BuildingSystems.Fluid.BaseClasses.PartialResistance

# .BuildingSystems.Fluid.BaseClasses.PartialResistance

Partial model for a hydraulic resistance

## Information

Partial model for a flow resistance, possible with variable flow coefficient. Models that extend this class need to implement an equation that relates `m_flow` and `dp`, and they need to assign the parameter `m_flow_turbulent`.

See for example BuildingSystems.Fluid.FixedResistances.PressureDrop for a model that extends this base class.

## Revisions

- April 14, 2020, by Michael Wetter:
  Changed `homotopyInitialization` to a constant.
  This is for IBPSA, #1341.
- February 26, 2020, by Michael Wetter:
  Changed icon to display its operating state.
  This is for #1294.
- October 25, 2019, by Jianjun Hu:
  Improved icon graphics annotation. This is for #1225.
- November 3, 2016, by Michael Wetter:
  Removed start value for pressure difference to simplify the parameter window.
  This is for #552.
- January 26, 2016, by Michael Wetter:
  Avoided assignment of `dp(nominal=0)` if `dp_nominal_pos = 0` and of `m_flow(nominal=0)` if `m_flow_nominal_pos = 0` as nominal values are not allowed to be zero.
- January 22, 2016, by Michael Wetter:
  Corrected type declaration of pressure difference. This is for #404.
- August 15, 2015, by Filip Jorissen:
  Implemented more efficient computation of `port_a.Xi_outflow`, `port_a.h_outflow` and `port_a.C_outflow` when `allowFlowReversal=false`. This is for #281.
- January 13, 2015, by Marcus Fuchs:
  Revised revisions section (there were two revisions statements)
- November 20, 2014 by Michael Wetter:
  Removed `start` attribute for `m_flow` as this is already set in its base class.
- October 8, 2013 by Michael Wetter:
  Removed propagation of `show_V_flow` to base class as it has no longer this parameter.
- December 14, 2012 by Michael Wetter:
  Renamed protected parameters for consistency with the naming conventions.
- February 12, 2012, by Michael Wetter:
  Removed duplicate declaration of `m_flow_nominal`.
- February 3, 2012, by Michael Wetter:
  Made assignment of `m_flow_small` `final` as it is no longer used in the base class.
- January 16, 2012, by Michael Wetter:
  To simplify object inheritance tree, revised base classes `BuildingSystems.Fluid.BaseClasses.PartialResistance`, `BuildingSystems.Fluid.Actuators.BaseClasses.PartialTwoWayValve`, `BuildingSystems.Fluid.Actuators.BaseClasses.PartialDamperExponential`, `BuildingSystems.Fluid.Actuators.BaseClasses.PartialActuator` and model `BuildingSystems.Fluid.FixedResistances.PressureDrop`.
- August 5, 2011, by Michael Wetter:
  Moved linearized pressure drop equation from the function body to the equation section. With the previous implementation, the symbolic processor may not rearrange the equations, which can lead to coupled equations instead of an explicit solution.
- June 20, 2011, by Michael Wetter:
  Set start values for `m_flow` and `dp` to zero, since most HVAC systems start at zero flow. With this change, the start values appear in the GUI and can be set by the user.
- April 2, 2011 by Michael Wetter:
  Added `m_flow_nominal_pos` and `dp_nominal_pos` to allow providing negative nominal values which will be used, for example, to set start values of flow splitters which may have negative flow rates and pressure drop at the initial condition.
- March 27, 2011, by Michael Wetter:
  Added `homotopy` operator.
- March 23, 2011 by Michael Wetter:
  Added homotopy operator.
- March 30, 2010 by Michael Wetter:
  Changed base classes to allow easier initialization.
- April 13, 2009, by Michael Wetter:
  Extracted pressure drop computation and implemented it in the new model BuildingSystems.Fluid.BaseClasses.FlowModels.BasicFlowModel.
- September 18, 2008, by Michael Wetter:
  Added equations for the mass balance of extra species flow, i.e., `C` and `mC_flow`.
- July 20, 2007 by Michael Wetter:
  First implementation.

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