Gregory J Sheard

Monash University (Australia) · Department of Mechanical and Aerospace Engineering, Clayton

The natural convective Cu-water nanofluid flow in l-shape cavity with an oscillating temperature profile is studied numerically. The cavity's lower horizontal and left vertical walls are heated sinusoidally with time about a high mean temperature (T¯H). In contrast, the cavity's right vertical wall and its nearby horizontal lower wall are kept cold...

Motivated by an interest in inciting instabilities and mixing for heat transfer enhancement in ducts, flow in a channel with repeated wedge-shaped protrusions is considered for various blockage ratios (wedge height to duct height), pitch (distance between wedges) and wedge angles. The stability of the two-dimensional base flow and its dependence on...

An entropy generation analysis is conducted for horizontal convection under the centrifugal buoyancy and the Boussinesq approximations in a relatively shallow enclosure at a fixed Prandtl number of unity to characterise irreversible losses across the conduction and convection-dominated regimes using the Bejan number (Be). A variable irreversibility...

A linear stability analysis is conducted for horizontal natural convection under a Gay-Lussac (GL) type approximation in a relatively shallow enclosure cavity. The GL type approximation is developed based on extending density variations to the advection term as well as gravity term through the momentum equation. Such a treatment invokes the GL para...

The transition to turbulence in ducts, pipes or other conduits is a crucial phenomenon. It determines the energy consumption and heat or mass exchange in countless processes: whether cooling circuits of heat exchangers, pipelines or chemical reactors to cite but a few. The transition occurs at relatively low flow rates as a response to perturbation...

The efficiency of photovoltaic (PV) cells is known to degrade with temperature, which limits their efficacy in the many regions around the world having a climate featuring high ambient temperatures. This study reports on field experiments supported by numerical modelling using Ansys steady-state thermal solver that demonstrates improved PV cell per...

Under the Boussinesq approximation for buoyancy driven flows, density variations are restricted to the gravity term. In contrast, the Gay-Lussac (GL) approach is developed based on considering density variations in any term of the Navier—Stokes equations in which density appears. In both incompressible approaches, a linear density state equation is...

The Gay‐Lussac (GL) approach is an incompressible‐based strategy for non‐Boussinesq treatment of the governing equations for free convection problems that is established based on extending the density variations beyond the gravity term. Such a strategy leads to emerging the GL parameter as a non‐Boussinesq pre‐factor of different terms in the gover...

Repeated flow-facing wedge-shaped protrusions are explored as a means to enhance the heat transfer behaviour in a quasi-two-dimensional magnetohydrodynamic duct flow. Quasi-two-dimensionality is achieved under a strong magnetic field which are prevalent in the cooling blankets of fusion reactors. Ranges of Hartmann friction parameters (50 ≤ H ≤ 500...

The well-known Boussinesq (also known as Oberbeck—Boussinesq) approximation is still the most common approach for the numerical simulation of natural convection problems. However, the accurate performance of this approximation is mainly restricted by small temperature differences. This encourages researchers and engineers to use other approaches be...

The stability of a pulsatile quasi-two-dimensional duct flow was numerically investigated. Flow was driven, in concert, by a constant pressure gradient and by the synchronous oscillation of the lateral walls. This prototypical setup serves to aid understanding of unsteady magnetohydrodynamic flows in liquid metal coolant ducts subjected to transver...

This manuscript has been accepted for publication in Physical Review Fluids, see https://journals.aps.org/prfluids/accepted/53075Se8O0b1b109b1cc0061b280aaa122f0f92dc. The stability of a pulsatile quasi-two-dimensional duct flow was numerically investigated. Flow was driven, in concert, by a constant pressure gradient and by the synchronous oscillat...

In this study, a benchmark natural convection problem is studied under a Gay‐Lussac type approximation incorporating centrifugal effects in the context of a new vorticity‐stream‐function approach. This approximation differs from the classic Boussinesq approximation in that density variations are considered in the advection term as well as the gravi...

This work investigates the mechanisms that underlie transitions to turbulence in a three-dimensional domain in which the variation of flow quantities in the out-of-plane direction is much weaker than any in-plane variation. This is achieved using a model for the quasi-two-dimensional magnetohydrodynamic flow in a duct with moving lateral walls and...

This manuscript has been accepted for publication in Physical Review Fluids, see https://journals.aps.org/prfluids/accepted/d5074S28J6b11905012b7cb06505e8f2149dd5f20. This work investigates the mechanisms that underlie transitions to turbulence in a three-dimensional domain in which the variation of flow quantities in the out-of-plane direction is...

A subcritical route to turbulence via purely quasi-two-dimensional mechanisms, for a quasi-two-dimensional system composed of an isolated exponential boundary layer, is numerically investigated. Exponential boundary layers are highly stable and are expected to form on the walls of liquid metal coolant ducts within magnetic confinement fusion reacto...

The link to the online abstract of this manuscript, accepted in Phys. Rev. Fluids, is https://journals.aps.org/prfluids/accepted/32074S4aH8b1c608e19768b42571f9001086a3f44. A subcritical route to turbulence via purely quasi-two-dimensional mechanisms, for a quasi-two-dimensional system composed of an isolated exponential boundary layer, is numerical...

Traditionally, the Boussinesq approximation is adopted for numerical simulation of natural convection phenomena where density variations are supposed negligible except through the gravity term of the momentum equation. In this study, a recently developed formulation based on a Boussinesq approximation is presented in which the density variations ar...

Solar panels that use photovoltaic (PV) cells are popular for converting solar radiation into electricity. One of the major problems impacting the performance of PV panels is the overheating caused by excessive solar radiation and high ambient temperatures, which degrades the efficiency of the PV panels remarkably. To overcome this issue, an alumin...

Horizontal convection has been used as an idealised model of the ocean overturning circulation, where some non-uniform buoyancy forcing profile is imposed along a horizontal boundary. Several different driving temperature profiles have been chosen for past numerical and laboratory studies, likely for convenience, yet the effect of the shape of the...

Horizontal convection is a distinct type of natural convection, where the flow is driven by non-uniform buoyancy supplied along a horizontal boundary. Horizontal convection has been studied in enclosures with aspect ratios (ratio of enclosure height H to length L) down to O(10⁻¹), though the flow dynamics and heat transfer in very shallow enclosure...

Natural convection in a square cavity filled with electrically conducting nanofluid that is driven by a periodic temperature profile along one of the vertical wall is studied numerically. The top and bottom horizontal walls are kept adiabatic. The right wall is maintained at low temperature while the temperature of the opposing vertical wall varies...

This study seeks to elucidate the linear transient growth mechanisms in a uniform duct with square cross-section applicable to flows of electrically conducting fluids under the influence of an external magnetic field. A particular focus is given to the question of whether at high magnetic fields purely two-dimensional mechanisms exist, and whether...

This study seeks to elucidate the linear transient growth mechanisms in a uniform duct with square cross-section applicable to flows of electrically conducting fluids under the influence of an external magnetic field. A particular focus is given to the question of whether at high magnetic fields purely two-dimensional mechanisms exist, and whether...

The heat transfer from the side-wall of a duct through which an electrically conducting fluid flows within a strong transverse magnetic field is numerically investigated using high-resolution numerical simulation. Parameter ranges considered are 0≤Ha≤2400 and 100≤Re≤3000 for a constant blockage ratio of 1/4. The gain in the heat transfer using obst...

The stability of the wakes of cylinders with triangular cross-sections at incidence is investigated using Floquet stability analysis to elucidate the effects of cylinder inclination on the dominant flow instability. The upper limit of the Reynolds numbers (scaled by the height projected by the cylinder in this study) at which the wake of the two-di...

The beating heart is known to produce pressure and airflow oscillations in the lungs of mammals. This phenomenon is often disregarded as detailed measurement of its effects in the lung have hitherto not been possible. Previous studies have attempted to measure the effect of these oscillations on gas mixing. However, the results have proven inconclu...

In this paper two-dimensional numerical investigation of the scaling and dynamics at the onset of horizontal convection (HC) is studied. Transient features during the onset of horizontal convection was studied to characterise the different flow features observed during the initial stages of development. A thorough characterisation of the transient...

This study seeks to characterise the breakdown of the steady two-dimensional solution in the flow around a 180-degree sharp bend to infinitesimal three-dimensional disturbances using a linear stability analysis. The stability analysis predicts that three-dimensional transition is via a synchronous instability of the steady flows. A highly accurate...

This study considers the linear stability of Poiseuille-Rayleigh-Bénard flows subjected to a transverse magnetic field, to understand the instabilities that arise from the complex interaction between the effects of shear, thermal stratification, and magnetic damping. This fundamental study is motivated in part by the desire to enhance heat transfer...

Horizontal convection driven by a linear temperature profile along the bottom of a two-dimensional rectangular enclosure is perturbed by a small tuned synthetic (zero-net-mass-flux/ZNMF) jet. The jet permits instability in the horizontal convection boundary layer to be investigated in a controlled manner. At a Prandtl number Pr = 6.14 and Rayleigh...

The linear stability of a magnetohydrodynamic duct flow with heating from below is investigated and serves as an extension to the class of Poiseuille–Rayleigh–Bénard flows. Such flows can be found in the blankets of nuclear fusion reactors and have been of great interest in recent times due to the endeavour of demonstrating the viability of nuclear...

The wakes of flows past bluff bodies are known to undergo several transitions leading to turbulence, each incurring changes to the dynamics of the flow. The initial transition to three-dimensional flow for several cylindrical geometries manifests as coherent structures along the cylinder span and have been the focus of many investigations. This tra...

The fluid dynamics and heat transfer characteristics of magnetohydrodynamic duct flow often degrade the laminarization caused by the magnetic field. The present work evaluates the performance of a system featuring alternating current injection from a point electrode as a vortex promoter for enhancement of the thermal-hydraulic characteristics. It i...

The effect of rotation on horizontal convection in a cylindrical enclosure is investigated numerically. The thermal forcing is applied radially on the bottom boundary from the coincident axes of rotation and geometric symmetry of the enclosure. First, a spectral element method is used to obtain axisymmetric basic flow solutions to the time-dependen...

The design of vortex promoters in a heated-wall duct is often limited by the considerations of practicality, especially in complex systems such as fusion blankets. The present study investigates the use of current injection to invoke a street of vortices in quasi-two-dimensional high transverse magnetic field magnetohydrodynamic duct flows to enhan...

The non-axisymmetric structure of an unstable Stewartson shear layer generated via a differential rotation between flush disks and a cylindrical enclosure is investigated numerically using both three-dimensional direct numerical simulation and a quasi-two-dimensional model. Previous literature has only considered the depth-independent quasi-two-dim...

A vortex decay model for predicting spatial evolution of peak vorticity in a wake behind a cylinder is presented. For wake vortices in the stable region behind the formation region, results have shown that the presented model has a good capability of predicting spatial evolution of peak vorticity within an advecting vortex across 0.1 ≤ β ≤ 0.4, 500...

The structure and stability of Stewartson shear layers with different heights are investigated numerically via axisymmetric simulation and linear stability analysis, and a validation of the quasi-two-dimensional model is performed. The shear layers are generated in a rotating cylindrical tank with circular disks located at the lid and base imposing...

A vortex decay model for predicting temporal evolution of peak vorticity in a wake behind a cylinder is presented. Where the wake vortices are in stable region, results have shown that the correlation has a good capability of predicting temporal evolution of peak vorticity within an advecting vortex across a wide range of flow parameters. The corre...

A confined laminar viscous flow past a two-dimensional bluff body in the presence of a strong uniform magnetic field is considered. The effects of Reynolds number (Re) and Hartmann number (Ha) on the dynamics of the wake are examined, with a focus on the shedding frequency and the distribution of the wake vortices. These two parameters are of prima...

The effect of rotation on the three-dimensional instability of a horizontal convection flow in a free-surface cylindrical enclosure driven by a radially increasing temperature over the base is numerically investigated using a linear stability analysis. The results demonstrate that higher rotation rates serve to inhibit the onset of non-axisymmetric...

This study considers the quasi-two-dimensional flow of an electrically conducting fluid subjected to a strong out-of-plane magnetic field in a rectangular duct. The effect of Hartmann number on flow features such as the length of the downstream recir-culation bubbles and the threshold Reynolds numbers between steady-state and unsteady flow regimes...

This study seeks to characterise the stability of a two-dimensional channel flow involving a 180-degree sharp bend, to infinitesimal three-dimensional disturbances by way of a linear stability analysis. A highly accurate global linear stability analysis of the flow is presented via the Reynolds number Re varies in the range 100 ≤ Re ≤ 700, this Re...

Heat transfer efficiency from a duct side-wall in which an electrically conducting fluid flows under the influence of a transverse magnetic field is investigated in the present work. The study intends to provide useful guidelines to improve efficiency of MHD cooling systems while also taking into account that practical designs should be both therma...

Horizontal convection has been used as an idealised model of the ocean overturning circulation, where some non-uniform buoyancy forcing profile is imposed along a horizontal boundary. A high order idealised numerical model is used to investigate the effect of different temperature forcing on the overturning circulation. Time independent, periodic a...

Red blood cell (RBC) deformation is a dominant factor in the rheological properties of blood in vessels smaller than 300 micrometers. The extensional flow within a microfluidic cross-slot microchannel has been proposed as a mechanism for measuring the deformation of cells. Three-dimensional simulations of red blood cell deformation in a microfluidi...

The generation of distinct polygonal configurations via the instability of a Stewartson shear layer is numerically investigated. The shear layer is induced using a rotating cylindrical tank with differentially forced disks located at the top and bottom boundaries. The incompressible Navier-Stokes equations are solved on a two-dimensional semi-merid...

An approach is studied for side-wall heat transfer enhancement in the magnetohydrodynamic flow of fluid in a rectangular duct that is damped by a strong transverse magnetic field. The mechanism employs the rotational oscillation of a cylinder placed inside the duct to encourage vortex shedding, which promotes the mixing of fluid near a hot duct wal...

The stability of a rotating system differentially forced at the lid and base of a cylindrical enclosure is numerically investigated. The shear layer arising from the angular velocity discrepancy at the disk-tank interface becomes unstable beyond a threshold of two governing parameters known as the Rossby number Ro, and Ekman number Ek. Beyond the t...

A numerical study on a shear layer produced via a differential-disk rotation system reveals that the flow is susceptible to instabilities under certain conditions. The incompressible Navier–Stokes equations are solved on a two-dimensional meridional semi-plane. The steady-state axisymmetric base flows demonstrate strong depth-independence and symme...

A fluid dynamics video is linked to this article, which have been submitted to the Gallery of Fluid Motion as part of the 65th American Physical Society meeting of the Division of Fluid Dynamics, held in San Diego, California, USA, over 17-20 November 2012. The video serves to visualize flows generated in a rectangular enclosure that are subjected...

Particle image velocimetry (PIV) selects the maximum of the cross-correlation map to represent the modal displacement, and a wealth of information stored in the cross-correlation is discarded. We introduce a novel method, termed polynomial element velocimetry (PEV), which results in continuous velocity and velocity gradient measurements. PEV utiliz...

A novel system to image and reconstruct a 3-dimensional map of the refractive index based on the diffraction of light through a transparent sample is presented. This method is tested and validated on computer-generated data sets. The proposed system is an advanced variation of an imaging technique used in engineering for the study of aerodynamics....

A Batchelor vortex represents the asymptotic solution of a trailing vortex in an aircraft wake. In this study, an unequal-strength, counter-rotating Batchelor vortex pair is employed as a model of the wake emanating from one side of an aircraft wing; this model is a direct extension of several prior investigations that have considered unequal-stren...

The flow of blood past an axisymmetric thrombus analogue, within an in vitro geometry, is computed via solution of the discrete three-dimensional (3D) Navier-Stokes equations. Particle tracking is used to model the behaviour of thrombocytes (platelets) moving throughout the domain and to investigate behaviour with respect to the platelets. The syst...

The transient response of optimal linear perturbations of liquid metal flow under a strong axial magnetic field in an electrically insulated rectangular duct is considered. The focus is on the subcritical regime, below the onset of von Karman vortex shedding, to determine the role of optimal disturbances in developing wake instabilities. In this co...

This paper examines the shock cell structure, vorticity and velocity field at the exit of an underexpanded jet nozzle using a hydraulic analogy and the Reference Image Topography technique. Understanding the flow in this region is important for the mitigation of screech, an aeroacoustic problem harmful to aircraft structures. Experiments are conduc...