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Sound impossible? Well, it's true. Engineers can analyze problems and develop solutions faster and more cost effectively than just a few years ago. Gone is the need to tie strings and blow smoke over a prototype to find out what is going on inside the enclosure. CFD (Computational Fluid Dynamics), once the domain of PhDs and their mainframe computers is now the tool of engineers with a Pentium class desktop and undergraduate knowledge of fluid dynamics and thermal analysis. Once affordable only by cash-rich projects, CFD now analyzes simple power supplies and subsystems while recouping its cost on the first design assignment. CFD software models the system under investigation by solving the partial differential equations that describe the environment within the system. The output is displayed as 2D and 3D simulations that show airflow and temperature distribution. With a few mouse clicks, the user easily can play "what if" scenarios by repositioning components or substituting ones with different thermal characteristics. Fans or other cooling devices can be manipulated for optimum effectiveness and the size of enclosures and vents altered. "A number of companies don't use CFD because they think it is difficult to implement and also costly, requiring a dedicated individual with the appropriate background," states Dan Jones, senior principal engineer, Raytheon Systems Co. "That is not true anymore." Jones, a designer of military communications gear, uses Coolit, an MS-Windows based CFD package from Daat Research Corp., Hanover, NH that is specifically tuned for ease-of-use when modeling electronic enclosures. Notes Jones, "Within a day, I installed the software, built my first model and had an answer that I felt pretty confident was correct." Building The Model
To build a Coolit model, the user first outlines the system, identifying the boundaries of the enclosure and then places the components and complex objects, such as the motherboard, inside. These components and objects are retrievable from the software's library of readily customizable shapes which includes such objects as pin fin heat sinks. A user can create special objects by combining simple forms and add these new shapes to the software library for future use. Another option would be to import the whole system from CAD software. Once the geometry is constructed, the material properties of components are specified, including the thermal conductivity for heat conductors and the fan curve for air movers. These properties also can be retrieved from the software library; for example, if the material is specified as aluminum 6061-T4, the associated properties for this metal will be transferred automatically into the appropriate dialog box. Fan curves can be either drawn with the mouse or entered through a table. A specialty of Coolit is its ability to easily handle any combination of units; length in inches, thermal conductivity in W/m.K, power in BTUs and temperature in Fahrenheit. The software will automatically convert disparate units into compatible ones. Furthermore, the computed results can be displayed in one set of units, then re-displayed in another. "It's great not to have to keep converting units every time you have to model," declares Rohit Gupta, mechanical engineer, Teradyne Corp. Gupta uses Coolit to analyze thermal conditions within the large card cages that make up automated test equipment. "Manufacturers of fans give air flow in cubic feet per minute, and static pressure in inches of water; our hardware design group gives me numbers for thermal dissipation in watts, and my reference book has thermal conductivity in BTU per hour-square feet-degrees Fahrenheit," he explains. "This is a nightmare! I have to constantly monitor and convert units, and make sure I don't make a mistake while converting, or make sure that I consistently use either SI or English units. With Coolit you don't have this problem." Meshing The Model
To solve a thermal problem, Coolit automatically divides the domain under investigation into a contiguous set of grid cells (finite volumes) and calculates its way from cell to cell. An expert user can override the automatic grid generation and create his own or edit the automatically created grid. Typically, the finer the mesh used, the more accurate the solution and also the longer it will take to calculate it. As a rule of thumb, finer meshes should be used where a variable is changing rapidly, such as near inlets and outlets and around components. In theory, the grid cell size should be reduced to the point at which further shrinkage will not change the results. This produces what is known as a grid independent solution. In practice, however, it becomes a tradeoff between reasonable compute time and the perfect solution. The finer the grid the longer it takes to calculate it, but most applications do not need an answer accurate to the last decimal place. Accurate results depend on the user selecting the appropriate flow conditions, i.e. laminar or turbulent flow. If a laminar model is used on turbulent flow, the results will be inaccurate. Theoretical guidelines can suggest which type of flow is likely to occur, and the CFD software program provides these guidelines to direct the user. However, in real life, the choice may not be obvious, as often flow is mixed, with co-existing regions of laminar, turbulent and transitional flow. An eddy viscosity model (Secundov) provided with Coolit has experimentally proven that it will handle such mixed conditions quite well. Calculating The Results
Once the model is built, the user selects the convergence precision (i.e. the number of significant digits desired in the computed answer), and the type and contents of output desired (listing, plot files, variables to output). As the calculations proceed, a convergence monitor graphically depicts the rate at which the program is converging on the solution. Convergence precision is displayed as a horizontal line, and when all the solution residuals cross the line, the solution has converged. Once the calculations are completed, the user can display the results. He or she can zoom in or out on the model, rotate it and view cross-sections. Temperature distribution patterns are color-coded making hot spots glaringly obvious, while velocity vectors indicate the direction and speed of air movement through the volume. Because the two graphics are overlaid, the interaction between airflow and temperature becomes immediately apparent. In addition to the temperature and velocity vectors, other useful information can be displayed: heat flux vector and its components, pressure, turbulence intensity, etc. Double click on a fan, and the program will identify the flow and heat flux rates through it. In addition, the user can draw a rectangle just about anywhere in the flow field and the program will report the mean values of variables and fluxes through that rectangle. Multiple streamlines can be simultaneously displayed to show how flow varies throughout the enclosure. "It is so easy to build a model, and once it is built, I can visualize the flow by adding streamlines or injecting particles that I can watch them dynamically circulate through the system," declares Teradyne's Gupta. To activate the animation, the user positions a mouse-controlled injector in the 3D view and presses the GO button. The injector starts emitting particles, which flow through the enclosure at a speed proportional to the local flow velocity. As they travel, the particle color changes based on the magnitude of the selected variable (e.g. temperature). The user can examine these particles from any angle by dynamically rotating the domain and zooming in as the particles traverse the region. Payback
A CFD software package can be licensed for a particular period or purchased outright; fees usually include technical support and upgrades. An annual license can be currently obtained for under $13,000, and shorter term licenses are available to meet short term project needs. "Without a doubt," declares Gupta, "CFD analysis increases my productivity by orders of magnitude." Because CFD is so effective in increasing an engineer's productivity and reducing the development costs, the license fee can be returned many times over, even when a single system is analyzed. Typically, users save anywhere from $20-$30k and one month of engineering time for a small project, to $750k and several months or more when analyzing a large system. These savings come primarily from reducing the need for expensive prototyping. "A CFD package like Coolit can save weeks, if not months of work, depending on the complexity of the design and how intuitive you are in developing the initial cooling scheme," states Jeff Estrella, designer of 19-inch rack-mounted equipment at telecommunications manufacturer, Tellabs . "It eliminates having to stamp sheet metal, build physical models, test with thermocouples, and the whole nine yards." CFD also easily does what a physical test cannot; it identifies the temperature at every spot within the volume. A physical test is limited by the number of temperature sensors placed within the volume. If they are put in the wrong spots, the hot spot may be missed. CFD also shows airflow. An engineer may anticipate one flow pattern only to find that an upstream device is blocking cooling air from reaching a critical downstream component. "CFD is a very strong tool for catching thermal problems early," adds Anthony J. Hanford, Ph.D. thermal analyst at a major space mission systems and services company in Houston, TX. "If you have to wait until after a design is completed before actually testing it, then you box yourself into a corner with respect to your design options. Changes late in the game are more costly than those that can be implemented while there is still flexibility in component placement and choice." Making CFD Easier To Use
CFD tools have traditionally suffered from awkward operational sequences, complex menus, limited flexibility and have required advanced engineering skills to use effectively. In short, they were not easy to use. Daat Research attacked these problems by tapping windows functionality, adding "intelligent" decision-making to object-oriented software, and using large doses of common sense in laying out intuitive operational sequences (easier said than done). "When I first encountered CFD 15 years ago, the user had to understand both the physics and the numerical details," remembers Anthony J. Hanford. "Today, the code moves you in the right direction and you only have to have a very limited knowledge to make some smart decisions about what's happening with your solution." An example of this smart decision-making is Coolit's ability to select the computational resolution. "The software automatically picks the appropriate grids based on default parameters, yet an advanced user still has the option of overriding these selections with his own inputs," notes mechanical engineer, Mike Staiano who designs ruggedized computers at Miltope Corp., "Formerly, many of the parameters had to be specified by the user." "This allows you to focus on the modeling instead of on the numerics," adds Hanford, "something you couldn't do a few years ago." Taking advantage of the flexibility inherent in "drag and drop" functionality enables users to quickly change the shape of the enclosure or components, and reposition components within the enclosure. "It really helps to be able to move a component around with the mouse," notes Hanford, "particularly if it doesn't have to be in a specific position. You can also use "drag and drop" to add finer resolution where you want it; if there is a potential trouble spot, you just drag the gridlines closer around that area. The whole process is very, very visual and intuitive." "Unlike other packages, Coolit supports solid frame," adds Teradyne's Gupta, "which makes it much easier to distinguish individual components inside a complex model." Even the best functionality is wasted if the user can't easily tap it. Daat addressed this problem by designing simple pop-up menus and readily recognizable point & click icons. "Many other software packages put you on a "menu-hunting tip" which makes the modeling process very tedious," declares Gupta. "You have to pull down complex menus and submenus, and the whole process, quite frankly, becomes tedious and boring." Tying all the functionality into intuitive operational sequences sometimes can be the most difficult goal to achieve. The solution usually isn't based on technology, but on an understanding of how engineers think, and what a Ph.D. or software developer thinks is intuitive may not match the expectations of the average engineer or occasional user. Daat Research Corp. set aside its own views about what was logical and observed the thinking processes of others. It tapped senior engineering students from nearby Dartmouth College's Thayer School of Engineering as test subjects in an experiment to improve its CFD product. A thermal problem was explained to the students on paper and they were given a computer loaded with the software and told to go at it---without a manual! "We watched where the students stumbled and set about to eliminate those stumbling blocks," says Dr. Arik Dvinsky, president, Daat Research Corp. "We found that areas we thought were intuitive were not always so obvious to the first time user. The students made us aware of inconsistent and sometimes confusing terminology, and they made us rethink the layout of dialogs and menus." When all the intelligent decision-making and ease-of-use features were combined, Daat Research found a pleasant surprise. It no longer took a lengthy manual to describe how to use CFD; it had shrunk the manual to only 79 pages including 23 pages of background theory. "I was impressed with how small and to the point the Coolit manual is," says Gupta. "Other manuals are much larger, which leads to an information overload and makes it time consuming and frustrating to learn and use CFD." |
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