Tutorial: Solving Transonic Flow over a Turbine Blade with Turbo-Specific NRBCs Introduction The standard pressure boundary conditions for compressible flow fix specific flow variables at the boundary (e.g., static pressure at an outlet boundary). As a result, pressure waves incident on the boundary will reflect in an unphysical manner, leading to local errors. The effects are more pronounced for internal flow problems where boundaries are usually close to geometry inside the domain, such as compressor or turbine blade rows. The turbo-specific non-reflecting boundary conditions (NRBCs) permit waves to “pass” through the boundaries without spurious reflections. The method used in FLUENT is based on the Fourier transformation of solution variables at the non-reflecting boundary. This tutorial demonstrates how to do the following: • Set up and solve the turbine blade flow field using the standard pressure outlet bound- ary treatment. • Activate the turbo-specific NRBCs and solve the problem again. • Compare the results for the standard and non-reflecting pressure boundaries. Prerequisites This tutorial assumes that you are familiar with the FLUENT interface, and have a good understanding of basic setup and solution procedures. In this tutorial, you will use turbo- specific NRBCs, so you should have some experience with them. This tutorial will not cover the mechanics of using this feature. Instead, it will focus on the application of turbo-specific NRBCs to a turbine blade flow field. If you have not used this feature before, Refer section 7.23.1 : Turbo-specific Non-Reflecting Boundary Conditions in the FLUENT 6.3 User’s Guide c Fluent Inc. January 25, 2007 1
16
Embed
58799487 Tutorial Solving Transonic Flow Over a Turbine Blade With Turbo Specic NRBCs Nrbc Turbine Cascade
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Tutorial: Solving Transonic Flow over a Turbine Blade with
Turbo-Specific NRBCs
Introduction
The standard pressure boundary conditions for compressible flow fix specific flow variablesat the boundary (e.g., static pressure at an outlet boundary). As a result, pressure wavesincident on the boundary will reflect in an unphysical manner, leading to local errors. Theeffects are more pronounced for internal flow problems where boundaries are usually closeto geometry inside the domain, such as compressor or turbine blade rows.
The turbo-specific non-reflecting boundary conditions (NRBCs) permit waves to “pass”through the boundaries without spurious reflections. The method used in FLUENT is basedon the Fourier transformation of solution variables at the non-reflecting boundary.
This tutorial demonstrates how to do the following:
• Set up and solve the turbine blade flow field using the standard pressure outlet bound-ary treatment.
• Activate the turbo-specific NRBCs and solve the problem again.
• Compare the results for the standard and non-reflecting pressure boundaries.
Prerequisites
This tutorial assumes that you are familiar with the FLUENT interface, and have a goodunderstanding of basic setup and solution procedures. In this tutorial, you will use turbo-specific NRBCs, so you should have some experience with them. This tutorial will not coverthe mechanics of using this feature. Instead, it will focus on the application of turbo-specificNRBCs to a turbine blade flow field.
If you have not used this feature before, Refer section 7.23.1 : Turbo-specific Non-ReflectingBoundary Conditions in the FLUENT 6.3 User’s Guide
Solving Transonic Flow over a Turbine Blade with Turbo-Specific NRBCs
Problem Description
This tutorial considers the transonic flow around a turbine blade cascade with a shortenedexit boundary. This configuration is frequently encountered in stage analyses where thespacing between adjacent blade rows is small, and hence, the exit boundary of the upstreamrow must be placed very close to the trailing edge of the blade. Using the traditionalpressure outlet boundary treatment can lead to spurious pressure distributions on the bladesurface since the exit pressure is typically being assumed to be uniform in the blade-to-bladedirection. NRBCs can eliminate this problem by permitting pressure waves to pass throughthe boundary without reflection, thereby leading to a more accurate solution.
Note: Non-reflecting boundary conditions can only be used with the density-based solver.
Figure 1: 2D Stator Blade
Preparation
1. Copy the mesh file 2d-stator.msh to the working folder.
Solving Transonic Flow over a Turbine Blade with Turbo-Specific NRBCs
Step 4: Materials
1. Modify the properties of air.
Define −→Materials...
(a) Select fluid from the Material Type drop-down list.
(b) Select ideal-gas from the Density drop-down list under Properties.
Note: FLUENT will automatically enable solution of the energy equation whenthe ideal gas law is used.
(c) Retain the default values for all other properties.
(d) Click Change/Create and close the Materials panel.
Step 5: Operating Conditions
1. Set the operating conditions.
Define −→Operating Conditions...
Here, you will set the operating pressure is set to zero and the boundary conditioninputs for pressure will be defined in terms of absolute pressures. Boundary conditionsfor pressure should always be relative to the value of operating pressure.
(a) Enter 0 for Operating Pressure.
(b) Click OK to close the Operating Conditions panel.
Step 6: Boundary Conditions
1. Set the conditions for the inlet (pressure-inlet).
Define −→Boundary Conditions...
(a) Enter 1.5 atm for the Gauge Total Pressure.
(b) Enter 1.0 atm for the Supersonic/Initial Gauge Pressure.
(c) Enter 1.0 for the X-Component of Flow Direction.
(d) Enter 0 for the Y-Component of Flow Direction.
Note: You must use the Direction Vector specification method for the pressureinlet in order to use the NRBCs.
(e) Select Intensity and Viscosity Ratio from the Specification Method drop-down listunder Turbulence.
(f) Enter 1% for the Turbulent Intensity.
(g) Enter 1.0 for the Turbulent Viscosity Ratio.
(h) Click the Thermal tab and enter 300 K for Total Temperature.
Solving Transonic Flow over a Turbine Blade with Turbo-Specific NRBCs
3. Create an XY plot of the static pressure distribution on the blade surface.
Plot −→XY Plot...
(a) Select Pressure... and Static Pressure in the Y Axis Function drop-down lists.
(b) Select stator-blade, in the Surfaces list.
(c) Keep the default Plot Direction of X.
This will plot temperature vs. the x coordinate along the selected surface (stator-blade).
(d) Click Plot.
The resulting XY plot for static pressure distribution is displayed in Figure 14.
Static PressureFLUENT 6.3 (2d, dbns exp, ske)
Position (m)
(atm)Pressure
Static
0.060.050.040.030.020.010
1.50e+00
1.40e+00
1.30e+00
1.20e+00
1.10e+00
1.00e+00
9.00e-01
8.00e-01
7.00e-01
6.00e-01
5.00e-01
stator-blade
Figure 14: XY Plot of Static Pressure Distribution on Stator Blade
(e) Save the plot data to a file.
i. Select the Write to File option and click the Write... button to open the SelectFile dialog box.
ii. Enter pdata-nrbc.xy in the XY File text entry box and click OK to closethe dialog box.
4. Read the plot files you saved for the two solutions and compare them in a single plot(Figure 15).
Notice that the shock wave position for the NRBC case is moved closer to the trailingedge of the vane. This is due to the fact that pressure variations in the vicinity ofthe shock are permitted to pass through the exit boundary without being artificiallyconstrained by assuming a constant (uniform) exit pressure, as is the case when theNRBCs are disabled.
Solving Transonic Flow over a Turbine Blade with Turbo-Specific NRBCs
Static PressureFLUENT 6.3 (2d, dbns exp, ske)
Position (m)
(atm)Pressure
Static
0.060.050.040.030.020.010
1.60e+00
1.40e+00
1.20e+00
1.00e+00
8.00e-01
6.00e-01
4.00e-01
NRBCStandard BC
Figure 15: Comparison of XY Plots
Summary
This tutorial demonstrated the salient points of setting up and solving of a problem withFLUENT’s turbo-specific NRBCs.
It was shown that the location of the shockwave is dramatically different when standardpressure BCs are used. This is not surprising since you are forcing the static pressure at theexit to be uniform. With NRBCs, the pressure waves are not constrained and are permittedto vary along the boundary such that waves are not spuriously reflected.
NRBCs can be used in 2D or 3D and with FLUENT 6.3 can be used with the coupledimplicit solver.