Hydrodynamic Coefficients of Yawed Square Cylinder in Oscillating Flow

Experimental and Computational Hydraulic

Observations of pumping and vortex dynamics due to a cylinder oscillating normal to a plane wall

Understanding the fluid dynamics associated with a circular cylinder oscillating normal to a plane wall is important for safe design of offshore infrastructure, such as power cables and pipeline risers. This paper investigates the fluid dynamics of an oscillating cylinder with no imposed incident current experimentally using flow visualisation and force measurements where the ratio of the cylinder Reynolds number (Re) to Keulegan–Carpenter number (KC) is β = 500 and KC varies between 2 and 12. The minimum distance between the cylinder and wall was between 12.5 % and 50 % of the diameter. Across this parameter space three primary vortex flow regimes were observed: (i) for KC ≤ 5, the flow field is approximately symmetric about the cylinder centreline and the velocity field between the cylinder and the wall resembled a pumping flow in phase with cylinder motion, which is well predicted by potential theory for most of the cycle; (ii) for 5 < KC < 8, the flow field is increasingly asymmetric but with frequent switching of the side associated with vortex shedding; and (iii) for KC ≥ 8, the flow field is consistently asymmetric due to vortex shedding. The in-line force increases when the cylinder is near the wall due to dynamic pressures associated with pumping. This increase can be estimated using potential theory superimposed onto the force time history for an isolated cylinder at the same KC and Re. This study complements recent numerical modelling focused on low Reynolds number conditions and provides important insights into the fluid mechanics associated with trenching beneath cable and pipeline risers

Calibration of linear contact stiffnesses in discrete element models using a hybrid analytical-computational framework

Efficient selections of particle-scale contact parameters in discrete element modelling remain an open question. The aim of this study is to provide a hybrid calibration framework to estimate linear contact stiffnesses (normal and tangential) for both two-dimensional and three-dimensional simulations. Analytical formulas linking macroscopic parameters (Young's modulus, Poisson's ratio) to mesoscopic particle parameters for granular systems are derived based on statistically isotropic packings under small-strain isotropic stress conditions. By taking the derived analytical solutions as initial approximations, the gradient descent algorithm automatically obtains a reliable numerical estimation. The proposed framework is validated with several numerical cases including randomly distributed monodisperse and polydisperse packings. The results show that this hybrid method practically reduces the time for artificial trials and errors to obtain reasonable stiffness parameters. The proposed framework can be extended to other parameter calibration problems in DEM

Turbulent energy scale-budget equations for nearly homogeneous sheared turbulence

For moderate Reynolds numbers, the isotropic relation between second-order and third-order moments for velocity increments (Kolmogorov's equation) is not respected, reflecting a non-negligible correlation between the scales responsible for the injection, transfer and dissipation of the turbulent energy. For (shearless) grid turbulence, there is only one dominant large-scale phenomenon, which is the non-stationarity of statistical moments resulting from the decay of energy downstream of the grid. In this case, the extension of Kolmogorov's analysis, as carried out by Danaila, Anselmet, Zhou and Antonia, J. Fluid Mech. 391, 1999 359–369) is quite straightforward. For shear flows, several large-scale phenomena generally coexist with similar amplitudes. This is particularly the case for wall-bounded flows, where turbulent diffusion and shear effects can present comparable amplitudes. The objective of this work is to quantify, in a fully developed turbulent channel flow and far from the wall, the influence of these two effects on the scale-by-scale energy budget equation. A generalized Kolmogorov equation is derived. Relatively good agreement between the new equation and hot-wire measurements is obtained in the outer region (40 < x+3 < 150) of the channel flow, for which the turbulent Reynolds number is Rlambda asymp 36

Three-dimensional numerical simulation of oscillatory flow around a circular cylinder at right and oblique attacks

Sinusoidal oscillatory flow around a circular cylinder at right and oblique attacks is investigated by three-dimensional numerical simulation. Calculations are carried out for oblique angles of α=0°, 15°, 30°, 45° and 60°, Reynolds number of 2000 and KC numbers ranging from 6.75 to 30. The oblique angle of α=0° corresponds to the right attack case. The predicted vortex shedding regimes for α=0° agree well with those found in physical experiments. It is found from the numerical simulations that when KC number is in the middle range of a vortex shedding regime (i.e. KC=10, 13, 17.5 and 26.2), the flow is in single mode. In each single mode, the number of vortices shed in each flow period is constant and the sectional hydrodynamic force in the cross-flow direction fluctuates at a unique frequency. As the KC number is close to the boundary between two vortex shedding regimes (KC=6.75, 15, 20 and 30), the number of vortices shed from the cylinder varies from period to period and the time series of the transverse force contains more than one predominant frequencies, implying the flow switching from one mode to another. This flow mode is referred to as multi-modes. The spanwise correlation factor obtained according to the sectional transverse force is close to 1 for single-mode flows. The correlation length of multi-mode flows depends on the KC number and is generally smaller than that of single-mode flows. Comparison between the numerical results of α=0° and those of α>0° shows that the independent principle is applicable for the calculated KC number range and the oblique angle (α≤45°). For α=60°, the maximum lift coefficient CL, max and mode-averaged lift force have distinct difference from their counterparts for α=0°

Direct numerical simulation of three-dimensional flow past a yawed circular cylinder of infinite length

Direct numerical simulation of flow past a stationary circular cylinder at yaw angles(α) in the range of 0–60° was conducted at Reynolds number of 1000. The three-dimensional(3-D) Navier–Stokes equations were solved using the Petrov–Galerkin finite element method. The transition of the flow from 2-D to 3-D was studied. The phenomena that were observed in flow visualization, such as the stream wise vortices, the vortex dislocation and the instability of the shear layer, were reproduced numerically. The effects of the yaw angle on wake structures, vortex shedding frequency and hydrodynamic forces of the cylinder were investigated. It was found that the Strouhal number at different yaw angles (a) follows the independence principle. The mean drag coefficient agrees well with the independence principle. It slightly increases with the increase of α and reaches a maximum value at α=60°, which is about 10% larger than that when α=0°. The root-mean-square(r.m.s.)values of the lift coefficient are noticeably dependent on α

Numerical simulation of vortex-induced vibration of a square cylinder at a low Reynolds number

Vortex-induced vibrations (VIV) of a square cylinder at a Reynolds number of 100 and a low mass ratio of 3 are studied numerically by solving the Navier-Stokes equations using the finite element method. The equation of motion of the square cylinder is solved to simulate the vibration and the Arbitrary Lagrangian Eulerian scheme is employed to model the interaction between the vibrating cylinder and the fluid flow. The numerical model is validated against the published results of flow past a stationary square cylinder and the results of VIV of a circular cylinder at low Reynolds numbers. The effect of flow approaching angle (α) on the response of the square cylinder is investigated. It is found that α affects not only the vibration amplitude but also the lock-in regime. Among the three values of α (α = 0°, 45°, and 22.5°) that are studied, the smallest vibration amplitude and the narrowest lock-in regime occur at α = 0°. It is discovered that the vibration locks in with the natural frequency in two regimes of reduced velocity for α = 22.5°. Single loop vibration trajectories are observed in the lock-in regime at α = 22.5° and 45°, which is distinctively different from VIV of a circular cylinder. As a result, the vibration frequency in the in-line direction is the same as that in the cross-flow direction

Using e-tutor program and pre-lab assessment task to enhance laboratory experience of civil engineering students

Before attending a lab session, students are usually required to be familiarized with laboratory tasks through pre-lab preparation. While it was found pre-lab preparation was an effective way to promote student learning, the benefits of running pre-lab preparation were not fully investigated. Traditional lab description sheets may provide general information on safety operation procedures and pre-lab questions. However, if these pre-lab questions were not assessed, they could easily be ignored by students. In this research, pre-lab problems were designed to give unique parameters to each student. Students were required to solve the problems and obtain theoretical values which were verified later in a practical lab session. To minimize the marking load, the pre-lab submissions were automatically marked using the e-tutor program. Results showed that the pre-lab preparation program was successful in enhancing students’ learning interests, in preventing plagiarism, and in increasing the efficiency of labs with the duration of a lab session being significantly reduced