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Edits made to 2.2 Release Notes

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Lew Rossman
2018-09-03 10:34:03 -04:00
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ReleaseNotes2_2.md
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Release Notes for EPANET 2.2 (Draft)
============================
@@ -49,15 +47,18 @@ These new parameters augment the current `EN_ACCURACY` option which always remai
## 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.
EPANET's original node re-ordering scheme has been replaced by the more powerful **Multiple Minimum Degree (MMD)** algorithm.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%.
EPANET's original node re-ordering scheme has been replaced by the more powerful **Multiple Minimum Degree (MMD)** algorithm.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 58%.
## 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.
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>*.
To implement pressure dependent analysis four new parameters have been added to the [OPTIONS] section of the EPANET input file:
*D = D<sub>f</sub> [ (P - P<sub>min</sub>) / (P<sub>req</sub> - P<sub>min</sub>) ]<sup>P<sub>exp</sub></sup>*
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>*.
To implement pressure driven analysis four new parameters have been added to the [OPTIONS] section of the EPANET input file:
| Parameter | Description | Default |
@@ -81,7 +82,8 @@ for the thread-safe API. Some additional points regarding the new **PDA** option
- 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>*.
- *P<sub>min</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>*.
## Code Changes