Motivated by the intriguing motility of spirochetes (helically-shaped bacteria that screw through viscous fluids due to the action of internal periplasmic flagella), we examine the fundamental fluid dynamics of superhelices translating and rotating in a Stokes fluid. A superhelical structure may be thought of as a helix whose axial centerline is not straight, but also a helix. We examine the particular case where these two superimposed helices have different handedness, and employ a combination of experimental, analytic, and computational methods to determine the rotational velocity of superhelical bodies being towed through a very viscous fluid. We find that the direction and rate of the rotation of the body is a result of competition between the two superimposed helices; for small axial helix amplitude, the body dynamics is controlled by the short-pitched helix, while there is a cross-over at larger amplitude to control by the axial helix. We find far better, and excellent, agreement of our experimental results with numerical computations based upon the method of Regularized Stokeslets than upon the predictions of classical resistive force theory

We study the behavior of an elastic loop embedded in a flowing soap film. This deformable loop is wetted into the film and is held fixed at a single point against the oncoming flow. We interpret this system as a two-dimensional flexible body interacting in a two-dimensional flow. This coupled fluid-structure system shows bistability, with both stationary and oscillatory states. In its stationary state, the loop remains essentially motionless and its wake is a von Kármán vortex street. In its oscillatory state, the loop sheds two vortex dipoles, or more complicated vortical structures, within each oscillation period. We find that the oscillation frequency of the loop is linearly proportional to the flow velocity, and that the measured Strouhal numbers can be separated based on wake structure.PACS numbers: 47.32.ck, 47.54.De, The wake flow behind a rigid obstacle is a central object of study in fluid mechanics. When the oncoming flow velocity exceeds a threshold, vortices are shed behind the obstacle [1]. A typical wake is composed of successive eddies of alternating sign -the "von Kármán vortex street" -and is observed over a wide range of flow velocities and body shapes [2,3]. The frequency of vortex shedding (f ) is determined by the flow velocity (V ) and the object size (d), whose relation is captured by the near constancy of the Strouhal number,The dynamics of a rigid object which moves freely in the direction perpendicular to the flow is of interest in many industrial and biological applications [4,5,6]. Lateral motion of an object can be induced by interaction with the flow and is often called the vortex-induced vibration (VIV). At low flow velocities, the body starts to oscillate sideways with small amplitude (less than 0.4 times body diameter). Its associated wake structure is again a von Kármán vortex street. However, further increase of flow velocity causes the obstacle to oscillate in phase with the vortex shedding, and as a result, a series of dipoles are shed instead [7,8].Settling bodies or rising bubbles, where the balance of gravitational and drag forces set the velocity, also exhibit transitions as they interact with their wakes. For example, a slowly settling sedimenting sphere falls straight downwards [9] but above a certain sedimentation velocity, the sphere's motion becomes periodic and its trajectory a spiral or zigzag [10]. A deformable object, such as a droplet or bubble, can behave similarly even as its shape now changes [11,12].Finally, studies have shown the instability (and bistability) of slender deformable bodies to lateral oscillations in quasi-2D soap-film flows [13], and of heavy deformable sheets to lateral oscillations in fast 3D flows [14,15,16,17]. In these cases, the system corresponds to the flapping of a flag in a stiff breeze.Flowing soap film provides a practical template upon which to study the dynamics of a nearly 2D flow [18,19]. The experimental setup has been introduced earlier [13,18,19,20]. In this work, we introduce a deformable closed body into a fast flowing so...