the generator is included in the computational domain to allow for the flow of air into and out of t...

The flow unsteadiness generated in the draft tube cone of hydraulic turbines affects the turbine operation. Therefore, several swirling flow configurations are investigated using a swirling apparatus in order to explore the unsteady phenomena. The swirl apparatus has two parts: the swirl generator and the test section. The swirl generator includes...

A mixing plane interface provides a circumferentially averaging rotor-stator coupling interface, which is extremely useful in practical turbomachinery simulations. It allows fundamentally transient problems to be studied in steady-state, using simplified mesh components having periodic properties, and with the help of a multiple reference frames (M...

Improvements in ventilation and cooling offer means to run hydro power generators at higher power output and at varying operating conditions. The electromagnetic, frictional and windage losses generate heat. The heat is removed by an air flow that is driven by fans and/or the rotor itself. The air flow goes through ventilation channels in the stato...

Particle-level simulations are used to study the rheology of monodispersed suspensions of rigid and flexible fibers in a creeping shear flow of a Newtonian fluid. We also investigate the influence of different equilibrium shapes (straight and curved) of the fibers on the behavior of the suspension. A parametric study of the impacts of fiber flexura...

The strongly swirling turbulent flow through an abrupt expansion is investigated using highly resolved LES and SAS, to shed more light on the stagnation region and the helical vortex breakdown. The vortex breakdown in an abrupt expansion resembles the so-called vortex rope occurring in hydro power draft tubes. It is known that the large-scale helic...

A particle-level rigid fiber model is used to study flocculation in an asymmetric planar diffuser with a turbulent Newtonian fluid flow, resembling one stage in dry-forming process of pulp mats. The fibers are modeled as chains of rigid cylindrical segments. The equations of motion incorporate hydrodynamic forces and torques from the interaction wi...

A highly swirling turbulent flow engendered by the rotor-stator interaction of a swirl generator is investigated using LES and DES. The delayed DES Spalart-Allmaras (DDES-SA), improved DDES-SA, shear stress transport DDES (DDES-SST) and a dynamic k-equation LES are studied. A mesh sensitivity study is performed on the hybrid methods, including the...

The need for electric power is continuously increasing. The power output of existing electric generators is forced to its limit, and the new intermittent electric energy sources increase the variations of the operating conditions of the electric generators in the power system. This requires better cooling of the heat that is generated by the electr...

Stent graft migration influences the long-term durability of endovascular aortic repair. Flow-induced displacement forces acting on the attachment zones may contribute to migration. Proximal fixation of aortic stent grafts has been improved by using hooks, while distal fixation and stent graft interconnections depend on self-expansion forces only....

The dynamics of individual flexible fibers in a turbulent flow field have been analyzed, varying their initial position, density and length. A particle-level fiber model has been integrated into a general-purpose, open source computational fluid dynamics code. The fibers are modeled as chains of cylindrical segments connected by ball and socket joi...

A flexible fiber model has been implemented in a general purpose open-source Computational Fluid Dynamics code. The fibers are modeled as chains of cylindrical segments, and all the degrees of freedom necessary to realistically reproduce the dynamics of real fibers, are taken into account. Each segment is tracked individually and their equations of...

A particle-level rigid fiber model has been integrated into a general-purpose, open source computational fluid dynamics code to carry out detailed studies of fiber--flow interactions in realistic flow fields. The fibers are modeled as chains of cylindrical segments, and their translational and rotational degrees of freedom are considered. The equat...

The dynamics of individual flexible fibers in a turbulent flow field have been studied. A particle-level fiber model has been integrated into a general purpose, open source Computational Fluid Dynamics (CFD) code. The fibers are modeled as chains of cylindrical segments connected by ball and socket joints. The fiber equations of motion contain the...

Computational fluid dynamics (CFD) analyses were made to study the unsteady three-dimensional turbulence in the ERCOFTAC centrifugal pump test case. The simulations were carried out using the OpenFOAM Open Source CFD software. The test case consists of an unshrouded centrifugal impeller with seven blades and a radial vaned diffuser with 12 vanes. A...

Most simulation studies done to deeper understand high-intensity welding arcs address axi-symmetric configurations and use the electric potential formulation. This formulation involves the assumption of a one-dimensional magnetic field. The assumption is justified in its original frame: rather long arcs (about 10 mm), and when the electrode tip is...

The paper presents a novel mathematical model for a-priori computation of the swirling flow at Kaplan runners outlet. The model is an extension of the initial version developed by Susan-Resiga et al [1], to include the contributions of non-negligible radial velocity and of the variable rothalpy. Simple analytical expressions are derived for these a...

This work presents an OpenFOAM case-study, based on the experimental studies of the swirling flow in the abrupt expansion by Dellenback et al.[1]. The case yields similar flow conditions as those of a helical vortex rope in a hydro turbine draft tube working at part-load. The case-study is set up similar to the ERCOFTAC Conical Diffuser and Centrif...

The flow in the Turbine-99 Kaplan draft tube was thoroughly investigated
at three workshops (1999, 2001, 2005), which aimed at determining the
state of the art of draft tube simulations. The flow is challenging due
to the different flow phenomena appearing simultaneously such as
unsteadiness, separation, swirl, turbulence, and a strong adverse
pres...

Electric arcs used in welding manufacturing often involve 3-dimensional configurations. However, a large fraction of the numerical simulations done to deeper understand high intensity arc in the frame of welding is based on axi-symmetric models.
The electric potential formulation introduced by Hsu and Pfender is the most commonly used electromagn...