When a Savonius rotor is used on a current meter as a sensor for fluid speed, it is usually surrounded by a number of support rods. These rods interfere with the flow to the rotor and, depending on their orientation to the flow, can be found to cause quite large variations in rotor speed. Experimental results show that generally the rotor speed asymmetries are larger for the rotor used by Aanderaa Instruments than for a true Savonius rotor. The asymmetries can be reduced to near zero if the number of rods used is such that near cancellation of the positive and negative asymmetries occurs. The asymmetry pattern for one support rod can be used to deduce the pattern for any number of rods; thus, for a specified rotor, support rod diameter and pitch circle, the number of rods can be chosen to give the minimum azimuthal asymmetry.

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