Theory 37/Strouhal Number/Fish swimming
Swimming Peaks at a Strouhal
The claim
The propulsive efficiency of swimming and flying animals peaks at a narrow Strouhal number, so robots tuned to that number should be more efficient at any size.
01Mechanism
Thrust and wake structure depend on the ratio of oscillation frequency to forward speed; efficiency peaks where the wake forms optimal vortices.
02Prediction
Efficiency should peak at a Strouhal number near 0.3.
03Refutation
Efficiency increasing monotonically with frequency would remove the optimum.