ORCID Profile
0000-0001-5255-8741
Current Organisation
The University of Auckland
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Publisher: ASME International
Date: 04-2011
DOI: 10.1115/1.4003875
Abstract: For high Reynolds number flows the total pressure remains constant along stream lines. At low Reynolds numbers the total pressure decreases in a global sense due to the actions of viscosity, but it may increase locally in regions such as stagnation points. Previous studies have considered the case of constant viscosity flow. However, gradients in the effective viscosity can occur normal to the wall for the flow of lubricating oils, and for turbulent flows calculated using an eddy viscosity model. In this paper the effect of these viscosity gradients on the stagnation point pressure are examined.
Publisher: Elsevier BV
Date: 07-2013
Publisher: Elsevier BV
Date: 09-2018
Publisher: Elsevier BV
Date: 10-2019
Publisher: Elsevier BV
Date: 12-2018
Publisher: Wiley
Date: 05-07-2013
DOI: 10.1002/WE.1525
Publisher: Cambridge University Press (CUP)
Date: 14-09-2020
DOI: 10.1017/JFM.2020.621
Publisher: American Institute of Aeronautics and Astronautics
Date: 09-01-2012
DOI: 10.2514/6.2012-538
Publisher: American Institute of Aeronautics and Astronautics
Date: 09-01-2012
DOI: 10.2514/6.2012-536
Publisher: Cambridge University Press (CUP)
Date: 30-08-2013
DOI: 10.1017/JFM.2013.392
Abstract: This study considers the convective-type instability of the near-field flow of a planar, pure thermal plume with a finite area source. Previous studies revealed the existence of an off-axis thermal boundary-layer instability, driving a puffing instability in the central ascending column, and qualitatively showed correlations between instabilities in these two flow regions. This paper extends the analysis to examine the effect of Prandtl number on transitional near-field behaviours and reports on the stability characteristics of a near-field, pure thermal plume based on a direct stability analysis. The variations in flow behaviours in response to symmetric and asymmetric disturbances suggest the existence of coupled instability mechanisms in the off-axis thermal boundary layer and the central ascending column.
Publisher: Wiley
Date: 19-03-2013
DOI: 10.1002/FLD.3789
Publisher: Wiley
Date: 03-2011
DOI: 10.1002/FLD.2234
Publisher: Elsevier BV
Date: 09-2015
Publisher: Elsevier BV
Date: 06-2013
Publisher: The Company of Biologists
Date: 15-11-2010
DOI: 10.1242/JEB.040741
Abstract: Blind Mexican cave fish (Astyanax fasciatus) sense the presence of nearby objects by sensing changes in the water flow around their body. The information available to the fish using this hydrodynamic imaging ability depends on the properties of the flow field it generates while gliding and how this flow field is altered by the presence of objects. Here, we used particle image velocimetry to measure the flow fields around gliding blind cave fish as they moved through open water and when heading towards a wall. These measurements, combined with computational fluid dynamics models, were used to estimate the stimulus to the lateral line system of the fish. Our results showed that there was a high-pressure region around the nose of the fish, low-pressure regions corresponding to accelerated flow around the widest part of the body and a thick laminar boundary layer down the body. When approaching a wall head-on, the changes in the stimulus to the lateral line were confined to approximately the first 20% of the body. Assuming that the fish are sensitive to a certain relative change in lateral line stimuli, it was found that swimming at higher Reynolds numbers slightly decreased the distance at which the fish could detect a wall when approaching head-on, which is the opposite to what has previously been expected. However, when the effects of environmental noise are considered, swimming at higher speed may improve the signal to noise ratio of the stimulus to the lateral line.
Publisher: Wiley
Date: 13-02-2007
DOI: 10.1002/FLD.1425
Publisher: Wiley
Date: 12-04-2013
DOI: 10.1002/WE.1501
Publisher: American Geophysical Union (AGU)
Date: 12-2012
DOI: 10.1029/2012WR012218
Publisher: Cambridge University Press (CUP)
Date: 21-10-2021
DOI: 10.1017/JFM.2021.839
Abstract: The present study concerns a temporally evolving turbulent natural convection boundary layer (NCBL) adjacent to an isothermally heated vertical wall, with Prandtl number 0.71. Three-dimensional direct numerical simulations (DNS) are carried out to investigate the turbulent flow up to $\\textit {Gr}_\\delta =1.21\\times 10^8$ , where $\\textit {Gr}_\\delta$ is the Grashof number based on the boundary layer thickness $\\delta$ . In the near-wall region, there exists a constant heat flux layer, similar to previous studies for the spatially developing flows (e.g. George & Capp, Intl J. Heat Mass Transfer , vol. 22, 1979, pp. 813–826). Beyond a wall-normal distance $\\delta _i$ , the NCBL can be characterised as a plume-like region. We find that this region is well described by a self-similar integral model with profile coefficients (cf. van Reeuwijk & Craske, J. Fluid Mech. , vol. 782, 2015, pp. 333–355) which are $\\textit {Gr}_\\delta$ -independent after $\\textit {Gr}_\\delta =10^7$ . In this Grashof number range both the outer plume-like region and the near-wall boundary layer are turbulent, indicating the beginning of the so-called ultimate turbulent regime (Grossmann & Lohse, J. Fluid Mech. , vol. 407, 2000, pp. 27–56 Grossmann & Lohse, Phys. Fluids , vol. 23, 2011, 045108). Solutions to the self-similar integral model are analytically obtained by solving ordinary differential equations with profile coefficients empirically obtained from the DNS results. In the present study, we have found the wall heat transfer of the NCBL is directly related to the top-hat scales which characterise the plume-like region. The Nusselt number is found to follow $\\textit {Nu}_\\delta \\propto \\textit {Gr}_\\delta ^{0.381}$ , slightly higher than the empirical $1/3$ -power-law correlation reported for spatially developing NCBLs at lower $\\textit {Gr}_\\delta$ , but is shown to be consistent with the ultimate heat transfer regime with a logarithmic correction suggested by Grossmann & Lohse ( Phys. Fluids , vol. 23, 2011, 045108). By modelling the near-wall buoyancy force, we show that the wall shear stress would scale with the bulk velocity only at asymptotically large Grashof numbers.
Publisher: Wiley
Date: 06-03-2015
DOI: 10.1002/WE.1722
Publisher: Elsevier BV
Date: 10-2011
Publisher: EDP Sciences
Date: 2018
DOI: 10.1051/E3SCONF/20184005028
Abstract: In this study we investigate the influence of grid resolution on a near-wall resolved LES model of a lock-exchange particle-driven gravity current. The simulations are performed using the finite volume Boussinesq code SnS with a Smagorinsky turbulence model for a buoyant Reynolds number of 60,000 on 4 grid sizes. According to previous studies, two-point correlations are most appropriate to estimate LES resolution. With the largest scales of the flow being resolved by more than 20 cells, well-resolved LES is obtained for grid resolutions of 1925×62×125 and finer. In addition, in order to apply the turbulence model correctly, we show that the velocity power spectrum densities provide useful information for the maximum cell size. The ratio of the subgrid scale viscosity to the molecular viscosity and the subgrid scale shear-stress to the resolved Reynolds stress show good convergence with grid refinement. The ratios v SGS / .3 above the current and τ SGS (u'v') ave 0.05 inside the mixing layer, are chosen as threshold values, based on our evaluation study.
Publisher: The Company of Biologists
Date: 15-11-2010
DOI: 10.1242/JEB.040790
Abstract: Blind Mexican cave fish (Astyanax fasciatus) are able to sense detailed information about objects by gliding alongside them and sensing changes in the flow field around their body using their lateral line sensory system. Hence the fish are able to build hydrodynamic images of their surroundings. This study measured the flow fields around blind cave fish using particle image velocimetry (PIV) as they swam parallel to a wall. Computational fluid dynamics models were also used to calculate the flow fields and the stimuli to the lateral line sensory system. Our results showed that characteristic changes in the form of the flow field occurred when the fish were within approximately 0.20 body lengths (BL) of a wall. The magnitude of these changes increased steadily as the distance between the fish and the wall was reduced. When the fish were within 0.02 BL of the wall there was a change in the form of the flow field owing to the merging of the boundary layers on the body of the fish and the wall. The stimuli to the lateral line appears to be sufficient for fish to detect walls when they are 0.10 BL away (the mean distance at which they normally swim from a wall), but insufficient for the fish to detect a wall when 0.25 BL away. This suggests that the nature of the flow fields surrounding the fish are such that hydrodynamic imaging can only be used by fish to detect surfaces at short range.
Publisher: Royal Institution of Naval Architects
Date: 12-2016
DOI: 10.3940/RINA.IJSCT.2016.B2.156
Abstract: This paper investigates an inverse process for the design of yacht sails. The method is described and then applied to the design of optimal sails for a specific yacht. The proposed inverse method generates the three-dimensional shapes of a headsail and mainsail from prescribed loading (i.e. differential pressure) distributions, accounts for the effect of the sea surface, and also simulates the twist and shear of the incoming flow. The uncoupled iterative routine solves a sequence of analysis steps so that the sail shapes are deformed in such a way that their updated loading distributions converge to the specified target distributions. During each iteration equations derived from two-dimensional Thin Aerofoil Theory, calculate a geometry correction from the difference between the current and target loading distributions. This correction is applied to the sail geometry, and a vortex lattice method code calculates the updated three-dimensional differential pressure distributions, which are again compared to the target distributions. Usually only five iterations are required to converge to sail shapes that have the target loading distributions. The inverse method has been validated by inverting the traditional way of analysing sails, i.e. a set of sails with known geometry has been analysed and the loading distributions on the headsail and mainsail were calculated. These distributions were then used as an input for the inverse code. It was found that the difference in camber between the original sails and the calculated geometry is less than 0.01% of camber at the mid-span of the sails. The second part of the paper presents two methods for the design of optimal sails for a yacht. One of the methods uses the more traditional analysis approach, while the other employs the inverse method described in this paper. The optimisation is performed for a Transpac 52 yacht in 12 knots (6.5 m/s) of true wind speed to obtain the best velocity made good. Results from both methods are presented and discussed and it is found that the results in terms of boat speed are similar although the trims differ slightly. However, the new inverse method is approximately nine times faster than the traditional analysis approach.
Publisher: Cambridge University Press (CUP)
Date: 24-07-2023
DOI: 10.1017/JFM.2023.521
Abstract: This study investigates the coherence of turbulent fluctuations in a turbulent vertical natural convection boundary layer immersed in a stably stratified medium (turbulent buoyancy layer). A turbulent buoyancy layer of a fluid having a Prandtl number of $0.71$ at a Reynolds number of $800$ is numerically simulated using direct numerical simulation. The two-point correlations reveal that the streamwise velocity fluctuations are coherent over large streamwise distances, with the length scale of the streamwise coherence being greater than the boundary layer thickness. This is due to large-scale motions (LSMs), similar to the LSMs observed in canonical wall-bounded turbulence despite the stark differences in flow dynamics. Both high-speed (positive) and low-speed (negative) streamwise velocity fluctuations form LSMs, with their streamwise length scales increasing with increasing wall-normal distance. High-speed LSMs are composed of upwash flow with high temperatures, while low-speed LSMs are composed of downwash flow with low temperatures. Both high-speed and low-speed LSMs meander appreciably in the streamwise direction, with the degree of meandering being correlated with the sign of the spanwise velocity fluctuations. The LSMs exhibit coherence across significant wall-normal distances and contribute significantly to the turbulence production in the outer layer. Examining the one-dimensional energy spectra of the turbulent buoyancy layer shows that the LSMs are the dominant energy-containing motions, implying that the length scale of the energy-containing range is of the order of boundary layer thickness. Notably, wall-normal velocity, spanwise velocity and buoyancy fluctuations do not form LSMs with streamwise length scales comparable to streamwise velocity fluctuations.
Publisher: IOP Publishing
Date: 16-06-2014
Publisher: Elsevier BV
Date: 12-2016
Publisher: Elsevier BV
Date: 06-2013
Publisher: Elsevier BV
Date: 02-2018
Publisher: IWA Publishing
Date: 22-06-2018
DOI: 10.2166/WST.2018.286
Abstract: Existing studies on sediment retention ponds (SRPs) have examined the effects of pond layout, inlet and outlet geometry and the installation of baffles on the performance of the SRPs. However, the effects of a temperature difference between the ambient water in the pond and the inflow are often neglected, and the buoyancy forces arising from these differences in temperature can potentially change the flow in the pond and hence its hydraulic performance. This study has experimentally evaluated the effect of these temperature differences on the flow field and residence time in a retention pond for a range of temperature differences. As expected a cold inflow sinks to the bottom of the pond while a hot inflow remains at the surface, but in both cases the inflow flows more rapidly towards the outlet than is the case for isothermal inflow. A counter-current was observed at the bottom or the surface of the pond for colder or hotter influents, respectively. These thermally induced flows significantly reduced the residence time of the pond, reducing the hydraulic performance of the pond and causing severe short-circuiting. The results have also shown that the temperature differences in the pond decrease with time, yet small temperature differences persist with the pond remaining thermally stratified.
Publisher: Cambridge University Press (CUP)
Date: 09-02-2015
DOI: 10.1017/JFM.2014.711
Abstract: Direct numerical simulations (DNS) of turbulent stratified flow in an open channel with an internal heat source following the Beer–Lambert law from the surface are used to investigate the transition from neutral to strongly stable flow. Our buoyancy bulk parameter is defined through the ratio of the domain height ${\\it\\delta}$ to $\\mathscr{L}$ , a bulk Obukhov length scale for the flow. We cover the range ${\\it\\lambda}={\\it\\delta}/\\mathscr{L}=0{-}2.0$ , from neutral conditions to the onset of the stable regime, with the Reynolds number range $Re_{{\\it\\tau}}=200{-}800$ , at a Prandtl number of 0.71. The result is a boundary layer flow where the effects of stratification are weak in the wall region but progressively stronger in the outer layer up to the free surface. At ${\\it\\lambda}\\simeq 1$ the turbulent kinetic energy (TKE) budget is in local equilibrium over a region extending from the near-wall region to a free-surface affected region a distance $l_{{\\it\\nu}}$ from the surface, with $l_{{\\it\\nu}}/{\\it\\delta}\\sim Re^{-1/2}$ . In this equilibrium region the flow can be characterised by the flux Richardson number $R_{f}$ and the local Obukhov length scale ${\\it\\Lambda}$ . At higher ${\\it\\lambda}$ local mixing limit conditions are observed over an extended region. At ${\\it\\lambda}=2$ the flux Richardson number approaches critical limit values of $R_{f,c}\\simeq 0.18$ and gradient Richardson number $Ri_{c}\\simeq 0.2$ . At high ${\\it\\lambda}$ , we obtain a flow field where buoyancy interacts with the smallest scales of motion and the turbulent shear stress and buoyancy flux are suppressed to molecular levels. We find that this regime can be identified in terms of the parameter $Re_{\\mathscr{L},c}=\\mathscr{L}u_{{\\it\\tau}}/{\\it\\nu}\\lesssim 200{-}400$ (where $u_{{\\it\\tau}}$ is the friction velocity and ${\\it\\nu}$ the kinematic viscosity), which is related to the $L_{\\ast }$ parameter of Flores and Riley ( Boundary-Layer Meteorol. , vol. 139 (2), 2011, pp. 241–259) and buoyancy Reynolds number $\\mathscr{R}$ . With energetic equilibrium attained, the local buoyancy Reynolds number, $Re_{{\\it\\Lambda}}={\\it\\Lambda}\\langle u^{\\prime }w^{\\prime }\\rangle ^{1/2}/{\\it\\nu}$ , is directly related to the separation of the Ozmidov ( $l_{O}$ ) and Kolmogorov ( ${\\it\\eta}$ ) length scales in the outer boundary layer by $Re_{{\\it\\Lambda}}\\simeq \\mathscr{R}\\equiv (l_{O}/{\\it\\eta})^{4/3}$ . The inner wall region has the behaviour $\\mathscr{R}\\sim Re_{\\mathscr{L}}Re_{{\\it\\tau}}$ , in contrast to stratified boundary layer flows where the buoyancy flux is non-zero at the wall and $\\mathscr{R}\\sim Re_{\\mathscr{L}}$ .
Publisher: Elsevier BV
Date: 07-2015
Publisher: Elsevier BV
Date: 04-2013
Publisher: Trans Tech Publications, Ltd.
Date: 2014
DOI: 10.4028/WWW.SCIENTIFIC.NET/AMM.493.215
Abstract: Parallel performance of a fractional step Navier-Stokes solver is investigated. Parallelisation is performed using Message Passing Interface, with domain partitioning. Block preconditioning is applied to the solution of the pressure Poisson equation, which is often the bottleneck in the computation of the fractional step method. Preconditioners tested are classes of incomplete matrix decompositions and sparse approximate inverses. The computational domain is decomposed into eight parts of about equal size in terms of the number of cells, and solved on eight parallel processors. Several aspects of the parallelisation, such as domain splitting directions, speed-up and scalability of the preconditioners, are discussed.
Publisher: Elsevier BV
Date: 09-2016
Publisher: Cambridge University Press (CUP)
Date: 14-12-2022
DOI: 10.1017/JFM.2022.962
Abstract: The O-type transition caused by a pair of small- litude oblique waves in a vertical buoyancy layer of a fluid with Prandtl number $0.71$ at a Reynolds number of $200$ is investigated using linear stability analysis and three-dimensional direct numerical simulation. The small- litude oblique waves experience linear growth and undergo nonlinear interactions to generate streamwise vortices/streaks, two-dimensional streamwise waves and harmonic oblique waves. The streamwise vortices/streaks and two-dimensional streamwise waves have twice the spanwise or streamwise wavenumber of the original perturbation, respectively. Unlike the O-type transition in isothermal flat-plate incompressible and compressible boundary layers where streaks dominate the transition, in the vertical buoyancy layer, either streaks or two-dimensional streamwise waves can dominate the flow field during the early stages of oblique transition. The growth rates of streaks and two-dimensional waves are dependent on the wavenumber of the initial oblique waves. Streaks dominate the flow for high streamwise wavenumbers, while two-dimensional streamwise waves dominate the flow for low streamwise wavenumbers. Analysis of the turbulent kinetic energy production and the Reynolds stresses reveals that the early stages of the transition differ depending on the wavenumber of the oblique waves. An increase in the initial litude of the oblique waves causes a faster transition from laminar flow however, the growth rates of the streaks and two-dimensional streamwise waves are independent of the initial litude. Even though different modes are dominant during the early stages of the O-type transition, the onset of chaotic flow is caused by the breakdown of streak modes.
Publisher: Elsevier BV
Date: 06-2017
Publisher: Elsevier BV
Date: 10-2015
Publisher: IOP Publishing
Date: 16-06-2014
Publisher: Cambridge University Press (CUP)
Date: 02-09-2019
DOI: 10.1017/JFM.2019.639
Abstract: The stability properties of a natural convection boundary layer adjacent to an isothermally heated vertical wall, with Prandtl number 0.71, are numerically investigated in the configuration of a temporally evolving parallel flow. The instantaneous linear stability of the flow is first investigated by solving the eigenvalue problem with a quasi-steady assumption, whereby the unsteady base flow is frozen in time. Temporal responses of the discrete perturbation modes are numerically obtained by solving the two-dimensional linearized disturbance equations using a ‘frozen’ base flow as an initial-value problem at various $Gr_{\\unicode[STIX]{x1D6FF}}$ , where $Gr_{\\unicode[STIX]{x1D6FF}}$ is the Grashof number based on the velocity integral boundary layer thickness $\\unicode[STIX]{x1D6FF}$ . The resultant lification rates of the discrete modes are compared with the quasi-steady eigenvalue analysis, and both two-dimensional and three-dimensional direct numerical simulations (DNS) of the temporally evolving flow. The lification rate predicted by the linear theory compares well with the result of direct numerical simulation up to a transition point. The extent of the linear regime where the perturbations linearly interact with the base flow is thus identified. The value of the transition $Gr_{\\unicode[STIX]{x1D6FF}}$ , according to the three-dimensional DNS results, is dependent on the initial perturbation litude. Beyond the transition point, the DNS results erge from the linear stability predictions as nonlinear mechanisms become important.
Publisher: Elsevier BV
Date: 04-2011
Publisher: Elsevier BV
Date: 04-2013
No related grants have been discovered for Stuart Norris.