Validity and assumptions in the use of hyperbolic secant function for wake survey
I have been trying to measure the drag characteristics of a wind tunnel model by using a pitot-static tube set up. From which the transducer will output to me the stream-wise velocity profile based on the pressure differential measured.
Following which, in order to evaluate the drag coefficient of the model, i have used the common method of fitting the velocity profile using a squared hyperbolic secant function. However, i realised that across all the different profiles measured, there shoulder of the profiles do not seem to be able to fit nicely into the hyperbolic function.
I would like to know if this is usually the case, or is there any other underlying assumption in the usage of hyperbolic secant functions which is not being considered here? For my case, the reynolds number of the flow is around 500k, is this an issue?