CFD: a complex mix of Physics, Engineering and Art!

The word "Computational" in the phrase "Computational Fluid Dynamics" is simply an adjective to "Fluid Dynamics". Hence, while dealing with aspects of CFD tool or process, it is vitally important to keep the physical understanding of fluid dynamics uppermost in user's mind as CFD has to do with physical problems.
Adapted from John D. Anderson, Jr (Computational Fluid Dynamics - The Basics with Applications).

We at CFDyna.com believe that any numerical simulation process is not just "Meshing, setting Boundary Conditions, Running Solver and Making Colourful Contour / Vector Plots". The results ultimately needs to be converted into a set of inputs for a robust design of the component of system. The sound knowledge of "underlying fluid mechanics principles and operating conditions of the problem set-up" are more important than just knowing how to use the software. Some of the requirements which will help a "CFD practitioners" take correct design decisions based on CFD results are:
  1. Basic of Fluid Mechanics such as Bernoulli's Equation and actual physics behind this principle
  2. Fluid Mass Conservation and Momentum Conservation with underlying mathematics
  3. Shear stress formulation on fluid flow, Empirical Calculations, Experimental Data
  4. Flow behaviour such as Developing Flow, Developed Flow, No Slip Condition, Separation and Re-attachment
  5. Complete theoretical and experimental behaviour of following flow conditions
    • Flow inside a circular duct
    • Flow over a circular cylinder
    • Flow over a flat plate
    • Flow between two parallel plates
    • Natural Convection in an Enclosed Cavity
Though the industrial flow configurations are far from being closer to these simple geometries, the fundamental ideas contained in them are indispensable to a good understanding of modern computation methods. The methods and results arrived at are important not only for these simple flow but also for the extension of our fundamental knowledge of turbulent flows in general. Methods for dealing with turbulent flow of any industrial applications could be devised only on the basis of the detailed experimental results obtained for them. For example, according to measurements performed by H. Kirsten, the entrance length of a turbulent flow in a pipe is about 50 to 100 diameters. This knowledge is very important in deciding the inlet boundary condition for any industrial internal flow configuration.

CFD: How to avoid it being the "Colourful Fluid Dynamics"?

CFD is a great tool when used with appropriate procedure and guidelines because of its inherent nature of multi-disciplinary science leading to technically unlimited potential and applications. Yet, "CFD is not a panacea of all your Flow and Heat Transfer problems without experience-base insight". Any result must be looked at by an experienced engineer in that field and must go through an "order-of-magnitude-check" before accepting the results.
  • CFD simulations are capable of predicting good qualitative results (trends). It will not make decisions for design engineers but certainly help them take more informed judgment. When no information is available about flow structure in a system, CFD is certainly an economical start into detail analysis of the performance of the system.
  • Even inaccurate CFD results, so long it is ensured mathematically physical, possesses many features which make it very useful:
    • The sheer capability of detailed visualization is rich in information.
    • CFD results give an insight which is not possible by experiments and other theoretical means.
    • Trends are usually reliable and leads to right direction in terms of design evaluations.
  • In many cases, quantitative information is predicted with sufficient accuracy to justify engineering design changes on commercial plant. CFD can even predict more useful information than testing because the measurement point (typically based on user experience) may not be at appropriate location.
  • Historical knowledge obtained from plant operation is a great validation tool for such numerical (also called Virtual) simulation results.
  • Notwithstanding the limitations mentioned above, CFD models drastically reduce implementation of "Design Modifications & Scale-up of a System"
  • CFD can be also used early in the design stage for performance evaluation, for optimization and enhancement during the development stage and for diagnostics in the later stage of the produce development cycle.
Excerts from H. Schilchting
All methods for the calculation of turbulent boundary layers are approximate ones and are based on the integral forms of the momentum and energy equations. Since, however, no general expressions for shear and dissipation in turbulent flow can be deduced by purely theoretical considerations, it is necessary to make additional suitable assumptions. These can only be obtained from the results of systematic measurements and, consequently, the calculation of turbulent boundary layers is semi-empirical.

While the usage of CFD simulations in industry is on rise at rapid pace, the credibility of results of any such calculation is still an area of concern. Most organizations using such codes, over time have evolved their own best practice guidelines to minimize the chances of "critical errors". There are many such guidelines issued by ERCOFTAC and AIAA. Following diagram summarizes classifications used to designate the types of error which needs to be addressed when CFD simulations are used to make design decisions beyond extant know-how of the company.

CFD, cfdyna.com

The content on CFDyna.com is being constantly refined and improvised with on-the-job experience, testing, and training. Examples might be simplified to improve insight into the physics and basic understanding. Linked pages, articles, references, and examples are constantly reviewed to reduce errors, but we cannot warrant full correctness of all content.