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Merge pull request #210 from LRossman/contributor-lr
Pressure Dependent Demands added to address issue 163
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| Release Notes for EPANET 2.2 (Draft) | ||
| ============================ | ||
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| This document describes the changes and updates that have been made to version 2.2 of EPANET. | ||
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| ## Thread-Safe API Functions | ||
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| A duplicate set of the version 2.1 API functions has been provided that allow multiple EPANET projects to be analyzed concurrently in a thread-safe manner. These functions maintain the same name as the original but use a `EN_` prefix instead of `EN`. In addition, the first argument to each of these functions is a pointer to an `EN_Project` structure that encapsulates the network data for the particular project being analyzed. For example, instead of writing: | ||
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| `ENgetnodevalue(nodeIndex, EN_ELEVATION, &elev)` | ||
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| one would use: | ||
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| `EN_getnodevalue(pr, nodeIndex, EN_ELEVATION, &elev)` | ||
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| where `pr` is the pointer to an `EN_Project`. | ||
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| Two new functions have been added to the API to manage the creation and deletion of project pointers. `EN_createproject` creates a new project along with a pointer to it, while `EN_deleteproject` deletes a project. An example of using the thread-safe version of the API is shown below: | ||
| ``` | ||
| #include "epanet2.h" | ||
| int runEpanet(char *finp, char *frpt) | ||
| { | ||
| EN_Project *pr = NULL; | ||
| int err; | ||
| err = EN_createproject(&pr); | ||
| if (err) return err; | ||
| err = EN_open(pr, finp, frpt, ""); | ||
| if (!err) err = EN_solveH(pr); | ||
| if (!err) err = EN_report(pr); | ||
| EN_close(pr); | ||
| EN_deleteproject(pr); | ||
| return err; | ||
| } | ||
| ``` | ||
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| ## Additional Convergence Parameters | ||
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| Two new analysis options have been added to provide more rigorous convergence criteria for EPANET's hydraulic solver. In the API they are named `EN_HEADERROR` and `EN_FLOWCHANGE` while in the `[OPTIONS]` section of an EPANET input file they are named `HEADERROR` and `FLOWCHANGE`, respectively. | ||
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| `EN_HEADERROR` is the maximum head loss error that any network link can have for hydraulic convergence to occur. A link's head loss error is the difference between the head loss found as a function of computed flow in the link (such as by the Hazen-Williams equation for a pipe) and the difference in computed heads for the link's end nodes. The units of this parameter are feet (or meters for SI units). The default value of 0 indicates that no head error limit applies. | ||
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| `EN_FLOWCHANGE` is the largest change in flow that any network element (link, emitter, or pressure-dependent demand) can have for hydraulic convergence to occur. It is specified in whatever flow units the project is using. The default value of 0 indicates that no flow change limit applies. | ||
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| These new parameters augment the current `EN_ACCURACY` option which always remains in effect. In addition, both `EN_HEADERROR` and `EN_FLOWCHANGE` can be used as parameters in the `ENgetstatistic` (or `EN_getstatistic`) function to retrieve their computed values (even when their option values are 0) after a hydraulic solution has been completed. | ||
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| ## Improved Linear Solver Routine | ||
| EPANET's hydraulic solver requires solving a system of linear equations over a series of iterations until a set of convergence criteria are met. The coefficient matrix of this linear system is square and symmetric. It has a row for each network node and a non-zero off-diagonal coefficient for each link. The numerical effort needed to solve the linear system can be reduced if the nodes are re-ordered so that the non-zero coefficients cluster more tightly around the diagonal. | ||
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| EPANET's original node re-ordering scheme has been replaced by the more powerful **Multiple Minimum Degree (MMD)** algorithm. On a series of eight networks ranging in size from 7,700 to 100,000 nodes **MMD** reduced the solution time for a single period (steady state) hydraulic analysis by an average of more than 50%. | ||
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| ## Pressure Dependent Demands | ||
| EPANET has always employed a Demand Driven Analysis (**DDA**) when modeling network hydraulics. Under this approach nodal demands at a given point in time are fixed values that must be delivered no matter what nodal heads and link flows are produced by a hydraulic solution. This can result in situations where required demands are satisfied at nodes that have negative pressures - a physical impossibility. | ||
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| To address this issue EPANET has been extended to use a Pressure Driven Analysis (**PDA**) if so desired. Under **PDA**, the demand *D* delivered at a node depends on the node's available pressure *P* according to: | ||
| $$D =D_f\left(\frac{P-P_{min}}{P_{req}-P_{min}}\right)^{P_{exp}} for P_{0}<=P<=P_{req}$$where *D<sub>f</sub>* is the full demand required, *P<sub>min</sub>* is the pressure below which demand is zero, *P<sub>req</sub>* is the pressure required to deliver the full required demand and *P<sub>exp</sub>* is an exponent. When *P < P<sub>min</sub>* demand is 0 and when *P > P<sub>req</sub>* demand equals *D<sub>f</sub>*. | ||
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| To implement pressure dependent analysis four new parameters have been added to the [OPTIONS] section of the EPANET input file: | ||
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| | Parameter | Description | Default | | ||
| |--|--|--| | ||
| | DEMAND MODEL | either DDA or PDA | DDA | | ||
| | MINIMUM PRESSURE | value for *P<sub>min</sub>* | 0 | ||
| | REQUIRED PRESSURE | value for *P<sub>req</sub>* | 0 | ||
| | PRESSURE EXPONENT | value for *P<sub>exp</sub>* | 0.5 | | ||
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| These parameters can also be set and retrieved in code using the following API functions | ||
| ``` | ||
| int ENsetdemandmodel(int modelType, double pMin, double pReq, double pExp); | ||
| int ENgetdemandmodel(int *modelType, double *pMin, double *pReq, double *pExp); | ||
| ``` | ||
| for the legacy API and | ||
| ``` | ||
| int EN_setdemandmodel(EN_Project *pr, int modelType, double pMin, double pReq, double pExp); | ||
| int EN_getdemandmodel(EN_Project *pr, int *modelType, double *pMin, double *pReq, double *pExp); | ||
| ``` | ||
| for the thread-safe API. Some additional points regarding the new **PDA** option are: | ||
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| - If no DEMAND MODEL and its parameters are specified then the analysis defaults to being demand driven (**DDA**). | ||
| - This implementation of **PDA** assumes that the same parameters apply to all nodes in the network. Extending the framework to allow different parameters for specific nodes is straightforward to do but is left as a future feature to implement. | ||
| - *P<sub>0</sub>* is allowed to equal to *P<sub>req</sub>*. This condition can be used to find a solution that results in the smallest amount of demand reductions needed to insure that no node delivers positive demand at a pressure below *P<sub>min</min>*. | ||
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| ## Code Changes | ||
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| - The header file `vars.h` containing global variables has been eliminated. Instead a number of new structures incorporating these variables has been added to `types.h`. These structures have been incorporated into the new `EN_Project` structure, also defined in `types.h`, which gets passed into each of the thread-safe API functions as a pointer. | ||
| - Each of the legacy API functions now simply calls its thread-safe counterpart passing in a pointer to a default global`EN_Project` variable that is declared in `types.h`. | ||
| - Throughout all code modules, global variables that were previously accessed through `vars.h` are now accessed using the `EN_Project` pointer that is passed into the functions where the variables appear. | ||
| - The exceedingly long `hydraul.c` file has been split into four separate files: | ||
| - `hydraul.c` now contains just the code needed to initialize a hydraulic analysis, set demands and control actions at each time step, and determine the length of the next time step to take. | ||
| - `hydsolver.c` implements EPANET's hydraulic solver at a single point in time. | ||
| - `hydcoeffs.c` computes values of the matrix coefficients (derived from link head losses and their gradients) used by the hydraulic solver. | ||
| - `hydstatus.c` checks for status changes in valves and pumps as requested by the hydraulic solver. | ||
| - The Multiple Minimum Degree re-ordering algorithm appears in a new file named `genmmd.c`. This is 1990's legacy code that is readily available on the web and can be found in several linear equation solver libraries. |
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