4 results for Armfield, SW

  • Convectively unstable turbulent open channel flow with stable surface stratification

    Williamson, NJ; Kirkpatrick, M; Armfield, SW; Norris, Stuart (2016)

    Journal article
    The University of Auckland Library

    We use direct numerical simulations to examine fully developed turbulent open channel flow where the near wall region is unstably stratified and the outer boundary layer is stably stratified. The simulations are a model for flow in shallow turbid river channels with incident solar radiation. The aim is to determine under what conditions and by what mechanism the stably stratified layer is overturned. The flow is attained by applying a radiative heat flux at the free surface of the open channel. The absorption and transmission of the radiation follows the Beer--Lambert law with a constant absorption coefficient. We examine conditions where approximately 20% of the incident radiative heat flux penetrates through to the wall, releasing heat at the wall as a heat flux. The problem is specified by our buoyancy parameter which is analogous to the bulk Obukhov length scale. In the stable outer boundary layer we observe that the flux Richardson number reaches the limiting value, as was observed in the atmospheric boundary layer under sheared convective conditions.

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  • Transitional High Prandtl Number Plumes with an Isothermal Source

    Hattori, T; Armfield, SW; Kirkpatrick, MP; Norris, Stuart (2011)

    Conference item
    The University of Auckland Library

    Plume flows in the transitional range, rising from an iso-thermal source, are numerically investigated for a fluid having a Prandtl number (Pr) of 7.0 to reveal the conditions under which an intermittent puffing behaviour takes place. Simulations have been conducted with varying Reynolds number (Re), planar and axisymmetric configurations, no-slip (all the velocity components are zero) and free-slip (Neumann conditions with zero derivative for tangential velocities and zero normal velocity) conditions at the bottom boundary. It has been found that the intermittent puffing behaviour only occurs in planar plumes. More noteworthy is that with the use of the free-slip condition at the bottom boundary, the intermittent puffing behaviour does not take place. Non-dimensional puffing frequencies have been obtained, and it has been found that the Re variation in the frequencies is small. The mean momentum and buoyancy balance equation terms have also been plotted, and similar trends to those for a turbulent plume have been observed.

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  • A Parallel Non-Staggered Navier-Stokes Solver Implemented on a Workstation Cluster

    Armfield, SW; Norris, Stuart; Morgan, P; Street, R (2002)

    Conference item
    The University of Auckland Library

    Non-staggered, fractional step Navier-Stokes solvers are presented and analysed in comparison to a standard staggered solver. It s shown analytically that the non-staggered schemes have the same order of accuracy as the staggered scheme, although one of the schemes is non-elliptic in the pressure field at the grid scale. Numerical results are used to validate the analysis, with only the non-elliptic non-staggered scheme showing a reduced accuracy and efficiency compared to the staggered scheme. Details of a computer cluster built using Pentium 4 processors with a fast ethernet switch are presented. A serial benchmark demonstrates the excellent performance of the P4 for large array size compute intensive jobs, while both MPI and HPF coded parallel benchmarks show the effective parallel performance of the cluster.

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  • Transition to stably stratified states in open channel flow with radiative surface heating

    Williamson, N; Armfield, SW; Kirkpatrick, MP; Norris, Stuart (2015)

    Journal article
    The University of Auckland Library

    Direct numerical simulations 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 δ to L, a bulk Obukhov length scale for the flow. We cover the range λ = δ/L = 0−2.0, from neutral conditions to the onset of the stable regime, with the Reynolds number range Reτ = 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 λ 1 the turbulent kinetic energy budget is in local equilibrium over a region extending from the near wall region to a free surface affected region a distance lν from the surface with lν/δ ∼ Re−1/2. In this equilibrium region the flow can be characterised by the flux Richardson number Rf and the local Obukhov length scale Λ. At higher λ local mixing limit conditions are observed over an extended region. At λ = 2 the flux Richardson number approaches critical limit values of Rf,c 0.18 and gradient Richardson number Ric 0.2. At high λ, we obtain a flow field where buoyancy interacts with the smallest scales of motion the turbulent shear stress and buoyancy flux are suppressed to molecular levels. We find this regime can be identified in terms of the parameter ReL,c = Luτ /ν 200 − 400 which is related to the L∗ parameter of Flores & Riley (2011) and buoyancy Reynolds number R. With energetic equilibrium attained, the local buoyancy Reynolds number ReΛ = Λ u w 1/2/ν, is directly related to separation of the Ozmidov (lO) and Kolmogorov (η) length scales in the outer boundary layer by ReΛ R ≡ (lO/η)4/3. The inner wall region has the behaviour R ∼ ReL Reτ in contrast to stratified boundary layer flows where the buoyancy flux is non-zero at the wall and R ∼ ReL .

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