ORCID Profile
0000-0003-0073-4525
Current Organisation
University of Newcastle Australia
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Publisher: Cambridge University Press (CUP)
Date: 28-01-2019
Abstract: Steady and unsteady linearised flow past a submerged source are studied in the small-surface-tension limit, in the absence of gravitational effects. The free-surface capillary waves generated are exponentially small in the surface tension, and are determined using the theory of exponential asymptotics. In the steady problem, capillary waves are found to extend upstream from the source, switching on across curves on the free surface known as Stokes lines. Asymptotic predictions are compared with computational solutions for the position of the free surface. In the unsteady problem, transient effects cause the solution to display more complicated asymptotic behaviour, such as higher-order Stokes lines. The theory of exponential asymptotics is applied to show how the capillary waves evolve over time, and eventually tend to the steady solution.
Publisher: Wiley
Date: 06-02-2020
DOI: 10.1002/PS.5736
Abstract: A suite of plant retention spray models has been developed to simulate spray retention using virtual surfaces (either single leaves or whole plants) and their outputs compared with experimental data for the equivalent spray scenarios. The results for a single formulation (0.1% v/v lecithin mixture in water) and difficult to wet plant species Chenopodium album L (common lambsquarters) are presented. They include experimental observations with single leaves, as well as simulations of virtual impaction events, conducted to provide for the first time estimates of f (the proportion of theoretical impact drop diameter at shatter). With this factor prescribed, multi-plant simulations using a range of nozzle types and droplet sizes (volume mean diameter (VMD) range 241 to 530 μm) are compared with equivalent experimentally determined spray retention by real plants. The simulations demonstrated that impaction resulted predominantly in shatter with the production of daughter droplets, and that retention is mainly due to re-capture of these droplets. Overall the simulations show the same trends as experimental retention results from different nozzle applications, but at best predicted retention results were 68% to 79% of experimental percentage retention, depending on plant spacing. Retention is the result of some primary drop capture but predominantly by recapture of shatter droplets as the modelling illustrates. The value of f affects the droplet shatter outcome and can result in fewer, more energetic daughter droplets, or more droplets but with lower energies. However, this effect alone cannot explain the discrepancy between actual and simulated results. Possible operational influences are discussed. © 2020 Society of Chemical Industry.
Publisher: Cambridge University Press (CUP)
Date: 03-05-2018
DOI: 10.1017/JFM.2018.254
Abstract: We consider steady nonlinear free surface flow past an arbitrary bottom topography in three dimensions, concentrating on the shape of the wave pattern that forms on the surface of the fluid. Assuming ideal fluid flow, the problem is formulated using a boundary integral method and discretised to produce a nonlinear system of algebraic equations. The Jacobian of this system is dense due to integrals being evaluated over the entire free surface. To overcome the computational difficulty and large memory requirements, a Jacobian-free Newton–Krylov (JFNK) method is utilised. Using a block-banded approximation of the Jacobian from the linearised system as a preconditioner for the JFNK scheme, we find significant reductions in computational time and memory required for generating numerical solutions. These improvements also allow for a larger number of mesh points over the free surface and the bottom topography. We present a range of numerical solutions for both subcritical and supercritical regimes, and for a variety of bottom configurations. We discuss nonlinear features of the wave patterns as well as their relationship to ship wakes.
Publisher: Cambridge University Press (CUP)
Date: 06-12-2017
DOI: 10.1017/JFM.2016.753
Abstract: A spectrogram is a useful way of using short-time discrete Fourier transforms to visualise surface height measurements taken of ship wakes in real-world conditions. For a steadily moving ship that leaves behind small- litude waves, the spectrogram is known to have two clear linear components, a sliding-frequency mode caused by the ergent waves and a constant-frequency mode for the transverse waves. However, recent observations of high-speed ferry data have identified additional components of the spectrograms that are not yet explained. We use computer simulations of linear and nonlinear ship wave patterns and apply time–frequency analysis to generate spectrograms for an idealised ship. We clarify the role of the linear dispersion relation and ship speed on the two linear components. We use a simple weakly nonlinear theory to identify higher-order effects in a spectrogram and, while the high-speed ferry data are very noisy, we propose that certain additional features in the experimental data are caused by nonlinearity. Finally, we provide a possible explanation for a further discrepancy between the high-speed ferry spectrograms and linear theory by accounting for ship acceleration.
Publisher: Elsevier BV
Date: 06-2018
Publisher: Cambridge University Press (CUP)
Date: 31-03-2021
DOI: 10.1017/JFM.2021.193
Publisher: Elsevier BV
Date: 10-2021
Publisher: Cambridge University Press (CUP)
Date: 09-10-2014
DOI: 10.1017/JFM.2014.530
Abstract: While the half-angle which encloses a Kelvin ship wave pattern is commonly accepted to be 19.47°, recent observations and calculations for sufficiently fast-moving ships suggest that the apparent wake angle decreases with ship speed. One explanation for this decrease in angle relies on the assumption that a ship cannot generate wavelengths much greater than its hull length. An alternative interpretation is that the wave pattern that is observed in practice is defined by the location of the highest peaks for wakes created by sufficiently fast-moving objects, these highest peaks no longer lie on the outermost ergent waves, resulting in a smaller apparent angle. In this paper, we focus on the problems of free-surface flow past a single submerged point source and past a submerged source doublet. In the linear version of these problems, we measure the apparent wake angle formed by the highest peaks, and observe the following three regimes: a small Froude number pattern, in which the ergent waves are not visible standard wave patterns for which the maximum peaks occur on the outermost ergent waves and a third regime in which the highest peaks form a V-shape with an angle much less than the Kelvin angle. For nonlinear flows, we demonstrate that nonlinearity has the effect of increasing the apparent wake angle so that some highly nonlinear solutions have apparent wake angles that are greater than Kelvin’s angle. For large Froude numbers, the effect on apparent wake angle can be more dramatic, with the possibility of strong nonlinearity shifting the wave pattern from the third regime to the second. We expect that our nonlinear results will translate to other more complicated flow configurations, such as flow due to a steadily moving closed body such as a submarine.
Publisher: Elsevier BV
Date: 06-2021
Publisher: The Royal Society
Date: 03-08-2020
Abstract: The Stokes phenomenon is a class of asymptotic behaviour that was first discovered by Stokes in his study of the Airy function. It has since been shown that the Stokes phenomenon plays a significant role in the behaviour of surface waves on flows past submerged obstacles. A detailed review of recent research in this area is presented, which outlines the role that the Stokes phenomenon plays in a wide range of free surface flow geometries. The problem of inviscid, irrotational, incompressible flow past a submerged step under a thin elastic sheet is then considered. It is shown that the method for computing this wave behaviour is extremely similar to previous work on computing the behaviour of capillary waves. Exponential asymptotics are used to show that free-surface waves appear on the surface of the flow, caused by singular fluid behaviour in the neighbourhood of the base and top of the step. The litude of these waves is computed and compared to numerical simulations, showing excellent agreements between the asymptotic theory and computational solutions. This article is part of the theme issue ‘Stokes at 200 (part 2)’.
Publisher: Elsevier BV
Date: 07-2014
Publisher: Wiley
Date: 03-03-2020
DOI: 10.1002/PS.5796
Publisher: AIP Publishing
Date: 06-2015
DOI: 10.1063/1.4921918
Abstract: Linear water wave theory suggests that wave patterns caused by a steadily moving disturbance are contained within a wedge whose half-angle depends on the depth-based Froude number FH. For the problem of flow past an axisymmetric pressure distribution in a finite-depth channel, we report on the apparent angle of the wake, which is the angle of maximum peaks. For moderately deep channels, the dependence of the apparent wake angle on the Froude number is very different to the wedge angle and varies smoothly as FH passes through the critical value FH = 1. For shallow water, the two angles tend to follow each other more closely, which leads to very large apparent wake angles for certain regimes.
Publisher: American Physical Society (APS)
Date: 29-10-2021
Publisher: Wiley
Date: 17-10-2018
DOI: 10.1002/FLD.4469
No related grants have been discovered for Ravindra Pethiyagoda.