nakedfluiddynamics-blog

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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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I mentioned in the previous post drafting in swimming. Here you can see a nature example of wave drafting! More about this soon! (at Lac du Ballon)

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Drafting is an important part of the strategy in competitive swimming in open water or even in pools (practice and meets). It is well known that similarly to biking, car racing and running, you can save precious energy by positioning yourself (here after call the drafter) in the wake of another swimmer (the leader). In an experimental study, Westerweel et al. looked at this effect and found that the lead swimmer also benefits from the drafter but at a lower extend and can also take the advantage for certain positions during passing. To do so they used passive swimmer models (held fixed in a current) and looked at the impact of the drafter on the leader drag for a typical Froude number of 0.28 (equivalent of an average speed of 1.23m/s or 40” per 50m). They found that when the drafter is just behind, it saves up to 64% on the drag, while the leader has a maximum reduction of 20%. At almost a body length distance, the drafter still saves about 50% while the leader has no benefit. When swimming side-by-side, the drag is the same for the two swimmers and the drag can increase up to 50%. They looked at the drag in between these two configurations (when the drafter tries to pass the leader). Quite surprisingly they found that there exists a position near the shoulder of the leader for which the leader can take an advantage on the drafter! This could be used as a strategy to impede a passing swimmer in open water race and maybe even in a swimming pool where the lines would need to be accounted for. These results are for passive swimmers yet. The arms and legs motion can perturb the wake, but still this is quite exciting! Source: Westerweel, J., et al. "Advantage of a lead swimmer in drafting." arXiv preprint arXiv:1610.10082 (2016). Picture: From @ffnatation #nakedfluiddynamics #fluiddynamics #fluidmechanics #physics #physique #scienceisart #science #artinscience #fluidartgallery #beautiful #picoftheday #awesome #swimming #drafting #swimmer #swimmingfun

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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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Since 1991 “the Ig Nobel Prizes honor achievements that first make people laugh, and then make them think.” Fluid dynamics won two prizes this year in the category Physics Prize and in the Fluid Dynamics Prize. The first one concerned a question that you probably have wondered about watching cats online: “are cats fluid or solid?”. This question was tackled by Marc-Antoine Fardin of Univ. of Lyon in 2014 [1] where he discusses the solid and fluid like behaviours of Felis Catus (or simply cats). In the first picture you can see different state of the Felis Catus materials: a) a solid state and (b-d) a fluid state. The second concerns a particular fluid, awarded of now three prizes by the community: COFFEE! [2] The first fluid dynamics prize on spilling coffee was awarded in 2012 to H. Mayer and R. Krechetnikov [3]. In the second picture, Jesse gave away a technique to record the acceleration of your coffee cup! Simply use your phone (and an apps like #phyphox). You better be in a not-spilling regime… [1] M.A. Fardin, On the rheology of cats, Rheology Bulletin, 83(2) July 2014 [2] J. Han, A Study on the Coffee Spilling Phenomena in the Low Impulse Regime, Achievements in the Life Sciences 10 87-101 (2016) [3]H. C. Mayer and R. Krechetnikov, Walking with coffee: Why does it spill?, Physical Review E 85, 046117 (2012) #nakedfluiddynamics #fluiddynamics #fluidmechanics #physics #physique #scienceisart #science #artinscience #fluidartgallery #beautiful #picoftheday #awesome #ignobel #ignobelprize #scienceisfun

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“Turbulence is the most important unsolved problem of classical physics” said Feynman and remain an active field of current research. In a recent study Cardesa et al. made tremendous progress in observing the way turbulence works. The main theory of turbulence was proposed by the Russian mathematician and physicist Andrei Kolmogorov in the early 1940s. In his picture the energy spreads from large swirls to smaller eddies nearby till reaching a critical scale at which the viscous effects become important and dissipate the energy in heat. This “cascade of energy” explains how even low viscosity fluid like gazes (air) are able to dissipate rapidly energy. Onsager (one of the geniuses of the 1940s that even Feynman found intimidating to talk to) announced in 1949 that even in the absence of viscosity a fluid could in certain cases dissipates energy through turbulence: “a shocking idea” that was proven right mathematically in a recent paper of Isett (2016) but in a rather unrealistic flow (a more realistic case was proposed by Buckmaster et al. in 2017). The picture shows a drawing of Leonardo da Vinci on the movement of water showing with astonishing details the turbulent structures, the swirls and eddies. Sources: Castelvecchi, Mysteries of turbulence unravelled, Nature Comm. 548 (2017) Cardesa, Vela-Martin, Jimenez, The turbulence cascade in five dimensions, Science, 2017 Isett, A proof of Onsager’s conjecture (2016) https://arxiv.org/abs/1608.08301 Source image: image extracted from Lavin, Leonardo’s watery chaos (1993) #nakedfluiddynamics #fluiddynamics #fluidmechanics #physics #physique #scienceisart #science #artinscience #fluidartgallery #beautiful #picoftheday #awesome #turbulence #complex #complexphysics #chaos “When I meet God, I am going to ask him two questions: Why relativity ? And why turbulence ? I really believe he will have an answer for the first.” said Heisenberg.

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When I saw this picture I did not know what I was looking at but I was like “wow that looks cool!” and it is!!!! In the first post I showed a drop of oil bouncing off an oil bath. The reason is the time you need to drain the thin air layer which separates the oil drop from the oil bath (same liquid!). Here we see another manifestation of this thin air layer which separates the oil jet from the oil bath. But why does it rebound? This is called the “Kaye effect”. One thing you need to know is that the bath moves to the right here. A jet hitting the surface will normally plunge into the liquid and entrain a bit of air and the jet will bend a bit to the right since the bath moves. If you increase the velocity of the bath, the layer of air will not have time to drain and the liquid jet will plunge and keep the layer of air which will act like a buoy or a air jacket if you like. The buoyancy will make then the jet curve up and float back to the surface creating an arc. But still no rebound. Now from this stage if you slow the bath down the curved surface will act like a ramp and the inertia of the jet will make it take off creating this rebound! The authors mentioned two others way to reach this state: move a rod through the jet which will destabilise it , starts from a higher flow rate of the jet then rapidly reduced it which will help curve the surface for lower flow rate jets. In this experiment the bath is moving to the right at a speed of 15.7cm/s and a oil jet is impacting the surface with a flow rate of 0.35cm^3/s (that is one can of coke every 17 min) from a 5cm height. The liquid is silicone oil with viscosity 102mPa (100 times more viscous than water). Source: M. Thrasher, S. Jung, Y. Kwong Pang,‡ C.P. Chuu, and H. L. Swinney. The Bouncing Jet: A Newtonian Liquid Rebounding off a Free Surface, 2007, Phys. Rev. E 76, 056319, with permission PS: sorry for the long absence of post. I started writing my thesis and it is time consuming!!! I will try to keep it up ;) #nakedfluiddynamics #fluiddynamics #fluidmechanics #physics #physique #scienceisart #science #artinscience #fluidartgallery #beautiful #picoftheday #awesome #mindblown#surfacetension

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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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Sharks are indeed amazing animals. First they are not completely cartilaginous fishes! They have bones!!!Well in facts more like teeth with bones in it but on their skins. Their skin is covered in denticles which are composed of an outer enameloid layer and an inner bone-like layer. But most interestingly to us here is the effect of these denticles on their hydrodynamics. Indeed sharks are amazing swimmers and their skin has to do with it. George Lauder and co-authors have investigated this using 3 D printed inspired shark skin. The benefit is that contrary to traditional technique of shark skin study, where actual shark skin is used and then sanded (thus removing material), here the skin is 3D printed and the amount of mass well control as well as the reference flat “skin”. The results are even more interesting: they found that at “low speed” the shark like skin experienced drag reduction of about 9% and that while control in motion they could in certain swimming program measure an increase in self propelled velocity and a reduction of cost of swimming at the same time. It means that shark can save a lot of energy while swimming at low speed and as they swim all the time on a lifetime this is a significant save of energy. This study also shows that the roughness can help achieve highest speed at lower cost making shark great swimmers! The figure here shows a lemonshark and the denticles. credit images: image1: By Albert kok image2: By Pascal Deynat Reference: Biomimetic shark skin: design, fabrication and hydrodynamic function, Li Wen, James C. Weaver and George V. Lauder, J. of Exp. Biology (2014) 217 (1656-1666 doi:10.1242/jeb.097097 ) [openaccess] #nakedfluiddynamics #fluiddynamics #fluidmechanics #physics #physique #science #shark #sharkweek #sharkskin #dragreduction #swimming #swimsuit #picoftheweek #picoftheday #awesome #mindblown #beautiful #scienceisart #scienceisfun #scienceisawesome #share #like #follow

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Highspeed impact of small hydrophobic objects can generate several interesting physical phenomena (Rayleigh-Plateau instability leading to pinch of the air cavity, water curtain sealing which leads to Rayleight-Taylor instability). Here, the authors looked at the water entry of a dense millimetric sphere with a highspeed camera. The frames are separated by 1.9 ms. The 1mm radius sphere is coated to be hydrophobic and impact the water at 540cm/s. At the impact a splash forms at the surface and close inward sealing the air cavity which detaches from the surface and sinks with the spheres. The air being lighter than water, on top of the cavity there is water above (a bit like when you flip a glass of water). A Rayleigh-Taylor Instability arises and is clearly visible in a jet like form. In the mean time the  cavity pinches off due to surface tension.  The jet produced by the instability will then meet the end of the broken cavity and generate an under water splash. These events repeat with smaller bubbles. Reference: The water-entry cavity formed by low Bond number impacts,Jeffrey M. Aristoff, Tadd T. Truscott, Alexandra H. Techet and John W. M. Bush, Phys. of Fluids, 20 (2008) See also: Water entry of small hydrophobic spheres, Jeffrey M. Aristoff and John W. M. Bush, J. Fluid Mech., 619, (2009) #nakedfluiddynamics #fluiddynamics #fluidmechanics #physics #physique #splash #waterentry #waterimages #waterimpact #softmatter #scienceisart #science #artinscience #fluidartgallery #beautiful #picoftheday #awesome #mindblown #picoftheweek

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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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To splash or not to splash? This is the question researchers have been struggling with for several decades. If it is clear now that the air ambient pressure has an importance, how it affects the splashing remains to be discovered. To tackle this problem, Nagel and his group looked at the airflow using a modified Schlieren technique. The video shows the impact of a drop which would splash on a smooth dry substrate (glass). The substrate in the video is voluntary scratched to delete splashing and allow clear visualisation of the airflow above the drop without significantly disrupting it. They reveal #beautiful and #elegant vortex structures after impact. They don’t appear to affect the splashing threshold parameters. The video shows the impact of a drop of silicon oil of radius 1.4mm, viscosity 20mm^2/s at Reynolds in air 685 on a rough dry substrate. Credit: Irmgard Bischofberger, Bahni Ray, Jeffrey F. Morris, Taehun Lee and Sidney R. Nagel. Airflows generated by an impacting drop. Soft Matter Vol. 12, 3013-3020 (2016) [with permission] #nakedfluiddynamics #fluiddynamics #fluidmechanics #physics #physique #splash #splashart #drop #softmatter #scienceisart #science #artinscience #fluidartgallery #beautiful #picoftheday #awesome #mindblown

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Waves are reflected and transmitted when they interact with a submerged plate. Vortex shedding affects significantly the linear potential prediction by dissipating large amount of energy. It also affects the mean velocity field. This picture shows a long time exposition picture of the shore side of the submerged plate. The fluid is seeded with buoyant particles lighten by a pulse laser. The long exposition enables to see the particle path. Vortices are clearly visible. A clockwise rotating vortex on the top of the plate and two counter rotative vortices advected toward the bottom of the tank. Credit: Brossard groups for this picture and special thanks to Gaële Perret for sending me this beautiful picture. For more detail on it please refer to: A. Poupardin, G. Perret, G. Pinon, N. Bourneton, E. Rivoalen , J. Brossard, Vortex kinematic around a submerged plate under water waves. Part I: Experimental analysis, E. J. of Mech B/Fluids (2012) G. Pinon, G. Perret, L. Cao, A. Poupardin, J. Brossard, E. Rivoalen, Vortex kinematics around a submerged plate under water waves. Part II : Numerical computations, E. J. of Mech B/Fluids (2016) #nakedfluiddynamics #fluiddynamics #fluidmechanics #physics #physique #waves #waterwaves #piv #breakwater #scienceisart #science #artinscience #fluidartgallery #beautiful #picoftheday #awesome #mindblown

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The contact time of a drop on a hydrophobic surface is affected by the presence of macro defects (like macro wires). Video shows the top view and side view of a drop of radius 1.3 mm impacting cylindrical wires of radius 100 micrometers also treated to be hydrophobic similarly to the flat surface at V=1.0m/s. The drop splits in 4 lobes that quickly forms 2 symmetric sub drops. Takeoff occurs 7.6 ms after impact. They take off the surface about square root of 2 time faster than one on a similar flat hydrophobic plate with no defects. This findings might help design even better water repellent or anti-freezing surface taking advantages of this reduced contact time. credit: Gauthier, Symon, Clanet and Quere, Water impacting on superhydrophobic macrotextures, Nat. Comm. 6:8001 (2015) #nakedfluiddynamics #fluiddynamics #fluidmechanics #physics #physique #drops #dropsart #dropsplash #splash #splashart #physicisart #artinscience #scienceisart #beautiful #mindblown #awesome 


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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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When you inject a fluid into another one in a quasi-two dimensional flow beautiful patterns appears. They are called fingerings. Here is a picture showing such patterns for two miscible fluids (the fluid is injected fast enough, large Peclet number, that they don’t mix, 1mL/min here) the transparent one (outer) being more viscous than the other (inner). The different frames show increasing viscosity ratio. In the bottom right frame the fingers disappear. The ratio is close to (in/out) = 0.3. Credit: Bischofberger, Ramachandran & Nagel, Fingering versus stability in the limit of zero interfacial tension, Nat. Comm. 5:5265 (2014) (with permission)
 To do this experiment, use two circular glass plates separated by a small gap (use washers to keep the distance for instance). Drill a hole in the center of the top one and inject the first liquid using a seringue. Then inject the second one at a constant rate! That is how you make Hele-Shaw cell #nakedfluiddynamics #fluiddynamics #flowdynamics #fluidmechanics #physics #physique #heleshaw #artinscience #scienceisart #scienceisbeautiful #scienceisawesome #scienceandart #beautiful #picoftheday #awesome #like #follow