4.1. Flow Phenomena
The development of the air flow field induced by shock impingement on a perforated-plate array is a highly unsteady process.
Figure 5a–c shows the calculated instantaneous pressure gradient, pressure, and velocity vector distributions under the condition of the model B perforated-plate array and
Ms = 1.41, respectively.
It can be seen that at the t = 0 moment, the planar incident shock front (abbreviated as IS) arrived exactly at the frontal edge of the array and manifested as a peak pressure gradient or a strong pressure discontinuity. The distributions of pressure and velocity vector on both sides of the IS were still uniform. At the t = 4.2 μs moment, a series of bow-shaped reflected shock waves RS1 propagating upstream and transmitted shock waves TS1 propagating downstream in the holes formed after the IS impacted the first perforated plate. The post-shock pressure of the TS1 changed slightly relative to that of the IS, meaning that the change of the shock wave intensity was insignificant. In addition, nearly stationary oblique shock waves appeared at the upstream edges of the inlet of each hole in the first plate, surviving the whole of the studied time.
At the
t = 8.3 μs moment, the transmitted shock wave TS
1 completely passed through the hole channels and entered the area between the two plates. Due to the sudden expansion structure of the flow channel at the rear side of the first plate, the TS
1, which is subject to disturbances from the outlet edges of the holes, diffracted so that its shock front shifted from the original planar structure to a curved surface consisting of a series of “mushroom head” shaped substructures, which are shown as bow-shaped shock waves in
Figure 5a,b. A jet starting from each hole is preceded by each of the bow shock waves. Consequently, the pressure inside the holes decreases with the discharge of the jets. Oblique shock waves begin to appear at the downstream edges of the outlet of each plate. According to the study of Baird [
27], the generation of these oblique shocks is due to the acceleration of expansion waves inside the hole. Moreover, because of shock–shock and shock–wall interactions, the fluctuant shock front of the RS
1 continuously levels out to develop into a nearly planar structure. The portion close to a side wall propagates faster than that close to the symmetric plane. As a result, the RS
1 shock front gradually becomes an inclined plane. The main reason is that the shock reflection from a near-wall, corner region is stronger than that from the remaining non-passage, solid body regions at the upstream side of the perforated plate. This is well proven by the significant, locally high pressure in the post-shock region at both the
t = 4.2 μs and
t = 8.3 μs moments.
At the
t = 13.8 μs moment, the first significant decrease in the intensity happens to the TS
1, as demonstrated by the continuous decrease in the post-shock pressure. As the transmitted bow-shaped shock waves extend, the interference happens first to the neighboring ones. Consequently, locally high pressures appear in the interference areas IA (i.e., the post-shock), overlapping portions of the TS
1. Meanwhile, a “strengthening point” SP appeared at a position where two neighboring transmitted bow-shaped shock fronts intersect. The superimposed effects cause increases in the local shock wave intensity and propagation speed, meaning that the SP moves downstream faster than the remaining portions of the TS
1 front. This explains the similar phenomena that the fluctuant fronts of the TS
1 and RS
1 become smooth gradually. Each jet gradually develops to form a nearly toroidal, locally high-speed region. A vortex ring is generated due to the development of a shear layer between the jet and the external fluid. The neighboring portions with opposite vorticities of two ring vortices (i.e., vortex cores) merge into a “heart” shaped structure, as shown in
Figure 5c. This merging of vortex cores dissipates the energy of themselves and the surrounding oblique shock and compression waves.
Furthermore, due to the suppression effects to the flow from the first perforated plate, the pressure upstream of the first plate maintains a level higher than that of the initial post-shock pressure of the IS. This provides conditions for the further development of the jets.
At the
t = 21.9 μs moment, the transmitted shock wave TS
1 only reached the upstream side of the second perforated plate. The transmitted shock wave from the hole in the first perforated plate closest to the wall encounters the wall, thus leading to the formation of a reflected shock wave RS
2, which interferes with the TS
1. As a result, the local pressure near the wall increases. At this point, the pressure inside the holes of the first plate has basically decreased to the level equivalent to the initial post-shock pressure of the IS. Reflected and transmitted compression waves following the RS
1 and the TS
1 can be clearly seen upstream and downstream the first plate, respectively. This observation agrees with the experimental work of Kontis et al. [
15]. Citing the work of Torrens and Wrobel [
28], it is believed that the compression waves are generated as a result of the reflections of the transmitted shock (i.e., TS
1 here) in both directions inside the perforated medium. Furthermore, the reflected and transmitted shocks are strengthened due to the superposition of these compression waves. Furthermore, the high-speed region of each jet becomes simply connected and develops further, dragging the vortex cores travelling downstream. The streamwise size of each merged vortex core structure is stretched. Meanwhile, the diffusion of vorticity dissipates the energy of each ring vortex and the surrounding waves.
At the t = 26.4 μs moment, a transmitted shock wave TS2 propagating downstream through the holes in the second perforated plate and a bow-shaped reflected shock wave RS3 comes into being simultaneously. Similar, but weaker oblique shocks appeared at the upstream edges of the inlet of each hole in the second plate.
In the time range of 30.6 ≤
t ≤ 37.3 μs, each jet flow that discharges from the first perforated plate develops an oblique shock pattern. The RS
3 first collides with the ring vortices. As a result, multiple layers of shock waves are generated. This observation is also reported in the experimental study of Kontis et al. [
15]. Furthermore, the local intensity of shock wave at intersections of the downstream cross section of the vortex cores with the central lines of the holes is remarkably increased. This finding is consistent with that from the numerical simulations by Takayama et al. [
29]. They attributed the local increase in shock intensity to a double-step mechanism. The head-on collision of the reflected shock with the high-speed flow inside the vortex causes a first slight intensification. Then, a second intensification is due to the convergence of the outside diffracted wave around the vortex on the intensified portion near each central line. Thereafter, the RS
3 with the multiple layer structure continues to propagate upstream and interacts with the oblique shock pattern within each jet flow, producing “α”-shaped shock waves. The oblique shocks represented by the two legs of “α” are strengthened during this interaction. The evolution of the transmitted shock wave TS
2 is basically the same as that of the TS
1. Successive interference of the TS
2 from different holes changes the post-shock parameters of the flow field. It is inferred from the pressure gradient and pressure contours (
Figure 5a,b) that the TS
2 significantly attenuated relative to the IS. In other words, the second remarkable decrease happened to the intensity of the leading shock wave.
At the t = 50.4 μs moment, one part of the reflected shock wave RS3 collides with the oblique shocks at the hole outlets of the first perforated plate. Consequently, the local shock intensity of this part is increased and propagates upstream through the holes. The remaining part is reflected by the plate. As a result, the shock intensity of the reflected part decreases due to the viscous dissipation and the energy loss during the collision process. The angle between the transmitted shock TS2 and the wall reaches the critical value for transition from regular reflection to Mach reflection. As a result, a Mach reflection starts to form. Each jet originating from the first perforated plate is stretched along the travelling direction. The inside flow velocity decreases due to dissipation effects as it moves downstream further. The merged vortex core structure splits into two individual ones. This irreversible process naturally brings some amount of energy dissipation. It is found that the size of each vortex core becomes greater than that at previous moments because of the diffusion of vorticity. This is another dissipation mechanism of the ring vortices. It should be noted that the interaction of the neighboring vortex cores should also contribute significantly to the dissipation of the ring vortices. The separated vortex cores lie side by side in the transverse direction, except for the farthest two from the symmetric centerline. We believe that the asymmetry due to the sidewall leads to the change in their arrangement pattern. A weaker behavior of ring vortices and jets induced by the TS2 similar to that described for the moment of t = 13.8 μs occurs downstream of the second plate.
In addition, it is found that the upstream pressure of the first perforated plate maintains a high level for an extended period of time. Although the upstream pressure is also locally increased near the second plate due to the reflection of the TS2, both the pressure level and the duration were significantly below that corresponding to the first plate, indicating that the first perforated plate plays a major role in the inhibition of the flow.
Figure 6a–c presents the later instantaneous pressure gradient, pressure, and velocity vector distributions at the downstream of the perforated-plate array, respectively. It can be found that in the time range of 68.1 ≤
t ≤ 101.4 μs, Mach reflections of the transmitted shock waves from the different holes in the second plate occur at the side wall, characterized by a Mach stem perpendicular to the wall, a curved reflected shock wave RS
4, and the remaining TS
2. Surprisingly, the TS
2 is followed by secondary shock waves SSW. We infer that these SSW are formed due to two mechanisms. One is the superimposed effects of multiple Mach reflections of the transmitted shock waves from the holes in the second plate except for the one closest to the side wall. The other is the superposition of the transmitted compression waves caused by the reflections of the transmitted shock (i.e., TS
2 here) inside the holes of the second plate. The latter was proposed in single shock tube studies of Torrens and Wrobel [
28] and Kontis et al. [
15]. The Mach stem extends toward the symmetric plane, and the SSW chases the leading TS
2. Consequently, the local intensity of the TS
2 near the side wall is increased, thus travels faster than the remaining portion, flattening the TS
2 front. The post-shock pressures of both the RS
4 and SSW are increased. Because the SSW moves faster than the leading TS
2, the former catches up and superimposes with the latter at a moment of around 164.1 μs, forming a new, stronger leading shock wave. Additionally, jets discharged from the holes in the second plate continue to develop. The vortex cores induced by the TS
2 gradually become weak because of the dissipation mechanisms described above.
The long-time interference of the TS
2 from different holes makes the distribution of the post-shock pressure tend to be uniform. On the other hand, before being affected by the reflected shock wave from the end wall, a nearly steady jet flow with a typical oblique shock pattern develops. Baird [
27] found a similar flow pattern and attributed its generation to the acceleration of expansion waves inside the shock tube (i.e., each hole in the second plate). As a result, almost completely quiescent pressure subdomains in a regular arrangement, in which each line alternates between low and high pressures repeatedly following a hole in the second plate, come into being inside each of the jets. It can be noticed that the leading transmitted shock wave propagated downstream, far away from the second plate at the late stages. Therefore, these jets should not be induced by the leading shock directly and different from those generated at earlier stages (e.g.,
t = 50.4 μs). We believe that an oscillation chamber for the inside dynamic wave system is produced between the two perforated plates, making the internal pressure maintain a temporarily stable level much higher than that downstream the second plate at these stages. It is a high enough pressure drop between two sides of the plate that maintains the existence of such jets.
In summary, in the case of shock impact on an array of perforated plates, the reflection, transmission, diffraction, interference behaviors, and the superimposed effects of compression waves and secondary shock waves following the leading transmitted shock wave are the main reasons for the process of attenuation, local enhancement, attenuation, enhancement and attenuation experienced by the leading shock wave. Additionally, many flow phenomena occur in this process such as Mach reflections of the transmitted shock wave, generation of the secondary shock waves, superposition of reflected compression waves and the reflected shock wave, production of jet flows and ring vortices, reflected shock interactions with the ring vortices and oblique shock patterns in the jets, interaction of neighboring vortex cores, deformation and travelling of the ring vortices, occurring of nearly steady jets, and so on. For further understanding regarding the details, the reader is directed to the excellent studies of Kontis et al. [
15], Baird [
27], Torrens and Wrobel [
28], and Takayama et al. [
29], among others. In this study, we only focused on the quantitative analysis for the attenuation of the leading shock wave.
4.2. Effect of Shock Mach Number
Figure 7a–d presents the transient pressures at different shock Mach numbers (
Ms = 1.21, 1.41, and 1.61) for an array of type B perforated plates at monitoring points P
1, P
2, P
3, and P
4, respectively. It can be seen in
Figure 7a that a pressure spine following a short platform appears on each curve of pressure in the time range of 37.8 ≤
t ≤ 55.1 μs. It can be concluded from the peak values of the first spines that the intensity of the reflected shock wave RS
1 increases with the increase in the incident shock Mach number. In the case of
Ms = 1.61, pressure spines with decreasing amplitudes appear periodically. It can be inferred that the first spine is due to the interference of neighboring reflected shock waves, while the following ones are caused by the reflected compression waves. Furthermore, the stronger the incident shock intensity, the more obvious the spine structures and the more active the interference behavior.
It is observed from
Figure 7b that first pressure spines appear on the pressure curves corresponding to the monitoring point P
2 in the time range of 17.6 ≤
t ≤ 24.9 μs. By comparing the peak values of the spines, it can be concluded that the transmitted shock intensity increases with the increase in the incident shock Mach number. Within the following around 20 μs, the transient pressure experiences a process consisting of first a rapid decline, small rise, second decline, and drastic rise. Combining
Figure 5a–c, it can be inferred that the first decline and the subsequent rise are caused by the diffraction of the TS
1 and its interference, respectively. The second decline is due to the arrival of the high-speed and low-pressure region inside the jet at the position of P
2, while the drastic rise happens when the RS
2 arrives at the same position. In the time range of 29.4 ≤
t ≤ 35.8 μs, the extent of pressure drop for
Ms = 1.41 is more remarkable relative to that for the remaining cases. This is because the collision of the ring vortex and the RS
2 happens at the downstream region of P
2, thus the transient pressure reflects the minimum level in the jet in the
Ms = 1.41 case. In the
Ms = 1.21 case, the jet develops relatively slowly, whereas in the
Ms = 1.61 case, both the TS
1 and the RS
2 move faster. The similar consequences in the latter two cases conclude that the collision happens at a position close to the vertical split of the two plates, leading to the formation of locally high-pressure regions and therefore the appearance of more distinct second pressure spines in the time range of 34.6 ≤
t ≤ 43.2 μs. Thereafter, as the RS
2 dissipates and degrades gradually, the oscillation of pressure at the P
2 position is mitigated. Furthermore, the decrease in the incident shock Mach number is beneficial for the flow field to reach a uniform and steady state more rapidly.
It can be seen from
Figure 7c that each pressure curve corresponding to monitoring point P
3 has an obvious spine in the time range of 51.6 ≤
t ≤ 69.2 μs, and the peak value decreases significantly compared with the transient post-shock pressure of the IS (see the headmost horizontal straight line in
Figure 7a). Moreover, the pressure declines more seriously as the incident shock Mach number increases, meaning that the attenuation of the leading shock wave is more remarkable, and the suppression effects on the shock wave are stronger. Furthermore, the pressure spines occur repeatedly in the studied time range of
t ≤ 300 μs. We believe that the first spine is due to the interference of neighboring transmitted shock waves, while the remaining are caused by the passage of transmitted compression waves and secondary shock waves following TS
2. It is also observed from
Figure 7d that the pressure amplitude decreases gradually, thus each pressure curve finally maintains at quite a stable level. The decrease in the incident shock Mach number leads to the decrease in the amplitudes of pressure oscillations when approaching a steady state. In combination with the analysis for the generation of the nearly steady jets downstream the array of perforated plates, it can be concluded that these small-amplitude pressure oscillations are mainly caused by the oscillation waves travelling downstream with the fluid medium above-mentioned.
Figure 8 shows a comparison of attenuation rates of the leading shock wave at the different incident shock Mach numbers. The pressure-based attenuation rate
βp is defined as
where
pL is the post-shock pressure of leading shock wave at monitoring point P
4. In the
Ms = 1.21 case, the post-shock pressure of leading shock wave
pL/
p1 is 1.37, thus the shock attenuation rate
βp = 11.0%. In the
Ms = 1.41 and 1.61 cases, the
pL/
p1 are equal to 1.60 and 1.81, respectively, and therefore the pressure-based attenuation rates are calculated as 25.6% and 36.8%, respectively. This indicates that the increase in the incident shock Mach number improves the attenuation of the leading shock wave during its propagation through an array of perforated plates.
4.3. Effect of Porosity
Figure 9a–d presents the transient pressures at
Ms = 1.41 for different porosities (
α = 13.4%, 23.4%, and 33.4%) at the monitoring points P
1, P
2, P
3, and P
4, respectively. It can be observed from
Figure 9a that the first pressure spines with small amplitudes appeared in the time range of 47.5 ≤
t ≤ 50.4 μs on the curves corresponding to the monitoring point P
1. Each of these spines is caused by the arrival of a reflected shock wave RS
1. By comparing the peak values of the spines, it can be found that the intensity of the reflected shock wave increases with decreasing porosity. This can be readily explained as the decrease in the porosity of the perforated plates restrains the flow capacity. The first spine on each curve is followed by a series of spines with gradually decreasing amplitudes. Particularly for the minimum porosity case, a nearly periodic appearance of the pressure spines is evident. A notable phenomenon is that the decrease in the porosity appears to decrease the amplitude of the first pressure spine. We infer that this is related to the shapes of the individually reflected shock waves when they interfere with the neighboring ones.
It can be seen from
Figure 9b that the evident first pressure spines appear in the time range of 18.9 ≤
t ≤ 20.8 μs on the curves corresponding to the monitoring point P
2. Each of these spines is due to the arrival of a transmitted shock wave TS
1. According to the peak values of these spines, it can be inferred that the transmitted shock intensity increases with the increase in porosity. As delineated above, the first spine is followed by the stages including first decline, small rise, second decline, and sharp rise within the subsequent, around 20 μs. It was found that the amplitude of the first pressure decline for the minimum porosity or of the first pressure rise for the maximum porosity is more remarkable than that in the other cases. The pressure spine with the maximum peak value can be seen on each pressure curve in
Figure 9b within a time range of 39.4 ≤
t ≤ 39.7 μs. Furthermore, the peak value increases with increasing porosity. All of these can be explained by the effect of porosity on the flow capacity summarized above. Furthermore, it can be found that the pressure recovery in the region between the first two plates decreases as the porosity increases. One can see that the transient pressure for the minimum porosity case tends to overtake that for the other cases. The reflected shock wave RS
2 and its subsequent reflected waves propagate back and forth inside an oscillation chamber between the two plates. The decrease in the porosity is beneficial for the reflection, while adverse to the transmission of these waves. As a result, the superimposed pressurization effect due to the repeated shock reflections is increased as the porosity decreases.
It can be seen from
Figure 9c,d that the evident first pressure spines appear in the time ranges of 52.3 ≤
t ≤ 59.3 μs and of 151.6 ≤
t ≤ 170.2 μs on the curves corresponding to the monitoring point P
3 and P
4, respectively. By comparing the peak values of these spines, we can conclude again that the transmitted shock intensity increases as the porosity increases.
A comparison of pressure-based attenuation rates of the leading shock wave at different porosities is shown in
Figure 10. It can be found that when
α = 13.4%, 23.4%, and 33.4%, the pressure-based attenuation rates are 34.5%, 25.6%, and 22.1%, respectively. Thus, it can be summarized that the decrease in the porosity strengthens the inhibition effect of the perforated plates on the leading shock wave, and therefore decreases its intensity more significantly.