## 2 Experimental Setup and Procedures  
### Facilities and Setup  
In order to measure the velocity of the flow field under the condition of double-walled confinement, an experiment was conducted and the results could clearly show the development and transformation of liquid flow. As is shown in Figure 1, a diagram was used to illustrate the experimental setups, which included three parts: water circulation system, PIV system and pressure test system.  
In the water circulation system, the pressurized tap water was supplied from a centrifugal pump that has a pump head of 250 m and pump capacity of 46 m\({}^{3}\)/h. In the experiment, the operating pressure of the centrifugal pump could be continuously regulated through the control table by changing the working frequency of the motor powering the pump. The flow rate of the pump could be directly read out from the pipeline flowmeter. Considering the limitation of the pump head, the strength of the plexiglass tank and the accuracy of the PIV, the operating pressure was 0.34 MPa in this experiment.  
The water tank had a volume of 3 m\({}^{3}\). Taking a small amount of water consumption and pipe or connections leakage in the course of the experiment into consideration, the volume of the water tank was sufficient to meet the qualification of the operation. In addition, another feeding pipe was used.  
As illustrated in Figure 1, the facilities of the measuring pressure and flow rate were applied in the water line. To be more specific, the pressure was monitored by a dynamic pressure transducer DMP 333 (BD Sensors, Thierstein, Germany) that had been calibrated by the manufacturer with an accuracy of \(\pm\)0.5% FS in advance. Further, the flow rate was measured by an electromagnetic flowmeter Optiflux 4100 C (Krohne, Krohne, Germany) that had an accuracy of \(\pm\)0.3%, velocity error of \(\pm\)1 mm/s and repeatability error less than 0.1%.  
The PIV system, Ultra-II (TSI, Knoxville, USA), was employed to measure the velocity field of the double-walled confined waterjet. The basic premise of PIV is to measure the velocity of makers or seed particles in the flow within a plane. PIV provides an Eulerian description of the velocity, since the  
Figure 1: Schematic diagram of the experimental setup.  
velocity vector is obtained by calculating correlations for interrogation areas. Specifically, the velocity is determined by using the basic definition.  
\[V(X,t)=\frac{\Delta S(X,t)}{\Delta t},\ \Delta t\to 0 \tag{1}\]  
where \(V(X,t)\) is the particle velocity vector at position \(X\) and time \(t\), and \(\Delta S\) is the displacement vector of the seeding particle during the elapsed time \(\Delta t\).  
In order to create a plane through the flow field, a pulsed laser generation device, YAG (TSI, Knoxville, TN, USA), was applied and the detailed information is shown in Table 1. The speed information of the measurement area is mainly from pictures taken by the charge-coupled device camera and the detailed information is shown in Table 2. The velocity range of the PIV was about 1-100 m/s. In this experiment, the region of 90 \(\times\) 130 mm was large enough for our measurement. To make the results more reliable, each testing section comprised 160 series of instantaneous data to analyze the characteristics of mean velocity and turbulence intensity.