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Experimental methods in Marine Hydrodynamics

Experimental methods in Marine Hydrodynamics
TMR7 Experimental methods in Marine Hydrodynamics – week 35 Instrumentation (ch. 4 in Lecture notes) • Measurement systems – short introduction • Measurement using strain gauges • Calibration • Data acquisition • Different types of transducers 100 90 80 70 60 Instrumentation 50 40 and data 30 20 acquisition 10 0 0.5 1 1.5 2 2.5 Speed m/s Measurement result Physical process 1 (numbers) Resistance NThe old resistance measurement system x kg Towing Carriage Ship model Transducer = weights, wheels and string Data acquisition = writing down total weight 2The new resistance measurement system Data acquisition and signal conditioning system A/D Filter Amplifier Towing Carriage Ship model Transducer based on strain gauges 3Measurement systems Analog signals Digital signals + 10 mV + 10V DC Amplifier Filter A/D Transducers 4Strain gauges 5D R R R Wheatstone bridge B Force K •DR is change of resistance due to elongation of the strain gauge Strain 1 2 gauges • R is known, variable resistances in the amplifier A V B G g • V is excitation – a known, in constant voltage source • V is signal g C Side view Front view V in Supply of constant voltage 6 D R+ R RWheatstone bridge • Constant voltage (can also be current) is supplied between A and C • The measured voltage (or current) between B and G depends on the difference between the resistances R R 1 4 • One or more of the resistances R R are strain gauges 1 4 • If all resistances are strain gauges, it is a full bridge circuit • If only one resistance is a strain gauge it is a quarter bridge Supply of constant voltage circuit 7 Output voltage measurementD R R R Force transducer with two strain gauges, using a Wheatstone half bridge B Force K Strain 1 2 gauges A V B G g C Side view Front view V in 8 D R+ R RCalibration • How to relate an output Voltage from the amplifier to the physical quantity of interest Adjust calibration Known load Known measurement value factor Analog signals Digital signals + 10V DC + 10 mV Amplifier Filter A/D Transducers In a measurement: Measurement value = transducer output  amplification  calibration factor In a calibration: Calibration factor = Known load / (transducer output amplification ) 9What is the calibration factor dependent on • Type of strain gauges used (sensitivity) Sensor dependence • Shape of sensor and placement of strain gauges • Excitation voltage Amplifier settings • Amplification factor (gain) dependence This means that one shall preferably calibrate the sensor with the same amplifier and same settings as will be used in the experiment 10Zero level measurement • The measurement is made relative to a known reference level – Typically, the signal from the unloaded transducer is set as zero reference • Two options: – Balancing the measurement bridge by adjusting the variable resistances in the amplifier • Tare/Zero adjust function in the amplifier – First making a measurement of the transducer in the reference condition (typically unloaded), and then subtract this measured value from all subsequent measurements • This is usually taken care of by the measurement software (Catman) • In hydrodynamic model tests, we usually use both options in each experiment 11RDR R Amplifiers • Many different types: – DC – AC – Charge amplifier (for piezoelectric sensors) – Conductive wave probe amplifier • Provides the sensor with driving current (V ) in • Amplifies the sensor output from mV to (usually) 10V DC • Tare/zero adjust function (bridge balancing) – Adjusting the resistances R , R , R , R in the Wheatstone bridge 1 2 3 4 to get zero V in unloaded condition B G Force K Strain 1 2 gauges Analog signals Digital signals A V B G g + 10V DC Amplifier Filter A/D C 12 Transducers Side view Front view V in D R+ R RA/D converters • Conversion of analog 10V DC signal to digital • Typically 12 to 20 bits resolution • Typically 8 to several hundred channels • Each brand and model requires a designated driver in the computer, and often a custom data acquisition software • Labview works with National Instruments (NI) A/D converters, but also other brands provides drivers for Labview • Catman is designed to work only with HBM amplifiers Analog signals Digital signals + 10V DC Amplifier Filter A/D 13 TransducersA/D conversion – sampling of data • The continuous analog signal is sampled at regular intervals the sampling interval h s – The analog value at a certain instant is sensed and recorded • The analog signal is thus represented by a number of discrete – digital – values (numbers) • The quality of the digital representation of the signal depends on: – The sampling frequency f=1/h Hz – The accuracy of the number representing the analog value • The accuracy means the number of bits representing the number 8 • 8 bit means only 2 =256 different values are possible for the number representing the analog value = poor accuracy 20 • 20 bit means 2 =1048576 different values = good accuracy – The measurement range vs. the range of values in the experiment – High sampling frequency and high accuracy both means large amounts of data being recorded = large data files • The reason not to use high sampling frequency is mainly to reduce file size 14Sampling frequency Nyquist frequency f c 1 f c 2h Means: •You need at least two samples per wave period to properly represent the wave in in the digitized data •You should have more samples per period to have good representation … •Less than two samples per wave period will give “false signals” (downfolding) 15Effect of folding Response spectrum • To avoid folding: S – Make sure f is high enough c that all frequencies are correctly recorded f c or – Apply analogue lowpass filtering of the signal, removing all signal frequency components at frequency above f before the signal is c sampled 16Filters – to remove parts of the signal Amplitude Ideal characteristic Real characteristic Removes high frequency part of signal (noise) Low pass filter Removes low frequency part of High pass filter signal (mean value) Retains only signals in a certain frequency band Band pass filter Frequency Analog signals Digital signals + 10V DC Amplifier Filter A/D 17 TransducersFiltering – low pass filter Asymmetric filtering (used in realtime) 2.5 2 1.5 1 0.5 0 Averaging window 0 10 20 30 40 50 60 70 80 90 100 0.5 1 1.5 2 2.5 Symmetric filtering (can only be used after the test) 2.5 2 1.5 1 0.5 Averaging window 0 0 10 20 30 40 50 60 70 80 90 100 0.5 1 1.5 2 2.5 Now Real time filters always introduce a phase shift – a delay 18Data acquisition without filtering • It is OK to do data acquisition without filtering as long as there is virtually no signal above half the sampling frequency – so there is no noise that is folded down into the frequency range of interest • Requires high sampling frequency – (100 Hz, depending on noise sources) • Requires knowledge of noise in unfiltered signal – Spectral analysis, use of oscilloscope • Unfiltered data acquisition eliminates the filter as error source, and eliminates the problem of phase shift due to filtering – Drawbacks: • Must have good control of highfrequency noise • Large sampling frequency means large data files 19Selection of filter and sampling frequency • The problem with high sampling frequency is that result files become large – Double the sampling frequency means double the file size – This is less of a problem for measurement of lowfrequency phenomena (ship motions etc.) • Lowpass filter should be set just high enough to let the most highfrequency signal of interest to pass unmodified • Sampling frequency should then be set to at least twice the lowpass filter cutoff frequency, preferably 510 times this value – 20 Hz LowPass filter  minimum: 40 Hz sampling recommended: 200 Hz sampling 20Data acquisition software • Communicates with the A/D converter • Conversion from 10V DC to physical units • Records the time series • Common postprocessing capabilities: – Graphical presentation of time series – Calculation of simple statistical properties (average, st.dev.) – Zero measurement and correction for measured zero level – Storage to various file format 21Data Acquisition with digital amplifiers Transducer MGC+ amplifier PC computer (strain gauge) Analog Digital signal in mV/V signal Physical units Change of resistance Strain gauge excitation (amplification) Data acquisition software due to elongation Bridge balancing Collecting data Analog to digital conversion Statistical analysis Zero correction (tare) Presentation Filtering and signal conditioning Storage to various file format Conversion to physical units 24Length of records of irregular wave tests and other randomly varying phenomena • The statistical accuracy is improved with increasing length of record. The required duration depends on: – The period of the most low frequent phenomena which occur in the tests – The system damping – The required standard deviation of the quantities determined by the statistical analysis • Rule of thumb: 100 times the period of most low frequent phenomena of interest 25Length of records Typical full scale record lengths: • Wave frequency response: 1520 minutes • Slowdrift forces and motions: 35 hours (ideally 10 hours) • Slamming • Capsize • To study and quantify very rarely occurring events, special techniques must be applied 26Transducer principles for strain and displacement measurements • Resistive transducers – Change of resistance due to strain – strain gauges • Inductive transducers • Capacitance transducers 27Inductive transducers • Measures linear displacement (of the core) • Needs A/C excitation • Used also in force measurements in combination with a spring or membrane Linear variable differential transformer 28Force measurement instruments: Dynamometers • 16 force components can be measured • Strain gauge based sensors are most common • One multicomponent dynamometer might be made of several one, two or three component transducers • Many different designs are available • Custom designs are common • Special dynamometers for special purposes like: – Propeller thrust and torque – Rudder stock forces 29Propeller dynamometer for measurement of thrust and torque 30Threecomponent force dynamometer 316 component dynamometer 32Pressure Measurements Transducer principles Inductive Strain gauge Piezoelectric 34Pressure Measurements Requirements • Stability is required for velocity measurements – Strain gauge or inductive • Dynamic response (rise time and resonance frequency) is important for slamming and sloshing measurements – Piezoelectric – Strain gauge 35Position measurements • Mechanical connection: – Inductive transducers – Wireoverpotentiometer – Wire with spring and force measurement • Without mechanical connection: – Optical and video systems – Acoustic systems – Gyro, accelerometers, Inertial Measurement Units (IMU) 36Mechanical position measurements Axial force transducer Potentiometer Measuring rotation Spring Wire connected Wire connected to model to model Ship model 37Optical position measurement • Remote sensing, nonintrusive measurement • Using CCD video cameras • Each camera gives position of the marker in 2D • Combination of 2D position from two cameras gives position in 3D by triangulation • Use of three markers on one model gives position in 6 DoF by triangulation • Calibration is needed for the system to determine: – Camera positions and alignment • The relative positions of the markers on the model must be known to the system 38Optical position measurement principle 3941Velocity measurements • Intrusive measurement (probe at point of measurement) – Pitot and prandtl tubes for axial or total velocity measurement – Three and five hole pitot tubes for 2 and 3D velocity measurement – Various flow meter devices • Nonintrusive measurement (no probe at point of measurement) – Laser Doppler Anemometry (LDA or LDV) • Measures velocity in a single point at each time instance – Particle Image Velocimetry • Measures flow field (2D) in one instant 43Prandtl (pitotstatic) tube 2 1 DPV 2 V 2DP 44Pitot tube • Smaller size than Prandtl tube • Less accurate, due to sensitivity to static pressure 2 1 PPPVghgz tot dyn stat 2 P tot z V h 451 80 350 170 330 150 325 320 310 130 290 110 Prandtl tube rake for propeller wake measurements 1.035 Axial wake 0.45 0.828 0.40 0.35 0.621 0.30 0.25 0.20 0.414 0.15 0.310 0.10 0.05 0.00 270 90 Axial wake shown as color contours Propeller disk indicated by dashed line 46 250 70 230 50 45 40 210 20 190 018 0 350 1 70 330 150 325 320 310 130 290 0 11 Reference vector 0.1 1.035 Fivehole pitot tube 0.828 Axial wake 0.45 0.621 0.40  V 0.35 0.30 0.414 T  0.25 C Radial wake com ponent 0.20  0.15 B (Horisontal) 0.10  V 0.05 270 90 0.00 VIEW FR OM SIDE =20 degrees   P  C Tan g en tial w ak e co m p o n en t  S (V e rt i c a l )   VIEW FR OM AB OVE Axial wake shown as color contours 47 Radial and tangential wake shown as vectors Propeller disk indicated by dashed line T S P C B VIEW FR OM THE FR ONT 250 70 230 50 45 40 210 20 190 0Particle Image Velocimetry (PIV) • Velocity distribution in a plane is found from the movement of particles in a short time interval • Highspeed video is used to capture images • A sheet of laser light is used to illuminate the particles in the water • Finding the velocity by comparing the two pictures is not trivial • ”Seeding” the water with suitable particles is another practical challenge 483D Particle Image Velocimetry (PIV) • Like 2D PIV, except that two cameras are looking at the particles from different angles • You obtain 3D velocity vectors in a plane 49Laser Doppler Velocimetry (LDV or LDA) Photo courtesy of Marin, the Netherlands • Point measurement – must move the probe to measure at different locations • Calibration free • Give 3D flow velocity – also time history  can measure turbulence intensity 50Practical arrangement for stereo LDV and PIV 60Applications of velocity measurement systems • Pitot and Prandtl tubes: – Intrusive measurement of velocity at a single (or few) points – Cheap, simple and reasonably accurate average • LDA/LDV – Very accurate, very high resolution point measurements, useful for turbulence measurements – Nonintrusive – Doesn’t require calibration – Costly and time consuming • PIV – Measurement of flow fields – Nonintrusive – Tedious calibration required for each new test setup – Very costly and time consuming 61Wave probe amp. output wave probe 10V DC Wave probes + + Measurement of resistance, Conversion to +10V DC Conductive wires Water will shortcircuit between the wires 62Relative wave measurements 63Acoustic wave probes • Working principle: A sound pulse is emitted, and the time it takes the reflected sound to reach the probe is used to calculate the distance to the water • Benefits: – Works also at high forward speeds – Nonintrusive – Calibration free • Drawbacks: – More costly – Steep waves in combination with smooth surface (no ripples) causes dropouts, when no reflected sound reach the probe 64
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