FSI in hydraulic machinery - Applying AcuSolve to identify hydrodynamic damping and hydroelastic instabilities

1 Hübner & Seidel | Altair Technology Conference | Munich | 2014-06-26

FSI in hydraulic machinery

Applying AcuSolve to identify

hydroelastic damping and instabilities

B. Hübner & U. Seidel / Altair Technology Conference / Munich / 2014-06-26


Outline

1. Motivation

2. Numerical analysis of fluid-structure interaction (FSI)

3. Hydrodynamic damping of blades

4. Hydroelastic instability of a bypass valve

5. Conclusions


Motivation


Fluid-structure interaction

in hydraulic machinery (1)

Fluid effects on structural dynamics

• Added-mass effects may reduce

natural frequencies considerably.

• Hydrodynamic damping may clearly

reduce the resonance magnification.

• Flow induced stiffness effects have

minor influence in hydroelasticity.

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Mode Shape

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Frequency [Hz]


Fluid-structure interaction

in hydraulic machinery (2)

FSI induced instabilities

• Self-excited vibrations due to flow

conditions in deforming gaps, e.g.

runner seals.

• Self-excited vibrations due to flow

separation at moving edges, e.g.

intake gates.

• Lock-in effects due to von Kármán

vortex shedding excitation in the

resonance vicinity.


Numerical Analysis of Fluid-Structure Interaction

coupled system response

geometry and physical modeling

mechanical system

CFD

(fluid)

forces (tractions)

displacements

CSM

(structure)


Computational models for

multi-physics simulations

Fluid Dynamics

Structural Dynamics

Thermal Dynamics

DNS

LES / DES

RANS

Euler

Full Potential

Potential / Acoustics

Static Pressure

Non-linear FEM

Network Models

Linear FEM

Prescribed Flux/Temperature

Rigid Walls

Rigid Body (6 DOF)

Modal Analysis

Linear FEM

Non-linear FEM

Rupture / Cracking

Conjugate

Heat Transfer:

Generator

Cooling

Multi-Physics of

Friction Bearings

Acoustic FSI:

Added-Mass Effects

Advanced Hydroelasticity:

Hydrodynamic Damping,

Hydroelastic Instabilities


AcuSolveʼs practical FSI approach

• Time domain solution of structural dynamics in modal

coordinates together with flow equations.

• Only a single iteration loop including equations for

continuity & momentum, turbulence, mesh, and structure.

• Good convergence behavior of coupled system solution

even for high density fluids.

• Efficient and stable solution procedure for strongly

coupled systems including complex turbulent flows.


Chapter Divider with Example Image, Arial 40 pt white

Hydrodynamic Damping of Blades


Introduction to hydrodynamic damping

idealized blade in air

➜ only structural dynamics

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idealized blade in still water

➜ added mass effect (acoustic FSI)

idealized blade in flowing water

➜ added mass + hydrodynamic damping

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Hydrodynamic damping at a hydrofoil

FSI simulations compared to experiment

• Thin hydrofoil situated in

a turbulent channel flow.

• Damping determination

from free vibrations after

intial loading using the

logarithmic decrement.

• Flow velocity influence

on hydrodynamic

damping effects.


Experimental damping determination

in a high speed cavitation tunnel

• Impulse excitation due to pressure

wave induced by a spark plug.

• Recording of structural response

with a laser vibrometer.


Numerical setup of hydrofoil FSI

AcuSolve

• Practical FSI with structural dynamics

in modal coordinates.

• Water as (slightly) compressible fluid.

• Sufficient mesh with 400 000 nodes.

• Small time step with 400 steps / period.

Comparing to "reference"

• Iteratively coupled “industry standard"

solvers for CFD and structural FEM.

• Larger time step size, but even higher

numerical effort.


Natural modes and frequencies (in air)

mode 1

1st bending

f = 583 Hz

mode 2

1st torsion

f = 985 Hz

mode 3

2nd bending

f = 1366 Hz


Ve

rtic

al D

ispla

cem

ent

Time

FSI simulation results - 1st bending mode

Decay behavior for different flow velocities


Comparison of results

Hydrodynamic damping over flow velocity

Hydro

dynam

ic D

am

pin

g R

atio

Normalized Inflow Velocity

Reference

Experiment

AcuSolve


Hydroelastic Instability of a Bypass Valve


Bypass valve of a spherical valve


Hydroelastic instability of a bypass valve

Situation

• At a unit with 500 m head, strong vibrations occured

during opening of the bypass valve of a spherical valve.

• Due to blockage of the servo-motor hydraulics,

the bypass valve stopped at a quite small opening

leading to high speed flow through narrow gaps.

• The interaction of gap flow and structural motion caused

strong self-excited vibrations of plunger and casing.

• Hydroelastic stability characteristics are investigated

using AcuSolve’s practical FSI approach.


Modal analysis of the valve structure

FEA model with 565 000 elements


Natural modes and frequencies (in air)

mainly involved in the unstable motion

Mode 7

mainly plunger bending

f = 440 Hz

Mode 10

mainly plunger bending

f = 516 Hz


CFD model of the valve with 3 mm gaps

Unstructured mesh with 6 million elements


Transient CFD result for increasing inflow

Streamlines and pressure field at 5 m/s

• Maximum gap velocity of about 125 m/s

• Inlet pressure of 4.5 MPa corresponding to 450 m head

24

FSI analysis with 20 structural modes

• Initial pertubation of mode 7 (vertical plunger bending)

• Slowly increasing inflow velocity starting at 3 m/s

• Corresponding to initial head difference of approx. 160 m

Inlet pipe velocity Inlet pipe pressure

Hübner & Seidel | Altair Technology Conference | Munich | 2014-06-26

25


Time history of modal displacements

Hydroelastic stability limit

Stability limit

vinlet = 6.0 m/s

Δp = 6.4 MPa

ΔH = 640 m

stability limit

damped: stable self-excited: unstable


Alternating velocity field


Alternating velocity field

visualized by streamlines


Conclusions

29

Conclusions


forces (tractions)

displacements

CSD

(structure)

CFD

(fluid)

Time

Dis

pla

ce

me

nt

v0

v1

v2

v3

v4

• Analyses of submerged components considering

FSI require problem adapted solution strategies.

• Hydrodynamic damping effects can be reliably

predicted using AcuSolve’s practical FSI.

• Hydroelastic instabilities can be identified and

prevented using AcuSolve’s practical FSI.

• AcuSolve’s practical FSI is a stable, accurate, and

efficient tool for solving complex turbulent flows

interacting with linearized structural dynamics.

• Further enhancements are highly desirable, e.g.

- Cavitation modeling including 2-phase flow

- Convenient post processing


Dr.-Ing. Björn Hübner

Phone +49 7321 37 6693

[email protected]

Voith Hydro Holding GmbH & Co. KG

Corporate Technology − Basic Development

Heidenheim − Germany

Engineering

FSI in hydraulic machinery - Applying AcuSolve to identify hydrodynamic damping and hydroelastic instabilities