The discussion of the formation and performance of the coherent structures in the dependency of jet operation conditions, e.g. jet exit velocity, jet nozzle size, jet-strip distance, is the scope of this paper. Four case studies with varied parameter setups are presented in Table 1. In an attempt to identify the appearance of vortices in the room between the nozzle and the strip, a series of lines are created with a constant step of 1.
1 mm, starting from the jet nozzle (see Figure 1). These lines situate on the middle plane in the span-wise direction. Consequently, the static pressure on each line is recorded over the period of simulation. The observed data shows that on the particular line near the nozzle, i.
e. it’s distance to the nozzle less than 4dp, the pressure level oscillates remarkably with relative high frequencies at two points, called active points found to be around half of jet lip gap, dp above and below the jet centreline. These positions gradually move away from the jet centreline as the line go further away from the jet nozzle. It is appropriate with the locations of shear layers where the vortices form, develop, and convect downstream. In the far field region, x > 4dp, or in the impinging region near the strip, there are more than two active points on each line.
Accompanying with the two main points, some minor points appear when small vortices are resulted from the interaction among large coherent structures.The maximum level of pressure fluctuation on a line indicates the location the main active points where the center of large vortices passes across the line. Negative peak pressure is achieved when a vortex center is exactly on the points, and the pressure increases to achieve positive peak when the points are moving out of the vortex. Figure 2 illustrates the time series plot of the static pressure at one of the abovementioned points on line 1 for the four case studies. One can say that these signals represent the occurrence of vortex structures at that point. To be more specific, a complete cycle of signal between two positive peaks reflects the fact that one vortex has passed the point.
Once that vortex comes to another point on the next line, it will generate pretty much the same pressure signal if its shape does not change much. Therefore, the signal at a point is the mapping of the signal at the previous point with some certain amount of time shift. By doing cross correlation between two signals (Eq.1), the time shift is revealed as shown in Figure 3.