Questions tagged [fluid-dynamics]
The study of the properties of fluids and gases in motion
552
questions
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0answers
55 views
Good non oscilliatory derivatives for an exsisting grid
I'm calculating the entropy production of a shockwave by utilizing the equations:
\begin{equation}
\sigma = J'_q\frac{\partial}{\partial x}\left(\frac{1}{T}\right) +\frac{1}{T}\frac{4\eta}{3}\left(\...
3
votes
1answer
57 views
Jos Stam's stable fluids — why is the timestep multiplied by the number of grid cells?
One more question about Jos Stam's GDC tutorial on stable fluids: in the advection step on page 8, the timestep for each dimension is implemented as dt * N, where <...
1
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1answer
37 views
Jos Stam's stable fluids — why multiply by the grid size when subtracting out the curl-free part?
Considering page 10 of Jos Stam's GDC tutorial on stable fluids, the function project() first obtains the divergence of the velocity field: ¹
...
0
votes
1answer
83 views
Simulating the behavior of falling peas
I hope this is the right place for my question.
I am a university student who designed a pea-pouring machine.Now I‘m really interested in simulating the behavior of those peas.
Basically, they are ...
2
votes
1answer
71 views
Discretizing the viscous component in 1 - D Navier stokes compressive flow
I've been working on modelling the NS equations in order to simulate shock waves. The equations are set up on the form:
\begin{equation}
\frac{\partial U}{\partial t} + \frac{\partial F(U)}{\partial x}...
1
vote
1answer
137 views
Non-reflective boundary condition
I'm currently solving incompressible Navier-Stokes system of equations with periodic flow and high viscosity.
Is there any outlet boundary types that avoids the reflection of flow from the outlet back ...
2
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0answers
57 views
Multigrid method: linear solver and modified residual
I am trying to better understand the FAS multigrid algorithm for Euler equation in FV discretization. The usage of the modified residual (the residual with forcing) inside the different cases:
...
2
votes
2answers
74 views
Filter size of large eddy simulation with Smagorinsky sub-grid stress model
I am trying to implement Large eddy simulation for solving air flow simulation with large Reynolds number using Smagorinsky sub-grid stress model. I read that the filter size should be calculated ...
1
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0answers
92 views
Weird "oscillatory" modes appearing in FEM simulations
I am using COMSOL to solve a mathematical model involving thermoelectric hydrodynamic (TEMHD) flow. I am running a very large parameter sweep and using the solutions obtained to make some plots. ...
5
votes
1answer
162 views
Write incompressible Navier Stokes as ODE in $(\mathbf{u},p)$
Consider the Navier stokes equation after the discretization with conforming finite elements with source term $f=0$. We have the algebraic structure of a saddle point problem:
$$M \dot{u} = f- Au -B^...
3
votes
1answer
159 views
Time discretization Navier Stokes equation
This question is a follow-up of this one.
The weak form of Navier Stokes equation is (assuming $v,q$ test functions for the velocity and the pressure, respectively)
$$(\frac{du}{dt},v)_{\Omega} + (\...
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0answers
50 views
How to numerically solve PDE that governs the free vortex wake model?
Crossposted at Math SE
I am reading a paper on the free vortex wake model for a helicopter rotor blade, which is described by the following PDE:
$$\frac{\partial \vec{r}}{\partial \psi} (\psi, \zeta) ...
0
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0answers
52 views
What is the rationale of second-order finite volume discretization?
When it comes to a second-order accurate finite volume discretization of Navier-Stokes equations, which one of the two following rationales is adopted?
1- Second-order accuracy is a direct consequence ...
1
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0answers
54 views
Why does including the pressure in this FVM for Stokes 2nd Problem lead to wrong solutions?
I'm trying to learn how to use finite volume methods and I want to solve a more general case of Stokes' second problem i.e. an infinite half-plane oscillating harmonically with no-slip boundary ...
3
votes
1answer
74 views
Why do we have to resort to Higher order schemes for solving the 1-D advection equation/ continuity equation?
\begin{equation}
\begin{aligned}
\frac{\partial N}{\partial t} &+ \frac{\partial J}{\partial r} = 0, \\
\frac{\partial N}{\partial t} &+ \frac{\partial }{\partial r}(N \upsilon ...
0
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0answers
35 views
Insert a boundary condition without removing periodicity assumption
I've to perform a multi-fluid internal flow simulation with a code which intrinsically assumes periodicity on a given direction (say z) on a cartesian grid. Nonetheless, the problem I'm trying to ...
-3
votes
1answer
79 views
How to avoid negative concentration from numerical solution using FDM scheme?
$\frac{\partial C}{\partial t} + u \frac{\partial C}{\partial x} + w \frac{\partial C}{\partial x} = D \left(\frac{\partial^2C}{\partial x^2}+\frac{\partial^2C}{\partial y^2}\right)-C \cdot \left(\...
0
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0answers
117 views
Tool that calculates total drag for a submarine
Can you please suggest a tool (software or online tool) that can analyze a submarine hull shape and give the total drag at a specific velocity.
A simplified tool would work too, where I select a ...
3
votes
0answers
81 views
Galerkin Least-Squares stabilization for FEM solution advection (hyperbolic) equations
I am playing with Galerkin Least-Squares stabilization to solve advection diffusion problem in the context of the finite element method. This works very well for steady-state advection-diffusion ...
5
votes
1answer
161 views
Projection method FVM poisson part, adding source term
The idea of the method is to decompose the Navier-Stokes equation into the solenoidal and irrotational parts.
$$\frac{\partial u}{\partial t}+u(\nabla \cdot u)=-\frac{1}{\rho}\nabla p+\nabla ^2 u$$
...
3
votes
2answers
116 views
Efficient schemes for solving the extended Saddle point problem
I am interested in knowing some efficient techniques for solving the following extended Saddle point problem.
\begin{align}
\begin{bmatrix}
A & B^T & C^T \\
B & 0 & 0 \\
C & ...
2
votes
1answer
112 views
What exactly are WENO schemes and where are they used?
I am currently trying to understand what WENO schemes are and most of the literature on web talks about cell-face reconstruction. What I am unable to understand is the origin of these discontinuities ...
1
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0answers
100 views
Lumped mass matrices for higher-order finite elements for CFD
Given that some of the mass lumping techniques, for example, row-sum lumping does not produce practically viable lumped mass matrices for all the element shapes, what are the techniques used for mass ...
2
votes
0answers
59 views
What is temporal order of accuracy of the PISO algorithm?
A few Computational Fluid Dynamics (CFD) codes implement the so called PISO (Pressure-Implicit with Splitting of Operators) algorithm for pressure-velocity coupling.
My concern is what is actual ...
0
votes
3answers
206 views
How to apply the boundary condition when global stiffness matrix is stored in csr format? [duplicate]
I am solving the poisson equation and I constructed the global stiffness matrix in compressed row storage format. Then I wrote the preconditioned conjugate gradient solver for solving the system of ...
-1
votes
1answer
105 views
Implementation of Source Panel Method as Described in Katz Plotkin Book
I am currently trying to implement Source Panel method as described in Katz and Plotkin in Low Speed Aerodynamics. I have successfully implemented two previous methods. However, I am fully blocked on ...
0
votes
1answer
106 views
Unstructured mesh preprocessing
For solving PDE with self written code it is needed to preprocess the data from mesh generators. I recently started shifting from cartesian grid to unstructured.
I finished reading up to FVM part of ...
0
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0answers
44 views
Boundary layer and high order finite element methods
When modeling boundary layers, it is typically required to have a highly refined mesh along the boundary. My question is if there has been success with high order finite element methods to overcome ...
2
votes
0answers
92 views
How are finite volume method boundary conditions implemented without using ghost-cells?
I'm currently trying to implement my own FVM code in cpp, but when I try to calculate the laplacian of a test function, given by \begin{align}\phi_0=\sin(2\pi x)\sin(2\pi y)\end{align}, I get ...
0
votes
1answer
141 views
L2 projection with bounds
In some problems we are currently working on, we are working with discontinuous functions that are defined on a finite element mesh and are established using Lagrangian particles. To obtain them on a ...
0
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0answers
50 views
Simulating a combustion process
I want to try simulation-(and not experimental)-driven approach to design custom fireplace fuel burners.
What software applications, libraries, code and model templates can I use to model and ...
0
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0answers
49 views
When to stop iterations in SOR solver for 3D Poisson equation
I'm writing a solver (in C) for 3D incompressible fluids, using the finite-differences method, and I'm finding a somewhat surprising behaviour: the solver provides "good-looking" solutions, ...
0
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0answers
28 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 ...
3
votes
1answer
68 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, ...
0
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0answers
46 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} - \...
1
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1answer
123 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:
$$...
3
votes
2answers
154 views
Why are fluid simulations so hard?
Fluid simulations solving the hydrodynamic (HD) or the magneto-hydrodynamic (MHD) equations are very useful in physics, the latter being particularly useful for modeling plasmas.
Of course these ...
0
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0answers
20 views
Probability Density Function of DNS velocity field [duplicate]
I am currently working on a DNS turbulent solver and I would like to compare my IHT simulations to papers. Those papers show the Probability Density Function of the velocity field:
I would like to ...
0
votes
1answer
63 views
Handling time derivative as source term in SDIRK methods
I am currently facing some challenge implementing a less traditional PDE which take a form similar to the Navier-Stokes equation, except that the continuity equation is modified such that:
$$\epsilon \...
3
votes
0answers
61 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
49 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 ...
1
vote
2answers
126 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 ...
2
votes
1answer
60 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 ...
0
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0answers
192 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
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0answers
57 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 ...
1
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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
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0answers
55 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
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2answers
68 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?
0
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1answer
110 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
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1answer
274 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 ...