Page 353 - ExtendSim User Guide

P. 353

```
Introduction 327
Blocks for building discrete rate models
☞ While the ExtendSim discrete rate architecture preserves mass balance in the system, no explicit consideration is given to pressure, energy, momentum, or temperature, since these are beyond its scope.
Blocks for building discrete rate models
The blocks in the Rate library are optimized for creating discrete rate models. In addition, you can build custom discrete rate blocks using the ExtendSim development environment.
Rate library
The Rate library allows you to simulate a wide range of flow systems by connecting blocks together and entering parameters. The complexity of calculating the effective flow rate and the generation of events that dic- tate a new rate calculation are handled within the blocks, alleviating the need to do any programming in the ModL language.
Rate library blocks are optimized for modeling anything flowing through a system. They incorporate concepts like constraints, goals, flow prioriti- zation, mixing, batching, unbatching, level indicators, and so forth. The blocks have been designed to meet most rate-based flow needs so you can quickly and easily perform complex high-volume/high-speed model- ing tasks.
As mentioned in the Tutorial module, discrete rate models can use con- tinuous blocks from the Value library for data management and model- specific tasks. Using Value blocks with Rate library blocks does not change the fundamental architecture of discrete rate models; they will still be event-based rather than use the time-based architecture of contin- uous models. Discrete event blocks from the Item library can also be used in discrete rate models; they are helpful for representing entities such as tankers, airplanes, people, and so forth that interface with flow.
☞ See “Rate Library Blocks” on page 859, “Item Library Blocks” on
page 851, and “Value Library Blocks” on page 843 for a listing and brief description of the blocks in those libraries.
Creating custom discrete rate blocks
Rate library blocks
Before building a discrete rate model, it is helpful to understand the terminology that will be used and to have an overview of ExtendSim discrete rate architecture.
LP technology
To provide global oversight to calculate the effective rates in a discrete rate model, ExtendSim uses linear programming (LP) technology. The purpose of the LP calculation is to determine the maximum effective flow rates in the system given the constraints defined by block settings
Because of the Rate library’s extensive capability, it is not likely that you
would need to program your own discrete rate blocks. If you do want to
do this, it is important to note that discrete rate blocks use different data structures and pro- gramming methods than continuous or discrete event blocks. It is suggested that you start with an existing discrete rate block as a base, using a copy of a Rate library block similar to the one you want to build. Read the Developer Reference before modifying discrete rate blocks so you have a better understanding of how those blocks work internally.
Terminology and architecture
Discrete Rate
```