Chapter 32: Fourier Transformation Method for an Inlet Disturbance Case

This tutorial includes:

Important:  This tutorial requires file TimeInletDistIni_001.res, which is produced by following tutorial Time Transformation Method for an Inlet Disturbance Case.

 

32.1. Tutorial Features

In this tutorial you will learn about:

Component

Feature

Details

CFX-Pre

User Mode

Turbo Wizard

General Mode

Analysis Type

Transient Blade Row

Fluid Type

Air Ideal Gas

Domain Type

Single Domain

Stationary Frame

Turbulence Model

k-Epsilon

Heat Transfer

Total Energy

Boundary Conditions

Inlet (Subsonic)

Outlet (Subsonic)

CFD-Post

Plots

Contour

Animation

 

32.2. Overview of the Problem to Solve

The goal of this tutorial is to set up a transient blade row calculation to model an inlet disturbance (frozen gust) using the Fourier Transformation model. The tutorial uses an axial turbine to illustrate the basic concepts of setting up, running, and monitoring a transient blade row problem in Ansys CFX. The full geometry of the axial rotor-stator stage contains 21 stator blades and 28 rotor blades.

In this tutorial, rotational phase-shifted periodic boundaries are used to enable only a small section of the full geometry to be modeled. The schematic below shows three stator blades along with the profile boundary showing a disturbance in the total temperature of the flow:

The geometry to be modeled consists of the stator blade row. When using the Fourier Transformation model, two passages of the bladed geometry must be used. This is required to enable a clean signal to accumulate at the sampling interface between the two passages where the Fourier coefficients will also be accumulated. In the stator blade component, a 34.28° section is being modeled (2*360°/21 blades). The rotor is upstream of the stator and creates a disturbance in the total temperature of the flow, which is then imposed at the inlet.

The flow is modeled as being turbulent and compressible. The inlet boundary condition serves to model the disturbance coming from the upstream rotor. It consists of a total temperature Gaussian profile with a pitch of 12.86° (360°/28 blades) and rotating about the Z axis at 6300 [rev min^-1]. The outlet boundary condition is a static pressure profile. The inlet and outlet boundary profiles are provided in .csv files. The outlet boundary profile was obtained from a previous simulation of a downstream stage.

When starting a new run, it is good practice to initialize Transient Blade Row simulations using results from steady-state cases. In this case, you will incorporate the steady-state results obtained from a previous tutorial. You will also use the Turbomachinery wizard feature, which facilitates the setup of a Fourier Transformation simulation. In order to do this, you have to:

  1. Define the Transient Blade Row simulation using the Turbomachinery wizard in CFX-Pre.

  2. Import the stator mesh, which was created in Ansys TurboGrid.

  3. Enter the basic model definition.

  4. Set the profile boundary conditions using CFX-Pre in General mode.

  5. Run the transient blade row simulation using the steady-state results from Time Transformation Method for an Inlet Disturbance Case as an initial guess.

 

32.3. Preparing the Working Directory

  1. Create a working directory.

    Ansys CFX uses a working directory as the default location for loading and saving files for a particular session or project.

  2. Download the fourier_inlet_disturbance.zip file here .

  3. Unzip fourier_inlet_disturbance.zip to your working directory.

    Ensure that the following tutorial input files are in your working directory:

  4. Set the working directory and start CFX-Pre.

    For details, see Setting the Working Directory and Starting Ansys CFX in Stand-alone Mode.

 

32.4. Defining a Transient Blade Row Case in CFX-Pre

The following sections describe the transient blade row simulation setup in CFX-Pre.

This tutorial uses the Turbomachinery wizard in CFX-Pre. This preprocessing mode is designed to simplify the setup of turbomachinery simulations.

  1. In CFX-Pre, select File > New Case.

  2. Select TurboMachinery and click OK.

  3. Select File > Save Case As.

  4. Under File name, type FourierInletDist.

  5. Click Save.

 

32.4.1. Basic Settings

  1. In the Basic Settings panel, configure the following:

    Setting

    Value

    Machine Type

    Axial Turbine

    Axes

    > Rotation Axis

     

    Z

    Analysis Type

    > Type

     

    Transient Blade Row

    Analysis Type

    > Method

     

    Fourier Transformation

  2. Click Next.

 

32.4.2. Components Definition

The Fourier Transformation method requires two stator blade passages. You will define a new component and import the stator mesh.

  1. Right-click in the blank area and select Add Component from the shortcut menu.

  2. Create a new component of type Stationary, named S1 and click OK.

  3. Configure the following setting(s):

    Setting

    Value

    Mesh

    > File

     

    TBRInletDistStator.gtm[a]

    1. You may have to select the CFX Mesh (*gtm *cfx) option under Files of type.

  4. Expand the Passages and Alignment section.

  5. Click Edit.

  6. Configure the following setting(s):

    Setting

    Value

    Passages and Alignment

    > Passages to Model

     

    2

  7. Click Done

    You will see that the stator blade passage is correctly replicated and the resulting mesh now contains two stator blade passages. This will also create the Sampling Interface (S1 Internal Interface 1) required for the Fourier Transformation model.

  8. Click Next.

 

32.4.3. Physics Definition

In this section, you will set properties of the fluid domain and some solver parameters.

  1. In the Physics Definition panel, configure the following setting(s):

    Setting

    Value

    Fluid

    Air Ideal Gas

    Model Data

    > Reference Pressure

     

    0 [atm][a]

    Model Data

    > Heat Transfer

     

    Total Energy

    Model Data

    > Turbulence

     

    k-Epsilon

    Inflow/Outflow Boundary Templates

    > P-Total Inlet P-Static Outlet

     

    (Selected)

    Inflow/Outflow Boundary Templates

    > Inflow

    > P-Total

     

     

    200000 [Pa]

    Inflow/Outflow Boundary Templates

    > Inflow

    > T-Total

     

     

    500 [K] [b]

    Inflow/Outflow Boundary Templates

    > Inflow

    > Flow Direction

     

     

    Cylindrical Components

    Inflow/Outflow Boundary Templates

    > Inflow Direction (a,r,t)

     

    1, 0, -0.4

    Inflow/Outflow Boundary Templates

    > Outflow

    > P-Static

     

     

    175000 [Pa] [b]

    1. To define the simulation using absolute pressure, set this value to 0 atm.

    2. These values are temporary. They will be replaced with profile data later in the tutorial.

  2. Click Next.

    Under the Interface Definition section you can observe that both the Fourier coefficient sampling interface S1 Internal Interface 1 as well as the phase shifted interface S1 to S1 Periodic 1 are automatically created.

  3. Click Next.

 

32.4.4. Disturbance Definition

In this section, you will specify the periodicity of the disturbance being imposed. In this case the inlet profile has a pitch of 12.857 [Degrees] or 1/28 of the wheel, so you need to specify 28 for the value of Passages in 360.

  1. Configure the following setting(s):

    Setting

    Value

    Disturbances

    > External Boundary

    > Passages in 360

     

     

    28

  2. Continue clicking Next until the Final Operations panel is reached.

  3. Ensure that Operation is set to Enter General Mode because you will continue to define the simulation through settings not available in the TurboMachinery wizard.

  4. Click Finish.

Note:  You may ignore the physics validation errors for the moment. You will correct these errors in the steps that follow.

 

32.4.5. Modifying the Fluid Model Settings

You will include additional settings to improve the accuracy of the simulation.

  1. Edit S1.

  2. Configure the following setting(s):

    Tab

    Setting

    Value

    Fluid Models

    Heat Transfer

    > Incl. Viscous Work Term

     

    (Selected)

    Turbulence

    > High Speed (compressible) Wall Heat Transfer Model

     

    (Selected)

  3. Click OK.

 

32.4.6. Initializing Profile Boundary Conditions

The inlet and outlet boundary conditions are defined using profiles in your working directory. Boundary profile data must be initialized before they can be used for boundary conditions.

  1. Select Tools > Initialize Profile Data.

    The Initialize Profile Data dialog box appears.

  2. Beside Profile Data File, click Browse  .

    The Select Profile Data File dialog box appears.

  3. From your working directory, select TBRInletDistOutlet.csv.

  4. Click Open.

  5. Click OK.

    The outlet profile data is read into memory.

    Next, you will prepare the inlet profile. Since the supplied profile file, TBRInletDistInlet.csv, only covers a single passage, you need to expand the profile so that it covers at least both passages. In this case you will expand the profile so that it covers the full wheel.

  6. Select Tools > Edit Profile Data.

    The Edit Profile Data dialog box appears.

  7. Under Source Profile, click Browse  .

    The Select Profile Data File dialog box appears.

  8. From your working directory, select TBRInletDistInlet.csv and click Open.

  9. Set Write to Profile to TBRInletDistInlet_FullWheel.csv.

  10. Ensure that Initialize New Profile After Writing is selected so that the inlet profile data will be automatically initialized using the expanded profile.

  11. In the Transformations frame, click Add new item  , set Name to Transformation 1, and click OK.

  12. Configure the following setting(s):

    Setting

    Value

    Transformation 1

    > Option

     

    Expansion

    Transformation 1

    > Expansion Definition

    > Rotation Option

     

     

    Principal Axis

    Transformation 1

    > Expansion Definition

    > Axis

     

     

    Z

    Transformation 1

    > Expansion Definition

    > Passages in Profile

     

     

    1

    Transformation 1

    > Expansion Definition

    > Passages in 360

     

     

    28

    Transformation 1

    > Expansion Definition

    > Expansion Option

     

     

    Expand to Full Circle

    Transformation 1

    > Expansion Definition

    > Theta Offset

     

     

    0 [degree]

  13. Click OK.

Note:  After profile data has been initialized from a file, the profile data file should not be deleted or otherwise removed from its directory. By default, the full file path to the profile data file is stored in CFX-Pre, and the profile data file is read directly by CFX-Solver each time the solver is started or restarted.

 

32.4.7. Creating the Local Rotating Coordinate Frame

Create a local rotating coordinate frame that will be applied to the inlet boundary in order to cause it to rotate:

  1. Select Insert > Coordinate Frame.

  2. Accept the default name and click OK.

  3. Configure the following setting(s):

    Setting

    Value

    Option

    Axis Points

    Coord Frame Type

    Cartesian

    Ref. Coordinate Frame

    Coord 0

    Origin

    0, 0, 0

    Z Axis Point

    0, 0, 1

    X-Z Plane Pt

    1, 0, 0

    Frame Motion

    (Selected)

    Frame Motion

    > Option

     

    Rotating

    Frame Motion

    > Angular Velocity

     

    6300 [rev min^-1]

    Frame Motion

    > Axis Definition

    > Option

     

     

    Coordinate Axis

    Frame Motion

    > Axis Definition

    > Rotation Axis

     

     

    Global Z

  4. Click OK.

 

32.4.8. Modifying Inlet and Outlet Boundary Conditions

Here, you will apply profiles to the inlet and outlet boundary conditions. In addition to this, you will also be applying the local rotating frame to the inlet boundary.

  1. Edit S1 Inlet.

  2. Configure the following setting(s):

    Tab

    Setting

    Value

    Basic Settings

    Profile Boundary Conditions

    > Use Profile Data

     

    (Selected)

    Profile Boundary Setup

    > Profile Name

     

    inletTo

  3. Click Generate Values.

You can create a moving disturbance by applying a moving coordinate frame to a boundary. Add rotational motion to the boundary condition values on the inlet by applying the local rotating coordinate frame that you made earlier:

  1. Configure the following setting(s):

    Tab

    Setting

    Value

    Basic Settings

    Coordinate Frame

    (Selected)

    Coordinate Frame

    > Coordinate Frame

     

    Coord 1

  2. Click OK.

  3. Edit S1 Outlet.

  4. Configure the following setting(s):

    Tab

    Setting

    Value

    Basic Settings

    Profile Boundary Conditions

    > Use Profile Data

     

    (Selected)

    Profile Boundary Setup

    > Profile Name

     

    outlet

  5. Click Generate Values.

  6. Click OK.

 

32.4.9. Setting up a Transient Blade Row Model

In this section, you will make some modifications to the Transient Blade Row Models object:

  1. Edit Transient Blade Row Models.

  2. Configure the following setting(s):

    Setting

    Value

    Fourier Transformation

    > Fourier Transformation 1

    > Signal Motion

    > Option

     

     

     

    Rotating

    Fourier Transformation

    > Fourier Transformation 1

    > Signal Motion

    > Coordinate Frame

     

     

     

    Coord 1

    Transient Method

    > Time Period

    > Option[a]

     

     

    Automatic

    Transient Method

    > Time Steps

    > Timestep Multiplier[b][c]

     

     

    20

    Transient Method

    > Time Duration

    > Option

     

     

    Number of Periods per Run

    Transient Method

    > Time Duration

    > Periods per Run

     

     

    10

    1. The passing period is automatically calculated as: 2 * pi / (Passages in 360 * Signal Angular Velocity). The Passing Period setting cannot be edited.

    2. The number of time steps per period should always be larger than 2 * Number of Fourier Coefficients + 1 to be used for postprocessing.

    3. The time step size is also automatically calculated as: Passing Period / Number of Timesteps per Period. The Timestep setting cannot be edited.

  3. Click OK.

 

32.4.10. Setting the Output Control and Creating Monitor Points

For transient blade row calculations, a minimal set of variables are selected to be computed using Fourier coefficients. It is convenient to postprocess total (stagnation) variables as well. Here, you will add Total Pressure and Total Temperature variables to the default list.

Monitor points can be used to effectively compare the Fourier Transformation results against a reference case. They provide useful information on the quality of the reference phase and frequency produced in the simulation. They should also be used to monitor convergence and, as the simulation converges, the user points should display a periodic pattern.

Note:  When comparing to a reference case, make sure monitor points are placed in the same relative locations with respect to the initial configuration in both cases.

It is important to check that the solver equations are being solved correctly. Monitoring pressure provides feedback on the momentum equations while monitoring temperature provides feedback on the energy equations.

Set up the output control and create monitor points as follows:

  1. Click Output Control  .

  2. Click the Trn Results tab.

  3. Configure the following setting(s):

    Setting

    Value

    Transient Blade Row Results

    > Extra Output Variables List

     

    (Selected)

    Transient Blade Row Results

    > Extra Output Variables List

    > Extra Output Var. List

     

     

    Total Pressure, Total Temperature[a]

    1. Click Multi-select from extended list   and hold down the Ctrl key while selecting each of the listed variables.

  4. Click Apply.

  5. Click the Monitor tab.

  6. Configure the following setting(s):

    Setting

    Value

    Monitor Objects

    > Monitor Points and Expressions

     

    Create a monitor point named Monitor Point 1[a]

    Monitor Objects

    > Monitor Points and Expressions

    > Monitor Point 1

    > Option

     

     

     

    Cylindrical Coordinates

    Monitor Objects

    > Monitor Points and Expressions

    > Monitor Point 1

    > Output Variables List

     

     

     

    Pressure, Temperature, Total Pressure, Total Temperature[b]

    Monitor Objects

    > Monitor Points and Expressions

    > Monitor Point 1

    > Position Axial Comp.

     

     

     

    0.1 [m]

    Monitor Objects

    > Monitor Points and Expressions

    > Monitor Point 1

    > Position Radial Comp.

     

     

     

    0.32 [m]

    Monitor Objects

    > Monitor Points and Expressions

    > Monitor Point 1

    > Position Theta Comp.

     

     

     

    5 [degree]

    1. To create a new item, you must first click the Add new item   icon, then enter the name as required and click OK.

    2. Click Multi-select from extended list   and hold down the Ctrl key while selecting each of the listed variables.

  7. Create additional monitor points with the same output variables. The names and Cylindrical coordinates are listed below:

    Name

    Cylindrical Coordinates

    Monitor Point 2

    0.16 [m], 0.32 [m], 4 [degree]

    Monitor Point 3

    0.16 [m], 0.32 [m], 11.6 [degree]

    Monitor Point 4

    0.06 [m], 0.32 [m], -6.5 [degree]

  8. Click OK.

 

32.4.11. Setting the Execution Control

Here you will prepare the case for execution and initialize the solution with steady-state results. Instead of obtaining steady-state results by setting up and running the steady-state solution for this case, which has a double passage configuration, you will import a results file from another steady-state simulation, which happens to have a single passage (see Defining a Steady-state Case in CFX-Pre). Because that other simulation involves only a single passage, you will use replication control settings to apply those results to both passages in this simulation.

  1. In the Outline tree view, right-click Simulation Control and select Insert > Execution Control.

  2. Configure the following setting(s):

    Tab

    Setting

    Value

    Run Definition

    Run Settings

    > Double Precision

    (Selected)

    Initial Values

    Initial Values Specification

    (Selected)

    Initial Values Specification

    > Initial Values

     

    Initial Values 1

    Initial Values Specification

    > Initial Values

    > Initial Values 1

    > File Name

     

     

     

    TimeInletDistIni_001.res [a]

    Initial Values Specification

    > Initial Values

    > Initial Values 1

    > Interpolation Mapping

     

     

     

    Create an interpolation mapping object named Interpolation Mapping 1 [b]

    Initial Values Specification

    > Initial Values

    > Initial Values 1

    > Interpolation Mapping

    > Interpolation Mapping 1

    > Replication Control

     

     

     

     

     

    (Selected)

    Initial Values Specification

    > Initial Values

    > Initial Values 1

    > Interpolation Mapping

    > Interpolation Mapping 1

    > Replication Control

    > Passages in 360

     

     

     

     

     

     

    21

    Initial Values Specification

    > Initial Values

    > Initial Values 1

    > Interpolation Mapping

    > Interpolation Mapping 1

    > Replication Control

    > Total Num. Instances

     

     

     

     

     

     

    2

    Initial Values Specification

    > Initial Values Control

     

    (Selected)

    Initial Values Specification

    > Initial Values Control

    > Continue History From

     

     

    (Selected)

    1. You can select a file to load by clicking Browse   and selecting the appropriate file from your working directory, then clicking Open.

    2. Add an item by clicking the Add New Item  icon and clicking OK to accept the default name.

  3. Click OK.

 

32.4.12. Writing the CFX-Solver Input (.def) File

  1. Click Define Run  .

    The CFX-Solver input file FourierInletDist.def is created.

    CFX-Solver Manager automatically starts and, on the Define Run dialog box, Solver Input File is set.

  2. Ignore the message and click Yes to continue.

  3. If using stand-alone mode, quit CFX-Pre, saving the simulation (.cfx) file at your discretion.

 

32.5. Obtaining a Solution to the Transient Blade Row Case

Ensure that the Define Run dialog box is displayed in CFX-Solver Manager.

  1. Click Start Run.

    CFX-Solver runs and attempts to obtain a solution. This can take a long time depending on your system. Eventually a dialog box is displayed.

    Note:

    • Before the simulation begins, the "Transient Blade Row Post-processing Information" summary in the CFX-Solver Output file displays the time step range over which the solver will accumulate the Fourier coefficients.

    • The CFX-Solver Output file contains a "Fourier Transformation Information" summary as well as the time step at which the full Fourier Transformation Model is activated.

    • Monitor points of similar values can be grouped together by right-clicking to the right of the User Points tab, selecting New Monitor, and clicking OK. In the New Monitor dialog box, you can set the name for the new monitor point and select the variables you want to monitor in the Monitor Properties dialog box.

    • After the simulation has proceeded for some time, observe the periodic nature of the monitor point values.

    • If the monitor points do not establish a periodic nature in a Fourier Transformation run, you can try applying frequency filtering.

      Frequency filtering is a powerful tool to deal with instabilities. It filters out all frequencies that are not harmonics of the blade passing frequency (or blade vibration frequency for flutter cases) and that could trigger instabilities. These typically occur in elongated domains where the amplitude of the periodic signal becomes very weak at the furthest point away from the source of the disturbance.

      To apply frequency filtering in CFX-Pre, edit the Transient Blade Row Models object; in the details view, on the Advanced Options tab, select Fourier Transformation Control > Frequency Filtering.

  2. When CFX-Solver is finished, select the check box next to Post-Process Results.

  3. Click OK.

 

32.6. Viewing the Fourier Transformation Results in CFD-Post

In this section, you will work with the Fourier coefficients compressed data in transient blade row analysis. The solution variables are automatically set to the transient position corresponding to the end of the simulation.

 

32.6.1. Creating a Turbo Surface

  1. You will see a dialog box named Transient Blade Row Post-processing. Click OK.

  2. Click the Turbo tab.

  3. Click Initialize All Components.

  4. A dialog box will ask if you want to auto-initialize all turbo components. Click Yes.

  5. Select Insert > Location > Turbo Surface.

  6. Change the name to Span 50.

  7. Click OK.

  8. Click Apply.

  9. Turn off the visibility of Span 50.

 

32.6.2. Creating a Contour Plot

  1. Click Insert > Contour and accept the default name.

  2. Configure the following setting(s):

    Tab

    Setting

    Value

    Geometry

    Locations

    Span 50

    Variable

    Temperature

    Range

    User Specified

    Min

    465 [K]

    Max

    605 [K]

    # of Contours

    21

  3. Click Apply.

 

32.6.3. Animating Temperature

Create an animation of the contour plot:

  1. Click Animation  .

    The Animation dialog box appears.

  2. Set Type to Timestep Animation.

  3. Click To First Timestep   in order to load the first time step.

  4. Click Play the animation  .

  5. When you have finished, quit CFD-Post.