Solitary Motion Research Articles

Developmental adaptations of trypanosome motility to the tsetse fly host environments unravel a multifaceted in vivo microswimmer system.

The highly motile and versatile protozoan pathogen Trypanosoma brucei undergoes a complex life cycle in the tsetse fly. Here we introduce the host insect as an expedient model environment for microswimmer research, as it allows examination of microbial motion within a diversified, secluded and yet microscopically tractable space. During their week-long journey through the different microenvironments of the fly´s interior organs, the incessantly swimming trypanosomes cross various barriers and confined surroundings, with concurrently occurring major changes of parasite cell architecture. Multicolour light sheet fluorescence microscopy provided information about tsetse tissue topology with unprecedented resolution and allowed the first 3D analysis of the infection process. High-speed fluorescence microscopy illuminated the versatile behaviour of trypanosome developmental stages, ranging from solitary motion and near-wall swimming to collective motility in synchronised swarms and in confinement. We correlate the microenvironments and trypanosome morphologies to high-speed motility data, which paves the way for cross-disciplinary microswimmer research in a naturally evolved environment.

Two-dimensional instability of the bottom boundary layer under a solitary wave

The objective of this paper is to establish a detailed map for the temporal instability of the bottom boundary layer (BBL) flow driven by a soliton-like wave induced pressure gradient in a U-tube-shaped tunnel, which serves as an approximation to the BBL under surface solitary waves. Both linear stability analysis and fully nonlinear two-dimensional simulations using high-order numerical methods have been carried out. The process of delineation of the stability regions as a function of boundary layer thickness-based Reynolds number of the temporally evolving base flow, Reδ, consists of two parts. In the first part, we assess the lower limit of the Reδ range within which the standard, quasi-steady, linear stability analysis is applicable when considering individual base flow profiles sampled during its transient evolution. Below this limit, transient linear stability analysis serves as a more accurate predictor of the stability properties of the base flow. In the second step, above the Reδ limit where the BBL is determined to be linearly unstable, the base flow is further classified as unconditionally stable, conditionally unstable, or unconditionally unstable in terms of its sensitivity to the amplitude and the insertion time of perturbations. Two distinct modes of instability exist in this case: post- and pre-flow-reversal modes. At a moderate value of Reδ, both modes are first observed in the wave deceleration phase. The post flow reversal mode dominates for relatively low Re and it is the one observed in Sumer et al. [“Coherent structures in wave boundary layers. Part 2. Solitary motion,” J. Fluid Mech. 646, 207 (2010)]. For Re above a threshold value of the base flow in the unconditionally unstable regime, the pre-flow reversal mode, which is longer in wavelength than its post-reversal counterpart, becomes dominant. In the same regime, the threshold Reδ value above which instability is observed in the acceleration phase of the wave is also identified. In this case, the base flow velocity profiles lack any inflection point, suggesting that the origin of such an instability is viscous. Finally, the lower Reδ limit above which quasi-steady linear stability analysis is valid may be independently obtained by adapting to the surface solitary wave BBL, an instability criterion which links the average growth rate and wave event timescale, as previously proposed in studies of the instability of the interior of progressive and solitary internal waves.

RANS modelling of the turbulent boundary layer under a solitary wave

The flow field in the boundary layer at the bottom of a solitary wave is computed by means of the Reynolds averaged Navier–Stokes equations. Reynolds stresses are determined by means of the two-equation turbulence model by Saffman (Saffman, P.G., 1970. A model for inhomogeneous turbulent flow. Proceedings of the Royal Society of London. Series A: Mathematical and Physical Sciences 317 (1530), 417–433.) and Saffman and Wilcox (Saffman, P.G., Wilcox, D.C., 1974. Turbulence-model predictions for turbulent boundary layers. AIAA J. 12, 541–546.) which allows both the laminar and turbulent regime to be investigated along with the transition process. The results obtained for the smooth wall case show a fair agreement with the experimental measurements of Sumer et al. (Sumer, B.M., Jensen, P.M., Søerensen, L.B., Fredsøe, J., 2010. Coherent structures in wave boundary layers. Part 2. Solitary motion. J. Fluid Mech. 646, 207–231.) and with the direct numerical simulations of Vittori and Blondeaux (Vittori, G., Blondeaux, P., 2008. Turbulent boundary layer under a solitary wave. J. Fluid Mech. 615, 433–443.). The model allows to investigate the rough wall case, too. Attention is focused on the bottom shear stress and on its maximum positive (in the direction of wave propagation) and negative (in the direction opposite to the direction of wave propagation) values.

Coherent structures in wave boundary layers. Part 2. Solitary motion

This study continues the investigation of wave boundary layers reported by Carstensen, Sumer & Fredsøe (J. Fluid Mech., 2010, part 1 of this paper). The present paper summarizes the results of an experimental investigation of turbulent solitary wave boundary layers, simulated by solitary motion in an oscillating water tunnel. Two kinds of measurements were made: bed shear stress measurements and velocity measurements. The experiments show that the solitary-motion boundary layer experiences three kinds of flow regimes as the Reynolds number is increased: (i) laminar regime; (ii) laminar regime where the boundary-layer flow experiences a regular array of vortex tubes near the bed over a short period of time during the deceleration stage; and (iii) transitional regime characterized with turbulent spots, revealed by single/multiple, or, sometimes, quite dense spikes in the bed shear stress traces. Supplementary synchronized flow visualization tests confirmed the presence of the previously mentioned flow features. Information related to flow resistance are also given in the paper.

Transition to strictly solitary motion in the Burridge-Knopoff model of multicontact friction

We show that, in the continuous 1D Burridge-Knopoff model of multicontact friction, motion occurs via stick-slip sliding on a finite length rather than in avalanches, excluding the occurrence of self-organized criticality. We present strong numerical evidence that a transition from collective to strictly solitary motion occurs at a critical value of the interblock interactions. The solitary motion corresponds to successive stick-slip motion of one block between immobile neighbors, repeated periodically in time. This state persists also with open boundary conditions and moderate temperature.

Quantum aspects of self-localized solutions in discrete systems

We present a special unitary transformation U = e s , S = − S +, the parameters of which are deduced from the classical amplitude pattern of the self-localized (solitary) mode (SLS). By means of this transformation the Hamiltonian is transcribed in such a way that the SLS is represented by a single anharmonic oscillator, and the quantal correspondence to the solitary motion appears to be the quasi-coherent state of this anharmonic oscillator. In this manner the dominant non-classical decay behaviour is incorporated in the time-enhancement of the position-momentum uncertainty product.

Quantal properties of self-localized solitons in anharmonic oscillatory systems

We present a special unitary transformation in which the parameters are deduced from the classical amplitude pattern of the self-localized (solitary) mode (SLS). By means of this transformation the Hamiltonian is transcribed in such a way that the SLS is represented by a single anharmonic oscillator, and the quantal correspondence to the solitary motion appears to be the quasi-coherent state of this anharmonic oscillator.

A canonical description of the solitary quantum decay

A quantum picture of solitary motion is presented. Time evolution of a quantumN-soliton is described as a transition through infinitely many unitarily inequivalent representations of the canonical commutation relations. The statistical nature and the irreversibility of the process naturally emerge. Their origin is found in the nonunitary character of the transformation among the unitarily inequivalent representations.