More and more often we have to perform combined analyzes, i.e. thermal (CFD) plus strength. Ansys offers for us one-way or two-way FSI analyzes (FSI - fluid structure interaction).
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| One way Fluid Structure Interaction in Ansys Workbench |
One-way analyzes are based on the assumption of the boundary conditions from CFD analysis to structural analysis. Most often these are temperature distributions on Heat Transfer Analysis transfered to mechanical program. Often in Ansys we use a combination of Fluent / CFX and Static Structural.
In two-way analyzes, we have a feedback link from mechanical analysis to CFD where, based on iteration, the volume and shape of the analyzed elements are updated. It is a closed system in which a certain defined condition must exist for the analysis to be considered as finished. You can read more about the two-way FSI analysis here.
Microsoft PowerPoint - ANSYS UGM_Wolmarans_Craig_3Sep2018.pptx (qfinsoft.co.za)
In today's post, we'll tackle one-way FSI analysis. The programs in which we will perform this analysis will be Fluent (transient analysis) and Static Structural (static analysis with reference points).
As it is known, the temperature distribution on e.g. cooled elements changes over time, or vice versa - it increases. Therefore, it is important to examine in the structural analysis at least a few reference points related to the temperature distribution at different CFD time steps. In Ansys Workbench it is possible to perform one static analysis with at least a few reference points. Such a simulation gives us a full view of what phenomena may occur in a given element, determines trends related to the increase or decrease of critical values in time intervals.
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| Example of One-Way Fluid Structure Interaction Analysis |
In the first step, we perform a transient analysis in Fluent. Please note that our model and material properties should be described in the same way in both programs. After performing the simulation, we combine the Fluent geometry and solution options with Static Structural program.
In the second stage in Statc Structural (1), we define the number of analysis steps, which is the same as the number of reference points (see number of steps option). We can also define the pseudo-time step length for each step. This is helpful in situations where we want that in our analysis steps will be different sizes (so that the increments of individual variables are different sizes in the subsequent steps of the simulation).
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| Stages of Imported Load and reference points in Ansys Static Structural |
Depending on how many elements our model consists of - this is how many imports results from Fluent we need to do (2). In the red frame we indicate the object which will have defined conditions from CFD analysis. In the transfer definition (blue frame), we assign named selection from the fluent program (which we defined in the previous analysis at the meshing stage). It is important that the selected part is correctly associated with the named selection from Fluent. It is worth making a preview of the results from the CFD analysis to check the validity of the definition at this stage.
In the Imported Body Temperature table, we define the reference points. From the Source Time drop-down list, we select a time step from Fluent and assign it to the next iteration in Static Structural (green frame, Analysis Time). We can also slightly modify our CFD boundary conditions at this stage by scaling the value (Scale). This is the ratio we are relating as a percentage value (eg 1.1 is 110%).
After defining the output data (e.g. displacement) and completing the solver's work, we get the results for each simulation reference point (brown frame). Of course, we can define hundreds of such reference points to better reflect the mechanics of deformation of our material based on the previous CFD analysis (3).
In the next posts, I would like to show you how to correctly define and export to a CSV file Point Cloud in post-processing.
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