My goal is to study the pressure drop in the tube for different combination of number of fins, fin height and fin width. The internal fins have a certain helix angle and there are a number of fins. The tube has one inlet and one outlet. Since the fins are arranged in a helical pattern, a single helical strip of one helix pitch length is to be modeled. I can understand that a transnational periodic boundary condition can be applied between the inlet and outlet of one strip of the fin, but I am not sure as to how would the sides of the fin can be modeled. I have one reference where he applies rotational periodicity to the sides of that one single strip.I went ahead with the reference, but I got absolutely wrong results. The pressure drop was of the -10^4 order,which is not even close to the reference measurements. The residuals were close to 10^-6 for all the equations.I am using 4 meshed-domains ;2 for connecting inlet and outlet and 2 for connecting the side faces of the helical strip. I applied the periodic conditions through "Mesh--> modify zones--> make periodic" through TUI option in Fluent,however I got a bit confused when a started to input the "mass flow rate" in periodic operating conditions.There was only periodic operating conditions input, so for which PBC I was entering the mass flow rate-rotation and translational ? The model being used is RANS SST k-omega. I also went through the "periodic repeats " option thinking that I have two periodic zones adjacent to each other, but it's usage was for something completely different. Also,since I have gone into a considerable detail, I also want to know whether it's possible to have mixed type boundary conditions ,for example, can I have translational and rotational boundary conditions on two faces of an hellical geometry, such as this pipe?