## 5 Results
### Relation Reynolds Frequency for All Dimensional Modifications Performed
In order to evaluate which is the temporal net momentum applied to the incoming jet, it will be required to know the instantaneous mass flow through both (FC) outlets as well as the temporal pressure at these two sections. In [18] it was concluded the oscillators studied appeared to be pressure driven, the same conclusion was reached in [12], where it was demonstrated that the forces triggering the oscillation were mostly due to the pressure difference at the feedback channels outlets. In the present section the different forces acting on the jet lateral surfaces will be analyzed for each of the different geometry modifications evaluated and as a function of the Reynolds number. The average net momentum applied to the jet entering the (MC), is for all Reynolds numbers and (FO) modifications, having a value close to zero, as observed for a Reynolds number of 16034  
Figure 16: Relation Reynolds number versus mass flow output frequency, for all dimensional modifications studied. in Figures 5d, 9d, 12d and 15d. Regarding the evolution of the average net momentum applied to the jet at the (MC) inlet, it was observed that whenever the pressure at the (MC) is particularly high and the Reynolds number increases, the average net momentum applied to the jet decreases. This happens for the following cases, lowest outlet width, lowest outlet angle and for the baseline case. The equations presented in Table 4 characterize this evolution. On the other hand, whenever the pressure at the (MC) is particularly low, which happens for the highest outlet width, the lowest inlet width, the highest inlet width and the highest outlet angle, the average net momentum increases with the Reynolds number increase, Table 4 states this evolution. Nevertheless, and as a general trend it can be stated that, regardless of the (FO) modification performed, as the Reynolds number increases the average pressure in the (MC) also increases. The equations characterizing the evolution of the average non-dimensional stagnation pressure at the (MC) converging walls, and the average non-dimensional net momentum acting on the jet entering the (MC), are respectively presented as a function of the Reynolds number in Tables 3 and 4. These equations are valid for a range of Reynolds numbers between \(8711<\text{Re}<32068\), and were obtained from the data presented in Figures 5, 9, 12 and 15, as well as from similar figures obtained at Reynolds numbers 8711 and 32068. Some geometry modifications, regardless of the Reynolds number, generate a decrease of the (MC) average pressure when compared to the one existing in the baseline case, these are, the highest and lowest inlet widths, the highest outlet angle and the highest outlet width. At Reynolds number 32068, the (MC) average pressure decrease versus the baseline one was respectively of 23.5  
peak amplitude, is smaller than the one associated to the net momentum amplitude for the same case. This is particularly relevant for the following cases, highest inlet width, highest inlet angle, lowest outlet angle and lowest outlet width. In any case, and regardless of the case studied, the equations from Tables 5 and 6, show a direct link between the peak to peak stagnation pressure amplitude and the net momentum amplitude, giving therefore strength to the thesis established in references [12; 18] and in the present paper, regarding the origin of the forces driving the oscillation. To understand, for each of the cases studied, the origin of the net momentum driving the oscillation, the relation between the peak to peak net momentum term due to the static pressure acting on the (FC) outlets, was compared with the peak to peak net momentum term due to the feedback channels mass flow.