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aries1988 : turbulence   10

What is turbulence—and how can you calm down about it?

Rough air happens everywhere, from ground level to far above cruising altitude. But the most common turbulence experienced by flyers has three common causes: mountains, jet streams, and storms.

At NCAR, Sharman has been working since 2005 to build much more precise “nowcasting” turbulence tools.

Here’s how it works: an algorithm currently installed on around 1,000 commercial airliners analyzes information from onboard sensors to characterize each plane’s movement at any given moment. Using data on forward velocity, wind speed, air pressure, roll angle, and other factors, the algorithm generates a local atmospheric turbulence level, which is fed back into a national system every minute. Used in conjunction with national weather forecasts and models, the tool annotates forecasts with real-time conditions, which in turn helps to strengthen weather prediction models.
turbulence  aviation  travel  plane  engineering  app 
march 2019 by aries1988
Turbulence, the oldest unsolved problem in physics

because our understanding of turbulence over time has stayed largely ad-hoc and limited, the development of technology that interacts significantly with fluid flows has long been forced to be conservative and incremental. If only we became masters of this ubiquitous phenomenon of nature, these technologies might be free to evolve in more imaginative directions.

Motions of fluids are usually hidden to the senses except at the interface between fluids that have different optical properties.

For example, you can see the swirls and eddies on the surface of a flowing creek but not the patterns of motion beneath the surface.

The Navier-Stokes equation is difficult to solve because it is nonlinear. This word is thrown around quite a bit, but here it means something specific. You can build up a complicated solution to a linear equation by adding up many simple solutions. An example you may be aware of is sound: the equation for sound waves is linear, so you can build up a complex sound by adding together many simple sounds of different frequencies (harmonics). Elementary quantum mechanics is also linear; the Schrödinger equation allows you to add together solutions to find a new solution.

The idea is that the interesting dynamics occur at larger scales, and grid points are placed to cover these. But the subgrid motions that happen between the gridpoints mainly just dissipate energy, or turn motion into heat, so don’t need to be tracked in detail. This approach is also called large-eddy simulation (LES), the term eddy standing in for a flow feature at a particular length scale.

The idea is that, while the low-speed solution is valid at any speed, near a critical speed another solution also becomes valid, and nature prefers that second, more complex solution. In other words, the simple solution has become unstable and is replaced by a second one. As the speed is ramped up further, each solution gives way to a more complicated one, until we arrive at the chaotic flow we call turbulence.
turbulence  Physics  science  history  explained  example  LES 
december 2018 by aries1988
TURBULENCE MODELS IN PHOENICS
1. Why turbulence models are needed
The small-eddy problem
The variations in space and time can be thought of as caused by a population of eddies of a wide range of sizes, orientations and speeds of rotation, which are in ceaseless and random motion.

Why the eddies cannot be ignored.

Although the details of the small-scale eddies are seldom of interest, their existence cannot be ignored; the reason is that their existence has large-scale effects.
These effects include:

Increased macro-mixing, an example of which is that the heat transfer from a hot turbulent fluid to the colder wall of the pipe through which it is flowing is much greater than it would be for a laminar flow.
Increased micro-mixing, an example of which is the greatly increased rate of chemical reaction between co-flowing streams of fuel and oxidant gases, when the velocity is raised to the critical level.
It is the role of a turbulence model to enable the effects of the eddies to be computed without simulating them in detail; whereby it should be stated that immensely more attention has been paid to the macro-mixing than to the micro-mixing effects.

A third approach to the problem of predicting the behaviour of turbulence phenomena can be regarded as combining some aspects of both DNS and macroscopic modelling. It is called 'Large-Eddy Simulation', commonly abbreviated to LES. Its nature is this:

- The CFD code is run in stepping-through-time mode, even when, turbulence fluctuations apart, the flow is steady.
- A macrocopic turbulence model is employed, but with modifications intended to represent only the eddies which are small compared with the cells of the computational grid which is in use.
eddy  explained  turbulence 
september 2018 by aries1988
[SINUMEF Lab] Enseignement
Module 1 : Aérodynamique et turbulence
doctorat  turbulence  course 
september 2012 by aries1988
漩涡
有趣的是,如果把空间从三维变成二维,情况会大不相同。上世纪三十年代开始人们就证明了在二维空间里流体方程解的存在性,并且意识到二维流体和三维流体在数学上表现出迥然相异的性质。在数学上,这种区别的根源在于在三维空间中「旋度」是一个矢量场而在二维空间中是一个标量场。简单地说,这意味着在三维空间中复杂的湍流在二维空间中就变成了人们更为熟悉的对象:漩涡。
maths  turbulence  moi 
january 2011 by aries1988

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