.AixLib.Fluid.Sources.Outside_CpData

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 ports ports.

The pressure p at the fluid ports is computed as:

p = p_w + C_p,act C_s v² ρ ⁄ 2,

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

The wind pressure coefficient C_p,act is a function of the surface wind incidence angle and is defined relative to the surface azimuth (normal to the surface is 0). The wind incidence angle incAng is computed from the wind direction obtained from the weather file with the surface azimuth azi as the base of the angle. The relation between the wind pressure coefficient C_p,act and the incidence angle incAng is defined by a cubic hermite interpolation of the users table input. Typical table values can be obtained from the "AIVC guide to energy efficient ventilation", appendix 2 (1996). The default table is appendix 2, table 2.2, face 1.

The wind speed modifier C_s can be used to incorporate the effect of the surroundings on the local wind speed.

Definition of angles

The angles incAngSurNor for the wind incidence angle relative to the surface normal are measured counter-clock wise. The figure below shows an example entry, which is also used in the model AixLib.Fluid.Sources.Examples.Outside_CpData_Specification.

The wind incidence angle and surface azimuths are defined as follows: The wind indicience angle is obtained directly from the weather data bus weaBus.winDir. This variable contains the data from the weather data file that was read, such as a TMY3 file. In accordance to TMY3, the data is as shown in the table below.

For the surface azimuth azi, the specification from AixLib.Types.Azimuth is used, which is as shown in the table below.

Related model

This model differs from AixLib.Fluid.Sources.Outside_CpLowRise by the calculation of the wind pressure coefficient C_p,act. The wind pressure coefficient is defined by a user-defined table instead of a generalized equation such that it can be used for all building sizes and situations, for shielded buildings, and for buildings with non-rectangular shapes.

References

• M. W. Liddament, 1996, A guide to energy efficient ventilation. AIVC Annex V.

Revisions

• February 2, 2022, by Michael Wetter:
  Revised implementation.
  This is for IBPSA, #1436.
• Apr 6, 2021, by Klaas De Jonge:
  First implementation.