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Questions tagged [fluid-dynamics]

The study of the properties of fluids and gases in motion

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1answer
844 views

How to choose the relaxation time in the Lattice Boltzmann Method?

We know that the relaxation time is very important in LBM. I have searched lost of papers, but can't find some systematic introductions about the choice of relaxation time in SRT LBM. Could you give ...
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0answers
127 views

Stability of the explicit MacCormack Scheme to solve the Navier Stokes equations with Wilcox's K-Omega Turbulence Model

I am solving turbulent pipe flow with an explicit MacCormack scheme and Wilcox k-omega model. The laminar version of the code had three distinct stability criteria which worked fine after ...
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2answers
181 views

Flow past square cylinder benchmark in 2D. Famous papers to compare

I'm looking for a benchmark for the flow past square cylinder problem in 2D to compare my results. I have found some papers, but it seems that there are different results in different papers. I want ...
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1answer
235 views

How can an engineering student become a computational scinece expert in a short time [closed]

How can a student with zero computing or programming language knowledge, few engineering mathematics knowledge, understand computational science especially Finite Element Modelling (FEM) from ...
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2answers
198 views

Benchmark setup for incompressible Rayleigh-Taylor flow

I'm trying to benchmark a new numerical method for the incompressible Navier-Stokes equations. I'd like to use the most widely-accepted test cases possible. In particular, I'm looking for a ...
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0answers
266 views

Algorithm for face based data-structure - CFD

Good morning I'm trying to develop an unstructured CFD code to solve Euler equations in a finite-volume (cell-centered) context (learning purposes). I was able to build from a cgns file some basic ...
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1answer
1k views

Is there an advantage of using a staggered grid over a regular one when combined with high order methods?

The title says is all. This question is in the contest of an incompressible Navier-Stokes solver. Specifically, I am currently working on a new solver while referring myself to an older code for ...
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1answer
340 views

Outlet boundary condisions in lattice Boltzmann Method

Here is the flow past a square cylinder configuration. The problem is a viscous and incompressible with parabolic velocity profile using freestream velocity U across single cylinder. I use the single ...
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0answers
97 views

What is a good algorithm to solve a discrete continuity equation in Cylindrical coordinates?

The equation is: $\partial f/\partial t + \nabla \cdot (v f) = 0$ $, \;\; f \in [0,1] $ and $v$ is a velocity known at every grid cell. A more precise constraint is that $f$ is either 0 or 1 but ...
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3answers
9k views

The real myth of GPU (specifically CUDA) really speed up FEM/CFD

Now I have been believing that FEM/CFD is supposed to be faster on a GPU unit - here I am using CUDA as solid example. However, I have not been able to find a convincing paper where the benchmark ...
3
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2answers
537 views

Discrete conservation and Finite Element methods

What would be the rigorous mathematical expression of the fact that a conservation law discretized with a Finite Element method with Galerkin discretization does not result in a conservative scheme ?
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1answer
108 views

How to discretize the surface of a prolate spheroid?

I need to discretize the surface of prolate spheroid given by the equation $$\frac{x^2}{L^2} + \frac{y^2}{D^2} + \frac{z^2}{D^2} = \frac{1}{4}$$ The surface has to be divided to 500 equal panels to ...
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1answer
262 views

Pressure-Pressure BC

My FDM code simulates Backward Facing Step flow when I use conventional BCs such as defining velocity profile at inlet and fully developed condition at outlet. I have validated the results and it ...
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0answers
67 views

Cavity Flow CFD Boundary conditions and strange waves

So I have a PDE that I use to describe how material flows through a volume(2D or 3D). $$\frac{\partial C}{\partial t} + \vec{u} \cdot \nabla C = (D' + D )\nabla^2C$$ Now using finite differences I get ...
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1answer
343 views

Difficult bug in my 2D Compressible Euler solver

For the past few days, I have been writing a numerical solver for the 2D compressible Euler equations for an ideal gas. My numerical method has been the Local Lax Friedrichs or "Rusanov's method." ...
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2answers
104 views

Definition of inflow boundary in CFD

If $w$ is the vector of conservative variables, $f=f(w)$ the flux function, I think have read somewhere (I can not find it anymore) that the inflow boundary $\Sigma$_ is characterized by: $\Sigma_{-}...
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1answer
65 views

Conservatives in shock tube

We know shock tube problem will give discontinuous solution of primitive variables ($\rho$, $v$, $p$). Will it give discontinuous result in flux terms? $F =[\rho u, \rho u^2 +p, \rho e_v]^T$. I tried ...
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0answers
50 views

Square error estimate for adaptive mesh refinement

In a particular implementation(Finite volume advection using upwind) of adaptive mesh refinement the error square estimate for a cell C is given as $$ \sum_{i = x,y,z} vol * \frac{1}{12} * h^{2} * (\...
3
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1answer
155 views

Relaxation Parameters for Steady Navier-Stokes

I am working on a project involving steady solutions for the Navier-Stokes Equations. In the past I've only worked with the unsteady Navier-Stokes, so some of this is new to me. In particular, at ...
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2answers
326 views

Can I model laminar incompressible fluid flow and heat transfer in MATLAB's PDE toolbox?

I have a system of PDEs in cylindrical coordinates that needs to be solved: 1. Continuity equation 2. Incompressible Navier stokes ( in r & z coordinates) 3. Heat transfer equation with both ...
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0answers
81 views

Oscillations in Chorin's method due to the BC

I am pretty new to the CFD and I wanted to start with Chorin's projection. The starting problem is just a free jet flowing in the investigated area. I got terrible oscillations almost immediately and ...
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1answer
202 views

Heat Equation in 3D mass Matrix set-up

I am solving a 3D heat transfer equation with variable boundaries (insulated, convective, radiative or free) using a F.D.M. technique. My geometry of choice is a cube. The purpose of my work is to get ...
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2answers
178 views

Implicit projection method with inflow boundary conditions

I am trying to use a projection method that deals with the viscous effects implicitly to model flow around a cylinder. I'm having trouble figuring out what the boundary conditions should be, ...
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2answers
673 views

How should I set the velocity conditions when you have a pressure inlet and pressure outlet?

I know that when modeling incompressible flow in a pipe, it is common practice that when you have a velocity inlet, there should be a zero pressure gradient at the inlet as well. Also, when you have ...
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0answers
208 views

Implementing pressure inlet boundary condition

I am interested in implementing pressure inlet boundary conditions for the 2D compressible Euler equations. My equation of state is an ideal gas ($p=\rho R T$) which is thermally prefect but not ...
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1answer
68 views

Implementing temperature depending viscosity in a finite-difference scheme

I have a little question that might be basic for some experts, but right now, its not clear for me. I want to implement temperature depending viscosity in a finite difference scheme (incompressible ...
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2answers
986 views

New to CFD, Lattice Boltzmann or Navier-Stokes?

I apologize if some of my questions are naive; I am very new to computer simulations and fluid-dynamics. I am going to start a PhD in early 2017, and I would like to bone-up on some Computational ...
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0answers
294 views

Collocated Grid Navier Stokes Solver

I want to solve Navier Stokes equations on a collocated grid. Earlier, I was using a MacCormick scheme based solver where I discretized predictor step in forward differences and corrector step in ...
4
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2answers
2k views

Recovering pressure from velocity or streamfunction fields

I am interested in 2D channel flow of an incompressible Stokes fluid (Re << 1), with periodic boundary conditions in the x-direction and no-slip at the walls in the y-direction. I have existing ...
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3answers
1k views

Constructing random divergence-free velocity fields

I am trying to simulate decaying homogeneous isotropic turbulence. As initial condition I want a divergence-free vector-field, i.e, $\mathrm{div} U = 0$. How do I initialize random velocity field in ...
3
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1answer
188 views

Generalized Eigenvalue Problem from linear stability analysis

I also posted this in the physics forum, but maybe here it fits better. I am trying to solve a generalized eigenvalue problem raised by linear stability analysis $$AV=\lambda BV.$$ $A$ and $B$ are ...
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1answer
79 views

Convergence Criteria for Two Fluid Flow Solver

Which one of the following is suitable for judging convergence in Two-Fluid Flow Solver? 1) Absolute Residual (L^2-Norm). 2) Relative Residual. 3) Fraction Change in Velocity, Pressure and Volume ...
2
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1answer
406 views

How to define residual in multigrid approach?

I wish to solve the two-dimensional Navier Stokes equations using multigrid method on a collocated grid using the Predictor-Corrector method mentioned below. But first, let me elaborate on what I had ...
3
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1answer
141 views

Decaying turbulence and simulation

I am a beginner in CFD having written few codes for laminar flow cases using SIMPLE and some other explicit solvers. Now, I want to use my solvers and some other models to solve for the turbulent flow ...
3
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1answer
210 views

Stability Criterion for this Explicit Scheme

I am solving an unsteady flow using the dual-time Navier-Stokes equation in which I write my momentum equation as: $$\frac{\partial u}{\partial \tau} + \frac{\partial u}{\partial t} + \frac{\partial u^...
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1answer
298 views

Outflow boundary condition

I know that in outflow boundary we assume a zero normal gradient condition and use upwind scheme for approximation. However, I saw this sentence in a book which I do not understand; "Convective fluxes ...
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0answers
117 views

CFD implementation in software [closed]

I've been learning CFD theory for 4 months (which includes FEA, FDM AND FVM) and now I want to start running simulations. As a novice to CFD software, I would appreciate some advice on where to start, ...
2
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3answers
137 views

combination of field and particle methods for fluid dynamics

In numerical fluid dynamics there are field methods like finite-volume, finite-element, etc. and particle methods like Smoothed-Particle-Hydrodynamics – SPH and others. Both approaches have advantages ...
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1answer
49 views

Different Meshes Different Maximum velocities

I am working on a model in COMSOL and I am usin different meshes from coarse to fine keeping the other parameters constant. And, I got different maximum velocities for all the meshes. What can I do ...
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1answer
110 views

In which cases an interface tracking/capturing method needed along with Navier-Stokes solver for flow?

In my view, it is needed, To save computational time. To define accurate boundary conditions at interface where properties changes drastically. To solver 2 phase problems as single phase only. To ...
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3answers
837 views

Simple Simulation Examples in Computational Fluid Dynamics

I would like to incorporate a CFD topic in a project I currently have. The deadline is a little less than a month from now. Basically, what I would like to do is solve some equations and simulate ...
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1answer
46 views

How to introduce a distance vector when decompose a force?

I read reference (1),but I am confused about how to introduce a distance vector in Lorentz force like the author does it in equation (11): $$\rm J\times B = -J\cdot \nabla \left( B\times r\right)+\...
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0answers
347 views

Periodic boundary conditions for solving Navier Stokes Equations on a Staggered Grid

I want to solve two dimensional Navier Stokes equations on a staggered grid for the case of Taylor-Green Vortex. My initial conditions are standard sine and cosine functions. As I am aware, I should ...
2
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1answer
397 views

Is the Finite Volume Method conservative when the source term depends on the variable?

I trying to do a simulation where there are two quantities, $\Delta\theta(x,t) = \theta(x,t) - \theta_{o}$ and $\Delta\nu(x,t) = \nu(x,t) - \nu_{o}$. These quantities are chemical concentrations ...
10
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3answers
1k views

Can compressible flow solvers be used to solve incompressible flow?

I know that incompressible and compressible flow solvers are specifically designed to solve different types of problems with different fluid properties/flow conditions. Clearly, among the advantages ...
3
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3answers
1k views

Time step size for transient simulations of vortex shedding

I am simulating unsteady flow around a circular cylinder (using FLUENT). I am facing the following problem. Kindly please help me. OBJECTIVE: To find out the Reynolds number at which vortex shedding ...
4
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1answer
501 views

Continuous vs discontinuous pressure elements in fluid flow problems

When solving fluid flow problems using finite elements you typically end up requiring that $p\in L^2$. For Darcy flow problems, a popular choice of elements is the Raviart-Thomas element and ...
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0answers
75 views

PDE discretization (via finite difference sheme) question

So after posting this question and reading all your comments I would like to make this new question (update). If you consider the three equations presented here: $$\frac{\partial \rho}{\partial t} +\...
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0answers
61 views

Calculate fluid speed in a conduit with obstacles

I need some advices to start to solve some problems about fluid motion in a conduit. The problem domain is shown below: a conduit with some obstacle of different geometric properties. The fluid is ...
6
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1answer
169 views

A simple PDE solution question

I need to ask a question about partial derivatives. I want to solve this equation (steady state, one dimensional continuity equation): $$\frac{\partial (\rho u)}{\partial z}=0$$ which is equivalent to:...