BuildSysPro.IBPSA.Airflow.Multizone.BaseClasses.windPressureLowRise

# .BuildSysPro.IBPSA.Airflow.Multizone.BaseClasses.windPressureLowRise

Wind pressure coefficient for low-rise buildings

## Information

This function computes the wind pressure coefficient for low-rise buildings with rectangular shape. The correlation is the data fit from Swami and Chandra (1987), who fitted a function to various wind pressure coefficients from the literature. The same correlation is also implemented in CONTAM (Persily and Ivy, 2001).

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

G = ln(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 as shown in the figure below.

Based on the wind incidence angle α 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, i.e., α = 0. 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 attenuation factor is

Cp ⁄ Cp0 = ln(1.248 - 0.703 sin(α ⁄ 2) - 1.175 sin2(α) - 0.131 sin3(2 α G) + 0.769 cos(α ⁄ 2) +0.071 G2 * sin2(α ⁄ 2) + 0.717 cos2(α ⁄ 2)),

where Cp is the wind pressure coefficient for the current angle of incidence.

This function is used in Buildings.Fluid.Sources.Outside_CpLowRise which can be used directly with components of this package.

#### 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.

#### Implementation

Symmetry requires that the first derivative of the wind pressure coefficient with respect to the incidence angle is zero for incidence angles of zero and π. However, the correlation of Swami and Chandra has non-zero derivatives at these values. In this implementation, the original function is therefore slightly modified for incidence angles between 0 and 5 degree, and between 175 and 180 degree. This leads to a model that is differentiable in the incidence angle, which generally leads to better numeric performance.

#### Interface

>     function windPressureLowRise
>       extends Modelica.Icons.Function;
>       input Real Cp0 "Wind pressure coefficient for normal wind incidence angle";
>       input Modelica.Units.SI.Angle alpha "Wind incidence angle (0: normal to wall)";
>       input Real G "Natural logarithm of side ratio";
>       output Real Cp "Wind pressure coefficient";
>     end windPressureLowRise;

## Revisions

- February 16, 2022, by Michael Wetter:
  Changed argment name to `alpha` for consistency with figure.
- March 15, 2016, by Michael Wetter:
  Replaced `spliceFunction` with `regStep`. This is for issue 300.
- January 26, 2016, by Michael Wetter:
  Removed `constant` keyword for `a180` as its value depends on the input of the function.
- October 27, 2011 by Michael Wetter:
  First implementation.

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