COMSOL Step-by-Step Tutorial - PowerPoint Presentation

Slide1

COMSOL Step-by-Step Tutorial

Expt 140 – Tank Discharge

March 4, 2013

Team 2

Joseph Duffy

Zlatko

Sokolikj

Andrew SuellentropSlide2

This

Comsol

tutorial demonstrates how to perform the velocity profile calculations using the simplest model possible

It utilizes the 2D axisymmetric reference systemSlide3

Before starting anything remember to have the preferences set so that

Comsol

does not save a recovery file (Box should be unchecked). For more information regarding this step, please go through Chris Golding’s Tutorial about starting

Comsol on a computer, located in Transport Phenomena II BalckboardSlide4

So we pick our space dimension to be 2D axisymmetric

And press nextSlide5

We pick laminar flow

We pick laminar flow from our physics menu

and press nextSlide6

We pick Stationary Study Type

and press FinishSlide7

In the actual Exp140 Tank Discharge, our system is not stationary, yet it is time dependent as one of our boundaries, the water level, changes. However to make these calculations easier we assume a quasi steady state as outlined on page 16 of

Exp140 Tank Discharge – Theoretical Background

by Loren B. Schreiber. That means that

Comsol will perform the calculation for the velocity while the water level is kept constant. This presents an obstacle for us as we would like to know how the velocity profile changes as the water level drops. We can overcome this obstacle with a feature called parametric sweep which will be covered later down this tutorial Before we start drawing our model we must realize the significance of our choice of dimension. We use this geometry when our 3D model is symmetric in both the x and y axis. What we need to draw in Comsol is a sketch which, when rotated around the z axis 360 degrees, will give us our model So for our model… Slide8

The tank is a 3D model that is symmetric in the x and y coordinate

In the 2D axisymmetric geometry our model looks like….

Rotated around the z axis, our model will give us the tank in 3D

Z-axisSlide9

However before we start drawing the system we need to set the values of the parameters we are going to use, such as density, Pipe Diameter, Tank Diameter etc.Slide10

We right click on Global Definitions and select parameters

Slide11

To insert a parameter click on the cell and insert the appropriate descriptions

For more information regarding Parameter Input, please go through Chris Golding Intro for

ComsolSlide12

Remember to use square brackets when plugging in the units

In case you plan to use these parameters for multiple models you can save them as text file which you can open later on instead of having to write them every time Slide13

In this tutorial we use the following parameters Slide14

In this tutorial we shall use the simplest geometry possible. We will have one rectangle represent our tank and another one represent our pipeSlide15

To build our first rectangle that will represent our pipe, we right-click on Geometry section and select RectangleSlide16

So first we need to choose what part of our rectangle will be our reference for its position.

We pick Corner

Therefore the corner of our rectangle should be at (0,0)

Note*: In Comsol, whenever you can enter a number, you can also enter a equation using the parametersTherefore for width and height we write our Dp/2 and Lp.

Remember that our model will represent only one half of the side section. That is why the width of the rectangle that is supposed to represent the pipe has a width of the radiusSlide17

After we are done setting up the rectangle, to materialize the rectangle on our sketch we right-click on the

Rectangle1

from the Geometry Sections and select

Build selectedSlide18

We repeat this process for the second rectangle. The corner of this rectangle will be located at(0,Lp) while the width and the height will be (

Dt

/2) and Lt

We materialize the rectangle on our sketchSlide19

The next step is to form a union with the rectangles we have built, merging them togetherSlide20

We need input the rectangles by selecting them first( which will redden the rectangle)Slide21

And then click add

We repeat this process for the other rectangle Slide22

The objects, once included in the union, should have the color of blue, and should appear in the Input Object sectionSlide23

In this step, we introduce the material that is used in the experiment

To do that, right-click on Materials and select Material Slide24

Make sure that this material is represented in both domains(the rectangles), which should be colored blue

We also input the density and viscosity. We can do this using values we know or using the parameters we previously entered Slide25

We enter the values in the Value Column

For density we enter

Rho

For viscosity we enter Rho*Mu – the product of the kinematic viscosity and density Slide26

The next step is to select the inlet of our system

Right click on Laminar Flow and select InletSlide27

The inlet section will appear

We need to select which wall of our system will be the inlet. Therefore we select the top wall and press addSlide28

We also choose the boundary condition

For this tutorial we choose the pressure to be our boundary condition and we set it equal to our Atmospheric Pressure Slide29

We repeat the process for the outlet. Right click on the Laminar Flow section, press Outlet and select and add the wall of the system that is to be your outlet

We use the same boundary condition as the inletSlide30

Here we select the

I

nitial Values

We select the Domain to be our rectanglesWe select the Pressure to be our atmospheric pressure To make our calculations faster, since we know that our velocity will be negative, we give the velocity field a small negative number in the z-direction Slide31

We introduce gravity to our Model

We do that by introducing the Volume Force which is the gravitational pull on a unit of volume. The Volume force is the Density multiplied the Earth’s Acceleration

Right-click on

Laminar Flow, and select Volume Force. In Selection, select the domains on which this force will act, which are our two rectangles. Also in regards to the magnitude, input the negative product of Density and the Earth’s Acceleration Slide32

The next step would be to perform the calculations for the velocity in these conditions

Right-click on the

Study

Section and press ComputeSlide33

Once the calculation is done we can see that the predicted velocity profile of the fluid is much smaller in the tank than in the pipe

We zoom in on the pipeSlide34

We can see that the velocity profile in the pipe is laminar.

We can also see how the fluid speeds up as it enters the pipe and propagates farther downSlide35

If we press on the Velocity3D section we can see our model in 3D. We can also zoom in to get a better look at the velocitySlide36
Slide37

Now we tackle the issue of constant boundary. To be able to get a feel of how the velocity changes as the tank level changes, using a constant boundary, we would have to perform separate calculations for each level of the fluid tank. In each calculation we would have to change the value of the parameter that gives us our tank level.

Comsol

can do these calculations all at once using a feature called parametric sweep.

With parametric sweep we select a parameter that will change and we choose the range in which it will change. Comsol performs the velocity calculations for each value of the parameter Slide38

To select Parametric Sweep we right-click on Study and select Parametric Sweep Slide39

Here we select the Parameter to be changed.

First we select add…Slide40

We select the parameter that describes our tank liquid level

In this tutorial that is LtSlide41

And now we need to select the range in which the parameter will be changed.Slide42

The following window appears

* In this tutorial we chose the step method. With this method we pick a start value(the highest level of the liquid in the tank) a stop value( the lowest level of the liquid in the tank) and the step at which it will change. The other method is similar with the exception that we do not choose the steps, but rather we choose the number of values for that parameter and

Comsol

chooses the steps. So in this tutorial- Our start value is the liquid level in the beginning- Our end value is the liquid level at the end of the experiment- Our step, to make this calculation short, is 0.1. Since we are decreasing our parameter our step has to be negative, hence it is written as -0.1. You can also choose a smaller step size; however it will make the calculation take longer and use up more RAM.- After inputting the values, press add. **Do not forget to input the unitsSlide43

After adding the values right-click on Study and press ComputeSlide44

After the calculation is done, we can see how the velocity profile changes as we change the tank height.

Select Solution 2, select the height, and press plot .Slide45

Now we can

calculacte

the average velocity and see how it changes for each height.

In the geometry we are using the equation is

Comsol

can find both the numerator and the denominator for us and then we can use other computational tools to calculate

.

 Slide46

We need to set up both integrals.

We do that by right-clicking Derived Units in the Results section, selecting Integration, and then selecting Line Integration.Slide47

Now we first evaluate the numerator

- We select Solution 2 for Data set; for Parameter Selection we select From list. Make sure all the values of the parameter are selected. Select the Outlet as the boundary over which we integrate

- In Expression, write

spf.U(the velocity profile)*r- Select EvaluateSlide48

When the integration is complete, the results will appear below in a spreadsheet format. So far we have Lt(the level in the tank) and the integral of

spf.U

*r which corresponds to our volumetric flow rate.

Now we determine the integral in the denominator which represents the areaSlide49

In Expression we input r, while everything else is kept as it was previously.

-Press Evaluate

- The new integral will appear in the third column

- We select the whole data set and press Copy Selection to Clipboard.Slide50

We copy the data in an Excel Spreadsheet and find the velocity at each height by dividing the Volumetric Flow rate by the Area, as given by the equation on the Previous Slide