The boundaries for the turbomachinery topology are as follows (see Figure 14.25: Turbomachinery Boundary Types):

Figure 14.25: Turbomachinery Boundary Types

After completing the turbo topology definition detailed in Defining the Turbomachinery Topology, the following turbomachinery-specific variables are available for postprocessing in Ansys Fluent:

These variables are contained in the Mesh... category of the variable selection drop-down list. See Field Function Definitions for their definitions.

Turbo surfaces are created using the Iso-Surface Dialog Box. The Turbomachinery-Specific Variables are available under the Mesh... category for the Surface of Constant drop-down list. For details on creating iso-surfaces, see Iso-surfaces.

Figure 14.26: Iso-Surface Dialog Box for Turbo Surfaces

The turbo surfaces will be used to display contours or velocity vectors. It is recommended that you create an iso-surface for each zone separately.

In turbomachinery analysis, it is common that only a portion of the machine is modeled. Therefore, in order to visualize the periodic repeating pattern of a turbomachine, you can use Periodic Instancing to replicate passages around an axis.

Periodic Instancing is accessed from the Turbomachinery ribbon:

  Domain → Turbomachinery → Periodic Instancing...

For more information on Periodic Instancing, see Mirroring and Periodic Repeats and Periodic Instancing Dialog Box.

The real representation of Fourier coefficients is used and the following and are provided in the list of variables available to postprocess. This representation allows for multiple (M) base frequencies to be captured (), each having its own harmonic content represented by . The Fourier coefficient accumulation method used is based on the moving averages. Its equivalent closed period integration form is given below where the integration period must be the common period between all frequencies captured.

Fourier coefficient definitions

(14–15)

The reconstructed signal can then be written as:

(14–16)

and is provided as a Fourier related variable.

Additional variables available for post-processing are as follows:

(14–17)

(14–18)

where for example is the time step change in coefficient .

Utilizing General Fourier Coefficient Postprocessing requires the following pre-requisites.

There are two distinct scenarios for general Fourier coefficient postprocessing:

There are two methods for creating spectral content for Fourier coefficient postprocessing/

There are additional settings found under the console command /define/turbo-model/graphics-extra-settings for changing defaults associated with spectral content creation of the graphics/post-processing Fourier coefficients. In most cases, the default values are sufficient and need not be changed. A sample dialog is shown below.

/define/turbo-model> graphics-extra-settings
Fourier coefficients relaxation [1] 1
Number of initialization cycles [0] 0
Use the default number of Fourier harmonics for each fundamental frequency? [yes] yes
Enter the default number of Fourier harmonics of each fundamental frequency: [10] 

The following options are available:

If the case is multi-row, you can specify a frequency selection algorithm option. This provides an easier way to select the fundamental base frequency for each row.

Enter the multi-row frequency selection algorithm option 
(1- nearest neighbors, 2- extended neighbors, 3- user-selected)

There are also additional commands to list, append, and delete postprocessing spectral content:

/define/turbo-model/list-graphics-spectral-content

/define/turbo-model/append-graphics-spectral-content

/define/turbo-model/delete-graphics-spectral-content

When Fourier coefficients are defined, they will be available for postprocessing. They are grouped by flow variable and labeled to reflect the mode and frequency.

Figure 14.28: Fourier Coefficients Available for Graphical Postprocessing

Once you have defined your turbomachinery topologies, as described in Defining the Turbomachinery Topology, you can report a number of turbomachinery quantities, including mass flow, swirl number, torque, and efficiencies.

To report turbomachinery quantities in Ansys Fluent, you will use the Turbo Report Dialog Box (Figure 14.29: The Turbo Report Dialog Box).

 Results → Model Specific → Turbo Post → Report...

Figure 14.29: The Turbo Report Dialog Box

The procedure for using this dialog box is as follows:

 

14.10.5.1.1. Computing Turbomachinery Quantities

 

14.10.5.1.1.1. Mass Flow

The mass flow rate through a surface is defined as follows:

(14–19)

where
	= is the area of the inlet or outlet
	= the velocity vector
	= the fluid density
	= a unit vector normal to the surface

 

14.10.5.1.1.2. Swirl Number

The swirl number is defined as follows:

(14–20)

where
	= the radial coordinate (specifically, the radial distance from the axis of rotation)
	= the tangential velocity
	= the velocity vector
	= a unit vector normal to the surface,
	= the inlet or outlet

 

14.10.5.1.1.3. Average Total Pressure

The area-averaged total pressure is defined as follows:

(14–21)

where is the total pressure and is the area of the inlet or outlet.

The mass-averaged total pressure is defined as follows:

(14–22)

where,
	= the total pressure
	= the area of the inlet or outlet
	= the velocity vector
	= the fluid density
	= a unit vector normal to the surface

 

14.10.5.1.1.4. Average Total Temperature

The area-averaged total temperature is defined as follows:

(14–23)

where is the total temperature and is the area of the inlet or outlet.

The mass-averaged total temperature is defined as follows:

(14–24)

where,
	= the total temperature
	= the area of the inlet or outlet
	= the velocity vector
	= the fluid density
	= a unit vector normal to the surface

 

14.10.5.1.1.5. Average Flow Angles

The area-averaged flow angles are defined as follows:

(14–25)

in the radial direction, and

(14–26)

in the tangential direction, where , , and represent the axial, radial, and tangential velocities, respectively.

The mass-averaged flow angles are defined as follows:

(14–27)

in the radial direction, and

(14–28)

in the tangential direction.

 

14.10.5.1.1.6. Passage Loss Coefficient

The engineering loss coefficient is defined as follows:

(14–29)

where,
	= the mass-averaged total pressure at the inlet
	= the mass-averaged total pressure at the outlet
	= the density of the fluid
	= the mass-averaged velocity magnitude at the inlet

The normalized loss coefficient is defined as follows:

(14–30)

where is the mass-averaged static pressure at the outlet.

 

14.10.5.1.1.7. Axial Force

The axial force on the rotating parts is defined as follows:

(14–31)

where,
	= the surfaces comprising all rotating parts
	= the total stress tensor (pressure and viscous stresses)
	= a unit vector normal to the surface
	= a unit vector parallel to the axis of rotation

 

14.10.5.1.1.8. Torque

The torque on the rotating parts is defined as follows:

(14–32)

where,
	= the surfaces comprising all rotating parts
	= the total stress tensor
	= a unit vector normal to the surface
	= the position vector
	= a unit vector parallel to the axis of rotation

 

14.10.5.1.1.9. Efficiencies for Pumps and Compressors

The definitions of the efficiencies for compressible and incompressible flows in pumps and compressors are described in this section. Efficiencies for turbines are described later in this section. Consider a pumping or compression device operating between states 1 and 2 as illustrated in Figure 14.30: Pump or Compressor. Work input to the device is required to achieve a specified compression of the working fluid.

Figure 14.30: Pump or Compressor

Assuming that the processes are steady state, steady flow, and that the mass flow rates are equal at the inlet and outlet of the device (no film cooling, bleed air removal, and so on), the efficiencies for incompressible and compressible flows are as described in the following subsections.

 

14.10.5.1.1.9.1. Incompressible Flows

For devices such as liquid pumps and fans (2D boundaries or 3D cell zones) at low speeds, the working fluid can be treated as incompressible. The efficiency of a pumping process with an incompressible working fluid is defined as the ratio of the head rise achieved by the fluid to the power supplied to the rotor/impeller. This can be expressed as follows:

(14–33)

where,
	= volumetric flow rate
	= total pressure
	= net torque acting on the rotor/impeller
	= rotational speed

This definition is sometimes called the “hydraulic efficiency”. Often, other efficiencies are included to account for flow leakage (volumetric efficiency) and mechanical losses along the transmission system between the rotor and the machine providing the power for the rotor/impeller (mechanical efficiency). Incorporating these losses then yields a total efficiency for the system.

 

14.10.5.1.1.9.2. Compressible Flows

For gas compressors that operate at high speeds and high pressure ratios, the compressibility of the working fluid must be taken into account. The efficiency of a compression process with a compressible working fluid is defined as the ratio of the work required for an ideal (reversible) compression process to the actual work input. This assumes the compression process occurs between states 1 and 2 for a given pressure ratio. In most cases, the pressure ratio is the total pressure at state 2 divided by the total pressure at state 1. If the process is also adiabatic, then the ideal state at 2 is the isentropic state.

From the foregoing definition, the efficiency for an adiabatic compression process can be written as

(14–34)

where,
	= total enthalpy at 1
	= actual total enthalpy at 2
	= isentropic total enthalpy at 2

If the specific heat is constant, Equation 14–34 can also be expressed as

(14–35)

where,
	= total temperature at 1
	= actual total temperature at 2
	= isentropic total temperature at 2

Using the isentropic relation

(14–36)

where is the ratio of specific heats specified in the Reference Values Task Page.

The efficiency can be written in the compact form

(14–37)

Note that this definition requires data only for the actual states 1 and 2.

Compressor designers also make use of the polytropic efficiency when comparing one compressor with another. The polytropic efficiency is defined as follows:

(14–38)

 

14.10.5.1.1.10. Efficiencies for Turbines

Consider a turbine operating between states 1 and 2 in Figure 14.31: Turbine. Work is extracted from the working fluid as it expands through the turbine. Assuming that the processes are steady state, steady flow, and that the mass flow rates are equal at the inlet and outlet of the device (no film cooling, bleed air removal, and so on), turbine efficiencies for incompressible and compressible flows are as described below.

Figure 14.31: Turbine

 

14.10.5.1.1.10.1. Incompressible Flows

The efficiency of a turbine with an incompressible working fluid is defined as the ratio of the work delivered to the rotor to the energy available from the fluid stream. This ratio can be expressed as follows:

(14–39)

where,
	= volumetric flow rate
	= total pressure
	= net torque acting on the rotor/impeller
	= rotational speed

Note the similarity between this definition and the definition of incompressible compression efficiency (Equation 14–33). As with hydraulic pumps and compressors, other efficiencies (for example, volumetric and mechanical efficiencies) can be defined to account for other losses in the system.

 

14.10.5.1.1.10.2. Compressible Flows

For high-speed gas turbines operating at large expansion pressure ratios, compressibility must be accounted for. The efficiency of an expansion process with a compressible working fluid is defined as the ratio of the actual work extracted from the fluid to the work extracted from an ideal (reversible) process. This assumes that the expansion process occurs between states 1 and 2 for a given pressure ratio. In contrast to the compression process, the pressure ratio for expansion is the total pressure at state 1 divided by the total pressure at state 2. If the process is also adiabatic, then the ideal state at 2 is the isentropic state.

From the foregoing definition, the efficiency for an adiabatic expansion process through a turbine can be written as

(14–40)

where
	= total enthalpy at 1
	= actual total enthalpy at 2
	= isentropic total enthalpy at 2

If the specific heat is constant, Equation 14–40 can also be expressed as

(14–41)

where
	= total temperature at 1
	= actual total temperature at 2
	= isentropic total temperature at 2

Using the isentropic relation

(14–42)

the expansion efficiency can be written in the compact form

(14–43)

Note that this definition requires data only for the actual states 1 and 2.

As with compressors, one may also define a polytropic efficiency for turbines. The polytropic efficiency is defined as follows:

(14–44)

Turbo averaged contours are generated as projections of the values of a variable averaged in the circumferential direction and visualized on an - plane. A sample plot is shown in Figure 14.33: Turbo Averaged Filled Contours of Static Pressure.

 

14.10.5.2.1. Steps for Generating Turbomachinery Averaged Contour Plots

You can display contours using the Turbo Averaged Contours Dialog Box (Figure 14.32: The Turbo Averaged Contours Dialog Box).

 Results → Model Specific → Turbo Post → Averaged Contours...

Figure 14.32: The Turbo Averaged Contours Dialog Box

The basic steps for generating a turbo averaged contour plot are as follows:

1. Select All or a specific predefined turbomachinery topology from the Turbo Topology drop-down list.

2. Select the variable or function to be displayed in the Contours of drop-down list. First select the desired category in the upper list; you may then select a related quantity in the lower list. (See Turbomachinery-Specific Variables for a list of turbomachinery-specific variables, and see Field Function Definitions for an explanation of the variables in the list.)

3. Specify the number of contours in the Levels field. The maximum number of levels allowed is 100.

4. Set any of the options described below.

5. Click Display to display the specified contours in the active graphics window.

The resulting display will include the specified number of contours of the selected variable, with the magnitude on each one determined by equally incrementing between the values shown in the Min and Max fields.

Note that the Min and Max values displayed in the dialog box are the minimum and maximum averaged values. These limits will in general be different from the global Domain Min and Domain Max, which are also displayed for your reference (see Figure 14.32: The Turbo Averaged Contours Dialog Box).

Figure 14.33: Turbo Averaged Filled Contours of Static Pressure

In postprocessing a turbomachinery solution, it is often desirable to display contours on surfaces of constant spanwise coordinate, and then project these contours onto a plane. This permits easier evaluation of the contours, especially for surfaces that are highly three-dimensional.

 

14.10.5.3.1. Steps for Generating Turbo 2D Contour Plots

You can display contours using the Turbo 2D Contours Dialog Box (Figure 14.34: The Turbo 2D Contours Dialog Box).

 Results → Model Specific → Turbo Post → 2D Contours...

Figure 14.34: The Turbo 2D Contours Dialog Box

The basic steps for generating a turbo 2D contour plot are as follows:

1. Specify a specific predefined turbomachinery topology using the Turbo Topology drop-down list.

2. Enter a value for the Normalized Spanwise Coordinates (0 to 1) for the spanwise surface you want to create.

   ---

   Important:  If shroud and hub are the curved surfaces, the isosurface very close to them may contain void spaces as Ansys Fluent displays only a plane cut surface.

   ---

3. Select the variable or function to be displayed in the Contours of drop-down list.

   First select the desired category in the upper list; you may then select a related quantity in the lower list. See Turbomachinery-Specific Variables for a list of turbomachinery-specific variables, and see Field Function Definitions for an explanation of the variables in the list.

4. Specify the number of contours in the Levels field. The maximum number of levels allowed is 100.

5. Click Display to display the specified contours in the active graphics window.

The resulting display will include the specified number of contours of the selected variable, with the magnitude on each one determined by equally incrementing between the values shown in the Min and Max fields.

When comparing numerical solutions of turbomachinery problems to experimental data, it is often useful to plot circumferentially averaged quantities in the spanwise and meridional directions. This section describes how to do this in Ansys Fluent.

 

14.10.5.4.1. Steps for Generating Turbo Averaged XY Plots

To create an XY plot of circumferentially averaged solution data, you will use the Turbo Averaged XY Plot Dialog Box (Figure 14.35: The Turbo Averaged XY Plot Dialog Box).

 Results → Model Specific → Turbo Post → Averaged XY Plot...

Figure 14.35: The Turbo Averaged XY Plot Dialog Box

The procedure for generating a turbo averaged XY plot are as follows:

1. Select the variable or function to be plotted in the Y Axis Function drop-down list. First select the desired category in the upper list; you may then select a related quantity in the lower list. (See Turbomachinery-Specific Variables for a list of turbomachinery-specific variables, and see Field Function Definitions for an explanation of the variables in the list.)

2. Select All or a specific predefined turbomachinery topology from the Turbo Topology drop-down list.

3. Select the variable or function to be plotted in the X Axis Function drop-down list. The choices are Hub to Casing Distance and Meridional Distance.

4. Specify the desired value in the Fractional Distance field. The definition of the fractional distance depends on your selection of X Axis Function:

   • If you select Hub to Casing Distance, the fractional distance will be Inlet to Outlet.

   • If you select Meridional Distance, the fractional distance will be Hub to Casing.

5. (optional) Modify the attributes of the axes or curves as described in Modifying Axis Attributes and Modifying Curve Attributes.

6. Click Plot to generate the XY plot in the active graphics window.

   Note that you can use any of the mouse buttons to annotate the XY plot (see Adding Text to the Graphics Window).

If you want to write the XY data to a file, follow these steps instead of Step 5 above:

1. Enable the Write to File option. The Plot button changes to a Write... button.

2. Click Write....

3. In The Select File Dialog Box, specify a name for the plot file and save it.

In some cases, that is, isosurface creation, Ansys Fluent enables you to globally set the current turbomachinery topology for your model using the Turbo Options Dialog Box (Figure 14.36: The Turbo Options Dialog Box).

 Results → Model Specific → Turbo Post → Options...

Figure 14.36: The Turbo Options Dialog Box

To set the current topology, select a topology from the Current Topology drop-down list and select OK.

You can define and evaluate performance characteristics for a turbomachine using the Turbo Performance dialog box (Figure 14.37: The Turbo Performance Dialog Box) accessed from:

 Domain → Turbomachinery → Turbo Performance...

Figure 14.37: The Turbo Performance Dialog Box

You can also access this tool by using the following console command and following the prompts:

/define/turbo-model/performance

You can compute the values of isentropic and polytropic efficiencies for a turbomachine using Named Expressions as shown in Figure 14.38: Expression Dialog Box with Efficiency Calculation.

  → User Defined → User Defined → Named Expressions → New...

Figure 14.38: Expression Dialog Box with Efficiency Calculation

Functions for both efficiency types are listed under Functions → Reduction as EfficiencyIsentropic and EfficiencyPolytropic.

Permeable boundaries of various types (for example, inlets, outlets, interfaces), as well as cut-planes and iso-surfaces, can be chosen as locations. Also, groups of surfaces for postprocessing is supported.

For the argument Inlets, you can select one or more locations of mixed types. However, multiple locations will still be treated as the single-stream inlet with properties averaged over all selected locations. This averaged inlet can be considered as the starting point for efficiency calculation. Similarly, the end point will be determined by locations selected for the argument Outlets. Options for the argument Process(Expansion and Compression) specify whether the flow expands or compresses when moving from the inlet to the outlet. Options for the argument Ratio (TotaltoTotal, TotaltoStatic and StatictoStatic) determines the values used in computing the efficiency.

After you have defined Turbo Topology (as described in Defining the Turbomachinery Topology), an additional option is available to compute efficiency using the Turbo Report dialog box as described in Generating Reports of Turbomachinery Data.

  → Results → Model Specific → Turbo Post → Report...

Figure 14.39: Turbo Report Dialog Box

By default, the Turbo Report dialog box computes the Total to Total values of isentropic and polytropic efficiencies assuming constant values of the specific heat capacity Cp and heat capacity ratio γ. You can switch to the more general equations which are implemented in the EfficiencyIsentropic and EfficiencyPolytropic functions, by using the following console command:

/report/efficiency/use-in-turbo-report?

Note that for an ideal gas with constant Cp value, the two methods give nearly identical results. However, for other cases, only the methods from the EfficiencyIsentropic and EfficiencyPolytropic functions are able to provide appropriate efficiency values.

The Turbo Performance tool uses the MassFlowAveAbs function to compute the values of total pressure and total temperatures ratios. Named Expressions for these values are defined as follows:

MassFlowAveAbs(Field,['location_1'])/MassFlowAveAbs(Field,['location_2'])

where the first argument Field is TotalPressure or TotalTemperature. For the Expansion process, the second argument ['location_1'] is the list of inlet zones and ['location_2'] is the list of outlet zones selected in the dialog box. For the Compression process, the locations are reversed with ['location_1'] being the list of outlet zones and ['location_2'] being the list of inlet zones selected in the dialog box.

The MassFlowAveAbs function is listed under Functions → Reduction in the Expression dialog box. For more information on this and other functions from the Named Expressions library, refer to Fluent Expressions Language.

The Turbo Performance tool uses the MassFlowInt function to compute mass flow at inlets and outlets. Named Expressions are defined as follows:

MassFlowInt(1, ['locations'])

The argument ['locations'] is the list of inlet or outlet zones selected in the dialog box for the definition of the corresponding expression.

The MassFlowInt function is listed under Functions → Reduction in the Expression dialog box. For more information on this and other functions from the Named Expressions library, refer to Fluent Expressions Language.