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  1. pja

    3 Node Triangular Shell Elements

    The stiffness matrix for the workhouse quad shell element in ANSYS (until shell181 came along) SHELL63 consists of a superposition of the stiffness matrices of 4 DKT (triangular) elements. Many commercial codes have shell elements like this..which is what I think he was talking about.
  2. pja

    How to Find Similar Eigenvectors

    if for some reason you want a quantitative measure you could find the angle between the two vectors in n-space.
  3. pja

    Stress at integration points or at nodes ?

    Prost, Chapter: Isoparametric Elements Section: Stress calculation
  4. pja

    Stress at integration points or at nodes ?

    Concepts and Applications of Finite Element Analysis
  5. pja

    Stress at integration points or at nodes ?

    Prost, The superconvergent-like behavior of stresses at Gauss points for Q4 or Q8 elements is shown to be true in many textbooks..for example look on page 231 of Cook fourth edition. My example gave the reason for this and was actually taken directly from Cook.
  6. pja

    Stress at integration points or at nodes ?

    The reason that the statement is often made that stresses are more accurate at the Gauss points, even for linear problems, is that for elements like Q4 and Q8 the stresses are superconvergent and their accuracy is comparable to the accuracy of computed displacements. For example impose on an...
  7. pja

    2d elements for membrane

    why 2D? is the loading nonuniform in both directions?
  8. pja

    how i can mondel a rope?

    If in fact it is something that needs to be done with FEM, the link180 element for example can be used to model rope.
  9. pja

    FEA for entertainment

    The first two references in the paper you cite are good places to start...
  10. pja

    Hydro acoustics

    Look for a book by Blevins for such solutions
  11. pja

    FVM and FEM

    Not sure what you mean by full-blown RANS..the act of averagin g the Navier-Stokes equations introduces unclosed stresses which is why you need turbulence (closure) models. So full-blown in the context of RANS always means some sort of closure approximation. Maybe what you meant was full blown...
  12. pja

    Worth it (at this point) to take an FEA class?

    How can you explain shear locking without "spraying equations all over the page" ? You need to at least write the expression for strain energy or at the very least the expression for shear strain for the concept to make any sense. Some of you sound as if you are afraid of mathematics. You...
  13. pja

    Why do flags flutter?

    I think one of the the basic questions here is whether or not the oscillation of the flag is fundamentally a forced oscillation or self-excited instability. If it is the former than the flag should be fluttering with a frequency near one of the fluid frequencies. For example if it is due to...
  14. pja

    CFD problem using FEM, Understanding the Fluid element?

    In FEM for fluids the strong form of the equations (differential equations) are converted to the weak form via the Galerkin method. These are the integral forms you are seeing. Then to get [K] these integrals are numerically integrated using some sort of quadrature scheme. This has to be...
  15. pja

    textbook on vibrations

    Meirovitch is pretty good
  16. pja

    FEA om multiple computers

    Finite element formulations have an inherent problem with scalability as compared to, for example, finite difference solutions. The reason for this is that while the assembly procedure in FEM is basically what one calls perfectly parallel, the solution of large sparse systems of equations...
  17. pja

    Why do flags flutter?

    I'm guessing flags flutter in same way that airplane wings flutter. A self-excited instability in which the structure extracts energy from the air. In nonlinear dynamics terminology, a Hopf bifurcation. The resulting limit cycle which you see is probably due to a combination of nonlinearity due...
  18. pja

    Which Solver can do eigenvalues with Lagrande multipliers

    You can use dggev although it won't be as efficient as a solver which is coded specifically for symmetric semi-definite problems like yours. Many of the sparse solvers (MUMPS for example) so you may be able to massage an eigenvalue computation out of them....
  19. pja

    Unsymmetric CG method failed

    By changing the specific heat you must be decreasing the "nice" properties of the system of equations which you are trying to solve. I.E the condition number may be getting too large. The convergence or lack thereof of iterative methods is function of the conditioning of the system. Have you...
  20. pja

    interpolating the beam nodal curvature values (Finite element question

    Your FEM based upon Hermites is only C1 continuous so your curvature will not be continuous across interelement boundaries. There is no way around that. If you want interelement continuous moments you will have to either use C2 continuos elements, which nobody does, or use a mixed functional...

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