, International Council for the C.M. 1990I C:30 Exploration of the Sea Hydrography Committee A FJELD INTERCOMPARISION BETWEEN AN ACOUSTIC CURRENT PROALER I ADCP AND TWO SAVONIUS TYPE CURRENT METERS By Trygve Gytre and 0ivind 0stensen lost. ofMarine Research P.O. Box 1870n2 Nordnes 5024 Bergen Abstract During an introductory experiment in Lofoten May 1990 data from avessei mounted ADCP on an anchored research vessel were intercompared with corresponding data from two ancho­ red rotor current meters in a region where the current speed varied between 0 and 30 ern/s. • The results showed good agreements when the ships heading was stable. When the anchored ship described "S" or "0"- shaped horizontal motions due to varying wind stress, the ADCP added up to 15 crn/s. to its output signal. Introduction. For many years the majority ofmoored eurrent meters have exploited different versions ofthe Savonius rotor as a sensor for eurrent speed. When such a a rotor rotates with a frequeney F due to a driving eurrent speed V, the eurrent speed may be expressed as V = g(F) where g(F) is a polynominal ofthe form: AO +A1xF +A2 x F2 +A3 x F3 AO represents the eurrent speed threshold velocity which depends on the rotor's hearing frieti­ on and its moment ofinertia. Most instruments measure F by eounting the number of rotor rotations N during a fixed time interval T and then ealculating Nff . In order to express the veetorial eurrent speed eomponents during eaeh T, the direetion must be eontinuously or very frequently measured by a compass. • In the last 10 years a new generation ofcurrent meters -the Acoustic Doppler Current Profiler (ADCP) which is based on remote measurements of the Doppler frequeney in different layers ofthe water has gradually entered the market. This new sensor has a technical potential to dominate future current measurements. Therefore the field hehaviour of the ADCP compared to that of rotor based current meters under different field eonditions has a signifieant interest. This paper describes an initial intercalibration experiment during a survey which was primarily dedicated to turbulence and recruitment research. That meant that the available ship time could not be solely dedicated to this experiment. During survey with the research wessei 0.0. Sars outside the Lofoten Islands May 1990 the data from an anehor station containing 1 Aanderaa model RCM-7 rotor eurrent meter and 1 Sensordata model SD-2000 "MINI" rotor current meter ( both manufactured in Bergen) were intercompared with the current speed data presented by a ship mounted Acoustic Doppler Cur­ rent Profiler (ADCP) (made by RD Instruments,Califomia. USA) Page 1 , Instrument description. Fig. 1 shows the basic mechanical design ofthe Aanderaa model RCM-7 vector averaging cur­ rent meter. It consists of a recording unit and avane assembly mounted on each side of a gim­ ble. The current speed is measured by a modified Savonius type rotor with a given threshold velocity of2 cm/s. The current direetion is measured by a compass with an analog eleetric output resolu­ tion equal to 0.35 degrees.. The real accuracy in direction depends on the ability of the vane to direct the instrument in the current direction. Since the instrument recording unit and the vane are on opposide sides cfthe girnble, the instrument drag reduces the net direetional force. The factory states an accuracy of +/- 5 degrees for speeds from 0- 100 cm/s and +/-7.5 degrees for 2.5- 5 crn/s. Oata from the RCM-7 are vector integrated during the measuring period and then successively recorded in a RAM memory. Readollt is made by opening the instrument and transfer the recor­ ded infonnation to computer via a readollt unh. VN'E. AANDERAA 1 MODEL RCM·7 • 495MM 1 WEIGHT IN AIR 25.8 KG .....1--_-'-_865 MM, _ • Fig. 1 Model RCM-7 Current meter' Fig. 2 shows the mechanical design of the 5ensordata model 50-2000 "Mini Current Meter" The instrument consists of a recording unh and a vane assembly on one side of a gimble and a balancing rod on the opposite side. This design rriakes the drag ofthe recording unh to coinside with the vane drag. Model 50-2000 measures the current speed with the same rotor as the RCM-7 . The rotor revolutions are counted and the compass is measured once during periods of 4 minu­ tes. The compass has an accuracy of +/- 7.5 degrees. After each 4 minute period the instrument generates a programmable delay of N x4 minutes until next period. (N can be set from 1-99) Recorded data from 50-2000 memory are delivered non galvanic as coded flashes oflight from the instrument transparent top cap into a pe via an optical "readout head ". Fig. 3 shows the principle for data readout. Page 2 .. 'iPiiilfl--........ VPI'E. I SENSOROATA 490 MM MOO. SO·2000 l~~·~ MI-(jIJ!'i' 1""5~~Trc IN I 1'1A~t-FT ~1jI:ö'" I FOlO'l WEIGHT IN AIR: 3.5 KG .,CI -rQClM.r."rt OArA .. 600 MM T~ YOLP De Fig. 2 Mod. SD-2000 current meter Fig. 3 Principle for data readout from SD-2000 • The RD INSTRUMENTS ADCP which is insta1led on the M/S 0.0. Sars transmits 150 kHz acoustic pulses of programmable duration along four inc1ined beams defmed by highly directio­ nal transducers. Backscattered sound from small panicles in the water is received by the transducers with a Doppler frequency shift proportional to the relative velocity between the scatterers and the transducers. During the experiment the ADCP was set to measure the current speed referred to the bottom in 8 m deep bins. The signals were spatially and temporally integrated and processed to give the North and East velocity component each 10. minute. Fig. 4 shows how the data were acquired from the ADCP. The ship was anchored in direction 220 degrees. Due to variations in the wind stress, the ship's heading direction angle varied within +/- 7 degrees. During the intercomparision the signals from bin 2 (17-25) meter were used. • N - - 9 M-- ______ 1~ • ..: 17 M I " '-\ Bin 1 Cl I \ a> "0 ,'8M \ BIN 2 o \ , oo::t I \ : A;/~20 +/- 7 degr. ,.,,##~ " '- BIN 3 ..... I \ --------~------------~--------_. I \ E Fig. 4 Setup of the RD Instruments ADCP Page 3 :0- 1 18-r u PERIODl (,J PERIOD2 en 16-- ...... 14- ~ I ,- ... 12- 10- / \ I 0. (,J I \ j (,J 8- a.. I ~ en 6- \I \ ./ j Q 4- J \ ...z :3: ~=I I I I I I I I I I 0 ::: 4 6 8 10 tZ t<t 18. 18 ::!O 2Z :os 27 :::g 3 t 33 3S. Hour Fig. 6 Wind velocity during: the intercomparision • :O-r------------"":""""---------------- - 18-1-------------1------------------ :J ~ 16-~------:_=_=-_:__,/-}--------__::_:__;o1r_------ ~ 14-1------.....;,.=;~__t~r+~~~-----~:f'7_+\:_-----­ :J 0 3 4 6 7 9 10 1::! 13 1S 16 18 19 ::!t ~ :S :6 :s ~ 31 Hou.. ~'l :.J :0- 2 :.::I 19-; :.J I ::l 16-' I - 14-1 ~'l 1:::-' :5 10-' '! • -~ ... -=: > g JO. h 1 ""1 ::l 0- --=: I I (,J 0 3 4 6 7 9 10 1::: 13 1S 16 18 19 :::t ::2 :::s :::6 :::9 :::9 31 3::! Haur Fig.7 Horizontal ship morlons and recorded. current speeds from ADCP. RGvf-7 and SD-2000 Fig. 7 shows that the current speed measured by the ADCP was significantly higher than the speed. measured by the rotor current meters during period 1. ' When the horizontal ship motions were small (most of period 2), the ADep and the RQyf-7 showed dose to idenrical data. Duting this period the datafrom SD-2000 showed a lower current .' Page 5 ,.------------------------ - ----- -----,1 speed than the other instruments. The anchor station shown on fig. 3 was operative for 8 days making a prolonged intercomparisi­ on between RCM-7 and SD-2000 possible. Fig. 8 shows an 8 day intercomparision between the two rotor current meters. File naMe: ninicM Current 'speed (CM/sect. 20-.-------------------------------- 18-1----------------------------- 16-1-----------------:------;-- ------- 14-1----------------:+----- ----~-- 12-'"""·----~---__t_-----____rl:!f~-----r;----'-+_---_:tT-- 10- ~--....---f!t_:---+_~-.___--_1Hfii:+_-__+h___.__;_:_-----1r,__-­ 8-1---*--1i!1!--Hnfr---:--+.--+-r:;,,-----1-4i1--i-+_---rHr.--!-~----;__';_-­ 6-1-t--...l-J\r---l~I4i4J!--..;...:....-1b...-+.:.J.h~---l----!1ll.:-;I\tHII4~if.--,,....---_t_----'__T-­ 4- Hh+-f--11+-l--1----jlF\'!tt:h:-:-:f-l\W---4:-:ftlh-lT~~~H--FIWJ--lm1htt.dtif__t7i:""- . 2- I o 3 4 5 6 7 8 Da", File nat'\e : aande'~8a • Current speed (CM/sec). 20-.-------------------------------- 18-1----------------------------­ 16-1---------------------.....,,------­ 14-1----------------+-------j;7-"-----:---- ~ 12-1---...,A.,.-----,---:r-t,.:i----;----fffl----.'/----r--o--------:'+---J, ' ,{ " r; \ r~ L ~ I, 11 ,.f 11, I, 11 10-1---4----t\-1;-~1....!-r ........--__Pr1-f-__;~-ft,__-+~----++--- 8-1----t--l---,-1f-iII-----I4+-~-'---+\---_"L...!+_-II'li_----l..f-_+_-r-----+,I\ h n, f],{' I! ,.'! plI' Ji! 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