Solution in Ansys: Depending on the analysis you are doing Ansys gives you options on applying different loads and boundary conditions. Thermal analysis: -Heat flux : surface load (dimensions are for ex. W/mm2, look at direction of the vector). If you have a total power to apply on an area, you first have to divide it for the area and then apply it in Ansys. Please note that if you apply a heat flux on an external area everything is ok, if you apply it on an internal area that is shared by two volumes
Ansys count it as double, so you have to apply half of it. -Heat generation: body load (dimensions are for ex: W/mm3) -Heat flow: concentrated nodal loads. Use them only in lineelement models (conducting bars, convection links, etc.) where you cannot specify convections and heat fluxes Loads as boundary conditions: -Temperature -Convection coefficient H (ex. of dimensions W/mm2K). This coeff. represents the effect of the water or other fluids. You can calculate it considering fluid theory (Re, Pr, and so on). I can suggest a
spreadsheet made from Neal Hartman ( I have a copy, so ask if you need it.) This convection load will require a value for H and a value for the Tb of the water, meaning the temperature of the bulk of the water, this last value in Ansys represents only a reference point, so if you are interested in Delta T, you can also put Tb=0. This doesnt mean that your Tb of the water in reality will be at 0 deg. The convection coefficient that you calculated was based on the properties of the water (or general fluid) at a certain temperature and that temperature is the important one.
-Thermal radiation: radiation analysis are non linear analysis, Ansys solves them pretty well, but there is a different procedure that will not be covered in this tutorial. If you need any help on those let me know. It is a tricky analysis. You have to be careful in following all the right steps in the right sequence. I was thinking on writing a procedure also for this at least for a simple radiation problem to a space node. Structural analysis: Loads: -force, moment -pressure (surface load)
-structural temperatures -pre- stresses Loads as boundary conditions: -Displacements, constraints, simmetries Thermal-structural analysis There are two ways of doing a thermal structural analysis, one is called sequential way, the other one is called direct way. I will only show you the sequential way because it is simple and reliable and it is the only one that I use. Steps: -do your thermal analysis, and solve it.
-find your temperatures as result of the solution ( temperatures at each node) These will be written in a .rth file. -in the same model (nodes and elements must be the same) switch elements from thermal to structural (Ansys will automatically choose the structural element correspondent to the thermal element). -apply the structural temperatures to the nodes of your model from the file .rth that your previous analysis generated. With the command plotctrls- symbols body load symbols (structural temperatures) you can see the temperatures you applied to your body. This is an easy way to check that you grabbed the right temperatures. -apply boundary constraints (fix your model in space). -solve.
Solution preferences. If you are not familiar with the solvers, in this case of steady state thermal structural analysis you can let Ansys automatically decide what solver is better for your analysis just using the command -fast solution option, In which you will define the degree of accuracy you like. If you have to do more complicated analysis like non linear ones and so on, you must get familiar with the solvers to decide which one is the most accurate to use.
Postprocessor: allows to give a look to your results. There are two postprocessors: -General postprocessor You use the general postprocessor to review analysis results over the entire model, or selected portions of the model, for a specifically defined combination of loads at a single time or frequency. -Time History postprocessor: You enter the time history processor to process time or frequency related results data. We will consider the General Postprocessor General Postprocessor
-Read results -Plot Results Contour plot Thermal analysis: temperatures, heat flow Structural analysis : displacements, stresses -Path Operation it allows you to plot a graph of parameters along a specified direction. You can also list the variables to build your own excel table. -define path -map variables onto path
-operate on variables if needed (differentiate exponentiate) -plot path items -list path items -List Results -Query results Temperature plot of something that will not work. How to change the look of your results: PlotCtrls options Style:
-edge options: edge only , all -size and shape-number of facets per edge -contours -colors -background Window controls: This allows you to modify your window look. Animate I will not spend any time on this because this is just a plus, default settings are ok for most of the cases. If you need to change them you can learn how to do it.
Conclusions: Anytime you approach a FEA with whatever software you are using you must know what you are doing. You must understand units and properties of material, you must understand the thermal behavior and the structural behavior of your part. Constraints are very important, you must understand them. If you dont, you can make big mistakes. You dont try to convince yourself of results that make no sense, You dont believe results that you dont understand. If you have any doubt, ASK!!!. There are a lot of ways of being sure that your results are right ,for ex. comparing results, doing test analysis to become familiar with the
behavior of a part, ask people with more experience. Important contacts: 1) My contact: Daniela Cambie x 6234 2) Ansys support group: MCR Ph.: 916-787-1636 Mark and Scott Rodamaker are the technical people. You can reach them also by email: [email protected][email protected] 3) Chuck Lawrence is the CAD person in charge of Ansys.
4) Some people at the Lab have a lot of experience with Ansys and can probably help you, just to give you a couple of names: Carol Corradi and Schlomo Caspi.
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