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 m³/h

Liquid flow rate 21 24 87 m³/h

Effective density 36 36 82 kg/m³

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 simulation time was two weeks per case on 32 cores, and just over one week on 64 cores.

– TRN files updated every 1E-4s, giving a frequency resolution of 0.25Hz to 5kHz

– Longer simulation time is not required for the frequency range of interest, and could be shortened.

wave6 was run on a standard workstation – Set up of basic case/workflow: ½ day

– Basic forced response calculated in one hour, if the workflow exists

– Further post processing / solving can extend this time, overall << 1 day

Conclusions

This presentation outlines the approach of using the Energy Institute guidelines to assess flow-induced vibration.

Detailed analysis can be used for: – Component-level analysis e.g. vortex shedding from intrusive elements

– System-wide analysis combining both CFD & FEA

The system-wide analysis showed: – It is possible to use STAR-CCM+ to determine frequency of pressure fluctuations.

– wave6 can use the pressure fluctuations to provide magnitudes of response and stress information to be predicted.

– The STAR-CCM+ & wave6 approach is relatively low in computational expense.

– A fully coupled flow and structural response analysis is also now possible – example in later presentation.