Happy Holidays! Hope everyone has a Merry Christmas and a Happy New Year! PS: Way cooler animation by EpsilonFEA: http://www.epsilonfea.com/happy-holidays-17/ PPS: Archived V18.2 file: Link

and other thoughts...

and other thoughts...

Showing posts from 2017

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Where should the accelerometers be placed? - Test Person Test People are tough to work with! I was one before so I would know. Have a bit of sympathy for them though as they could be frantically troubleshooting equipment in the middle of no-where or spend countless hours rolling-and-unrolling cables. Goal of Measurement Points Selection OK, we have a model with natural frequencies and mode shapes. If your company is well endowed, this is a simple question with pretest software like FEMtools or some other software that optimizes the MAC matrix . This would also help with correlation later. Generally speaking, the software will maximize the diagonal matrix while minimize the off diagonal matrix values. Loosely defined, this means the modes are relatively unique (mathematically speaking) for the chosen measurement locations. Poor Man's Method for Picking Measurement Locations Alternately, we can pick measurement locations by hand based on past experience and liberal use of e

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Solving for many cases with small variations using DesignXplorer is quite convenient. After setting up an initial model, a parameter could be defined in SpaceClaim or elsewhere. The solution for multiple design points are then generated as I sip on my beer (legally at home). As shown in this YouTube Video , the output parameters are single values like natural frequency, displacements and other maximum or minimum of available solution results. Unfortunately if you are interested in more complicated items that aren't easily available through the user defined results or *GET commands, how do you capture that output parameter? One way would be to retain all design point results for later post processing but that would be overkill especially since I'll have to go into everyone of them to extract out the information which is a pain. Alternately, a post-processing APDL script could be written to extract out key information at each run. An example would be the mode shape o

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The Original Eye * SpaceClaim has a pretty neat feature of being able to import pictures. This comes in useful when trying to compare a picture of a part cross-section to the CAD. Rough dimensional analysis can be done. To take it a step further, the model could be updated so that the contours match the cross section picture. Other uses for it is to help 'inspire' CAD when the models are created from scratch. More accurate proportions and curvatures could be represented in the model. An example of which is shown below where the vitreous humor and crystalline lens of a Wikipedia picture was approximated. The Scary Eye We could insert the picture here: Insert Picture File here With the picture imported, it could be used as a background image for tracing splines to recreate key features. For example, two areas were recreated here: Adding Splines to Create Areas Another view of the model: Areas at Different Plane from Picture Given a bit m

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Tait-Brian Angles Z-X'-Y'' * After a simulation is complete, it is sometimes useful to know how much the overall object is rotated and translated. This is true especially for rigid body objects. While simple in 2D, it gets more complicated for 3D rotations. Euler angles always makes my head spin. A rotating reference frame? * Ansys uses the Z-X'-Y'' sequence for defining the coordinate system but there is also the *GET command to extract the Euler Angles of a Local Coordinate System. So let's say you have 3 nodes on a rigid beam that has been rotated and translated in 3D space. One could do it the hard way by starting with the general definition of R in the Rotation Matrices and go in reverse to get the angles with some fun scripting in your math package of choice. Or... the easy way: define a local coordinate system using NWPLAN at the deformed nodes at each result time step and use the *GET command to extract angles. *dim , MyAngles,

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It is sometimes useful to extract the mass and stiffness matrix from Ansys. *SMAT, MatK, D, IMPORT, FULL, file.full, STIFF *PRINT, matk, matk, txt Exporting mass matrix would be similar: *SMAT, MatM, D, import, full, file.full, MASS The above script uses APDL Math to get the job done. (Please see previous post for another example). The ordering of the matrix is unfortunately not concurrently exported. To verify the sequencing is as expected, we will work to replicate a truss example in the Finite Element Trusses course notes by Bob Greenlee. Figure 1: Truss Problem Setup Model Creation Script to create model: /prep7 !! Creates Model to reflect course notes ! Properties et ,1,1 mp , ex, 1, 29.5e6 r , 1, 1 ! Geometry n ,1 $ n ,2, 40 $ n ,3, 40, 30 $ n ,4, 0, 30 e ,1,2 $ e ,2,3 $ e ,1,3 $ e ,3,4 ! Boundary Conditions d ,1,ux,0 $ d ,1,uy,0 d ,2,uy,0 d ,4,ux,0 $ d ,4,uy,0 f ,2,fx,20e3 f ,3,fy,-25e3 ! solves /solu eqslv , sparse

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For those starting out, here's a few commands I find pretty neat. If you're not familiar with them, it's worth checking out. NDNEXT & ELNEXT When looping through a bunch of nodes or elements, this provide some convenience. /post1 cmsel , s, NodesOfInterest *get , ncount, node, 0, count *dim , uxsave,, ncount, 2 nnow = 0 *do ,ct,1,ncount nnow = ndnext(nnow) uxsave(ct, 1) = nnow *get, uxsave(ct, 2), node, nnow, u, x *enddo *stat , uxsave Alternately, one could instead use fancier commands: *VGET with *VMASK cmsel , s, NodesOfInterest *get , nmx, node,, num, max *get , ncount, node,, count *dim , ndata,, nmx, 3 *dim , nkeep,, ncount, 2 *vget , ndata(1,1), node, 1, nsel *vfill ,ndata(1,2),ramp,1,1 *vmask , ndata(1,1) *vget , ndata(1,3), node, 1, u, x *vmask , ndata(1,1) *vfun , nkeep(1,1), comp, ndata(1,2) *vmask , ndata(1,1) *vfun , nkeep(1,2), comp,

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In a previous post , I mentioned mesh convergence study is not done as often it should. I've been reminding myself to be careful in reporting stress results using sanity checks. A nagging question persists in my mind: "How much should the mesh be refined before calling it good? Is a factor of 2 good enough?" A presentation here at Ruhr-UniversitÃ¤t Bochum is quite interesting. It presented a linear Richardson Extrapolation being: f ex = f n + k(f m - f n ) where... f ex is the exact solution f m is the solution at mesh size m f n is the solution at mesh size n k is what I'll call Richardson Extrapolation Factor: k = (m Î± ) / (m Î± - n Î± ) where Î±=2 for linear elements and Î±=3 for quadratic elements. To plot this factor k relative to the reduction factor ( n/m ) in Figure 1: Figure 1: Richardson Extrapolation Factor It is interesting that for quadratic elements, with a mesh size reduction by a factor of two, the factor is 114.3%. Hypothe

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In Ansys Mechanical, there are a lot of drop-down selections. Instead of clicking on the down arrow, try double-clicking on the current selection. The selection would cycle to what's next on the list. This is convenient when I want to create multiples of an object (e.g. contacts or bolt pretension) with the only difference being the named selection. After setting up the first object as I like it, I then Right-Mouse-Button "Duplicate" and double-click on the named selection to cycle through the named selection. This of course only works if the named selection was already created as part of the workflow that is named sequentially (e.g. bolt1, bolt2, bolt3... (or) fric1a, fric1b, fric2a, fric2b...) Double-clicking also works for a slew of drop-downs in Analysis Settings. p/s: Creating an ACT script would be more efficient but the conventional/lazy way above could be faster if future projects would rarely need it.

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If you're not careful, you'll end up like me with unintended interference. The problem can be illustrated with a pin and hole-part assembly. Three identical geometry assemblies are created. From left to right, firstly a Tet mesh, then two others of different Hex mesh controls (Figure 1). Though the pin has the same diameter as the hole diameter, there may be interference in the model due to geometry discretization. Figure 1: Identical Geometry but Different Mesh The difference in results are clear from the contact pressure shown in Figure 2: Figure 2: Contact Pressure at Frictionless Contacts The coarse Tet mesh does not 'curve' well (Figure 3) thus bumping into each other. The interference in the second assembly with hex mesh is less acute but still present (Figure 4). The third assembly with hex mesh with nodes of both the pin and hole aligned has no interference (Figure 5) hence almost no pressure. Figure 3: End View Tet Mesh Shows Interference at

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Recently I came across a problem where the large deflection solution that has Body-to-Body beam connection that wouldn't converge. Ansys bisected at the maximum number of iterations (26) and then halved the load step again... and again... The trick was to add a command snippet in the solve section: ...with the following text containing NEQIT to increase the maximum number of iterations per substep : neqit, 75 and VOILA! It Solves!

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Triangle Wave by Fourier Series * In the Ansys APDL documentation of Mode-Superposition Harmonic Analysis and other Verification Manual documentation (e.g. VM76 & VM149 ), only load step file method was described. Personally, I favor Multiple Solve Method instead because you can vary the loads for different frequencies. This could come in handy for different loads at various engine orders or rotating unbalance force. Or say if you have the Fourier coefficients of the periodic force of irregular form (e.g. triangle wave ), the steady state response can be computed. The APDL script below replicates VM183 but uses the Multiple Solve Method. Do let me know if you could recommend ways to improve the script. Script !! Modified from VM183 to use Multiple SOLVE Method !! Results should be identical to VM183 ! Author: Sze Kwan (Jason) Cheah ! Modified: August 27, 2017 ! Disclaimer: Use at own risk! /PREP7 ANTYPE, MODAL ! MODE - FREQUENCY ANALYSIS MODOP

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