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Using CFD to Help Increase the Safety of Driverless Cars
Pointwise uses a multiple block approach simplifying complex CFD studies into modular grids that can easily be linked together. This approach gives users full control of the process, from the smallest element to the largest, and enables them to troubleshoot and improve grid quality early in the process. It also saves time and increases accuracy by allowing the reuse of grids in different configurations for additional CFD studies.

“For the motorway study, I did not have to re-mesh the car. I did not have to re-extrude viscous layers or re-mesh the volume,“ says Khalil. “It saves a remarkable amount of time and ensures that the results are exactly the same because we’re using the exact same data and mesh. Pointwise’s T-Rex feature was also critical to accurately capturing the boundary layer flow, which is especially important for turbulence studies.”
cfd  example  driving  automation  car  autoroute 
4 weeks ago by aries1988
Case Study: CFD and AI Can Aid Driverless Cars | Another Fine Mesh
Working with his professors and advisors, Khalil began by gathering data about road conditions on the UK’s M1 roadway such as traffic patterns, accidents, winds, and speed. He then used Pointwise to create individual high-fidelity meshes for the subject car and a variety of vehicles. After inputting the road conditions and running simulations on the individual aerodynamics of the main vehicle, he joined all the meshes together using Pointwise’s multiple block feature for the final study of how the moving parts interacted together.
cfd  example  driving  automation 
4 weeks ago by aries1988
Implementation of boundary conditions for FVM -- CFD Online Discussion Forums
[4] WHAT IS THE DIFFERENCE BETWEEN IMPLICIT AND EXPLICIT TREATMENT OF BOUNDARY CONDITIONS AND HOW DOES IT EFFECT THE DISCRETIZED EQUATION? I.E. HOW IS THE DIAGONAL INFLUENCED AND AND WHAT ABOUT THE EXPLICIT SOURCE TERM DUE TO THE BOUNDARY CONDITION?

for boundary cells the face value is "replaced" by the particular boundary assumption. Thus, when re-arranging the variables, you have the choice to either put the contribution in the source or in the matrix or to split it. Generally spoken, larger source terms (explicit) will slow convergence down.
explained  cfd 
5 weeks ago by aries1988
SOLUTION TECHNIQUES
Coupled Equations

The momentum and continuity equations are linked, in that the momentum equations share the pressure, and the velocities (and pressure via the density in compressible flows) enter the continuity equation.

There is no direct equation for the pressure. The task of all CFD codes is to join the variable without an equation (pressure), to the equation without a variable (continuity).

PHOENICS does this using a variant of the SIMPLE algorithm, namely SIMPLEST (= SIMPLEShorTened).
cfd 
7 weeks ago by aries1988
CONVERG.HTM
Linear relaxation is almost always preferable to the false-time-step alternative. The reason is that the appropriate value for this time step is hard to choose; and what has been found suitable for one size of domain may be entirely unsuitable for another, even though other aspects of the flow may be identical.

A common error of inexperienced users is to set very small values of the false time step, e.g. 1.0e-3 sec for the velocity. They then find that the oscillations which have alarmed them disappear; and they are comforted by the apparently smooth progress of the solution procedure, not recognising that it is proceeding so slowly that an intolerably large number of sweeps will be needed to attain the true solution.

For a wind-farm simulation for example, in which a 10 km width and a 10 km/sec wind imply that effects take 1000 sec to be convected across the domain, even a 1.0 sec false time step imposes a severe delay on the attainment of the correct solution.
cfd  explained 
7 weeks ago by aries1988
CONVERGENCE MONITORING AND CONTROL
The most widely-known technique for achieving convergence is the use of relaxation. This slows down (relaxes) the changes made to the variables from sweep to sweep. Relaxation does NOT alter the final solution, only the way in which it is achieved. Two types of relaxation are available: linear and false timestep.
explained  cfd 
7 weeks ago by aries1988
What y+ should I run? Part 1 – The underlying physics
I would also like to make a note that its recommended to either resolve the viscous sublayer at y+1 or the log region at y+>30 and not to place the first cell in-between them. The model has been calibrated either case and CFD software deals with transition between the layers differently.
cfd  explained 
11 weeks ago by aries1988
Explanation bounding k and bounding epsilon -- CFD Online Discussion Forums
It's bit technical to explain in detail, so please refer to the literature. In short, the terms dissipation error/diffusion are used with to indicate the same thing, meaning the numerical diffusion/viscosity added by the scheme. This error is due to the presence of non-zero imaginary part in the modified wavenumber. The real part of the wavenumber is responsible of what is called dispersion error. Upwinded schemes are affected by both, while central schemes are only affected by the dispersion error.
cfd  explained 
may 2019 by aries1988
I’m Charles Schnake and This Is How I Mesh | Another Fine Mesh
I think that a big problem I see being wrestled with right now is the scalability of CFD, and by that I don’t just mean the ability of the solvers to run efficiently on many nodes/cores. How can we (engineering software vendors) enable CFD engineers to run more simulations, evaluate more designs, and gain more insight faster?

I’m a relatively laidback person who prefers a comfortable casual atmosphere, so I really enjoy a hole-in-the-wall joint serving up street & comfort foods. That said, if we’re talking bucket-list type establishments I would choose a three Michelin star establishment like Alinea (Chicago), Jiro (Tokyo), or Meadowood (Napa). The formality of it all would probably make me a little uncomfortable at first, but I imagine that the quality of the food & service would eradicate that in short order and leave me with an unforgettable experience.
peer  cfd  advice  restaurant 
december 2018 by aries1988
Toolkit Produced for Optimizing Floating Renewables Devices
The toolkit consists of two main components: Computational Fluid Dynamics (CFD) using Proteus open-source software and Multi-Body Dynamics (MBD) using the Chrono open-source solver. Both models have been validated separately and together. Using a CFD model early in development can reduce costs in design optimization by performing full-scale simulations under realistic sea states, before performing laboratory tests which may be subject to practical limitations. 
ocean  energy  wave  eolien  offshore  cfd 
december 2018 by aries1988
FvVectorMatrix questions -- CFD Online Discussion Forums

Continuity Eq: div(U) = 0
and with source term: div(U) = S

Discretised momentum eq. A*U = H - grad(p)
->
U = H/A -(1/A)*grad(p) = Ustar - (1/A)*grad(p)
H/A is the momentum-predictor value of U, ie Ustar.
if we insert this in the continuity eq. it yields

div(U) = div(Ustar - (1/A)*grad(p)) = S
->
div(Ustar) - laplacian(1/A, p) = S
or
laplacian(1/A, p) = -S + div(phi)
where phi is Ustar evaluated on faces.
have a look at icoFoam and you will easily see where to insert S.
explained  cfd 
november 2018 by aries1988
phi -= pEqn.flux() vs. linearInterpolate(U) & mesh.Sf() - Page 2 -- CFD Online Discussion Forums

After re-reading your last and previous posts and also Issa's paper and Jasak's Thesis, I think I should clarify this a bit. My previous consideration is NOT right, since H(U) is updated between every two consecutive PISO correctors (so k-1 times along the whole time step, if k is the number of PISO correctors), but ONLY in terms of the cell-centered neighbour velocities U_N, and NOT in terms of the phi contribution inside discretization coefficients (a_N, but also the owner coefficient a_P, which contributes in the construction of the HbyA vector).
algorithm  piso  cfd  programming  ofm 
november 2018 by aries1988
phi -= pEqn.flux() vs. linearInterpolate(U) & mesh.Sf() -- CFD Online Discussion Forums

Now lets take a look into the PISO algo itself. I will not cite the code but it should be easy to follow.
1) create the UEqn using the last- known phi. Note that any "XEqn" is like a black box, with a void space for the unknown. The imporatant stuff are the coeffs.
2) if you like, solve momentum predictor (this is unnecessary and can be dropped for time saving).
3) extract the semi-central coeffs from the UEqn, reverse them and call rAU. This is the famous "operator splitting".
4) pressure loop:
4.1) recalculate velocity: U = rAU * H();
4.2) recalculate fluxes: phi = interpolate(U) * S; note that the flux field is NOT divergence free;
4.3) solve for pressure using the non divergence free flux
4.4) solve it several times...
5) now, we solved for pressure with non-div-free phi. But the literature (Jasak's thesis, Issa et al.) tells us, that we can correct the fluxes using the pressure field and this way ensure div-free condition. This is the famous phi -= pEqn.flux();
6) Finally, we correct the velocity field, acquiring a good approximation of the velocity field.
algorithm  cfd 
november 2018 by aries1988
OpenFOAM guide/The PISO algorithm in OpenFOAM - OpenFOAMWiki
The PISO (Pressure Implicit with Splitting of Operators) is an efficient method to solve the Navier-Stokes equations in unsteady problems. The main differences from the SIMPLE algorithm are the following:

No under-relaxation is applied.
The momentum corrector step is performed more than once.
explained  cfd 
november 2018 by aries1988
OpenFOAM guide/The SIMPLE algorithm in OpenFOAM - OpenFOAMWiki
The SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) allows to couple the Navier-Stokes equations with an iterative procedure, which can be summed up as follows:

Set the boundary conditions.
Solve the discretized momentum equation to compute the intermediate velocity field.
Compute the mass fluxes at the cells faces.
Solve the pressure equation and apply under-relaxation.
Correct the mass fluxes at the cell faces.
Correct the velocities on the basis of the new pressure field.
Update the boundary conditions.
Repeat till convergence.
The steps 4 and 5 can be repeated for a prescribed number of time to correct for non-orthogonality.
algorithm  cfd  explained 
november 2018 by aries1988
IcoFoam - OpenFOAMWiki
A highly annotated version of the code is shown below. Ferziger and Peric, below, give a fairly standard overview of PISO, but OpenFOAM uses a notation and form closer to Jasak and Rusche's theses. References to PISO documentation are:
explained  cfd  algorithm 
november 2018 by aries1988
I’m Sebastian Desand and This Is How I Mesh | Another Fine Mesh
What is some of the best CFD advice you’ve ever received?

Keep it simple and use the computational power available. The more you model, the more sources of error and uncertainty.
Never forget to ask “so-what” after concluding something. The value is very often in the implications.
cfd  temoignage  advice  entrepreneurial  sweden  cloud 
november 2018 by aries1988
I’m Kelly Senecal and This Is How I DON’T Mesh | Another Fine Mesh
But the problem was that it took so long to create the first mesh, there was no way we were going to create more meshes with different resolutions. As a result, we really didn’t know if we were anywhere near grid convergence. Hence we would tune models to account for what was, most likely, error from being under-resolved.

JC: Grid convergence, for better or worse, remains an open issue if only from one of runtime. And the manner in which one obtains a family of meshes suitable for a grid convergence study is a bit more challenging with hybrid unstructured meshes than it was with structured grids.

What do you see are the biggest challenges facing CFD in the next 5 years?
Maintaining a good balance between accuracy and runtime. Industry needs faster simulations to make design decisions quicker; however, for CFD to be truly predictive we need higher mesh resolution, less empirical modeling, and fully transient simulations.
industry  cfd  mesh  interview 
october 2018 by aries1988
Advantages of Structured Grids Over Unstructured Grids
The elements generated are typically either triangles or quadrilaterals in 2D and volumes are generally tetrahedral or hexahedral in 3D.

In a structured mesh, a repeated pattern is followed where every node is uniquely defined according to a specified algorithm; while in an unstructured grid all nodes are typically arbitrarily defined automatically and no regular pattern is followed. In a structured grid, a node or vertex is surrounded by an equal number of nodes or vertices across space while there is no such requirement in unstructured grids.

Traditionally, structured meshes were the norm but currently unstructured meshes have become commonplace due to their advantages in faster grid generation, ability to handle complex geometry and not much need of grid generation experience.
opinion  comparison  mesh  cfd 
september 2018 by aries1988
National Institute of Aerospace/ Computational Fluid Dynamics Seminar
A place to share ideas and problems for barrier-breaking developments
cfd  seminar  resource  research 
september 2018 by aries1988
Rayleigh-Benard Convection (two-dimensional and very turbulent) - YouTube
Numerical simulation of highly turbulent two-dimensional Rayleigh-Benard convection. Be sure to watch this in HD!
Hot (red) fluid rises from the hot bottom plate while cold (blue) fluid falls down from the cold top plate. The mean temperature corresponds to white.
The Rayleigh number is 10^13 and the Prandtl number is 1; the aspect ratio of the simulation is 16:9.
Two seconds of the video correspond to one so-called free-fall time unit, i.e. the travel-time from plate to plate of a parcel of hot fluid that is accelerated by gravity and not slowed down by surrounding fluid.
The numerical resolution is 7680x4320 gridpoints, i.e. one pixel of the 1080p-Video corresponds to 4x4 gridpoints.
visualization  cfd  mecaflu  painting 
july 2018 by aries1988
I’m Tom Chan and This Is How I Mesh | Another Fine Mesh
Tecplot and Pointwise bookend the simulation process – Pointwise on the pre-processing side and Tecplot on the post-processing end. We have worked with the folks at Pointwise for many years and have many joint customers.

What do you see are the biggest challenges facing CFD in the next 5 years?

LES

What is some of the best CFD advice you’ve ever received?

Avoid the “black box” syndrome – don’t blindly depend on automated “black box” techniques.

Our customers are some of the smartest, most experienced engineers in the world. They work hard year after year, developing a deep knowledge of their craft. The experience and intuition they acquire can never be replaced by a “black box.”

But, much can be done to help engineers with the more tedious and tiresome tasks, which would give them more time for analysis and digging deeper into their research and designs.
tecplot  interview  cfd  fun  vietnam  chinese  usa 
june 2018 by aries1988
SimpleFoam k and epsilon bounded -- CFD Online Discussion Forums
In general, upwind schemes tend to suppress numerical oscillations and would be more stable than linear schemes.
This means that they also tend to suppress the turbulent fluctuations. So if you are only interested in computing the mean flow (and hence using a RANS model like k-epsilon), then upwind schemes are a good option. For cases where you want to capture the unsteady flow (say, in LES) you will have to use linear schemes.
cfd 
may 2018 by aries1988
高薪互联网对决CFD
CFD教授级高工自己写的了CFD代码(具备高水平的计算机编程水平),推得了算法(张量、离散求解、甚至自己提出模型)。这是基本素养,是CFD教授级高工和CFD工程师的区别。
CFD教授级高工,可以从你的云图看出你的网格数量,从流场看出是否调用高阶格式,从几何外形看出哪里需要网格加密。

重申,CFD不同于实验学科,这一行,充满了学术传奇和商业传奇:
开源CFD软件架构师Weller只是本科生,从不写文章,创立CFD软件OpenFOAM
Wilcox逝世之前一直都是讲师,生前提出kOmega模型
Gosman在帝国理工时期,提出各种Gosman模型,后创立CFD软件STAR-CD
Hirt提出VOF模型,单篇被引10000+,创立CFD软件Flow-3D
Peric放弃帝国理工大学教授职位创立公司,即为CD-adapco的前身
Spalding提出CFD模型无数,学术不倒,同时创立CFD软件PHOENICS
他们都是纯粹的CFD教授级高工,是神。或有教职,或创立公司,或商业咨询,或身兼数职。

CFD应届生去公司要高薪,要走【一事一议】。发挥经验优势,越老越值钱。
谈薪水,靠的自己的技能。赚多少钱,取决于你能拿多少东西上台面。但这里,问一下自己,你的CFD技能是否能做到一事一议?算例发散了怎么办?是否会植入新的大气流动模型?如何增加一个地面的造火源项?
拿多少工资,取决于你创造的效益。

在某一天,各大公司将不再看重花花绿绿的云图,而是真正的需要CFD做产品设计的时候。
他们终将填充一批强有力的CFD教授级高级工程师。
那个时候,将需要一个CFD理论功底扎实,行业经验丰富的人,以领导者开创者的身份,负责万亿体量产品的设计变革。
career  cfd  moi  future  today  history  comparison  internet  2018  industry  bat 
april 2018 by aries1988
QNET
Welcome to the world's foremost repository of structured knowledge and advice designed to underpin quality and trust in the industrial application of CFD.
cfd  database 
march 2018 by aries1988
Pimplefoam - residuals not converging -- CFD Online Discussion Forums
(2) get back to meshing and try a better mesh. if you are running latest OpenFOAM version 1706, you may run chekcMesh -writeAllFields to get scalar fields depicting the mesh quality parameters in paraview.
convergence  cfd  explained 
march 2018 by aries1988
I’m ZJ Wang and This Is How I Mesh | Another Fine Mesh
What software or tools do you use every day?

We use Qt Creator as our development environment, Git for version control, Matlab or Mathematica for symbolic derivations, Tecplot 360 and ParaView for visualization, and Gmsh and Pointwise for mesh generation. I use the Microsoft Office suite for emails, papers and presentations, and skype for web conferences.
cfd  interview  list 
march 2018 by aries1988
pimpleFoam vs simpleFoam vs pisoFoam vs icoFoam? -- CFD Online Discussion Forums
Pimple being an Implicit numerical method (cf Wikipedia), you can use a large Co, greater than 1. Co needs to be smaller than 1 for explicit method (see here).
By the way, as owayz said, I took a great care to define correctly the time scales of the large eddies I am simulating.
In my case, there was no problem at Co=10. Nevertheless, Co=100 crashed, as well as Co=50. At Co=30, the results were not satisfactory. So the limit (in my case, which was a simple rectangular rod in a laminar flow, for vortex shedding estimation), the limit is between Co=10 (OK) and Co=30 (KO)
cfd 
february 2018 by aries1988
計算の確認と計算結果の処理
計算の確認と計算結果の処理 2018年2月5日 はじめに 計算の確認と計算結果の処理について。 使用バージョン OpenFOAM 5.x 残差の確認 function object を用いる方法 バージョン 2.4.0 から、残差を出力する function object が使える。system/controlDict…
post  cfd  ofm 
february 2018 by aries1988
CFD Education Center
This website is a platform on which visitors can discuss Computational Fluid Dynamics (CFD) and get some feedback from CFD experts or other visitors.

One major advantage of this blog is that it works in parallel with different courses taught in fluid mechanics and fundamental books in numerical methods.

The structure of the blog is based on different levels of CFD (elementary, basic, intermediate and advanced) which will be mentioned in the topic of each post. Many common problems of fluid mechanics are approached by proposing discussions about their main aspects and some codes for solving them.

All visitors are welcomed to participate in discussions and suggest solutions. Furthermore, the questions or problems, which are not fully solved during the discussion, will be answered frequently (e.g. every 10 days) by experts.
cfd  primer  explained  moi  resource  fluid-mechanics  learn 
january 2018 by aries1988
A Hybrid Quadcopter for Longer Flights, Quieter Skies
Video from a computational fluid dynamics simulation of the Straight Up Imaging (SUI) Endurance quadcopter hybrid design in forward flight. The video starts with NASA’s novel hybrid design modification from the original configuration, where the front rotors have been undermounted and the back rotors are kept overmounted. The video then shows the vortex wake using the Q-criterion isosurfaces. If you look closely, you can see that the upper surfaces of the blades are blue (lower pressure), and the lower surfaces are red (higher pressure). The difference in pressure is what generates thrust to fly the drone. The hybrid design reduces the rotor wake interactions and yields much better performance. Timothy Sandstrom, NASA/Ames
drone  nasa  simulation  cfd  video 
january 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
Étude des performances aérodynamiques d'un drone à aile rhomboïdale | andheo — Ingénierie et Conception par Simulation — Région de Paris IdF
Andheo a simulé avec le code CFD elsA les performances aérodynamiques d'un drone à aile rhomboïdale en tenant compte de la transition laminaire-turbulent.
cfd 
december 2017 by aries1988
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