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CMQ
Solid finite elements- Continuum elements correct answer Pin-jointed (line) 2D plane stress (flat plane) 2D plane strain (flat plane) Axisymmetric (flat projection) 3d (volume) Solid finite elements- Structural elements correct answer Beam (line) Plate (surface) Shell (surface) 2D plane stress correct answer 'Thin' geometries in z direction Stress across thickness is neglected strain across thickness is not 0 2D plane strain correct answer 'Thick' geometries in z direction Strain across thickness is neglected Stress across thickness is not 0 z is infinite, xy plane remote from end Types of FEA error correct answer Modelling error- If geometry of boundary conditions modelled incorrectly. Mesh error- Not a good mesh, too little nodal points for good accuracy, too many to get good answer out. Numerical errors- Round off error in computation -Solution becomes ill conditioned
SIMPLE correct answer Semi Implicit Pressure Linked Equation Solving the pressure field on a staggered grid, uses iterations to find the right answer. Corrections used to get closer to the correct value. Why is it needed? For the 2D grid, a checkerboard pattern can form, alternate values. The staggered grid prevents this but means the SIMPLE method is needed to work out the p, v and u terms Peclet number correct answer Ratio between convection and diffusion, if above 2 then then central differencing scheme may be unstable. Used in convection diffusion FEA 1D things. Pe=(ρu)/(Γ/Δx)< Eulerian frame of reference correct answer Considering a fluid moving through a fixed cube. More simpler than Lagrangian Lagrangian frame of reference correct answer Considering a fluid cube moving in space as a particle of fluid moves around. More difficult. Continuity equation, simplified and expanded correct answer div(u)= (∂u/∂x)i+(∂v/∂y)j+(∂w/∂z)k= How direct stress is formed in fluid mechanics correct answer Due to stretching of the fluid. Done by viscous action of fluid sliding past each other. ∇ correct answer i(∂/∂x)+j(∂/∂y)+k(∂/∂z)
Structured mesh correct answer Uniform shapes on full geometry, more computationally efficient Unstructured mesh correct answer Made with triangular or tetrahedral cells, form in different directions and size depending on position on the geometry, larger computation but gives a more reliable representation of the model. Boundary elements correct answer Solution matrix is divided into small boundary segments. This uses surface modelling and does not discretise the area/volume of the model. Finite element analysis correct answer Whole solution domain is DIVIDED IN TO ELEMENTS all the way the way through the volume. Solution matrix is sparsely populated. Used for more irregular geometries. Strain in a 3D continuum and long-thin uni axel bar correct answer 6 terms describe the strain in a 3D object. These are all described by the displacement of the surfaces. Direct and shear stress in the x, y, z directions. A uni axel bar strain=x/l Thin and thick beam elements in FEA correct answer Thin beams do not include shear deformation Perpendicular cross sections stay perpendicular to the mid point. Thick beams include shear deformation Perpendicular cross sections do not need to stay perpendicular.
FE stress contour plots and the degree of approximation used correct answer Produced by extrapolating and interpolating values of stress to create the colour contours. This gives a visual representation of the stresses. In the programme, the contours are often smoothed between two contours as stresses are gradients. Meaning of finite volume correct answer Small part of the sub-devided fluid volume. It is a control volume representing velocity, pressure and mass. Has a node point in the centre where flow data is stored. Meaning of discretisation correct answer Defines numerical integration formulae for each volume node. Divides the overall flow equation in to formula that links adjacent nodes. Finite differencing method correct answer Whole solution domain is divided in to A GRID OF POINTS where equations are applied at each point. Used in more simpler shapes as it is tricky to change the pattern of the points. Degrees of freedom in Pin-jointed elements Beam elements 3D continuum elements Axi symmetric continuum elements
Rigid body motion correct answer If system is not constrained well enough and is allowed to rotate around a point then rigid body motion will occur. Causing movement of the whole body without any stresses. More constraints are needed to keep it stationary. Nodal forces will never be completely zero causing the movement. Round off errors correct answer A numerical error in FEA These occur in all computation with multiplications and divisions but are very small. Accuracy of result gets worse with more and more equations, over 100. 4 types of turbulence model correct answer Boussinesq Prandtl Kolmogrov k-e RSM Under relaxation correct answer Used as a dampener, factor applied to the updated iteration to reduce the change in value. To prevent divergence Convergence in a simple algorithm, the amount of residual correct answer The point where the result has converged is 10^(-3) Non-slip condition Slip condition
In walls correct answer Non-slip condition, fluid at the walls has zero relative velocity Slip condition, gives fluid a certain amount of slip at the wall, like in small passages Velocity inlet Profile velocity inlet correct answer Uniform velocity over the inlet, this has to be stated. This may not reflect reality as not all elements will have exactly the same amount of velocity. Profile velocity inlet gives the input a set profile, which would be caused by upstream effects Pressure inlet correct answer Specific pressure is set for the inlet, allows a difference in velocities of the inlet. Pressure outlet correct answer Specific pressure set for outlet, does not allow for any non perpendicular velocities, should have an extended tube to prevent this. Outflow correct answer Can only be used when there is no pressure or velocity gradients at the outlet. Meshless correct answer Does not require a mesh, used when object has discontinuities or it goes under a lot of changes during deformation. Skewness correct answer Shows how much a cell is skewed. The more skewed the less the cell is tolerated by the solution algorithm. Skewness=(Optimal cell size area- actual cell size area)/ (Optimal cell size area)
