Dingyong Yu

Bi-stable flow around tandem cylinders of different diameters at low Reynolds number

The flow around two circular cylinders in tandem arrangement with different diameters has been investigated numerically. The upstream to downstream cylinder diameter ratio was kept constant at a value of d/D=2/3 and the centre-to-centre distance was varied from 1.2D to 5D. The Reynolds number based on upstream cylinder diameter and free-stream velocity used in this study is equal to 200. Force coefficients and flow frequencies have been obtained from 2D fluid flow finite-element simulations. Single-wake shedding, reattachment and co-shedding regimes have been observed. The results show that, depending on initial perturbations, reattachment and co-shedding can be observed on a wide range of centre-to-centre distances (L/D[1.8, 3.8]), which defines the bi-stable flow range. The lower part of the bi-stable range is characterized by a lock-in phenomenon for which vortex-shedding from the upstream and downstream cylinders are synchronized when there are co-shedding flow conditions.

Wake flow behaviour behind a smaller cylinder oscillating in the wake of an upstream stationary cylinder

The flow patterns around a cylinder oscillating freely in the wake of a larger cylinder upstream were investigated using the particle image velocimetry technique. The upstream cylinder was fixed at both ends while the downstream smaller cylinder was held by springs such that it was free to oscillate in the transverse direction. The flow patterns, amplitudes of oscillation and vortex shedding frequencies were compared with those of a single cylinder. In the presence of the upstream cylinder, the three parameters characterizing the oscillation response of the smaller cylinder—amplitude of oscillation, vortex shedding frequency and Reynolds stresses—were greatly reduced. While their magnitude increased with gap ratio, these three parameters were still smaller than the corresponding magnitudes for a single oscillating cylinder. The peak values of turbulence statistics such as Reynolds shear stress and normal stress behind the oscillating downstream cylinder were similarly reduced, and increased with gap ratios.

Bayesian Inversion of Multimode Interface-Wave Dispersion From Ambient Noise

This paper applies nonlinear Bayesian inversion to estimate seabed shear-wave speed profiles and their uncertainties using interface-wave dispersion curves extracted from ocean ambient noise, and compares the resolution of seabed structure for fundamental mode and multimode data. Ambient noise recordings were collected for 2.15 h at hydrophones of an entrenched ocean bottom cable in the North Sea. Scholte-wave dispersion curves for the fundamental mode and several higher order modes within the frequency range 0.25-3.9 Hz are extracted from cross correlations of noise recordings at sensor pairs via the slowness-frequency transform. The Bayesian information criterion is used to determine the preferred model parameterizations in terms of the number of sediment layers supported by the data for inversions based on the fundamental mode alone and on the first three modes. Adaptive hybrid optimization and Metropolis-Hastings sampling are applied to estimate the optimum a posteriori shear-wave speed models and to compute marginal posterior probability distributions and profiles. The results show quantitatively that multimode inversion provides higher resolution of shallow shear-wave speed structure with smaller uncertainties at all depths than inversion of the fundamental mode alone.

Feasibility study on common methods for wave force estimation of deep water combined breakwaters

China’s newly enacted Breakwater Design Specifications (JTS154-2011) explicitly state that breakwaters with water depths greater than 20 m are categorized as deep-water breakwaters, and emphasize that design principles, methods and construction requirements are different from those of common shallow water breakwaters. However, the specifications do not make any mention of how to choose wave force calculation methods of deep-water breakwaters. To study the feasibility of different formulae for wave force estimation of deep water combined breakwaters, the wave force calculated by the Sainflou’s, Goda’s, modified Goda’s and specifications’ methods are compared for various water depths and wave heights in this paper. The calculated results are also compared with experimental data. The total horizontal forcing and the pattern of pressure distributions are presented. Comparisons show that the wave pressure distributions by the four methods are similar, but the total horizontal forces are different. The results obtained by the Goda’s method and the specified formulae are much closer to the experimental data. As for the wave force estimation for the deepwater mixed embankment foundation bed parapet, the Goda’s formula is applied in the case with a water depth of less than 42 m. The Specifications method is suitable for standing waves. In the wave force estimates of breastworks, Sainflou’s and the modified Gaoda’s formulae are no longer applicable for the foundation bed of mixed embankment.

Numerical Study of an Oscillating Smaller Cylinder in the Wake of an Upstream Larger Cylinder

A numerical study of flow around two tandem cylinders with unequal diameters was carried out. The upstream larger cylinder was fixed and the downstream smaller cylinder was allowed to oscillate in the transverse direction only. Comparisons of the experimental and numerical results were made to investigate the effects of the gap ratio on the maximum vibration amplitude and vortex shedding frequency. The results showed that the vibration response of the smaller cylinder was significantly affected by the presence of the upstream larger cylinder, and resulted in greatly reduced vibration amplitudes. With an increasing gap ratio, the vibration amplitude increased. However, the magnitude was lower than that corresponding to a single cylinder (with the same diameter as that of the downstream smaller cylinder) under the same flow conditions.

Bayesian inversion of seabed interface-wave dispersion from ambient noise

This paper applies Bayesian inversion to estimate seabed shear-wave speed profiles and their uncertainties using interface-wave dispersion curves extracted from ocean ambient noise, and compares the resolution of seabed structure for fundamental-mode and multi-mode data. Ambient-noise recordings were collected for 2.15 h at hydrophones of an entrenched ocean-bottom cable in the North Sea. Scholte-wave dispersion curves for the fundamental mode and several higher-order modes within the frequency range 0.26–3.8 Hz are extracted from cross-correlations of noise recordings at sensor pairs via slowness-frequency transform. The Bayesian information criterion is used to determine the preferred model parameterizations in terms of the number of sediment layers supported by the data for inversions based on the fundamental mode alone and on the first three modes. Adaptive-hybrid optimization and Metropolis-Hastings sampling are applied to estimate the most-probable shear-wave speed models and to compute marginal pos...

Flow Behavior Behind Two Side-by-Side Circular Cylinders With Unequal Diameters

In this study, flow behavior behind two side-by-side circular cylinders with unequal diameters, was investigated by using the technique of particle-image-velocimetry (PIV). The experiments were conducted in a re-circulating open water channel within the subcritical Reynolds number regime and a center-to-center pitch ratio T/D range of 1.2 to 3.6. Instantaneous and average velocity patterns, vorticity and Reynolds stress contours acquired with the PIV were used to analyze the flow behavior behind the two cylinders. The effects of the gap ratio and Re on vortex shedding were studied. The results showed that three flow patterns including the asymmetric biased flow, symmetric biased flow and two wake modes with different scales for different gap ratios respectively were observed.© 2010 ASME

Crest-Height Distribution in Nonlinear Random Wave

Many empirical and theoretical distribution functions for wave crest heights have been proposed, but there is a lack of agreement. With the development of ocean exploitation, waves crest heights represent a key point in the design of coastal structures, both fixed and floating, for shoreline protection and flood prevention. Waves crest height is the dominant parameter in assessing the likelihood of wave-in-deck impact and its resulting severe damage. Unlike wave heights, wave crests generally appear to be affected by nonlinearities; therefore, linear wave theory could not be satisfied to practical application. It is great significant to estimate a new nonlinear wave crest height distribution model correctly. This paper derives an approximation distribution formula based on Stokes wave theory. The resulting theoretical forms for nonlinear wave crest are compared with observed data and discussed in detail. The results are shown to be in good agreement. Furthermore, the results indicate that the new theoretical distribution has more accurate than other methods presented in this paper (e.g. Rayleigh distribution and Weibull distribution) and appears to have a greater range of applicability.Copyright