Visualization Tool Helps Engineers Solve Fluid Flow Problems

AEA Technology Engineering Software, Inc. (Pittsburgh, Pa.) has dramatically improved engineers' ability to visualize the results of its software
N/Aany%> (Pittsburgh, Pa.) has dramatically improved engineers' ability to visualize the results of its software by taking advantage of powerful visualization capabilities offered by a standard graphics toolkit. AEA Technology's CFX-Visualize post-processor allows users to easily visualize fluid flow, heat transfer, chemical reactions and other complex engineering phenomena involving irregular geometries.

This visualization tool was developed in only 18 person-months using AVS5, a graphics toolkit from Advanced Visual Systems Inc. (Waltham, Mass.) that provides a comprehensive suite of data-visualization and analysis tools. AEA Technology developers estimate that providing the same capabilities from scratch would have taken approximately three or four times as much work.

Prior to the introduction of computational fluid dynamics (CFD) software such as AEA Technology's CFX, the only way to address a fluid flow problem was to build an experimental model. The problem with this approach is that it takes at least a week to build the model and the cost is also quite high. Any changes to the model, which are required to investigate alternative configurations, take additional time and money. Viewing an experiment of this type gives a general idea of how the fluids flow but provides only very crude quantitative information. The rapid increase in the performance of engineering workstations, combined with the release of commercial CFD products that are much easier to use than earlier codes, has led to a rapid proliferation of this technology

CFD involves the solution of the governing equations for fluid flow, heat and mass transfer, and chemistry at several thousand discrete points on a computational grid in the flow domain. The use of CFD enables engineers to obtain solutions for problems with complex geometries and boundary conditions. A CFD analysis yields values for fluid velocity, fluid temperature, and fluid concentration throughout the solution domain. Based on the analysis, a designer or an engineer can optimize fluid flow patterns or temperature distribution by adjusting either the geometry of the system or the boundary conditions such as inlet velocity or temperature, wall heat flux and so on.

Most commercial CFD codes contain three main elements: a preprocessor, a solver, and a post-processor. The pre-processor provides a platform for the user to define the geometry of the region of interest (the computational domain). The user subdivides the domain into a grid or mesh of cells or elements, selects the physical, chemical and fluid properties of the materials that make up the domain, and specify boundary conditions at the cells which touch the domain boundary.

The solver provides the numerical solution method. The set of equations that describe the processes of momentum, heat and mass transfer is known as the Navier-Stokes equations. These are partial differential equations, but can be discretized and solved numerically for real engineering flows. Equations describing other processes-such as combustion-can also be solved with the Navier-Stokes equations. Often, an approximating model is used to derive these additional equations, turbulence models being a particularly important example.

The final main component of CFD software is the post-processor, used to depict the results of the simulation. Modern post-processors present the numerical simulation results, such as pressure, velocity, temperature, etc. superimposed upon the image of the problem under study. This type of visualization provides the best opportunity for engineers to gain maximum understanding of the problem they are studying.

As one of the leading suppliers of CFD software, AEA Technology previously wrote their own visualization software using X-Windows graphics routines. According to David Clayworth of AEA Technology, this graphics tool was "limited to a small number of colors, did not provide interactive mouse control and had no animation or light shading." Faced with a challenging development project that would detract from core modeling and analysis improvements, AEA Technology management assigned a selection team to find a third-party package that would provide leading capabilities in both graphics and visualization. One critical issue was the need to support the wide range of UNIX platforms used by AEA Technology users.

AEA Technology development selected AVS because it offered the most powerful combination of graphics and visualization tools and supported all popular UNIX platforms. Clayworth said that by using this toolkit, the company was able to put together its post-processor package in only 18 man-months. The only work required was customizing the user interface supplied with AVS and writing code to convert the results file of their solver to the AVS format. "If we had tried to create the same functionality from scratch, it would have taken us at least ten times as long," Clayworth concluded.

The result, called CFX-Visualize, provides graphics and visualization capabilities equal to or greater than any other program in its class. With the software, analysts can create three-dimensional perspective views with hidden-line removal and light shading. They can trace the path of a marker traveling along with the fluid through a series of animated views. Engineers can depict an isosurface, a surface running through the points where a numeric value such as fluid velocity is constant, colored with contour plots depicting another numeric value such as temperature. These and the many other visualization techniques provided by the program can also be combined such as by plotting pressure contours on various surfaces while streamlines illustrate the flow.

AEA uses AVS' software in its CFX-Visualize program for
computational fluid dynamics.

In a typical application of this technology, CFD was used to optimize the design of a thermal mixer/flow straightener that helped improve the efficiency of a thermal electric generating plant. The mixer reduced the turbulent flow and temperature stratification of air supplied to the mills, which pulverize coal for the plant. Reducing thermal stratification improved the performance of the pulverizer, which in turn increased plant efficiency. Reducing flow turbulence and thermal stratification improves the accuracy of flow measurement equipment. The simulation optimized mixer design to the point that temperature stratification was reduced to 73°F while pressure drop was cut from 1.5 to 0.5 in H(subscript: 2)O.

The new visualization software also helps to design laboratory equipment that accurately models the performance of pilot and production process equipment. One of the most difficult jobs in the chemical industry has long been converting processes developed at the bench scale first to pilot plants and then to production plants. With computational fluid dynamics (CFD), Nalco engineers perform parametric simulations on a wide range of reactor designs. By designing a laboratory reactor that matches larger scale processes, particularly in the areas of mixing and shear, they have greatly reduced the amount of time and expense required to scale up processes.

The AEA Technology software also helped to improve the uniformity of fiber dyeing, finish application and solvent removal, generating over $1 million in additional revenues through grade enhancement. Over the past 20 years, Monsanto has experimented with a number of different designs of a multi-purpose device used to remove solvent from and apply various liquids to wet-spun fibers. Process variability made it difficult to objectively determine the relative performance of each of these designs. Analyzing each of the designs of the device with CFD made it possible to identify the one that provided the most uniform application, resulting in a 4 percent improvement in Grade A product over the plant's entire production volume.

AEA Technology developers are currently working on a new generation of post-processor based on a new graphics toolkit from Advanced Visual Systems called AVS/Express. Clayworth says that AVS/Express provides an object-oriented environment that significantly reduces the amount of time required to construct graphics applications. In addition, the new programming environment provides the developer with greater freedom in designing the user interface. That, according to Clayworth, will allow AEA to make the user interface of their next generation post-processor match the look and feel of their other applications to a tee.

For further information contact Advanced Visual Systems, 300 5th Ave, Waltham, MA 02151. Tel: 617-890-4300; Fax: 617-890-8287.

By Nick Basta