TM NO. 377 



Thus, the cylindrical shroud of the ducted meter greatly increases its 

 response to axial directed pressure perturbations „ But most important (if 

 not imperative) for wave measurements, the cylindrical shroud makes the 

 impeller response a. predictable function of the angle formed by the instan- 

 taneous fluid flow and the cylinder axis. In other words, if the response 

 of each ducted meter is known as a function of speed and off angle ; then, 

 by coupling two meters orthogonally, it should be possible to sense a two- 

 dimensional field. of motion in the plane of the orthogonal cylinder axes. 



A discussion of the dynamic properties of such an instrument coupling 

 would be premature at this point. The geometry of flow response was inter- 

 preted only after some calibration measurements were performed. These are 

 discussed in later sections. 



Another interesting feature of the impeller is that the angular velocity 

 is essentially independent of the density of the fluid. Thus, steady cali- 

 bration can be made with no required water density information. In fact, the 

 wave meter can be calibrated in a wind tunnel. The response time, however, 

 is dependent upon the viscosity and density of the fluid. 



Another important aspect in the choice of an instrument is that of 

 mounting or suspending the wave meter in the ocean. Any force detector or 

 deflection device in which minute beam displacements are measured by means 

 of strain gauges or linear differential transformer systems requires a 

 completely rigid support system in the fluid medium. This is because the 

 operation depends upon the flexure or strain (with respect to a rigid co- 

 ordinate system) of a drag device in response to the pressures imparted by 

 the fluid, flow. These flexures ara usually of the order of a few thousandths 

 of a centimeter,, 



A deflection device must interfere with or inhibit the fluid flow in 

 order to work properly. The impeller device, however, does not offer rigid 

 resistance to flow, but responds or reacts rather completely to the flow. 

 There is, of course, some interaction with the cylinder housing, which must 

 be kept reasonably fixed in space. 



The main point is that a force deflection device requires completely 

 rigid mounting! whereas an impeller device can be suspended by means of 

 wires and weights, since displacements of the order of a few millimeters 

 would in no way affect its sensitivity. The ability to suspend the impeller 

 system greatly favors the use of this device in preference to a force- 

 deflection system. 



Orthogo nally Mounted Ducted Meter - OMDIM I 



The OMDUM I system is the result of the first attempt made at NUWS to 

 fabricate a particle velocity meter for sensing two-dimensional wave motions. 



Design Characteristics — OMDUM I consists of a coupling of two identical 

 ducted flow meters manufactured by Marine Advisors of La Jolla, California^ 

 (I96l). The two meters are mounted together in an adjacent orthogonal posi- 

 tion (figure 11=3) and sense fluid flow components in the planes defined by 



21A 



