Mark C Runacres

Vrije Universiteit Brussel | VUB · Thermo and Fluid Dynamics (FLOW)

A novel optimization methodology is presented for optimizing high-altitude propellers operating in the lower stratosphere. The proposed methodology combines a Bayesian optimization approach for most design variables and Vortex Theory for the optimization of twist distribution and rotational velocity. For the Bayesian optimization part of the proble...

This research explores cycle-to-cycle variability in dynamic stall through wind tunnel experiments conducted on a pitching NACA 0018 aerofoil at a Reynolds number of 2.8×10 ⁵ . Multiple cycles are considered, and different clusters are identified based on inspection of the lift time series. Experiments reveal that the blind application of a convent...

Recent studies have shown that the placement of paired vertical-axis wind turbines (VAWTs) in close proximity is a promising approach to design efficient VAWT farm layouts, mainly due to the increased power output. This study aims to experimentally investigate the impact of the rotational direction of paired counter-rotating VAWTs on their wake cha...

The aerodynamic interaction of vertical-axis wind turbines (VAWTs) in paired configurations can be advantageous in wind farms. However, these turbines operate in the atmospheric boundary layer which is characterised by high turbulence and may also face high-turbulence levels associated with downstream wakes. Therefore, the main goal of this study i...

The development of simple, low-order and accurate unsteady aerodynamic models represents a crucial challenge for the design optimisation and control of fluid dynamical systems. In this work, wind tunnel experiments of a pitching NACA 0018 aerofoil conducted at a Reynolds number $Re = 2.8 \times 10^5$ and at different free-stream turbulence intensit...

A known challenge when building nonlinear models from data is to limit the size of the model in terms of the number of parameters. Especially for complex nonlinear systems, which require a substantial number of state variables, the classical formulation of the nonlinear part (e.g. through a basis expansion) tends to lead to a rapid increase in the...

High-altitude propeller optimization aims to discover blade designs that yield superior performance in the low air density environment present at the lower levels of the stratosphere. In our work, a multi-fidelity, multi-objective (MFMO) optimization framework is developed and tested for the application of high-altitude propeller optimization. The...

Performance evaluations for propellers operating at high altitudes are subject to increased uncertainty due to scarce experimental or flight data and difficulties in modeling low Reynolds number flows. For this reason, the Polynomial Chaos Expansion (PCE) method is used in this paper to assess the performance uncertainty of propellers operating at...

Performance evaluations for propellers operating at high altitudes are subject to increased uncertainty due to scarce experimental or flight data and difficulties in modeling low Reynolds number flows. For this reason, the Polynomial Chaos Expansion (PCE) method is used in this paper to assess the performance uncertainty of propellers operating at...

This work discusses the experimental challenges and processing of unsteady experiments for a pitching wing in the low-speed wind tunnel of the Vrije Universiteit Brussel. The setup used for unsteady experiments consisted of two independent devices: (a) a position control device to steer the pitch angle of the wing, and (b) a pressure measurement de...

Multivariate functions emerge naturally in a wide variety of data-driven models. Popular choices are expressions in the form of basis expansions or neural networks. While highly effective, the resulting functions tend to be hard to interpret, in part because of the large number of required parameters. Decoupling techniques aim at providing an alter...

While modern wind turbines have become by far the largest rotating machines on Earth with further upscaling planned for the future, a renewed interest in small wind turbines (SWTs) is fostering energy transition and smart grid development. Small machines have traditionally not received the same level of aerodynamic refinement as their larger counte...

Accurate unsteady aerodynamic models are essential to estimate the forces on rapidly pitching wings and to develop model-based controllers. As system identification is arguably the most successful framework for model predictive control in general, in this paper we investigate whether system identification can be used to build data-driven models of...

This paper aims to improve the understanding of the wind turbine tower shadow effect by developing a model that combines potential flow and blade-element momentum theory. The change in the flow field due to the presence of the tower is discussed in detail, together with the resulting variations in the aerodynamic forces acting on the rotating blade...

In this paper, the performance of propellers is evaluated with 3D RANS and Vortex Theory for a wide range of geometries and advance ratios at high-altitude conditions. For this purpose, the results of a high-altitude propeller optimization are used as a CFD database and discussed briefly. A comparison is made between the predictions of 3D RANS and...

While modern wind turbines have become by far the largest rotating machines on Earth with further upscaling planned for the future, a renewed interest in small wind turbines is fostering energy transition and smart grid development. Small machines have traditionally not received the same level of aerodynamic refinement of their larger counterparts,...

The focus of this study is to investigate under high turbulence intensity the performance and wake characteristics of a pair of closely spaced counter-rotating vertical axis wind turbines (VAWTs) in wind tunnel-controlled conditions. To achieve this, wind tunnel experiments for an isolated VAWT and dual VAWTs are conducted. A squared-mesh wooden gr...

Multivariate functions emerge naturally in a wide variety of data-driven models. Popular choices are expressions in the form of basis expansions or neural networks. While highly effective, the resulting functions tend to be hard to interpret, in part because of the large number of required parameters. Decoupling techniques aim at providing an alter...

In this paper, we present an analytical solution to the nonlinear optimization problem encountered in the context of the filtered canonical polyadic decomposition. It is shown that the originally proposed alternating least squares (ALS) approach can be significantly accelerated through a thorough analysis of one of the matrix factors, that is, the...

In this paper, we present an analytical solution to the nonlinear optimization problem encountered in the context of the filtered canonical polyadic decomposition. It is shown that the originally proposed alternating least squares (ALS) approach can be significantly accelerated through a thorough analysis of one of the matrix factors, that is, the...

The increasing sophistication of wind turbine design and control generates a need for high‐quality wind data. The relatively limited set of available measured wind data may be extended with computer generated data, for example, to make reliable statistical studies of energy production and mechanical loads. Here, a data‐driven model for the generati...

View Video Presentation: https://doi.org/10.2514/6.2021-2597.vid A surrogate-based, multipoint and multiobjective optimization was performed in this work for a High-Altitude Propeller. For each blade design, the performance was evaluated with 3D RANS at three advance ratios for cruise and one advance ratio for climb, while the ability to take off,...

During the last three decades, rapid growth of wind energy has led to questions regarding the possible impacts of wind farms on local weather and microclimates. Physically, the increased turbulence due to the wind turbine operation affects the mixing processes, may slightly disturb the pressure and temperature distributions downstream of wind farms...

Nonlinear state-space modelling is a very powerful black-box modelling approach. However powerful, the resulting models tend to be complex, described by a large number of parameters. In many cases interpretability is preferred over complexity, making too complex models unfit or undesired. In this work, the complexity of such models is reduced by re...

In this work recurrent neural networks of the LSTM type are used to describe unsteady fluid loading. Indirect load measurements are especially useful for structural health monitoring applications. Relying on discrete spatiotemporal measurements of the velocity field, both the forces and the corresponding displacement of a cylinder subjected to vort...

Dynamic fluid loading is of major concern to the off shore industry. Impacted structures may exhibit accelerated fatigue and even failure. Load monitoring has therefore become an integral part of structural health monitoring. Obtaining direct load measurements can be cumbersome, e.g. obtaining strain gauge or pressure taps measurements. In this wor...

Two vertical-axis wind turbines (VAWTs) benefit from a power increase when placed side by side in close proximity. To study the potential of paired VAWTs for integration in wind farms, wind tunnel wake measurements of lift-driven VAWTs are compared for isolated and three counter-rotating configurations. Because the wake of an isolated VAWT is defle...

Closely spaced counter-rotating vertical-axis wind turbines (VAWTs) exhibit a considerable power improvement compared to the same turbines in isolation. It thus makes sense to study their potential for wind farm production optimisation. With this objective in mind, the wake of an isolated VAWT is compared experimentally to the wake of counter-rotat...

In aerodynamics, as in many engineering applications, a parametrised mathematical model is used for design and control. Often, such models are directly estimated from experimental data. However, in some cases, it is better to first identify a so-called nonparametric model, before moving to a parametric model. Especially when nonlinear effects are p...

Synthetic wind-speed generators can provide a detailed characterisation of the wind variability at different time scales. A keen interest in the availability of synthetic wind speeds has recently risen in wind power modelling applications. In particular, a proper simulation of the diurnal and annual variability of the wind speed is sought that can...

A persisting challenge in nonlinear dynamical modelling is parameter inference from data. Provided that an appropriate model structure was selected, the identification problem is profoundly affected by a choice of initialisation. A particular challenge that may arise is initialisation within a region of the parameter space where the model is not co...

As the structural limits for the conventional horizontal‐axis wind turbine are being reached, innovative concepts are needed to ensure increased reliability and reduced costs. One option is limiting the fatigue to which a rotor is subjected. This study, therefore, intends to experimentally investigate the tower shadow effect for a deeper understand...

A nonlinear model relating the imposed motion of a circular cylinder, submerged in a fluid flow, to the transverse force coefficient is presented. The nonlinear fluid system, featuring vortex shedding patterns, limit cycle oscillations and synchronisation, is studied both for swept sine and multisine excitation. A nonparametric nonlinear distortion...

A nonlinear model relating the imposed motion of a circular cylinder, submerged in a fluid flow, to the transverse force coefficient is presented. The nonlinear fluid system, featuring vortex shedding patterns, limit cycle oscillations and synchronisation, is studied both for swept sine and multisine excitation. A nonparametric nonlinear distortion...

Small Darrieus vertical-axis wind turbines (VAWTs) have recently been proposed as a possible solution for adoption in the built environment as their performance degrades less in complex and highly-turbulent flows. Some recent analyses have even shown an increase of the power coefficient for the large turbulence intensities and length scales typical...

In cities, the tallest rooftops have the best potential for electricity production. They often combine good wind and solar conditions with available space. Rooftop-mounted photovoltaic (PV) panels have already proven their efficiency and profitability. For tall rooftops with good wind conditions, also small wind turbines can be economically viable....

This work proposes a deterministic methodology to generate synthetic wind-speed time series consistent with both a probability density function and a spectral density function. The method relies on the random-phase multisine signal to generate an initial sequence conforming to the target spectral density. A following iterative rank-reordering proce...

Nonlinear state-space modelling is a very powerful black-box modelling approach. However powerful, the resulting models tend to be complex, described by a large number of parameters. In many cases interpretability is preferred over complexity, making too complex models unfit or undesired. In this work, the complexity of such models is reduced by re...

A persisting challenge in nonlinear dynamical modelling is parameter inference from data. Provided that an appropriate model structure was selected, the identification problem is profoundly affected by a choice of initialisation. A particular challenge that may arise is initialisation within a region of the parameter space where the model is not co...

As rooftops in cities are mostly underused, they have a large potential for decentralised electricity production. In that context, photovoltaic (PV) panels have proven to be an effective solution. Meanwhile, the market of small wind turbines is increasing, and some building owners have already installed one or more units on their roof. While the ec...

The potential of vertical-axis wind turbines (VAWTs) for specific applications (floating off-shore or urban in- stallations) has been reconsidered in recent years. This renewed interest has led to an increasing number of experimental studies on VAWTs. A challenge of experimental work is correctly interpreting experimental results but just as import...

Turbulence is one of the main characteristics of urban flows, but its effect on the performance of small VAWTs (often used in urban wind installations) has not been thoroughly researched. This experimental study focuses in testing a H-Darrieus VAWT prototype in different turbulent conditions inside the wind tunnel, in order to study the influence o...

Wind tunnel tests have been performed of individual and paired H-type Darrieus vertical-axis wind turbines. The turbines in the paired configuration are closely spaced, at 1.2 and 1.3 rotor diameters shaft to shaft, and are counter-rotating. Two directions of rotation were studied, one where the facing (inner) blades move along with the incoming fl...

Multivariate polynomials are omnipresent in black-box modelling. They are praised for their flexibility and ease of manipulation yet typically fall short in terms of insight and interpretability. Hence often an alternative representation is desired. Translating the coupled polynomials into a decoupled form, containing only univariate polynomials ha...

In Belgium, the installation of a small wind turbine on rooftops is still very uncommon. Currently, there is no dedicated legal framework governing the installation on a building. Nevertheless, over the past years, our research group has received several demands from building owners wanting to install one or several turbines on their rooftop. The l...

Vertical-Axis Wind Turbines are receiving the attention of the wind energy community for urban wind harvesting. However, their practical application is still far from maturity, due to the lack of understanding of urban flows. High turbulence is one of the main characteristics of wind in complex environments, so special attention has been paid to mo...

In this work we study the effect of using different types of excitation signals as training data when constructing black box nonlinear models. We focus in particular on the class of nonlinear systems which exhibit autonomous oscillations. Three type of excitation signals are considered: random-phase multisines, filtered white noise and swept sines....

In this article, we develop a nonlinear data-driven model of the aerodynamic force on an aerofoil made to oscillate in pitch in a wind tunnel under constant flow conditions. The proposed model structure is a polynomial nonlinear state-space model (PNLSS) which is an extension of the classical linear state-space model with nonlinear functions. In th...

Vertical-axis wind turbines (VAWTs) have received a renewed interest in the wind energy research community, mainly for off-shore applications. One advantage is that installing a pair of counterrotating VAWTs on the same floating platform would result in thrust reduction and potential cancellation of the mooring yaw moment. In addition, such configu...

The flow-induced vibration of bluff bodies is an important problem of many marine, civil, or mechanical engineers. In the design phase of such structures, it is vital to obtain good predictions of the fluid forces acting on the structure. Current methods rely on computational fluid dynamic simulations (CFD), with a too high computational cost to be...

Polynomial nonlinear state-space (PNLSS) models have proven to be very useful in modelling highly nonlinear systems, encountered over a variety of engineering applications. In this work, we focus on modelling the kinematics of an oscillating circular cylinder, submerged in a low Reynolds number flow. Such a set up is typically used to study vortex...

Due to the complexity and uncertainty involved in the process of making power from wind, more and more advanced tools are being developed to maintain the sustainability and the growing trend of the wind industry. Prior to the development of a wind farm project, measured data are provided by limited installed wind masts at the site under investigati...

Following the growth of wind sector and the scarcity of available land, the market share of onshore wind energy installation on complex terrain is expected to increase. Wind resource assessment is a crucial process for the successful development of a wind farm project. To estimate the future energy production on a specific site, developers investig...

The development of large scale wind energy projects created the demand for increasingly accurate and efficient models that limit the project’s uncertainties and risk. Wake effects are of great importance and together with the complex terrain effects are employed to the optimization process of wind farms. Despite the growing body research there are...

The development of large-scale wind energy projects has created the demand for increasingly accurate and efficient models that limit a project's uncertainties and risk. Wake effects are of great importance and are relevant for the optimization of wind farms. Despite a growing body of research, there are still many open questions and challenges to o...

In this work a black-box identification approach is used to identify a nonlinear model for nonlinear systems exhibiting autonomous oscillations. We focus on the proces of vortex-induced vibrations where fluid-structure interactions cause vibrations. A polynomial nonlinear state space model structure is proposed and fitted to the data acquired from...

Vertical-axis wind turbines (VAWTs) have experienced a renewed impulse during the last few years with important research efforts focused on them. This work explores whether the global profitability of VAWTs can be improved through improved manufacturing techniques. We studied how large-series production techniques from the sheet-metal industry can...

In this work we explain the basic ideas of a novel modelling methodology for unsteady aerodynamics based on system identification techniques. We aim to construct transfer functions (as they are often used in noise and vibration engineering) to model periodic unsteadiness. This is of direct use for the study of limit-cycle oscillations (LCO) in flut...

In this paper we address the use of system identification techniques as a means for constructing a model relating fluid variables (pressure, velocity, vorticity) measured in the wake to the oscillatory behaviour of a cylinder in a flow. We describe how the model can be decomposed in three separate transfer functions, linked in a loop configuration....

Vertical axis wind turbines have experienced a renewed impulse during the last few years, with important research efforts focused on them. This work is focused in the development of new blade designs capable of notably reduce their manufacturing cost and, consequently, improve global profitability. Thanks to the aerodynamic and structural analyses...

A classical problem in vortex-induced vibration is to know the flow field past an oscillating cylinder. In this paper we use system theory to identify the oscillatory behaviour of a circular cylinder from flow variables in the wake. We use numerical simulations (CFD) of the flow past a cylinder oscillating in the cross-flow direction at different o...

In this paper we present the results of a feasibility study about small wind turbines in Flanders. We performed wind speed measurements at various sites and conducted an exhaustive review of small wind turbines to compare the different characteristics. Combining these two aspects, we predicted the annual energy production and compared the payback t...

In building a reliable aeroelastic model of a newly developed aircraft, one key factor is the coherence between ground vibration and flight flutter tests. This complex relation entails the complementarity of the ground and flight tests, and the integration of the numerical model with experimental (ground and flight) data. Recent research projects h...

In this paper, consistent multiple-input frequency-domain estimators will be presented based on a right matrix-fraction description of the frequency response function. This matrix-fraction description leads to a fast algorithm in the same way as the least-squares complex frequency-domain estimator (LSCF). The use of multiple inputs simultaneously i...

A numerical study of the combined implementation of terrain roughness and velocity shear of the atmospheric boundary layer is presented in this paper. The consistency of these boundary conditions is tested by simulating the flow over a flat terrain. The results are described in the first part of this paper. Once these conditions are tuned, they are...

Design optimization can be very time-consuming depending on the complexity of the model to be optimized. This manuscript describes the development of an adaptive response surface method for reliability-based design optimization of computation-intensive models, capable of reducing optimization times significantly. The method applied in this paper ma...

Ambient noise is generally seen as an unwanted excitation that disturbs the estimation of vibration parameters. Averaging techniques are then used to decrease as much as possible the influence of the noise. However, this noise also excites the mechanical structure and thus increases the vibration response level. Moreover, it is possible that (broad...

The star Cyg OB2 No. 9 is a well-known non-thermal radio emitter. Recent theoretical work suggests that all such O-stars should be in a binary or a multiple system. However, there is no spectroscopic evidence of a binary component. Re-analysis of radio observations from the VLA of this system over 25 years has revealed that the non-thermal emission...

We present one-dimensional, time-dependent models of the clumps generated by the line- deshadowing instability. In order to follow the clumps out to distances of more than 1000 R�, we use an efficient moving-box technique. We show that, withinthe approximations, the wind can remain clumped well into the formation region of the radio continuum.

We present one-dimensional, time-dependent models of the clumps generated by the line-deshadowing instability. In order to follow the clumps out to distances of more than 1000 stellar radii, we use an efficient moving-box technique. We show that, within the approximations, the wind can remain clumped well into the formation region of the radio cont...

HD 167971 is a triple system consisting of a 3.3-day eclipsing binary (O5-8 V + O5-8 V) and an O8 supergiant. It is also a well known non-thermal radio emitter. We observed the radio emission of HD 167971 with the Very Large Array (VLA) and the Australia Telescope Compact Array (ATCA). By combining these data with VLA archive observations we constr...

We present a model for the non-thermal radio emission from presumably single O stars, in terms of synchrotron emission from relativistic electrons accelerated in wind-embedded shocks. These shocks are associated with an unstable, chaotic wind. The main improvement with respect to earlier models is the inclusion of the radial dependence of the shock...