This document summarizes a company's evaluation of using computational fluid dynamics (CFD) as a practical tool for analyzing large passenger vehicles. The company tested CFD on internal air flow, external air pressure distribution, and transient thermal analysis of a target vehicle model. For internal air flow, CFD required 80% CAD work and 20% model building and simulation took 108 hours. External air flow simulations converged within 5.9 hours on 128 cores. A thermal transient simulation took under 7 hours on 64 cores. The company found CFD gave good visualization results and was an excellent starting point to evolve their in-house CFD capabilities.

1. CFD a practical tool for large
passenger carrying vehicles
• Laurence Wood
• Team leader test & development
• Plaxton (Part of the ADL Group)
• Engineer
– mechanical
– Structural and Systems Analysis
– Structural and Systems Testing

2. Background
• As a business we are increasingly
challenged by product diversity and reduced
time to market.
• Our CAE approach is based around
improved problem understanding and
iterative advancement.
• With respect to CAE simulation tools we tend
to be Product rich and Resource Poor.
• But we are continually evolving.

3. We have evolved our CAE
capabilities over the years
• Hand Calcs
• Punched card analysis
• 1D FEA analysis, implicit
• 2D FEA analysis, implicit
• 2D FEA Crash analysis, explicit
• ~CFD~

4. The Opportunity
• As a rapidly growing business we face
increasing technical challenges which CFD
could potentially assist
• Cooling
• HVAC
• Aerodynamic & Fuel efficiency
• Poor weather Visibility

5. The Project
• Evaluate the practicalities of CFD with
respect to large passenger carrying vehicles
• Ease of model creation and interrogation
• Required knowledge to get simple results
• Required computing power to get simple
results
• Method of validation, (Visual Engineering
Validation)

6. Target Vehicle (E400)

7. Project outline
• Static internal air flow
• Static exterior pressure distribution
• Transient internal thermal analysis

8. Internal air flow
• HVAC Schematic
Out
Out
In InIn
Out
Extract
In
Out
InIn
Air Chiller

9. Level of Model Detail (Interior Air Flow)

10. Domain Inlets & Outlets
Heater Inlets into
driver cabin: 14.3m/s
Inlet fan: 7.5
m/s
Inlets
Top deck
floor: 1.7
m/s
Inlet Chiller: 180
ft3/min
Inlet from
Outside: 8.03m/s
Upper
Deck
Demister
s: 5.3 m/s

12. External Air Flow

13. Level of Model Detail (Exterior Air Flow)

14. External Air Flow

15. External pressure distribution

16. Reversed Air Flow

17. External air flow what ifs
Original Vehicle Geometry

18. External air flow what ifs
HyperMesh Mesh Morphing

19. Morphed Mesh Geometry

20. CD = 0.494 ; CL = 0.019Original Geometry

21. Iteration 1 CD = 0.461 ; CL = 0.020

22. Iteration 2 CD = 0.486 ; CL = 0.011

23. Iteration 4 CD = 0.452 ; CL = 0.007

24. Iteration 3 CD = 0.413 ; CL = 0.118

25. External Air Flow comparison results
Drag (N) Lift (N) CD CL L/D
Original 1241 48.3 0.494 0.019 0.039
Design1 1158 49.1 0.461 0.020 0.042
Design2 1220 27.6 0.486 0.011 0.023
Design4 1135 17.2 0.452 0.007 0.015
Design3 1036 297.1 0.413 0.118 0.287

26. Transient Thermal Analysis
Heater Inlets
into driver
cabin:
14.3m/s,
30°C
Inlet fan: 7.5
m/s, 30°C
Inlets Top
deck floor:
1.7 m/s, 30°C
Inlet Chiller: No Flow
Inlet from
Outside:
8.03m/s, 30°C
Upper Deck
Demisters:
5.3 m/s,
30°C

27. Temperature Results 566 second

28. Time Dependent Temperature Results

29. Summary of Resource
• Internal Air Flow:
– 80% CAD manipulation, 20% Model build, 108 hours
– Simulation Pre and Post Processing 32 hours
• External air Flow:
– Lift converged within 120 time steps: 5.9 hours run
time on 128 cores
• Thermal Transient:
– 300 sec simulation time required <7 hours on 64
cores

30. Internal CFD investigations Coach Racks

31. Outlet air velocities

32. Rack Air Flow, Stream Lines

33. Rack Air Flow, Particles

34. Rack Plenum Pressure Profile

35. Summary of Evaluation
• Current HyperMesh skills a good start point
• Acusolve CFD give good visualisation
results for a relatively simplistic modelling
approach
• For broad-based CFD problem appreciation
and understanding good engineering
visualisation correlation was achieved
• Excellent starting point to evolve in-house
CFD capabilities

36. CFD a practical tool for large
passenger carrying vehicles
Thank You