35 research outputs found
A mesh-free framework for high-order simulations of viscoelastic flows in complex geometries
Full text linkThe accurate and stable simulation of viscoelastic flows remains a significant computational challenge, exacerbated for flows in non-trivial and practical geometries. Here we present a new high-order meshless approach with variable resolution for the solution of viscoelastic flows across a range of Weissenberg numbers. Based on the Local Anisotropic Basis Function Method (LABFM) of King et al. (2020), highly accurate viscoelastic flow solutions are found using Oldroyd B and PPT models for a range of two dimensional problems — including Kolmogorov flow, planar Poiseulle flow, and flow in a representative porous media geometry. Convergence rates up to 9th order are shown. Three treatments for the conformation tensor evolution are investigated for use in this new high-order meshless context (direct integration, Cholesky decomposition, and log-conformation), with log-conformation providing consistently stable solutions across test cases, and direct integration yielding better accuracy for simpler unidirectional flows. The final test considers symmetry breaking in the porous media flow at moderate Weissenberg number, as a precursor to a future study of fully 3D high-fidelity simulations of elastic flow instabilities in complex geometries. The results herein demonstrate the potential of a viscoelastic flow solver that is both high-order (for accuracy) and meshless (for straightforward discretisation of non-trivial geometries including variable resolution). In the near-term, extension of this approach to three dimensional solutions promises to yield important insights into a range of viscoelastic flow problems, and especially the fundamental challenge of understanding elastic instabilities in practical settings
High-order simulations of isothermal flows using the local anisotropic basis function method (LABFM)
No full textMesh-free methods have significant potential for simulations of flows in complex geometries, with the difficulties of domain discretisation greatly reduced. However, many mesh-free methods are limited to low order accuracy. In order to compete with conventional mesh-based methods, high order accuracy is essential. The Local Anisotropic Basis Function Method (LABFM) is a mesh-free method introduced in King et al. (2020) [20], which enables the construction of highly accurate difference operators on disordered node discretisations. Here, we introduce a number of developments to LABFM, in the areas of basis function construction, stencil optimisation, stabilisation, variable resolution, and high order boundary conditions. With these developments, direct numerical simulations of the Navier Stokes equations are possible at extremely high order (up to 10th order in characteristic node spacing internally). We numerically solve the isothermal compressible Navier Stokes equations for a range of geometries: periodic and channel flows, flows past a cylinder, and porous media. Excellent agreement is seen with analytical solutions, published numerical results (using a spectral element method), and experiments. The potential of the method for direct numerical simulations in complex geometries is demonstrated with simulations of subsonic and transonic flows through an inhomogeneous porous media at pore Reynolds numbers up to Rep = 968
Large eddy simulations of bubbly flows and breaking waves with smoothed particle hydrodynamics
Full text linkFor turbulent bubbly flows, multi-phase simulations resolving both the liquid and bubbles are prohibitively expensive in the context of different natural phenomena. One example is breaking waves, where bubbles strongly influence wave impact loads, acoustic emissions and atmospheric-ocean transfer, but detailed simulations in all but the simplest settings are infeasible. An alternative approach is to resolve only large scales, and model small-scale bubbles adopting sub-resolution closures. Here, we introduce a large eddy simulation smoothed particle hydrodynamics (SPH) scheme for simulations of bubbly flows. The continuous liquid phase is resolved with a semi-implicit isothermally compressible SPH framework. This is coupled with a discrete Lagrangian bubble model. Bubbles and liquid interact via exchanges of volume and momentum, through turbulent closures, bubble breakup and entrainment, and free-surface interaction models. By representing bubbles as individual particles, they can be tracked over their lifetimes, allowing closure models for sub-resolution fluctuations, bubble deformation, breakup and free-surface interaction in integral form, accounting for the finite time scales over which these events occur. We investigate two flows: bubble plumes and breaking waves, and find close quantitative agreement with published experimental and numerical data. In particular, for plunging breaking waves, our framework accurately predicts the Hinze scale, bubble size distribution, and growth rate of the entrained bubble population. This is the first coupling of an SPH framework with a discrete bubble model, with potential for cost-effective simulations of wave-structure interactions and more accurate predictions of wave impact loads. © The Author(s), 2023. Published by Cambridge University Press
Pumpjets in de binnenvaart
No full textBigger ships are replacing the small ships, also at the Amsterdam-Rijnkanaal. A direct consequence is the use of larger main propellers and increased installed engine power in order to realize acceptable ship speeds. To improve the maneuverability, the power of auxiliary bow and stern propulsion systems has increased as well. This results in higher flow velocities and thus more damage to bed protections at berthing places. Especially pump jet thrusters as a bow propulsion system may attack the bed protections more severely due to a vertical component of the flow field induced by the pump jet which is directed towards the bed. The aim of this research is, in cooperation with Rijkswaterstaat, to develop an analytical model for the flow field induced by the pump jet thruster. Subsequently, the required dimensions of a bed protection has been determined as well as an investigation on the possible scour effects in case of an unprotected bed of the Amsterdam-Rijnkanaal. In order to meet this goal, an inventory of the bow propulsion systems in inland vessels has been made by conducting an inquiry. Subsequently, the different propulsion systems have been compared qualitatively. Since existing formulas are based on the circular free jet, this system is also treated in the comparison. In addition, a water jet (hydro jets can be distinguished in low-powered and high-powered jets), often used as propulsion system by ferries, has been taken into account due to the strong analogy with the circular free jet. As a result, both the ratio of decrease of axial velocities and diffusion of the jets in radial direction of a circular free jet strongly correspond to a high-powered water jet. However, the free propeller and transverse tunnel thrusters are considered to have a faster decrease of axial velocities and a faster diffusion of the flow field. The applied power and the induced turbulence seem to be the main reasons for the flow field characteristics. An essential part of this research is the derivation of an analytical model for the pump jet thruster induced flow field. The principle is based on the free propeller theory. In the case of a pump jet, part of the contraction takes place inside the unit. Furthermore, the flow field has been subdivided in three parts: initial outflow velocity, axial flow velocity and the flow velocity distribution in the region of the established flow. The coefficients in the model have been empirically determined using a simulation with a CFD model. This model has been compared to flow fields induced by different other propulsion systems. It results in a similar axial flow velocity decrease as a low-powered water jet. However, the pump jet diverges faster and with a greater angle than any other jet. The derived model has been used to investigate the stone stability at the toe of a sheet pile wall. Firstly, the flow velocities at the bed have been evaluated by looking at berthed ships in different conditions (fully loaded, partly loaded and unloaded). Secondly, a stability analysis of the bed material has been executed by using a method presented by Deltares. However, in order to protect the bed against the high flow velocities, large stone diameters are required. An alternative is allowing scour holes to occur, but the length of the sheet piling should be extended in order to maintain stability of the entire quay wall. The predicted maximum scour depths caused by a pump jet are in the same range of measured scour depths at the Amsterdam-Rijnkanaal. This means that scour holes may cause instability problems of the existing sheet pile walls.Hydraulic EngineeringCivil Engineering and Geoscience
Boundary conditions for simulations of oscillating bubbles using the non-linear acoustic approximation
Full text linkWe have developed a new boundary condition for finite volume simulations of oscillating bubbles. Our method uses an approximation to the motion outside the domain, based on the solution at the domain boundary. We then use this approximation to apply boundary conditions by defining incoming characteristic waves at the domain boundary. Our boundary condition is applicable in regions where the motion is close to spherically symmetric. We have tested our method on a range of one- and two-dimensional test cases. Results show good agreement with previous studies. The method allows simulations of oscillating bubbles for long run times (5×1055×105 time steps with a CFL number of 0.8) on highly truncated domains, in which the boundary condition may be applied within 0.1%0.1% of the maximum bubble radius. Conservation errors due to the boundary conditions are found to be of the order of 0.1%0.1% after 105 time steps. The method significantly reduces the computational cost of fixed grid finite volume simulations of oscillating bubbles. Two-dimensional results demonstrate that highly asymmetric bubble features, such as surface instabilities and the formation of jets, may be captured on a small domain using this boundary condition
A mesh-free framework for high-order direct numerical simulations of combustion in complex geometries
No full textThe multiscale nature of turbulent combustion necessitates accurate and
computationally efficient methods for direct numerical simulations (DNS). The
field has long been dominated by high-order finite differences, which lack the
flexibility and adaptivity for simulations of complex geometries and
flame-turbulence-structure interactions in realistic settings. In this work we
introduce a new approach to DNS of premixed combustion, based on a high-order
mesh-free discretisation in combination with finite differences, enabling
high-order simulations in non-trivial geometries. The approach is validated
against a range of two- and three-dimensional flows, both laminar and
turbulent, and reacting and inert. The present method a) has the resolving
power for DNS of both laminar flames and inert turbulence with comparable
accuracy to high-order finite differences, b) can capture the dynamics of
unsteady bluff body stabilised flames, and c) is capable of simulating
flame-turbulence interactions, with results comparing qualitatively well with
published data. This work paves the way for DNS of combustion in complex
geometries, offering an alternative approach to methods based on structured
grids with immersed boundaries, or unstructured meshes. Further studies with
the present method are proposed, which will aid understanding of fundamental
flame dynamics in non-trivial geometries. Planned developments in adaptivity
and extension of the mesh-free construction to all three dimensions will
increase the value of the method, and support the push towards DNS of real
geometries.Comment: revision submitted CMAME Jan 202
NewProfiler
No full textA report about the development of NewProfiler, a behavioral target market system that we developed for NewNomads. The report describes the various stages of development of the software, the problems we encountered during the development and our solutions to these problems.Electrical Engineering, Mathematics and Computer Scienc
Oxyhalogenation of activated arenes with nanocrystalline ceria
No full textArenes can be chlorinated, brominated, and iodinated in the presence of CeO2 nanoparticles under aerobic conditions. In a biomimetic approach, active electrophilic halogen species are generated from either organic or inorganic halogen compounds and are trapped by arenes. This C-H transformation can be applied to the synthesis of naturally occurring products. © 2012 American Chemical Society.A.L.-P. thanks CSIC for a contract. D.C.-M. thanks the Spanish MICINN for a postgraduate scolarship (FPI). J.R.C.-A. thanks MCIINN for the concession of a FPU contract. Financial support by Consolider-Ingenio 2010 (proyecto MULTICAT), PLE2009 project from MCIINN, and King Saud University is also acknowledged
High order difference schemes using the Local Anisotropic Basis Function Method
Full text linkMesh-free methods have significant potential for simulations in complex geometries, as the time consuming process of mesh-generation is avoided. Smoothed Particle Hydrodynamics (SPH) is the most widely used mesh-free method, but suffers from a lack of consistency. High order, consistent, and local (using compact computational stencils) mesh-free methods are particularly desirable.Here we present a novel framework for generating local high order difference operators for arbitrary node distributions, referred to as the Local Anisotropic Basis Function Method (LABFM). Weights are constructed from linear sums of anisotropic basis functions (ABFs), chosen to ensure exact reproduction of polynomial fields up to a given order. The ABFs are based on a fundamental Radial Basis Function (RBF), and the choice of fundamental RBF has small effect on accuracy, but influences stability. LABFM is able to generate high order difference operators with compact computational stencils (4th order with 25 nodes, 8th order with 60 nodes in two dimensions). At domain boundaries (with incomplete support) LABFM automatically provides one-sided differences of the same order as the internal scheme, up to 4th order. We use the method to solve elliptic, parabolic and mixed hyperbolic-parabolic partial differential equations (PDEs), showing up to 8th order convergence. The inclusion of hyperviscosity is straightforward, and can effectively provide stability when solving hyperbolic problems.LABFM is a promising new mesh-free method for the numerical solution of PDEs in complex geometries. The method is highly scalable, and for Eulerian schemes, the computational efficiency is competitive with RBF-FD for a given accuracy. A particularly attractive feature is that in the low order limit, LABFM collapses to Smoothed Particle Hydrodynamics (SPH), and there is potential for Arbitrary Lagrangian-Eulerian schemes with natural adaptivity of resolution and accuracy
The Kaye effect: New experiments and a mechanistic explanation
No full textThe Kaye effect is a phenomenon whereby a jet of fluid poured onto a surface appears to leap on impact, rather than stagnate or coil as expected. Since it was first described in 1963, several authors have attempted to explain the mechanism by which the phenomenon occurs, although to date no complete explanation for the behaviour exists. Current evidence points towards the existence of an air layer between the jet and the heap which enables slip. We show that the Kaye effect does not occur in a vacuum, indicating that the air layer is crucial for the effect to occur. By use of control volume analysis we show that viscoelasticity plays a key role in the Kaye effect, and this role is two-fold. Firstly, viscoelasticity appears to increase air entrainment, and secondly, it reduces the pressure required to bend the jet, allowing a thicker air layer to be sustained. Shear thinning behaviour reduces this viscoelastic response. These findings provide new insight into a problem that has puzzled rheologists for over half a century
