Hongwei An

Direct numerical simulation of oscillatory flow around a circular cylinder at low Keulegan-Carpenter number

The Honji instability is studied using direct numerical simulations of sinusoidal oscillatory flow around a circular cylinder. The three-dimensional Navier–Stokes equations are solved by a finite element method at a relatively small value of the Keulegan–Carpenter number KC . The generation and subsequent development of Honji vortices are discussed over a range of frequency parameters by means of flow visualization. It is found that the spacing between Honji vortices is only weakly dependent on the frequency of oscillation, but is strongly correlated to KC because it is the terms within the governing equation containing KC that dominate the three-dimensional features of the flow. An empirical relationship between KC and the spacing between neighbouring vortices is proposed. The three-dimensional steady streaming structure within the vortices is identified and it is found that at high frequencies the steady streaming is two-dimensional although the instantaneous flow structure is itself fully three-dimensional.

Steady streaming around a circular cylinder near a plane boundary due to oscillatory flow

Steady streaming due to an oscillatory flow around a circular cylinder close to and sitting on a plane boundary is investigated numerically. Two-dimensional (2D) Reynolds-averaged Navier-Stokes equations are solved using a finite element method with a k-ω turbulent model. The flow direction is perpendicular to the axis of the cylinder. The steady streaming around a circular cylinder is investigated for Keulegan-Carpenter (KC) number of 2≤KC≤30 with a constant value of Stokes number (β) of 196. The gap (between the cylinder and the plane boundary) to diameter ratio ( e/D ) investigated is in the range of 0.0–3.0. The steady streaming structures and velocity distribution around the cylinder are analyzed in detail. It is found that the structures of steady streaming are closely correlated to KC regimes. The gap to diameter ratio ( e/D ) has a significant effect on the steady streaming structure when e/D<1.0 . The magnitude of the steady streaming velocity around the cylinder can be up to about 70% of the vel...

Stability of subsea pipelines during large storms

On-bottom stability design of subsea pipelines transporting hydrocarbons is important to ensure safety and reliability but is challenging to achieve in the onerous metocean (meteorological and oceanographic) conditions typical of large storms (such as tropical cyclones, hurricanes or typhoons). This challenge is increased by the fact that industry design guidelines presently give no guidance on how to incorporate the potential benefits of seabed mobility, which can lead to lowering and self-burial of the pipeline on a sandy seabed. In this paper, we demonstrate recent advances in experimental modelling of pipeline scour and present results investigating how pipeline stability can change in a large storm. An emphasis is placed on the initial development of the storm, where scour is inevitable on an erodible bed as the storm velocities build up to peak conditions. During this initial development, we compare the rate at which peak near-bed velocities increase in a large storm (typically less than 10−3 m s−2) to the rate at which a pipeline scours and subsequently lowers (which is dependent not only on the storm velocities, but also on the mechanism of lowering and the pipeline properties). We show that the relative magnitude of these rates influences pipeline embedment during a storm and the stability of the pipeline.

Direct numerical simulation of flow around a surface-mounted finite square cylinder at low Reynolds numbers

With the aid of direct numerical simulation, this paper presents a detailed investigation on the flow around a finite square cylinder at a fixed aspect ratio (AR) of 4 and six Reynolds numbers (Re = 50, 100, 150, 250, 500, and 1000). It is found that the mean streamwise vortex structure is also affected by Re, apart from the AR value. Three types of mean streamwise vortices have been identified and analyzed in detail, namely, “Quadrupole Type” at Re = 50 and Re = 100, “Six-Vortices Type” at Re = 150 and Re = 250, and “Dipole Type” at Re = 500 and Re = 1000. It is the first time that the “Six-Vortices Type” mean streamwise vortices are reported, which is considered as a transitional structure between the other two types. Besides, three kinds of spanwise vortex-shedding models have been observed in this study, namely, “Hairpin Vortex Model” at Re = 150, “C and Reverse-C and Hairpin Vortex Model (Symmetric Shedding)” at Re = 250, and “C and Reverse-C and Hairpin Vortex Model (Symmetric/Antisymmetric Shedding...

Three-dimensional numerical simulation of flow around a circular cylinder under combined steady and oscillatory flow

Combined steady and oscillatory flow past a circular cylinder is investigated numerically by three-dimensional Direct Numerical Simulation (DNS). The incompressible Navier-Stokes equations are solved by finite element method (FEM). The aim of this study is to investigate influence of the existence of steady current on the flow regime and hydrodynamics forces. The computational results of pure oscillatory flow past a circular cylinder agree well with the experimental data. The flow ratios (ratio of current velocity to oscillatory velocity amplitude) are 0.0, 0.2 0.5 and 1.0 in the study. The influence of the flow ratio on the vortex shedding regime and hydrodynamic forces on the cylinder are investigated numerically.

Time Scale of Local Scour around Pipelines in Current, Waves, and Combined Waves and Current

AbstractThis paper presents results of an experimental investigation into the time scale of local scour around subsea pipelines in current-only, wave-only, and combined wave and current flow condit...

Flow and flow-induced vibration of a square array of cylinders in steady currents

Flow and flow-induced vibration of a square array of cylinders are investigated by two-dimensional numerical simulations. Flow past 36 cylinders in an inline arranged square array and 33 cylinders in a staggered arranged square array is firstly simulated, for Re = 100 and the spacing ratios of L/D = 1.5, 2, 3, 4, 5. Only one vortex street is observed in the wake of the cylinder array when the spacing ratio is 1.5 in the inline arrangement and 1.5 and 2 in the staggered arrangement, indicating that the critical spacing ratio for the single-vortex street mode in the staggered arrangement is higher than that in the inline arrangement. The vortex shedding from the cylinders is suppressed at L/D = 3 for both inline and staggered arrangements. Vortex shedding from each individual cylinder is observed when L/D = 4. Flow-induced vibration of 36 cylinders in an inline square arrangement is studied for a constant Reynolds number of 100, two spacing ratios of 2 and 5, a constant mass ratio of 2.5 and a wide range of reduced velocities. It is found that for a spacing ratio of 2, the vibration of the cylinders in the four downstream columns does not start until the reduced velocity exceeds 4.5. The vibration of the cylinders progresses downstream with increasing reduced velocity. For a spacing ratio of 5, the vibrations of the cylinders in the most upstream column are similar to that of a single cylinder. The vibration amplitudes of the downstream cylinders peak at higher reduced velocities than that of a single cylinder. The maximum possible response amplitudes occur at the most downstream cylinders.

Stable state of Mode A for flow past a circular cylinder

A stable three-dimensional (3D) wake structure for flow past a circular cylinder has been discovered through 3D direct numerical simulations (DNS). The stable 3D wake structure occurs over a small range of Reynolds number (Re) below the critical Re for Mode A∗ (i.e., Mode A with large-scale vortex dislocations, where Mode A is the first 3D instability mode which will evolve into Mode A∗) instability. It is believed that the stable 3D wake structure discovered in this study is the stable state of Mode A wake structure inferred by Williamson [“Three-dimensional wake transition,” J. Fluid Mech. 328, 345–407 (1996)]. This confirms the wake transition sequence of 2D → A → A∗ → B suggested by Williamson. Compared with conventional Mode A structure, the stable state of Mode A structure has much weaker amplitude and does not evolve into large-scale vortex dislocations. The stable state of Mode A structure is triggered by small-scale spanwise disturbance introduced upstream of the cylinder, due to energy amplifica...

Calibration of UWA’s O-tube flume facility

The O-tube facility, designed and established at the University of Western Australia, is an innovative closed loop flume in which a random storm sequence can be reproduced via control of a large pump system. The O-tube facility is capable of simulating hydrodynamic conditions near the seabed and the interaction with seabed sediment and any infrastructure that is resting on it.The purpose of carrying out the O-tube calibration described in this paper is to obtain the relationship between the motor rotation movement and the flow velocity generated in the O-tube, such that any required storm history within the performance envelope of the O-tube can be reproduced. A range of flow velocities and the corresponding pump speeds were measured under steady current, oscillatory flow and combined flow conditions. It was found that the relationship between the pump speed and the flow velocity varies with the oscillatory flow period. Based on the pump characteristic curves and O-tube system curves, the correlation between the motor speed and the flow velocity was derived by applying hydraulic theory and the principle of energy conservation.The derived correlation is validated by reproducing a wide range of target storm series, including a (1:5.8) scaled 100-year return period storm from the North West Shelf of Western Australia in 40 m water depth.Copyright

Numerical Simulation of Oscillatory Flow Around Two Circular Cylinders of Different Diameters

A detailed study of oscillatory flow around two circular cylinders of different diameters is carried out numerically. The Reynolds-averaged Navier-Stokes equations are solved using a finite element method (FEM) with a k–ω turbulence closure. The numerical model is validated against oscillatory flows past a single circular cylinder where the experimental data are available in literature. Then it is employed to simulate the flow around two circular cylinders. It’s found that the fluid flow field around two cylinders is different from the single cylinder case, especially when the small cylinder diameter increases. The orientation of the small cylinder and the gap between two cylinders have significant effects on the vortex shedding process and force coefficients on the cylinders.Copyright