Computing Nusselt number

As far as I am aware, this is the correct definititon of the Nusslet number.
Note that in the paper you are considering they have scaled the temperature \theta=\frac{T-T_{cold}}{T_{hot}-T_{cold}} which you have not done in your equation, and can account for a difference.

Note that just supplying a snippet of your code is not very helpful, as you have not provided what problem you are solving (mathematically), a minimal working code example, and what values you expect (and which you actually achieve).

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