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Multi-Physics

Multi-physics is the art of combining multiple physics disciplines to solve a complex problem. Thermodynamics analysis, computational fluid dynamics analysis, and structural analysis are all serious disciplines on their own. Combining them creates exciting possibilities for solving comples problems Traditionally pyrotechnic inflators are designed using a set of ballistics equations, with final characterization through "tank testing". This trial-and-error process makes inflator development slow and expensive. Moreover, the tank test does not provide the temperature of the inflator gas. Various methods are in use to calculate the inflator mass flow rate from an estimated gas temperature. These methods go by names such as "Constant Temperature Method", "Dual Pressure Method", "Linear Burn Method", and others. They all predict slightly different mass flow rate curves that can affect the results of airbag simulation analysis. By combining fluid dynamics analysis, thermodynamics analysis, and structural analysis it is possible to model the inflator combustion process from measurable parameters, such as burn rate, flame temperature, and reaction heat.

The simulation analysis allows for detailed study of the stresses that occur in the inflator wall for design optimization. It further provides the combustor pressure, which varies both in the time and spatial domains. The mass flow rate and exit gas temperature directly follow from the analysis. For airbag simulation analysis, one can use the dynamic gas temperature in combination with the exit gas velocity for the most accurate approach to out-of-position occupant analysis.

This is a tremendously demanding computer simulation application that taxes even the most powerful computer clusters to the max. Lithium-Ion batteries are the popular choice for electric and hybrid-electric vehicles, because of their high energy density. For vehicle applications the conditioning of the temperature of the batteries is of vital importance to get the highest possible performance under a variety of environmental conditions. Coupled Thermal-Fluid analysis is used to analyze the cooling requirements under various loading conditions.

The animation shows how a Li-Ion battery cell warms up due to an increased duty cycle. When it reaches the top of its operating temperature a fluid pump is activated. The fluid flow carries the heat away and cools the battery down to its optimum operating temperature. Once sufficiently cool, the fluid pump is deactivated to preserve the available electrical power.

Acoustics

Ultrasonic transducers comprise a piezo-electric element that deforms when an electric voltage is applied to it. It deforms in the opposite way if the polarity is reversed. By rapidly changing the polarity the transducer can be made to vibrate and emit sound. Ultrasonic sound ranges in frequency from about 25 kHz to about 80 kHz in air. Lower frequencies become audible sound, whereas higher frequencies are absorbed too much by air to be of practical use. Analysis of these transducers involves fluid-structure interactions. Depicted here is the core of a -so called- open type transducer. It comprises a base and the piezo electric element onto which an impedance matching cone is mounted. The unit is less than 10mm in diameter. In the simulation the transducer is excited with a number of pulses at its resonant frequency, making the cone vibrate at increasing amplitude. This creates alternating wave of low and high pressure in the column of air above the cone, which is what creates the ultra-sound. When the drive pulse is halted the cone continues to vibrate for a while in a number of eigen modes. These cause complex wave patterns that need to be thoroughly understood. Sound waves travel through air as longitudinal pressure variations. Because they use air as their transport medium, they are affected by changes in that medium due to temperature, humidity, or flow. Because they are waves, they are diffracted, and attenuated and can form complex interference patterns. Understanding those diffraction patterns and their relationship with the characteristics of the transducer is subject of study.
On the right is an example of the study of sound waves, using the explicit finite element analysis program Dyna3D. In this simulation a sound wave is traveling through a straight channel until it reaches an opening at the end. There it forms the expected defracting pattern. What is of interest here though is that the corners of the channel act as sound wave sources and reflect pressure waves right back into the channel. Although in magnitude much smaller than the outgoing sound wave, such waves can obscure echoes from nearby objects and are therefore undesirable. By varying the shape of the channel, defraction and reflection patterns can be affected. Visualization of the pressure patterns by the analysis software, helps to understand the otherwise invisible sound waves. This study was made in conjunction with the development of the ATI/Autoliv Occupant Spatial Sensor System.

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