Cfx_tbr_aeromechanics_cfd_18.0_module03_introduction_to_tbr_methods

18.0 Release

Module 03: Introduction to Transient Blade Row Methods Aeromechanics of Turbomachinery Blades

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Welcome • Welcome to the ANSYS Aeromechanics training course! • This training course covers many important aspects involved in aeromechanical analysis of turbomachinery blades.

• It is intended for all new or occasional ANSYS CFX and Mechanical users − Emphasis in this course is given to ANSYS CFX • Course Contents: − Transient blade row methods for turbomachinery CFD analysis − Fundamentals and how to perform blade flutter analysis

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Introduction • Lecture Themes: − Transient Blade Row Methods can be used to solve many CFD applications across industries involving systems or devices with moving parts. ANSYS CFX offers many different models for rotating machinery, for arbitrary prescribed motion and for objects whose motion is determined by the flow.

• Learning Objectives: − You will be able to use domain interfaces and will become familiar with the CFX models for systems with moving parts and when a particular model is applicable.

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Blade Row Methods Overview and Objectives • Provide blade row methods to perform − Aerodynamic, Aeromechanical and Aerothermodynamic Analyses

• Methods for both steady-state & transient simulations

• Provide fast & accurate transient blade row solution − Using range of pitch-change methods:

Full-wheel

• PT, TT and FT (Full-wheel  Reduced geometry)

• Harmonic Analysis (hybrid frequency/time solution method) 4

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Reduced geometry

ANSYS Blade Row Analysis Methods

Steady Stage/ Mixing-Plane • • • •

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Single Passage per Row Very accurate over broad range of performance map Does not account for unsteady interaction Low computational expense

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Transient with Pitch-Change • • • •

Reduced domain model One or few passages per row Accuracy of full domain Account for unsteady interaction medium comp. expense

Transient Full-Domain • • •

Requires Full or Partial wheel modeling Accurate account for unsteady interactions Large comp. expense − Memory − CPU

Steady State Simulations Upstream

• In general the steady state mixing plane model is a very efficient blade row analysis and design tool − Can be applied with pitch-change − Computationally more efficient than transient simulations − Can often predict machine performance well • Using the constant total pressure option for the stage interface can improve predictions as shown in the following example

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Downstream

Stage Numerics Example: Modified Hannover Compressor

• Modified Hannover compressor − 2 ½ stage − IGV=24, R1=21, S1=27, R2=30, S2=33 − Modeled with stage, multistage TT, full wheel transient 7

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Steady-State Stage Interface (Mixing-Plane) Mixing Plane • Steady-state (can be combined with other TBR pitch-change methods) • Blade pitch change by “mixing”, standard periodicity enforced • Conservative implicit circumferential mixing interface • No unsteady interaction between blade rows (wakes, vortices) • In general MP model is very efficient blade row analysis and design tool • Can quickly produce performance map • Solution can be used to initialize transient simulation

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Mixing-Plane Enhancements • Default options: “Constant Total Pressure” and enhanced numerics

− Improved Rothalpy distribution − Minimize reflections for high−

speed flows Overall improved aerodynamic performance predictions

Removed streaking from Rothalpy contours

• MP results move closer to transient predictions R17.X & R18.0: • Use default “Constant Total Pressure” with the following Expert Parameter stage energy closure option = 1

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Without enhancements

With enhancements

New numerics remove reflections due to Shock downstream of MP

MP Enhancements: Minimize High Speed Flow Reflection in MP Reflection downstream of MP

Default setting since R17 will minimize this type of reflections

Reflection and numerical artifacts at downstream of MP interface 10

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Removing reflection with default MP settings August 10, 2017

Reflection upstream of MP

Turn on: - High speed numerics - Implicit stage averaging option to minimize this reflection

Reflection and numerical artifacts at upstream of MP interface

Removing reflection with use of improved numerics settings

Transient Simulations • Steady state solutions are fast and practical, however, there are cases that where the unsteady effects of blade row interaction do affect performance • Full domain model not always required • TBR methods can capture unsteady blade to blade interaction − Typically one or two passages are modeled

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Problem: How to obtain the full-wheel transient solution, but at low cost?

Solution: • The ANSYS TBR Transformation family of pitch-change methods • New models minimize number of simulated passages • Provide enormous efficiency gains and reduced infrastructure requirements

Pitch Change Problem • Adjacent blade rows typically have different blade counts

Full-wheel Model

• A single periodic sector will have a different pitch angle • ANSYS CFX has three TBR approaches to handle this pitch change

• Difference in pitch change will determine best approach

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Reduced Model Periodicity treatment

R/S Interface & pitchwise periodic determine pitch-change method

ANSYS CFX Transient Pitch Change Models Profile Transformation (PT)

Time Transformation (TT)

Fourier Transformation (FT)

Small/Moderate Pitch

Small/Moderate Pitch

Large Pitch

• •

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• • •

Single Stage Multistage

Frozen gust Single Stage Multistage

• • • •

Transient Solution Method

Time marching Current offerings

Harmonic Solution Method

Frequency based

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Full-wheel Model

Frozen gust Single Stage Multistage Blade Flutter

Reduced Model

Pitch-Change Method Profile Transformation (PT) • If Pitch-Change: the profiles across the rotor/stator interface are stretched or compressed by the pitch-ratio while full conservation is maintained • Standard periodicity applied on pitchwise boundaries • Maintains true blade counts & geometry • Computationally efficient and fast (fully implicit)

• Single-Stage and Multistage modeling − Accurate prediction for machine performance for small pitch ratio − For larger pitch ratios, the accuracy can be maintained by adding more passages per row to reduce the ensemble pitch-ratio 14

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Implicit & Conserving profile exchange via GGI

Standard Periodicity

Pitch-Change Method Time Transformation (TT) • Based on the Time-Inclining method (Giles ‘88) • Fully implicit, turbulence & transition models • Transform equations in time so that instantaneous periodicity can be applied on pitchwise boundary with no approximation. Solution advanced in computational time but results will be displayed in physical time. Q E G   0 t X Y X ' X

Y' Y

t '  t  Y



T Pr

T 

Ps  Pr Us

 (Q  G ) E G   0 t ' X ' Y '

• Inlet-disturbance, single-stage, and multistage analysis

− Moderate pitch-ratio

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Implicit & Conserving profile exchange via GGI

Standard Periodicity applied in computational time

Pitch-Change Method Fourier Transformation (FT)

 (t  T )

Sampling plane (GGI)

• Based on the Shape-Correction method of L. He (1989) and Chorochronic interface periodicity of Gerolymos (2002) • Fully implicit, turbulence & transition models • Fourier–series are used for reconstruction of solution history on pitchwise boundary and inter-row interfaces for efficient data storage & convergence Pitchwise Boundary Inter-row interfaces

 (t ) 

N

A e

k  N

 (t ,  ) 

M

 j ( kt )

k

N

 A

l  M k  N

k ,l

e  j ( kt l )

• Double-passage strategy (faster convergence than single passage) • Supports Inlet disturbance, Single-stage analysis, multidisturbance, blade-flutter • Works for Large-pitch ratios 16

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 (t  T )

Guidelines for Usage

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Profile Transformation

Time Transformation

Fourier Transformation

Small pitch ratio

Yes

Yes

No

Incompressible

Yes

No

Yes

Frequency preserved

No

Yes

Yes

Mesh Motion

Yes

No

Yes

Frequencies not at blade passing

No

No

Yes

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Considerations: Time Transformation • Pitch Ratio: − limit depends on compressibility − CFX will warn you if you are outside the limit, but not stop the solution

• Solution Monitoring: − Does not support monitor points with CEL expressions − Use solution variables instead

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Considerations: Fourier Transformation • Use Double Precision for solver when running in serial • Use Double Precision for partitioning, optional to run solver in double precision, but recommended

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Publications: ANSYS TBR Transformation Methods • GT2010-22762 Siemens & ANSYS “Unsteady CFD Methods in a Commercial Solver for Turbomachinery Applications” • GT2011-45820 GE & ANSYS “A Comparison of Advanced Numerical Techniques to Model Transient Flow in Turbomachinery Blade Rows” • GT2011-46635 ANSYS “Investigation of Transient CFD Methods Applied to a Transonic Compressor Stage” • GT2012-69019 GE & ANSYS “The Efficient Computation of Transient Flow in Turbine Blade Rows Using Transformation Methods • GT2012-69151 PCA & ANSYS “Impeller-Diffuser Interaction in Centrifugal Compressors” 20

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Publications: ANSYS TBR Transformation Methods • GT2013-95059 GE & ANSYS “Efficient Computation of Large Pitch Ratio Transonic Flow in a Fan With Inlet Distortion” • GT2013-94639 Siemens & ANSYS “Experimental and Computational Analysis of a Multistage Axial Compressor Including Stall Prediction by Steady and Transient CFD Methods” • GT2013-95005 PCA & ANSYS “Investigation of Efficient CFD Methods For The Prediction of Blade Damping ” • GT2013-94739 Honeywell “Study of Steady State and Transient Blade Row CFD Methods in a Moderately Loaded NASA Transonic High-Speed Axial Compressor Stage” 21

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Publications: ANSYS TBR Transformation Methods • GT2014-27097 Dresser Rand & ANSYS “Investigation of Efficient CFD Methods for Rotating Stall Prediction in a Centrifugal Compressor Stage” • GT2014-26846 Siemens & ANSYS “Efficient Time Resolved Multistage CFD Analysis Applied to Axial Compressors” • GT2015-43624 ANSYS “Time Transformation Simulation of 1.5 Stage Transonic Compressor”

• GT2015-42632 Altsom & ANSYS “CFD Modeling of Low Pressure Steam Turbine Radial Diffuser Flow by Using a Novel Multiple Mixing Plane Based Coupling- Simulation and Validation” 22

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Applications for Transient Blade Row Methods

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TBR Methods with Pitch-Change • Provide efficient and fast solution to:

Aerodynamic Analysis

Aeromechanical Analysis Aerodynamic damping

Aerothermodynamic Analysis Surface Temperature Distribution

Fluid Solid Thermal Response

EO Forcing 24

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Aerodynamic Analysis • Speedline performance: pressure ratio, efficiency, choke limit , stall onset, flow instabilities • Transient simulation can improve performance predictions. Exact blade passing frequency (bpf) is not always required

Low speed, mostly subsonic flow

Steady MP solution as good as transient Ref, PT, TT 25

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High speed, transonic flow

Accurate capturing of shock across R/S interface is essential

Aerodynamic Analysis • Accurate aerodynamic performance for multistage can be predicted with TT & PT interface combinations

IGV

TT

• For high speed flow region it is very important to place the TT interface where the shock cross the interface • If shock cross both interfaces then combination of TT and STT can be used

• Lower speed flow stages can even be modeled with PT or MP interfaces only. − One reason to do so is less restriction (PT and MP) on pitch ratio

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R1

IGV

S1

TT

TT

PT

R1

R2

S1

STT

S2

PT

R2

TT

S2

STT

Aeromechanical Analysis • Machine operability & durability: Flutter margin, High Cycle Fatigue (HFC) • Fundamentally transient multiphysics problem − FSI (expensive)

• Alternate modeling/analysis methods − Blade Flutter & Aerodamping Calculations: Determine if the aerodynamic loads damp out blade vibration at natural frequencies − Forced Response: Determine blade response (motion & stresses) due to excitations from neighboring blade rows. • Tuned • Mistuned – Based on stiffness variation – Aerodamping introduced 27

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EO Forcing

Aeromechanical Analysis: EO Forcing • TT and FT pitch-change methods can be used

FT or TT -TRS FT- ID Single-Stage, Multi-stage Fan inlet distortion

TT-TRS for 1.5 Stage 28

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FT-TRS Impeller in Vaneless volute

FT-TRS Fan crosswind

FT-ID multi-disturbance

Aerothermodynamic Analysis • Aerodynamic performance & durability: blade surface temperature, hot streaks migration, thermal cycle fatigue (yielding & creep failure) • Temperature distribution is needed to design blade cooling systems • Fundamentally transient flow simulation with conjugate heat transfer (CHT) • ANSYS TBR methods can solve for range of thermal distribution problems • Simultaneously can solve for aerodynamics, CHT, and predict max surface temperature

TT & PT

Uniform distribution hot streaks 29

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FT

Non-uniform distribution hot streaks

Aerothermodynamic Analysis Single-Stage Hot Streak Modeling • PT or TT can be used • All will give similar thermal distribution • PT frequency errors have little impact on predicted max, min, average temperatures • Steady MP result is significantly in error

The measurements on the test case "Aachen Turbine" were carried out at the Institute of Jet Propulsion and Turbomachinery at RWTH Aachen, Germany

Instantaneous Temperature Contours

Time Averaged Temperature Contours on Rotor PT

MP

PT

TT

TT 30

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Aerothermodynamic Analysis Multistage Hot Streak Modeling PT MP

• Choice of interface to be used in down stream rows depend on: 1- How far you want to track the hot streaks. 2- Aerodynamic accuracy • For example following combination could be used: − PT or TT followed by MP − PT or TT followed by PT − TT followed by STT Temperature distribution On each blade 31

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R1

IGV PT

PT

S2

Aerothermodynamic AnalysisHot Streak Modeling + CHT • Modeled with TT/MP combo + CHT • Altering the solid thermal response

Rotor blade (solid)

MP

TT

Solid Thermal Response

Simulation timestep = 9.718 x 10-6 s

Solid time scale Ave : 10.6 s Fluid 398k-450k 32

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Solid 399k-406k

Timescale Factor = 1.0 x 106

𝐿2𝑠𝑐𝑎𝑙𝑒 = 𝑆𝑜𝑙𝑖𝑑 𝑡ℎ𝑒𝑟𝑚𝑎𝑙 𝑑𝑖𝑓𝑓𝑢𝑠𝑖𝑣𝑖𝑡𝑦 =

𝑆𝑜𝑙𝑖𝑑 𝑡𝑖𝑚𝑒 𝑆𝑐𝑎𝑙𝑒 𝑆𝑖𝑚𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑡𝑖𝑚𝑒𝑠𝑡𝑒𝑝

Aerothermodynamic Analysis Modeling Non-uniform Hot Streak • Complex non-uniform 360 hot streaks enter the IGV • Two modeling techniques in addition to full domain modeling Aero + Thermo + CHT CHT

CHT

+

MP FT-TRS

Transient Coupled 33

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MP MP

Steady MP

+

FT-ID

Transient FT-ID

MP

Aerothermodynamic Analysis Modeling Non-uniform Hot Streak Temperature from FC A0 Solution for the entire domain Is a reconstruction from solution on two rotor passages

Solid Temperature

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Gas Temperature

Setup and Post-Processing

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Setup • Very similar to a standard transient setup • New TBR panel defines timestep

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Passage Duplication • FT Method requires two passages • PT and TT may require additional blade passage to obtain near unity pitch ratio

• Two methods in CFX-Pre: − Right click on Mesh in Tree 1 − Transform Mesh>Turbo Rotation 2 − Tools > Turbo Mode

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Domain Settings • For a TBR simulation a Passage Definition group box is availbel in the Basic Settings tab of each Domain − Ensure that the correct number of Passages in Component and Passages in 360 is set

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Transient Blade Row Panel Special Panel: • Insert > Transient Blade Row Models − Specify: • Method (PT, TT, FT) − Timestep will be determined on this panel • Recommendation: Let CFX select suitable value where possible − Number of timesteps per run will be determined

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Method: Profile Transformation When PT is selected: • User needs to define relevant Time Period − Passing Period of a blade row (recommended) − Value • Time Steps − Number of Timesteps per period • Timestep is calculated

• Time Duration − Number of periods per run − Maximum number of periods

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Method: Time Transformation When TT is selected: • Create TT interface • Define Option − Rotor Stator − Rotational Flow Boundary Disturbance • For Rotor Stator − Select Domain Interface − Select Side 1 and Side 2 • Automatic • Domain List • None 44

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Method: Time Transformation • For Rotational Flow Boundary Disturbance − Select Signal motion • Rotating or Stationary − External Passage definition • # Passages in 360 • # Passages in Component − Specify Transient details similar to PT method • Option Automatic (recommended) – Based on blade counts, selects a suitable min. timesteps per period – Timestep multiplier (can be left to default value of one for most cases)

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Method: Fourier Transformation Options for

• Rotational Flow Boundary Disturbance • Rotor Stator • Blade Flutter − Covered in later lecture • Need to define Sampling Domain Interface &Phase Corrected Interfaces • Transient Method − Classic Time Intergration − Faster Harmonic Balance 46

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Sampling Interface (GGI)

Note for Inlet Disturbance cases • Inlet Profile always needs to be defined in a separate coordinate frame • Even if profile is stationary!!!

• See CFX Tutorials 32 and 33 for details

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Output Control • Writing out .trn files is unnecessary • Data is stored as Fourier Coefficients • Solution is reconstructed in CFD-Post

• Default Option (Essential) only writes out Solver variables • Use Extra Output Variables List and multi select any variable of interest • Data compression − How many Fourier Coeffs. to store − Max 10, more can be added via CCL 48

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Output Control Monitor Points • Create several Monitor Points by Coordinates − These will include also any additional frequencies which are not accounted for in the TBR method (e.g due to vortex shedding or scale resolving turbulence models) − In Post you get only the frequencies related to modelled inlet disturbances and blade row motion

• TT specific − Be cautious creating monitors in CFX-Pre for integral quantities: • Use also the CFX-Solver expert parameter "monitor raw tt data = t" − Can also create such integral monitors by CEL in CFD-Post and visualize by charts − Example from WS01: forces on rotor = sqrt(force_x()@ R1 Blade ^2 + force_y()@ R1 Blade ^2 + force_z()@ R1 Blade ^2)

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Post Processing • When loading, case is identified as TBR

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Fourier Coefficients • Time Transformation and Fourier Transformation simulations store variables as Fourier Coefficients • Example for Pressure:

• User can specify value for N in CFX Pre

• More Fourier Coeffs. − More accurate − Larger .res file

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Fourier Coefficients • Data will be filtered in CFD Post: • Only frequency of interest and harmonics are captured • Frequency of interest is defined in CFX-Pre, and could be: − Blade flutter frequency − Blade passing frequency (of rotor or stator) − Inlet disturbance frequency • Frequencies not captured: − Vortex shedding − Rotating stall − Define apporpiate coordinate-based Monitor Points in CFX-Pre to capture such frequencies 52

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Variables in CFD-Post • Fourier Coefficients

• Standard Variables are calculated as instantaneous (based on timestep selected) • Reconstructed from Fourier Coeffs.

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Plotting Fourier Coefficients • Plotting Pressure shows instantaneous values • Plotting FC A0 for Pressure shows time averaged values • Can also plot An, Bn coefficients if desired

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Timestep Selector Phase=39.5

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Timestep Selector Phase=40.0

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Timestep Selector For a TBR case: • Fourier Coefficients collected over last period • Timestep Selector − Will display many periods − Up to number of cycles solved − Phase will repeat, each cycle identical − Exposed to facilitate comparing to reference cases and consistent post processing

• Example: − Run for 40 cycles, 44 timesteps per cycle − Results at Phase @ 39.75 would be the same as 38.75, 37.75, etc. 57

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Plotting TBR Cases: Changing Timestep

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Plotting TBR Cases: Changing Timestep

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Transient Statistics • Transient Statistics created automatically − Arithmetic Average − Root Mean Square − Standard Deviation • Values are averaged and therefore do not change when timestep is changed

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Example Showing RMS and Standard Deviation

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Expanding FT Data – Data Instancing

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Data Instancing • Only available for TBR cases • Calculates data in additional blade passages • Applied on Domain(s)

• Data is created in Post • Plots will take longer to generate • CFD Post essentially treats this as a full data set

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Animations • Animations with TBR cases: • Much higher fidelity compared to Transient cases • Any number of timesteps per cycle can be used − Each timestep calculated from Fourier Coefficients • No need for .trn files with TBR cases • Use Don’t Encode Last MPEG frame to make sure start/end point in period are not repeated

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Comparison of Animations Reference Case

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TBR Case

Summary • Introduction to Transient Blade Row Methods • Descriptions of Profile, Time, and Fourier Transformation Methods

• Applications for TBR methods • Setup and Post Processing Considerations

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References • IGTI Turbo Expo Papers (Listed on pages 19-21) • Chapter 6 in CFX Modeling Guide • Transient Blade Row Modeling

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Workshop 01 • Time Transformation modeling for a 1.5-stage machine

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