#nakedfluiddynamics

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Turbulence is beautiful! #science #turbulence #artscience #nakedfluiddynamics https://www.instagram.com/p/Bs5xxe_h-Nw/?utm_source=ig_tumblr_share&igshid=hzw5zq2el9z3

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Vortex rings are mind-blowing and are an important feature of aquatic propulsion (we will be talking soon about fish, squid, human and other kind of animals swimming). It is really easy to form vortex rings (or half rings, see the video of @thepyhicsgirl): you just need to displace a limiting amount of fluid out of some kind of nozzle into a tank using a piston. There is so much mass in front in the nozzle exit that the fluid jet exiting is “blocked” and starts to roll up on itself, therefor forming a vortex ring! Once a vortex ring is formed, it starts to propagate on its own. If you push your piston at a given speed on a distance of about 2 nozzle diameters and then stop, the vortex will look like the one of the top figure and it will propagate “forever”. You can continue to grow the vortex size by pushing more fluid (increase the distance the piston travel). This is the case of the middle figure (L/D=3.8). But if you continue to increase the distance, the pattern changes! There is a leading vortex of about the same size as (L/D=3.8) and followed by a detached a trailing jet-like region. Gharib et al. showed that this occurs at a characteristic “formation number” L/D=3.6-4.5. Explanation: the formed vortex travels at its own speed which depends linearly on the its “size” (or circulation). As long as the speed of the jet is larger than the own speed of the vortex ring, the jet “feeds” the vortex and increases its size and hence its speed, till the critical time the velocity of the vortex is as large as the one of the jet. This leads to pinch-off! Sources: Gharib et al., 1998, A universal time scale for vortex ring formation, JFM Shusser and Gharib, 2000, Energy and velocity of a forming vortex ring, Ph. of Fluids Credit: figure from the JFM paper with permission #nakedfluiddynamics #fluiddynamics #fluidmechanics #physics #physique #scienceisart #science #artinscience #fluidartgallery #beautiful #picoftheday #awesome #vortexring #vortex #propulsion

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Can Michael Phelps swim faster than a shark? Iosilevskii and Weihs have found the physical limits on swimming speed of lunate tail propelled aquatic swimmers: Dolphins, tunas, sharks and Michael Phelps! Well the study does not actually mention MP but with his monofin the results should apply. They found that large swimmers (not humans) were limited by cavitation! They have enough power to swim so fast that the water literally boils around their tail. The cavitation will induce pain to the swimmer due to the violent shocks generated during the collapse (see previous posts). Cavitation happens when the speed of the swimmer exceeds 10-15 m/s referring to the paper. For dolphin this is pretty much the top speed observed. Some fish like tunas don’t have sensors on their tails and thus can exceed this speed but not by much due to the drastic reduction of lift induced by the cavitation. The top speed of monofin swimmer is of the order of 3-4 m/s... We stand no chance... We are still limited by the available power in the human body... Let’s evaluate the top speed of MP: A professional athlete can produce about 20 W/kg (for fish it is in between 10 to 160!). MP is about 80 kg so P = 1.6 kW. Using the paper data we find a top speed due to power limitation of the order of 3 m/s! In good agreement with the actual top speed of finswimmers and the results of the race (38.1 sec on a 100m) Source: Iosilevskii and Weihs, Speed limits on swimming of fishes and cetaceans, J. R. Soc Interface, 2008, 5, 329–338 #nakedfluiddynamics #nakedfluid #fluidmechanics #cavitation #physicsflow #physics #swimming @m_phelps00 #shark #racing #physicsisfun #scienceisfun #scienceisawesome

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The Liebau’s pump is a model of the embryonic heart. It is a pump that has no valve nor impeller. Yet it can generate a unidirectional pulsating flow under certain conditions. Its response is highly non-linear. A blood flow is crucial for the well development of the embryonic heart. In the human embryo, the first beats and the blood circulation start well before the heart with its chambers and valves to ensure unidirectional flow is formed. The flow starts around the day 22 of the embryo development, while the heart is fully functional after 50-60 days. At the beginning the blood flow is driven by impedance pumping (Forouhar et al. 2006) and not by peristaltic pumping as previously described. Gharib and its group showed this by studying the development of the heart of a zebrafish. They showed that the blood velocity of the embryo was not linearly linked to the heart frequency. To do so they varied the temperature in a short range. The embryo develop normally under the tested conditions and the blood did not change viscosity. The first conceptual idea of this special way of pumping flow was first proposed by Liebau in 1954, thus the name of the pump. The system is composed of a flexible tube (white part in the video) and a rigid tube (transparent glass tube here). The flexible tube is pinched at an off-centred position and depending on the pinching amplitude, frequency, actuation position and duty cycle a flow develops. You can imagine that the pincher represent a simple muscle cells. So the pumping at the beginning is rather simple to achieve. A peristaltic pump is more challenging to design and in the embryo it would require several muscle cells plus coordination. In this video you can see the circulation thanks to the particle tracer in the tubes. The flow change direction when the actuation is on the opposite side. video credit: Gharib’s group reference: Forouhar, Liebling, Hickerson, Nasiraei-Moghaddam,Tsai, Hove,Fraser, Dickinson, Gharib, The Embryonic Vertebrate Heart Tube Is a Dynamic Suction Pump, Science, 312 (2016) Libeau, Über ein ventilloses pumpprinzip. Naturwissenschaften 41, (1954) #nakedfluiddynamics #fluiddynamics #fluidmechanics

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“Elastic spheres can walk on water”! This is the title of the paper of today post. Summer is coming and this means water and playing. Water-skipping is popular in this time of the year and you might have noticed in the past years the emergence of balls specially designed for long skip events. These “elastic spheres” deform to adopt an optimal disk shape upon impact favourable for skipping and at each impact the normal restitution being smaller than the tangential the skip becomes “easier” until a certain regime is reached (object of the post!)… Using highspeed imaging Tadd and his team investigated why these balls skip more easily than stiffer ones and the fluid-elastic body interaction. This video was obtained using a highspeed camera. At the impact, the sphere dramatically deforms and elastic waves propagate through the sphere and hit the air-water interface generating a cavity called “Matryoshka cavities”. It is due to the too long contact time with the water. The demise of a multiple skip event starts with this kind of cavity formation. The restitution decreases rapidly when this occurs and eventually the sphere will enter the water. In the side view it seems that the sphere rotates while from the top view it appears that it is actually indeed a wave that propagates and genrates the cavities! credit: Jesse Belden, Randy C. Hurd, Michael A. Jandron, Allan F. Bower & Tadd T. Truscott, Elastic spheres can walk on water. Nature Comm. 2016 [open access] #nakedfluiddynamics #fluiddynamics #fluidmechanics #physics #physique #splash #waterskipping #softmatter #bouncingball #rockskipping #ballskills #highspeedcamera #slowmotion #scienceisart #science #artinscience #fluidartgallery #beautiful #picoftheday #awesome #mindblown

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At the meeting point of two fluid drops a striking transformation occurs. There is a singularity. When the distance is below the Van der Waals radius the coalescence starts. The surface tension tries to minimize the surface and the initial small neck starts growing. The viscosity limit the expansion till inertia dominates the resistance to the growth. Coalescence happens in an outer fluid. In this study we investigated the effect of the outer fluid viscosity on the coalescence neck growth rate. label : Coalescence of 1cP drops in 0.5cP silicone oil. This snapshot shows the evolution of the bridge growing between two coalescing drops of about 5mm in diameter. The inner fluid (inside the drop) is a none saturated NaCl/water solution of viscosity 1cP. The outer fluid (surrounding the drops) is a low viscosity (0.5cP) silicone oil. The drops radii are approximatively 2mm and the time step between each picture is 147μs. The first picture is chosen as the first picture before the coalescence. credit: personal work. Figure extracted from Ecole Polytechnique report. Allrights reserved. Publication: for more insight please refer to our paper: Paulsen, Carmigniani, Kannan, Burton and Nagel, Coalescence of bubbles and drops in an outer fluid, Nature Comm. 5:3182-2014 Camera: #phantomV7 #phantomV12 #phantom #nakedfluiddynamics #physics #physique #fluiddynamics #fluidmechanics #softmatter #surfacetension #coalescence #drops #artinscience #scienceisart #scienceandart #scienceisbeautiful #scienceisawesome #picoftheday #follow #like #awesome #mindblown #beautiful

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Cavitation bubbles have a strong erosion power (see prev post). This comes from the micro-jet and shock waves emission during collapse. This pictures show the expansion and collapse of such a bubble near a free surface. A micro-jet forms from the top of the bubble and pierce the bottom part of the bubble resulting in shock emissions. All this happens in less than 4ms after the formation of the bubble. To form the bubble, they use a converged laser beam to locally "boil" the water. The pressure is 0.1 atm. Figure adapted from: Supponen, Obreschkow, Kobel and Farhat, Detailed Jet Dynamics in a collapsing bubble, J. Phys. CS 656 (2015). (With author permission) Special thanks to Outi S. for sending us this nice image and a video i will post soon if granted permission by AIP #PhotronSA11 #Photron #fluiddynamics #nakedfluiddynamics #picoftheday #science #physics #physique #physicsisfun #experiment #scienceisfun #scienceisart #scienceisawesome #scienceisbeautiful #artinscience #mindblown #amazing #awesome #like #follow