Fluid Mechanics of Swimming Propulsion 



radius R (see Fig. D2), while the second foil is fixed in the downwash of the 

 oscillating foil. By this arrangement the vorticity in the unsteady downwash 

 is reduced and the efficiency of the system is enlarged. 



Applying the results obtained by Schiele for alternating vortex streets, the 

 axial efficiency in ideal flow foil can be estimated. In Fig. D2, the efficiency 

 is plotted against the induced axial velocity nondimensionalized by the speed of 

 the undisturbed flow. The nondimensionalized angular velocity of the oscillat- 

 ing foil is taken as a parameter, and represents a reduced freeway. As can be 

 seen from the figure, the free-running efficiency is unfavorable even for light 

 loading. 



When the propeller is placed behind a vehicle of similar configuration as 

 the propeller, for example, a vehicle as shown in Fig. D3, the total efficiency 

 in ideal flow can be considerably enlarged. The curves in Fig. D3 are obtained 

 by applying Weinig's interaction theory, and are valid for a total ideal efficiency 

 equal to unity. As can be seen from the diagram also at medium loadings a 

 high total efficiency can be obtained supposing the frequency of oscillation is 

 favorably chosen and the flow interaction resistance is of sxifficient value. 



Fig. D2 - The well-propeller in 

 the free -running condition 



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