Master of Engineering

(Mechanical)

Assessment Instructions:

1. Please familiarise yourself with the EIT Academic Honesty and Misconduct Policy, in order to understand your requirements and responsibilities as a student of EIT.

1. Please refer to our Assessment, Moderation and Student Progress Procedure for information relating to extensions. Extension requests should be submitted to your LSO at least 3 days prior to the due date.

1. Assessments submitted via email will not be accepted.

1. Assessments must be submitted through Turn-it-in (unless otherwise stated). Your submission must:

1. Be a single document (Word or PDF only)

1. Include at least 20 words of machine-readable text, and

1. Not exceed 10MB.

1. You must use the provided assessment cover page available on your Moodle student homepage. Submissions without a cover page will not be accepted.

1. You must correctly title your document/s. For example:

UNIT#_ASSESSMENT#_YOURNAME_DATE

E.g. MME505_Assessment3_SteveMackay_01Aug2019

1. You must reference all content used from other sources including course materials, slides, diagrams, etc. Do not directly copy and paste from course materials or any other resources. Refer to the referencing section of the EIT eLibrary on Moodle for referencing guides.

1. It is your responsibility to check that you have submitted the correct file, as revised submissions are not permitted after the due date and time.

Important note: Failure to adhere to the above may result in academic penalties. Please refer to the unit outline or EIT Policies and Procedures for further information.

Please complete your answers on the assessment cover page document available on Moodle.

Clearly label your question numbers (there is no need to copy the full question over). Include all working out.

You are to use the ANSYS FLUID software program to answer all the questions in this assessment. This software is available on EIT’s Remote Labs. All students completing this unit have been provided with access to this software.

While completing this assessment, you are required to provide screenshots at various stages of the ANSYS FLUID model to demonstrate your proficiency with using the software and the attainment of the required level of knowledge for this assessment.

You are also required to submit your ANSYS FLUID.cfx, ANSYS FLUID.def and ANSYS FLUID.res files as a part of this assessment.

Problem Statement:

In this practical assessment, you have to model smoke coming out from a chimney stack. The smoke is released into the atmosphere and mixes with an oncoming side wind. This is a time dependent problem, so you have to model transient simulation. Initially no smoke is being released. In the second part of the problem, the chimney starts to release smoke and you have to capture how the plume of smoke above the chimney develop with time.

Figure 1: Geometry of the chimney stack

The volume mesh file CircVentMesh.gtm is provided in Moodle in the appropriate section. This is the starting point of the solution. Import the mesh file into ANSYS FLUID CFX-Pre. Develop the model in CFX-Pre.

Figure 2: volume mesh of the chimney stack

Set up the Circular Vent Model:

Creating an Additional Variable

In this practical assessment, an additional variable (non-reacting scalar component) will be used to model the dispersion of smoke from the vent.

Note: While smoke is not required for the steady-state simulation, including it here prevents the user from having to set up time value interpolation in the transient simulation. You can create the additional variable in the following way

1. From the main menu, select Insert > Expressions, Functions and Variables > Additional Variable or click Additional Variable
2. Under Name, type smoke.
3. Click OK.
4. Under Variable Type, select Volumetric.
5. Set Units to [kg m^-3].
6. Click OK.

In order to simulate a transient problem, first you have to develop and run a steady state problem. The results of the steady state problem will be used as an initial file for simulating transient problem

Creating the Domain

The fluid domain will be created that includes the additional variable. To Create a New Domain

1. Select Insert > Domain from the main menu, or click Domain, then set the name to CircVent and click OK.

2. Apply the following settings Fluids List: Air at 25 C Reference Pressure:0 [atm] Heat transfer: None

   Select the Kinematic Diffusivity of the additional variable to 1.0E-5 [m^2 s^-1]

Creating the Boundary Conditions

This is an example of external flow since fluid is flowing over an object and not through an enclosure such as a pipe network (which would be an example of internal flow). In such problems, some inlets will be made sufficiently large that they do not affect the CFD solution. However, the length scale values produced by the Default Intensity and Auto Compute Length Scale option for turbulence are based on inlet size. They are appropriate for internal flow problems and particularly, cylindrical pipes. In general, you need to set the turbulence intensity and length scale explicitly for large inlets in external flow problems. If you do not have a value for the length scale, you can use a length scale based on a typical length of the object, over which the fluid is flowing. In this case, you will choose a turbulence length scale which is one-tenth of the diameter of the vent.

Note: In selecting boundary conditions, where the flow direction changes, or is unknown, an opening boundary condition can be used. An opening boundary condition allows flow to both enter and leave the fluid domain during the solution.

Inlet Boundary

Create a Wind inlet boundary condition and apply the following settings Cart. Vel. Components

Mass and Momentum > U      1 [m s^-1] Mass and Momentum > V                       0 [m s^-1] Mass and Momentum > W                     0 [m s^-1] Turbulence Intensity and Length Scale Value 0.05

Eddy Len. Scale 0.25 [m]

Additional Variables > smoke > Value 0 [kg m^-3]

Opening Boundary

Create an Atmosphere opening boundary and apply the following settings Opening Pres. and Dirn

Mass and Momentum > Relative Pressure    0 [Pa]

Flow Direction > Option> Normal to Boundary Condition Turbulence Intensity and Length Scale

Value 0.05

Eddy Len. Scale 0.25 [m]

Additional Variables > smoke > Value 0 [kg m^-3]

Inlet for the Vent

Create Vent inlet boundary condition and apply the following settings Normal Speed 0.01 [m s^-1]

Turbulence intensity and Eddy Viscosity Ratio Additional Variables > smoke > Value 0 [kg m^-3]

Setting Initial Values

Click Global Initialization and select Turbulence Eddy Dissipation.

Setting Solver Control and run

ANSYS CFX-Solver has the ability to calculate physical timestep size for steady-state problems. If you do not know the time step size to set for your problem, you can use the Auto Timescale option.

Select the maximum iterations 75 and writing the Solver (.def) file. Set the convergence criteria Max Value to 10-6

Monitor the residual plots for six equations: U - Mom, V - Mom, W - Mom, P - Mass, K-TurbKE and E-Diss.K. The Momentum and Mass tab contains four of the plots and the other two are under Turbulence Quantities. The variable smoke is also plotted but registers no values since it is not initialized.

When the steady state simulation is completed, keep the results file .res in a safe place. This file will be used as an initial boundary condition for the transient run.

Now reload the simulation into ANSYS CFX-Pre to define the transient simulation.

Opening the Existing Simulation:

1. Start ANSYS CFX-Pre.
2. Select File > Open Simulation and open the simulation file CircVentIni.cfx.
3. Save Simulation as CircVent.cfx and save the file

Modifying the Simulation Type

In this step, you will make the problem transient. Later, you will set the concentration of smoke to rise exponentially with time, so it is necessary to ensure that the interval between the timesteps is smaller at the beginning of the simulation than at the end.

Click Simulation type and change the option to Transient with

Time Duration > Total Time 30 [s] and Time Steps as 4*0.25, 2*0.5, 2*1.0, 13*2.0 [s] Set the initial time to 0 [s]

Modifying the Boundary Conditions

The only boundary condition which needs altering is the Vent boundary condition. In the steady- state calculation, this boundary had a small amount of air flowing through it. In the transient calculation, more air passes through the vent and there is a time-dependent concentration of smoke in the air. This is initially zero but builds up to a larger value. The smoke concentration will be specified using the CFX Expression Language.

Modify the Vent inlet boundary condition as

Mass and Momentum > Normal Speed 0.2 [m s^-1]

You need to create an expression for smoke concentration. The concentration is zero for time t=0 and builds up to a maximum of 1 kg m^-3.

Create new expressions: TimeConstant = 3 [s], FinalConcentration =1 [kg m^-3] and ExpFunction = FinalConcentration*abs(1-exp(-t/TimeConstant))

Next you will visualize how the expressions have scheduled the concentration of smoke issued from the vent.

In the next step, apply the expression ExpFunction to the additional variable smoke as it applies to the boundary Vent.

Initialization Values

The steady state solution that you have finished calculating is used to supply the initial values to the ANSYS CFX-Solver.

Modifying the Solver Control

Set Convergence Control > Max. Coeff. Loops to 3.

Output Control

To allow results to be viewed at different timesteps, it is necessary to create transient results files at specified times. The transient results files do not have to contain all solution data. In this step, you will create minimal transient results files.

To Create Minimal Transient Results Files

1. From the main menu, select Insert > Solver > Output Control.
2. Click the Trn Results tab.
3. Click Add new item and then click OK to accept the default name for the object.

This creates a new transient results object. Each object can result in the production of many transient results files.

Set the pressure, velocity, and smoke with output frequency 1, 2, and 3 sec to Transient Results 1. Now create a second item with the default name Transient Results 2 and set pressure, velocity and smoke with time interval 4 sec.

A transient results file will be produced every 4 s (including 0 s) and at 1 s, 2 s and 3 s. The files will contain no mesh and data for only the three selected variables. This reduces the size of the minimal results files. A full results file is always written at the end of the run.

Obtaining a Solution to the Transient Problem

You have to use the results file from the steady state solution as a initial file for the transient run

Post-processing the results 

1. Draw the velocity vectors at the mid plane of the circular vent and show the direction of smoke.
   1. Draw the velocity vectors at the mid plane of the circular vent and describe the flow pattern near to the Chimney.
   2. Draw the pressure contours at the mid plane of the circular vent.
   3. Draw the velocity concours at the mid plane of the circular vent.
   4. Describe the flow coming out from chimney and mixing with air using velocity vectors and pressure contours.
   5. Draw the iso-surface of smoke with a value = 0.005 kg m-3 and show the spread of the smoke at the end of transient simulation.

Requirements:

1. Provide all screenshots of all the new variables and expressions in the model.       (10 marks)
2. Explain and show all the screen shots of setting the transient model in CFX-Pre.    (15 marks)
3. Provide a screenshot of the convergence.                                                                 (5 marks)
4. Post-processing of the results:
   • Draw the velocity vectors at the mid plane of the circular vent and show the direction of smoke (10 marks)
   • Draw the velocity vectors at the mid plane of the circular vent and describe the flow pattern near to the Chimney (10 marks)
   • Draw the pressure contours at the mid plane of the circular vent (10 marks)
   • Draw the velocity concours at the mid plane of the circular vent (10 marks)
   • Describe the flow coming out from chimney and mixing with air using velocity vectors and pressure contours (10 marks)

• Draw the iso-surface of smoke with a value = 0.005 kg m-3 and show the spread of the smoke at the end of transient simulation (10 marks)

1. Create a 5-minute demonstration                                                                             (10 marks)

Select any one of the questions above. Create and submit a Loom video with webcam ON (showing your face) and screen capture showing how you use ANSYS CFX software to complete the tasks. The video length should NOT be more than 5 minutes. So, practice well before recording.

Important: This video is one of the mandatory requirements. If you do not submit this video, your assessment marks will be zero.

The guidelines on how to create a Loom video: https://www.loom.com/share/159e007948ce4cb8acfee8a99615f388

The requirements are as follows:

• Select any one of the above questions in this assessment.
  • Create a Loom Video: (a) Introduce yourself (b) Demonstrate how you use ANSYS CFX software and explain (speak) the solution/simulation while you are running it.
  • Paste the link of your recording within your assessment report.

Microsoft Word Document and Circular Vent Model Submission Process

There is no word limit for the Microsoft Word document report. However, your answer should be specific. See the marks distribution and put emphasis accordingly.

You must submit the following executable files in the following format

MME505A_Ass3-Pract_StudentNumber_StudentName.cfx MME505A_Ass3-Pract_StudentNumber_StudentName.def MME505A_Ass3-Pract_StudentNumber_StudentName.res

The Word document (docx) and the circular vent model (.cfx, .def, .res) must be uploaded into the assessment submission box on Moodle, which may be accessed on the unit page.

This assessment is not to be uploaded onto TurnItIn.

END OF ASSESSMENT