Coupled Field Analysis using the ANSYS/Multiphysics Commercial FEA Code

Transcription

1 Industry Sector RTD Thematic Area Date Deliverable Nr Consumer Goods Multi Physics and Analysis 11th Sept 2002 Coupled Field Analysis using the ANSYS/Multiphysics Commercial FEA Code David Ellis Idac Ltd, London, United Kingdom Summary This presentation will describe the capabilities of the ANSYS program for solving Multiphysics Coupled Field solutions. An example analysis will be used to describe these capabilities in a practical sense.

2 Introduction - What is FEA? Finite Element Analysis (FEA), is a way to simulate and determine the response of a design to certain excitation and loading conditions FEA is widely used in industry to: - reduce amount of prototype testing - simulate designs not suitable for prototype testing Huge cost and time savings can be achieved with better designs.

3 Introduction - About ANSYS ANSYS is a complete FEA software package used by engineers worldwide in virtually all fields of engineering: Structural, Thermal, Electromagnetics, Electrical/electrostatics, and fluids, including Computational Fluid Dynamics (CFD) A partial list of industries in which ANSYS is used: - Aerospace - Electronics & appliances - Automotive - Power generation - Biomedical - Micro electromechanical - Civil & construction systems (MEMS) etc

4 Introduction - About ANSYS ANSYS/Multiphysics is the flagship ANSYS product which includes all physics capabilities in all engineering disciplines i.e. structural, thermal, low/high frequency electromagnetics, electrical/electrostatics, and CFD. There are three main Coupled-Field analysis component products derived from ANSYS/Multiphysics: ANSYS/Mechanical - Structural & Thermal capabilities ANSYS/Emag Electromagnetics & Electrostatics ANSYS/FLOTRAN - CFD & Thermal capabilities AI*EMAX High Frequency Electromagnetics (e.g. Antenna Design)

5 Coupled-Field Analysis Definition A coupled-field analysis is an analysis that takes into account the interaction (coupling) between two or more disciplines (fields) of engineering. The fact that each field depends upon another makes it impossible to solve the disciplines independently. Example Heating of a bimetallic bar requires a thermal-structural coupled field analysis. Piezoelectric analysis handles the interaction between the structural deformation and electric fields.

6 Applications in Electromagnetic/Mechanical Industries Structural-electromagnetic coupling. Examples are armature motion for a voltage fed actuator and induced current in a coil due to motion of a permanent magnet. Thermal-electromagnetic coupling. Examples are induction heating of a bar to temper (heat treat) the bar, bus bar temperature rise and coil temperature rise of a voltage fed winding. Fig. 1 An actuator armature rotates, then opens and closes the air gap

7 Applications in Electromagnetic/Mechanical Industries Electrostatic-structural. Example is electrostatic radial comb drive used in MEMS Electromagnetic-electrical circuit simulation. Example is a coil connected to an external circuit with resistors, inductors, capacitors and current/voltage supply Fig. 2 Radial comb drive Combinations of the above disciplines, e.g. electrostatic-thermal-structural coupling, and electro-thermal actuator Fig. 3 Induction heating of a bar

8 Types of Coupled-Field Analysis ANSYS is capable of performing steady state (static), harmonic and timevarying transient analyses for 2D, axisymmetric and 3D models. Two methods are available for coupled-field analysis. The sequential method involves two or more sequential analysis, each belonging to a different field. The fields are coupled by applying results from the first analysis as loads for the second analysis. The sequential method is more efficient and flexible for coupling situations which do not exhibit a high degree of non-linear interaction.

9 Types of Coupled-Field Analysis Using the direct method, special coupled-field elements having structural, thermal and electromagnetic degrees of freedom (DOF) are used. Direct coupling is advantageous when the coupled-field interaction is highly non-linear and is best solved in a single solution using a coupled formulation containing all the necessary terms in the element matrices or element load vectors. Examples of direct coupling include piezoelectric analysis, conjugate heat transfer with fluid flow, and circuit-emag analysis.

10 Example - Loudspeaker Analysis Solid model can be created in ANSYS, or imported from other CAD products in IGES, SAT, Pro/ENGINEER, Unigraphics, CATIA, or Parasolid format. Fig. 5 Half model of the loudspeaker A wide range of Boolean operations in 2D or 3D are supported in ANSYS

11 Loudspeaker Analysis Model A coupled-field analysis using the direct method was chosen. The 3D model was simplified into a 2D axisymmetric model. Two regions of air (stationary and moving) were added to the structural model using Boolean operation Fig. 6 Axisymmetric model

12 Loudspeaker Analysis FE Mesh A mixture of 4-noded electromagnetic elements and coupled-field elements are used for the stationary and moving regions, respectively. A robust, easy-to-use ANSYS Mesh Control was used to obtain finer mesh in the gap region and coarse mesh at the perimeter. Fig. 7 The FE mesh for the analysis

13 Loudspeaker Analysis FE Mesh Spring element (Combin14) and mass element (Mass21) are added to the moving coil to represent the structural stiffness and mass of the speaker cone and the moving coil Fig. 8 FE model with spring and mass element added

14 Loudspeaker Analysis Material Properties ANSYS supports both linear and non-linear, as well as temperaturedependent material properties. Typical material properties may be permeability, resistivity, coercive force or non-linear B-H curve for electromagnetic analysis; elastic modulus, Poisson s ratio, density and plasticity (if required) for structural analysis; and conductivity and film coefficient for thermal analysis. In this analysis, a non-linear B-H curve was used for the steel and the magnetisation curve of the ceramic magnet was input. A transient solution was performed for 20 cycles at 250Hz.

15 Loudspeaker Analysis Excitation and Boundary Conditions Goals for this analysis were to find the coil displacement and spring force over a range of excitation frequencies. Eddy current is also of interest. Flux parallel condition was applied to the edge of the model. Excitation was provided by the current density input into the coil element. The moving coil, structural air and spring elements were restrained in the radial direction. The displacement of all nodes in the coil have been coupled in the axial (vertical) direction. All the unused thermal DOF (Temp) have been constrained.

16 Loudspeaker Analysis Results - General Results are generally reviewed in the forms of contour plots, vector plots or result listings. Results may be values of DOF s (magnetic vector/scalar potential, displacement, temperature etc.) or their derivatives such as flux density for electromagnetic analysis, stress/strain and reaction forces for structural analysis. A time-history postprocessor is also available to review analysis results at specific locations in the model as a function of time. The results can be displayed as graph, or tabular listings, and math operations such as differentiation and integration can also be performed.

17 Loudspeaker Analysis Results Fig. 9 Plot of flux density, overlapped with the flux lines at 0.02 second

18 Loudspeaker Analysis Results Fig. 10 Graph of coil displacement vs time showing the effect of the excitation frequency on the natural frequency of speaker cone/coil

19 Loudspeaker Analysis Results Fig. 11 Eddy current generated in the steel at 0.02 second

20 Loudspeaker Analysis Conclusion Using a coupled field analysis the effect of the coil motion upon the various design parameters of the electromagnetic system can be fully investigated. The results may even be used in further analysis such as thermal response and/or structural deformation if needed. AVI shows results coupled with acoustics.

21 Other Applications This technology offered by ANSYS/Multiphysics has also been used to solve many other electromagnetic/structural problems such as actuators, motors and alternators. Hopefully this short presentation enables you to see and explore the whole new world of opportunities for multi-disciplinary engineering solutions in ANSYS!!!