**4. Experimental Study**

The goal of the conducted experiments was to obtain the pressure changes during the rapid water hammer phenomenon at the downstream end of a viscoelastic pipeline system with sudden contractions and expansions. Experimental data were later used to validate the numerical model. Four types of experiments were conducted (Figure 3).

**Figure 3.** Experimental setup.

The main elements of the experimental setup were serially connected HDPE pipes of the same length with various inner diameters (labelled 1 in Figure 3) attached to an upstream pressure tank (labelled 2 in Figure 3) equipped with a pressure gauge (labelled 3 in Figure 3). The tested pipes were rigidly fixed to the floor. In all experiments, the absolutely sharp inlet and outlet as well as the abrupt diameter changes were considered. The water hammer positive pressure surge was induced by manual, rapid and full closure of the ball valve at the downstream end of the pipeline system (labelled 4 in Figure 3). The valve closing time was measured with an electronic gauge. In all tests, the pressure wave period was longer than the closure time, which did not exceed 0.015 s. The water pressure was measured by a high-frequency relative pressure sensor (labelled 5 in Figure 3) with a range of −0.1~1.2 MPa and a measurement uncertainty of 0.5%. Pressure samples with a frequency of 1000 Hz were converted through an analogue-to-digital card connected to a laptop.

The pipeline systems denoted as "Type A" and "Type B" consisted of two serially connected pipes, whereas the systems denoted as "Type C" and Type D" consisted of three serially connected pipes. The internal diameters of the individual pipes are denoted with "L" (as in "left"), "M" (middle) or "R" (right) subscripts. For the purpose of this investigation, 6 experiments were conducted (2 tests each for Type A and Type B experiments and 1 test each for Type C and Type D experiments). The main characteristics of the experimental installation and registered steady flow parameters before initiation of the water hammer phenomenon are reported in Table 1.

Due to the influence of temperature on the creep parameters, the measurements were carried out with care to maintain a constant water and ambient temperature. The measured pressures are presented in the next section of the paper along with the results of the numerical calculations.


**Table 1.** Parameters of the pipes used in experiments.

where *L* is the length of the individual pipe, *Q* is the initial steady-state volumetric flow rate and *v*<sup>0</sup> is the initial flow velocity.
