TM No. 377 



Calibration of the OMDUM III System. The calibration procedures for the 

 orthogonally mounted ducted meter system ( OMDUM III) in the Converse Towing 

 Tank were similar to those used for the individually mounted ducted meters . 

 A series of tests was made to obtain the response of each cylinder in the 

 coupled position at various speeds for end-on flow. Then the system was 

 towed at various constant speeds and at various angles of O as depicted in 

 figure 11-10. Finally, both procedures were repeated under acceleration 

 and deceleration of the towing carriage. 



Results of steady towing at various speeds with 0=0 are shown in 

 figure 11-21 (crosses). The slope of the curve for the orthogonally 

 mounted system is distinctly greater than that obtained for the individually 

 mounted meters. The slope of the upper plot (OMDUM III) is about 0.383 cm" 1 , 

 whereas the slope of the curve for the individual meters is 0.322 cm"-. The 

 geometric configuration of the orthogonally mounted meters affects the flow 

 through the forward looking impeller so as to render the impeller more re- 

 sponsive or more efficient. This is due to a greater impeller angular velo- 

 city per unit flow speed than with the single ducted meter. 



This modified wave meter was designed to obtain precise knowledge of 

 system response characteristics as a function of off -angle flow, and to 

 determine how off=angle response varies with the absolute speed of flow. 

 This information is particularly critical for the orthogonal meters be- 

 cause covariance functions between the horizontal and vertical velocities 

 within ocean waves must be estimated. Since the covariance function is 

 derived by averaging the products of pairs of velocity values, it would 

 be a severe handicap to have to contend with products of data values 

 containing errors; e.g., a 10 percent error in the speed value of the 

 velocity components could give a 20 percent error in the covariance 

 functions . 



The tests to determine the response to off -angle flow were similar to 

 those made with the individually mounted cylinders. Runs were made at 

 various steady speeds to measure the response of each meter as a function 

 of <.\ Again the results were evaluated on a basis of how closely the out- 

 put of the OMDUM system compared to the actual tow speed. This is shown in 

 figure 11-23 where R s is the ratio of the flow velocity detected by the wave 

 meter to the actual tow speed. The numerator of R g is the square root of 

 the sum of the squares of the values of u and w obtained at various angles 

 of 9- with respect to the fluid flow. (Velocity outputs were transformed 

 into speeds using the OMDUM III calibration curve in figure 11-21.) The 

 average value of R^ is I.069 (+ O - 0,055). It is clear that the error 

 in the calculated flow velocity (i.e., the velocity appearing in the de- 

 nominator of R s ) is essentially independent of the actual, value of the speed. 



The lower curve ( OMDUM III) in figure 11=22 shows that between and 25 

 the OMDUM III system deviates only slightly from a value of unity., as con- 

 trasted with the single meter curve, This deviation increases gradually to a 

 value of about 1.17 for ©■ = 70°. Between 70° and 90° there is a region of 

 apparent instability where the value of R & becomes erratic. This same pheno- 

 menon of gross deviation from the cosine law was displayed inthe rotating boom 

 calibration of OMDUM II. 



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