Postprocessing and Visualization

1. Postprocessing and Visualization

2. Outline • Solution monitoring • Solution data • Graphical display: • vectors • contours • pathlines • XY plots, histograms • 2D vs. 3D • Alphanumeric reports: • field data, fluxes, forces, integrals • Custom Field Functions • Data export

3. Solution monitoring • Graphical display of the solution provides: • quick visual check of solution status • indication of equations that are not converging • The actual solution quantity plotted depends on the software continuity equation convergence trouble affects convergence of all equations Fidap example here

4. Other Solution Monitors • You can also monitor and display other quantities during calculation • Value of a variable • at a boundary • at a point • Integrals/averages on a surface • Force: lift, drag, moment • Another indication of convergence is when the monitored value stops changing Lift coefficient vs. iteration

5. Solution Data (1) r mo2 e k • Pressure • static, dynamic, total, pressure coefficient, ... • Velocity • magnitude, components, stream function, vorticity, helicity, ... • Temperature • static, total, enthalpy, internal energy, entropy, ... • Turbulence • kinetic energy, dissipation rate, effective viscosity, wall y+, ... • Species • mass fraction, mole fraction, concentration, deposition rate, ...

6. Solution Data (2) • Properties • density, viscosity, conductivity, specific heat, ... • Wall quantities • shear stress, heat flux, heat transfer coefficient, ... • Grid • coordinates, areas, volumes, quality measures, ... • Derived quantities • shear rate, stresses, other derivatives, … • Custom quantites • User-defined Functions • Field Function Calculator

7. Graphical Display of Data • What would “colorful fluid dynamics” (CFD) be without graphics?! • Graphical display should be more than just pretty pictures • The goal is visualization and understanding of the flow • capture of gross features: • is the flow headed the right way • does it look correct? Have I done something wrong? • Visualization of finer features: • where does the flow separate • what does the local heat transfer coefficient look like on the wall • Many options: contours, vectors, path lines, ...

8. Contours • Color-coded contours are an effective means of visualizing scalar quantities such as pressure, velocity magnitude, stream function, … • Display is essentially 2-D • You can identify the min/max and gradients slide coater contours of stream function filled contours

9. Vector Plots • Velocity vectors indicate the direction and magnitude of the flow • Usually colored by velocity magnitude • Usually displayed on a surface, but shows “three-dimensionality”

10. Path Lines • Path lines trace the flow pattern using massless “particles” • 3-D version of stream function contours • Can be colored/twisted by any scalar value • Be careful! You specify the starting point for the path lines, but... • if the particles start outside a recirculating region, you won’t see it! Particles released here particles released here Flow structure is actually quite complex! nothing here?!

11. XY Plots • “Quantitative” data can be viewed as XY plots • Useful for comparing against experimental or other data • Examples: • heat transfer coefficient along a wall • pressure distribution on lift surfaces • time dependent behavior (e.g., shedding) • Can be plotted on a surface, but usually along a line velocity distribution at exit of a coating die

12. Histograms • Histograms are a good way to visualize distributions • cell skewness • other statistics (e.g., variation of density throughout flowfield) Cell skewness distribution

13. 2D vs. 3D • In 2D, data is normally plotted on the entire flow domain • In 3D, this is not feasible — too much data!! • Thus, for 3D problems, you normally plot data on: • a surface or surfaces • a line or lines • All outer boundaries are available as plotting surfaces • Most postprocessors provide tools for creating arbitrary plotting surfaces: • points or lines • planes, iso-surfaces, clipped surfaces

14. Alphanumeric Reports • It is not practical to view “raw data” — perhaps on a small, “mapped” grid • However, other alphanumeric reports are quite helpful: • fluxes: e.g., summary of mass flux in/out of each boundary • forces: e.g., summary of xyz components of forces/moments on a surface • integrals: e.g., evaluate mass flow rate weighted average temperature at the outlet L D

15. Custom Postprocessing Values • Sometimes, the value you want to plot isn’t provided • What then? Some software programs provide: • user-defined subroutines — calculation of “custom” quantities which then can be visualized • “field function calculator” — you can enter a formula in a special GUI panel. The resulting quantity can be contoured, XY plotted, etc.

16. Data Export • Most CFD codes can write out data in other formats • For people who want to: • view the data in another CAD program (I-DEAS, PATRAN, etc.) • use a third-party postprocessor (FieldView, Ensight, Data Explorer, Tecplot, etc.) • make some further calculations with the data using their own software

17. Animation • CFD data lends itself nicely to animation: i.e., moving pictures of the data • Examples: • particles moving with the fluid • fly-by’s, fly-through’s • visualizations of moving geometries (e.g., mixing tanks) • visualizations of other transient phenomena (e.g., movement of a shock) • data on a plane passing through the domain • Its very helpful if the CFD software provides animation support: • composition of animation • proper file output formats

18. Summary • Postprocessing includes all the tools needed for interpretation and visualization of the CFD solution • Postprocessing occurs at two levels: • qualitative check: are b.c.’s correct, does flow look “right” • extracting the quantitative results that are needed for design • 2D postprocessing is fairly straightforward and trivial • Effective visualization of 3D flows is assisted by: • preparing appropriate plotting surfaces for viewing scalar data • path line plots or particle tracks to “see” overall flow patterns • animation to “bring the flow to life”