ability to record continuously across the line of zero flow from the 

 negative to the positive domain. The threshold frequency determines how 

 well the meter responds to the oscillatory flow under waves. However, an 

 ideal sensor should equally be responsive to quasi-steady motions and the 

 capillary wave range. The limiting factor for ducted meters is the iner- 

 tia of the impeller's mass (and indirectly by the fit and wear of the 

 bearings); the B-10 sensors response time to an impulse force is 0.1 

 second. The system has three options for the time constant, 0.22, 2.2, 

 and 22.2 seconds. 



Directivity of the meter, commonly referred to as a cosine response , 

 is important for determining the velocity of the flow at the point of 

 measurement, i.e., both the rate of flow through the sensor and its direc- 

 tion. Since the B-10 meters have near-cosine response, speeds recorded by 

 three sensors placed in an orthogonal configuration can be resolved into 

 a resultant vector representing velocity. 



Information available on tests performed with the B-7 sensors (Fig. 11) 

 discussed below demonstrates the utility of a directed, impeller-type 

 meter for measuring currents which include wave components. 



a. Laboratory Tests . Four B-7 current meters were tested in 1972 in 

 a 29.26- by 0.457-meter (96 by 1.5 feet) wave tank in 0.61 meter of water. 

 The sensors were attached to a carriage driven by a bulkhead frame with 

 the bulkhead removed, and oscillated in still water using a programable 

 wave generator. The reason for a simulated wave condition rather than 

 real waves was to eliminate reflected waves from either end of the tank. 

 Two amplitudes of the sine wave input (a! = 25.4 centimeters (10 inches) 

 and a 2 = 50.8 centimeters or 20 inches) were used with frequencies of 

 0.05, 0.0625, 0.0714, 0.08, 0.1, 0.125, 0.0538, 0.2, 0.25, and 0.4 hertz. 

 The sensors were tested for linearity of response, phase lag, voltage, 

 and pulse output as functions of frequency and amplitude of oscillation. 



Figure 23 shows that the relationship between the maximum voltage, 

 Amax. and the corresponding maximum velocity, V ma x> is linear: 



Vmax (— ) = "1-499 + 1.983 A max (mV) 

 sec 



Since a high-frequency oscillation and a long time-constant preclude the 

 output voltage to reach zero for zero velocity, the sensors' responses 

 were evaluated with respect to the offset voltage which follows the 

 relationship: 



Vmax (j^i = 3.95 + 2.095 A min (mV) 



as shown in Figure 24. The correlation coefficients were R 2 = 0.9955 and 

 0.9907, respectively. Calibration tests for the flow velocity versus 

 pulses per second output of the B-7 sensors indicated that towing tank- 

 generated response curves cannot be used in bidirectional flow conditions. 



The phase- frequency relationship is a concern in oscillating flow, i.e. 

 the real time associated with the function forcing the response recorded 



43 



