Questions tagged [fluid-dynamics]
The study of the properties of fluids and gases in motion
506
questions
1
vote
1answer
59 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 ...
6
votes
1answer
151 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 ...
0
votes
0answers
36 views
Running VTK simulation on Visit
I apologize if any of these questions are elementary, I have just begun research involving scientific computing and am still getting ahold of the basics.
My research advisor has given me a few ...
3
votes
0answers
46 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,...
0
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1answer
28 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
votes
1answer
378 views
General case Kutta condition
I'm working on a 2D inviscid fluid simulation using a "panel method", with Potential being used to enforce the no-through boundary condition. I'm trying to incorporate the Kutta condition, which says ...
8
votes
2answers
7k views
Use of machine learning in computational fluid dynamics
Background:
I have only built one working numeric solution to 2d Navier-Stokes, for a course. It was a solution for lid-driven cavity flow. The course, however, discussed a number of schemas for ...
1
vote
2answers
7k views
Fluent time step size (transient implicit solver)
I am studying fluid-structure interaction across integrated circuits packaging by using Ansys Fluent (I'm a beginner). Currently I had an issue about the time step size.
When I set time step size of ...
3
votes
0answers
82 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 ...
1
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0answers
25 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:
$$
\partial_t ...
0
votes
0answers
67 views
Numerical scheme to calculate the normal mode of a set of hyperbolic PDEs?
I would like to solve the linearised, ideal, MHD equations, where the gas pressure is zero.
$$\frac{\partial u_x}{\partial t}=v_A^2(x,z)\left[\nabla_{||}b_x - \frac{\partial b_{||}}{\partial x}\right],...
1
vote
1answer
96 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 ...
4
votes
1answer
138 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 ...
0
votes
1answer
175 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 ...
3
votes
1answer
81 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 ...
0
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0answers
26 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 ...
0
votes
0answers
64 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 ...
11
votes
8answers
5k 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 ...
1
vote
0answers
20 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 ...
0
votes
0answers
27 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 ...
0
votes
1answer
104 views
What's a time centered Riemann problem?
I am trying to understand the meshless methods as described in https://arxiv.org/pdf/1409.7395.pdf. I'm having trouble understanding the following step: (Page 7, just after equation 17)
Now, rather ...
2
votes
0answers
38 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. ...
0
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0answers
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 ...
2
votes
1answer
375 views
1-D turbulent energy spectra in homogeneous direction (non-isotropic)
I am trying to compute the one-dimensional energy spectra for my channel-flow simulation. I have already written a post-processing script to achieve this; however, I need to validate my code before ...
5
votes
1answer
510 views
Finite volume a posteriori error estimation
I'm wondering what alternatives there are to a grid convergence study to judge solution accuracy for a given grid resolution when doing steady-state RANS simulations on an automatically generated ...
2
votes
3answers
187 views
Value of density when there are no or very few neighbours in SPH simulation
I am trying to implement SPH using the directions shown in this paper.
The density needs to be updated using the formula
$$
\rho(\mathbf{x}_i)=\sum_j m_jW_\text{default}(\mathbf{x}_i-\mathbf{x}_j,h)
...
0
votes
1answer
48 views
Numerical simulation of water layer over terrain
I'm looking for a grid-based numerical method that allows simulating water over a grid-based terrain, presumably something like shallow water equations. I have a square grid of terrain elevation ...
2
votes
3answers
153 views
How is central difference scheme second-order accurate?
In an arbitrarily unstructured mesh, shown in the figure below, in the context of finite volume method, I want to obtain an approximation of $\phi_f$, where $N$ and $P$ are cell centers of adjacent ...
6
votes
4answers
235 views
In Matlab, how can I be consistent with units?
I am modeling some aerodynamics equations and am using meters / centimeters, kilograms, and seconds.
I've heard that, "matlab doesn't know units". So, how can I make sure that it does? Just ...
9
votes
2answers
599 views
Time discretization of the variational formulation of the Navier-Stokes equation
I've asked this question on mathoverflow too.
Let
$T>0$
$I:=(0,T]$
$d\in\mathbb N$
$\Lambda\subseteq\mathbb R^d$ be nonempty and open, $$\mathcal V:=\left\{\phi\in C_c^\infty(\Lambda,\mathbb R^d):...
2
votes
2answers
2k views
openFOAM RAS to LES
I successfully ran a RANS simulation of industrial fan with all wall BCs,
pimpleDyMFoam solver and spallart-almaras RAS model.
I got results upto some time, I would like to continue that simulation ...
1
vote
0answers
32 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 ...
1
vote
1answer
70 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
votes
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 ...
2
votes
4answers
683 views
Finite volume piecewise linear 2D advection develops instability
I'm developing a finite volume solver for the simple twodimensional advection equation with constant velocities $u, v$ and constant mesh spaces $\Delta x$:
$$ \frac{\partial \rho}{\partial t} + u \...
2
votes
1answer
189 views
Order of Accuracy Measurements on 1D Advection Methods
I am trying to learn about basics of computational fluid dynamics, at the moment on the simple example of linear advection in 1D.
I am am currently testing the theoretical predictions of the order of ...
1
vote
0answers
55 views
Implementing Level Sets
I am trying to implement a basic level set program for fluid simulation in C++ and visualize it using SFML, so far I have the following simple program for propagating a circular curve:
https://gist....
0
votes
0answers
32 views
Why it doesn't matter to use velocity gradient or shear rate tensor to calculate wall shear stress?
The wall shear stress is defined based on Matyka et. al. eq. A.4:
$$\vec{\tau} = 2 \mu (\mathbf{S} \cdot \vec{n} - (\vec{n} \cdot \mathbf{S} \cdot \vec{n})\vec{n})$$
Where $\mathbf{S} = \frac{1}{2} (...
2
votes
0answers
61 views
Defining appropriate test function spaces for the finite element solution of Euler's fluid equations
I have the following coupled equations for the conservation of mass and momentum of a compressible fluid :
\begin{equation}
\rho_t + (\rho u)_z = 0,
\end{equation}
$$
(\rho u)_t + (\rho u^2)_z + \...
1
vote
1answer
171 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-...
2
votes
0answers
126 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....
2
votes
1answer
163 views
mesh dependence of numerical adjoint solution
I am solving the steady, two-dimensional adjoint Euler equations, $$A_x^T \partial_x \Psi + A_y^T \partial_y \Psi = 0$$, where $A_x = \partial F_x/\partial U$ and $A_y= \partial F_y/\partial U$ are ...
2
votes
1answer
87 views
What is the correct way to calculate deviatoric stress tensor in lattice Boltzmann method?
Due to my previous question, where I asked about flux calculation in lattice Boltzmann (LB) method here, I have more or less same question for deviatoric stress tensor calculation due to pseudo-...
0
votes
2answers
331 views
Boundary conditions for streamlines in enclosed flow
I am trying to solve Lid driven square cavity flow problem of Stokes equation using finite element method. I have boundary conditions for velocity as zeros on every boundary but u=1 on top boundary. ...
-1
votes
1answer
248 views
Finite Element - Flux Calculation
I am solving an advection-diffusion equation using the FEM and am having trouble calculating my fluxes.
I start with the equation,
$$\frac{\partial n}{\partial t} = \frac{\partial j_{n}}{\partial x}\...
1
vote
0answers
157 views
Immersed boundary method in FEniCS?
I have looked at the FEniCS tutorials and documentation but I cannot find any mention to the possibility of implementing an immersed boundary method (IBM) for fluid dynamics.
In particular, I want ...
4
votes
1answer
96 views
Modelling flow through pipe networks
I'm trying to educate myself on modelling solute flows through pipe networks.
This is a follow up of my previous post here
$$\frac{\partial C}{\partial t} = - v\frac{\partial C}{\partial x}$$
While ...
1
vote
0answers
36 views
How to avoid density getting “deleted” in two way rigid body coupling with LBM CFD?
I've been reading this paper recently, which talks about using Lattice Boltzmann methods and two way coupling. Specifically, it outlines fluid solid coupling, and solid fluid coupling, and how simply ...
2
votes
1answer
99 views
Does mass balance hold in convective diffusion
I'm trying to understand how convection-diffusion equations are solved in pipe flow modules available in CFD solvers.
$$
\frac{\partial C}{\partial t} + \nabla \cdot (\mathbf{v} C) = \nabla \cdot (D \...
1
vote
0answers
48 views
Question regarding 1D implementation of the DG method
I'm pretty new to the DG method and have been writing a 1D code to help me understand the coding aspect. With respect a reference, I've been following these notes https://www3.nd.edu/~zxu2/...
