Comparison of model tests and calculations

Transcription

1 Comparison of model tests and calculations Experimental Methods in Marine Hydrodynamics Lecture in week 45 Covers chapter 12 in the lecture notes 1

2 Contents Validation or verification? Numerical vs. Physical models Steps in a validation procedure for wave induced ship motions Comparison of calm water performance 2

3 Validation or Verification? Validation: Are the computer program valid for a certain case? Compare the end result with realistic cases for which the program shall be valid Verification: Verify that the computer program is working correctly according to the theory Compare with idealized cases corresponding to the assumptions and simplifications in the theory 3

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5 Question: You are selecting a model to verify a computer program based on thin-ship theory. What kind of ship model would you choose? You are selecting a model to validate the use of the same computer program to calculate wave resistance of offshore supply vessels. What kind of ship model would you choose? Thin-ship theory is a potential-flow sink-source method where sinks and sources are placed on the centreplane of the vessel to represent the geometry 5

6 Physical versus Numerical Models Qualities Physical Models Numerical Models Representation Very Good Limited by available theories and computer power Accuracy Good Good within validity limits Scale Effects Yes No (depends ) Reliability Very Good (?) Risk of human errors Credibility Very Good Prima facie not good, improving! Flexibility Not Good Good Execution Long Low with standard programs Can be high for complex problems and long time series Cost High Development cost high 6

7 Model tests Benefits Can represent complex geometries, configurations and physics Accurate results for integrated (overall) forces and motions Gives a visual impression Can reveal unknown physical effects Drawbacks Scale effects Difficult to measure pressure and velocity distributions Time and cost 7

8 CFD calculations Benefits Can yield physical understanding through detailed pressure and velocity distributions Can (to a limited degree) provide results directly in full scale Quicker and cheaper than model tests (usually, but not for complex problems like sea keeping) Drawbacks Integrated forces (and motions) not very accurate Less good credibility Can hardly answer questions that is not asked 8

9 A note on numerical calculations and scale effects 9 For model tests, the difficult scaling problems are related to viscous flow effects Turbulence Flow separation and vortex shedding For numerical calculations, the difficult problems are related to viscous flow effects Difficult to perform calculations at Reynolds numbers for full scale ships ship resistance calculations are still frequently done in model scale Flow separation is very difficult to handle with RANS type CFD Must use Large Eddy Simulation (LES) or similar techniques that are extremely demanding with respect to computer power The problems of scale effects are not easily overcome by switching to numerical methods

10 Steps in a validation procedure for wave induced ship motions Equal model Geometry Vessel draft and trim Metacentric height (GMT and GML) Radius of gyration rxx ryy and rzz Equal environmental data Wave height H and period T at position of testing for regular waves. For irregular waves Hs, TP and spectral shape. Possible deviation in wave data along test track. Effect of water depth? If yes, shallow water to be included in calculations Test results influenced by diffracted and reflected waves, including tank wall effects? Equal test condition Forward speed Wave heading Transient effects in model tests (and also in calculations if time simulations) Equal natural periods Causes to possible deviations to be explained (mass terms or restoring terms, if decay; damping contributions). Influence of difference in conditions between model tests and prototype Establish by numerical calculations the effect of possible differences between model test conditions and calculation condition case on the results. Influence of error ranges on results Establish by numerical calculations the effect of error ranges for the different parameters on the final results. Comparison of wave results 10

11 Steps in a validation procedure for wave induced ship motions Equal model Equal environmental data Equal test condition Equal natural periods Influence of difference in conditions between model tests and prototype Influence of error ranges on results Comparison of wave results 11

12 Equal model Geometry Vessel draft and trim Metacentric height (GM T and GM L ) Radius of gyration r xx r yy and r zz 12

13 Equal environmental data Wave height H and period T at position of testing for regular waves. For irregular waves H S, T P and spectral shape. Possible deviation in wave data along test track. Effect of water depth? If yes, shallow water to be included in calculations Test results influenced by diffracted and reflected waves, including tank wall effects? 13

14 Equal test condition Forward speed Wave heading Transient effects in model tests (and also in calculations if time simulations) 14

15 Natural periods Compare calculated and measured resonance periods Heave, roll and pitch. Causes to possible deviations to be explained Mass terms or restoring terms if decay; damping contributions 15

16 Influence of difference in conditions between model tests and prototype Establish by numerical calculations the effect of possible differences between model test conditions and calculation condition case on the results. 16

17 Influence of error ranges on results Establish by numerical calculations the effect of error ranges for the different parameters on the final results Establish uncertainty of the model test Uncertainty of different result variables 17

18 Comparison of calm water performance Total resistance in model scale Wave profiles Along hull (read from pictures or video) In a longitudinal cut (from wave probe) Wake field measured in propeller disk 18

19 Comparison between experiment and Waveres for RoPax ferries Optimisation of C M Residual Resistance Coef. CR [-] 2.5E E E E E-04 M Experimental M Calculated M Experimental M Calculated M Experimental M Calculated M Experimental M Calculated 0.0E Froude Number F n [-]

20 Resistance coefficients (Labtest 2) x 10-3 Totalmotstandskoeffisient for skip C Ts Modelforsøk C Ts Beregnet nummerisk C R Modelforsøk C R Beregnet nummerisk C Ts [1] Fn [1] Gr. 2

21 Wave elevation along hull (Labtest 2) 4 3 Wave elevation (from WL) [m] Model test - 18 knots Calculation - 18 knots Model test - 20 knots Calculation - 20 knots Model test - 22 knots Calculations - 22 knots Model test - 24 knots Calculations - 24 knots -1 BOW Stern Model test: Data by Group Distance from Lpp/2 (pos. aft) [m]

22 Summary The difference between validation and verification Pro s and con s of model testing vs. numerical calculations Steps in a validation procedure for wave induced ship motions Aspects of comparison of calm water performance 22

23 Comparison of wave results Regular Waves: RAO Phase Added resistance Irregular waves: Standard deviation Statistical distribution Extreme values RAO Added resistance / speed reduction (Green water, Impact Loads etc., depending on actual case) 23

24 Comparison of Vertical Force Estimator (VFE) for a catamaran Model Tests Veres calc RAO a v (g) WavePeriod (sec) 24

25 Comparison of Pitch RAO for a catamaran RAO Pitch 4 /ka Model Tests Veres calc WavePeriod (sec)