Dear David,

I can provide some reference values for you.

However, it may not be the best case to use any “reference” data due to the strong dependence between Cp on the top of the stack and the building/surroundings geometry.

The values below are based on the wind tunnel experiments performed for this study:
Unsteady natural ventilation at model scale—Flow reversal and discharge coefficients of a short stack and an orifice
Building and Environment, Volume 43, Issue 9, September 2008, Pages 1491-1506
D. Costola, D.W. Etheridge

The goal of the experiments was not determining the Cp value you need, but rather studying the unsteady effects on the flow rate.

In this sense, Cp is a secondary result and you can consider that the uncertainty on these experimental results is high. After this disclaimer, here are the values:

No obstruction on the roof:

    Dir (°) / Cp

    0 / -0.7

    90 / -0.1

    180 / -0.6

With obstruction on the roof:

    Cp between 0.8 and -1.0 depending on the obstruction geometry and position

If I understood your problem correctly, the chimneys will be highly influenced by the presence of the clerestory, so you should use the data with obstruction on the roof.

As you can see, the results vary in a very wide range.

My advice for you is to perform sensitivity analysis for Cp, and go for wind tunnel tests if you need more precise data.

Kind regards,

Daniel Cóstola