Journal of Fluid Mechanics


Unsteady swimming of small organisms

S. Wanga1 and A. M. Ardekania1 c1

a1 Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA


Small planktonic organisms ubiquitously display unsteady or impulsive motion to attack a prey or escape a predator in natural environments. Despite this, the role of unsteady forces such as history and added mass forces on the low-Reynolds-number propulsion of small organisms, e.g. Paramecium, is poorly understood. In this paper, we derive the fundamental equation of motion for an organism swimming by means of the surface distortion in a non-uniform background flow field at a low-Reynolds-number regime. We show that the history and added mass forces are important as the product of Reynolds number and Strouhal number increases above unity. Our results for an unsteady squirmer show that unsteady inertial effects can lead to a non-zero mean velocity for the cases with zero streaming parameters, which have zero mean velocity in the absence of inertia.

(Received February 01 2012)

(Reviewed March 14 2012)

(Accepted April 01 2012)

(Online publication June 01 2012)

Key Words:

  • micro-organism dynamics;
  • propulsion;
  • swimming


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