scholarly journals Elastic-instability–enabled locomotion

2021 ◽  
Vol 118 (8) ◽  
pp. e2013801118
Author(s):  
Amit Nagarkar ◽  
Won-Kyu Lee ◽  
Daniel J. Preston ◽  
Markus P. Nemitz ◽  
Nan-Nan Deng ◽  
...  
Keyword(s):  
Symmetry Breaking ◽  
Scaling Laws ◽  
Multiple Scales ◽  
Symmetric Cone ◽  
Simple Approach ◽  
Inertial Forces ◽  
Elastic Instability ◽  
Anisotropic Friction ◽  
Buckling Instability ◽  
Elastic Instabilities

Locomotion of an organism interacting with an environment is the consequence of a symmetry-breaking action in space-time. Here we show a minimal instantiation of this principle using a thin circular sheet, actuated symmetrically by a pneumatic source, using pressure to change shape nonlinearly via a spontaneous buckling instability. This leads to a polarized, bilaterally symmetric cone that can walk on land and swim in water. In either mode of locomotion, the emergence of shape asymmetry in the sheet leads to an asymmetric interaction with the environment that generates movement––via anisotropic friction on land, and via directed inertial forces in water. Scaling laws for the speed of the sheet of the actuator as a function of its size, shape, and the frequency of actuation are consistent with our observations. The presence of easily controllable reversible modes of buckling deformation further allows for a change in the direction of locomotion in open arenas and the ability to squeeze through confined environments––both of which we demonstrate using simple experiments. Our simple approach of harnessing elastic instabilities in soft structures to drive locomotion enables the design of novel shape-changing robots and other bioinspired machines at multiple scales.

Vibration of Tube Bundles in Two-Phase Cross-Flow: Part 2—Fluid-Elastic Instability

1989 ◽  
Vol 111 (4) ◽  
pp. 478-487 ◽  
Author(s):  
M. J. Pettigrew ◽  
J. H. Tromp ◽  
C. E. Taylor ◽  
B. S. Kim
Keyword(s):  
Tube Bundle ◽  
Cross Flow ◽  
Design Guidelines ◽  
Experimental Program ◽  
Elastic Instability ◽  
Two Phase ◽  
Critical Flow Velocity ◽  
Flexible Tube ◽  
Elastic Instabilities ◽  
Tube Bundles

An extensive experimental program was carried out to study the vibration behavior of tube bundles subjected to two-phase cross-flow. Fluid-elastic instability is discussed in Part 2 of this series of three papers. Four tube bundle configurations were subjected to increasing flow up to the onset of fluid-elastic instability. The tests were done on bundles with all-flexible tubes and on bundles with one flexible tube surrounded by rigid tubes. Fluid-elastic instabilities have been observed for all tube bundles and all flow conditions. The critical flow velocity for fluid-elastic instability is significantly lower for the all-flexible tube bundles. The fluid-elastic instability behavior is different for intermittent flows than for continuous flow regimes such as bubbly or froth flows. For continuous flows, the observed instabilities satisfy the relationship V/fd = K(2πζm/ρd2)0.5 in which the minimum instability factor K was found to be around 4 for bundles of p/d = 1.47 and significantly less for p/d = 1.32. Design guidelines are recommended to avoid fluid-elastic instabilities in two-phase cross-flows.

On the relation between viscoelastic and magnetohydrodynamic flows and their instabilities

2003 ◽  
Vol 476 ◽  
pp. 389-409 ◽  
Author(s):  
GORDON I. OGILVIE ◽  
MICHAEL R. E. PROCTOR
Keyword(s):  
Viscoelastic Medium ◽  
Magnetic Reynolds Number ◽  
Deborah Number ◽  
Elastic Instability ◽  
Magnetorotational Instability ◽  
Electrically Conducting ◽  
Electrically Conducting Fluid ◽  
Magnetohydrodynamic Flows ◽  
Elastic Instabilities ◽  
The Stability

We demonstrate a close analogy between a viscoelastic medium and an electrically conducting fluid containing a magnetic field. Specifically, the dynamics of the Oldroyd-B fluid in the limit of large Deborah number corresponds to that of a magnetohydrodynamic (MHD) fluid in the limit of large magnetic Reynolds number. As a definite example of this analogy, we compare the stability properties of differentially rotating viscoelastic and MHD flows. We show that there is an instability of the Oldroyd-B fluid that is physically distinct from both the inertial and elastic instabilities described previously in the literature, but is directly equivalent to the magnetorotational instability in MHD. It occurs even when the specific angular momentum increases outwards, provided that the angular velocity decreases outwards; it derives from the kinetic energy of the shear flow and does not depend on the curvature of the streamlines. However, we argue that the elastic instability of viscoelastic Couette flow has no direct equivalent in MHD.

A Simple Approach to the Wing Flutter Problem

1933 ◽  
Vol 37 (273) ◽  
pp. 783-792 ◽  
Author(s):  
B. Lockspeiser
Keyword(s):  
Elastic Energy ◽  
Simple Approach ◽  
Inertial Forces ◽  
Initial Displacement ◽  
Wing Flutter ◽  
Wing Section ◽  
Air Damping ◽  
Gain Energy ◽  
Wing Structure ◽  
Set Up

Let us first consider the oscillations, in still air, of a monoplane wing whose aileron is supposed locked to the wing in such a way that it behaves as though it were an integral part of the wing structure. When the wing is displaced from its position of equilibrium and released it will, in general, vibrate both in flexure and torsion. The initial displacement may be purely flexural, but if the inertial forces called into play, over any wing section, produce a twisting moment about the centre of twist (i.e., the centre about which the wing section twists on the application of a pure torque at that section) torsional as well as flexural oscillations will be set up. Inertia, in general, robs the two kinds of oscillation of their independence, and, when they are interdependent, we may conveniently speak of “inertial couplings” between the two motions. In still air these vibrations must, of necessity, die down. One part of the wing may gain energy at the expense of another, but the store of elastic energy given to the wing by the initial displacement must grow progressively less as the wing does work against the viscous air damping and structural hysteresis forces.

scholarly journals Flow Direction-Dependent Elastic Instability in a Symmetry-Breaking Microchannel

Micromachines ◽  
2021 ◽  
Vol 12 (10) ◽  
pp. 1139
Author(s):  
Wu Zhang ◽  
Zihuang Wang ◽  
Meng Zhang ◽  
Jiahan Lin ◽  
Weiqian Chen ◽  
...  
Keyword(s):  
Symmetry Breaking ◽  
Flow Direction ◽  
Weissenberg Number ◽  
Viscoelastic Flow ◽  
Elastic Instability ◽  
Directed Flow ◽  
Polyacrylamide Solution ◽  
Nozzle Structure

This paper reports flow direction-dependent elastic instability in a symmetry-breaking microchannel. The microchannel consisted of a square chamber and a nozzle structure. A viscoelastic polyacrylamide solution was used for the instability demonstration. The instability was realized as the viscoelastic flow became asymmetric and unsteady in the microchannel when the flow exceeded a critical Weissenberg number. The critical Weissenberg number was found to be different for the forward-directed flow and the backward-directed flow in the microchannel.

scholarly journals Quark confinement and the renormalization group

2011 ◽  
Vol 369 (1946) ◽  
pp. 2718-2734 ◽  
Author(s):  
Michael C. Ogilvie
Keyword(s):  
Renormalization Group ◽  
Symmetry Breaking ◽  
Scaling Laws ◽  
Dyson Equation ◽  
String Tension ◽  
Real Space ◽  
Equation Approach ◽  
Zero Temperature ◽  
Quark Confinement ◽  
Basic Concepts

Recent approaches to quark confinement are reviewed, with an emphasis on their connection to renormalization group (RG) methods. Basic concepts related to confinement are introduced: the string tension, Wilson loops and Polyakov lines, string breaking, string tension scaling laws, centre symmetry breaking and the deconfinement transition at non-zero temperature. Current topics discussed include confinement on R 3 × S 1 , the real-space RG, the functional RG and the Schwinger–Dyson equation approach to confinement.

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