Wake oscillator equations in modelling vortex-induced vibrations at low mass ratios.

The current research explores the capabilities of Rayleigh and Van der Pol equations from the standpoint of accuracy of vortex-induced vibrations (VIV) modelling for low mass ratio cases. The two degree-of-freedom rigid structure model suggested by Postnikov et al. [1] is used as the base case and the fluid equations are modified to create seven options as alternatives to this model. The considered options constitute variation in damping terms, including introduction of additional damping coefficients (as different Van der Pol or Rayleigh parameters). Then the calibration is performed to identify the best set of coefficients to provide accurate match with the experimental data. The main aim is to predict correctly the development of the super-upper branch [2]. Experimental results by Stappenbelt and Lalji [3] for mass ratio 2.36 are utilised for the model calibration. Then the obtained models are validated using data from the series of experiments by Stappenbelt and Lalji [3] and published experimental data from other sources [2, 4]. The obtained results demonstrate the advantages of changes in damping terms. Overall, it is concluded that Rayleigh oscillator can be recommended to approximate the lift coefficient for low mass ratios

Hydrodynamic loads on a group of six structures of different cross-sections in uniform and sheared flow.

The estimates of hydrodynamic forces for a group of structures represent a challenge for the design of offshore systems, as they are subject to changes with a variation in flow profiles. The fluctuating effects may be more pronounced or, on the contrary, suppressed if the cross-sectional shape of structures in an array is altered. The present work performs a series of 2D numerical simulations for the flow past six identical stationary cylinders of three distinct geometrical shapes arranged in a 2 × 3 matrix configuration. The flow profiles considered have an averaged velocity corresponding to the critical flow regime of a Reynolds number of 2.5 × 105. The detached eddy simulation k–ω SST turbulence model is employed to perform a comprehensive investigation of the fluid force coefficients, their frequencies and vortex formation patterns. The effect of the spacing ratio varied simultaneously among the structures from 2 to 7 is considered in conjunction with the change in the flow profile and the cylinders’ cross-section. The results of simulations show a higher mean drag on the upstream cylinders, reduced mean drag on the mid- and downstream cylinders with the second cross-sectional shape, and a higher mean drag on the cylinders with the third cross-sectional shape, compared to the original circular cylinders

Numerical simulation of flow-induced forces on subsea structures in a group under uniform and sheared flow.

Subsea production systems design requires estimates of hydrodynamic loads related to characteristics of structures and the external flow. The current work investigates flow-induced forces for a group of stationary rigid structures modelled in 2D including one structure with a squared cross-section and three smaller circular cylinders located in proximity of each other. Uniform flow and planar sheared flow conditions are considered in this work, with three different arrangements of smaller structures. Flow characteristics are obtained using CFD method and k-ω SST turbulence model. Simulation results include time histories of hydrodynamic coefficients, FFT data and velocity fields. Results for the planar sheared flow in the cases considered show a reduction of mean drag coefficients, increase of frequencies and amplitudes of the fluctuating drag and lift coefficients compared to values observed for the uniform flow

Flow-induced forces for a group of one large and several small structures in the sheared turbulent flow.

Evaluating the hydrodynamic force fluctuations acting on each structure in a group of subsea objects of different cross-section shapes, sizes and relative positions represents a challenge due to the sensitivity of the vortex shedding process, especially for a variety of sheared flows. The present study uses the numerical 2D computational fluid dynamics model to estimate the flow-induced forces on a group of small circular and D-shaped cylinders in the linear and parabolic sheared flow, which are placed in proximity to a larger structure of the squared cross-section. This allows us to evaluate loads, which are affected by the presence of subsea equipment located on the seabed. The average Reynolds number of the considered linear flow profile is 3900, while the parabolic flow profile has the maximum Reynolds number of 3900. The k-ω SST turbulence model is used for simulations. The work demonstrates the effect of the cross-sectional shape of smaller cylinders on hydrodynamic coefficients, explores the effect from the spacing in between the structures and highlights differences between loads in the linearly sheared and parabolic flow. The results obtained show that the presence of the squared cylinder notably influences the mean drag coefficient on the first cylinder, for both circular and D-shaped cylinders. The parabolic sheared flow profile in this series leads to the highest mean drag and the highest amplitudes of the fluctuating drag and lift coefficients

VIV of flexible structures in 2D uniform flow

Acknowledgements V.K. would like to acknowledge the support of the Industrial University of Tyumen, Tyumen, Russia, and the State Program ”Global Education”, Russia. This work has been performed using the Maxwell High Performance Computing Cluster funded by the University of Aberdeen. Authors would like to express their gratitude to Dr Andrew Starkey for the advices regarding optimisation procedures, to Naveed Khan for the technical advices on accelerating computations and to Dr Andrey Postnikov for the productive discussions.Peer reviewedPostprin

Fluid nonlinearities effect on wake oscillator model performance

Peer reviewedPublisher PD

Calibration and comparison of VIV wake oscillator models for low mass ratio structures

V.K. would like to acknowledge the support of the Industrial University of Tyumen, Tyumen, Russia, and the State Program ”Global Education”, Russia. This work has been performed using the Maxwell High Performance Computing Cluster funded by the University of Aberdeen. Authors would like to express their gratitude to Dr Andrew Starkey for advices regarding optimization procedures and to Naveed Khan for technical advices on accelerating computations.Peer reviewedPostprin

Optimization of the Wake Oscillator for Transversal VIV

Funding Information: Funding: V.K. would like to acknowledge the support of the National Project “Science and Universities” of the Ministry of Science and Higher Education of the Russian Federation, grant number FEWN-2021-0012. G.R.F. thanks the Brazilian National Council for Scientific and Technological Development (CNPq) for the grant 305945/2020-3.Peer reviewedPublisher PD

A one-dimensional mechanistic model for tracking unsteady slug flow.

A novel one-dimensional slug tracking mechanistic model for unsteady, upward gas-liquid slug flow in inclined pipes is presented. The model stems from the first principles of mass and momentum conservation applied to a slug unit cell consisting of a slug body of liquid and a region of stratified flow containing an elongated bubble and a liquid film. The slug body front and rear are treated as surfaces of discontinuity where mass and momentum balances or "jump laws"are prescribed. The former is commonly applied in mechanistic models for slug flow, whereas the latter is typically overlooked, thereby leading to the assumption of a continuous pressure profile at these points or to the adoption of a pressure drop due to the fluid acceleration on a heuristic basis. Our analysis shows that this pressure change arises formally from the momentum jump law at the slug body front. The flow is assumed to be isothermal, the gas is compressible, the pressure drop in the elongated bubble region is accounted for, the film thickness is considered uniform, and weight effects in the pressure from the interface level are included. Besides specifying momentum jump laws at both borders of the slug body, another novel feature of the present model is that we avoid adopting the quasi-steady approximation for the elongated bubble-liquid film region, and thus the unsteady terms in the mass and momentum balances are kept. The present model requires empirical correlations for the slug body length and the elongated bubble nose velocity. The non-linear equations are discretized and solved simultaneously for all the slug unit cells filling the pipe. Timespace variation of the slug body and film lengths, liquid holdup and void fraction, and pressures, among other quantities, can be predicted, and model performance is evaluated by comparing with data in the literature