Software for Fluid Power Technology

From Editor

The purpose of the Software Review section of the Journal is to present information to the reader about engineering software, including simulation programs, to highlight their specific features and their "fitness to purpose" in the unique field of fluid power and motion control. It is, of course, impossible to establish evaluation criteria matching the needs of all readers, therefore readers should not look for absolute ratings but more or less "fuzzy" opinions of the reviewer. A software program is like a wrench, just a tool to solve problems. It is good to solve some problems and not so good for others and this depends on both the nature of the problem and the users' attitude - and generally when we review software we do not know either. A software tool can be highly specialised and great for a some applications but not so well suited for others, on the other hand another software tool can be more flexible and generally applicable but without outstanding features. It is impossible, and even misleading, to say which one is better. What we hope to accomplish is to give the reader information necessary to take his/her own decision.

CFD with FLUENT and its Application in the Fluid Power Industry

Introduction

Researchers, equipment designers and process engineers are increasingly using computational fluid dynamics (CFD) to analyze the flow and performance of fluid power components such as pumps, valves, actuators, tanks and reservoirs, filters, pneumatic systems, motors, and much more. CFD allows for an in-depth analysis of the fluid mechanics and local effects in these types of equipment. In many cases this results in an increased system insight, improved performance, better reliability and more confident design. Thecontinual and exponential increase in computer power, improved physical models, and better user interfaces, now enables non-experts to use CFD as a design tool on a day to day basis. As a consequence, CFD has progressed from the domain of the mainframe to the high-end engineering workstation, and even laptop PCs. Typical benefits of CFD are as follows:

CFD can predict relevant data and system performance prior to any physical modelling.
CFD can be used when design correlations or experimental data are not available.
CFD provides comprehensive data that is not easily obtainable from experimental tests.
When evaluating design problems, CFD highlights the root cause, not just the effect.
CFD can be used to complement physical modelling. Some design engineers actually use CFD to analyze new systems before deciding which and how many validation tests need to be performed.
Many "what if" scenarios can often be analyzed in a very short time.

Fluent is the world largest provider of CFD Software and Services, specialized on CFD for more than 20 years. Fluent’s flagship software product, called FLUENT, is the leading CFD software in the world. The name, Fluent, is suggestive of smooth and rapid progress in engineering. Fluent strives to provide its customers with technology that integrates smoothly into their design process and enables them to make progress quickly. In 2002 the FLUENT-Code was the winner of the IMechE (UK) Heritage Award in recognition of its significant impact on knowledge, excellence and innovation in mechanical engineering.

Performing CFD Analyses with FLUENT

Computational Fluid Dynamics (CFD) is the science of predicting fluid flow, heat & mass transfer, and related phenomena by solving the mathematical equations which govern these processes using a numerical process (that is, on a computer). The result of CFD analyses like pressure drops, flow forces and moments or cavitation is relevant engineering data used in conceptual studies of new designs, detailed product development, troubleshooting and redesign.
To apply CFD, the geometry of interest is first divided, or discretized, into a number of computational cells. Discretization is the method of approximating the differential equations by a system of algebraic equations for the variables at some set of discrete locations in space and time. The discrete locations are referred to as the grid or the mesh (see Fig. 1). The continuous information from the exact solution of Navier-Stokes equation partial differential equations is now replaced with discrete values.
The number of cells can vary from a few thousand for a simple problem to millions of cells for very large and complicated problems. Cells can have a variety of different shapes. Triangular and quadrilateral cells are usually used for 2-D problems, in which the flow depends only on two spatial coordinates (for example, an axisymmetric actuator). For 3-D problems, in which the flow depends on all three spatial coordinates, for example, a complex safety relief valve, hexahedral, tetrahedral, pyramid, and prism shaped cells can be used. In the past, CFD codes required the use of structured grids containing one cell type, such as the brick shaped hexahedral elements, in which the cells were positioned in a regular pattern. FLUENT allows cells to be located in an irregular, unstructured pattern, giving much greater geometric flexibility. Additionally, FLUENT can accept grids consisting of a combination of different cell types, or hybrid grids, to address very complex geometry, providing flexibility to the CFD analyst.
Geometries are often created using computer aided design (CAD) software (e.g. SolidWorks, AutoCAD or Pro/ENGINEER). The geometry, either a wire frame or solid model, is exported to the grid generation software program GAMBIT to create the CFD-quality grid. This phase of the CFD analysis is referred to as pre-processing.
Once the grid has been created, boundary conditions need to be applied. Pressure, velocities, mass flows and scalars such as temperature, may be specified at inlets; temperature, wall shear rates or heat flux may be specified at walls; and pressure or flow rate splits may be specified at outlets. Additional, arbitrary kinematics for moving parts like valve slides or spools can be defined, e.g. by external definition, by results of sum of forces etc. The component material properties such as density and viscosity need to be prescribed as constant or selected from a database. These can be functions of temperature, pressure or any other state variable. The fluids like air or hydraulic oil can be modelled as either incompressible or compressible.

Fig. 1: The continuous physical domain of the pipe is spatially discretized into a number of computational cells represented by the surface grid shown

With the grid created, the boundary conditions and physical properties defined, the calculations can start. The FLUENT code will solve the appropriate conservation equations for all grid cells using an iterative procedure. FLUENT is the CFD solver of choice for all kinds of complex flows. Providing multiple choices of solver options, combined with a convergence-enhancing multigrid method, FLUENT delivers optimum solution efficiency and accuracy for a wide range of regimes. The wealth of physical models in FLUENT allows you to accurately predict laminar and turbulent flows with various modes of heat transfer, multiphase flows, cavitation, and other phenomena with complete mesh flexibility and solution-based mesh adaption. With FLUENT’s dynamic mesh modelling capabilities it is easy to ac-count for moving objects with mesh motion and deformation automatically handled by solver (see Fig. 2a and 2b.).

Fig. 2a: Valve motion: Initial position of the piston

Fig. 2b: Valve motion: Translatory moved piston with deformed mesh

The final result of the flow, turbulence, heat transfer and multiphase calculations will be a detailed map of the local velocities, pressure distribution, temperatures, flow forces, eventually cavitating zones, flow-induced noise generation, etc. These results can be analyzed in detail in FLUENT, using graphical visualization, calculation of overall parameters and integral volume or surface averages, and comparisons with experimental data (Fig. 3). This analysis phase is referred to as post-processing. Because of improvements in computer power and enhanced graphics software, it is now much easier for CFD analysts to create animations of their data (Fig. 4). Animations often help in understanding complex flow phenomena that are sometimes difficult to see from static plots. Examples of animations of the flow in various types of components are available on the web (www.fluent.com). Application specific templates may be easily defined in this way, that they compose self-controlled, in-detail post processing report (Fig. 5a and 5b).
In the past, CFD was the realm of high-powered computer systems. But much of today's modelling work can be accomplished on low-end Unix workstations, or high end PCs. A typical configuration might be a one or two-processor Intel Pentium or Compaq Alpha system, running Windows XP or Linux, and having between one-half and one gigabyte of memory. Unix workstations or PCs with one or two or more processors are also commonly used. These systems are adequate for the typical steady state and transient analysis. For complicated models and large problems requiring time-dependent calculations, the simulation can easily be distributed on a cluster of multiple inexpensive PCs running Windows XP or Linux as a parallel-computing network.

Fig. 3: Typical post processing of calculated results with flow path lines, contours of static pressure on wall surfaces, x-y-diagram of static-pressure-distribution along a flow-path line and a model summary

Fig. 4: Pressure Contours on wall surfaces during a transient flow calculation for a valve with moving parts

The user-friendliness of CFD software has also increased significantly. In the past, CFD software was characterized by text or command file based "interfaces" and difficult to configure solvers, that made fluid flow analysis the exclusive domain of highly trained experts. However, FLUENT and GAMBIT have been developed specifically to be used through graphical user interfaces, to have much more stable and robust solvers, and to allow easy geometry exchange between CAD programs and the CFD solver. This has allowed engineers who are not experts in fluid dynamics to make efficient use of CFD and use this technology on a day-to-day basis in their design and optimization work. Fluent provides training and continual technical support with the software licenses. The average engineer typically requires one week of training to get started using GAMBIT and FLUENT.

Fig. 5a: Automatic generated output report of a design template for valve analysis. The different piston positions are evaluated self-controlled by the application template

Fig. 5b: Contours of Velocity Magnitude for one valve design, automatically generated within a template report

Conclusion

In the past decade, the applicability of CFD in the fluid power industry has grown considerably. CFD is used as an analysis tool capable of providing extensive and detailed information about flow-related phenomena in many different types of components that can not be obtained in any other way. A broad range of newer and better models now exists, including models for Moving/Deforming objects, cavitation and flow induced noise generation. The technology is still improving and what was once the exclusive domain of highly specialized experts is now accessible to most engineers through increased computational power of common desktop computers and better interfaces to the CFD codes.

MST

Vendor	Fluent Inc.
Contact Person:	Markus Stephan, Fluent Deutschland GmbH
Address	Birkenweg 14a, D-64295 Darmstadt, Germany
Phone	+49 6151 3644 142
Fax	+49 6151 3644 44
Email	mst@fluent.de
Internet	http://www.fluent.com
Platforms	many different platforms including Linux and Windows-PC

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