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Questions tagged [fluid-dynamics]

The study of the properties of fluids and gases in motion

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1answer
57 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
52 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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44 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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29 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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47 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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71 views

Can this vector field be made divergence-free?

Consider an oscillating shear flow given by the velocity vector field \begin{align} &v_x = e^{\cos(x(t))} y\\ &v_y = 0 \end{align} where $x$ and $y$ are in the usual lab-frame coordinate ...
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1answer
82 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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30 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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19 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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62 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
246 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 ...
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1answer
77 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 ...
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1answer
63 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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51 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
37 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 ...
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85 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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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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69 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
143 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 ...
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1answer
91 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
231 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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28 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
97 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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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 ...
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21 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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38 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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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 ...
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41 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
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 ...
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3answers
160 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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33 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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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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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....
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1answer
192 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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35 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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190 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
71 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 ...
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0answers
63 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 + \...
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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....
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1answer
189 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-...
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1answer
164 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 ...
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185 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 ...
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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 ...
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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 ...
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1answer
104 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 \...
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1answer
97 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-...
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51 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/...
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1answer
153 views

What is eighth order central difference?

The origin of the question can be found here. I know the details about forward, backward and central differences. If $u$ is the variable, does eight order means it approximates the $u_{xx}$ using $u$...
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103 views

CFD and finite volume method: Dirichlet boundary conditions for the Euler equations

Please point me to an answer if one already exists, but after some searching, I still can't find the answer to what seems like a very simple question. There are plenty of references out there for ...

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