Questions tagged [fluid-dynamics]
The study of the properties of fluids and gases in motion
473
questions
2
votes
1answer
64 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.
$$
\frac{\partial C}{\partial t} + \nabla \cdot (\mathbf{v} C) = \nabla \cdot (D \...
2
votes
0answers
14 views
What is the correct way to calculate deviatoric stress tensor in lattice Boltzmann method?
Due to my previous question, where I asked about flux calculation in lattice Boltzmann (LB) method here, I have more or less same question for deviatoric stress tensor calculation due to pseudo-...
1
vote
0answers
36 views
Question regarding 1D implementation of the DG method
I'm pretty new to the DG method and have been writing a 1D code to help me understand the coding aspect. With respect a reference, I've been following these notes https://www3.nd.edu/~zxu2/...
2
votes
1answer
83 views
What is eighth order central difference?
The origin of the question can be found here.
I know the details about forward, backward and central differences.
If $u$ is the variable, does eight order means it approximates the $u_{xx}$ using $u$...
2
votes
0answers
42 views
CFD and finite volume method: Dirichlet boundary conditions for the Euler equations
Please point me to an answer if one already exists, but after some searching, I still can't find the answer to what seems like a very simple question. There are plenty of references out there for ...
2
votes
1answer
42 views
Numerical integration of the dataset of a function
The energy equation for a spherically symmetric system is given by
$$\mathscr{E}=\frac{v^2(r)}{2}+\frac{c_s^2(r)}{\gamma-1}+\phi(r)$$
where $\mathscr{E}$ is the total energy, $v$ is the velocity of ...
2
votes
1answer
112 views
Physical interpretation of divergence theorem
In a diverging pipe section like the following,
the pipe of radius $r$ splits into two pipes of radius $r/2$.
Consider a solute transported by convection from node 1.
$$\frac{\partial C}{\partial t}...
0
votes
0answers
39 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
178 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
56 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
votes
0answers
26 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
83 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
42 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
votes
0answers
24 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
vote
2answers
160 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
51 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
vote
1answer
93 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
votes
1answer
46 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
47 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
vote
0answers
25 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 : $$\partial_t c - u\...
3
votes
0answers
81 views
numerical instabilities in Fluid Dynamics, Finite Element Method
I'm looking for references to understand where the numerical instabilities come from in hydrodynamics in general, and notably when the Péclet number: $Pe>1$. I'm using the finite element method.
...
1
vote
0answers
23 views
Solution errors when refining a static grid: Continuous vs. step-wise refinement
Let's assume I am working on a 2-D domain on $R^2$, with my coordinates $x \in[-1,1]$, $y \in[-1,1]$ and I want to solve a popular CFD problem, like the shallow water system or the Euler system.
At $x=...
0
votes
1answer
85 views
Well-posedness of Navier-Stokes equation
Before running a simulation for turbulence (e.g Rayleigh-Benard Convection), how do we check for well-posedness of Navier-Stokes with other equations for a given boundary condition??
Can someone ...
7
votes
1answer
209 views
Do computational scientists typically also become domain experts?
Let's say I'm interested in fluid dynamics, specifically in fluid-structure interactions -- and I want to get into modeling, simulations and experiments. I am a mathematics student by training, ...
3
votes
1answer
338 views
Limitations with dynamical systems vs. PDEs?
As a computational scientist, are there limitations to studying dynamical systems — systems of odes in which each state variable evolves with time — compared to studying PDEs?
For instance, it seems ...
2
votes
1answer
124 views
Why don't we call the simulation “a model for …”?
When a set of model equations, e.g. some coupled differential equations, has solutions that behave in ways similar to real-life phenomena such as blood flow in the heart, a wave movement, or a plate ...
1
vote
0answers
34 views
Pseudospectral method for Rayleigh-Benard with constant temperature gradient
$$
\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
42 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
59 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
votes
0answers
56 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
votes
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
votes
0answers
120 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
votes
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
vote
1answer
148 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
vote
0answers
47 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....
1
vote
0answers
61 views
OpenFOAM and CFDEM on GPU
I have a simulation project written for OpenFOAM and CFDEM and would like to find an alternative to run it on GPU since raising the number of cores already provided a promising speed up and ...
1
vote
1answer
57 views
How to get started with numerically solving a Stochastic Navier Stokes equation
I originally posted the question on the math stackexchange, and was told I should try here.
I’m researching Stochastic PDE, in particular the Navier Stokes Equation, and would like to estimate the ...
1
vote
0answers
31 views
Dealing with spurious oscillations in particle tracking methods
I work on modelling high intensity discharge xenon-filled lamps.
The model governing the discharge is quite complex and sadly includes fluid dynamics.
After some time, I managed to implement a finite-...
0
votes
0answers
11 views
Compute cell values and isosurface constant from density values of particles
I am trying to reconstruct the surface for a fluid simulation based on a list of particles using the Marching Cubes algorithm. From different resources, such as http://paulbourke.net/geometry/...
2
votes
2answers
153 views
Automatic timestep adjustment in a CFD solver
I have developed my own 3D Finite Volume Navier-Stokes solver based on projection method for nonuniform grid. I am looking to incorporate automatic timestep adjustment at each time step based on ...
0
votes
1answer
67 views
Is “Gradient Computation” in Finite Volume Discretization Really 2nd order accurate?
Based on this, pp 245, we go through these steps to discretize a gradient statement, namely $\nabla\phi$:
1- Gauss theorem reads,
$$
\int_V\nabla \phi dV = \oint_{\partial V}\phi dS
$$
2- Integral ...
0
votes
0answers
84 views
Double mach reflection at a inclined wedge
I am running into a strange problem when solving the 2D compressible Euler equations on a inclined wedge. To elaborate, my top boundary condition seems to emitting some type of instability. I have ...
-1
votes
1answer
56 views
Simulating magnetic particles in a field free point generated by two opposing magnets
This is probably a long shot with such a short time, but I've been trying to get theoretical data for a project I'm working on. The project involves using a very simplified version of magnetic ...
1
vote
0answers
42 views
Instability in Lattice Boltzmann Solver
I wrote a Lattice Boltzmann Solver in Rust a little while back using both the BGK approximation and the TRT (two relaxation time) method on the D2Q9 lattice. In both cases I run into major stability ...
0
votes
1answer
79 views
Imposition of Dirichlet BC for Fourier pseudospectral in this paper
I was trying to implement the algorithm from the paper "Adapting a Fourier pseudospectral method to Dirichlet boundary conditions for Rayleigh–Benard convection".
I am having a hard time to ...
0
votes
0answers
31 views
3D Tollmien-Schlichting Waves Imposed in a Channel Flow (Are Physics correct?, etc)
So I am trying to do some further tests on a 2nd-order code Incompressible Navier Stokes equations, by studying transition to turbulence in a Poiseuille flow. Specifically, I'm interested to see ...
0
votes
1answer
139 views
Finite Element - Flux Calculation
I am solving an advection-diffusion equation using the FEM and am having trouble calculating my fluxes.
I start with the equation,
$$\frac{\partial n}{\partial t} = \frac{\partial j_{n}}{\partial x}\...
6
votes
3answers
275 views
Why is the FVM traditionally used in CFD, and FEM in computational structures?
Most CFD codes use FVM. Most computational structures codes use FEM.
Why is the FEM not frequently used in CFD, and why is FVM not frequently used in FEM?
3
votes
2answers
118 views
Finite volume discretization of non-conservative linear hyperbolic equation
Problem. Consider the one-dimensional adjoint Euler equations for $(x,t) \in \Omega \times [0,T]$ with $\Omega \subset \mathbb{R}$ and $T > 0$
$$ \varphi_t + \Big(\frac{\mathrm{d}F}{\mathrm{d} U}(x)...
