**6. Conclusions**

In the presented paper, the MacCormack explicit method was used to numerically solve transient flow equations in polymeric pipes. In order to take into account a sudden change in the pipeline diameter, the connection section boundary was used, which satisfies the head condition but takes into account two different values of the flow velocity on the left-hand side and the right-hand side of the connection node.

Including a term of retarded deformation in the continuity equation made it possible to take into account the viscoelastic behaviour of the pipe walls. The generalised one- and twoelement Kelvin–Voigt model was used for numerical simulations. Despite including only a steady friction factor in the momentum equation, the calculated pressure fluctuations showed a satisfactory agreement with the laboratory data in terms of both phase and amplitude.

The use of the connection boundary condition made it possible to reproduce the pressure disturbances in the initial pressure increase related to the change in the diameter of the series-connected pipes.

Furthermore, a constant value of pressure wave velocity for the entire pipeline system was assumed. Further work needs to be performed to study water hammer in variableproperty pipeline systems. Future studies will concentrate on simulating water hammer in serially connected pipes with various inner diameters and made of different materials. More broadly, research is also needed to address local losses during hydraulic transients due to diameter changes in pipeline systems.

**Author Contributions:** Conceptualisation, M.K. and A.M.; methodology, M.K., A.M. and A.K.; software, M.K.; validation, A.M., A.K., K.U. and M.S.; formal analysis, M.K., A.M. and K.U.; investigation, A.M. and A.K.; resources, A.M. and A.K.; data curation, M.K. and A.M.; writing—original draft preparation, M.K. and A.M.; writing—review and editing, A.K., K.U. and M.S.; visualisation, M.K.; supervision, M.K and A.M.; project administration, K.U. and M.S.; funding acquisition, M.K. and A.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The code generated during the study and experimental data are available from the corresponding author by request.

**Conflicts of Interest:** The authors declare no conflict of interest.
