## 3 Suppression Methods for Low-Frequency Pressure Fluctuation
### Vortex Generators  
The incorporation of various vortex generators at the nozzle of an open-jet wind tunnel is a widely used passive control method for mitigating low-frequency pressure fluctuations [3; 16; 23; 49; 51; 52; 53; 61].  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{2}{c}{**Suppression Methods**} & **Configuration** & **Effectiveness** & \begin{tabular}{c} **Side** \\ **Effect** \\ \end{tabular} & **Cost** & \begin{tabular}{c} **Application** \\ **Range** \\ \end{tabular} \\ \hline \multirow{4}{*}{\begin{tabular}{c} Nozzle Vortex \\ Generator (VG) \\ \end{tabular} } & \multirow{2}{*}{2D} & Vanes, tabs, teeth, spoiler, Seiferth & \(++\) & \(-\) & \(++\) & \(++\) \\ \cline{2-6}  & & & & & & & \\ \cline{2-6}  & \multirow{2}{*}{3D} & Tetrahedrons, FKFS best [4; 54; 55] & \(++\) & 0 & \(++\) & \(++\) \\ \cline{2-6}  & & & & & & & \\ \cline{2-6}  & \multirow{2}{*}{CVG} & 3D VG on a square nozzle corners [23; 49] & \(++\) & 0 & \(++\) & \(+\) \\ \hline \multirow{4}{*}{\begin{tabular}{c} Collector \\ optimization \\ \end{tabular} } & Breathing gap & \(\begin{array}{c}\text{gaps/slots/holes in collector}\\ 15; 21; 58; 59] \\ \end{tabular} & \(++\) & \(+\) & \(+\) & \(++\) \\ \cline{2-6}  & \multirow{2}{*}{\begin{tabular}{c} Shape \\ optimization \\ \end{tabular} } & \begin{tabular}{c} Inclined inlet, increasing inlet size, reshaping inlet [4; 6; 17; 21; 23; 50; 57] \\ \end{tabular} & \(++\) & \(++\) & \(-\) & 0 \\ \hline \multirow{4}{*}{Helmholtz resonator} & \multirow{2}{*}{\begin{tabular}{c} Chambers connected to wind tunnel \\ through a tube [31; 55; 60] \\ \end{tabular} } & \multirow{2}{*}{\(++\)} & \(++\) & 0 & \(+\) \\ \hline \multirow{4}{*}{\begin{tabular}{c} Modifying the \\ circuit loop \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} Entrainment \\ system \\ \end{tabular} } & Entrainment & Entrainment intels beside nozzle and entrainment outlets around the first corner [11] & \(+\) & \(+\) & \(-\) & 0 \\ \cline{2-6}  & \multirow{2}{*}{\begin{tabular}{c} Cross-section \\ area changing \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} Abrupt changing of cross-section area \\ at certain locations of circuit loop [9] \\ \end{tabular} } & \(++\) & \(+\) & \(-\) & 0 \\ \cline{2-6}  & \multirow{2}{*}{\begin{tabular}{c} Membrane \\ covert \\ apertures \\ \end{tabular} } & Adding membrane-covered apertures at certain locations of the circuit loop [19] & \(+\) & \(++\) & 0 & \(+\) \\ \hline \multirow{4}{*}{\
\begin{tabular}{c} Active control \\ methods \\ \end{tabular} } & \multirow{2}{*}{ARC} & A plenum-mounted microphone and a wall-mounted loudspeaker in circuit loop [26; 32] & \(++\) & \(++\) & 0 & 0 \\ \cline{2-6}  & \multirow{2}{*}{Movable flaps} & Electrically driven movable flaps at nozzle [19] & \(++\) & \(-\) & 0 & 0 \\ \cline{2-6}  & Synthetic jet & Synthetic jet exciters at nozzle [27] & \(++\) & \(-\) & 0 & 0 \\ \cline{2-6}  & \multirow{2}{*}{
\begin{tabular}{c} Flow-follow \\ device \\ \end{tabular} } & Airflow injection around nozzle [50] & \(+\) & \(+\) & 0 & 0 \\ \hline \hline \end{tabular}  
* The symbols \(-\), 0, and \(+\) refer to bad, reasonable, and good, respectively. \(++\) is better than \(+\).  
\end{table}
Table 3: Summary of suppression methods \({}^{\ddagger}\).  
The jet nozzle is located at the initial stage of the jet shear layer development, where even small perturbations introduced into the jet can have a significant effect on its downstream evolution. Therefore, this location is considered optimal for controlling the jet. Conventional vortex generators installed in the jet nozzle modify both the frequency and intensity of the edgetone feedback by introducing small-scale vortices into the jet shear layer, thereby enhancing mixing within the jet shear layer and altering the pattern of the downstream large-scale vortices.  
Nozzle-vortex-generators are a cost-effective and straightforward solution. However, the small-scale vortices produced by them contribute to high-frequency noise levels [2; 53; 62; 23].