The "Horsepower Lab 1D" Handbook: Introduction

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1. Introduction

Package "Horsepower Lab 1D" (or hpl1d) is designed for numerical simulation of unsteady flow of gaseous fluid in engines and other pipelined facilities using 1D gas dynamics models.

In this version of hpl1d only the thermo- and gas dynamical processes can be modeled, with strong emphasis on simulation of entire operation cycle of internal combustion engine with wave action in engine ducting.

1.1. General Information

Computer simulation of complex systems is done here by breaking down the modeled system into small subsystems, for which standard models exist. This can be considered as applying modular approach, in which typical elements of ducting are represented by modules -- elements of an assembly. Assemblies consist of 2 kinds of modules: components and connectors, with links drawn between ports of modules of different types.
Components are modules that contain conserved quantities (masses of mixture constituents, momentum and total internal energy). Change of components' state is governed by conservation laws of 1D gas dynamics (click here). Interaction of each component with the "outer world" is done through its ports, to which modules of another kind (connectors) are linked. Connectors, as it becomes clear from their name, serve to connect components to each other. They do not contain conserved quantities, but rather account for components' interactions and evaluate fluxes of (named before) conserved quantities; components, connectors and links between them comprise assembly that (to some extent of detailization) represent the simulated physical system.

      The most important programs found in hpl1d package are graphical user interface (GUI) and solver. GUI ("hpl1dw.exe") ensures visual manipulation of assembly, editing modules' properties, running solver and also some pre- and post-processing. Solver does numerical simulation; it is written as a standalone program ("hpl1ds.exe"), this is done for simplicity and for flexibility. The latter means that the user may engage the solver as a computational utility in his/her own ways, e. g. for automated simulation/optimization, etc. Simulation results can be visualized, stored or post-processed and interpreted in any other way.
      There are also other programs in the package; they can be used mostly as "tools" (or "plug-ins").
      The only physical domain considered in this version of hpl1d is the one-dimensional (1D) gas dynamics. All components and connectors are based on gas-dynamical models. Thermodynamic parameters and flow velocity have zero- or one-dimensional representation within these modules.
      Working fluid is considered as a mixture of two (at most) gases. Both mixture constituents are considered to be ideal gases with (that is optional) non-linear dependence of their specific internal energy on the temperature e(T), that is represented by a polynomial:

e(T)=e₀+e₁T+e₂T²+...

Modules of Component kind implemented to date are:

Atmosphere that simply contain the ambient parameters;
Vessel that accounts for constant capacity vessels;
Duct that accounts for unsteady flow of gaseous mixtures in ducts;
Cylinder -- to simulate in-cylinder processes;
Crankcase -- such an engine-specific module accounting for processes in a chamber under piston;

Among these 5 component types only Duct has real internal spatial x-coordinate and can hold moving waves. One-dimensional unsteady conservation laws are used to describe the motion of the gaseous mixture within the Duct and a high-resolution finite-volume numerical method is used to solve these equations for each computational cell on each time step. Other components are simpler, all them are vessel-like and are modeled in a similar way -- as a zero-dimensional open thermodynamic system.

Modules of another kind are connectors, their job is to evaluate interactions between the components. Only two kinds of connectors are present in this version:

flow Restriction that connects 2 gas-dynamical components;
flow Splitter that connects 3 components and, therefore, acts as a triple junction;

      In turn, each of the 2 types of connectors listed above can be subdivided further due to basic nature of components connected.
      Actually Restriction connector is the "facade" for 3 different models used when appropriate: Diaphragm that interconnects two Ducts, Valve that links one Duct and another vessel-like component and a connector of Window sub-type -- to serve as link between two vessel-like components of any type (i. e., any component except Duct).
      As for Splitter-type connectors (used for triple junctions), these can be represented by 4 imaginable subtypes of connection, defined by the kind -- channel or vessel -- of connected components. In this version only 2 most usual subtypes/sub-models are implemented: (1) a model for Triple sub-type accounts for a junction of three Ducts and (2) a Chink model that represents a junction such as a side hole (connected to some vessel-like component) at the connection of two Ducts.

      Each model used to compute flow on connector (except Window) uses characteristic-based solution procedure to compute gas-dynamical fluxes. Every connector needs some kind of empirical data to properly account for its behavior in unsteady gas flow.
      More detailed theory and instructions for usage can be found in User Manual.
      Explanation of terms used throughout the documentation can be found in Glossary.

1.2. Implemented Features

Project management:

Input data needed to simulate each case is being collected as files within the project directory -- .\prj\[project-name]\. Projects can be created, copied and edited independently. Editing the assembly layout, modules' properties, running and terminating solver and visualizing the simulation results are managed with the help of GUI.
The main project file is input. Herein placed are modules layout and module properties with (optional) links to external data files. Another obligatory input file is thermo, which holds data that is ready to represent thermodynamic properties of gases used in simulation. Other files may be optional. Some files, (screen, track) are used for visualization and contain data generated by the solver program. More detailed description can be found in User Manual.

Output and visualization:

Output is done using multiple "channels". First, solver reports (if needed) the current step number (in time). Also, the simulated parameters on special "probes" during a given period are (optionally) collected and stored in file screen. This file then can be "played" and post-processed (scanned, exported as text and saved as graphical plots); Similarly, parameter curves along [quasi-]spatial coordinate x can be stored and then animated, producing an impressive record of wave action, e. g. in entire engine ducting system. This animation is stored in track file.

Visualization is possible for the following flow parameters: pressure, temperature, flow velocity, mass fraction of the 1^st mixture constituent (both on screen and on track). Also (in case of screen) port opening areas, and mass flow rates of the mixture and of its 1^st constituent (typically, fresh air) can be output. In case of track it is possible to view the entropy, both Riemann invariants and some kind of specific work of gas along the ducting.

Importing data on characteristics:

Tables of empirical data needed for simulation (found in sub-directories of .\data) are arranged in categories. These are:

thermodynamic tables for individual gases;
stagnation pressure loss curves for Restrictions of sub-types Diaphragm and Valve and curves for discharge coefficients for Windows (TODO!);
unsteady flow maps for Splitters of both sub-types: Triples and Chinks;
scavenging curves for Cylinders;

These all are documented and accessible with special tools.

1.3. Changes

Changes (in chronological order) are listed in file CHANGES.TXT

The current version of "Horsepower Lab 1D" is still a pre-release, so it may be worth looking at the list of major changes planned for the future versions:

automated computation planning, optimization, and report generation capabilities;
ensuring backward compatibility using XML-like format for project files;
rearranging the package to achieve capabilities for cross-platform and distributed computation (in version 2.0);
extending the set of physical domains, implementing models of differing levels of fidelity (in version 2.0);

More detailed list of planned changes is placed in file TODO.TXT

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