Questions tagged [fluid-dynamics]
The study of the properties of fluids and gases in motion
524
questions
8
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2answers
8k 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
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:
$$...
0
votes
0answers
13 views
How to define "Characteristic length in Ansys CFX Power Syntax script?
I am trying to evaluate Reynolds number in Ansys CFX 2020 R2 using CCL/Perl script on different domains in a radial turbine. But for that, I need to define the Characteristic length. So I define the ...
0
votes
1answer
66 views
Smoothed Particle Hydrodynamics: Weird clustering of particles. Is that normal?
I implemented a rather simple SPH simulation using a cubic-spline-kernel and a simple non-iterative pressure solver as described in this PDF in equation 9. I followed algorithm 1 of that paper (...
3
votes
1answer
56 views
How does the diffusion of a finite volume method with a WENO scheme compare with that of spectral methods?
I know that, in general, finite volume (FV) methods are more (numerically) diffusive than spectral methods. However, I can't find any information on how the advection scheme changes that.
For example, ...
4
votes
1answer
151 views
What's a time centered Riemann problem?
I am trying to understand the meshless methods as described in https://arxiv.org/pdf/1409.7395.pdf. I'm having trouble understanding the following step: (Page 7, just after equation 17)
Now, rather ...
3
votes
1answer
449 views
1-D turbulent energy spectra in homogeneous direction (non-isotropic)
I am trying to compute the one-dimensional energy spectra for my channel-flow simulation. I have already written a post-processing script to achieve this; however, I need to validate my code before ...
6
votes
1answer
551 views
Finite volume a posteriori error estimation
I'm wondering what alternatives there are to a grid convergence study to judge solution accuracy for a given grid resolution when doing steady-state RANS simulations on an automatically generated ...
0
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0answers
44 views
Unsteady Stokes equations in ALE framework
I'm trying to solve Unsteady Stokes equations on a moving domain, using an ALE formulation, that is
$$\frac{\partial \mathbf{u}}{\partial t} - \mathbf{w}\cdot \nabla\mathbf{u} = \nu\Delta\mathbf{u} - \...
0
votes
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
votes
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 ...
0
votes
0answers
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 ...
0
votes
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:
$$\epsilon \...
1
vote
2answers
181 views
Finite Volume Method flux integration
I was reading through this document about FVM. I understood all up to the point where we have on page 15 the following
$$(\bar u^{n+1}_i - \bar u^n_i) \Delta x + \int^{t^{n+1}}_{t^n} f(u(x_{i+1/2},t))...
3
votes
0answers
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}(\...
0
votes
0answers
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 ...
3
votes
0answers
277 views
Simulation of surface tension-dominated interfaces
I want to simulate the shape of the free surface in a small fuel tank in microgravity, which is very slowly being emptied.
The tank is not symmetric, the geometry is given by CAD (e.g. step file).
...
5
votes
6answers
3k views
What is the difference between MATLAB and FORTRAN?
In our university some Ph.D students for computational methods prefer FORTRAN over MATLAB. I can't understand why? What is the difference between them when are used in computational methods like ...
3
votes
1answer
410 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 ...
0
votes
1answer
103 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 ...
4
votes
4answers
3k views
How to deal with nonlinear term in Navier Stokes equations (finite element code)
I am trying to solve the Navier Stokes equations using the finite element method. I plan on using the pressure correction method to deal with the pressure and an implicit time stepping scheme for ...
0
votes
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?
1
vote
1answer
77 views
How to remove sigularities from 3D Vortex Lattice Method
I'm trying to solve aerodynamics of whole aircraft by vortex lattice method (or deeper here). The problem is that sometimes trailing vortex filaments from Horse Shoe Vortexes of main wing hits panels ...
1
vote
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 ...
1
vote
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 ...
6
votes
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 ...
2
votes
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 ...
1
vote
0answers
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 ...
3
votes
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 ...
1
vote
0answers
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 ...
1
vote
0answers
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 ...
1
vote
0answers
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: ...
0
votes
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 ...
1
vote
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 ...
2
votes
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 ...
3
votes
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,...
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
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 ...
1
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0answers
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 ...
0
votes
0answers
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.
$$\frac{\partial u_x}{\partial t}=v_A^2(x,z)\left[\nabla_{||}b_x - \frac{\partial b_{||}}{\partial x}\right],...
1
vote
1answer
101 views
Why is my Cahn-Hilliard simulation separating out so finely?
I am trying to simulate the Cahn-Hilliard equation using Python, but the 2 fluids aren't separating into big blobs, as desired, under any conditions. I'm setting up (what I think is) an orthogonal ...
4
votes
1answer
154 views
Without positive definiteness, does an iterative solver work?
Question
Does lacking positive definiteness of the matrix of coefficients in a system of equations, make using iterative solvers impractical?
Description
Using the finite volume method, I have ...
1
vote
1answer
427 views
Can Julia be used to create a large-scale CFD software like OpenFOAM?
I have some experience with some programming languages like C/C++, Fortran, Python, etc but recently, I am getting interested in Julia.
But, I am wondering if Julia could be used to create some large ...
3
votes
1answer
183 views
Finite difference methods in cylindrical and spherical co-ordinate systems
I am quite familiar with finite difference schemes in cartesian coordinates. The key point here is that every point in the cartesian grid is treated equally as the spacing between consecutive points ...
0
votes
0answers
30 views
How to solve this mass transport diffusion equation?
My goal is to numerically solve the mass transport diffusion equation of the form:
$$\ \nabla (D \nabla C) =0$$
$C$ is concentration and $D$ is the diffusivity constant.
I was solving this for a ...
0
votes
0answers
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 ...
11
votes
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 ...
1
vote
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 ...
0
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0answers
80 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 ...
2
votes
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. ...
