Tim Stallard | The University of Manchester (original) (raw)

Dr Stallard joined the University of Manchester in October 2006 after two years of interdisciplinary research concerning the performance and economics of marine energy converters. This followed completion of a DPhil in the Ocean Engineering Dynamics group at Oxford University where he developed an interest in both vortex- and wave-induced forces. Since 2006 research activities have focused on arrays of wave and tidal stream devices.

Staff webpage: http://www.mace.manchester.ac.uk/people/staff/academic-staff/profile/?staffId=463

Recent and ongoing projects include:

ALL-TT (2014-15): Arrays of Long Life Tidal Turbines (Lead). EPSRC Newton UK-China Marine Energy development feasibility project with Northeast Normal University, Jilin Province, China

StepWEC (2013-16): Step Change for Wave Energy Conversion (Co-I). EPSRC Challenge research project led by Manchester University with University of Oxford, University of Bath.

X-MED (2012-15): Extreme Loading of Marine Energy Devices (Co-I). EPSRC Challenge research project led by Manchester University with University of Plymouth, University of Edinburgh, Scottish Assoc. Marine Sci.

WEC Wakes (2012-13): Experimental study of wave farms at DHI, Denmark (Co-investigator). EU FP7 Hydralab IV project led by University Ghent with 6 EU academic partners

ReDAPT (2010-14): Reliable Data Acquisition Platform Tidal: Energy Technologies Institute (ETI) industry-academia project led by Alstom Ocean. Co-investigator of Manchester contribution on behalf of EdF. Contributed to development and evaluation of massively parallel CFD methods for prediction of unsteady loads on a full-scale tidal stream turbine.

PerAWaT (2009–13): Performance of Wave and Tidal Array Systems. Energy Technologies Institute (ETI) industry-academia project led by GL-Garrad Hassan. Lead of work package: Designed, conducted and evaluated Turbine Array Experiments and supported development of commercial software for farm energy yield predictions.

EquiMAR (2008-11): Equitable Appraisal of Marine Energy: EU FP7 Collaboration project led by University of Edinburgh with 22 partners. Lead of work package: Developed and published procedures for assessing the economic viability of commercial scale wave- and tidal-stream projects. (www.equimar.org)

Tidal Stream Turbulence Research (2007–08): (Lead) Funded by NWDA Joule Centre.

Supergen Marine Phase I (2004-06): Economic Assessment of Large Scale Marine Energy Project, University of Lancaster.

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Papers by Tim Stallard

Commercially available wind yield assessment models rely on superposition of wakes calculated for... more Commercially available wind yield assessment models rely on superposition of wakes calculated for isolated single turbines. These methods of wake simulation fail to account for emergent flow physics that may affect the behaviour of multiple turbines and their wakes and therefore wind farm yield predictions. In this paper wake-wake interaction is modelled computationally (CFD) and physically (in a hydraulic flume) to investigate physical causes of discrepancies between analytical modelling and simulations or measurements. Three effects, currently neglected in commercial models, are identified as being of importance: 1) when turbines are directly aligned, the combined wake is shortened relative to the single turbine wake; 2) when wakes are adjacent, each will be lengthened due to reduced mixing; and 3) the pressure field of downstream turbines can move and modify wakes flowing close to them.

In Proceedings of 3rd International Conference on Hydroelasticity in Marine Technology Proceedings of 3rd International Conference on Hydroelasticity in Marine Technology 2003, May 1, 2003

Page 1. An Experimental Study of Closely Spaced Point Absorber Arrays Tim Stallard, Peter K. Stan... more Page 1. An Experimental Study of Closely Spaced Point Absorber Arrays Tim Stallard, Peter K. Stansby and Alan J. Williamson School of Mechanical, Aerospace and Civil Engineering, University of Manchester. Manchester, UK. ABSTRACT ...

Deployment of co-located wind and tidal stream turbines is proposed as a method for reducing cost... more Deployment of co-located wind and tidal stream turbines is proposed as a method for reducing cost of electricity generation from either technology individually. Energy yield for wind turbines is modelled using an eddy viscosity wake model and for tidal turbines using a method of self-similar superposition of wake deficits. Yaw strategy is considered for the tidal turbines, finding that although a continuous yaw strategy generates highest yield, a slack-tide strategy offers a suitable compromise with mechanical complexity. A case-study of the MeyGen site in the Pentland Firth is considered for co-location. The addition of 12MW of wind capacity to a 20MW tidal array results in a twofold increase in annual energy yield, compared to operating the tidal turbines alone. Phasing of the tidal cycle means that during a neap tide, the combined system may be entirely dependent on wind generation, but during a spring tide there is a regular tidal supply. Steady state loads for wind and current ...

Numerical Weather Prediction (NWP) models such as Weather Research and Forecasting (WRF) are wide... more Numerical Weather Prediction (NWP) models such as Weather Research and Forecasting (WRF) are widely used for prediction of wind resource at potential wind farm sites and, increasingly, for energy yield prediction. Such models solve a reduced form of the Navier-Stokes equations with typical resolution of 20-1000 m in the vertical axis and 1-2 km in the horizontal axes. Sub-grid models have previously been developed to represent wind farms including by modification of momentum sink and turbulence kinetic energy source terms within cells occupied by turbines. Here, semi-empirical wake models are employed to assess the extent of losses between turbines within a small group such as within a single WRF. Variation of thrust and power with wind speed and direction were obtained using the modified PARK and Eddy Viscosity methods in OpenWind. The influence of wake-losses on yield was evaluated through WRF simulations of resource only and with standard and modified turbine parameterizations. A...

Numerical studies have shown that the power capture and displacement amplitude of a body located ... more Numerical studies have shown that the power capture and displacement amplitude of a body located within an array can be significantly greater than that of the same body in isolation. Linear models of array interactions, such as those compared by Mavrakos and McIver (1997) [1] have received only limited comparison to experimental measurements and may be inadequate for predicting the response of even an isolated device, particularly if the float is of shallow draft (Vantorre et al., 2005 [2]). This paper compares numerical predictions of the response of an array of shallow draft heaving floats to experimental measurements. Numerical simulations are based on hydrodynamic forcing obtained from analysis of the full array using WAMIT. Comparisons are drawn between the displacement amplitude of an isolated device and devices within linear arrays of varying orientation. The main focus is on a linear array of 5 hemispherical floats similar to the arrangement studied in [1] and [3]. Response ...

Water Wave Basin of DHI, in Denmark, on large arrays of up to 25 heaving point absorbers for a ra... more Water Wave Basin of DHI, in Denmark, on large arrays of up to 25 heaving point absorbers for a range of layout configurations and wave conditions. Float response and modification of the wave field are measured to provide data suitable for the evaluation of array interaction models and environmental scale models. Each wave energy converter unit has a diameter of 0.315 m and power absorption is due to friction of both a power take off system and bearings. Response is measured on all floats and surge force on five floats. Wave gauges are located withinand around the array. Wave conditions studied include regular waves and both long-and short-crested irregular waves. A rectilinear arrangement of support structures is employed such that several float configurations can be studied. A summary is presented of the experimental arrangement with particular emphasis on the individual wave energy converters and wave conditions employed. Reasonable agreement is observed between measured response for single floats and power output and float response predicted using a linear time domain model. For an array of 25 floats, up to 16.3% reduction of significant wave height is observed down-wave and 10.8% increase observed upwave for unidirectional irregular waves due to wave radiation by the heaving WECs. Spectra at different locations within and around the array show the wave field modifications.

Many studies have been published concerning the influence of the immersed shape (in still water) ... more Many studies have been published concerning the influence of the immersed shape (in still water) of a floating body on its response and power capture from ocean waves. With a few notable exceptions, much of this analysis has assumed small amplitude motion and linear models have been employed to predict response. The form of the upper surface of such a body has received little attention. Here, it is shown that the upper (top) surface of a floating body can be designed to ensure that the response amplitude of the body is within a specified value. This is of considerable importance to the survivability of wave energy devices. The approach used is to affect a large increase of both natural period and hydrodynamic damping for only a small change of float mass. These two factors impose a hydrodynamic limit on the displacement which may be exploited to avoid the 'end-stop' problem often encountered in wave device design. To demonstrate the change of response, experimental measureme...

International Journal of Heat and Fluid Flow, 2013

This paper presents results from numerical simulations of a 3-bladed horizontal axis tidal stream... more This paper presents results from numerical simulations of a 3-bladed horizontal axis tidal stream turbine. Initially, Reynolds Averaged Navier Stokes (RANS) k-x Shear Stress Transport eddy-viscosity and Launder-Reece-Rodi models were used for code validation and testing of a newly implemented sliding mesh technique for an unstructured finite volume code. Wall-and blade-resolved large-eddy simulations (LES) were then performed to study the complete geometry at various tip speed ratios (TSR). Thrust and power coefficients were compared to published experimental measurements obtained from a towing tank for a range of TSR (4, 5, 6, 7, 8, 9 and 10) at a fixed hub pitch angle. A strong meandering is observed downstream of the supporting tower due to interaction between the detached tip vortices and vortex shedding from the support structure. The wake profiles and rate of recovery of velocity deficit show high sensitivity to the upstream turbulence intensities. However, the mean thrust and power coefficients were found to be less sensitive to the upstream turbulence. Comparisons between RANS and LES are also presented for the mean sectional blade pressures and mean wake velocity profiles. The paper also presents an overview of modelling and numerical issues relating to simulations for such rotating geometries.

Many studies have been published concerning numerical predictions of wave energy device and array... more Many studies have been published concerning numerical predictions of wave energy device and array responses in both idealised and realistic wave-fields. However, there are few comparisons of predicted array response to experimental measurements, particularly for irregular waves. Although time domain models are widely used for modelling the behaviour of isolated devices, the response of arrays has typically been predicted by assuming linear superposition of steady-state response to regular waves. In this experimental study, the validity of approaches based on linear theory is investigated for an isolated device and array of two devices at 4-and 3-radius centre-to-centre spacing. A range of regular wave frequencies over which response is linear with wave height is identified and these response measurements are used to predict time-varying response to an irregular wave. In irregular waves, the influence of previous wave cycles, the time required for the device to reach a steady respons...

Experiments have been performed in the DHI Shallow Water Wave Basin (Denmark), on large arrays of... more Experiments have been performed in the DHI Shallow Water Wave Basin (Denmark), on large arrays of up to 25 heaving point absorber Wave Energy Converters (WECs), for a range of geometric layout configurations and wave conditions. WEC response, surge forces on the WECs and modification of the wave field are measured to provide data for the understanding of WEC array interactions/effects. Wave conditions studied, include regular, polychromatic, long- and short-crested irregular waves. The experimental arrangement and the obtained database are presented. For irregular long-crested waves, up to 18.1% attenuation of significant wave height is observed downwave a rectilinear array of 25 heaving WECs.

Journal of Ocean Engineering and Marine Energy, 2015

ABSTRACT A moored multi-body line absorber is an attractive option for offshore wave energy conve... more ABSTRACT A moored multi-body line absorber is an attractive option for offshore wave energy conversion. Laboratory studies have been undertaken to determine capture width with multi-mode excitation and heave resonance for the three-float system M4 where the adjacent float spacing is about half a typical wavelength giving anti-phase forcing. The floats increase in diameter and draft from bow to stern and the bow and mid float are rigidly connected by a beam. A hinge with a damper above the mid float absorbs power from the relative rotation between the bow/mid float and the stern float. The resonant heave frequency for each float is different. Anti-phase surge forcing between mid and stern floats is substantial, while there is no hydrostatic stiffness producing resonance. This represents a hydrodynamically complex system and the laboratory experiments indicate high overall capture widths in irregular waves across a range of peak periods without damping optimisation. With different spectral peakedness and directional spread, the capture width is greater than 20 % of a wavelength (based on the energy period) across a range of peak periods typical of an offshore site for floats with a rounded base. The maximum capture width was about 37 % of a wavelength with rounded base floats; having rounded rather than flat bases increased energy capture by up to 60 % by reducing energy losses due to drag. For floats with flat bases comparisons with a geometrically scaled device five times larger and with similar magnitudes of equivalent damping showed similar capture widths as a proportion of wavelength.

Journal of Fluids and Structures, 2014

The mean wake of a three-bladed horizontal axis tidal stream turbine operating at maximum power c... more The mean wake of a three-bladed horizontal axis tidal stream turbine operating at maximum power coefficient has been investigated experimentally in a wide flume with width 11 times the depth, providing minimal restriction to transverse wake development and behaviour of large-scale horizontal turbulence structures. This is an important first stage for understanding wake interaction in turbine arrays and hence large-scale power generation. The rotor diameter has a typical value of 60% of the depth and the thrust coefficient is representative of a full-scale turbine. The shear layers originating from the rotor tip circumference show classic linear expansion downstream, with the rate of a plane shear layer vertically and 1.5 times that horizontally. These shear layers merge by around 2.5 diameters downstream forming a self-similar two-dimensional wake beyond eight diameters downstream with a virtual origin at two diameters downstream of the rotor plane. The spreading rate is somewhat less than that for solid bodies. The detailed velocity measurements made in the near wake show rotation and vorticity similar to that measured previously for wind and marine turbines although with asymmetry associated with bed and surface proximity. The longitudinal circulation in a transverse plane is conserved at about 1% of the swept circulation from the blade tip within two diameters downstream, the extent of detailed measurement. Turbines are usually designed using blade element momentum theory in which velocities at the rotor plane are characterised by axial and tangential induction factors and it is now possible to see how this idealisation relates to actual velocities. The axial induction factor corresponds to velocity deficits at 0.4-0.8 radii from the rotor axis across the near wake while the tangential induction factor at the rotor plane corresponds to velocities at 0.4-0.6 radii between 1-2 diameters downstream, indicating some general correspondence. For the two-dimensional selfsimilar far wake the two parameters defining the centreline velocity deficit and the transverse velocity profiles are likely to be insensitive to Reynolds number in turbulent conditions.

Renewable Energy, 2015

ABSTRACT A line absorber consisting of three cylindrical floats is shown to have high crest captu... more ABSTRACT A line absorber consisting of three cylindrical floats is shown to have high crest capture widths for wave energy conversion across a broad band of frequencies. The bow, mid and stern floats are small, medium and large respectively; the floats are spaced about half a wavelength apart so that forces and motion of adjacent floats are substantially in anti-phase. The bow and mid float are rigidly connected by a beam and a beam from the stern float is connected to a hinge above the mid float for power take off. The draft of the stern float enables heave resonance at a prominent wave frequency and the smaller draft of the mid float provides resonance at a somewhat lower frequency. Experimental results at about 1:8 scale show capture widths greater than 25% of a wavelength in regular waves and greater than 20% of a wavelength in irregular waves across a broad range of wave periods. A time-stepping model for regular waves with coefficients from linear diffraction theory showed similar power prediction with a generic drag coefficient of 1.8. The model shows the importance of surge forcing and heave resonance. The model also shows that reducing drag coefficient will increase capture width.

International Journal of Heat and Fluid Flow, 2013

In Proceedings of 3rd International Conference on Hydroelasticity in Marine Technology Proceedings of 3rd International Conference on Hydroelasticity in Marine Technology 2003, May 1, 2003

International Journal of Heat and Fluid Flow, 2013

Journal of Ocean Engineering and Marine Energy, 2015

Journal of Fluids and Structures, 2014

Renewable Energy, 2015

International Journal of Heat and Fluid Flow, 2013

This paper offers a holistic approach to the evaluation of an ocean renewable energy (ORE) techno... more This paper offers a holistic approach to the evaluation of an ocean renewable energy (ORE) technology type or specific project in order to provide a comprehensive assessment of both narrow economic and broader socioeconomic performance. This assessment incorporates methods from three pillars areas: Economic - financial returns and efficient use of resources, Social -employment, social and community cohesion and identity, and Environmental - including the physical environment and pollution. These three pillars are then considered in the broader context of governance. In order to structure this evaluation, a novel parameter space model was created, defined by the three pillars and by the scale of the system under assessment. The scale of the system ranged from individual components of an ORE project; to projects comprising of a number of devices; through to a geographic regions in which multiple farms may be deployed. The parameter space consists of an inner circle representing the boundary of interest for a private investor, or a firm, developing an ORE project. The outer circle is characterised by assessment tools typically employed at the broader stakeholder level including economic, social, and environmental methods that can be employed at local, regional or national scale and which are typically employed to inform policy and decision making regarding ORE. Governance sets the stage within which management occurs. Wider impacts to the firm undertaking the project will take into account “externalities” of the project across the three fields. In this model, key methods identified are mapped onto this parameter space and the connectivity explored. The paper demonstrates that the three pillars are inter-connected and each must be considered in any meaningful assessment of ORE sustainability. An integrated assessment approach has the ability to address both the private and the public aspects of an ORE development. This analysis provides insights on existing best practice, but also reveals the potential for disconnect between an ORE project’s commercial viability and its contribution to environmental and social goals.