Ashkan Rafiee

• Doctor of Philosophy
• Senior Data Scientist at INPEX

This study investigates the efficacy of Generalized Polynomial Chaos (gPCE) and Bayesian Optimisation in the analysis of mooring loads for offshore floating wind turbines. Amidst inherent uncertainties, these methodologies provide robust frameworks for optimizing design and assessing mooring system fatigue. By leveraging advanced statistical techni...

Precise prediction of wave energy is indispensable and holds immense promise as ocean waves have a power capacity of 30-40 kW/m along the coast. Utilising this energy source does not generate harmful emissions, making it a superior substitute for fossil fuel-based energy. The computational expense associated with simulating and computing intricate...

In engineering design optimisations, the dimensionality of the design space plays a critical role in determining the number of computationally expensive evaluations required to explore the design space and reach convergence. Dimensionality reduction allows engineers to represent high-dimensional design spaces in a more compact form to reduce the co...

Simulation-based design plays an important role in many engineering fields enabled by advancements in computing resources, numerical algorithms and computer aided engineering software. In naval architecture, it opens new opportunities for improving hull form designs by reducing calm water resistance and hence minimising required propulsive energy....

Computational Fluid Dynamics (CFD) has become an indispensable tool in the field of engineering design evaluation and optimisation in the maritime industry. Accurate modelling of flow around a ship’s hull and superstructure requires high fidelity CFD simulations due to complex nonlinear phenomena. On the other hand, high fidelity CFD simulations ar...

In this paper we demonstrate how big data and state of the art machine learning techniques are used to build, test and deploy intelligent models to predict and optimize motion of High Speed Craft (HSC). We show that the motion performance can be improved by using machine learning models to predict vessel response and optimise controller settings on...

Computational Fluid Dynamics (CFD) has become an indispensable tool in the field of engineering design evaluation and optimisation. Existing numerical simulation methods are computationally expensive, memory demanding and time-consuming, thus limiting design space exploration and forbid generative design. In order to overcome these challenges, we p...

Presentation at Perth Machine Learning Group. For the video see: https://www.youtube.com/watch?v=KjWC-gDe7m4&t=1657s

In this paper the dynamics of a submerged axi-symmetric wave energy converter are studied, through mathematical models and wave basin experiments. The device is disk-shaped and taut-moored via three inclined tethers which also act as a power take-off. We focus on parasitic yaw motion, which is excited parametrically due to coupling with heave. Assu...

In previous work, a frequency-domain model was developed from linear potential theory to investigate the oscillation modes and efficiency of a single-tether 3 degree-of-freedom submerged spherical point absorber with asymmetric mass distribution (SPAMD). It was found that the trajectory of the device has a strong correlation with the performance of...

Wave energy converters and other offshore structures may exhibit instability, in which one mode of motion is excited parametrically by motion in another. Here, theoretical results for the transverse motion instability (large sway oscillations perpendicular to the incident wave direction) of a submerged wave energy converter buoy are compared to an...

This study presents dynamic simulation results of two point absorber wave energy converters comparing between linear, pseudo-nonlinear, and CFD models. When modelling wave energy converters, linear assumptions are commonly used to simplify calculations. One such assumption is that the hydrody-namic parameters do not change with pose. This study pro...

Theoretical and experimental investigation of yaw motion instability in a submerged axi-symmetric wave energy converter is presented. The device is a truncated vertical cylinder which is taut-moored via three tethers. Assuming linear hydrodynamics, but retaining non-linear geometry associated with the tethers, governing equations are derived in 6 d...

In this paper, the forced oscillation of a circular cylinder, known as the Radiation problem, close to the free-surface is studied for a range of Reynolds (Re) and Keulegan-Carpenterr (KC) numbers , using a semi-analytical linear solution and a fully non-linear Navier-Stokes solver. The fully non-linear solution is obtained using the OpenFOAM packa...

There are numerous designs and concepts that have been offered to extract energy from ocean waves. A heaving buoy is distinguished as the most popular device which predominantly harnesses energy from the vertical motion in waves. One such device is the bottom-referenced submerged heaving buoy represented by the Carnegie Clean Energy CETO system. Th...

In previous works [1], a single-tethered spherical point absorber with asymmetric mass distribution (SPAMD) was found to be 2-3 times more efficient than a generic single-tethered point absorber (PA) with uniform mass distribution under long-crested waves. However, the hydrodynamics of PAs were modelled using linear potential wave theory that inher...

Wave energy converters (WECs) with Coulomb-type damping are relatively common as this behaviour is typical of hydraulic power take-off (PTO) systems. This study investigates motion response of a mass-spring system under combined linear and Coulomb damping, as a simplified representation of a WEC with a Coulomb damping force in the PTO. Motivation f...

The development of new wave energy converters has shed light on a number of unanswered questions in fluid mechanics, but has also identified a number of new issues of importance for their future deployment. The main concerns relevant to the practical use of wave energy converters are sustainability, survivability, and maintainability. Of course, it...

Safe and economic design of any wave energy converter (WEC) requires an accurate estimation of extreme loads and motions of the device. The aim of this paper is to understand the characteristics of the wave groups which lead to an extreme response of a point absorber WEC. To do so, initially several long–duration experiments in extreme random seas...

In order to optimise energy extraction from ocean waves, the Wave Energy Converters (WECs) are generally designed to maximise the fluid-structure interactions between the device and the waves. Hence, it is of significant importance to accurately predict the hydrodynamic loads on the device during both normal operation and extreme conditions. This p...

Bottom hinged oscillating wave surge converters are known to be an efficient method of extracting power from ocean waves. The present work deals with experimental and numerical studies of wave interactions with an oscillating wave surge converter. It focuses on two aspects: (1) viscous effects on device performance under normal operating conditions...

Wave impacts on an oscillating wave surge converter are examined using experimental and numerical methods. The mechanics of the impact event are identified experimentally with the use of images recorded with a high-speed camera. It is shown that it is the device that impacts the wave rather than a breaking wave impacting the device. Numerical simul...

11 pages, 10 figures, 9 references. Other author's papers can be downloaded at http://www.denys-dutykh.com/

It has been known that bottom hinged Oscillating Wave Surge Converters (OWSCs) are an efficient way of extracting power from ocean waves (Whittaker et al. 2007). OWSCs are in general large buoyant flaps, hinged at the bottom of the ocean and oscillating back and forth under the action of incoming incident waves (Schmitt et al. 2012, Renzi and Dias...

This paper deals with numerical studies of wave interaction with an Oscillating Wave Surge Converters (OWSC) using a Godunov–type SPH method. The use of a Riemann solver in calculating the density field results in a smoother SPH pressure field. Hence, a more accurate estimation of loads on the OWSC is achieved. Furthermore, the Lagrangian form of t...

In this paper, an SPH method based on the SPH-ALE formulation is used for modelling two-phase flows with large density ratios and realistic sound speeds. The SPH scheme is further improved to circumvent the tensile instability that may occur in the SPH simulations. The two-phase SPH solver is then used to model a benchmark problem of liquid impact...

In this paper, we describe an SPH algorithm for multi‐fluid flow, which is efficient, simple and robust. We derive the inviscid equations of motion from a Lagrangian together with the constraint provided by the continuity equation. The viscous flow equations then follow by adding a viscous term. Rigid boundaries are simulated using boundary force p...

Wave impacts on an Oscillating Wave Surge Converter are examined using experimental and numerical methods. The mechanics of the impact event are identified experimentally with the use of images recorded with a high speed camera. It is shown that it is the device which impacts the wave rather than a breaking wave impacting the device. Numerical simu...

In this paper, we simulate an academic benchmark impact problem using a two-phase compressible Smoothed Particle Hydrodynamics (SPH) method and an incompressible Level-Set method. The benchmark problem was proposed in ISOPE 2009 (Sloshing symposium) and involves studying the impact of a falling liquid box on a rigid wall in an atmosphere of a gaseo...

Free-surface flows are of significant interest in Computational Fluid Dynamics (CFD). However, modelling them especially when the free-surface breaks or impacts on solid walls can be challenging for many CFD techniques. Smoothed Particle Hydrodynamics (SPH) has been reported as a robust and stable method when applied to these problems. In modelling...

Multiphase flows are present in many different industrial and research applications. The accurate tracking of interfaces is therefore an important part of numerical simulation of many physical phenomena. One of the challenges in modeling multiphase flows is to capture a moving interface with a large deformation, especially the breaking and merging...

In this paper, sloshing phenomenon in a rectangular tank under a sway excitation is studied numerically and experimentally. Although considerable advances have occurred in the development of numerical and experimental techniques for studying liquid sloshing, discrepancies exist between these techniques, particularly in predicting time history of im...

In modelling incompressible flows using the Smoothed Particle Hydrodynamics method (SPH), an equation of state with a large sound speed is typically used. This weakly compressible approach (WCSPH), results in a stiff set of equations with a noisy pressure field and stability issues at high Reynolds number. As a remedy, an incompressible SPH techniq...

In this paper, we apply our two-phase Smoothed Particle Hydro-dynamics (SPH) method for numerically simulating the slosh-ing phenomenon with varying ratio of densities between the liquid and gas. This method has recently been developed and validated on several benchmark problems. Here, we consider a closed rectangular tank subject to translational...

In this paper an incompressible Smoothed Particle Hydrodynamics (SPH) method is proposed for simulation of fluid–structure interaction problems, deploying the pressure Poisson equation to satisfy incompressibility constraints. Viscous fluid flow past rigid and hypoelastic solid surfaces is studied. The fluid is fully coupled with the solid structur...

Sloshing is an important issue for LNG tanks and in general when partially filled tanks are on-board of a vessel. This resonance phenomenon may be connected with complex motions of the filled liquid that can couple with ship motions and can represent a danger for the tank structure and for the stability of the ship. The Smoothed Particle Hydrodynam...

In this paper a smoothed particle hydrodynamics (SPH) method is introduced for simulating two-dimensional incompressible non-Newtonian fluid flows, and the non-Newtonian effects in the flow of a fluid which can be modelled by generalized Newtonian constitutive equations are investigated. Two viscoplastic models including Bingham-plastic and power-l...

In this paper a smoothed particle hydrodynamics (SPH) method is introduced for simulating two-dimensional incompressible non-Newtonian fluid flows, and the non-Newtonian effects in the flow of a fluid which can be modelled by generalized Newtonian constitutive equations are investigated. Two viscoplastic models including Bingham-plastic and power-l...

In this paper an incompressible Smoothed Particle Hydrodynamics (SPH) method is proposed for simulation of fluid-structure interaction problems, deploying the pressure Poisson equation to satisfy incompressibility constraints. Viscous fluid flow past rigid and hypoelastic solid surfaces is studied. The fluid is fully coupled with the solid structur...

In this paper, an incompressible smoothed particle hydrodynamics (SPH) method is presented to solve unsteady free-surface flows. Both Newtonian and viscoelastic fluids are considered. In the case of viscoelastic fluids, both the Maxwell and Oldroyd-B models are investigated. The proposed SPH method uses a Poisson pressure equation to satisfy the in...