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Using FLUENT in Design & Optimization. Devendra Ghate, Amitay Isaacs, K Sudhakar, A G Marathe, P M Mujumdar. Centre for Aerospace Systems Design and Engineering Department of Aerospace Engineering, IIT Bombay http://www.casde.iitb.ac.in/. Outline. CFD in design Problem statement

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using fluent in design optimization

Using FLUENT in Design & Optimization

Devendra Ghate, Amitay Isaacs,

K Sudhakar, A G Marathe, P M Mujumdar

Centre for Aerospace Systems Design and Engineering

Department of Aerospace Engineering, IIT Bombay

http://www.casde.iitb.ac.in/

outline
Outline
  • CFD in design
  • Problem statement
  • Duct parametrization
  • Flow solution
  • Results
  • Conclusion

FLUENT CFD Conference 2003

using cfd in design
Using CFD in Design
  • Simulation Time
    • CFD is takes huge amounts of time for real life problems
    • Design requires repetitive runs of disciplinary analyses
  • Integration
    • With optimizer
    • With other disciplinary analyses (e.g. grid generator)
  • Automation
    • No user interaction should be required for simulation
  • Gradient Information
    • No commercial CFD solvers provide gradient information
    • Computationally expensive and problematic ( ) to get gradient information for CFD solvers (finite difference, automatic differentiation)

FLUENT CFD Conference 2003

methodology

Problem Specification

New parameters

Parametrization

Geometry Generation

Grid Generation

CFD problem setup

Flow Solution

Methodology

Optimization using

Surrogate Models

(RSM, DACE)

FLUENT CFD Conference 2003

methodology1

Problem Specification

Parametrization

CFD problem setup

Flow Solution

Methodology

New parameters

Geometry Generation

Optimization using

Surrogate Models

(RSM, DACE)

Grid Generation

FLUENT

FLUENT CFD Conference 2003

3 d duct design problem

Geometry of duct from Entry to Exit ?

3-D Duct Design Problem

Entry Exit

Location and shape known

  • Pressure Recovery
  • Distortion
  • Swirl

FLUENT CFD Conference 2003

parametrization

Y

Z

X

X

Duct Centerline

  • Control / Design Variables
    • Ym, Zm
    • AL/3, A2L/3

A

X

Cross Sectional Area

Parametrization

FLUENT CFD Conference 2003

parametrization contd

Y

Z

X

X

Duct Centerline

  • Control / Design Variables
    • Ym, Zm
    • AL/3, A2L/3

A

X

Cross Sectional Area

Parametrization(contd.)

FLUENT CFD Conference 2003

typical 3d ducts
Typical 3D-Ducts

FLUENT CFD Conference 2003

grid generation
Grid Generation

Clustering Parameters

  • Complete grid generation process is automated and does not require human intervention
  • Complete control over
    • Distance of the first cell from the wall
    • Clustering
    • Number of grid points

Generation of entry and exit

sections using GAMBIT

Mesh file

Conversion of file format to

CGNS using FLUENT

Entry & Exit sections

Generation of structured volume

grid using parametrization

Grid parameters

Conversion of structured grid

to unstructured format

FLUENT CFD Conference 2003

turbulence modeling
Turbulence Modeling
  • Relevance: Time per Solution
  • Following aspects of the flow were of interest:
    • Boundary layer development
    • Flow Separation (if any)
    • Turbulence Development
  • Literature Survey
    • Doyle Knight, Smith, Harloff, Loeffer
      • Circular cross-section
      • S-shaped duct
    • Baldwin-Lomax model (Algebraic model)
      • Computationally inexpensive than more sophisticated models
      • Known to give non-accurate results for boundary layer separation etc.
  • k- realizable turbulence model
    • Two equation model
    • Study by Devaki Ravi Kumar & Sujata Bandyopadhyay (FLUENT Inc.)

FLUENT CFD Conference 2003

turbulence modeling contd
Turbulence Modeling (contd.)
  • Standard k- model
    • Turbulence Viscosity Ratio exceeding 1,00,000 in 2/3 cells
  • Realizable k- model
    • Shih et. al. (1994)
    • Cμ is not assumed to be constant
    • A formulation suggested for calculating values of C1 & Cμ
    • Computationally little more expensive than the standard k- model

Total Pressure profile at the exit section (Standard k-  model)

FLUENT CFD Conference 2003

distortion analysis
Distortion Analysis
  • DC60 = (PA0 – P60min) /q

where,

PA0 - average total pressure at the section,

P60min - minimum total pressure in a 600 sector, q - dynamic pressure at the cross section.

  • User Defined Functions (UDF) and scheme files were used to generate this information from the FLUENT case and data file.
  • Iterations may be stopped when the distortion values stabilize at the exit section with reasonable convergence levels.

FLUENT CFD Conference 2003

parallel execution
Parallel Execution
  • Parallel mode of operation in FLUENT
  • 16-noded Linux cluster
  • Portable Batch Systems for scheduling
  • Batch mode operation of FLUENT (-g)
  • Scale up depends on grid size

FLUENT CFD Conference 2003

results total pressure profile
Results: Total Pressure Profile

FLUENT CFD Conference 2003

results contd
Results (contd.)
  • Mass imbalance: 0.17%
  • Energy imbalance: 0.06%
  • Total pressure drop: 1.42%
  • Various turbulence related quantities of interest at entry and exit sections:

y+ at the cell center of the cells adjacent to boundary throughout the domain is around 18.

FLUENT CFD Conference 2003

slapping
Slapping
  • Methodology
    • Store the solution in case & data files
    • Open the new case (new grid) with the old data file
    • Setup the problem
    • Solution of (0.61 0.31 1 1) duct slapped on (0.1 0.31 0.1 0.1)

These are huge benefits as compared to the efforts involved.

FLUENT CFD Conference 2003

conclusion
Conclusion

Time for simulation has been reduced to around 20% using parallel computation and slapping.

  • Process of geometry & grid generation has been automated requiring no interactive user input
  • FLUENT has been customized for easy integration into an optimization cycle
  • CFD analysis module ready for inclusion in optimization for a real life problem

FLUENT CFD Conference 2003

future work
Future Work
  • Further exploration and improvement of slapping methodology
  • Identification and assessment of optimum optimization algorithm

FLUENT CFD Conference 2003

thank you

Thank You

http://www.casde.iitb.ac.in/mdo/3d-duct/

problem statement
Problem Statement
  • A diffusing S-shaped duct
  • Ambient conditions: 11Km altitude
  • Inlet Boundary Conditions
    • Total Pressure: 34500 Pa
    • Total Temperature: 261.4o K
    • Hydraulic Diameter: 0.394m
    • Turbulence Intensity: 5%
  • Outlet Boundary Conditions
    • Static Pressure: 31051 Pa (Calculated for the desired mass flow rate)
    • Hydraulic Diameter: 0.4702m
    • Turbulence Intensity: 5%

FLUENT CFD Conference 2003

duct parameterization
Duct Parameterization
  • Geometry of the duct is derived from the Mean Flow Line (MFL)
    • MFL is the line joining centroids of cross-sections along the duct
    • Any cross-section along length of the duct is normal to MFL
  • Cross-section area is varied parametrically
  • Cross-section shape in merging area is same as the exit section

FLUENT CFD Conference 2003

mfl design variables 1

y(Lm/2), z(Lm/2) specified

Cmerger

y2, z2

y(x), z(x)

y1, z1

Centry

x

0

Lm/2

Lm

MFL Design Variables - 1
  • Mean flow line (MFL) is considered as a piecewise cubic curve along the length of the duct between the entry section and merging section

Lm : x-distance between the entry and merger section

y1, y2, z1, z2 : cubic polynomials for y(x) and z(x)

FLUENT CFD Conference 2003

mfl design variables 2
MFL Design Variables - 2
  • y1(x) = A0 + A1x + A2x2 + A3x3, y2(x) = B0 + B1x + B2x2 + B3x3
  • z1(x) = C0 + C1x + C2x2 + C3x3, z2(x) = D0 + D1x + D2x2 + D3x3
  • y1(Lm)= y2 (Lm), y1’(Lm)= y2’(Lm), y1”(Lm)= y2”(Lm)
  • z1(Lm)= z2 (Lm), z1’(Lm)= z2’(Lm), z1”(Lm)= z2”(Lm)
  • y1’(Centry) = y2’(Cmerger) = z1’ (Centry) = z2’(Cmerger) = 0
  • The shape of the MFL is controlled by 2 parameters which control the y and z coordinate of centroid at Lm/2
  • y(Lm/2) = y(0) + (y(L) – y(0)) αy 0 < αy < 1
  • z(Lm/2) = z(0) + (z(L) – z(0)) αz 0 < αz < 1

FLUENT CFD Conference 2003

area design variables 1
Area Design Variables – 1
  • Cross-section area at any station is interpolated from the entry and exit cross-sections
  • A(x) = A(0) + (A(Lm)–A(0)) * β(x)
  • corresponding points on entry and exit sections are linearly interpolated to obtain the shape of the intermediate sections and scaled appropriately
  • Psection = Pentry +(Pexit - Pentry) * β

FLUENT CFD Conference 2003

area design variables 2

β(Lm/3) and β(2Lm/3) is specified

β(x)

β2

1

β1

0

x

Lm/3

2Lm/3

Lm

0

Area Design Variables - 2

β variation is given by piecewise cubic curve as function of x

A0 + A1x + A2x2 + A3x3 0  β < β1

B0 + B1x + B2x2 + B3x3 β1  β  β2

C0 + C1x + C2x2 + C3x3 β2< β  1

β =

FLUENT CFD Conference 2003