Swimming freely near the ground leads to flow-mediated equilibrium altitudes

Experiments and computations are presented for a foil pitching about its leading edge near a planar, solid boundary. The foil is examined when it is constrained in space and when it is unconstrained or freely swimming in the cross-stream direction. It was found that the foil has stable equilibrium altitudes: the time-averaged lift is zero at certain altitudes and acts to return the foil to these equilibria. These stable equilibrium altitudes exist for both constrained and freely swimming foils and are independent of the initial conditions of the foil. In all cases, the equilibrium altitudes move farther from the ground when the Strouhal number is increased or the reduced frequency is decreased. Potential flow simulations predict the equilibrium altitudes to within 3 %–11 %, indicating that the equilibrium altitudes are primarily due to inviscid mechanisms. In fact, it is determined that stable equilibrium altitudes arise from an interplay among three time-averaged forces: a negative jet deflection circulatory force, a positive quasistatic circulatory force and a negative added mass force. At equilibrium, the foil exhibits a deflected wake and experiences a thrust enhancement of 4 %–17 % with no penalty in efficiency as compared to a pitching foil far from the ground. These newfound lateral stability characteristics suggest that unsteady ground effect may play a role in the control strategies of near-boundary fish and fish-inspired robots.

The Multiple Lives of Affect: A Case Study of Commercial Surrogacy

This article intervenes into contemporary scholarship on affect by bringing different affect theories into the same analytical frame. Analysing commercial surrogacy in India through three different conceptualizations of affect found in the work of Michael Hardt, Sara Ahmed and Brian Massumi reveals how affect emerges as a malleable state in the practice of, as a circulatory force in the debates around, and as an ephemeral intensity in the spontaneous resistance to surrogacy. Based on this analysis, I suggest that integrating different theories of affect enables more holistic examinations of corporeal regulation by opening our understanding to the multiple lives of affect that operate on the level of political economy, cultural signification and material intensity simultaneously.

On Stability of a Circulatory System With Bilinear Hysteresis Damping

Dynamic stability problems of a two-degree-of-freedom system with bilinear hysteresis damping and subjected to a circulatory force are considered. Steady-state oscillations and critical loads for flutter of the system are investigated using energy considerations and the method of slowly varying parameters. Problems concerning stability limits (limiting values of the critical load) for vanishing hysteresis damping are examined and compared with those found by earlier investigators for vanishing viscous damping. It is shown that, in a circulatory system, small hysteresis damping may have a destabilizing effect similar to linear viscous damping, but the hysteretic model generally yields a satisfactory stability limit as the hysteresis damping effect approaches zero. The viscoelastic model generally leads to unsatisfactory stability limits as the viscous damping effect varies from small values to zero. Thus, the hysteretic model is shown to be more adequate than the viscoelastic model. The study also shows that for some different sets of initial perturbation there may exist two disparate states of steady-state oscillations of the system under the same loading.