## 3 Suppression Methods for Low-Frequency Pressure Fluctuation
### Collector Optimization
#### 3.2.1 Breathing Gap  
It is a common practice in open-jet wind tunnels to provide a breathing gap (slot) in the collector or diffuser to suppress low-frequency pressure fluctuations [6; 7; 14; 15; 21; 59].  
Experiments were conducted by NASA Langley Research Center in a 1:24-scale wind tunnel on six collectors of different configurations to reduce periodic airflow pulsations [21]. It was found that the configuration with a straight wall sloping outward and a slot between the collector and the diffuser was the most effective in reducing velocity pulsations. On the other hand, collectors with a bell mouth at the front had inferior performance. The optimized configuration has been successfully implemented in the full-scale subsonic wind tunnel with dimensions of 4 m \(\times\) 7 m.  
The 8 m \(\times\) 6 m opening test section of the German-Dutch DNW-LLF wind tunnel incorporated gaps at various positions along the collector [4]. However, it failed to effectively mitigate vibrations in the walls and ceiling of the plenum under high wind speeds, only demonstrating a reduction in vibrations at lower speeds.  
[PERSON] [6] investigated the cause of pressure fluctuations in 1/7-scale open-jet wind tunnels at the aerodynamics laboratory of the Institute for Aerospace Research of the National Research Council of Canada (NRC/IAR). The results showed that the installation of a ventilation gap downstream of the collector in a 1/7-scale wind tunnel reduced the RMS levels of the pulsating pressure coefficient in the jet to less than 0.01 for all wind speed ranges, with most speeds reduced to less than 0.005 [6]. According to [PERSON] [6], the ventilation gap can reduce the intensity of the pressure perturbations caused by the vortices colliding with the collector, thereby suppressing the edgetone feedback.  
After the implementation of a ventilation gap in the HAWT wind tunnel in South Korea, the RMS levels of the pulsating pressure coefficient at all wind speeds decreased significantly, reducing from approximately 6% of the dynamic pressure in the plenum to less than 0.5% [7].  
Gaps of varying widths were introduced in the collector throat of a scaled automotive wind tunnel by [PERSON] et al. [14; 15]. According to the experimental results, the size of the gap played a critical role, and the low-frequency pressure fluctuations could not be suppressed sufficiently if the gap was either too small or too large. By incorporating  
Figure 12: The velocity contours at the inlet cross-section of the collector, both with and without CVG, are shown using the velocity divided by the speed of sound at the center of the nozzle. Originally shown in [49].  
structures into the throat gap to prevent airflow without impeding acoustic waves, it was observed that the gap no longer effectively suppresses low-frequency pressure fluctuations. Therefore, [PERSON] et al. [14] concluded that the mechanism behind the collector gap is to change the flow state within the jet.  
[PERSON] et al. [59] employed the unsteady Reynolds-averaged Navier-Stokes (RANS) method to simulate low-frequency oscillations in the \(\phi\)3.2 m open-jet wind tunnel at the China Aerodynamics Research and Development Center. The study considered two scenarios: with ventilation holes on the collector wall either open or closed, as depicted in Figure 13. The results indicated that opening the ventilation holes led to a reduction of more than one order in the amplitude of pressure coefficient oscillations compared to closing them.