Pictures of the Future Spring 2006
Simulation – Applications
Current Events
Computer simulations are an indispensable tool in industry. They replace many physical tests, accelerate development times and cut costs. Dynamic systems that can be visualized in this manner range from wind and gas turbines to micro reactors, magnetic resonance tomographs and airbags.
Sometimes physics is really simple. Last summer, for instance, during a holiday in South Tyrol, Italy, Dr. Arno Steckenborn’s children threw leaves into the "Waale"—channels that have irrigated the orchards in Meran since the 13th century. At the shores, the leaves tended to rotate without really going anywhere, and only continued on their way when the children helped them along with sticks. Steckenborn, a physicist with Siemens Corporate Technology (CT) in Berlin, is also familiar with the phenomenon of dancing leaves—especially when it comes to channels. The only real difference is that the channels Steckenborn works with are as fine as a hair and are designed for micro reactors, minuscule devices in which chemicals and drugs will be manufactured in the future. (see Pictures of the Future, Fall 2002, Miniaturization).
To the fruit-growers of Meran the foliage is a nuisance because it blocks drainage filters. But the same process is at work in the channels of the micro reactors being tested in Steckenborn’s Berlin laboratory, where chemicals tend to anchor themselves in spots. But because it’s impossible to look inside a micro reactor, Steckenborn and his colleagues make the currents visible with the help of a simulation program in which all physical parameters, such as material characteristics, the geometry of the micro channels and the viscosity and temperature of the fluid are defined.
Simulations illustrate how turbulence helps two liquids (red and yellow) to mix better at a high flow velocity (right) than at a lower velocity (left)
The CFD program (computational fluid dynamics) then provides current profiles, temperature distribution—and sometimes surprises, as in the case of a micro mixer that was designed to mix two fluids warmed to different temperatures.
Here, a simulation showed that the two chemicals were flowing parallel to one another, like honey in a glass of cold milk. While vortexes are generally frowned upon, in this case they were the solution—the spoon in the milk, so to speak. Steckenborn’s team solved the problem by having one fluid flow in slightly under the other so that it overshot the target somewhat and mixed with the other fluid. The simulation showed that this resulted in completely equalized temperatures in two channels at the other end of the mixer.
Programmed Reality. "Simulation saves time and money, and it shows us which parameters we can adjust," says Steckenborn. The advantages are worthwhile even if designing a model in a computer takes months. After all, manufacturing numerous prototypes would be even more expensive and time-consuming. But there’s one thing simulations can’t do right now: replace experiments. Instead, they help researchers to prepare practical tests and make decisions regarding where changes are worthwhile. "Like a basketball player, we make a lunge and look at the model to see in which direction to play the ball," explains Steckenborn. The micro mixer model is thus first given the characteristics of known agents, like nitrogen and water. Only when the model and reality are in agreement are other fluids sent through the micro reactor virtually. But by then nearly any change to, for example, the mixer’s geometry or the temperature of the media flowing in, can be made with a mouse click.
The actual role of CFD software is to solve the Navier-Stokes equations, which describe fluid and gas flows. Much to the delight of physicists, these equations are so universal that they are as valid for hair-thin channels as they are for a 50-m rotor blade on a wind turbine (see Fault-free windmills). That’s why Jesper Laursen, an engineer with Siemens Wind Power in Brande, Denmark, also uses the CFD software, which, incidentally, costs tens of thousands of euros. Using the software, a rotor blade’s contours emerge on Jesper’s monitor, surrounded by lines depicting computed paths of air molecules. The air flows evenly onto the outer two-thirds of a blade, is slowed by it and creates suction that moves the blade sideways, driving the generator.
"Wind blade manufacturers have paid too little attention to the inner part of the rotor until now," says Laursen as he points to the area where the blade is attached to the shaft. The flow makes a sharp bend there and breaks off—and that costs energy. Laursen needed several weeks to model the rotor blade in a computer, but the effort has been worth it. With a few mouse clicks he can now change the position of the two million dots that envelop the surface of the rotor like a tightly-woven net and are stored on a 1-Gbyte file. It then takes a day before a cluster of several PCs has re-calculated the flow at each point.
Better MR images thanks to simulation
We don’t know if California’s Governor Arnold Schwarzenegger was examined using magnetic resonance imaging (MRI) after his motorcycle accident in January 2006. What is certain, however, is that the high-frequency power absorbed by the former Mr. Universe would have been relatively high, for "muscles have a high electrical conductivity and warm up more," says Dr. Dirk Diehl, a physicist with Siemens Corporate Technology in Erlangen. Because a specific absorption rate (SAR, in watts per kilogram body weight) may not be exceeded, MRI personnel have to enter the height and weight of each patient prior to an examination. The machine then limits the high-frequency electromagnetic pulses that stimulate tissues, causing the tissues to emit signals from which an image can be generated. Because height and weight reveal relatively little about the actual physical condition of the patient, however, MR tomographs adhere to a generous safety margin, which—on the other hand—increases examination time and reduces the sharpness of the image. To better utilize the leeway stipulated by statutory limits, Diehl is modeling an MR tomograph with Microwave Studio software. He is also placing virtual people of varying sizes in it. In total, 40 different types of tissue have been simulated. A red color immediately tells Diehl where the fields and the SAR values—and thus the temperature—are especially high. There are hotspots in the tissue, for instance, near the wall of the MR tube and also in many muscles. Heavy people have an advantage, says Diehl, because "fat absorbs less high-frequency power." He’s also keeping an eye on the homogeneity of the electromagnetic fields. In the newest MRI systems, which have a magnetic flux density of 3 tesla or higher, the intensity of the magnetic field fluctuates greatly depending on tissue penetration, making it necessary for a control program to compensate for this. But thanks to simulation, the program can be tailored to each patient, thus making it possible to make the field distribution more homogeneous and further improve image quality.
Laursen has saved his most promising results for an improved blade form with an even flow from root to tip. In his simulations, the sharp back blade edge no longer gently changes to a cylindrically shaped base, but runs, slightly flattened, all the way into the blade mounting on the shaft. Today, windmills can transform 45 % of the wind energy into power. "Two to three percentage points more efficiency could be achieved with the new rotor shape," Laursen suggests. That translates into 500 MWh more energy yield in a year—and that’s what really interests customers.
Designing Turbines that Don’t Resonate. Efficiency is also vital to gas power plant operators—especially since the price of natural gas have skyrocketed. Power authorities like Siemens because combined-cycle power plants made by Siemens have an energy efficiency of just under 60 %, and thus are the most efficient in the world (see Power Plants). This achievement is largely due to the shape of the turbine blades. In close cooperation with Siemens Power Generation (PG), the Simulation and Risk Analysis Competence Center at Siemens Corporate Technology in Munich simulates gas and steam turbine blades in order to squeeze additional efficiency out of them. In doing so, team members, who are all engineers and mathematicians, work in much the same way as their colleagues in Denmark who have been designing wind turbines. The rotor blades are described mathematically by a tightly woven net, and the computer solves the Navier-Stokes equations. Here, the target function is efficiency.
Computer simulations determine how air currents are affected by a change in the shape of rotor blades
Once a mathematical model has been created, a cluster of workstations requires a week to compute parameters that include blade curvature and twist, and to find the optimum blade geometry. "Flow simulation takes the lion’s share of the time," says the Competence Center’s Dr. Utz Wever. The specifications, however, are narrow because not every ideal mathematical solution can be produced in a PG plant. After all, the blades have to withstand extreme gas temperatures in excess of 1,500 °C, which limits the selection of materials and the range of practical shapes.
And there’s another problem that causes headaches. The combustion chamber of a gas turbine, which can contain up to 24 gas burners and the housing, behaves like the strings and body of a violin—the rhythmic oscillation of the gas flame can cause loud humming sounds at between 90 and 500 Hz. If their frequency is in harmony with the resonant frequency of the housing, the vibrations can build up to the point where they can damage the turbine. It’s a phenomenon that all gas turbine manufacturers have to contend with. Before things go too far, however, the gas turbine is shut down.
What’s the optimum shape for a turbine blade? How much does the pressure in a combustion chamber oscillate? Computer simulations provide the answers
In the meantime experts have managed to recreate the ring combustion chamber in a computer using a two-step simulation. They can also predict why and at which frequency the vibrations are created in different gas turbines. The first step, an acoustic simulation, is carried out in Mülheim an der Ruhr, Germany. This phase simulates the pure acoustics of the combustion chamber under the thermal conditions that are present during operation. Here, a finite element method is used and the flame isn’t taken into account. In this way, the resonances in the combustion chamber can be determined—just as in a concert hall in which the acoustics are measured.
CT researchers then combine these acoustic data with the characteristics of the burners and flames in a 3D stability analysis, which computes the pressure changes in the combustion chamber. The geometry can be easily changed and, for example, the gas injection point can be moved in the software. The simulation shows when the coupling of combustion instabilities reaches its lowest acoustic level. That’s when the turbine remains silent.
"The simulation matches the experiment very closely," explains Dr. Sven Bethke, a thermoacoustics expert at Siemens in Mülheim an der Ruhr. Until recently, Bethke and Wever computed only existing gas turbine types in order to figure out how to interpret the results of simulations. This year individual burners and combustion chambers will be put to the test, evaluated with the help of an acoustics simulation, and optimized even further. And in 2007, in Irsching, Bavaria, the first gas turbine with a thermoacoustic design that takes the new 3D stability analysis into account is expected to be operational. Output will change as a result because the turbine’s decreased tendency to hum will give it a markedly increased operating range. "It will be one of the most powerful gas turbines in the world," predicts Wever. Indeed, this combined-cycle power plant with its modified turbine is expected to reach the magical figure of 60 % energy efficiency—a first.
Bernd Müller
Dummies and simulations provide smart results
The wire mesh skull slowly moves forward and nestles into a plastic pillow. Then a computer simulation makes the dummy’s head spring back gently. "And this is what it looks like in reality," says Gerd Scholpp. In a video, the dummy’s head crashes against the vehicle’s A-pillar like a cannonball. If it had been a person rather than a dummy, the head would nevertheless only be bruised—if a simulation program developed by Siemens Restraint Systems in Alzenau, Germany had optimized the A-pillar. Otherwise such an impact would be fatal. Siemens’ Alzenau Development Center tests restraint and safety systems, such as seatbelts, airbags and pedestrian protection systems for the automobile industry. Nothing happens without simulations, because simulations save time and money. Recreating the model of a crash in a computer requires several months for two of Scholpp’s coworkers. But at a cost of up to a million euros, crashing a real prototype into a wall is considerably more expensive. In addition, the simulations can be repeated, and the parameters can be modified any number of times, in order to determine the best protection system. Even so, crash tests or sled experiments continue to be conducted in the 160-m-long hall under Scholpp’s office on a daily basis. "We need the genuine tests to check the computer model," says Scholpp, a mechanical engineer. Software tools are so sophisticated these days that the forces exerted on passengers and belts can be pre-defined, and the simulation then computes how the restraint system must be designed. The vehicle’s geometry is provided by the car manufacturers, and the Alzenau team provides the virtual dummies. At the same time, the trend is moving away from inflexible dummy models and toward simulated humans fashioned using the finite element method, which permits the simulation of tissue deformation and the breaking of bones. Rollover trials with dummies indicated significant need for further development in this area. Until now a virtual dummy behaved like a sack of sand and simply slid to one side. In such a case, the side airbag would have to trigger as soon as possible. In reality, however, passengers would instinctively tense their muscles and try to counteract the movement of the vehicle. Says Scholpp, "That gives us a few more fractions of a second."