Questions tagged [fluid-dynamics]

The study of the properties of fluids and gases in motion

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4answers
202 views

In Matlab, how can I be consistent with units?

I am modeling some aerodynamics equations and am using meters / centimeters, kilograms, and seconds. I've heard that, "matlab doesn't know units". So, how can I make sure that it does? Just ...
0
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2answers
224 views

How to represent CFD result when I use grid-centered FVM?

My variables are stored at the center of the cells. How can I transfer these values to grid points? If I calculate the algebraic average value there may be a shock.
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0answers
40 views

CFD Mass exchange across a reactive front

I have a CFD solver at hand which can deal with 2 phases and so far is able to exchange momentum and energy across the interface, where both phases meet. How this is done is well documented in the ...
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0answers
118 views

Definition of CFL number in Arbitrary Lagrangian-Eulerian framework

In an Eulerian frame of reference, the CFL number is defined as $$\sigma=\frac{u \Delta t}{\Delta x}$$ with $u$ the magnitude of the fluid velocity. A restriction such as $\sigma<1$ for time ...
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1answer
165 views

Robust/Tested Solver for incompressible 2D Euler (Fluid dynamics) Equation

I am trying to locate suitable computational algorithms for a optimization problem that requires repeated solution of transient 2D incompressible Euler equation on a 2D domain (say rectangular). My ...
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0answers
83 views

Reconstruction of cells information when given mesh in polyMesh format [duplicate]

The polyMesh format used in OpenFOAM is an intelligent format for which 'cells' file, i.e. the file giving list of nodes that make a cell, is not needed. The way to reconstruct geometry data such as ...
1
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1answer
75 views

Are there generic formulations of Riemann solvers?

I have been following Toro's book on Riemann solvers to implement a finite-volume scheme for computational fluid dynamics. The Riemann solvers presented in the book seem to be fairly tightly coupled ...
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0answers
300 views

Solution to 1D consolidation problem python implementation

A solution to the 1D consolidation problem is given by $$\frac{\partial}{\partial t} p = c_{v} \frac{\partial^{2}}{\partial y^{2}} p$$ where $p$ is the pore water pressure, $c_v$ is the ...
6
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3answers
2k views

Galerkin method: Test functions vs. Basis functions

I'm a novice to finite element and I'm finding quite hard to find the actual difference between Test function(s) and Basis function(s). I would be glad if somone could explain me that and point out ...
0
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2answers
264 views

Boundary conditions for streamlines in enclosed flow

I am trying to solve Lid driven square cavity flow problem of Stokes equation using finite element method. I have boundary conditions for velocity as zeros on every boundary but u=1 on top boundary. ...
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3answers
707 views

Pressure boundary condition in lid driven cavity using finite element method

Thank you all 1.) I am trying to solve lid driven cavity problem for an incompressible Stokes and Navier Stokes equations using general "Mixed" finite element method. dirchlet boundary conditions are ...
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0answers
47 views

Extrapolating to non-fluid cells (for Shallow Water Equations), for a shore/beach?

The water height $h$ and 2d velocity field $(u,w)$ are "extrapolated to non-fluid-cells, i.e., setting $h$ equal to the value in the nearest fluid cell." [Bridson] I'm using finite differences. ...
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3answers
131 views

Numerical Calculation of the Fluid Stress Tensor

I have been looking into simple ways of calculating numerically the total fluid stress tensor $ \pi_{ij}$ if the flow field is known in every $ x,y,z $ point. Applying the fluid stress equation (see ...
0
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1answer
178 views

Turbulent energy spectrum from isotropic turbulence field in a box (bis)

I'm trying to reproduce the work in computing turbulent energy spectrum from isotropic turbulence flow field in a box but I have a hard time understanding James' answer about the binning part. Would ...
2
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1answer
139 views

Order of accuracy in a combined scheme

I am trying to use a combination of JST and ROE scheme for the RANS solution of an aerodynamic problem. I notice that the ROE scheme performs poorly in terms of the convergence in the drag counts for ...
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0answers
36 views

How to implement boundary conditions in arbitrary geometries, in MD\Lagrangian FD simulations

Let's say I have a random arbitrary geometry, like this one https://ibb.co/dG8fUa (something I just made up). I make one of these in 3D in SolidWorks. I have molecules/particles coming in through the ...
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3answers
193 views

Best way to track parameters and results, in your experience (i.e. how do you do it)?

Computational Science has many more parameters than my previous programming, and I'm finding it difficult to keep track. What's the best way to track them? editing source, commenting-out. Good for ...
3
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1answer
147 views

Reasons to choose LES in stead of RANS models? (turbulence)

In oceanography, is there any particular reason why choosing large eddy simulations in stead of RANS (regardless of the type of flow)? In both cases, 2d simulations would be used (shallow water model)....
1
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1answer
294 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 ...
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0answers
47 views

Perfect filtering of high frequencies in 2D FFT (Multidimensional 2/3 Rule)

Let $u_n$ be an array containing discrete values of the function $u(x,y)$. Performing a 2D FFT to this array we obtain $\hat{u_n}$ representing the values of $\hat{u}(k_x,k_y)$. I would like to ...
3
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1answer
179 views

Resources for solving fluid-structure interaction problems

I would like to get started solving Fluid-Structure interaction problems. I already have some experience with Finite Elements, including my own MATLAB and Julia software packages for developing ...
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2answers
125 views

If a numerical solution remains constant for different grid sizes, what does it mean?

I'm testing a finite volume scheme, Godunov type solver, using a problem with analytical solution. I'm not able to reproduce the solution, which includes source terms. I tried with different mesh ...
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0answers
227 views

Physics of explosions: just vorticity?

eg 1 2. Is it just vorticity? What's actually happening? (Similar: steam engines, volcanoes, clouds). examples are grid-based, using "vorticity confinement" in Phoenix FD. EDIT Some techniques: "...
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0answers
47 views

Simulation of 2D impulse response using finite volume

Finite volume methods can be used to simulate the acustic wave equation (linearized Euler equations) in complex domains. I have developed a FV and a Discontinuous Galerkin code for 2D acoustic ...
2
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2answers
345 views

In practice, what are the most useful ways to visualize 2d fluid flow, to tell what is happening in the simulation? Esp for verification and debugging

My simulation creates a 2d grid of vectors and scalars (EDIT of velocity, depth etc), at 60 frames per second. Is it correct? It looks sort of right... but who knows? How can I tell what's happening - ...
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0answers
47 views

How is the Gastner-Newman equation implemented to create value-by-area cartograms?

There is a paper called "Density-equalizing map projections: Diffusion-based algorithm and applications" by Michael T. Gastner and M. E. J. Newman, which explains their algorithm (which is based in ...
2
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2answers
290 views

Can we simulate compressible flows by simple direct explicit calculation, without solving systems of linear equations (such as Poisson eq)?

Is this is plausible at all? It seems the most obvious/naive approach, so there's probably good reasons why it's not used - what are they? Viscosity is not important. Starting with inviscid Navier ...
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0answers
43 views

How do we implement Parameter free generalised Moment limiter in 1D Case in Discontinuous Galerkin methods?

I am referring to this paper:- "A Parameter-Free Generalized Moment Limiter for High- Order Methods on Unstructured Grids " by Michael Yang and Z.J. Wang. http://dept.ku.edu/~cfdku/papers/AIAA-2009-...
3
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1answer
295 views

Can we simulate incompressible flows using the (slight) density changes to give pressure?

A common approach says an incompressible flow has velocity divergence of 0; use this to solve for pressure in the Navier Stokes momentum equation. Or, using the Helmholtz Decomposition "project" the ...
4
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2answers
211 views

Preconditioner for the GMRES method in the Uzawa algorithm

I'm trying to solve \begin{equation}\left\{ \begin{split} \frac{\partial u}{\partial t}+(u\cdot\nabla)u-\nu\Delta u+\frac1\rho\nabla p&=f\;\;\;\text{in }\Lambda\\ u&=0\;\;\;\text{on }\partial\...
2
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1answer
88 views

Deposition model in laminar flow

I have a chamber full with a fluid flowing horizontally in laminar regime from one side to the other. It carries a suspension with concentration $c$. This suspension also falls to the bottom of the ...
2
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0answers
187 views

Outflow boundary condition - second derivative of velocity

Consider a fluid flow simulation in a pipe. At the outflow, instead of explicitly imposing a boundary condition, I linearly extrapolate information from the interior (for velocity components). This ...
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0answers
263 views

How to perform Taylor series expansion consisting of cell averaged derivatives in a computational element?

I have encountered the term cell "Cell averaged derivatives" and "Taylor series expansion using cell averaged derivatives about centroid" in the context of polynomial representation in any cell (...
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5answers
2k 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
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0answers
184 views

Rhie--Chow interpolation on PDE level

The Rhie--Chow [1] interpolation seems to be a standard tool in the Finite-Volume discretization of incompressible flows. It is commonly defined on the discrete level [2]. In the lecture notes [3] --...
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2answers
472 views

Reference request for computational fluid dynamics

I have good background of finite element methods and continuum mechanics and I am familiar with fluid mechanics. My aim is to understand the required theory and to write my own simple codes using ...
1
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4answers
276 views

Mass transport in porous media

My goal is to numerically solve the convection-diffusion equation of the form: $$\frac{\partial C}{\partial t} = \nabla (D \nabla C) - \nabla (v C)$$ $C$ is concentration and $D$ is the diffusivity ...
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0answers
95 views

How do we derive the elemental equation for Discontinuous Galerkin method using Centered Numerical Flux?

If we take a 1st order polynomial approximation in each cell, we can find the (2x2) -mass matrix, differentiation matrix, and flux matrix through the integration of Lagrange polynomials. However, I ...
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0answers
79 views

Modelling fracture flow in stress field in COMSOL [closed]

I am new to COMSOL and I want to model fluid flow in stress field. As starting point I want a simple 2D box having a fracture from right to left while pressure gradient vertically. I want to know the ...
3
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1answer
186 views

Is it possible to solve Euler equation numerically without using any flux limiter (in DG scheme)?

I have recently learned about Discontinuous Galerkin method to solve differential equations and I was trying to implement it to solve Euler equation. For now, consider the standard Sod Shock Tube Case....
2
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1answer
51 views

Can 3rd order TVD admit perfect shift for Upwind 1D Advection equation?

I recently coded a 1 stage and 3 stage optimal TVD-RK explicit scheme using eqn 3.3 here http://www.ams.org/journals/mcom/1998-67-221/S0025-5718-98-00913-2/ on the equation Ux+Uy=0, where x and y ...
3
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4answers
1k views

Practical coding resources for new CFD practicioners

There seem to be many books and papers that explain various CFD methods in great detail, but unfortunately I have not been able to find many good resources of such methods implemented in codes such as ...
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2answers
73 views

Avoid component mixing in eulerian simulation of plastic deformation of solid

Just for my interest I'm thinking how to make some cheap-and-dirty simulation of processes such as: penetration of metal plate by projectile explosion of shaped charge (for example https://www....
3
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1answer
154 views

Numerical computation of the velocity in the steady Navier-Stokes equation

I've asked this question on Math.SE too. Let $d\in\left\{1,\ldots,4\right\}$ $\Lambda\subseteq\mathbb R^d$ be bounded, nonempty and open and $\partial\Lambda$ be Lipschitz $V:=\left\{u\in H_0^1(\...
7
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1answer
533 views

Time discretization of the variational formulation of the Navier-Stokes equation

I've asked this question on mathoverflow too. Let $T>0$ $I:=(0,T]$ $d\in\mathbb N$ $\Lambda\subseteq\mathbb R^d$ be nonempty and open, $$\mathcal V:=\left\{\phi\in C_c^\infty(\Lambda,\mathbb R^d):...
3
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1answer
212 views

Software/code to extract a solenoidal (a.k.a. divergence-free) field from a 2D vector field numerically

Can somebody point me to software/code to extract a solenoidal (a.k.a. divergence-free) field from a 2D vector field numerically? There are a plethora of papers and documents describing how to do ...
0
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1answer
135 views

Initial Condition in a Numerical Problem

In a initial value problem does the initial condition has to satisfy the boundary condition and the governing equation? For example: If a non-homogeneous Neumann boundary condition for the pressure ...
4
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2answers
496 views

Time discretization: Runge-Kutta methods vs. standard backward difference

I've recently written a code that solves the incompressible/low-Mach number formulation of the Navier-Stokes equation with high-order methods for both time and space. My advisor insisted that I use ...
3
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1answer
639 views

1-D incompressible unsteady Couette Flow Explicit finite differece CFD

I am currently following J.Anderson Jr.'s CFD with basic application and I came into some troubles while coding for my very first CFD problem. As the title suggests I am solving an incompressible ...
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0answers
46 views

Shallow water equations with moving body

I was wondering if there are any shallow water equation solvers out there that can include rigid body motion for the surface the water flows on?