## 3 Suppression Methods for Low-Frequency Pressure Fluctuation
### Other Passive Control Methods  
In addition to the aforementioned passive control methods, commonly used techniques include the use of Helmholtz resonators to suppress specific frequencies in the plenum or pipeline circuit [31; 55; 60], as well as modifications to the wind tunnel pipeline to suppress the circuit standing wave frequency [9; 11].  
A Helmholtz resonator is a cavity that can be connected to a wind tunnel through a small tube. When the air column in the tube vibrates under the excitation of an external sound wave, a damped air spring system is formed due to friction. When the frequency of the incident sound wave matches the natural frequency of the resonator, the air column will resonate and vibrate violently, leading to the maximum consumption of sound energy, equivalent to the absorption of the sound wave by the resonator.  
When FKFS upgraded the full-scale aeroacoustic wind tunnel at the University of Stuttgart, Germany, in 2014, they installed a Helmholtz resonator adjacent to the plenum [55]. By precisely tuning the resonator frequency to suppress the Helmholtz frequency of the plenum and implementing the FKFS best vortex generators at the nozzle, they were able to achieve pulsating pressure RMS levels below 0.5% of the dynamic pressure over the entire velocity range [55].  
[PERSON] et al. [31] integrated a Helmholtz resonator into a scaled wind tunnel and adjusted its resonant frequency to match the frequency of the observed significant low-frequency pressure fluctuation in the wind tunnel. In addition, they introduced an auxiliary pipeline to the side wall of the collector, which was open upstream, closed downstream, and connected to the collector through an opening in its side wall. By carefully sizing this pipeline, the pulsating pressure within the collector could be balanced by the pulsating pressure output from the pipeline at a specific frequency band. These measures effectively reduced the low-frequency pressure fluctuations but resulted in a slight increase in noise in other frequency bands.  
[PERSON] et al. [60] successfully suppressed the low-frequency pressure fluctuations by using Helmholtz resonators in two BMW AVZ wind tunnels; the Helmholtz resonators also improved the pressure gradient in the test section.  
The suppression measures of the low-frequency pressure fluctuations in a 3/4-scale open-jet model wind tunnel at Mitsubishi Heavy Industries were introduced by [PERSON] et al. [11]. They discovered that the propagation of the low-frequency pulsation in the wind tunnel was mainly in the circuit. By taking in airflow through entrainment inlets on both sides of the nozzle and expelling airflow through entrainment outlets located before and after the first corner, an effective reduction of the low-frequency pressure fluctuation was achieved.  
During the acoustic upgrade in 2009, the DNW-NWB wind tunnel in Germany implemented a sudden change in the cross-sectional area between the cooling and driving sections of its pipeline. This alteration created an acoustic impedance that effectively suppressed low-frequency pressure fluctuations by preventing edgetone feedback from exciting the resonance mode of the wind tunnel circuit [9].  
[PERSON] et al. proposed in their patent that the addition of apertures covered by elastic membranes to wind tunnel pipelines can shift the circuit resonance to a higher frequency (see [19]).