Flow Induced Vibration - STAR Global .Flow Induced Vibration - flow internal to structures that,

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  • Flow Induced Vibration

    A Review of Current Assessment Methods

    David Fielding, Matt Straw (Norton Straw Consultants)

    Alex Graham, Phil Shorter (CD-adapco)

  • Introduction

    Presenting a joint study into flow-induced vibration (FIV) by Norton Straw Consultants

    CD-adapco

    Summary of work presented more fully at Institution of Mechanical Engineering Offshore Engineering 2015

    Designing for Flow Induced Vibration with Simulation - Webinar on CD-adapco website (linked from Norton Straw news page)

    Flow Induced Vibration (FIV) and assessment Energy Institute Guidelines approach

    Use of CFD & FEA (STAR-CCM+ & wave6)

  • Background to FIV

    Flow Induced Vibration - flow internal to structures that, due to a number of mechanisms, may cause vibration of the structure

    If oscillation frequencies are close to the natural frequencies of the structure, resonance can occur leading to excessive and potentially damaging vibration e.g. fatigue or fretting

    We will present a case study that : Introduces current energy industry practice

    Demonstrates what simulation has to offer us

  • Case Study

    INLET

    OUTLET

    THERMOWELL 2 (INTRUSIVE ELEMENT)

    THERMOWELL 1 (INTRUSIVE ELEMENT)

    DEADLEG (NON-FLOWING BRANCH)

    FLOW DIRECTION

    CLAMPED SUPPORT

    CLAMPED SUPPORT

    INTERNAL DIAMETER = 87 mm

    DEADLEG INTERNAL DIAMETER = 43 mm

    CLAMPED SUPPORT

  • Typical Approach EI Guidelines

    Energy Institute guidelines allow the potential for FIV to be assessed*

    A case study is presented initially using the EI guidelines approach

    The same case is then evaluated further using CFD and FEA methods

    * Guidelines for the avoidance of vibration induced fatigue failure in process pipework, Energy Institute, 2nd edition

  • Case Study

    Parameter Case 1 Case 2 Case 3

    Gas flow rate 921 1047 984 m/h

    Liquid flow rate 21 24 87 m/h

    Effective density 36 36 82 kg/m

  • Case Study Results EI Guidelines

    Mechanism Case 1 Case 2 Case 3

    Flow induced turbulence - - !

    Flow induced pulsation ! ! !

    Slug flow

    VIV from intrusive elements -

    ! !

    Small bore connections

    ! ! !

    ! Further action required: redesign, further detailed analysis and/or vibration monitoring

    - Further action: only visual inspection for good, as analysed construction

    Low liquid flow rate not estimated to occur (flow maps)

  • Case Study Results EI Guidelines

    Corrective actions suggested by the EI guidelines vary for the likelihood of failure (LOF) predicted: Redesigning or re-supporting the line or

    Carrying out detailed analysis and/or

    Carrying out vibration monitoring

    Detailed analysis is not defined Change in design is often the most simple action (and then re-evaluating using the EI

    guidelines approach)

    Design changes may be not be possible

    Re-design may be unnecessary if the EI approach is providing an over-conservative answer

    So what detailed analysis could we do?

  • What can CFD and FEA offer us?

    CFD: Local flow modeling of specific phenomena e.g. Flow-induced turbulence

    Impact of intrusive elements

    Pulsation

    Cavitation

    Multiphase flow e.g. presence of slugging

    System-wide modelling CFD to assess flow mechanisms and input to

    Finite Element Analysis (FEA) to assess Natural frequencies of system

    How the flow-induced forces may excite the system

    Coupled CFD & FEA analysis could offer the complete solution

  • Local Model - Intrusive Elements

    The EI guidelines use the following equation to predict a vortex shedding frequency: St = fL/v

    This equation is based on a long cylinder in free stream flow.

    Is this what we have? Not in most cases

    Not in our case study

    VORTEX SHEDDING AROUND CYLINDER IN CROSS-FLOW

  • Intrusive Elements

    SKETCH OF THERMOWELL GEOMETRY

    FLOW AROUND INTRUSIVE ELEMENT IS DIFFERENT TO LONG CYLINDER IN FREE STREAM

    Using CFD need to be consider:

    Time step size

    Grid resolution

    Turbulence modelling

  • Whole System Analysis

    CFD simulation can be used in conjunction with a finite element structural model

    Here STAR-CCM+ (CFD) was used with wave6 (FEA) CFD to assess the frequencies of flow phenomena

    FEA enables the assessment of whether flow frequencies coincide with natural frequencies of the system

    A transient flow solution from STAR-CCM+ imported for random vibration analysis in wave6 a frequency domain aero-vibro-acoustic solver Summary of this presented here

  • Whole System Analysis

    Flow modeling: Unsteady (time accurate)

    Volume of Fluid method for multi-phase flow (VOF)

    Large Eddy Simulation (LES) turbulence model

    Ideal gas law applied to gas phase, liquid phase assumed to be incompressible

    Objectives: Predict internal flow regime

    Record transient wall pressure data, apply this to FE structural model in wave6

  • Whole system analysis: CFD Domain

    Flow development region 20m upstream

    Domain extended 5m downstream in area of interest

    The main fluid region was meshing using polyhedral elements with a body fitted prism layer.

    Total element count = 2M cells

    Average cell size in core mesh ~4mm

    Flow Direction

  • Flow regime for case study for case 1

    All three cases exhibit annular flow regime

    Some low frequency pumping visible in deadleg

    Whole System Analysis: CFD

  • The intrusive element interacts with the fluid film and gas streams (vertical thermowell, case 2 shown)

    An unsteady pumping mechanism results From time domain data & visual inspection it is difficult to discern characteristic spatial

    and frequency content

    Whole System Analysis: CFD

  • Frequency Domain Analysis

    To quantify the risk of structural vibration, CFD results can be processed in the frequency domain.

    STAR-CCM+ has built in functions to perform spectral analysis on monitored data Fluctuating surface pressures can be compared to modal frequencies of the structure to estimate the

    response

    wave6 is a frequency domain aero-vibro-acoustic analysis tool which can read in transient wall pressure data from STAR-CCM+. wave6 can be used to create coupled vibro-acoustic models that combine Finite Element, Boundary

    Element and Statistical energy analysis methods.

    Structural/Acoustic FE model is directly excited by the fluctuating surfaces pressures calculated in CFD.

    Acceleration and Stress RMS levels calculated

  • A

    STAR-CCM+ surface pressure data mapped on wave6 model

    RMS pressure shows ~30dB dynamic range

    Highest levels associated with: Separated flow at the bend

    Near the first thermowell

    Around the deadleg tee

    RMS Pressure on Pipe Wall Case 1

    Overall RMS level

    B

    C

    A

    B C

    Flow

    direction

  • Spectral Diagnosis of RMS Pressure on Pipe Wall Case 1

    4Hz

    32Hz

    RMS pipe wall pressure shown as a function of frequency

    Highest levels at lower frequencies with distinct spectral peaks around 4 Hz

    Second peak around 32 Hz

  • Cause of 4Hz Peak Case 1

    Pressure PSD at 4 Hz

    B

    C

    C

    B

    Pressure fluctuations at 4Hz concentrated on sharp edges on which the annular flow impinges

    The liquid holdup trace from CFD run confirms that this is roughly the frequency of fluctuation in liquid loading

  • Cause of 32Hz Peak Case 1

    Pressure PSD at 32 Hz

    Pressure fluctuations at 32Hz comes from the first thermowell pumping

    The second thermowell does not produce a clear response

  • Structural Modes Free Response

    Mode 1

    17 Hz

    Mode 2

    39 Hz

    Mode 3

    42 Hz

    Mode 4

    60 Hz

  • Random Vibration Response

    0.54 gRMS

    0.49 gRMS

    0.25 gRMS

    0.24 gRMS

    17 Hz

    37 Hz 32 Hz Case 1

    Acceleration response is maximum at modal peaks

    Structure does not respond to general excitation at 4Hz

    32Hz excitation in thermowell 1 is just visible in system level response

  • Stress Response

    Case 1

    Highest Von-Mises stress responses occur at structural resonances (17Hz,

    37Hz and 42Hz) near the support locations

    Very small contribution from thermowell excitation at 32Hz

    17 Hz

    37 Hz

    1 MPa RMS

    0.85 MPa RMS 0.77 MPa RMS

    0.45 MPa RMS

  • Case Study Summary

    EI guidelines identifies flow-induced vibration due to a range of mechanisms.

    Issues identified can be further investigated by detailed analysis using CFD & FEA.

    The component-level CFD analysis for intrusive element showed the VIV issue identified in EI guidelines was not present.

    System-wide analysis using CFD & FEA showed: Characteristic frequency of multiphase flow around 4Hz

    Unsteady phenomena in thermowell found to occur at 32Hz

    Natural frequencies of system did not correspond to flow

    Using STAR-CCM+ coupled with wave6 little flow-related issues were identified

  • STAR-CCM+ / wave6 workflow

    STAR-CCM+ was run on a cluster in the CD-adapco office in Hammersmith, London The 2M cell model was run on up to 64 cores depending on availability.

    Runtime for 8s of simula