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aries1988 : les   14

CFD and others...: Sub-grid Scale (SGS) Stress Models in Large Eddy Simulation
the role of the SGS model, in the above scenario, was to stabilize the simulation by adding numerical dissipation.
For numerical methods which have natural dissipation at high-wave numbers, such as the DG, SD or FR/CPR methods, or methods with spatial filtering, the SGS model can damage the solution quality because this extra dissipation is not needed for stability.
LES  model  opinion 
4 weeks ago by aries1988
GitHub - HiFiLES/HiFiLES-solver: High Fidelity Large Eddy Simulation Solver
At the Aerospace Computing Laboratory we believe that high-order numerical schemes have the potential to advance CFD beyond the current plateau of second-order methods and RANS turbulence modeling, ushering in new levels of accuracy and computational efficiency in turbulent flow simulations. HiFiLES (High Fidelity Large Eddy Simulation) is released as a freely available tool to unify the research community, promoting the advancement and wider adoption of high-order methods. The code is designed as an ideal base for further development on a variety of architectures.
code  LES 
january 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
拷问LES大涡模拟
例如kEpsilon模型则是完整的,因为不需要给定参数。对于混合长模型,即为不完整的,因为需要指定混合长(位置和时间的函数)。如果两个CFDer对同一个流型指定了不同的混合长计算公式,结果是完全不同的。
LES  review 
june 2018 by aries1988
CFD中的LES湍流模型
LES中的亚格子尺度应力(SGS),其表示模化的速度分量对解析的速度分量的影响(τ在OpenFOAM的代码中用B来表示)。

我们需要对SGS进行模化,最简单的模型即为通过Boussinesq假定来实现,其也为其他更高级模型的本源。首先我们将τij表达为偏应力以及法向应力部分,且其中的法向应力部分被认为是各项同性的

SGS模型的主要作用就是进行湍流波动进行耗散9。因此SGS模型需要给定一个合适的耗散量。因此LES需要使用中心差分格式,因为这一类格式不会添加附加的耗散。所有的迎风格式都会对SGS模型添加附加的耗散特性。

周期边界的距离应该大于最大的涡旋尺寸的二倍以上。
explained  LES  ofm 
may 2018 by aries1988
Sub-grid Scale (SGS) Stress Models in Large Eddy Simulation
The simulation of turbulent flow has been a considerable challenge for many decades. There are three main approaches to compute turbulence: 1) the Reynolds averaged Navier-Stokes (RANS) approach, in which all turbulence scales are modeled; 2) the Direct Numerical Simulations (DNS) approach, in which all scales are resolved; 3) the Large Eddy Simulation (LES) approach, in which large scales are computed, while the small scales are modeled. I really like the following picture comparing DNS, LES and RANS.
numeric  cfd  LES  comparison  explained 
january 2018 by aries1988

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