Teaching How to use the CFD Approach by an Example: Hydrodynamics within a Passenger Car Compartment in Motion
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This document presents a teaching methodology for Computational Fluid Dynamics (CFD) using the example of airflow in a moving passenger car compartment. It details the modeling process, including problem definition, domain construction, boundary conditions, and result analysis at different car speeds. The conclusions emphasize that CFD is a valuable tool, but its success relies heavily on the correct understanding of fluid dynamics principles and accurate representation throughout the modeling process.
![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 problem
The residual is monitored in a graphical way
along 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]
gpolanco3@hotmail.com /
nelsongarciapolanco@gmail.com
12](https://image.slidesharecdn.com/paperfesmd2009-78014-140211083556-phpapp01/85/Teaching-How-to-use-the-CFD-Approach-by-an-Example-Hydrodynamics-within-a-Passenger-Car-Compartment-in-Motion-12-320.jpg)
Teaching How to use the CFD Approach by an Example: Hydrodynamics within a Passenger Car Compartment in Motion
- 1. 2009 ASME Fluids Engineering Division Meeting (FEDSM2009), Colorado, USA. Teaching How to use the CFD Approach by an Example: Hydrodynamics within a Passenger Car Compartment in Motion Geanette Polanco, Nelson García-Polanco, Luis Rojas-Solórzano Universidad Simón Bolívar, Venezuela ISBN: 978-0-7918-4373-4 | eISBN: 978-0-7918-3855-6 Copyright © 2009 by ASME gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 1
- 2. The target To 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 gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 2
- 3. The example The 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. gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 3
- 4. 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 behavior The mathematical scheme used corresponds to the finite volume method. gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 4
- 5. Modeling process steps - Summary • Definition of the problem: Physics considerations Whole physics involved A particular topic of the actual situation Numerical accuracy Spatial consideration 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 gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 5
- 6. Modeling process steps - Summary The degree of success of the modeling is based on a good definition of the objective and aim of 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 • The iterative process of checking strongly depends on the management of the Fluids Mechanics knowledge • Results, Analysis and conclusion also depend on the Fluids Mechanics knowledge of the research. The results not always are as 100 % explicit or accurate as desired gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 6
- 7. Modeling process • Simplified 2D version of a cabin, based on prismatic shapes. •Two different air-cabin relative velocities: 50 and 100 km/h W 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 gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 3W L 2L 7
- 8. Modeling process / Domain and mesh checking – Velocity flow field For a distance 2L at the back of the car flow recirculation is presented Therefore 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. gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 8
- 9. 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 gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 9
- 10. 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 gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 10
- 11. Modeling process / Boundary Conditions Constant ambient pressure Constant Constant ambient pressure velocity profile V=50 km/h or V=100 km/h Constant ambient pressure Constant pressure condition also implies that the velocity field is developed in the perpendicular direction to the border, therefore, no recirculation 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. gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 11
- 12. 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 problem The residual is monitored in a graphical way along 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] gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 12
- 13. 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 file gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 13
- 14. 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 gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 14
- 15. 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. gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 15
- 16. Results III / Velocity flow field at car speed of 50 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. gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 16
- 17. Results IV / Pressure field at car speed of 50 km/h. Steady state The maximum pressure is located at front of the cabin as expected due to the stagnation condition. The minimum pressure is located inside the cabin. gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 17
- 18. 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. gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 18
- 19. 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. gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 19
- 20. 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. gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 20
- 21. Conclusions 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 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 gpolanco3@hotmail.com / nelsongarciapolanco@gmail.com 21