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The Simulation Manager tab can be thought of as a dashboard for the run. The efficiency and accuracy of the solution are indicated by the runtime plots and solver text messages. Familiarize yourself with the various runtime diagnostic plots available in the dropdown list above the plot window:

The plots are generated at every History Data output step, which is, by default, 1/100th of the simulation finish time. All the data shown here is also available as General History data in the output flsgrf file. You can draw a box to zoom in on the plots by holding the left mouse button and dragging a zoom window. Use Ctrl+left-click to zoom back out.

1. Stability limit & time step size: Compares the time step stability limit (the smallest time step required in any cell to meet any explicit solver that is active) and dt, the actual time step being

used. Ideally the time step dt is the same as the stability limit but it may be smaller when there are too many pressure iterations or a stability limit is unexpectedly exceeded (e.g., due to splashing).

2. Time-step size: A simple plot of the dynamically-selected time step over time.

3. Epsi & maximum pressure residual: Epsi represents the pressure iteration convergence criteria that the solver uses to determine if the pressure solution is accurate. At every time step, pressures and velocities are solved in all cells with fluid until the residual Ū (a measure of

potential error) in each cell is less than some small cutoff value epsi. The max residual is for the cell with the maximum value of Ū after the pressure solution has either converged or reached

the maximum allowed number of iterations. The pressure iteration fails if the max residual is larger than epsi after the maximum number of pressure solver iterations, otherwise it is

converged.

4. Pressure convergence ratio: the ratio of maximum residual to epsi. Less than one means the pressure solution converged, greater than one means it did not. An “ideal” solution maintains

res/epsi less than one at all times. A few iteration failures are unlikely to damage the simulation

accuracy, especially when they are isolated or occur at the beginning of the run. When the maximum residual is much lower than epsi, it means that the solution is converging very quickly (the solution is “easy”). When it is close to epsi, it implies that the convergence criteria selected is an efficient choice. Non-default options for controlling the pressure solution are rarely needed or desirable, but are available on the Model Setup > Numerics tab and will be discussed in a later lecture.

5. Pressure iteration count: The number of iterations of the pressure solver. Different pressure solvers have different “best” values. In general, less than ten is a desirable value.

6. Convective volume error (% lost) and Multi-block volume error (% lost): Represents the amount of fluid gained (negative value) or lost (positive value) due to advection errors or inter- block interpolation errors, in percent. Usually much less than 1%, and should always be less than 3%.

7. Fill Fraction, Volume of fluid 1, and Fluid 1 surface area: self-explanatory. In this simulation, the units of volume are cm3 and surface area is in cm2. Constant fluid volume and surface area are two indicators that the simulation has reached steady state. Fill fraction is the dimensionless ratio of fluid volume to total open (non-solid) volume in all blocks of the domain.

8. Mass-averaged mean kinetic energy and Mass-averaged mean turbulent energy: the domain- averaged, time-mean, kinetic and turbulent kinetic energies. These are two more indicators of the steadiness of the flow. Average TKE is not shown in this case because the flow is being modeled as laminar according to the selections on the Model Setup > Physics tab.

9. Other plots are available depending on the physics models used: these include thermal energy, number of particles, and other output.

Take a few moments to look over the available runtime plots for the simulation. See if they make sense to you.

Also look at the solver text output results at the bottom of the screen. This is a copy of the hd3msg file that the solver writes to the simulation directory while it runs. Even if the user interface (GUI) crashes,

the solver will continue to run and write all the important output files. The text output lists a summary

of the simulation at each Short Print Data Interval. Each line gives:

1. output time t (usually in seconds, refers to time elapsed in the model)

2. output cycle (the number of time steps solved to date, including the current time step), 3. actual time step delt used during the cycle (less than or equal to dtstbl),

4. limiting time step required for stability by any explicit solver process dtstbl,

5. solver process that requires the limiting time step, as a two-letter code (cx = x-direction

convective fluid flux, for example)

6. number of pressure iterations during the cycle iter,

7. the ratio res/epsi of pressure solution maximum residual (any cell) to convergence criteria epsi 8. volume of fluid in the simulation domain,

9. the known %loss due to convective flux error and inter-block interpolation error 10. the fraction of the open domain volume that is filled with fluid frac,

11. elapsed real-world time and computer clock time el_time and clk_time (hh:mm:ss), 12. multi-processor efficiency ratio %PE, as percent