Questions tagged [fluid-dynamics]
The study of the properties of fluids and gases in motion
525
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When to stop iterations in SOR solver for 3D Poisson equation
I'm writing a solver (in C) for 3D incompressible fluids, using the finite-differences method, and I'm finding a somewhat surprising behaviour: the solver provides "good-looking" solutions, ...
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13 views
How to define "Characteristic length in Ansys CFX Power Syntax script?
I am trying to evaluate Reynolds number in Ansys CFX 2020 R2 using CCL/Perl script on different domains in a radial turbine. But for that, I need to define the Characteristic length. So I define the ...
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1answer
56 views
How does the diffusion of a finite volume method with a WENO scheme compare with that of spectral methods?
I know that, in general, finite volume (FV) methods are more (numerically) diffusive than spectral methods. However, I can't find any information on how the advection scheme changes that.
For example, ...
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44 views
Unsteady Stokes equations in ALE framework
I'm trying to solve Unsteady Stokes equations on a moving domain, using an ALE formulation, that is
$$\frac{\partial \mathbf{u}}{\partial t} - \mathbf{w}\cdot \nabla\mathbf{u} = \nu\Delta\mathbf{u} - \...
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1answer
76 views
Is there any way to have a better guess for initial condition of an ODE coupled to CFD as a boundary condition?
I'm doing CFD simulations for blood flow in unstructured grids. My boundary condition at the outlets is called three-element Windkessel which basically calculates the pressure by solving this ODE:
$$...
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2answers
132 views
Why are fluid simulations so hard?
Fluid simulations solving the hydrodynamic (HD) or the magneto-hydrodynamic (MHD) equations are very useful in physics, the latter being particularly useful for modeling plasmas.
Of course these ...
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20 views
Probability Density Function of DNS velocity field [duplicate]
I am currently working on a DNS turbulent solver and I would like to compare my IHT simulations to papers. Those papers show the Probability Density Function of the velocity field:
I would like to ...
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1answer
62 views
Handling time derivative as source term in SDIRK methods
I am currently facing some challenge implementing a less traditional PDE which take a form similar to the Navier-Stokes equation, except that the continuity equation is modified such that:
$$\epsilon \...
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57 views
Is it possible to shift the numerical results in order to compare it with the analytical solution?
I'm using lattice Boltzmann method (LBM) to do simulation of flow in a pipe. The diameter of the pipe is 0.02 m and its height is 0.1 m. I put a parabolic velocity profile at the inlet:
$$\mathbf{u}(\...
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48 views
Finding a CFD paper with extra degree of freedom variable in mass conservation
I am trying to find a paper that I saw about a year ago. I am not sure of the actual date of the paper.
I believe it was a finite difference CFD paper. The interesting part of the paper was the ...
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1answer
62 views
Imposing pressure variation instead of Dirichlet boundary conditions on Finite Element Method
I always see Finite Element codes solving PDE with Dirichlet or Neumann boundary conditions. But, I have a problem now consisting of a straight cylinder with a circular base (a simple 3D tube), with ...
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1answer
56 views
Understading memory sharing in a GPGPU using a lattice example
I am new in the GPUs world, I used them in Matlab ambient so I didn't need to appreciate the subtleties of these devices.
I know that a GPU can be divided into multiprocessors (also called Streaming ...
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79 views
How to calculate the average velocity over a surface through time using paraview?
My problem is a little more complex, but here is a simplified similar model.
Let us consider a fire source inside a box and the hot gases (burnt or unburnt) from the combustion are going out of the ...
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33 views
Effect of Peclet number on concentration profiles
I have been performing convection-diffusion transport studies on microfluidic channels like the following
I came across an article that illustrates the effect fo Peclet number on concentration ...
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48 views
computing time scale and steady state concentration in microfluidic channels
I have been performing convection-diffusion transport studies on microfluidic channels like the following
The inlet concentration is specific and I obtain the time-dependent concentration profiles of ...
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1answer
84 views
Would modeling a fluid flow that decreases in magnitude violate conservation of mass?
If I make a quasi-steady assumption in a model such as keeping the fluid density constant and assuming the flow is incompressible, does modeling a flow with decreasing velocity / magnitude, say, as ...
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37 views
Understanding STRUCTURED_POINTS dataset in VTK
Based on my readings this is what I have gathered:
Dimensions: Number of data points in the x, y, z directions
Spacing: Space between points in each dimension
Origin: Location of origin
Cell data: ...
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2answers
58 views
Grid Independence Study
Is the change in time step necessary for the grid independent study? As the CFL is based on the relation between dt and dx.
In mesh independent study, only change should be mesh i.e, dx isn't it so?
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1answer
70 views
Extracting data from VTK simulations using C++
I have been given a few numerical simulations regarding fluid mixing and have been asked to extract a few parameters from them using C++. Altogether there are about 1000 VTK files per simulation, and ...
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1answer
252 views
DNS simulation in industry
I have this idea in my mind that Direct Numerical Simulation (DNS) is only used in research since the Reynolds numbers of industrial applications are too high and require computer horsepower we don't ...
3
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1answer
81 views
Is the Immersed Boundary (IB) method a Direct Numerical Simulation?
Is the Immersed Boundary (IB) method considered a Direct Numerical Simulation?
A DNS code is the most detailed type of simulation and the most accurate but computationally expensive, right?
What makes ...
2
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1answer
106 views
Reading VTK file into C++ for analysis
I apologize in advance if this post is at all ignorant or elementary, I am a pure mathematician who is newly getting into the world of scientific computing.
For my research, my advisor would like me ...
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0answers
55 views
Choosing good modelling method for solving Boltzmann equation
I'm writing a solver for Boltzmann Equations (BE) including a force term in rarefied plasma, for my PhD. The aim is to see if an instability occurs inside an electric streamer (theoretically it should,...
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1answer
67 views
Getting streamline for a lid driven cavity flow in openFoam/ParaView
I have installed openFoam on Debian GNU/Linux and learning from official user guide. I have a problem with generating streamlines. I am trying to generate streamlines as explained in the post ...
3
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0answers
88 views
Is the matrix exponential and the Jordan canonical form actually useful for solving differential equations?
All of my yearlong graduate-level Linear Algebra course notes from my professor—an algebraist/representation theorist—shows his love for the exponential map $e^A$ and the Jordan canonical form—and one ...
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26 views
Entropy of a spatially and temporally varying fluid system?
I'm trying to analyze the change in system thermodynamic entropy of an ideal gas system.
For reference, I'm analyzing solution techniques for solving the 1D compressible Euler equations:
$$
\partial_t ...
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71 views
Numerical scheme to calculate the normal mode of a set of hyperbolic PDEs?
I would like to solve the linearised, ideal, MHD equations, where the gas pressure is zero.
$$\frac{\partial u_x}{\partial t}=v_A^2(x,z)\left[\nabla_{||}b_x - \frac{\partial b_{||}}{\partial x}\right],...
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1answer
154 views
Without positive definiteness, does an iterative solver work?
Question
Does lacking positive definiteness of the matrix of coefficients in a system of equations, make using iterative solvers impractical?
Description
Using the finite volume method, I have ...
3
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1answer
184 views
Finite difference methods in cylindrical and spherical co-ordinate systems
I am quite familiar with finite difference schemes in cartesian coordinates. The key point here is that every point in the cartesian grid is treated equally as the spacing between consecutive points ...
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1answer
427 views
Can Julia be used to create a large-scale CFD software like OpenFOAM?
I have some experience with some programming languages like C/C++, Fortran, Python, etc but recently, I am getting interested in Julia.
But, I am wondering if Julia could be used to create some large ...
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30 views
How to solve this mass transport diffusion equation?
My goal is to numerically solve the mass transport diffusion equation of the form:
$$\ \nabla (D \nabla C) =0$$
$C$ is concentration and $D$ is the diffusivity constant.
I was solving this for a ...
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1answer
101 views
Why is my Cahn-Hilliard simulation separating out so finely?
I am trying to simulate the Cahn-Hilliard equation using Python, but the 2 fluids aren't separating into big blobs, as desired, under any conditions. I'm setting up (what I think is) an orthogonal ...
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72 views
Open Boundary Conditions for Solving Navier Stokes in moving ALE domain
I'm trying to solve the problem of a body freely rising in a fluid due to gravity, the density of the body is just slightly less than that of the fluid.
The body is rigid, so I have to solve a ...
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0answers
24 views
Convection equation expanded in Legendre polynomials
In some areas of physics, for example radiation transport in plasmas, it is beneficial to re-express the convection equation in terms of a Legendre polynomial expansion. This results in a set of ...
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81 views
Import stl file into Gmsh and create a 3d mesh for CFD
I am trying to develop a case study for teaching CFD. To this end, I am using OpenVSP, Gmsh and SU2.
I have created a wing in OpenVSP, exported the CFD mesh as .stl file.
Opened the .stl file in Gmsh ...
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8answers
6k views
What programming language should I choose and why?
I am a mechanical engineer, intermediated/advanced level in MATLAB and MATHEMATICA, and beginner in Python. I intend to get a PhD in aeroelasticity (FEM + CFD) and coding my own program.
I intend to ...
2
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0answers
45 views
How does the “Stable Fluids” algorithm by Jos Stam relate to the SIMPLE and PISO algorithms?
The "Stable Fluids" paper (*) by Jos Stam starts by acknowledging that "Our method cannot be found in the computational fluids literature, since it is custom made for computer graphics applications. ...
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73 views
Numerical flow visualization in 2D for a moving boundary,
I have a rigid body that moves according to a set of governing ODEs, and I'd like to numerically visualize the vortices that are shed by this object. How could I proceed?
I've been reading up on the ...
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1answer
51 views
Numerical simulation of water layer over terrain
I'm looking for a grid-based numerical method that allows simulating water over a grid-based terrain, presumably something like shallow water equations. I have a square grid of terrain elevation ...
2
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3answers
187 views
How is central difference scheme second-order accurate?
In an arbitrarily unstructured mesh, shown in the figure below, in the context of finite volume method, I want to obtain an approximation of $\phi_f$, where $N$ and $P$ are cell centers of adjacent ...
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34 views
First-principles benchmark of CFD solver
A decade ago I saw a ~20 coupled state + random number problem that was used as a benchmark for meteorological CFD tools. It had a fractal dimension (of the chaotic attractor) around 2.3, I think. I ...
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0answers
34 views
Decoupling of the fluid viscosity and incompressibility constraint in a partitioned scheme
I am new here and this is my very first question: I hope I respect all the criteria and rules.
I am just getting started with partitioned approaches for solving FSI problems. I am interested in a ...
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0answers
55 views
Implementing Level Sets
I am trying to implement a basic level set program for fluid simulation in C++ and visualize it using SFML, so far I have the following simple program for propagating a circular curve:
https://gist....
2
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1answer
200 views
Order of Accuracy Measurements on 1D Advection Methods
I am trying to learn about basics of computational fluid dynamics, at the moment on the simple example of linear advection in 1D.
I am am currently testing the theoretical predictions of the order of ...
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0answers
39 views
Why it doesn't matter to use velocity gradient or shear rate tensor to calculate wall shear stress?
The wall shear stress is defined based on Matyka et. al. eq. A.4:
$$\vec{\tau} = 2 \mu (\mathbf{S} \cdot \vec{n} - (\vec{n} \cdot \mathbf{S} \cdot \vec{n})\vec{n})$$
Where $\mathbf{S} = \frac{1}{2} (...
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2answers
198 views
Why lattice Boltzmann despite its huge number of mesh points still has worse accuracy in comparison to FEM for calculating wall shear stress?
I'm just doing a very simple experiment. I'm calculating wall shear stress based on Poiseuille flow for a pipe by using lattice Boltzmann method (LBM) and FEM to compare their values with the ...
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1answer
83 views
Filtering out outliers in a vector field [closed]
I have a vector field that represents a incompressible fluid flow (ie. divergence-free, ideally) that contains a certain percentage of vectors that are completely incorrect, due to the procedure used ...
2
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0answers
65 views
Defining appropriate test function spaces for the finite element solution of Euler's fluid equations
I have the following coupled equations for the conservation of mass and momentum of a compressible fluid :
\begin{equation}
\rho_t + (\rho u)_z = 0,
\end{equation}
$$
(\rho u)_t + (\rho u^2)_z + \...
2
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0answers
130 views
Velocity self-advection stability problem with QUICK upwind scheme
I'm trying to implement a finite-difference fluid solver (incompressible, inviscous) using the QUICK scheme for advection. Mostly I've been following the Norris thesis chapter 2 (https://ses.library....
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1answer
298 views
Application of Poiseuille equation
I'd like to know whether the Hagen-Poiseuille equation can be used to solve for the velocity of fluid when the Reynolds number (Re) is less than 1.
From textbooks, I understand that the Hagen-...
