Calculate Discharge Coefficients

A discharge coefficient (Cd) is a crucial parameter in fluid dynamics and building physics, especially in the context of natural ventilation through windows.

It quantifies the ease with which air can pass through an opening, such as a window, under pressure differences. A higher Cd indicates a more efficient flow of air through the window.

Any prefered equation can be used to adopt a Cd value. Better Building recommends BB101, which adopts the definitions recommended by the CIBSE Natural Ventilation Group for free area, effective area and equivalent area. A spreadsheet can be downloaded below, for ease of calculation.

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Discharge_coefficient_calculator.xlsx

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Download BB 101: Ventilation, thermal comfort and indoor air quality 2018 here

Components of Discharge Coefficient Calculation

To calculate a Cd value, the following inputs need to be provided. These are then used within an Airflow Network to calculate the impact on a model. Instructions are provided in the spreadsheet.

Window Dimensions

The width (w) and height (h) of the window are essential dimensions that define the size of the opening through which air will flow. These dimensions influence the overall free area available for air passage.

Opening Angle (α)

This is the angle at which the window is opened. It significantly impacts the effective area (Aeff) through which air flows. For hinged windows, this angle can vary, whereas for sash windows, it is typically set to 90°.

Stroke Length (d)

This is the distance by which the window is opened. It contributes to determining the effective opening area.

Orifice Discharge Coefficient (Cd0)

This coefficient, typically a value between 0.62 and 1.0, represents the ideal discharge coefficient for a sharp-edged orifice under standard conditions.

Equivalent Area (Aeq)

This is a theoretical area that represents the window opening considering the discharge coefficient. It simplifies the comparison between different window designs and their airflow capabilities.

Effective Area (Aeff)

This is the actual area available for airflow, adjusted by the discharge coefficient. It reflects the true performance of the window in facilitating ventilation.

Free Area (Afree)

This is the physical area of the window opening without considering the discharge coefficient. It represents the maximum potential area for airflow.