## 1 Background  
Hydropower, small and large, remains the most important source of renewable energy for electrical power production providing 16.4% of the world electricity mix in 2016 [1]. In Switzerland, hydropower provides 56.7% of the electricity of the country, 5.7% coming from small-hydro (see [2]), considering plants with an \"annual average used\" capacity up to 10 MW [3]. Indeed, there are more than 1300 small-scale hydropower plants in operation, with an installed capacity of approximately 760 MW and an output of 3400 GWh per year. In the post-Fukushima era, Switzerland has decided to renounce to its nuclear energy power stations and to accelerate the transition to a sustainable energy future based on carbon-free renewable electricity sources. To increase the small hydropower production while limiting the environmental impact, the primary sustainable solution would be to harness the energy of existing infrastructure, such as drinking water networks or wastewater networks. To address such potential sites, axial flow turbines provide low CAPEX solution by limiting the civil engineering works. Along the years, different research teams developed various models of micro hydro turbines. [PERSON] et al. [PERSON] et al. (2018); [PERSON] et al. (2018) studied numerically and experimentally a new axial turbine designed for pressure recuperation on piping systems. The turbine consists of a stator with 12 blades and a rotor with 15 blades, with the possibility to modify the angle of the guide vanes. [PERSON] et al. [PERSON] et al. (2018) designed and tested four different axial microturbines for heads between 4 mWC and 9 mWC with a hydraulic efficiency around 68%. [PERSON] et al. [PERSON] et al. (2018) investigated experimentally the behavior of a five-blade axial turbine reaching 64% of efficiency for a head of 3.4 mWC. In order to achieve a higher efficiency and a stable operation of this type of microturbines, axial turbines with counter rotating runners have been developed. Accordingly, [PERSON] et al. [PERSON] et al. (2018) studied numerically such turbines with a calculated efficiency for both runners of 70%, with a drastically decreased efficiency for the rear runner at partial and large flow rates. In this framework, a research project to develop new technologies to harvest hydraulic energy on existing infrastructure has been set up by the HES-SO Valais//Wallis (Sion, Switzerland) and the EPFL-Laboratory for Hydraulic Machines, Lausanne, Switzerland. One of the results is a new axial microturbine with counter-rotating runners for drinking water networks [PERSON] et al. (2018); [PERSON] et al. (2018); [PERSON] et al. (2018), which should cover a part of the hydraulic energy potential that must be additionally exploited until 2050 in Switzerland. The microturbine with a shroud diameter of \(\phi 0.1\) m and a hub diameter of \(\phi 0.08\) m was designed to reach a hydraulic efficiency of more than 85%. The nominal conditions are expected for a discharge of 8.7 l\(\cdot\)s\({}^{-1}\), a head of 15 mWC on each runner and a rotational speed of 3000 min\({}^{-1}\) for both runners. The prototype turbine was manufactured and tested on the HES-SO Valais//Wallis--Switzerland test rig [PERSON] and [PERSON] (2018) to assess its performances. According to [PERSON] and [PERSON] and [PERSON] (2018), the budget allocated to the development for small-hydro is much more limited compared to large-hydro. The total men-hours investment dedicated to the performance measurements must be reduced as much as possible. To this end, with the main purpose of reducing the time necessary to perform the full hydraulic performance tests of a turbomachine, an alternative dynamic method has been implemented and validated. Based on dynamic measurements to retrieve the hydraulic characteristics, the so-called sliding-gate method was first tested by [PERSON] et al. [PERSON] et al. (2018); [PERSON] et al. (2018).