Wave Directional Spectrum Estimation Based On ADCP Current Measurement

Based on RD ADCP(acoustic Doppler current profiler) and Nortek AWAC(acoustic waveand current profiler), the paper works on wave directional spectrum estimation based onADCP wave current measurement, the paper also works on data processing and softwaretechnology research. Considering the problems of RD software that on wave directionestimation algorithm, the threshold of tilt correction, the threshold of background currentcorrection, the paper analysis the problems and then makes improvement.Wave directional spectrum estimation of ADCP can be decomposed into two parts: waveheight spectrum estimation and wave direction estimation. Velocity along the beam, measureddepth of ADCP, pressure data of ADCP are used in wave height spectrum estimation. Thereare two methods in wave direction estimation: estimation by velocity points array of ADCPand water particle velocity of ADCP beam center. The main factors that can affect ADCPwave directional estimation performance are: cross-spectrum estimation, wave directionestimation algorithm, measured points correction when ADCP is largely tilted, wavedirectional spectrum correction when background current is strong. The paper also works onwave directional spectrum estimation of ship borne ADCP. The theoretical research providestheoretical support for the follow-up data processing and software technology research.Wave direction is estimated by inverse calculation of equation that containscross-spectrum. Considering inverse calculation has different methods, so wave directionestimation algorithm is not unique. Fourier series algorithm(DFTM), iterative maximumlikelihood algorithm(IMLM) are used by RD and Nortek, main use of IMLM, IMLM canestimate unidirectional wave and bimodal wave during test, it cannot suppress the noiseenergy of other directions. Considering the problem of IMLM, the wave direction distributionmodel is introduced to effectively solve the problem of noise energy. The paper uses a hybridgenetic algorithm (HGA) to estimate parameters of the model, the estimation performance ofHGA is better than the genetic algorithm(GA), it show that HGA jumps out of local optimalsolution and convergence faster. During the simulation analysis of BBADCP(broadbandacoustic Doppler current profiler) and NBADCP(narrowband acoustic Doppler currentprofiler), HGA works better than IMLM, it shows that wave directional spectrum error ofHGA is less than IMLM. During ADCP measured data analysis, the estimated result of IMLMand HGA of the paper is consistent with RD WAVESMON and Nortek Quickwave, it provesthat data processing technique of wave directional spectrum estimation based on ADCP current measurement of the paper is correct; HGA and IMLM can both accurately estimateunidirectional wave and bimodal wave, but HGA can effectively inhibit the wave energy ofthe noise in the measured data, HGA is more accurate in wave direction estimation.When ADCP is largely tilted, the measured points of ADCP will obviously changelocations, and then wave directional spectrum estimation will has large error, so when it islargely tilted, ADCP needs correction for tilt before wave directional spectrum estimation. RDdoes not give correction method of measured points, the paper introduces the rotation matrixcalculation to derive correction method of measured points. When the tilt angle is bigger than10°, RD needs correction for tilt. During the simulation of paper, the wave directionalspectrum estimation error not only concerns with tilt angle, but also concerns with the ADCPdepth, the threshold of RD tilt correction is obviously not reasonable, during the test ofmeasured data, the simulation result is verified by the measured ADCP data. The paper sets upa new tilt correction threshold, that ADCP data is corrected or not considers the tilt angle andADCP depth, when the significant wave height change because of ADCP tilt is bigger than thespecified threshold, ADCP needs correction for tilt. The significant wave height change canbe gotten form simulation and it makes the tilt correction simpler. The tilt threshold of thepaper is more accurate than the threshold of RD tilt correction threshold.When background current is strong, it will affect ADCP wave directional spectrumestimation, wave directional spectrum needs correction for background current. Backgroundcurrent correction of RD considers background current amplitude, background currentdirection, wave direction and water depth, but the RD background current correctionthreshold considers only background current amplitude, namely0.75m/s, and it is obviouslyunreasonable. For example, when the background current amplitude is0.75m/s, thebackground current direction and wave direction angle has90°difference, wave directionalspectrum of ADCP estimation before and after the correction does not change. The paper setsup a new background correction threshold, that the data of ADCP is corrected for backgroundcurrent or not is related to background current amplitude, background current direction, wavedirection, water depth. When the significant wave height change because of backgroundcurrent is bigger than a given value, it needs correction for background current. During theADCP measured data, the background current threshold of the paper is more accurate thanRD threshold. Although the background current correction threshold of the paper will adds theamount calculation, but the threshold of the paper ensures accuracy of ADCP wave directionalspectrum estimation when background current is strong.