teaching how to use the cfd approach by an example: hydrodynamics within a passenger car compartment...

Teaching How to use the CFD Approach by an Example: Hydrodynamics within a Passenger Car Compartment in Motion

2009 ASME Fluids Engineering Division Meeting (FEDSM2009), Colorado, USA.

Geanette Polanco, Nelson García-Polanco, Luis Rojas-SolórzanoUniversidad Simón Bolívar, Venezuela

ISBN: 978-0-7918-4373-4 | eISBN: 978-0-7918-3855-6

Copyright © 2009 by ASME

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[email protected] / [email protected]

The targetTo teach in a effectible way Computational Fluid Dynamics

to student or engineers in a formation process

The aim of this work

To illustrate the CFD technique application using an example on the study of flow field of a passenger car compartment in motion

The aim of this work

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The exampleThe example taken represents the motion of a car compartment,

with indoor air flow produced by the interaction between the cabin inner air with the external flow through two glass windows (one in the front seat and one in the back seat).

This configuration could represent a common situation for the passenger car compartment. The study covers two different car speeds, 50 and 100 km/h.

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Governing equations and mathematical scheme

As a fluid mechanics problem the Navier-Stokes equations are using to represent the flow interaction with the car compartment, which is assumed completely solid without any deformation produced by the flow.

The k-ε turbulence was selected to reproduce the turbulence The k-ε turbulence was selected to reproduce the turbulence behavior

The mathematical scheme used corresponds to the finite volume method.

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• Definition of the problem:

Physics considerations

Whole physics involved

A particular topic of the actual situation

Numerical accuracy

Spatial consideration

Two-dimensional approach (2D)

Modeling process steps - Summary

Two-dimensional approach (2D)

Full three-dimensional approach (3D)

• Computational model and domain construction

• Application of suitable boundary conditions

• Domain and mesh checking process

• Results

• Results analysis

• Conclusion remarks

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The degree of success of the modeling is based on a good

definition of the objective and aimof the work, which will

define the rest of the steps for the modeling process!

The results to be obtained will obey directly to the definition

proposed

Modeling process steps - Summary

proposed

• The iterative process of checking strongly depends on the management

of the Fluids Mechanics knowledge

• Results, Analysis and conclusion also depend on the FluidsMechanics

knowledge of the research. The results not always are as 100 %

explicit or accurate as desired

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• Simplified 2D version of a cabin, based on prismatic shapes.

Modeling process

•Two different air-cabin relative

velocities: 50 and 100 km/h

3W

L

W

2L

The domain size is described based on the cabin length, L, and wide, W, of the model which correspond to 2 m and 1. 3 m, respectively

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Modeling process / Domain and mesh checking – Veloci ty flow field

For a distance 2L at the back of the car flow recirculation is presentedTherefore a larger distant behind the car is needed.This demonstrates the importance of the appropriate downstream length when complying the constant pressure-developed flow condition at outflows.

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Modeling process / Changes in the domain

• As the result of flow field analyses a

new domain was established with

reduced lateral dimensions from

twice the wide of the car up to one

time the wide of the car and larger

area behind the car

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Modeling process / Changes in the Mesh

• The mesh takes into account the walls and the internal

space of the cabin

• Mesh sensibility was also tested

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Modeling process / Boundary Conditions

Constant

velocity profile

V=50 km/h

or

V=100 km/h

Constant ambient pressure

Constant

ambient

pressure

Constant ambient pressure

Constant pressure condition also implies that the velocity field is developed in the

perpendicular direction to the border, therefore, no recir culation can be presented.

If occurs, this means the condition can not be fulfilled and a change in the domain

must be done or in some cases the applicability of this kind of boundary must be

reassessed.11


Modeling process / Convergence

Convergence criteria:

Usually the criteria to stop a simulation are based on the residual values of each

variable calculated during the simulation.

It is possible to change the defaults values pre-established for each variable or even it

is possible to prioritize the residuals of some variables over others which can be

ignored, due to the physics involved in the problemignored, due to the physics involved in the problem

The residual is monitored in a graphical wayalong with a text file which contains all the information available to further analysis.

The typical defaults values are:10-3 for mass flow [kg/s]10-3 for velocities [m/s]10-2 for pressure [Pa]

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Results/ Different formats

As part of the advantages of the CFD technique the results of a

simulation can be extracted under various formats, such as:

•Spatial vector flow profiles

•Contours profiles

•Profiles over a specific line

•Data file13


Results I / Velocity flow field at car speed of 50 km/h. Steady state

Velocity profile for the new domain tested.No recirculation is presented at the border. However, it is important to mention that uniform flow pattern is not achieved which suggests that a new length must be introduce to avoid any influence of the boundary condition on the results

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Results II / Velocity flow field at car speed of 50 km/h – Zoom

Steady state

vortex

As expected, the vortex shedding phenomenon appeared breaking the symmetry of the flow field.

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Results III / Velocity flow field at car speed of 5 0 km/h – Cabin.

Steady state

Flow field around the cabin does not show symmetry.These results have the same general trend of the simulation performed for transient conditions.

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Results IV / Pressure field at car speed of 50 km/h . Steady state

The maximum pressure is located at front of the cabinas expected due to the stagnation condition. Theminimum pressure is located inside the cabin.

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Results V / Velocity field at car speed of 50 km/h. Transient. Cabin

Internal flow recirculation and the interaction at the glass windows location between the external and internal flow is shown. The main direction of the flow is from the back window to the pilot window for both car speeds tested.

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Results VI / Velocity field at car speed of 50 km/h . Transient

The pressure field corresponding to transient cases also keeps the same characteristics of the steady state case.The max and min pressure are located at the same points of the steady state simulation and the other speed car velocity tested.

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Results VII / v - ε fields at car speed of 50 km/h. Transient

�No major differences are observed respect to the steady state condition.

� The zones with more energy dissipation are located on the outside part of the turbulent structure behind the cabin.

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The use of CFD technique allows the student to apply the basic concepts

of fluid dynamics in the study and analysis of a new designs or

prototypes in any area of engineering. CFD is a computational tool and

therefore it can not overcome in any situation the understanding of the

physics involved in the problem studied by the user. The success of the

CFD application to a particular problem is based on the correct

Conclusions

CFD application to a particular problem is based on the correct

representation of the reality in every single phase of the modeling

process and the correct interpretations of the obtained results.

REFERENCE: Paper No. FEDSM2009-78014, pp. 251-257; 7pages doi:10.1115/FEDSM2009-78014

From:ASME 2009 Fluids Engineering Division Summer Meeting, Volume 2: Fora, Vail, Colorado, USA, August 2–6, 2009

Conference Sponsors: Fluids Engineering Division, ISBN: 978-0-7918-4373-4 | eISBN: 978-0-7918-3855-6

Copyright © 2009 by ASME

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teaching how to use the cfd approach by an example: hydrodynamics within a passenger car compartment...

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