moving wall fluent tutorial

Med Mahmoud CFD: Final Project 1

IntroductionThis tutorial will go step-by-step through the process of simulating flow in a mixing tank with rotatingeccentric cylinders. This problem is relevant because it is a simplified model of an economical way to mixviscous fluids, which don’t readily exhibit turbulent mixing (the kind of mixing that makes it so easy to stirmilk into coffee) [1].

Figure 1: Diagram of the fluid region. The outer boundary is the inner boundary of the large cylinder, androtates with speed Ω1. The inner boundary is the outer surface of the small cylinder, and rotates with speedΩ2.

In the original problem formulation, the angular speed of the small inner cylinder actually oscillatessinusoidally, providing better mixing. Since the goal of this tutorial is primarily to exhibit a use case forANSYS rather than to solve the actual problem, we’ll skip that complication and assign constant angularspeed to the small inner cylinder.

Problem StatementThe problem involves a fluid domain bounded by two nested rotating cylinders whose axes are not aligned.The inner cylinder rotates faster than the outer cylinder.

Since the original problem formulation is an analytical one, we’ll have to select the parameters for theproblem in order to actually perform the simulation.

DimensionsDimensions were chosen somewhat arbitrarily:

• Outside Cylinder Diameter: 3 m

• Inside Cylinder Diameter: 1.5 m

• Center-to-center Distance: -0.5 m

Fluid ParametersWe selected glycerin as the fluid to model with because it’s quite viscous and its properties are available inthe FLUENT Fluid Database.


Operating ConditionsWe set up the problem so that the inner cylinder rotates at 10 rad/s, and the outer rotates more slowly at2 rad/s. In our model, they both rotate in the same direction.

Tutorial StagesTo set up and solve a problem with ANSYS/FLUENT, there is a standard workflow that the software guidesyou through.

• Set up the Geometry

• Mesh the Geometry

• Setup the solver parameters (boundary conditions, fluid properties, etc)

• Solve the actual problem

• Visualize the results

This tutorial will be divided into those stages as well.


GeometryThe Geometry setup is done in ANSYS DesignModeler, where the fluid domain is sketched. (This can beconfusing at first... usually the solid parts are being designed, but its the negative space between these partsthat FLUENT will fill with fluid and analyze.) For a more complicated Geometry, it’s possible to draw theGeometry in SolidWorks or a less wonky modeling package than DesignModeler and import the Geometry.To do this, the Geometry should be saved in .STEP format.

In this project, however, the Geometry is quite simple, so we just draw it in DesignModeler as a 2Dobject.

Drawing the Geometry

The basic procedure for our eccentric cylinders is this:

• Sketch the two circles aligned along y = 0.

• Dimension (constrain) their radii, and dimension the distance along the y-axis between their centers.

• Convert the sketch to an actual surface bounded by the two circles.


MeshingMeshing is done with ANSYS Meshing. This step very much makes-or-breaks the simulation, because (asI learned the hard way) tiny differences between two Meshes can make the simulation fail to converge to areasonable solution, let alone maintain comparable accuracy.

In ANSYS Meshing, the user specifies the Meshing algorithm and any embellishments, sets the parametersfor these choices, and Generates the Mesh. For our simple Mesh, we will essentially use the default Meshingalgorithm, but embellish it with Inflation, which increases the Mesh resolution near the boundaries of thedomain so that boundary effects can be modeled with greater fidelity.

Meshing Procedure

The basic procedure for our eccentric cylinders is this:

• Fix one parameter in the default Meshing algorithm (use a Fine Relevance Center)

• Insert an Inflation algorithm and tune its parameters

• Name some regions of the Geometry for use in FLUENT later (this could have also been done inDesignModeler)


Setup and SolveThis is where we finally open FLUENT and set up the problem.

Like ANSYS as a whole, FLUENT also has a workflow that it tries to guide the user along. Each stepappears in order in the Problem Setup pane on the left side of the FLUENT window. This process is dividedinto two sections of its own (omitting steps that aren’t relevant to the cylinder problem):

Problem Setup

• General, where the type of solver is selected and units are set

• Models, where the specific equations to be used are selected (we just use the defaults in this paper)

• Materials, where materials and their properties are made available to the later steps

• Cell Zone Conditions, where coordinate systems, materials, and other properties are assigned to“Zones” of the Mesh

• Boundary Conditions, where, obviously, the Boundary Conditions are specified

Solution

• Solution Methods, where the type of integrator is selected (the overall method was specified in theGeneral step, but precisely what variation on the method is selected here)

• Monitors, where required information about the computation as it occurs is requested

• Solution Initialization, where the Mesh is initialized with a guess at the solution (a requirement forevery iterative method of solving differential equations)

• Run Calculation, where computational parameters like Time Step Size are selected, and the compu-tation is actually started


VisualizingThe final step is to visualize the results of the computation. FLUENT provides tools for doing this in theResults section of the Problem Setup pane.

First, the quality of the calculation should be checked by verifying physical constraints (like balance offluxes through surfaces). We use Mass Flow Rate across a fictitious Boundary separating the upper andlower halves of the Geometry.

To find the Mass Flow Rate across an imaginary Boundary, we first need to specify that Boundary inFLUENT. This is done (counterintuitively) in Graphics and Animations. The actual flux validation is doneunder Reports.

Secondly, we should actually look at the flow pathlines to see if mixing is occurring. This is done byplotting the pathlines in the Graphics and Animations section.

Validation

The validation procedure is as follows:

• Set up a Boundary line in Graphics and Animations

• Report the Surface Integral of Mass Flow Rate across the Boundary; it should be near zero

Pathlines

The procedure for plotting the Pathlines is straightforward:

• Set up a Pathlines graphic under Graphics and Animations

• Display it!


Closing Remarks

Crappy ResultOur validation step revealed the crappiness of our result. I actually obtained better mass balance earlier,but upon repeating the procedure to document the steps, I was somehow unable to reproduce the Mesh!

Appended to this report is a table listing different solver parameters in attempts to get better massbalance. (This table isn’t a complete catalogue of my attempts-- only those attempts that I remembered towrite down.)

Notice the sad quality after I remeshed the Geometry.

MixingTo say a few words about the actual application at hand-- our crappy simulation fidelity notwithstanding--the mixing quality was exremely poor compared to the mixing quality with an oscillating inner cylinder.The constant speed inner cylinder is a gross simplification of the model, since the oscillating appears to becrucial. This could perhaps be implemented with a User-Defined Function for the oscillation.

References1. B. Y. Ballal & R.S. Rivlin. Flow of a Newtonian Fluid Between Eccentric Rotating

Cylinders: Inertial Effects. Archive for Rational Mechanics and Analysis,Volume 62, Issue 3. (1976).

First MeshMethod Gradient Pressure Momentum Time Iters/Step Tstep Steps Mass FluxSIMPLEC Cell Presto 2 2 300 0.1 20 0.0047SIMPLE Cell Presto 2 2 300 0.1 20 0.068SIMPLEC Cell Presto 2 2 300 0.2 20 0.27SIMPLEC Cell Presto 2 2 400 0.2 20 0.278SIMPLEC Cell Presto 2 2 500 0.2 20 0.278PISO Cell Presto 2 2 300 0.2 20 0.29Coupled Cell Presto 2 2 300 0.2 20 0.297SIMPLEC Cell Presto 2 2 300 0.1 40 0.31SIMPLEC Cell Presto 2 1 300 0.2 20 0.37SIMPLEC Cell Presto 2 1 400 0.2 20 0.371SIMPLE Cell Presto 2 1 300 0.2 20 0.408SIMPLE Cell Presto 2 1 400 0.2 20 0.41SIMPLE Cell Presto 1 1 300 0.2 20 0.46PISO Cell Presto 1 1 300 0.2 20 0.46SIMPLEC Node Presto 2 1 300 0.2 20 0.84PISO LSQ Presto 2 2 300 0.2 20 1.44SIMPLE LSQ Presto 2 1 300 0.2 20 1.6SIMPLEC LSQ Presto 2 1 300 0.2 20 1.66

Failed Attempt at Reproducing the First MeshMethod Gradient Pressure Momentum Time Iters/Step Tstep Steps Continuity Mass FluxSIMPLEC Cell Presto 2 2 300 0.2 20 0.0001 8.32PISO Cell Presto 2 2 200 0.2 20 0.0001 8.44SIMPLEC Cell 2 2 2 300 0.2 20 0.0001 9.43SIMPLEC LSQ 2 2 2 300 0.2 20 0.0001 4.43PISO LSQ 2 2 2 300 0.2 20 0.0001 4.43Coupled LSQ 2 2 2 300 0.2 20 0.0001 4.42Coupled LSQ 2 2 2 60 0.2 20 0.00001 4.44

moving wall fluent tutorial

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