From active stresses and forces to self-propulsion of droplets

We study the self-propulsion of spherical droplets as simplified hydrodynamic models of swimming micro-organisms or artificial micro-swimmers. In contrast to approaches that start from active velocity fields produced by the system, we consider active interface tractions, body force densities and active stresses as the origin of autonomous swimming. For negligible Reynolds number and given activity, we compute the external and internal flow fields as well as the centre of mass velocity and angular velocity of the droplet at fixed time. To construct trajectories from single time snapshots, the evolution of active forces or stresses must be determined in the laboratory frame. Here, we consider the case of active matter, which is carried by a continuously distributed rigid but sparse (cyto)-skeleton that is immersed in the droplet interior. We calculate examples of trajectories of a droplet and its skeleton from force densities or stresses, which may be explicitly time-dependent in a frame fixed within the skeleton.

Download Full-text

Experimental Analysis of Velocity Fields in Hot Extrusion of Aluminium Alloy 6351

Key Engineering Materials ◽

10.4028/www.scientific.net/kem.491.145 ◽

2011 ◽

Vol 491 ◽

pp. 145-150 ◽

Cited By ~ 1

Author(s):

Marcelo Martins ◽

Sérgio Tonini Button ◽

José Divo Bressan

Keyword(s):

Metal Forming ◽

Internal Flow ◽

Hot Extrusion ◽

Experimental Tests ◽

Velocity Fields ◽

The Finite Element Method ◽

Forming Process ◽

Complex Shapes ◽

Metal Forming Process ◽

Volume Method

Hot extrusion is a metal forming process with a huge importance in the manufacturing of long metallic bars with complex shapes, and because of this, academics and industries are especially interested in better understanding how metal flows during the process. In order to have a reliable computational tool that can help to solve and to obtain material internal flow, experimental tests and numerical simulation with the finite element method were carried out to obtain results of the velocity fields generated in hot direct extrusion of aluminum billets (aluminum alloy 6351). The experimental results of the velocity field will be used to validate a computational code based on the finite volume method.

Download Full-text

Effect of liquid viscosity on the aerodynamic breakup of non-spherical droplets

Proceedings ILASS–Europe 2017. 28th Conference on Liquid Atomization and Spray Systems ◽

10.4995/ilass2017.2017.4675 ◽

2017 ◽

Author(s):

Konstantinos Bergeles ◽

Georgios Charalampous ◽

Yannis Hardalupas ◽

Alex M. Taylor

Keyword(s):

Aspect Ratio ◽

Liquid Viscosity ◽

Centre Of Mass ◽

Water Droplets ◽

Kinematic Characteristics ◽

Experimental Findings ◽

Spherical Droplets

This paper studies the effect of liquid viscosity on the atomisation regimes of initially spherical and non-sphericaldroplets and also kinematic characteristics of non-spherical droplets. The droplets consisted of water-glycerolsolutions with viscosities ranging from 6.3 to 697 mPa s, and the initial aspect ratio was 1<AR<2. The range of Wenumber was from 10 to 200 and of the Oh number from 0.01 to approximately 4. The experimental findings showedthat the equivalent Weeq and Oheq numbers, proposed in previous work [1] for water droplets, are also applicableto spherical and non-spherical droplets in the range of Oh numbers of this study in order to classify the breakupregimes on the existing morphological charts. The kinematic characteristics of the centre of mass for droplets withAR=1.2 are evaluated and the role of viscosity examined in the no breakup and bag-stamen regimes.DOI: http://dx.doi.org/10.4995/ILASS2017.2017.4675

Download Full-text

Collision-based mechanics of bipedal hopping

Biology Letters ◽

10.1098/rsbl.2013.0418 ◽

2013 ◽

Vol 9 (4) ◽

pp. 20130418 ◽

Cited By ~ 5

Author(s):

Anne K. Gutmann ◽

David V. Lee ◽

Craig P. McGowan

Keyword(s):

Mass Velocity ◽

Mechanical Energy ◽

Large Fraction ◽

Center Of Mass ◽

Reaction Force ◽

Centre Of Mass ◽

Cost Of Transport ◽

Kangaroo Rats ◽

Muscle Work ◽

Collision Angle

The muscle work required to sustain steady-speed locomotion depends largely upon the mechanical energy needed to redirect the centre of mass and the degree to which this energy can be stored and returned elastically. Previous studies have found that large bipedal hoppers can elastically store and return a large fraction of the energy required to hop, whereas small bipedal hoppers can only elastically store and return a relatively small fraction. Here, we consider the extent to which large and small bipedal hoppers (tammar wallabies, approx. 7 kg, and desert kangaroo rats, approx. 0.1 kg) reduce the mechanical energy needed to redirect the centre of mass by reducing collisions. We hypothesize that kangaroo rats will reduce collisions to a greater extent than wallabies since kangaroo rats cannot elastically store and return as high a fraction of the mechanical energy of hopping as wallabies. We find that kangaroo rats use a significantly smaller collision angle than wallabies by employing ground reaction force vectors that are more vertical and center of mass velocity vectors that are more horizontal and thereby reduce their mechanical cost of transport. A collision-based approach paired with tendon morphometry may reveal this effect more generally among bipedal runners and quadrupedal trotters.

Download Full-text

The Centre-of-Mass Velocity of a Radially Pulsating Star: Insights from NLTE Models

International Astronomical Union Colloquium ◽

10.1017/s025292110003757x ◽

1995 ◽

Vol 155 ◽

pp. 375-376

Author(s):

Dimitar D. Sasselov

Keyword(s):

Mass Velocity ◽

Radiation Hydrodynamics ◽

Systematic Effect ◽

Centre Of Mass ◽

Line Asymmetry

AbstractThe problem of deriving the centre-of-mass velocity of a radially pulsating star is reexamined. New observations of line asymmetry and Non-LTE radiation hydrodynamics point to a systematic effect of about 1 km s−1 in Cepheids.

Download Full-text

Vertical distribution of HMXBs in NGC 55: Constraining their centre of mass velocity

Proceedings of the International Astronomical Union ◽

10.1017/s1743921319001236 ◽

2018 ◽

Vol 14 (S346) ◽

pp. 358-361

Author(s):

Babis Politakis ◽

Andreas Zezas ◽

Jeff J. Andrews ◽

Stephen J. Williams

Keyword(s):

Vertical Distribution ◽

Mass Velocity ◽

Milky Way ◽

Compact Object ◽

High Mass ◽

Centre Of Mass ◽

X Ray ◽

Star Forming ◽

X Ray Binaries ◽

The Vertical Distribution

AbstractWe analyse the vertical distribution of High Mass X-ray Binaries (HMXBs) in NGC 55, the nearest edge-on galaxy to the Milky Way. Our analysis reveals significant spatial offsets of HMXBs from the star forming regions, greater than those observed in the SMC and the LMC but similar with the Milky Way. The spatial offsets can be explained by a momentum kick the X-ray binaries receive during the formation of the compact object. The difference between the scale height of the vertical distribution of HMXBs and the vertical distribution of star-forming activity is 0.48±0.04 kpc. The centre-of-mass velocity of the distribution of HMXBs in NGC 55 is moving at a velocity of 52.4±11.4 km s−1, greater than the corresponding velocity of HMXBs in the SMC and LMC, but consistent with velocities of Milky Way HMXBs.

Download Full-text

Undulatory swimming in shear-thinning fluids: experiments with Caenorhabditis elegans

Journal of Fluid Mechanics ◽

10.1017/jfm.2014.539 ◽

2014 ◽

Vol 758 ◽

Cited By ~ 39

Author(s):

D. A. Gagnon ◽

N. C. Keim ◽

P. E. Arratia

Keyword(s):

Caenorhabditis Elegans ◽

Fluid Velocity ◽

Shear Thinning ◽

Velocity Fields ◽

Swimming Behaviour ◽

Kinematic Data ◽

Shear Thinning Fluids ◽

Fluid Environment ◽

Micro Organisms ◽

Undulatory Swimming

AbstractThe swimming behaviour of micro-organisms can be strongly influenced by the rheology of their fluid environment. In this article, we experimentally investigate the effects of shear-thinning (ST) viscosity on the swimming behaviour of an undulatory swimmer, the nematode Caenorhabditis elegans. Tracking methods are used to measure the swimmer’s kinematic data (including propulsion speed) and velocity fields. We find that ST viscosity modifies the velocity fields produced by the swimming nematode but does not modify the nematode’s speed and beating kinematics. Velocimetry data show significant enhancement in local vorticity and circulation and an increase in fluid velocity near the nematode’s tail. These findings are compared with recent theoretical and numerical results.

Download Full-text

Navigation of micro-swimmers in steady flow: the importance of symmetries

Journal of Fluid Mechanics ◽

10.1017/jfm.2021.978 ◽

2021 ◽

Vol 932 ◽

Author(s):

Jingran Qiu ◽

Navid Mousavi ◽

Kristian Gustavsson ◽

Chunxiao Xu ◽

Bernhard Mehlig ◽

...

Keyword(s):

Reinforcement Learning ◽

Optimal Strategies ◽

Laboratory Frame ◽

Nonlinear Flow ◽

Survival Strategies ◽

Environmental Cues ◽

Global Information ◽

Local Flow ◽

Flow Vorticity ◽

Micro Organisms

Marine micro-organisms must cope with complex flow patterns and even turbulence as they navigate the ocean. To survive they must avoid predation and find efficient energy sources. A major difficulty in analysing possible survival strategies is that the time series of environmental cues in nonlinear flow is complex and that it depends on the decisions taken by the organism. One way of determining and evaluating optimal strategies is reinforcement learning. In a proof-of-principle study, Colabrese et al. (Phys. Rev. Lett., vol. 118, 2017, 158004) used this method to find out how a micro-swimmer in a vortex flow can navigate towards the surface as quickly as possible, given a fixed swimming speed. The swimmer measured its instantaneous swimming direction and the local flow vorticity in the laboratory frame, and reacted to these cues by swimming either left, right, up or down. However, usually a motile micro-organism measures the local flow rather than global information, and it can only react in relation to the local flow because, in general, it cannot access global information (such as up or down in the laboratory frame). Here we analyse optimal strategies with local signals and actions that do not refer to the laboratory frame. We demonstrate that symmetry breaking is required to find such strategies. Using reinforcement learning, we analyse the emerging strategies for different sets of environmental cues that micro-organisms are known to measure.

Download Full-text

Electron-proton interaction in radio sources

Proceedings of the International Astronomical Union ◽

10.1017/s1743921320003634 ◽

2019 ◽

Vol 15 (S356) ◽

pp. 409-409

Author(s):

Halima Ugomma Obini

Keyword(s):

Cross Sections ◽

Laboratory Frame ◽

Radio Sources ◽

Centre Of Mass ◽

Elastic Collisions ◽

Proton Interaction ◽

Mass Frame ◽

The Cross

AbstractA method of treating electron-proton interaction is presented. The energies involved in the interaction are estimated. Only elastic collisions are considered. The cross sections of the processes are not taken into account. Calculations are carried out in the centre of mass frame. Relevant quantities are transformed into the laboratory frame. Results indicate that the energy per collision gained by an electron ranges from 0.5 MeV to 0.6 MeV, under suitable conditions.

Download Full-text