Questions tagged [fluid-dynamics]
The study of the properties of fluids and gases in motion
487
questions
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0answers
29 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 ...
2
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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 ...
1
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0answers
54 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
181 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 ...
0
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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} (...
6
votes
1answer
128 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 ...
1
vote
1answer
58 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
59 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}
$$
(\rho u)_t + (\rho u^2)_z + \...
2
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0answers
124 views
Velocity self-advection stability problem with QUICK upwind scheme
I'm trying to implement a finite-difference fluid solver (incompressible, inviscous) using the QUICK scheme for advection. Mostly I've been following the Norris thesis chapter 2 (https://ses.library....
1
vote
1answer
118 views
Application of Poiseuille equation
I'd like to know whether the Hagen-Poiseuille equation can be used to solve for the velocity of fluid when the Reynolds number (Re) is less than 1.
From textbooks, I understand that the Hagen-...
2
votes
1answer
152 views
mesh dependence of numerical adjoint solution
I am solving the steady, two-dimensional adjoint Euler equations, $$A_x^T \partial_x \Psi + A_y^T \partial_y \Psi = 0$$, where $A_x = \partial F_x/\partial U$ and $A_y= \partial F_y/\partial U$ are ...
1
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0answers
116 views
Immersed boundary method in FEniCS?
I have looked at the FEniCS tutorials and documentation but I cannot find any mention to the possibility of implementing an immersed boundary method (IBM) for fluid dynamics.
In particular, I want ...
1
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0answers
36 views
How to avoid density getting “deleted” in two way rigid body coupling with LBM CFD?
I've been reading this paper recently, which talks about using Lattice Boltzmann methods and two way coupling. Specifically, it outlines fluid solid coupling, and solid fluid coupling, and how simply ...
4
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1answer
90 views
Modelling flow through pipe networks
I'm trying to educate myself on modelling solute flows through pipe networks.
This is a follow up of my previous post here
$$\frac{\partial C}{\partial t} = - v\frac{\partial C}{\partial x}$$
While ...
2
votes
1answer
90 views
Does mass balance hold in convective diffusion
I'm trying to understand how convection-diffusion equations are solved in pipe flow modules available in CFD solvers.
$$
\frac{\partial C}{\partial t} + \nabla \cdot (\mathbf{v} C) = \nabla \cdot (D \...
2
votes
1answer
72 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-...
1
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0answers
42 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/...
2
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1answer
102 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 $u$...
2
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0answers
71 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 ...
2
votes
1answer
45 views
Numerical integration of the dataset of a function
The energy equation for a spherically symmetric system is given by
$$\mathscr{E}=\frac{v^2(r)}{2}+\frac{c_s^2(r)}{\gamma-1}+\phi(r)$$
where $\mathscr{E}$ is the total energy, $v$ is the velocity of ...
2
votes
1answer
133 views
Physical interpretation of divergence theorem
In a diverging pipe section like the following,
the pipe of radius $r$ splits into two pipes of radius $r/2$.
Consider a solute transported by convection from node 1.
$$\frac{\partial C}{\partial t}...
0
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0answers
43 views
Simulating flow in a branched pipe
I am trying to simulate 1D advection and convection of a solute in the following blood vessel segment.
I would like to know if this system can be simulated in COMSOL or MATLAB.
I have used pdepe ...
11
votes
1answer
182 views
Implications of thermodynamic inconsistency in CFD calculations
During my PhD work, I had to use tabulated values of thermodynamic properties of gases in some Computational Fluid Dynamics (CFD in short) simulations.
My tables are discretized in temperature and ...
2
votes
0answers
68 views
What is appropriate boundary condition for Poisson pressure equation?
I'm doing CFD simulations in unstructured grids. Well, it's a bit different from conventional unstructured grids that are used mainly in FEM or FVM as tetrahedral meshes. Mine is a voxelized mesh of ...
0
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0answers
44 views
What is Voronoi particle tracking?
I've been trying to track this down, but google is giving paywall papers that don't appear to be directly related to computational science, or simply don't explain the source algorithm.
There's an ...
1
vote
1answer
97 views
Are there any commercial CFD codes that implement a Discontinuous Galerkin scheme?
I've been reading about the Discontinuous Galerkin discretization scheme and it's application to CFD for fluid flow. It seems to be a promising method for simulating turbulent flows, by using higher-...
-1
votes
1answer
46 views
Smoothed Particle Hydrodynamics: Weird clustering of particles. Is that normal?
I implemented a rather simple SPH simulation using a cubic-spline-kernel and a simple non-iterative pressure solver as described in this PDF in equation 9. I followed algorithm 1 of that paper (...
0
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0answers
25 views
How to ensure values stay within range?
e.g. water in a height map
Choosing a range with a margin of error for typical model behaviour seems practical.
Could we instead (1). predict maximum values; or (2). have a natural maximum?
1. ...
1
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2answers
227 views
Lattice Boltzmann methods vs Navier stokes/ other eulerian methods for *water* simulation
Note, there is already a question here, however the answers don't answer the original question, let alone specific considerations when dealing with nearly in-compressible fluids (water). Another ...
1
vote
1answer
55 views
Should the derivative of an array be calculated array by array or element by element in CFD codes?
I am making my own finite difference computational magnetohydrodynamic code in Fortran 90. Looking at other codes they appear to calculate for example their $x$-derivatives, bb of their variables, e.g....
2
votes
0answers
22 views
Riemann solvers for metastable phases
Most Riemann solvers I've come across can solve the Riemann problem only under certain conditions such as convexity of the equation of state. But what happens if the fluid enters a metastable state or ...
1
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1answer
105 views
How to compute turbulent energy cascade
I need to compute the kinetic energy cascade using a finite volume solution in an equally spaced grid. I wonder if it is more correct to first compute the kinetic energy in the space (or time) domain, ...
2
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1answer
58 views
Confusion about Zabusky and Kruskal's stepper for the KdV equation
In Zabusky and Kruskal's paper about solitons, they derive the following update for the Korteweg de Vries equation (their footnote 6):
\begin{align*}
u_{i}^{j+1} = u_{i}^{j-1} - \frac{1}{3} \frac{k}{...
2
votes
0answers
73 views
Understanding MP-PIC implementation in OpenFOAM
The multiphase particle-in-cell (MP-PIC) method is characterized by mapping particle properties from the Lagrangian coordinates to the Eulerian grid. However, the implementation of this method in ...
1
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0answers
27 views
Finite difference/element method : time step and spatial resolution close to a finite singularity
I'm using the finite element method (FEM), but my question is quite a global question. It's related to this question but it is not the same.
Let's assume we have this equation : $$\partial_t c - u\...
3
votes
0answers
85 views
numerical instabilities in Fluid Dynamics, Finite Element Method
I'm looking for references to understand where the numerical instabilities come from in hydrodynamics in general, and notably when the Péclet number: $Pe>1$. I'm using the finite element method.
...
1
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0answers
23 views
Solution errors when refining a static grid: Continuous vs. step-wise refinement
Let's assume I am working on a 2-D domain on $R^2$, with my coordinates $x \in[-1,1]$, $y \in[-1,1]$ and I want to solve a popular CFD problem, like the shallow water system or the Euler system.
At $x=...
0
votes
1answer
90 views
Well-posedness of Navier-Stokes equation
Before running a simulation for turbulence (e.g Rayleigh-Benard Convection), how do we check for well-posedness of Navier-Stokes with other equations for a given boundary condition??
Can someone ...
7
votes
1answer
220 views
Do computational scientists typically also become domain experts?
Let's say I'm interested in fluid dynamics, specifically in fluid-structure interactions -- and I want to get into modeling, simulations and experiments. I am a mathematics student by training, ...
3
votes
1answer
349 views
Limitations with dynamical systems vs. PDEs?
As a computational scientist, are there limitations to studying dynamical systems — systems of odes in which each state variable evolves with time — compared to studying PDEs?
For instance, it seems ...
2
votes
1answer
124 views
Why don't we call the simulation “a model for …”?
When a set of model equations, e.g. some coupled differential equations, has solutions that behave in ways similar to real-life phenomena such as blood flow in the heart, a wave movement, or a plate ...
1
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0answers
36 views
Pseudospectral method for Rayleigh-Benard with constant temperature gradient
$$
\nabla\cdot \mathbf{u} = 0 \\
\frac{\partial \mathbf{u}}{\partial t}+\left(\mathbf{u}\cdot \nabla\right)\mathbf{u} = -\nabla p+\nu\nabla^2\mathbf{u}+\alpha g\theta\mathbf{e}_z\\
\frac{\partial\...
2
votes
0answers
43 views
Verification on pressure predictor method for CFD code
I have developed a python code for a lid-drive cavity model. However, my results are not converging. The algorithm of my code looks like this:
Euler Momentum Equation looks like this:
$$\frac{u^{n+1}...
1
vote
1answer
62 views
What is the exponential trick to include laplacian term in Rayleigh-Bernard simulation
I have come across a Rayleigh-Bernard simulation code which doesn't have the laplacian term but an integrating factor (in the exponential form) containing viscosity and diffusivity. I found out that ...
2
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0answers
59 views
Solving a simple Shallow Water model
Hi. I have a question at Mathematics and they suggested post here, once it's not common. I transcript as following. Many thanks
I need to solve with basic methods this simple Shallow Water Model:
$...
0
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0answers
36 views
Is semi-Lagrangian 1D advection identical to upwind Euler, when $|u|<\Delta x/\Delta t$?
This looked true to me, I worked through the algebra for 1D and confirmed it. But no one seems to mention it, so maybe I'm missing something...
I'm using Bridson's SIGGRAPH 2007 course notes. [5MB ...
6
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0answers
123 views
What are some good debugging habits for numerical simulation?
I'm currently writing a lid drive cavity CFD code on python. Currently, my code has some issues (values jumping bear b.c). I was wondering what are some good habits in debugging numerical codes. ...
0
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0answers
8 views
Universal formulation of adiabatic equations of state in compresible finite-volume simultions
I code some finite element solver which should work for broad variety of materials (i.e. gas, liquid, solid, plasma) and large span of compressions resp. densities. I want to simulate things like ...
1
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1answer
236 views
What will be the impact of quantum computing on existing numerical techniques (e.g. CFD)?
Quantum computing seems to be a very active and promising development area in computer science. However, I am curious as to what impact (if any) quantum computing will have on existing classical ...
1
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0answers
51 views
WENO scheme on curvilinear coordinates
I've been developing a curvilinear FVM code. So far I've implemented the PPM scheme and am looking into adding WENO schemes. So far I've been discretizing the grid metrics using a second-order central....
