DISADVANTAGES 



The bad features of the drag force meter from a hydrodynamic standpoint 

 are as follows: 



1. The meter has a nonlinear output, even in steady flow. 



2. Cq and C\f vary for oscillating flows. 



3. The values of Cf^j are not small compared with the values of Cq with 

 dV/dt unknown for data reduction use of inertia term. The neglect of this term 

 may result in sizable errors in solving for the dc level. 



4. The slope of the force (or output) vs velocity curve is zero at zero 

 velocity, which makes the zero inherently difficult to determine accurately. 



SUMMARY 



The drag force method is not suitable for measuring low -level dc signals 

 in the presence of these typical swell conditions. Even if a perfect electrical 

 transducer could be built, the inherent hydrodynamic problems would be too great. 

 This method is, however, being used successfully as a "surge recorder" where 

 only the maximum value of the oscillating flow is required. 



LAGRANGIAN METHOD 



The Lagrangian Method involves determining the position of artificial 

 tracers in space and time. Von Arx^*^ lists, as types of tracers, drift bottles, 

 drift cards, radio buoys, contaminants such as dye or radioactive waste, current 

 poles, deep drogues, neutrally buoyant floats, and ship drift. Current poles and 

 "deep" drogues can be used in shallow water. Even though measurements are 

 taken in shallow water, it is still desired to measure velocities at some predeter- 

 mined depth. Near the surface, current poles could possibly be used whereas deep 

 drogues would be needed for other depths. Current poles are basically long, thin, 

 vertical spar buoys, with only a small portion above the water line, which are 

 used for visual tracking. In the deep-drogue configuration, a surface float is 

 connected to a submerged object having large drag by a fine line. The float is 

 tracked visually. 



Of the serious faults and disadvantages inherent in the Lagrangian Method 

 when used in shallow water, the most obvious is the necessary support effort. 

 Setting, tracking, and picking up drogues is by no means an easy task either at 

 night or for long periods of time. 



The output, which is a function of space and time, is not electrical and 

 might not comply with the condition of a point measurement satisfactorily. Since 



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