Modelling and analysis of vibrations in flexible cylinders subjected to internal multiphase and external single-phase flows.

Ravindran Meenakumari, Hareesh Narain

Abstract

This study investigates the fluid-structure interaction phenomena of flexible straight cylinders subjected to internal two-phase slug flows and external vortex-induced vibrations (VIV) by taking into account the geometric and hydrodynamic nonlinearities. Flow-induced vibration (FIV) is a common phenomenon observed in various engineering systems such as heat exchangers, nuclear power plants, chemical process plants and hydrocarbon transportation. In many of these applications, internal flow generally occurs as multiphase flow, characterized by simultaneous flow of gas and liquid. Among different flow patterns that may emerge from multiphase flow, slug flows are widely recognised as problematic due to fluctuations in multiphase mass, velocities and pressure changes causing slug flow-induced vibrations (SIV). Most of the literature related to internal single and two-phase flow has largely focused on the use of a linearised tension beam model. Fundamental understanding of SIV in three-dimensional space and time, considering in-line and axial dynamics, is still lacking. A three-dimensional semi-empirical model is presented, incorporating nonlinear structural equations of coupled out-of-plane, in-plane and axial oscillations combined with equations involving centrifugal and Coriolis forces for the analysis and prediction of SIV. New dimensionless equations are derived from the dimensional model to analyse key flow features and improve overall understanding of the model. To capture forces due to VIV, the model uses a phenomenological wake oscillator model to emulate oscillatory drag and lift forces induced by VIV. An idealised slug unit model capable of capturing mass variations in the slug flow regime is utilised in this study assuming slug flows are fully developed and undisturbed by pipe oscillations. A numerical space-time finite difference scheme is implemented to analyse and solve the highly nonlinear partial differential equations present in the model. Model validations are performed by comparing the results with experimental and numerical data from literature related to SIV and VIV. Comparisons demonstrate qualitative and quantitative similarities with experimental results, highlighting the model's capability to capture similar amplitudes, frequencies and dominant modes excited by SIV and VIV. Parametric studies capture the effects of key SIV and VIV flow characteristics on the vibration responses of vertical and horizontal fluid-conveying cylinders. Results from SIV in submerged vertical straight cylinders revealed amplitude-modulation response, mean displacements, simultaneous three-dimensional oscillations and the influence of internal flow velocities. Lower flow velocities produced amplitude increase of up to 350% compared to higher velocities at specific slug formations. A new dimensionless parameter is introduced to predict scenarios of high amplitude oscillations for vertical cylinders conveying two-phase slug flows. SIV in flexible horizontal cylinders highlighted the significant effects of slug frequencies, internal flow velocities and fluid densities through large mean displacements, three-dimensional out-of-plane, in-plane and axial oscillations and parametric resonance. When the slug frequency to structure natural frequency ratio, fs/fn=1, was observed, it generated high amplitudes with an increase of up to 4 times compared to fs/fn=2 ratio at certain flow velocities. Additionally, frequency domain analysis revealed the presence of sub-harmonic frequencies and high frequency modulated response. Numerical results in the case of combined VIV-SIV excitation scenario exhibited variations in mean displacements and RMS amplitudes compared to VIV-only results in all three-directions of oscillations. In-line and axial oscillation amplitudes increased by up to 200% compared to VIV-only amplitude responses. Frequency domain analysis demonstrated frequency response with increased frequency components and sub-harmonics particularly in the in-line and axial oscillations. Moreover, parametric resonance conditions produced an oscillation amplitude increase of up to 2 times in the cross-flow direction and up to 8 times in the in-line direction, with increased frequency modulations, sub-harmonic frequencies and variations in obtained dominant modes.

Citation

RAVINDRAN MEENAKUMARI, H.N. 2024. Modelling and analysis of vibrations in flexible cylinders subjected to internal multiphase and external single-phase flows. Robert Gordon University, PhD thesis. Hosted on OpenAIR [online]. Available from: https://doi.org/10.48526/rgu-wt-2801155

• Thesis Type: Thesis
• Deposit Date: Apr 22, 2025
• Publicly Available Date: Apr 22, 2025
• DOI: https://doi.org/10.48526/rgu-wt-2801155
• Keywords: Slug flows; Multiphase flows; Fluid dynamics; Nonlinear dynamics; Vortex-induced vibrations; Parametric resonance
• Public URL: https://rgu-repository.worktribe.com/output/2801155
• Award Date: Oct 31, 2024