SIC^2: Simulation and Integration of Control for Canals

Time settings

David Dorchies — 2015-07-11T13:57:55Z

This tab is used to define the simulation time parameters and result writing time parameters for a scenario and its variants.

Simulation time parameters

Beginning time (Day: 0 Time: 00:00:00 by default)
End time (Day: 0 Time: 00:00:00 by default)
Calculation step (10 minutes by default)

Simulation time parameters for result writing

These parameters are used to reduce the size of written results during simulation. This allows to get simulations and operations on results faster but also to solve memory overflow problems that may occur with very long simulations on large systems.

Start time for writing (day: 0 Time: 0:00:00 by default): useful if one is interested in a phenomenon having place only after a certain time step.
Write results frequency (1 time step by default): useful if the results are not essential at each calculation time.

Steady flow simulation

By default, for a steady flow simulation (FLUVIA), only the first time step is executed. To run several successive steady flow regimes over time, the option "First time step computing only" in the "Steady calculation" tab should be unselected.

Unsteady flow simulation

The default value of the calculation time step is set at 10 minutes. One must be careful when changing this value since it has an influence on the numerical scheme and consequently the accuracy of the calculation. However, it has no influence on the stability of the numerical scheme since the scheme used is the implicit scheme Preissmann which is unconditionally stable (see Theoretical documentation).

We can choose local spaces step and the time step as desired according to the study carried out. What is known (cf. Cunge 1983, Malaterre 1994) when the Courant–Friedrichs–Lewy condition is very small (eg. <0.1) or very large (eg.> 10) then the scheme introduce numerical diffusion and phase difference. It is therefore desirable to keep the Courant–Friedrichs–Lewy condition in the range [0.5, 10].

Parameters for calculating and writing results

David Dorchies — 2015-07-11T10:19:57Z

The parameters for calculating and writing results are applied in steady flow calculations (FLUVIA) and unsteady flow calculations (SIRENE). These parameters are defined by scenario and will be used for all the variants of the scenario.

Access to the window can be made from:

The Project Explorer window by selecting a hydraulic scenario or variant and pressing the "Calculation settings" button
The main window in hydraulic mode by pressing the
The main window in hydraulic mode via the Tools menu > Settings.

The window consists of five tabs:

Time : definition of temporal parameters of the simulation (start time, end time, calculation time step, beginning time for writing results and write frequency of results)
Settings for results writing (definition of the data that will be usable in "Result" mode)
Steady flow calculation settings
Unsteady flow calculation settings
General settings

Steady flow calculation parameters

David Dorchies — 2015-07-10T19:49:09Z

This tab allows you to change the parameters influencing the calculation algorithm in steady flow, especially when the iterations in case of mesh network, and/or with offtakes in flow calculation mode.

The default settings allow almost all the time to compute a solution with very good accuracy. In some cases one can help, accelerate or make possible the convergence of the algorithm.

Relaxation coefficient at diversions

This option allows you to speed up or slow down how the flow distribution is corrected at diversions during iterations of the calculation mesh. It may be necessary to decrease it when the mesh algorithm stops because of negative flow in a reach.

Max iterations meshed system

One can increase the number of iterations if the number provided by default is not sufficient to ensure the convergence of the mesh calculation.

Precision at nodes

This is the maximum permitted deviation of the water elevation at offtakes and diversions (in meters) between the assumptions made in the current iteration and real dimensions calculated considering the hydraulic conditions (elevations and flow rates) calculated in the system.

In case of difficulty to calculate in steady flow, especially in highly meshed networks where it is difficult to know the direction of flow a priori, two possibilities are offered:

It is possible to find the hydraulic solution by greatly increasing the value of "precision at nodes" (eg by setting it to 1 or 2 meters and run the calculation in steady flow, and then start the calculation in unsteady flow which is able to reverse the direction of flow in the reaches if necessary, and find the hydraulic solution.
You can activate the option "Allow reversing the direction of the reaches"

Small space step

The "Small space step" mode is used to locally introduce new calculation sections (dividing the local space step by 10) if high water line slopes are detected (slope superior to 5/1000 including reverse slope). Please note that this mode is not supported in unsteady flow calculation, which is not a big problem, but this can introduce "waves" at the beginning of unsteady flow simulation.

Debug mode

You can activate the "Debug" mode that will record more intermediate variables on the log file during steady flow calculation. These informations allow to follow in more detail the computation iterations.

Allow reversing the direction of the reaches

As the name suggests, this option allows the mesh resolution algorithm to reverse the flow direction of the reaches during the calculation. Only "intermediate" reaches (ie not adjacent to a node located upstream or downstream of the system) may be reversed.

Allow calculation of supercritical

When this option is checked, FLUVIA calculates supercritical water lines and the location of hydraulic jumps. Otherwise, the supercritical flows are represented by a critical flow. To perform an unsteady flow calculation from a water line including supercritical flows, it is recommended not to check this option so that the calculation can be done with the "Simple supercritical" of SIRENE.

First time step computing only (in steady flow calculation)

This is useful for running only one time step even if the time settings specify more time steps to calculate. Indeed, the conventional use of the software to perform an unsteady flow calculation is to create a scenario specifying the simulation time, perform a calculation on the first time step in steady flow, then create a variant to perform calculation in unsteady flow with the same parameters.

This option will be unchecked only if it is desired to carry out a simulation with successive steady flow simulations.

Write in all calculation sections (to allow to initiate an unsteady flow computation

A calculation made with this option can then be selected for importing an initial condition for an unsteady flow calculation.

Unsteady flow calculation parameters

David Dorchies — 2015-07-09T09:44:08Z

Discretization settings

Discretization method:

The method called "Classic" is the one described in the theoretically documentation.
The "homogeneous permanent" method limits the "waves" that can appear when starting a unsteady flow calculation from a water line computed by the steady flow calculation (Fluvia). Indeed, in this case, the discretization of the transient equations gives exactly the equations of the steady state when one removes the derivative terms with respect to time. It is less conventional because it can lead to not very normal hydraulic phenomena in the case of change of geometry (cf Cunge book 1980). The small "waves" mentioned below can be explained by the fact that the stabilization regime of the classical scheme does not lead exactly to the initial permanent water line. The new equilibrium will not be very far, but to achieve this, a balancing of levels and therefore mass transfers will be realized.

Initialisation method:

Zero Variation: for a given time step, the calculation of the new water line is initialized from the water line calculated at the previous time.
Previous Variation: The calculation of the new water line is initialized from the water line calculated at the previous time at which we apply the flow and water elevation variations observed in the previous time step. This option can accelerate the convergence if hydraulic variables are often progressive. However, our experience indicates that convergence is very fast, and this option is not essential.

Theta coefficient of the Preissmann scheme:

This coefficient sets the implicit elements of the numerical scheme. It should be set between 0.5 (stability limit) and 1 (total implicitation). If this condition is met, the Preissmann implicit scheme is unconditionally stable. It is set to 0.6 by default. It is possible to modulate it automatically according to the Froude (cf below). This can be useful because a high Theta coefficient will generate more numerical dissipation and thus limit the numerical oscillations that can appear with high Froude. Putting it large (close to 1) all the time is not very good numerically, so the option to set it variable, just according to the Froude, is interesting.

Note: by default HEC-RAS advises to start modeling with Theta=1, which is quite violent anyway, because it generates a lot of numerical dissipation.

Simple super critical

This option allows, in the case of classical discretization as well as the homogeneous to steady flow discretization, to gradually eliminate the inertia terms of the dynamic equation of Saint-Venant when the Froude number exceeds a certain value and for example becomes close to 1. This is a classical method to better manage high Froude numbers, as well as supercritical flows in some cases. This stems from the fact that it is these terms that generate numerical problems in the Preissmann scheme at high Froude numbers. We delete these terms progressively and linearly: 0% for Fr = FrMin (default 0.6) up to 100% for Fr >= FrMax (default 0.9). The default values of FrMin and FrMax can be changed in the advanced settings of this option ("Detail" button). This option is to be used with moderation because it presents certain risks of numerical stability with Froude high and especially with low water depths.

No: The option is not applied and the calculation is stopped as soon as the Froude is greater than or equal to 1 on one of the sections of the model.
Yes local: The option is only applied to sections with high Froude, at the moment it happens and gradually as explained above.
Global Yes: The option is applied to all sections of the model and removing 100% of the selected terms during all the simulation time.
Sart-Preissmann: more advanced method with modification of the Preissmann scheme cf article Sart et al 2010 (http://hal.ird.fr/hal-00632009/document).

One can also, in the advanced options ("Detail" button), choose the term(s) to be deleted: the local acceleration term ${dQ/dt}$, and/or the term d convective acceleration ${\frac{d\frac{Q^2}{S}}{dx}}$. If we remove these two terms, we have the model of the diffusive wave. Older versions of SIC (prior to 5.37b) allowed to remove only the second term, convective acceleration which is the one that poses the problems of numerical stability at high Froude numbers. These terms are progressively removed locally or globally (depending on the option chosen above). For the local option, this or these terms are removed for Froude values between FrMin (0.6 by default) and FrMax (0.9 by default), but nothing prevents to put FrMin=0 and FrMax=0, which implies that these terms will always be removed whatever the Froude values. In this case we have an implementation of the diffusive wave model. The global option will give the same thing. Attention in case of variable Theta, this reduction will also apply on this coefficient Theta, in this case one would have then Theta=ThetaMax.

We currently do not allow to remove the term ${\frac{dy}{dx}}$ , which would then lead to the kinematic wave model. See for example http://www.agroparistech.fr/coursenligne/hydraulique/degoutte1.pdf for information on these simplified variants of the Saint-Venant equations.

Another option is to propose a variable Preissmann Theta implicitation coefficient, increasing when the Froudes are high. It is varied with the same logic as for the terms of inertia, locally, and linearly between Theta and ThetaMax depending on the Froude for the local option. For the global option one has in this case systematically Theta = ThetaMax, everywhere and all the time.

Another activatable option is to recalculate the torrential water line in steady state. Flow rates and water elevations are first calculated with the conventional method, and then the water elevations are recalculated with the steady state algorithm. This option is to be used without reserve and without guarantee, especially in the case of meshed and/or branched networks, and is no longer conservative in volume, since the dimensions of the water line are recalculated.

Numerical resolution method

The calculation can be done in linear (without iterations) or non-linear with the Newton or quasi-Newton method (the derivative is not calculated at each iteration). In the case of non-linear calculation, the iterations convergence criteria should be defined:

Precision of the non linear system

The convergence of the linear system is tested on flows of structures and boundary conditions. The iterations are also made on the Saint-Venant equations.

The calculation succeeds when the deviations of flow and water elevation are below the requested precisions. The requsted precisions can be defined in absolute terms (in m³/s for flows and in m for water elevations) or in relative values (in proportion of the flow rate or in proportion of the water elevation with reference to the lowest elevation in the studied model).

This test can be very demanding if the precision is set to a small value (eg.: 0.0001 m³/s). On systems with significant flow rates, it is desirable, even mandatory, to increase it. For example, the Rhône, which has flows of about 2000 m³/s the system will fail to converge to an accuracy of 0.0001 m³/s. However with a value of 0.01 m³/s the calculation is done smoothly and with a largely sufficient accuracy. If you have any messages indicating that convergence is not possible, decrease the coefficient within reasonable limits (ex .: 0.1% of nominal capacity).

Max iterations of the non-linear system

Maximum number of iterations to converge the system. If a significant accuracy is required on a system having difficulties to converge, it may be necessary to increase it.

Note that if the system fails to converge, SIRENE restarts the calculation again by halving the time step of calculation. This is repeated if necessary until divide by 8 the time step calculation chosen by the user.

Derivative calculation frequency (Quasi-Newton)

0: the calculation is linear (without iterations).
1: the calculation is performed by non-linear with Newton's method
>1: the calculation is performed by quasi-Newton (the calculation of the derivative is not performed at each iteration)

Do not stop the calculation in case of non convergence

By default, if the calculation did not reach the precision required after the maximum number of iterations and after the divisions of the time step of calculation, the simulation is stopped. Enable this option allows in this case to only display a warning message and switch to the calculation of the next time step.

Write in all calculation sections (to allow a hot start unsteady flow computation)

If selected, we can use the results of the simulation as the initial condition for a new unsteady flow simulation. For networks with many sections of calculation, it may be appropriate to increase the write frequency results in order not to burden the XML file and associated treatments (see Time Settings).

Settings for results writing

David Dorchies — 2014-12-16T13:08:52Z

Results recorded can be set for each scenario of the project.

The user can choose which data that will be available in the results for each network object (section, node, offtake, device). To do this, open the calculation settings window and check the data on the tab “Choice of variable to write” (see below).

In addition to the variables mentioned in the table above, every regulated variable (by law function of time, by objective regulation at an offtake or at a cross-structure, or variable regulated by a regulation module) will by add to the results.

General settings

Pierre-Olivier Malaterre — 2013-11-15T11:52:20Z

The tab "General settings" contains three blocks:

Discharge calculation at offtakes

This block rules the criteria of the iterative calculation at offtakes. The parameters are the maximum number of iterations and the precision to reach in m³/s. The default values are suitable for most cases. It could be necessary to refine theses criteria if one ask a greater precision inthe precision at nodes in steady calculation parameters or in the precision of the non linear system in unsteady calculation.

Ce bloc règle les critères du calcul itératif aux prises. Les paramètres sont le nombre maximum d'itération et la précision à atteindre en m³/s. Leur valeur par défaut convient pour la plupart des cas. Il peut être nécessaire d'affiner ces critères si on demande un précision importante dans la précision aux noeuds en calcul permanent ou dans la précision du système non linéaire en calcul transitoire.

Automatic head loss at expansions

An automatic head loss at expansions may be added to take into account the loss of hydraulic head at locations where the cross section is getting suddenly larger (width increasing and/or bed elevation decreasing). In this case, a head loss is added using the Borda formula $\Delta H = c (V_{upstream} - V_{downstream})^2 / 2g$ where ${c}$ is a parameter dependent on the angle ${\alpha}$ of the enlargement:

$c = 0.525 ln(tan(\alpha)) + 1$ (Neperian logarithm)
$c = 0$ for $tan(\alpha) < 0.15$
$c = 1$ for $tan(\alpha) \geq 1$

Writing simulation results

There are two options:

into the XML file project
into a separated binary file

This option, when selected, is valid for the current scenario and all its variants. But you can change this option at any time, and thus have, if desired, disparate results in the format that you want for the various calculations, either for a scenario or for one of its variants, whether for a steady flow or an unsteady flow calculation. In the case of choosing the XML file, all the results of calculations performed for this branch of the project (scenario and variants) after this choice will be written in the XML project file. The advantage is to have all the input and output data in a single file. But the downside is that the XML file can become very large, and depending on the power of the computer (memory in particular), it can sometimes cause problems with losses of pieces of the XML file. We have indeed noticed that when the XML file is over 60 or 70 MB (megabytes), these problems can appear. In addition to the risks related to the size of the XML file, the time of writing and reading the results of this XML file can become very long. It is for these reasons that we have added the option of writing the results on binary files. These files have the extension .res for the results in steady or unsteady flow, and .rci for the initial conditions. The filenames with the .res and .rci extensions are constructed from the name of the XML file with an additional _ns_nv, where ns is the number of the scenario and nv the number of the variant (or 0 if it is not a variant, but directly a scenario). All these files are written into the subdirectory where the XML file is located.

How to write the XML file results

There are two options:

Mono-thread
Bi-thread

The bi-thread option on computers with multiple cores, significantly increase the speed of writing the results of the XML file in a simulation with Fluvia or Sirene. It has the disadvantage of not being completely stable and, in rare cases, crashes the simulation program at the time of writing the results.

Note on possible instabilities of the software

If you experience random crashes of Fluvia or Sirene calculation programs at the time of writing the results, choose the mono-threaded option.

If you notice any problem with the instability of the software and possible data loss, or even a complete loss of the XML file, then use the option with the results on binary files. Automatic backup .bak files are also created in the subdirectory of the project, which generally allows to recover the old XML file. It may also be safer to work directly on a subdirectory with automatic archiving (eg Dropbox), which will allow to retrieve all the intermediate versions of the files in case of problems.