More-intuitive pre-processors and advanced solvers are making computational fluid dynamics (CFD) software easier to use, more accurate, and faster. CFD techniques involve the solution of the Navier-Stokes equations that describe fluid-flow processes. Using MSC/ PATRAN as a starting point, AEA Technology plc, Harwell, Oxfordshire, England, has developed a pre-processor for its software that is fully computer-aided design (CAD)-compatible and works with native CAD databases such as CADDS 5, CATIA, Euclid3, Pro /ENG INEER, and Unigraphics. The simplicity of modeling complex geometries in CFX allows more details to be included in models, such as gangways between coaches, bogies, and even some parts of the pantograph. CFX 5's coupled solver offers a radically different approach that solves all the hydrodynamic equations as a single system. CFX 5 has demonstrated its ability to deliver much faster pre-processing and shorter run times, thus increasing productivity for its users. CFX 5.2 should be a further step forward in commercial CFD, with its mixed element types combining the accuracy of prismatic meshes adjacent to surfaces with the speed and geometric flexibility of tetrahedral elements in the remainder of the grid.
THE PAST 15 YEARS have seen computati onal fluid dyn amics (CFD) become an integral part of the engineering design and analysis environment. With CFD, engineers can simulate fluid-flow and hea t- transfer phenomena in a design, and understand how that design will perform before they decid e wh eth er to use it for manufac ture. T he savings that can be realized are obvious: fewer iterations to the fi n al d es ign, sh o rt er l ead times, fewer expensive pro totypes to produce, and so on. On top of this, CFD provides a cost- effective means of testing novel designs and concepts that would other wise be too expensive and r isky to investi ga te.
CFD techniques involve the solution of the N avierStokes equations that describe fluid-flow processes. T hese equations are extremely complex, and for many years co nsiderable effort was spent in developing the basic modeling skills required to solve them. Initially, only research institutions and universities were developing CFD 'codes, and the lack of effective user interfaces in these early software packages restri cted their use in large part to a few experts.
The 1980s saw the commercialization of CFD. As the number of users increased and CFD's benefits became more apparent to industry, demand grew fo r usable software. For users of these co des, the advantages of a comp e titive CFD marke t were immense: N o t only was there accelerated development in the software's numerics and ergon omics but develop ers were comp elle d to li st e n to users' wishes as well. CFD software is big busin ess today, and any successful CFD ho use has to anticipate the needs of the market.
In October 1996, AEA Technology plc-a leading CFD provider in H arwell, Oxfordshire, England, with U.S. h eadquar ters in Pittsburgh- released its CFX 5 software, which represents a major advance in ease of use, robustness, accuracy, and speed. A few months later, AEA acquired Advanced Scientific Computing Ltd. in Waterloo, Ontario, a CFD software developer that had recently launched a new produ ct, TAScflow for CAD. AEA has combined the two product lines to create CFX 5.2, new joint software that is scheduled for release during 1998.
Creating The Geometry
Nowadays, engineers without previous CFD experience tend to j udge software based on how friendly the graphical user interface is and how easy it is to generat e a model. Frequently, engineers are already working within a CAD/CAE environment (for design or stress analysis), and they expect the CFD environment to be just as welconu ng. To satisfy this market demand, CFD houses are directing more effort into developing ergononuc fi-ont ends for their CFD codes. As development resources are rarely unlimited, this may be at the expense of further development in the core of the CFD software.
To avoid this fate, AEA is working with MacNealSchwendler Corp. in Los Angeles, the largest provider of finite-elelTlent-analysis products to the CAE market. Using MSC/PATRAN as a starting point, AEA has developed a preprocessor for its software that is fully CAD-compatible and works with native CAD databases such as CADDS 5, CATIA, Euclid3, Pro/ENGINEER, and Unigraphics. "With this preprocessor," said Graham Westmacott, AEA's CFX general nunager, "CFX 5 becomes an integrated part of the design environment, which engineers can use with the same geometric model that they work with throughout the design/ analysis process."
Adtranz-a train manufacturer in Kaimar, Sweden, that had been using CFD for many years-chose CFX as its CFD tool. "Being able to import CAD geometries directly makes CFX 5 very fast to work with," said Hakan Bjork, CFD engineer at Adtranz. "It alJows me to investigate so many more scenarios-running trains in singleand two-track tunnels, on open fields, through curves with the train tilted, and so on. Setting up the problem is no longer the time-consunung task it used to be, so I get a lot more results for my invested money."
In addition, the simplicity of modeling complex geometries in CFX allows more details to be included in models, such as gangways between coaches, bogies, and even some parts of the pantograph. A typical application of CFX 5 at Adtranz is to show the predicted surface-pressure distribution and streamlines along a tilted high-speed train. "Using CFX has greatly reduced our dependence on wind-tunnel testing and significantly shortened development times," Hakan said.
Meshing The Flow Domain
Creating the geometric model of the design is not the end of the story, however. The engineer still needs to build a grid covering the flow domain. From the user's point of view, the most demanding part of a CFD calculation is mesh generation. The engineer first has to create a numerical description of the ge0111.etry of the object to. be investigated, then must divide the flow domain around the object into small volumes (creating a mesh or grid) within which the flow solution is calculated.
An automatic meshing feature is included in the current CFX 5 software. The solver uses an unstructured tetrahedral mesh, for which the user only specifies the geometry and surface grid. The solver then automatically performs the volume meshing, using a very fast Delaunay algorithm. This greatly speeds preprocessing and produces high-quality meshes that in turn ensure faster convergence.
CFX 5.2 will increase meshing flexibility even further by allowing mixed element types-tetrahedrons, hexahedrons, pyran-uds, and wedges-to be used at the same time. Considerable controversy has revolved around the use of tetrahedral elements in CFD software. The main arguments in their favor are that the mesh-gei1eration procedure can be automated and that high-quality meshes are easier to obtain when refining the grid locally to resolve features of interest. On the down side, tetrahedrons may be less accurate than a corresponding hexahedral mesh, especially in boundary layers where the flow is aligned with the geometry. For these types of flow, an alternative method of meshing, now being developed by AEA and General Electric Corporate Research and Development in Niskayuna, N.Y, will be incorporated in CFX 5.2. Tlus uses a revolutionary technique that will grow prismatic elements normal to a triangulated surface mesh, then switch to tetrahedrons away from the surfaces. This pernuts the use of meshes that are structured normal to surfaces and are thus aligned with the flow in a boundary layer.
Solving The Flow
Once the mesh is created, a solution procedure has to be applied to solve the flow equations. The nonlinearity of the momentum conservation equations makes this especially difficult.
Traditional solution methods use the iterative segregated Simple algorithm. At each iteration, Simple uses the existing pressure field to calculate a velocity field that satisfies momentum conservation, then corrects the pressures and velocities to satisfy continuity. In making these corrections, however, momentutTl conservation is lost, so the iterative process must continue_ As the solution progresses, the necessary corrections become smaller, and the figures for both mass and momentum converge on a solution.
CFX 5's coupled solver offers a radically different ap~ proach that so lves all the hydrodynamic equations as a single system. After one iteration, the velocity field and pressure alm.ost satisfy both momentum and mass conservation- but not entirely, because the equations are nonlinear. Iteration is still needed, but as momentum and mass continuity are always satisfied, far fewer iterations are necessary than with the Simple algorithm. Typically, CFX 5 requires only a few dozen iterations to converge, where a segregated solver would need hundreds or thousands. Furthermore, processing times for CFX 5 scale much better with mesh size than do those for the Simple algorithm. For the large meshes often required in real engineering simulations, this approach leads to significant reductions in run times and faster project completion.
GEC Alsthom Mirrlees Blackstone, a diesel-engine manufacturer in Stockport, Cheshire, England, uses CFX 5 as a design tool for the optimization of its engines. In a recent study, the firm's engineers analyzed the water flow within the cylinder- head cooling passages of a 10-megawatt engine used for power generation. They wanted to optinuze the engine further by developing an alternative cylinder-head design. This involved changes to the injector- seat region to minimize stresses, but these alterations also necessitated the redesign of the water inlet.
The engineers used CFD to determine whether the flow p attern of the cooling water would be adversely affected by these modifications, resulting in undesirable flow restrictions or inadequate heat transfer. "The project was originally started using Simple-based software," said David Bishop, design technical- services manager at Mirrlees Blackstone, "but the use of CFX 5 accelerated progress and allowed greater geometric accuracy." The new cylinder head is now being pro to typed and should greatly improve overall performance.
In another example, Sam Simonian, se nior engineer at Schlumberger U.S. in Sugarland, Tex., recently completed a project using CFX 5 to predict the torque from a turbine impeller foe the oil-drilling industry. The equipment is used for measurement while drilling (MWD), where its primary functions are to indicate direction and inclination while the drill proceeds as well as to measure natural radioactivity, used to recognize sand/ shale sequences. This information is crucial for the driller as he or she attempts to navigate toward the oil reserve. Communication from the MWD tool occurs via a mud-pulse telemetry system, which uses the change in flow resistance of a rotary valve (modulator) to transmi t pressure pulses through the mud to the driller. These pulses represent a binary code, which can then be interpreted to recover the original information.
The modulator is composed of a spear point carrying a fin impeller, which is fixed to the rotating part of the valve. To transmit the data, a control system momentarily unlocks, and the torque applied by the mud flowing past the fins causes it to rotate. The mechanism then locks the impeller again after it has rotated to a position where the valve resistance has changed from nunimum to maximum, or vice versa. This change in the relative position of the valve parts creates the pressure pulses.
The environment in which the device operates is extremely hostile. The mech anism can suffer severe erosion from solid materials carried in the mud, and the fins are subjected to very high loads when the device is locked. Schlumberger's aim therefore was to design a new impeller that would perform better under these adverse conditio ns. " I started building the model from a single point ," sai d Simonian, adding that CFX 5's automatic meshing made the work go quickly. Initial benchmark calculations comparing CFX 5 results with experiments agreed within 10 percent and gave Simonian confidence in using CFX 5 as a design tool. With the power to investigate the performance of many alternative de signs quickly, a new generation of turbine has been developed and is now being prototyped.
CFX 5 has demonstrated its ability to deliver much faster preprocessing and shorter run times, thus increasing productivity for its users. CFX 5.2 should be a further step forward in commercial CFD, with its mixed element types combining the accuracy of prismatic meshes adjacent to surfaces with the speed and geolTletric flexibility of tetrahedral elements in the remainder of the grid.