CORRELATION OF MODEL-SCALE AND FULL-SCALE...

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Dessi, D., D’Orazio, D.

INSEAN-CNR

Rome - Italy

CORRELATION OF MODEL-SCALE AND FULL-SCALE DATA:

SENSOR VALIDATION AND ELASTIC SCALING EVALUATION

MSC RoadShow – University of Rome «La Sapienza», June, 12th 2012

This work was funded by the Italian Navy within the cooperative research project MOU-6dof-RANS, whose aim was the development of codes for the prediction of the ship behavior in wavy seas

3D-FE Analysis

Full-scale trials

Equivalent beam

simulations

Scaled model tests

Main investigation

areas

Project structure – hydroelastic side


Project structure – hydroelastic side

Equivalent beam

simulations Scaled

model tests

3D-FE analysis

Full-scale trials

response prediction & comparison


Motivation of this work

• Full-scale measurements will be more available in the future as long as monitoring systems are installed on-board. • Is it possible to validate the codes, or at least the scaled physical modes, with the full-scale measurements? In this presentation, with reference to global loads (VBM) we will focus on: • full-scale data validation and/or error recovering • analysis of intrinsic differences between the ship and the scaled model


Full-scale measurement

Motion reference unit for 6dof motions

WAVEX radar for wave field recongnition

GPS for trajectory meas.

Wave finder for relative wave meas.

Bow panel strain gages for slamming impulse loads

3 sg x 5 sections for bending moment distribution

Italian Navy patrol vessel

Lpp = 80 m


Real Ship 3D FE model (PATRAN)

• mass distribution • bending stiffness distribution • shear area distribution2

° modo.JPG • natural frequencies, mode shapes (NASTRAN – SOL 103)

Reference Ship Data 1D Equivalent Structure

Reduction to 1D ship beam

Segmented model

Ship data for validation

Real ship


Scaled model-tests


Comparison between full-scale and scaled meas.

Under which assumptions, scaled tests and full-scale trials can be compared?

• Compared quantities has to be homogeneous Strains can not be the same, better use of VBM in the same locations • Ship has to be in the same loading conditions

Unexpected heel angle of about 2 degs could not be reproduced with the experimental set-up.

• Speed and encountered sea are the same

The wave-maker reproduced the same sea-states in the towing tank. However the linear towing-tank basin imposed to ensure head and following wave conditions in the ship trials by cruising at given angles with the prevalent sea direction. Use of RAO allows to minimize uncertainties in the test input conditions (RAO = )


2ppsf LBhM

Practical determination of bending moments

However, unlike rigid-body motion, their determination is not straight-forward at full-scale and may be affected by several error sources.

Theoretical shear force and bending mom.

Given force input V

Mf

Meas. output voltage

k =Mf /V

Calibration factor

Elastic model strain-gage calibration



We have to rely on the virtual calibration of the FE model

k =Mf /V

Ship strain-gage calibration


Strain gage position

FEM model


k =Mf /V = k1 k2 V

sg

loc

Mf glob

vbend

Thermal strain

Local modes

Global modes other than vert. bending ones

Device block

Stra

in g

age

k1

Physical block

k2

Structural element

Strain-gage

Calibration rod sg

vbend sgyyvbendy zIEM /

measured

calculated

PFPPyy dFzI /


Preliminary results were not so satisfactorily [HYEL09], even if: • VBM is dependent on the longitudinal ship loading that, in the same sea state, should be close between model and full scale. Let us observe: • RAO (Response Amplitude Operators) were computed using preliminary wave spectrum data in low sea states ensuring linear response regime

Previous results

sgPifdFMdFzI

zIEMFPy

PFPPyy

sgyyvbendy

//

However, in practice: • Bending moment is dependent on sensor calibration and expected deformations • Computation of RAO is sensitive to inappropriate wave spectrum model


• Determination of k1 (sensor calibration) does not present any particular difficulty • Assessment of the disturbances and the strain - VBM relationship (k2) must be investigated • Thermal stresses due to reasonable thermal loading induce negligible structural deformations • Effects of global unsymmetric loading with respect to the longitudinal vertical plane are partially filtered out taking the average between the port and the starboard strains


T=15°-30°


In the past, we have used several techniques to identify ship elastic modes based on output-only approach (no-input measurement, ambient excitation) that: • Frequency domain decomposition [JSR 2008] • Proper Orthogonal Decomposition [JFS 2012,…] In this case, a less straightforward technique in the time domain known as Stochastic Subspace Identification is used because of noise on the data (it requires a certain expertise on modal analysis and a feeling on expected results)

Identification technique


Strain-modes analysis

Modes obtained with SSI in terms of strains: • points in the top side refer to upper deck measurements • points in the bottom side refer to the keel measurements

Modes are poorly described


Can we trust on the data? Is there any chance for an internal data validation?


The idea is to find a tool for independent validation / recovering of the data based on data internal coherence Criteria for internal coherence of strain-gage / VBM results: • if strain measurement are correct, strain modes should appear • an acceptable VBM RAO variation along the ship sections has to be preserved


Sensor validation / Data recovering

Timoshenko beam parameters from 3D-FE model

Experimental strain mode identification with SSI

Mode shape evaluation on strain-gage points

Same eigenvectors normalization

Comparison between Num. & Exp. 1-st bending mode

Correction of strain-gage calibration constants (only on faulty sensors)

Correlation between full-scale and model scale experiments is improved

Re-extract experimental modes

Orthogonality among exp. modes has improved?

Comparison between Num. & Exp. Is enhanced?

Check box

Strain vibration mode extraction from 1D Timoshenko beam


• Next, it is assumed that the local strain effect is proportional to the structural loading, i.e., to the global response represented by the VBM strain • Thus, since , , using the previous relationship yields:

It is reasonable to assume that other effects can be included in the calibration coefficients ( / Volt) of the strain gages. • The correction factor for the calibration coefficients is defined as

Strain-modes analysis via SSI

vbendsmodesloc

vbendmodeslocsg

vbendvbendvbendsg )1( sgsgvbend )1(

),,,(),,,(ˆ (exp)(exp) tzyxktzyx iiisgiiisg )()(

(exp)

)(

xx

ksg

numsg

First mode Overall strain gage time-history


Strain-modes analysis via SSI

Modes shapes

Modal assurance criterion

Validation Check



The comparison in terms of the response amplitude operator of the vertical bending moment is more reliable than the direct comparison of the outputs

2/1* )/( gLff ppee


2ppsf LBhM


The comparison in terms of the response amplitude operator of the vertical bending moment is more reliable than the direct comparison of the outputs


2ppsf LBhM

• Analysis of the effects due to the load segmentation • Analysis of the effects due to the difference between the model backbone and the ship beam • Analysis of the effects due to the differences between the ship-beam and the 3D-FE model

From scaled to full-scale tests

Also error sources inherent to the intrinsic differences in the physical models has to be considered using a chain of numerical models representing just one difference at time from the segmented elastic model to the real ship


Num. vs. exp. (true ship)

Lack of correspondence may depend on several reasons: •The ship is a quite complex and huge structure that may suffer of:

• design modifications • difficult in modelling some ships details

• Fot the “wet” real ship, estimation of damping is a critical point

Non-dimensional time

Non

-dim

ensi

onal

VB

M

0 5 10 15 20 25 30 35

Ship on-board measurement3D-FE simulation

0

0

In this comparisons, quite the “same” load has been applied.


Concluding remarks

• Use of 3D finite element modeling:

• Extraction of Timoshenko beam parameters • Strain-gage calibration • Vibration modes • Time-domain simulations

• Sensor validation / Data recovery by: • comparison with numerical model on 1st mode shape • checking over the other identified modes (shape and orthognality)


Concluding remarks

Thank you for your attention


References: 1. Coppotelli, G., Dessi, D., Mariani, R. and Rimondi, M. (2008). "Output-only analysis for modal parameters estimation of an elastically scaled ship" in Journal of Ship Research. 2. Dessi, D. and D'Orazio, D. (2010). "Analysis of the vessel global loads via ship trial investigations and segmented-hull tests“ in 28th Symposium on Naval Hydrodynamics, Pasadena (CA). 3. Mariani, R. and Dessi, D. (2012). "Analysis of the global bending modes of a floating structure using the proper orthogonal decomposition" in Journal of Fluids and Structures.

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