modeling rotating machinery using ansys fluent · 2018. 1. 9. · release 13.0 december 2010...

L6-1 ANSYS, Inc. Proprietary

© 2010 ANSYS, Inc. All rights reserved. Release 13.0

December 2010

Modeling Rotating

Machinery using

ANSYS FLUENT

Customer Training Material

Lecture 6

滑移网格 Sliding Mesh Model

(SMM)

Modeling Rotating Machinery using ANSYS FLUENT



December 2010

Customer Training Material Outline

•滑移网格介绍

• N-S方程: 动网格形式

•设置要点 – 交接面Grid Interfaces

– 预览网格Mesh Preview

– 时间步Choosing a Time Step

•常见问题及求解策略

• Summary

• Appendix




December 2010

Customer Training Material Introduction

• 精确的瞬态求解方式:

– Potential interactions - flow unsteadiness due to pressure waves which propagate both upstream and downstream

– Wake interactions - flow unsteadiness due to wakes from upstream blade rows advecting downstream

– Shock interactions - for transonic/supersonic flows, unsteadiness due to shocks waves striking downstream blade row

• 弱交互可简化用 MRF and Mixing Plane




December 2010

Customer Training Material Illustration of Unsteady Interactions

wake interaction

shock interaction

potential interaction

stator rotor




December 2010

Customer Training Material What is the Sliding Mesh Model?

•多个域

•非共节点interface

– interfaces must be surfaces of revolution about the

axis of rotation

– interfaces can be rotationally periodic, but adjacent

zones must have equal periodic angles

•绝对位移unsteady

– For each time step, the meshes are moved and the

fluxes at the sliding interfaces are recomputed




December 2010

Customer Training Material Navier-Stokes Equations: Moving Mesh Formulation

• In the sliding mesh (or moving mesh) formulation , the motions of

moving zones are tracked relative to the stationary frame

– No moving reference frames are attached to the computational

domain, which simplifies the flux transfers across the interfaces

• The motion of any point in the domain is given by a time rate of

change of the position vector ( )

– is also known as the grid speed

– Note that for rigid body rotation at constant speed

r

r

Urr




December 2010

Customer Training Material Moving Mesh Illustration

x

y

z

stationary

frame

axis of

rotation

Δt)(tr

Moving CFD domain

)(tr




December 2010

Customer Training Material Navier-Stokes Equations for a Moving Mesh (1)

gb Q

0)(

VF

VVpTkeUVdt

ed

FpVUVdt

Vd

UVdt

d

tt

b

(Continuity)

(Momentum)

(Energy)




December 2010

Customer Training Material Pros and Cons of the Sliding Mesh Model

• Advantages

– 更精确结果

– 可用于多运动区域

– 交接面类型灵活

• Disadvantages

– 瞬态求解

– 时间长、数据多




December 2010

Customer Training Material Sliding Interfaces

• 交接面要求圆周

“warped” interfaces aligned

at initial time level...

…become misaligned at a

subsequent time level!




December 2010

Customer Training Material Sliding Mesh Setup [1]

• 瞬态项.

• interface zone pair.

• “Mesh Motion”选项. – Enter rotational speed, axis, etc. in

the same manner as SRF.

• 边界类似SRF, MRF models.

• 插值格式discretizations – 时间First order time discretization.

– 空间Second order spatial discretizations.

– 压力基PRESTO! for pressure-based solver pressure discretization.




December 2010

Customer Training Material Sliding Mesh Setup [2]

•设置Solution controls – 默认选项

•监控Monitors – 时间相关.

– 可以做FFT.

•时间步长和子步.




December 2010

Customer Training Material Sliding Mesh Preview

• Fluent provides a sliding mesh preview option for checking sliding mesh motion before beginning the calculation

• To use this facility:

– Specify the time step and number of time steps

– Click on Preview

• You can display the grid motion and optionally save hardcopy images of the grid motion for later animation

• NOTE: Save your initial case and data files prior to running Mesh Preview so you can start from your original mesh positions




December 2010

Customer Training Material Solving SMM Problems

• Choose appropriate time step and

Max Iterations Per Time step to

ensure good convergence with each

time step

• Advance the solution until the flow

becomes time-periodic (pressures,

velocities etc. oscillate with a

repeating time variation)

– Usually requires several

revolutions of the grid

• Good initial conditions can reduce

the number of time steps needed to

achieve time-periodicity

– You can use either an MRF or

mixing plane solution as an

initial condition

• Data Sampling for Time Statistics

can be enabled to have Fluent save

time-averaged flow field variables.




December 2010

Customer Training Material Time Periodic Flow

Flow unsteadiness becomes

periodic after initial transient




December 2010

Customer Training Material Choosing a Time Step for the SMM

• Recommended time step size is based on the principal

that the time step should be no larger than the time it

takes for a moving cell to advance past a stationary point

• An estimate for the time step can thus be calculated as:

movingV

st

s = mesh spacing at sliding interface

Vmoving = velocity of the moving zone

cells at time t cells at time t+t

moving mesh zone s




December 2010

Customer Training Material 常见问题及求解策略Troubleshooting

• 交接面位置问题

– interface网格质量尽量高

– 分解交接面（多个简单面）

• Some other things to consider for troublesome cases

– 网格质量要求 (max cell skewness < 0.9 – 0.95)

• 双精度（网格高长宽比）

– MRF初始化

– 减小松弛因子或 Courant numbers

– 减小时间步或增大迭代步




December 2010

Customer Training Material 快速设置Accelerating Sliding Mesh Cases

•旋转参数设置moving mesh parameters.

– Rotational axes, velocity

– Translational velocity

•扩展设置 profile file or UDF. – The Zone Motion Function

can be employed to permit

all inputs to be defined in a

single UDF function.




December 2010

Customer Training Material Example: Flapping Airfoil

Oscillating inner zone

Stationary outer zone




December 2010

Customer Training Material Flapping Airfoil UDF

• UDF uses the TRANSIENT

_PROFILE macro to

prescribe the sinusoidal

oscillation of the domain.

/**********************************************/

/* flap.c */

/* UDF for specifying a time-varying omega */

/* */

/* Simulates +/- 8 deg flapping with cycle of */

/* of 1 sec. */

/* */

/* Version 13.0 */

/* */

/**********************************************/

#include "udf.h"

#define PI 3.141592654

DEFINE_TRANSIENT_PROFILE(speed, time)

{

real ampl = 2.0*PI/15.0;

real freq = 2.*PI;

real omega;

omega = 2.0*PI*ampl*cos(freq*time);

return omega;

}




December 2010

Customer Training Material Flapping Airfoil Animation

Velocity magnitude contours




December 2010

Customer Training Material Summary

• 真实更精确的仿真方式（包含动静域）

– 交接面位置非稳态模拟

• 计算时间长

• 设置与MRF类似

– MRF作为初步计算

• 预览网格特征The mesh preview option allows you the check the mesh motion prior to run the calculation

• Accelerating reference frames can also be handled using sliding mesh – See Appendix B for mode details.




December 2010

Customer Training Material Appendix A: Sliding Mesh Examples

•2-D turbine stage

•2-D blower

•1.5 stage research turbine (ERCOFTAC U1)




December 2010

Customer Training Material 2-D Turbine Stage

• 2-D subsonic turbine stage test case

• Planar geometry simulates midspan stream surface

• Equal blade counts for stator and rotor

• Motion of rotor modeled using linear y-velocity (29.445

m/s)

• Boundary conditions

– Stage Inlet

• Ptotal = 1 atm, Ttotal = 300 K, TU = 5%

– Stage Exit

• Pstatic = 0.963 atm




December 2010

Customer Training Material 2-D Turbine Stage (2)

• Numerical model

– Coupled-implicit solver, steady-state, compressible flow (air)

– mesh (total): 7917 tri cells

– Realizable k-e turbulence model

• Two cases examined

– Mixing plane model (with mass conserving mixing plane)

– Sliding mesh model

• MPM results used as initial condition for SMM




December 2010

Customer Training Material 2-D Turbine Stage: Mesh




December 2010

Customer Training Material Pressure Contours - Mixing Plane Solution




December 2010

Customer Training Material Mach No. Contours - Mixing Plane Solution




December 2010

Customer Training Material Stator Pressure Distribution Comparison




December 2010

Customer Training Material Rotor Pressure Distribution Comparison




December 2010

Customer Training Material 2-D Blower

• Simple 2-D model of a squirrel cage blower (44 blades)

• Pressure boundaries (inlet total pressure = 200 Pa), 2500

rpm

• Numerical Model

– Segregated solver, incompressible flow (air)

– Standard k-e turbulence model (TU = 5% at inlet)

– MRF solution computed first - used as initial condition for SMM

– time step = 0.0001333 sec (corresponds to 2 deg rotation of the

wheel)

– Calculation carried out for 10 revolutions

– Time averaged solution computed for one revolution




December 2010

Customer Training Material 2-D Blower: Grid

interfaces

inlet

outlet




December 2010

Customer Training Material 2-D Blower: Unsteady Total Pressure

time-periodic solution achieved




December 2010

Customer Training Material 2-D Blower - Static Pressure

MRF Solution SMM Solution

(time-averaged)




December 2010

Customer Training Material 2-D Blower Results

• MRF results show reasonable agreement with SMM for this

operating condition

• Other operating conditions may show larger discrepancies -

e.g. lower flowrates, where unsteadiness due to

separation/stall become more significant

Total Pressure

Rise (Pa)

Outlet Normal

Velocity (m/s)

MRF 532.0 23.8

SMM* 491.3 24.6

Error (%) 8.3 3.3

* time-averaged results




December 2010

Customer Training Material 1.5 Stage Research Turbine (1)

• Three blade row (stator-rotor-stator) turbine stage

– 36 stator blades, 41 rotor blades

• Rotor geometry modified to 40 blades to permit periodic

boundaries (9 stator blades, 10 rotor blades)

– Design conditions: speed = 3500 rpm, flowrate = 8.0

kg/s

• Numerical model

– Coupled solver, compressible flow (air), SMM

– 50 subiterations per time step, 4.3 time steps per

passing period

– tet mesh - 882,000 cells

– Spalart-Allmaras turbulence model




December 2010

Customer Training Material 1.5 Stage Research Turbine (2)

• Results

– Unsteady data time-averaged for comparison with

experiments

– Computed flowrate = 8.04 kg/s - agrees very well with

data

– Pitch-averaged profiles extracted from time-averaged

flowfield at

• 8.8 mm downstream of trailing edge of first stator blade row

(plane 1)

• 8.8 mm downstream of trailing edge of rotor blade row (plane 2)

• 8.8 mm downstream of trailing edge of second stator blade row

(plane 3)

– Profiles show good overall agreement with data




December 2010

Customer Training Material 1.5 Stage Turbine: Geometry




December 2010

Customer Training Material Surface Pressure Coefficient Contours




December 2010

Customer Training Material Pitch-Averaged Yaw Angles (1)

Yaw angle at plane 1

0

5

10

15

20

25

30

0.24 0.25 0.26 0.27 0.28 0.29 0.3

Radial position (m)

Yan

gle

Experiment

CFD




December 2010


Yaw angle comparisons at plane 2

60

65

70

75

80

85

90

95

0.24 0.25 0.26 0.27 0.28 0.29 0.3

Radial position (m)

Yaw

an

gle

CFD

Experiment




December 2010


Yaw angle at plane 3

0

5

10

15

20

25

30

35

0.24 0.25 0.26 0.27 0.28 0.29 0.3

Radial position (m)

Yaw

an

gle

CFD

Experiment




December 2010

Customer Training Material Pitch-Averaged Mach Number Profiles (1)

Mach number comparison at plane 1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.24 0.25 0.26 0.27 0.28 0.29 0.3

Radial position (m)

Mach

nu

mb

er

CFD

Experiment




December 2010



0

0.05

0.1

0.15

0.2

0.25

0.24 0.25 0.26 0.27 0.28 0.29 0.3

Radial position (m)

Mach

nu

mb

er

CFD

Experiment




December 2010



0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.24 0.25 0.26 0.27 0.28 0.29 0.3

Radial position (m)

Mach

nu

mb

er

CFD

Experiment

modeling rotating machinery using ansys fluent · 2018. 1. 9. · release 13.0 december 2010...

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