# Performance Study of a Fluidic Hammer Controlled by an Output-Fed Bistable Fluidic Oscillator
## 4 Results and Discussion
### The Effect of Parameters of the Impact Body on the Performance of Fluidic Hammers  
Apart from the single-impact energy mentioned above, another important performance parameter of the fluidic hammer is the output power which can be written as:  
\[P=Ef \tag{11}\]  
In this equation, the output power is proportional to both of the single-impact energy \(E\) and the impact frequency \(f\). Case 1-17 form four groups of CFD simulation tests to examine the effect of different parameters of the impact body on the performance of the fluidic hammer. In these simulations, the supply flow rate of the fluidic hammer remains the same at 200 L/min. As shown in Figure 7, the results are obtained from the computations of Case 1-5. It indicates that as the diameter of the piston becomes larger, both of the single-impact energy and the output power exhibit downward trends. On the other hand, the variation of impact frequency of the impact body is not monotone, changing with the diameter of the piston, and reaches its maximum at \(D_{\rm P}=26\) mm. This is attributed to the fact that when the diameter of the piston increases, the liquid action areas of the piston increases, which means lower fluid resistance for the output flow entering the working chamber, thus leading to a higher flow recovery and a correspondingly lower pressure recovery. As shown in Equation  
Cases 1, 6, 7, 8, and 9 form another group of computations. It can be seen in Figure 8 that there is no corresponding value of \(E,f,\) and \(P\) for Case 9. This is because the flow velocity in the supply nozzle does not reach the threshold flow velocity [21,22] of the fluidic hammer, thus leading to the result that the supply jet was unable to switch to the other side of the attachment wall. An increase in the diameter of the piston rod means the decrease in the action area of the front chamber of the cylinder, due to the previous conclusion obtained in the earlier group, the impact frequency of the impact body decreases. This is confirmed by the variations of impact frequency shown in Figure 8. The single-impact energy of the impact body firstly increases and then decreases. This increase is mainly due to the decrease of the action force in the front chamber while the decrease can be attributed to the lower rebound velocity and the more delayed switching attachment caused by the decrease of the backward impact velocity when the backward-moving impact body rebounds and starts to move forward. Compared with the impact frequency, the variation amplitude of single-impact energy is relatively weak, and hence as depicted in Figure 8, the output power of the fluidic hammer is in a steady decrease which is consistent with the changing trend of the impact frequency. In this group of simulations, it can be concluded that the cases of \(D_{\rm r}\) = 16 mm or 18 mm show better performance, as both of their single-impact energy and output power is relatively high. Figure 9 shows the computation results of the group including Cases 1, 10, 11, 12, and 13. As the mass of the impact body increases, the single-impact energy of the impact body increases while the impact frequency decreases. The output power of the fluidic hammer does not vary monotonically, and there exists a maximum value at \(m\) = 8 kg. In this group of computation cases, the cases of \(m\) = 8  
Figure 8: Variations of \(E,f\) and \(P\) versus diameter of the piston rod \(D_{r}\). Figure 7: Variations of single-impact energy \(E,\) impact frequency \(f,\) and output power \(P\) versus diameter of the piston \(D_{\rm p}\). kg or 10 kg shows better performance of the fluidic hammer, as both of their single-impact energy and output power is relatively high. The rest of the cases form the last group of test cases. As shown in Figure 10, an increase of the stroke increases the single-impact energy and decreases the impact frequency, while the output power first increases and then decreases.