using computational fluid dynamics (cfd) analysis to ... · step 3 –defining physics settings ......


Session overview

What is CFD?

How does it work?

When should CFD be used?

CFD in the context of flooding and drainage

Case study of CFD / TUFLOW coupling process

Final Thoughts


Reality Simulation

What is CFD?

4 |4 |

How does it work?

Step 1 – Geometry Build

Step 2 – Meshing

Step 3 – Defining Physics Settings

Step 4 – Solving and post processing

▪ Define objective & scope

▪ Identify design parameters

▪ Collect input data

▪ Construct CAD representation of

CFD domain ▪ Generate numerical grid used for solving

CFD model

▪ Discretises the model domain into

thousands (or millions) of smaller cells

▪ Set up the Physics Models

▪ Define the fluid properties

▪ Create the boundary conditions

▪ Check the model stability

▪ Validation

▪ Debugging

▪ Results post processing

▪ Design verification against

design parameters

Inlet

Outlet

Coupling CFD Analysis and 1D/2D Flood Modelling | 15 October 2018

5 |5 |

How does it work?


Objective & scope?

Define key assumptions

Collect Data

Geometry

Build Mesh

Setup Model

Solve

Convergence criteria

Post processing | Calibration | Validation

Summarize

Project Complete

• 2D or 3D?

• Single phase or multi phase?

• Steady or unsteady?

• Rating curves

• Design flows

• Riprap/roughness parameters

• Chemical dosing regimes

• How many flow scenarios?

• Is CFD actually required?

• What are we trying to understand?

• Supplied or created?

• STL or CAD format?

• Boolean / translate / scale operations

• Parametric operations

• Mesh type: tetrahedral or hexahedral?

• Targeted mesh refinement – volume controls & surface controls

• Boundary layers / mesh inflation

• Best practice for mesh resolution

• Fluid properties (density, viscosity, isothermal?)

• Turbulence models (K-epsilon, SST / DES / LES)

• Boundary conditions (inlets / outlets / solid walls / glass walls etc)

• VOF or Eularian/Eularian?

• Output controls

• Roughness parameters

• Generate scenes

• Post process animations

• Post process data (python or excel)

• Comparisons to observations or data from known events or PHM

• Understanding limitations of the CFD relative to these events

• Stable residuals?

• Stability (total volume of water in model, inflow vs. outflow, WL monitors)

• Consistent band of oscillation in transient results

• What is the best way to convey the result with respect to the client’s interests?

Mesh quality

Physics & solver

parameters

Not converging?

Poor

solution?


When should CFD be used?

Empirical Calcs

Simple, purpose specific models

CFD sweet spot

Physical scale models (laws of nature)

• When the phenomena isn’t adequately

described by empirical equations

• When the phenomena isn’t adequately

described by simpler, purpose specific

models (if any)

• When physical scale models are not viable

(these can be an order of magnitude more

expensive)

• When you need (or want) a smarter, non-

standard design

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7 |7 |

CFD in the context of flooding and drainage


Flood modelling is an essential analysis tool in the civil engineering

design of major transport projects

Some structures require simplified (1D) representations in broader 2D

flood models.

• Bridges

• Culverts

• Pits & Pipes

Associated assumptions around head losses in complex cases can lead

to misrepresentations of fluid behaviour – CFD can help!

9 |9 |

Case Study


10 |10 |

Case Study


11 |11 |

Case Study

23 m3/s

2900mm ∅Main Drain

Junction Pit

1800mm ∅Cross Drain

7 m3/s

??? m3/s

??? m3/s


12 |12 |

Case Study

23 m3/s


Junction Pit


7 m3/s

??? m3/s

??? m3/s

CFD Model

Domain


13 |13 |

Case Study


14 |14 |

Case Study


15 |15 |

Case Study


16 |16 |

Case Study


17 |17 |

Case Study

𝐻𝑇 = 𝑍 +𝑃

𝜌𝑔+𝑉2

2𝑔

∆𝐻 = 𝐻𝑇 𝑈𝑝𝑠𝑡𝑟𝑒𝑎𝑚 − 𝐻𝑇 𝐷𝑜𝑤𝑛𝑠𝑡𝑟𝑒𝑎𝑚


18 |18 |

Case Study

ComponentHead Loss (ΔH) Form Loss Coefficient (α)

TUFLOW CFD TUFLOW CFD

2900 mm drain 0.13 m 0.27 m 0.25 (input) 0.42 (derived)

1800 mm drain 0.35 m 0.65 m 0.20 (input) 1.67 (derived)


19 |19 |

Case Study

ComponentDischarge (m3/s)

Flow Distribution

(% relative to total flow)

TUFLOW CFD TUFLOW CFD

Upstream 2900 mm drain 23.0 m3/s 77%

Upstream 1800 mm drain 7.0 m3/s 23 %

Downstream 2900 mm drain 22.9 m3/s 26.0 m3/s 76 % 87 %



20 |20 |

Case Study

23 m3/s


Junction Pit


7 m3/s

First Pass TUFLOW:

CFD:

22.9 m3/s

26.0 m3/s

First Pass TUFLOW:

CFD:

7.1 m3/s

4.0 m3/s



Head losses under-represented in TUFLOW model in comparison to CFD

model

Flow distribution between drain lines post junction pit is not well

represented in the TUFLOW model

What changes can be implemented to the TUFLOW model to achieve the

same outcomes found from the CFD study?

Case Study

22 |22 |

Case Study

ComponentManning’s ‘n’ Values

First Pass TUFLOW Model Revised Manning’s ‘n’ TUFLOW Model

2900 mm drain 0.013 0.008

1800 mm drain 0.013 0.070

Component

Discharge (m3/s)Flow Distribution

(% relative to total flow)

Revised

TUFLOWCFD

Revised

TUFLOWCFD

Upstream 2900 mm drain 23.0 m3/s 77%

Upstream 1800 mm drain 7.0 m3/s 23 %




23 |23 |

Case Study

23 m3/s


Junction Pit


7 m3/s

First Pass TUFLOW:

Revised Manning’s TUFLOW:

CFD:

22.9 m3/s

25.6 m3/s

26.0 m3/s

First Pass TUFLOW:

Revised Manning’s TUFLOW:

CFD:

7.1 m3/s

4.4 m3/s

4.0 m3/s


24 |24 |

Final Thoughts


Mischaracterisation of flow distribution has broader implications for:

• Flood level afflux;

• Exposure to risk of flood damages (due to under-design);

• Excess cost of construction (due to over-design); and

• Associated approvals.

There is value in using CFD to resolve the behaviour of complex hydraulic structures in broader 1D/2D flood models

Unique problems require site specific, customised solutions.

CFD offers one such solution for fluid-flow problems.

25 |25 |

Final Thoughts


26 |26 |

Thank you for listening


For more information please contact:

[email protected]

Acknowledgements

Dr. Luana Stefanon – Senior Water Engineer – GHD

using computational fluid dynamics (cfd) analysis to ... · step 3 –defining physics settings ......

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