Fluid Dynamics

A high order Discontinuous Galerkin - Fourier incompressible 3D Navier-Stokes solver with rotating sliding meshes

by Esteban Ferrer

Journal of Computational Physics

Received 28 July 2011. Revised 23 April 2012. Accepted 27 April 2012. Available online 9 May 2012.

We present the development of a sliding mesh capability for an unsteady high order (order>3) h/p Discontinuous... more We present the development of a sliding mesh capability for an unsteady high order (order>3) h/p Discontinuous Galerkin solver for the three-dimensional incompressible Navier-Stokes equations. A high order sliding mesh method is developed and implemented for flow simulation with relative rotational motion of an inner mesh with respect to an outer static mesh, through the use of curved boundary elements and mixed triangular-quadrilateral meshes.

A second order stiffly stable method is used to discretise in time the Arbitrary Lagrangian-Eulerian form of the incompressible Navier-Stokes equations. Spatial discretisation is provided by the Symmetric Interior Penalty Galerkin formulation with modal basis functions in the it x-y plane, allowing hanging nodes and sliding meshes without the requirement to use mortar type techniques. Spatial discretisation in the z-direction is provided by a purely spectral method that uses Fourier series and allows computation of spanwise periodic three-dimensional flows. The developed solver is shown to provide high order solutions, second order in time convergence rates and spectral convergence when solving the incompressible Navier-Stokes equations on meshes where fixed and rotating elements coexist.

In addition, an exact implementation of the no-slip boundary condition is included for curved edges; circular arcs and NACA 4-digit airfoils, where analytic expressions for the geometry are used to compute the required metrics.

The solver capabilities are tested for a number of two dimensional problems governed by the incompressible Navier-Stokes equations on static and rotating meshes: the Taylor vortex problem, a static and rotating symmetric NACA0015 airfoil and flows through three bladed cross-flow turbines. In addition, three dimensional flow solutions are demonstrated for a three bladed cross-flow turbine and a circular cylinder shadowed by a pitching NACA0012 airfoil.

A simple, inexpensive system for digital particle image velocimetry (DPIV) in biomechanics

by Kurt Schwenk

A purely non-linear route to transition approaching the edge of chaos in a boundary layer

by stefania cherubini

S. Cherubini, P. De Palma, J.Ch Robinet, A. Bottaro, to appear on the special BIFD issue of Fluid Dynamics Research, 2012

The understanding of transition in shear flows has recently progressed along new paradigms based on the central role... more The understanding of transition in shear flows has recently progressed along new paradigms based on the central role of coherent flow structures and their nonlinear interactions. We follow such paradigms to identify, by means of a non-linear
optimization of the energy growth at short time, the initial perturbation which most easily induces transition in a boundary layer. Moreover, a bisection procedure has been used to identify localized flow structures living on the edge of chaos, found to be
populated by hairpin vortices and streaks. Such an edge structure appears to act as a relative attractor for the trajectory of the laminar base state perturbed by the initial finite amplitude disturbances, mediating the route to turbulence of the flow, via the triggering of a regeneration cycle of  and hairpin structures at different space and time scales. These findings introduce a new, purely non-linear scenario of transition in a boundary-layer flow.

Spectral structure of stratified turbulence: Direct numerical simulations and predictions by large eddy simulation

by Sebastian Remmler

published in "Theoretical and Computational Fluid Dynamics"

Density stratification has a strong impact on turbulence in geophysical flows. Stratification changes the spatial... more Density stratification has a strong impact on turbulence in geophysical flows. Stratification changes the spatial turbulence spectrum and the energy transport and conversion within the spectrum. We analyze these effects based on a series of direct numerical simulations (DNS) of stratified turbulence. To facilitate
simulations of real-world problems, which are usually beyond the reach of DNS, we propose a subgrid-scale turbulence model for large eddy simulations of stratified flows based on the Adaptive Local Deconvolution Method (ALDM). Flow spectra and integral quantities predicted by ALDM are in excellent agreement with
direct numerical simulation. ALDM automatically adapts to strongly anisotropic turbulence and is thus a suitable tool for studying turbulent flow phenomena in atmosphere and ocean.

Fluid Flow Control: a Vision-based Approach

by Romeo Tatsambon

Co-authored with Christophe Collewet, accepted for publication in the International Journal of Flow Control

Optimization of Mixed Convection in a Lid-Driven Enclosure with a Heat Generating Circular Body

by Sumon Saha

Numerical Heat Transfer, Part A: Applications 2011

The physical model considered here is a lid-driven enclosure with bottom heating and top cooling conditions, and a... more The physical model considered here is a lid-driven enclosure with bottom heating and top cooling conditions, and a heat generating circular body is placed at the center. The vertical
walls of the cavity are kept thermally insulated, and the top lid moves at a constant speed. The steady two-dimensional governing equations for the physical problem are transformed in a dimensionless form with dimensionless governing parameters that decide the fluid flow and heat transfer characteristics in the system. The solution of these transport equations is obtained numerically with the finite element approach using the Galerkin method of weighted residuals. The parametric study has been carried out for variation of the heat generation parameters, the Reynolds numbers, solid-fluid thermal conductivity ratios as well as the Richardson numbers. The working fluid is assigned as air with a Prandtl number of 0.71 throughout the simulation. Results are presented in the form of streamlines, isotherms, average Nusselt number, bulk temperature, and drag force for the afore mentioned parameters. The numerical results indicate the strong influence of the mentioned parameters on the flow
structure and heat transfer as well as average Nusselt number, average bulk temperature, and drag force. An optimum combination of the governing parameters would result in higher
heat transfer and lower drag force.

UTILIZAÇÃO DA MÁQUINA DE ATWOOD PARA ESTUDAR O MOVIMENTO DE UMA ESFERA NUM FLUIDO VISCOSO

by José Gonçalves

Co-authored with José Manuel Brochado Oliveira, published in "Física 2010, Livro de Resumos, SPF"

Uma esfera de aço colocada num dos lados da máquina de Atwood desloca-se no interior de um fluído viscoso – glicerina.... more Uma esfera de aço colocada num dos lados da máquina de Atwood desloca-se no interior de um fluído viscoso – glicerina. Uma das roldanas funciona também como sensor de rotação o que permite efectuar um registo computorizado do movimento de queda da massa em função do tempo. Contrariamente ao que sucede na queda livre de uma esfera em regime de Stokes, a velocidade da esfera diminui depois de atingir um valor máximo. O modelo utilizado para interpretar os resultados experimentais teve em conta não só o atrito e inércia das roldanas mas também os conceitos de número de Reynolds e massa acrescentada do fluído, para a determinação da força de arrasto do movimento da esfera e, em especial, sobre o seu fio de suspensão cuja parte imersa é variável. O modelo teórico sugerido ajustou-se perfeitamente aos resultados experimentais com a inclusão de um único parâmetro empírico.

Application of homotopy perturbation method to find an analytical solution for magnetohydrodynamic flows of viscoelastic fluids in converging/diverging channels

by Mostafa Safdari Shadloo

Improved Incompressible Smoothed Particle Hydrodynamics method for simulating flow around bluff bodies

by Mostafa Safdari Shadloo

Numerical modeling of Kelvin–Helmholtz instability using smoothed particle hydrodynamics

by Mostafa Safdari Shadloo

robust weakly compressible SPH method and its comparison with an incompressible SPH

by Mostafa Safdari Shadloo

The minimal seed of turbulent transition in a boundary layer

by stefania cherubini

S. Cherubini, P. De Palma, J. Ch. Robinet, A. Bottaro, accepted in Journal of Fluid Mechanics, 2011

This paper describes a scenario of transition from laminar to turbulent flow in a spatially developing boundary layer... more This paper describes a scenario of transition from laminar to turbulent flow in a spatially developing boundary layer over a flat plate. The base flow is the Blasius non-parallel flow solution; it
is perturbed by optimal disturbances yielding the largest energy growth over a short time interval. Such perturbations are computed by a non-linear global optimization approach based on a Lagrange multiplier technique. The results show that non-linear optimal perturbations are characterized by a localized basic building block, called the minimal seed, defined as the smallest flow structure  which maximizes the energy growth over short times. It is formed by vortices inclined in the streamwise direction surrounding a region of intense streamwise disturbance velocity. Such a basic structure appears to be a robust feature of the base flow since it is practically invariant with respect to the initial energy of the perturbation, the target time, the Reynolds number and the dimensions of the computational domain. The minimal seed grows very rapidly in time while spreading, and it triggers non-linear effects which bring the flow to turbulence in a very efficient manner, through the formation of a turbulence spot. This evolution of the initial optimal disturbance has been studied in detail by direct numerical simulations. Using a perturbative formulation of the Navier--Stokes equations, each linear and non-linear convective term of the equations has been analyzed.
The results show the fundamental role of the streamwise inclination of the vortices in the process. The non-linear coupling of the finite amplitude disturbances is crucial to sustain such streamwise inclination, as well as to generate dislocations within the flow structures, and local inflectional velocity distributions. The analysis provides a picture of the transition process characterized by a sequence of structures appearing successively in the flow, namely, Lambda vortices, hairpin vortices, streamwise streaks. Finally,  a disturbance regeneration cycle is conceived, initiated by the fast non-linear amplification of the minimal seed, providing a possible scenario for the continuous regeneration of the same fundamental
flow structures at smaller space and time scales.

Automobile Development

by Mity Wongkit

Transition and Flow-Induced Scattering of Acoustic Modes in Ducts.

by Alexander Smith

This is a copy of my PhD thesis (submitted version), which I completed at University College London from 2006-2011

My doctoral supervisors were Dr Nick Ovenden and Dr Robert Bowles

The financing for my PhD was provided by EPSRC

The propagation of unsteady disturbances in ducts of slowly-varying geometry, such as those typical of an aero-engine,... more The propagation of unsteady disturbances in ducts of slowly-varying geometry, such as those typical of an aero-engine, can be successfully modelled using a multiple scales approach. The multiple-scales approach has a number of distinct advantages over full numerical methods. The accuracy and usefulness of the multiple scales approach has been validated against finite element methods, using realistic aero-engine configurations.

Cut-on cut-off transition of acoustic modes in hard-walled ducts with irrotational mean flow is well understood. However, previous finite-element simulations of this phenomenon appear to indicate the possibility of energy scattering
into neighbouring modes at large Helmholtz numbers. In this thesis, an attempt is made to explain such scattering phenomena in slowly varying aero-engine ducts using multiple-scales techniques.

In order to model modal scattering a good understanding of cut-on cut-off transition is necessary. Here, the well known single turning point is revisited, and our understanding of cut-on cut-off transition is extended to include an
analysis of a double turning point. Then using a similar apparatus, modal scattering in the case where a mode undergoes cut-on cut-off transition is investigated. It is found that, for sufficiently high frequencies, a mechanism
exists whereby a propagating incident mode can be scattered into neighbouring modes provided that a mean flow exists within the duct. An asymptotic analysis of this mechanism is presented and, by solving numerically a composite
solution, results in a duct of rectangular cross section are obtained. The energy distribution of the incident and neighbouring scattered modes reveals an interaction and exchange of energy with the mean flow. This work now allows greater insight as well as more accurate and fast computations of high frequency mode propagation in slowly-varying hard walled ducts using multiple-scales approaches.

An experimental study of discharge partitioning and flow structure at symmetrical bifurcations

by Wouter Marra

Robert E. Thomas, Daniel R. Parsons, Steven D. Sandbach, Gareth M. Keevil, Wouter A. Marra, Richard J. Hardy, James L. Best, Stuart N. Lane & Jessica A. Ross, 2011, Earth Surface Processes and Landforms Special Issue on River Bifurcations.

Recent research has examined the factors controlling the geometrical configuration of bifurcations, determined the... more Recent research has examined the factors controlling the geometrical configuration of bifurcations, determined the range of stability conditions for a number of bifurcation types and assessed the impact of perturbations on bifurcation evolution. However, the flow division process and the parameters that influence flow and sediment partitioning are still poorly characterised. To identify and isolate these parameters, three-dimensional velocities were measured at 11 cross-sections in a fixed-walled experimental bifurcation. Water surface gradients were controlled, and systematically varied, using a weir in each distributary.

As may be expected, the steepest distributary conveyed the most discharge (was dominant) while the mildest distributary conveyed the least discharge (was subordinate). A zone of water surface super-elevation was co-located with the bifurcation in symmetric cases or displaced into the subordinate branch in asymmetric cases. Downstream of a relatively acute-angled bifurcation, primary velocity cores were near to the water surface and against the inner banks, with near-bed zones of lower primary velocity at the outer banks. Downstream of an obtuse-angled bifurcation, velocity cores were initially at the outer banks, with near-bed zones of lower velocities at the inner banks, but patterns soon reverted to match the acute-angled case. A single secondary flow cell was generated in each distributary, with water flowing inwards at the water surface and outwards at the bed. Circulation was relatively enhanced within the subordinate branch, which may help explain why subordinate distributaries remain open, may play a role in determining the size of commonly-observed topographic features, and may thus exert some control on the stability of asymmetric bifurcations. Further, because larger values of circulation result from larger gradient disadvantages, the length of confluence-diffluence units in braided rivers or between diffluences within delta distributary networks may vary depending upon flow structures inherited from upstream and whether, and how, they are fed by dominant or subordinate distributaries.

Numerical Analysis of Air-Flow and Temperature Field in a Passenger Car Compartment

by Nazri Kamsah

Authors: H M Kamar, N Kamsah, and A M Mohammad Nor

Submitted to the International Meeting on Advances in Thermo-Fluid 2011. The conference will be held on 4th October 2011, in Malacca, Malaysia.

This paper presents a numerical study on the temperature field inside a passenger’s compartment of a Proton Wira... more This paper presents a numerical study on the temperature field inside a passenger’s compartment of a Proton Wira saloon car using computational fluid dynamics (CFD) method. The main goal is to investigate the effects of different glazing types applied onto the front and rear windscreens of the car on the distribution of air-temperature inside the passenger compartment in the steady-state conditions. The air-flow condition in the passenger’s compartment is also investigated. Fluent CFD software was used to develop a three-dimensional symmetrical model of the passenger’s compartment. Simplified representations of the driver and one rear passenger were incorporated into the CFD model of the passenger’s compartment. Two types of glazing were considered namely clear insulated laminated tint (CIL) with a shading coefficient of 0.78 and green insulated laminate tint (GIL) with a shading coefficient of 0.5. Results of the CFD analysis were compared with those obtained when the windscreens are made up of clear glass having a shading coefficient of 0.86. Results of the CFD analysis show that for a given glazing material, the temperature of the air around the driver is slightly lower than the air around the rear passenger. Also, the use of GIL glazing material on both the front and rear windscreens significantly reduces the air temperature inside the passenger’s compartment of the car. This contributes to a better thermal comfort condition to the occupants. Swirling air flow condition occurs in the passenger compartment. The air-flow intensity and velocity are higher along the side wall of the passenger’s compartment compared to that along the middle section of the compartment. It was also found that the use of glazing materials on both the front and rear windscreen has no significant effects on the air-flow condition inside the passenger’s compartment of the car.

Direct and large eddy simulation of stratified turbulence

by Sebastian Remmler

presented at the 7th International Symposium on Turbulence and Shear Flow Phenomena (Ottawa, Canada)

Simulation of geophysical turbulent flows requires a robust and accurate subgrid-scale turbulence modeling. We propose... more Simulation of geophysical turbulent flows requires a robust and accurate subgrid-scale turbulence modeling. We propose an implicit subgrid-scale model for stratified fluids, based on the Adaptive Local Deconvolution Method. To validate this turbulence model, we performed direct numerical simulations of the transition of the three-dimensional Taylor--Green vortex and homogeneous stratified turbulence. Our analysis proves that the implicit turbulence model correctly predicts the turbulence energy budget and the spectral structure of stratified turbulence.

On a modified Monte-Carlo method and variable soft sphere model for rarefied binary gas mixture flow simulation

by Pouyan Jahangiri

The effect of new terms in the improved algorithm, the modified direct simulation Monte-Carlo (MDSMC) method, is... more The effect of new terms in the improved algorithm, the modified direct simulation Monte-Carlo (MDSMC) method, is investigated by simulating a rarefied binary gas mixture flow inside a rotating cylinder. Dalton law for the partial pressures contributed by each species of the binary gas mixture is incorporated into our simulation using the MDSMC method and the direct simulation Monte-Carlo (DSMC) method. Moreover, the effect of the exponent of the cosine of deflection angle (a) in the inter molecular collision models, the variable soft sphere (VSS) and the variable hard sphere (VHS), is investigated in our simulation. The improvement of the results of simulation is pronounced using the MDSMC method when compared with the results of the DSMC method. The results of simulation using the VSS model show some improvements on the result of simulation for the mixture temperature at radial distances close to the cylinder wall where the temperature reaches the maximum value when compared with the results using the VHS model.

Broadband trailing edge noise from a sharp-edged strut

by Con Doolan

Broadband trailing edge noise from a sharp-edged strut
Danielle J. Moreau, Laura A. Brooks, and Con J. Doolan, J. Acoust. Soc. Am. 129, 2820 (2011), DOI:10.1121/1.3569698

This paper presents experimental data concerning the flow and noise generated by a sharp-edged flat plate at... more This paper presents experimental data concerning the flow and noise generated by a sharp-edged flat plate at low-to-moderate Reynolds number (Reynolds number based on chord of 2.0 × 105 to 5.0 × 105). The data are used to evaluate a variety of semi-empirical trailing edge noise prediction methods. All were found to under-predict noise at lower frequencies. Examination of the velocity spectra in the near wake reveals that there are energetic velocity fluctuations at low frequency about the trailing edge. A semi-empirical model of the surface pressure spectrum is derived for predicting the trailing edge noise at low-to-moderate Reynolds number.

3D SBLI in Supersonic Corners

by W. Ethan Eagle