AixLib.Fluid.Sources.Outside_CpLowRise

# .AixLib.Fluid.Sources.Outside_CpLowRise

Boundary that takes weather data as an input and computes the wind pressure for low-rise buildings based on the equation from Swami and Chandra (1987)

## Information

This model describes boundary conditions for pressure, enthalpy, and species concentration that can be obtained from weather data. The model is identical to AixLib.Fluid.Sources.Outside, except that it adds the wind pressure to the pressure at the fluid port `ports`. The correlation that is used to compute the wind pressure is based on Swami and Chandra (1987) and valid for low-rise buildings with rectangular shape. The same correlation is also implemented in CONTAM (Persily and Ivy, 2001).

The wind pressure coefficient is computed based on the side ratio of the walls, which is defined as

s = x ⁄ y

where x is the length of the wall that will be connected to this model, and y is the length of the adjacent wall. The wind direction is computed relative to the azimuth of this surface, which is equal to the parameter `azi`. The surface azimuth is defined in AixLib.Types.Azimuth. For example, if an exterior wall is South oriented, i.e., its outside-facing surface is towards South, use `AixLib.Types.Azimuth.S`.

Based on the surface azimuth, the wind direction and the side ratio of the walls, the model computes how much the wind pressure is attenuated compared to the reference wind pressure `Cp0`. The reference wind pressure `Cp0` is a user-defined parameter, and must be equal to the wind pressure at zero wind incidence angle. Swami and Chandra (1987) recommend Cp0 = 0.6 for all low-rise buildings as this represents the average of various values reported in the literature. The computation of the actual wind pressure coefficient Cp is explained in the function Buildings.Airflow.Multizone.BaseClasses.windPressureLowRise that is called by this model.

The pressure p at the port `ports` is computed as

p = pw + Cp 1 ⁄ 2 v2 ρ,

where pw is the atmospheric pressure from the weather bus, v is the wind speed from the weather bus, and ρ is the fluid density.

This model differs from AixLib.Fluid.Sources.Outside_CpData by the calculation of the wind pressure coefficient Cp,act. The wind pressure coefficient is defined by an equation in stead of a user-defined table. This model is only suited for low-rise rectangular buildings.

#### References

- Muthusamy V. Swami and Subrato Chandra. Procedures for Calculating Natural Ventilation Airflow Rates in Buildings. Florida Solar Energy Center, FSEC-CR-163-86. March, 1987. Cape Canaveral, Florida.
- Andrew K. Persily and Elizabeth M. Ivy. Input Data for Multizone Airflow and IAQ Analysis. NIST, NISTIR 6585. January, 2001. Gaithersburg, MD.

## Revisions

- April 4, 2023, by Michael Wetter:
  Removed use of non-existent parameter in annotation.
  This is for IBPSA, #1724.
- February 2, 2022, by Michael Wetter:
  Revised implementation.
  This is for IBPSA, #1436.
- Jun 28, 2021, by Klaas De Jonge:
  Documentation changes to explain the difference with AixLib.Fluid.Sources.Outside_CpData.
- January 26, 2016, by Michael Wetter:
  Added `unit` and `quantity` attributes.
- October 26, 2011 by Michael Wetter:
  First implementation.

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