physical and cfd modeling as a design tool · good flow distribution however, “pillowing” on...

© 2014 HDR, Inc., all rights reserved.

Physica l and CFD Model ing as a Design ToolB r i a n T h o r s v o l d , P. E .

01 Introduction/Purpose

02 Physical vs. CFD Modeling

03 CFD Modeling Case Studies

04 Physical Modeling Case Studies

CFD = Computational Fluid Dynamics 1-D / 2-D Modelso HEC RASo WaterCADo Visual Hydraulics

3-D Models o Fluento Flow-3D

What is CFD?

Inexpensive insurance Optimize hydraulic performance Optimize operational efficiency Maximize equipment life Minimize maintenance Verify manufacturer’s claims

Why Model Dur ing Design?

CFD modeling Typically less expensive and faster

than physical modeling Sometimes precedes physical

modeling to “fine tune“ design to prevent having to build multiple physical models

Not recommended for turbulent flow

Physica l vs . CFD Model ing Considerat ions

Physical modeling Uses real water, gravity, and other

forces Necessary for turbulent flow

scenarios such as screening facilities and pumping stations

Important to select proper model scale (similitude):-Froude #: Inertial/Gravitational Force-Reynolds #: Inertial/Viscous Force-Weber #: Inertial/Surface Tension

Physica l vs . CFD Model ing Considerat ions

Expanding 40 MGD facility to 80 MGD by adding 8 filters

“Skewing” filter channel to avoid relocating high voltage power lines

Concerned about flow distribution changes based on past issues before “race track” was installed

Purpose was to demonstrate how sensitive distribution was to channel configuration

CFD Model ingF i l t e r I n f l u e n t C h a n n e l

Existing

Proposed

“Race Track”

CFD Model ing – F i l ter In f luent Channel

Pump 2

2

Layout 3 Final, Trial 1

Pump 5

46

810

12

14

16

13

57

9

11

13

15

17

18 Filters 1 – 18 in operation Filter pump 2 on at 24 MGD Filter pump 5 on at 24 MGD Total flow = 48 MGD Water surface elevation = 541.85

ft.


Filters 1 – 18 in operation Filter pump 2 on at 24 MGD Filter pump 5 on at 24 MGD Total flow = 48 MGD Water surface elevation = 541.85

ft.



ft.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Head

loss

(inc

hes)

Pipe Filter Number

Final Layout 3, Trial 1: Predicted Vs. Theoretical Headloss

Theoretical Headloss

Predicted Headloss


Results: Model demonstrated the problem prior to the

“race track” configuration Confirmed negligible flow variance between

filters from channel headloss for proposed filter alternatives

Allowed team to make design decisions based on cost and operability instead of concerns about potential process differences Filters 1 – 18 in operation

Filter pump 2 on at 24 MGD Filter pump 5 on at 24 MGD Total flow = 48 MGD Water surface elevation = 541.85

ft.

CFD Model ing – Compost ing Fac i l i ty

Maximizing operator comfort in the mixing room


ft.


Mixing Room – Preliminary Results


ft.


Mixing Room – Preliminary Recommendations Re-configure outlet duct to a single opening with bell-mouth located at the edge of

the mechanical room roofline. Modify fan locations to get optimum flow distribution at floor Run 4 fans during work hours to boost ventilation rate at floor if necessary


Mixing Room –Results with proposed changes


ft.


Results: Increased airflow at worker areas near floor Decreased pressure loss Improved air flow distribution Reduced cost of ductwork


ft.

Existing splitter box with significant flow imbalance

Deemed too undersized to be “fixable” even though CFD modeling might be able to improve flowsplit

Desired to ensure new splitter box would perform as intended

Physica l Model ingS p l i t t e r B o x

Physica l Model ing S p l i t t e r B o x

Initial Design Layout Plan and Section


ft.

Physica l Model ing – Spl i t ter Box

Geometric Similitude 1:3 scale


ft.


Dynamic Similitude Froude Number Re > 1x10-5

H over weir > 1”


ft.


Initial Design: Good flow distribution However, “pillowing”

on the back wall causes uneven flow over length of weirs


ft.


Recommended Improvement: Impact baffle


ft.


Results: Impact baffle

eliminates pillowing Even flow over length

of weir Confirmed even flow

distribution


ft.

Why Model? Space constraints causes

potential concern for “choking” Minimize grit deposition Confirm hydraulic performance Much cheaper to address any

issues in design than after startup Physical model because too

turbulent for CFD

Physica l Model ingS c r e e n i n g a n d G r i t F a c i l i t y

Model dimension layout


Model photos


Modeling results


Modeling Conclusions: Verified good hydraulic flow without

“choking” Verified no in channel “dead zones”

for grit deposition Indicated need to rotate channels in

operation to minimize grit deposition in front of off-line channels

No design changes necessary / recommended


Modeling: Becoming more prevalent as part of the design process Helps fine-tune and increase confidence in the performance of the structure Allows engineers to fix problems when they are easy and inexpensive to fix

CFD: typically preferred for non-turbulent flow structures like aeration basins, clarifiers, low velocity mixing applications, etc.

Physical Models: Necessary for turbulent flow such as pump intakes, headworks, etc.

SUMMARY

QUESTIONS?

physical and cfd modeling as a design tool · good flow distribution however, “pillowing” on...

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