Uppsala University
Information Technology Scientific Computing
September 29, 2011 Applied Scientific Computing Jonas Thies Martin Kronbichler (room MIC 2350) martin.kronbichler@it.uu.se
Application of Computational Fluid Dynamics
Objective
The objective of the assignment is to train two essential aspects in the appli- cation of computational fluid dynamics using a powerful software package: flow simulation and postprocessing. Another essential part of CFD is provided for you: the grid generation.
Introduction to Fluent
In this assignment you will solve a 3D turbulent fluid flow and heat transfer problem, using Fluent. The problem is to predict the flow and temperature fields in a “mixing elbow”. This is a common pipe structure in process plants and process industries, where fluids (water in this case) of different temperatures and velocities are mixed.
Fluentis a computational fluid dynamics (CFD) software produced by Fluent Inc. (http://www.fluent.com), which is in turn owned by Ansys Inc. Fluentis popular in the industry and there is a good chance that you will encounter and use it. Fluent’s numerical solver is based on the finite volume method (FVM) and handles structured and unstructured grids, different numerical fluxes, grid adaptation, parallel execution, and several turbulence models. Fluent relies on external tools for grid generation, e.g. Gambitor theANSYS Toolchain.
This assignment consists of aFluenttutorial with some supplementary ques- tions. Your tasks are to
• define material properties, initial and boundary conditions,
• solve the problem with different numerical methods,
• use grid adaptation, and
• study the solution using different post processing techniques.
Practical information
• There are two versions of Fluent available, version 6.3 and version 13.
The folder for the former is located in G:\Programs\Fluent\, and the latter atG:\Programs\Fluentv13\. In the respective folder, you will find a shortcut tofluent.exe. A selection window will pop up, where you start with the predefined settings in 3D.
• You are supposed to work through thecomplete first tutorial, see be- low. You need to use the input file containing the mesh dataelbow.msh, which you can download from the course homepage.
• Fluent 6.3: There are only two licenses available. Follow the tutorial 1, found at the course homepage and via Help → More Documentation → Tutorial Guide→Tutorial 1, Introduction to Using ANSYS FLUENT: Fluid Flow and Heat Transfer in a Mixing Elbow.
1
• Fluent v13: Newer version with 10 licenses. Unfortunately, some of the commands and output options have been changed. You will find a tutorial file on the course homepage and at Help → PDF → Tutorial Guide, Chapter 1, section 1.4.5. Below, you find tasks that should be performed in addition to the tutorial guide.
– Adapt the mesh according to the temperature gradient – Compare different discretizations
• To save figures inFluentuse File →Save pictures. . .
• The report should include figures and answers to the questions below, and be handed in at the seminar,October 11, 2011. For questions, contact Martin Kronbichler or Per L¨otstedt.
Problem description
Through the main inlet, the lower left corner of Figure 1.1 in the tutorial, water at 20◦C is flowing with the speed 0.4 m/s. Through the second inlet, on the lower right corner, water at 40◦C flows with the speed 1.2 m/s and mixes with the colder water of the main pipe. The objective is to compute the temperature profile at the outflow part of the upper right corner. Further description of the problem can be found in the tutorial.
Questions
Besides saving your own versions of Figures 1.6, 1.7, 1.9, 1.14, and 1.21 in the tutorial (Fluent 6.3), you should answer the following questions.
• Write down the equations which Fluent is solving for the mixing elbow problem (no equations for turbulence modeling). Which boundary and initial conditions are used?
• Why do we need a turbulence model in our example? Use the Reynolds number for your argumentation.
• How many degrees of freedom are there in the original computational grid?
Is this a large number for a computation like this?
• Give an outline the numerical method employed byFluent, how derivatives are approximated, etc. (up to 1/2 page).
• Record the mass flow at the outlet, i.e.
Z
∂Ωout
ρu·nds,
where∂Ωoutis the outlet boundary andnthe outward normal unit vector, for all the methods below. Can you find an exact value for the mass flow?
Explain!
• Present a figure with the outlet temperature profiles for all the methods below.
– First order method (First order upwind) on the original grid.
– Second order method (Second order upwind) on the original grid.
– Third order method (MUSCL) on the original grid (and second order for pressure).
2
– First order method on grid adapted twice.
Discuss the results, compare also with Figure 1.23 in the tutorial (Fluent 6.3). What is best for this kind of problem, a higher order method or an adaptation of the grid? What should one compare with? Discuss how a reference solution can be obtained and produce such a solution.
3
