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Unsteady Flow Computational Options

GeoHECRAS software provides some default computational options and tolerances for 1D and 2D unsteady flow models. The tolerances are used in the solution of unsteady flow equations. In general, it is recommended that the default computation options and tolerances be maintained. However, the user can override the default computational options to achieve model stability while maintaining computational accuracy. Extra care should be taken while overriding the default calculation tolerances as it could result in computational errors in the water surface profile.

Follow the steps below to override the default computational options and tolerances:

  1. From the Analysis ribbon menu, select the Unsteady Flow Computational Options command.
    Unsteady Flow Computational Options command
  2. The Unsteady Flow Computational Options dialog box will be displayed. This dialog box contains separate tabbed panels for 1D/2D Flow Options, 2D Flow Options, Restart Options, Detailed Output, Advanced Time Step Control, and Non-Newtonian Flow Options.
    Unsteady Flow Computational Options dialog box

The following sections describe how to override the default computational options and tolerances and interact with the above dialog box.

1D/2D Flow Options

By default, the 1D/2D Flow Options tabbed panel is selected when the Unsteady Flow Computational Options command is run.
1D/2D Flow Options tab

The above panel has different options to control the 1D unsteady flow calculation and mixed flow regime capabilities. Refer to this article in our knowledge base to learn more about the 1D/2D Flow Options panel and its underlying sections.

2D Flow Options

On selecting the 2D Flow Options tab, the following panel will be displayed.
2D Flow Options tab

The above panel contains several computational options and tolerances that can be set for the 2D module and options for controlling iterations between 1D and 2D hydraulic connections. Refer to this article in our knowledge base to learn more about the 2D Flow Options panel and its underlying sections.

Restart Options

On selecting the Restart Options tab, the following panel will be displayed.
Restart Options tab

The above panel allows the user to write out initial conditions file(s) for an entire simulation, sometimes called a Hot Start file. A Hot Start file can be used to set the system’s initial conditions for a subsequent run. It is commonly used in real-time forecasting, where the user wants to use the results at a specific time from a previous run to be the initial conditions of the next run. Refer to this article in our knowledge base to learn more about the Restart Options panel and its underlying sections.

Detailed Output

On selecting the Detailed Output tab, the following panel will be displayed.
Detailed Output tab

This data panel helps the user to find model stability problems. To pinpoint the stability issues, the user can generate the computational output file and/or detailed log output file. Refer to this article in our knowledge base to learn more about the Detailed Output panel and its underlying sections.

Advanced Time Step Control

On selecting the Advanced Time Step Control tab, the following panel will be displayed.
Advanced Time Step Control tab

The above panel provides three options to control for selecting and controlling the computational time step:

  • Fixed Time Step — Basic method.
  • Allow Variable Time Step Using Courant — A variable time step based on the Courant number.
  • Allow Variable Time Step Using Time Series Divisors — A variable time step based on a user-entered table of dates, times, and time step divisors.

Note that variable time step options can improve model stability and reduce computational time. However, not all models will be faster with the use of the variable time step.

Refer to this article in our knowledge base to learn more about the Advanced Time Step Control panel and its underlying sections.

Non-Newtonian Flow Options

On selecting the Non-Newtonian Flow Options tab, the following panel will be displayed.
Non-Newtonian Flow Options tab

The above panel allows the user to define the parameters necessary for performing a Non-Newtonian flow analysis with HEC-RAS. Refer to this article in our knowledge base to learn more about the Non-Newtonian Flow Options panel and its underlying sections.

Note that clicking the [Reset to Defaults] button resets unsteady flow computational options to default values.

When all the data has been defined, click the [OK] button to run the analysis.

About the Author Chris Maeder

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