Fluid Dynamics and Turbulence
Flowing liquids and gases, above all turbulence.
arXiv: physics.flu-dyn
13 topics
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In a flat flow, such as a soap film or a thin atmospheric layer, turbulence sends energy to larger scales instead of smaller ones and builds giant vortices. Some of its statistics have the same symmetries as critical points in statistical physics, for reasons nobody can explain.
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When a liquid is pulled apart hard enough it tears open into vapor bubbles, and when those bubbles collapse they can become hot enough to give off flashes of light. The questions are when the tearing starts, and what happens inside a collapsing bubble.
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Moving contact lines and dynamic wetting
5 problemsWhere a liquid, a solid and air meet there is a line, which moves when a drop spreads or a plate is dipped into a bath. The ordinary equations of fluid flow predict an infinite friction force at that line, so some extra physics must act at very small scales.
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A drop hitting a dry surface may spread smoothly or splash into a crown of droplets, and lowering the surrounding air pressure can stop the splash entirely. Related questions concern the instant a liquid neck pinches off or a bubble bursts, where the equations of motion produce singularities.
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A tiny amount of long polymer molecules dissolved in water can make a slow, syrupy flow chaotic, driven by stretching of the molecules instead of by inertia, and can cut the friction of fast turbulent pipe flow by up to about 80 percent. How these chaotic states arise in straight pipes and channels and what limits drag reduction are not known.
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Clouds, dusty gases and sediment-laden rivers carry particles that are flung out of whirls, cluster and collide. Warm clouds produce rain within about 20 minutes, faster than standard droplet-growth theory allows, and whether turbulence explains the difference is disputed.
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Quantum turbulence in superfluids
5 problemsIn superfluid helium and atomic Bose-Einstein condensates, rotation exists only in thin vortex lines carrying a fixed quantum of circulation. A tangle of such lines behaves at large scales like ordinary turbulence, but how its energy is finally dissipated near absolute zero is not settled.
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A fluid layer heated from below and cooled from above carries heat upward by turbulent convection. Theory predicts that at strong enough heating the thin layers of fluid next to the plates become turbulent and heat transport rises steeply, but experiments disagree on whether and when this happens.
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Transition to turbulence in shear flows
5 problemsWater in a pipe stays smooth at low speed even though tiny disturbances do not grow, and then becomes turbulent in patches that split, decay and spread. Physicists describe this onset as a phase transition similar to the spread of an epidemic, but several of its properties remain unmeasured or unexplained.
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In a turbulent fluid, energy fed in at large scales passes to ever smaller swirls until viscosity turns it into heat. The small-scale fluctuations are far more bursty than the simplest theory predicts, and nobody can yet compute their statistics from the equations of fluid motion.
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In the ocean and atmosphere, layers of different density and the rotation of the Earth constrain turbulent motion. How efficiently such turbulence mixes heat, salt and momentum, and in which direction it moves energy between scales, are open questions.
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A flame in a turbulent gas wrinkles and burns faster, and under some conditions it suddenly turns into a detonation, a supersonic shock wave sustained by the burning behind it. The questions are how fast turbulent flames burn and when and how detonation starts.
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Most of the drag on ships, aircraft and pipelines comes from turbulence next to solid walls. Basic properties of this layer, such as the constant in its logarithmic velocity law and whether near-wall fluctuations keep growing as flows get faster, are still debated.