## 4 Concluding Remarks  
The open-jet wind tunnel is a widely used form of wind tunnel configuration, and the occurrence of low-frequency pressure fluctuations is relatively common in this type of wind tunnel. Low-frequency pressure fluctuations have been extensively documented worldwide in various sizes of open-jet wind tunnels. * Mechanism  
The jet impinges on the collector, creating an edgetone feedback loop that serves as the primary source of low-frequency pressure fluctuations in open-jet wind tunnels. The occurrence of significant low-frequency pressure fluctuations is observed when the frequency of certain modes in the wind tunnel is close to the frequency of the edgetone feedback loop. The wind tunnel circuit standing wave and the plenum Helmholtz mode are widely reported as the two most common sources of excitation. More recently, planar standing wave modes in the plenum have been identified as a source of excitation. The authors contend that past studies may have underestimated the possibility of low-frequency pressure fluctuations due to planar standing waves in a plenum. In addition to resonance with edgetone feedback, if the frequencies of two passive modes are close enough to meet the resonance conditions, these two modes might also produce significant low-frequency pressure fluctuations through self-excitation. * Control methods  
Passive control methods are efficient and cost-effective, making them the most widely used low-frequency pressure fluctuation control techniques. In contrast, active control methods, while effective, require complex control schemes to achieve the desired results and are rarely used in large wind tunnels. A variety of low-frequency pressure fluctuation control techniques are commonly employed at two key locations where edgetone feedback occurs: the nozzle and the collector. The goal of these measures is to either attenuate the edgetone feedback to some degree or change its frequency to avoid resonance with the inherent modes of the wind tunnel. Incorporating vortex generators into the jet nozzle is a cost-effective control approach, although it does introduce broadband noise. Compared to two-dimensional sheet spoilers, three-dimensional vortex generators can partially mitigate the introduced broadband noise. The implementation of a breathing gap serves as a widely used optimization measure for collectors, and this method needs to be introduced at the design stage. In addition, the optimization of the collector shape is effective in some cases, but the selection of the optimized shape requires trial and error and is expensive. When a standing wave is excited in the transverse or vertical direction of the plenum, increasing the size of the collector inlet to fully enclose the jet appears to be an effective means of controlling significant low-frequency pressure fluctuations. However, widening the collector may not always be a practical option for large open-jet wind tunnels in operation. In such cases, the use of CVG can reduce the jet expansion angle to some extent and achieve similar results to widening the collector. The ability of CVG to control jet development by adjusting their expansion angle may also prove useful in jet situations other than open-jet wind tunnels. By adjusting their size, Helmholtz resonators are capable of suppressing pressure oscillations at specific frequencies in the plenum and pipe loop. Once the wind tunnel is in operation, the cost of applying this measure is relatively high. By reserving adjustable Helmholtz resonators at certain key locations in the wind tunnel pipe loop, such as near the collector and diffusers, these resonators can be adjusted after the wind tunnel is in operation to suppress pressure pulsations at specific frequencies. * Unsolved problems  
As the primary excitation source for low-frequency pressure fluctuations in open-jet wind tunnels, the general framework of edgetone feedback is well established.