## 4 Results and Discussion  
Figure 4 illustrates the measured pressure \(P\) (static pressure) at three different positions of a ZFT versus the measuring time \(T\) when the supply volume flow rate \(Q_{V}\) was 16 L/s. As displayed, periodic pressure pulses were produced at (i), (ii), and (iii), respectively. Both of the pressure values and the pulse amplitude measured at (i) were larger than that at (ii) and (iii), and the pressure values at (iii) were the smallest. However, in the study of [PERSON] et al. [19], the pressure values at the front chamber (iii) were larger than the pressure at the side passage. This difference is caused by the throttling action of the throttling plate, which raised the pressure in the front chamber to a certain level. In our study, we focus more on the pressure at (i), as this is the direct power actuating the oscillation assembly to implement oscillation movement. Besides, if we use the pressure at the outlet of the oscillation tool as the reference for the evaluation of pressure difference and ignore the kinetic energy at the inlet of the fluidic oscillator, the pressure drop of the system is equal to the pressure measured at (i). The pulse amplitude of pressure drop \(A\) is defined as:  
\[A_{P}=P_{max}-P_{min} \tag{1}\]  
where \(P_{max}\) and \(P_{min}\) are the maximum and minimum of the pressure at (i), respectively. Additionally, \(P_{max}\) is the mean value of the pressure drop (mean value of the pressure drop in a working cycle) of the system. Another dimensionless performance index is defined as the relative pulse amplitude of pressure drop \(A_{P}\)', which can be calculated from:  
\[A_{P}{}^{\prime}=A_{P}/P_{max} \tag{2}\]  
A larger pulse amplitude \(A_{P}\) implies a stronger ability of pressure pulse production and a smaller mean pressure drop \(P_{max}\) implies lower system power consumption. Combining both \(A_{P}\) and \(P_{max}\), it can be concluded that an FAOT exhibits a better performance at a higher value of relative pulse amplitude \(A_{P}\)'. In order to investigate the working process of a ZFT, a unit (cycle) of pressure fluctuation (curve i) is divided into four sections by five highlighted circles corresponding to four working states of the fluidic oscillation tool. At the a-b state, the value of pressure drop \(P\) decreases with increasing time \(T\) and reaches the minimum at b; this phenomenon conforms to the case when the piston is moving forward and the main jet is attached to the attachment wall flowing to the rear chamber of the cylinder. This is because, while the piston is moving forward, its velocity increases with the increase of time, which leads to an increase of flow recovery. An increased flow recovery means a lower hydraulic resistance downstream of the output channel connected to the rear chamber, as issuing flow can easily enter the working chamber of the cylinder. Therefore, a decrease of pressure drop will be obtained. Because of the existence of the piston rod, the liquid action area of the rear chamber is larger than that of the front chamber, and the forward stroke end velocity of the piston is greater than the backward stroke end velocity. That is why the pressure drop at d is higher than b, and the state of c-d should be the case when the piston is moving backward. The subsequent state of b-c represents the jet switching process which occurred at the forward stroke end of the piston. A pressure pulse caused by the sudden stop of the piston is produced and the value of \(P\) reaches the maximum at c. Analogously, the d-e state corresponds to the jet switching process when the piston moves to the backward stroke end and is rebounded to move forward again. It can be found that per unit of pressure fluctuation includestwo pulses--one large and one small. The pressure value at e (or a) is smaller than c, which can be attributed to the fact that the forward stroke end velocity is faster than the backward stroke end velocity, resulting in a higher water hammer pressure.