Low-frequency Pressure Fluctuations Research Articles

Study of the effects of modified draft tube with inclined conical diffuser on draft tube and upstream region

Various countermeasures including geometry optimization have been proposed and proved to have a huge impact on flow filed within the draft tube in order to mitigate the vortex rope and the induced pressure fluctuations for Francis turbine under part load operation. However, the effect of these approaches on the hydro-dynamics in upstream region still remains unclear, which is of great significance for the overall performance of the unit. This study aims to explore the influences of modified draft tube with inclined conical diffuser on the pressure fluctuations in whole flow passage. The results reveal that Generation-1 and Generation-2 draft tubes are effective in alleviating pressure fluctuations resided in the draft tube, but the former one would trigger a low-frequency pressure fluctuation with higher amplitude in the runner zone. In addition, the modified draft tube has a very limited effect on the high-frequency pressure fluctuations in the guide vane and vaneless areas. To eliminate the adverse effect of inclined conical diffuser, a design with a transitional section is put forward to smoothly connect the runner zone and inclined conical diffuser. This further developed Generation-2 draft tube presents a reasonably good performance in terms of improving flow stabilities, i.e. alleviating the pressure fluctuations not only in draft tube but also in upstream region such as runner zone. This study provides reference for obtaining a better mitigating effect on pressure fluctuations in the whole turbine flow passage.

Study on the Unsteady Flow Characteristics of a Pump Turbine in Pump Mode

Extensive research has been conducted on the performance of pump turbines, particularly focused on understanding the generation mechanism of S-shaped characteristics. However, there has been a lack of research on unsteady flow characteristics in hump characteristics with small guide vane openings. This study focuses on the hump characteristics of a pump turbine in pump mode. The unsteady numerical simulation method is used along with experimental testing to examine the internal flow characteristics and induced pressure fluctuations under pump operating conditions. The results indicate that flow separation occurs in the impeller when the flow rate decreases to the valley operating condition, and recirculation flow occurs near the impeller inlet at the partial flow rate. Moreover, the unstable flow on the positive slope exhibits a low-frequency characteristic of 0.15fn. The pressure fluctuation from the hub to shroud areas of the guide vane region diminishes sequentially. Notably, distinct vortex structures emerge at the draft tube cone section under the valley operating condition. These structures extend toward the elbow section of the draft tube as the flow rate decreases. This phenomenon generates low-frequency pressure fluctuation originating from the primary frequency of the vortex and dean vortex on the surface, located at 0.4 D of the draft tube under conditions of low flow rate.

Low-frequency fluctuation propagation of rotating stall in the centrifugal compressor and pipe system

The characteristic signal judgment of rotating stall in a centrifugal compressor is necessary to avoid the compressor becoming instability in operation. This study investigates rotating stall in a centrifugal compressor and pipe system using experimental and numerical simulation methods. In the experiment, a low-frequency (approximately 10% of the impeller's rotational frequency) pressure fluctuation is observed in the inlet and outlet pipes as the pressure ratio curve declined at small flow rate. The frequency spectrum results of different measuring locations suggest that the low-frequency disturbance is in the flow direction within the pipe system during rotating stall. To further analyze this pressure fluctuation, a transient numerical simulation of the centrifugal compressor with plenum model is conducted. Rotating stall can be captured by the numerical model, and the low-frequency pressure fluctuation is also observed in the transient simulation, aligning with the experimental results. The periodic evolution of the tip leakage vortex influences the flow in the impeller passage, causing a fluctuation in the flow direction that propagates upstream and downstream as revealed by flow field analysis. The low-frequency pressure fluctuation in the inlet and outlet pipe system is a characteristic signal, which can be a new stall judgment of rotating stall in the operation of centrifugal compressor.

Suppressing Methods of the Pressure Fluctuation in Open Jet Wind Tunnels

Due to the distinctive structure of the test section, the open jet wind tunnel generates low-frequency pressure fluctuations (LFFs) within the range of typical wind speeds. These fluctuations significantly compromise the quality of the flow field in the test section. The evolution of the flow structure and vortex is analysed through the improved delayed detached eddy simulations (IDDES). The LFFs and the control mechanism in the open jet wind tunnel of Jilin University are then studied. The interaction between the large-scale vortex shedding at the nozzle exit and the collector forms the edge feedback, which is the main reason for the pressure fluctuation. According to the feedback mechanism, the LFFs are suppressed using the throat gap and by improving the collector shapes. The results show that the increase of the throat gap length at the collector can significantly alleviate the pressure accumulation inside the collector. The change of the collector shapes can control the impact area and time of the incoming flow, or produce permanent vortex structure to affect the impact shape of the vortex and the flow field at the collector, which allows to control the LFFs. This study lays a solid foundation for further comprehension of the aerodynamic characteristics of the open jet wind tunnels.

Review of Mechanisms and Suppression Methods for Low-Frequency Pressure Fluctuations in Open-Jet Wind Tunnels

Low-frequency pressure fluctuations are common in open-jet wind tunnels, affecting test accuracy and posing safety risks to the wind tunnels. These oscillations can be caused by different mechanisms in different wind tunnels, and it is often necessary to identify the specific mechanism responsible for the oscillation and develop appropriate control methods. This paper presents a comprehensive review of the current state of research on low-frequency pressure fluctuations in subsonic open-jet wind tunnels, with a particular emphasis on their generation mechanisms and control strategies. The primary source of excitation is attributed to the edgetone feedback formed by the impingement of the jet on the collector. The sound wavelength corresponding to the edgetone frequency is close to that of the plenum scale, facilitating resonance with both plenum-associated vibration modes and specific-order standing wave modes within the circuit loop, resulting in significant low-frequency pulsations. Passive control methods such as nozzle vortex generators and collector breathing gaps have been extensively employed due to their cost-effectiveness and efficiency. The concluding section highlights some unresolved issues that require further investigation in this field.

Influence and multi-objective optimization on three-stage guide vane closure scheme of a pumped storage power plant

Selection of the guide vane closure scheme was a critical issue for the load rejection transients of a pumped-storage power plant. To fully and accurately assess the influences of the guide vane closure scheme on the transient performance and pressure fluctuation, the load rejection transients adopting the original two-stage and optimized three-stage guide vane closure schemes were simulated adopting the one- and three-dimensional coupled approach. Results suggested that the guide vane closure schemes significantly influenced the transient performances and pressure fluctuations during load rejection transients. Compared with adopting the original two-stage guide vane closure scheme, adopting the optimized three-stage guide vane closure scheme could improve the maximum rotation speed, maximum and minimum fluctuating pressures in the pumped-storage power plant, and suppress the local backflow vortexes close to the high-pressure inlet of runner and their induced low-frequency and high-amplitude pressure fluctuation close to the no-load conditions. The study results provided theoretical reference for selecting the optimized guide vane closure scheme.

Runner cone drilling and slotting to reduce vortex rope - induced pressure fluctuations in a Francis turbine

Pressure fluctuations caused by helical vortex rope in the draft tube is one of the main reasons for the instability of Francis turbines. Previous studies showed that drilling holes on runner cone can reduce the amplitude of pressure fluctuation. In this study, based on the runner cone which has been drilled one hole, two kinds of new runner cones are designed by means of drilling two more holes (2 holes runner cone) and drilling a slot (2 holes and 1 slot runner cone). Pressure fluctuation and flow pattern observation experiments as well as numerical simulations at partial load operation condition in a Francis turbine were carried out for the three types of runner cones. Analyses were performed to reveal the generation mechanism of the low frequency pressure fluctuations. The results showed that low frequency fluctuations are related to the cyclical rotation of helical vortex rope. Holes and slots on the runner cone have little impact on the efficiency of the Francis turbine. Slotting on the runner cone can reduce the amplitude of pressure fluctuation induced by vortex rope to a maximum of 29.41%. Hence, proper slotting on the runner cone can effectively reduce the low-frequency fluctuation caused by vortex rope in Francis turbines. These findings provide a new idea to improve the hydraulic instability of Francis turbines induced by vortex rope.

Analysis of flow characteristics and cavitation in the vanes of a reversible pump-turbine in pump mode

In reversible pump-turbines, cavitation erosion can be found at the leading edge of stay vanes (SVs), which is generally considered related to the undesirable flow regime in the double radial vanes in pump mode. In this study, the flow characteristics in the vanes were studied at small guide vane (GV) openings in pump mode to evaluate the easily-ignored stay vane (SV) cavitation. Numerical simulations for three-dimensional (3D) flow passages of pump-turbine revealed that a large pressure drop was induced at SV leading edge in the situation of large velocity magnitude in the GV-SV vaneless space with a backflow flow pattern in the SV channel. Since a quasi-two-dimensional characteristic was confirmed for the flow regime in the vanes, refined simulations for GV-SV region with a two-dimensional (2D) approach were conducted to analyze the evolution of flow regime and the influence of flow rate coefficient. A low-frequency pressure fluctuation was found at SV leading edge, in which the minimum pressure appeared when the flow separation occurred near SV inlet in the situation with backflow in SV channel. This special flow pattern transferred to the adjacent SV channel in the direction of runner rotation. Moreover, the increase of flow rate coefficient resulted in a larger pressure drop at SV leading edge and accelerated the evolution of the flow regime in the vanes. This implies that a high cavitation risk at SV leading edge appears when operating at small GV opening angles with large flow rate coefficients. The outcomes of this study may help researchers to focus on the SV cavitation and improve the start-up and shutdown processes in pump mode.

Circumferential groove on flow field and pressure fluctuation intensity of a semi-open centrifugal pump under design condition

The tip leakage flow (TLF) generated by the tip clearance has adverse effect on the performance of the impeller and increases pressure fluctuation. To suppress the effect of TLF on the instable flow in a semi-open centrifugal pump, the circumferential groove method is proposed in this paper. The effect of the circumferential groove on the internal flow and pressure fluctuation under the design condition is studied by combining numerical simulation with experiment. The results show that the head and efficiency were reduced by 1.1% and 1.0%, respectively. However, the circumferential groove provided a circumferential channel for leading edge overflow. The fluid in the groove formed a jet near the low-pressure region, which can effectively disperse the low-energy fluid of the blade inlet. The initial position and trajectory of the tip leakage vortex (TLV) move downstream, which decreases the relative flow angle and low-velocity region. The flow field in the blade tip region was improved remarkably. The low-frequency pressure fluctuation in the pressure surface and tip clearance of the blade inlet was suppressed by the circumferential groove, and pressure pulsation intensity decreased by 50% and 66%, respectively.

Influence of skew angle on the cavitation dynamics and induced low-frequency pressure fluctuations around a marine propeller

This study numerically investigated the effects of the skew angle on cavitation and the induced low-frequency pressure fluctuations. The numerical results compare well with available experimental data. The skew angle significantly impacts the cavitation dynamics as well as the pressure fluctuations. As the skew angle increases, the dominant cavity type gradually changes from sheet cavitation to tip vortex cavitation and the cavitation on the blade passing through the wake flow is delayed. Fast Fourier Transforms combined with Wavelet Transforms were applied to explore the relationship between the cavity volume acceleration and pressure fluctuations for various skew angles. The low-frequency pressure fluctuations around the propeller are mainly related to the first blade passing frequency (BPF) and its harmonics. The sheet cavity collapse significantly enhances the interaction of the low BPF components, while the tip vortex cavity collapse has the opposite effect. In addition, large skew angles weaken the interaction of the low BPF components which reduces the low-frequency pressure fluctuations.

Numerical study on cavitation–vortex–noise correlation mechanism and dynamic mode decomposition of a hydrofoil

The large eddy simulation model coupled with the modified Schnerr–Sauer cavitation model has been used to numerically simulate the unsteady cavitation and noncavitation flow of the three-dimensional NACA66 (National Advisory Committee for Aeronautics) hydrofoil under different operating conditions. The results show that the magnitude of the cavitation number plays a decisive role in the hydrofoil cavitation quasiperiodic phenomenon. The cavitation number of 1.25 is used as a typical working condition for analysis. Using the Ffowcs Williams–Hawkings acoustic analogy approach accompanied by the vorticity transport equation splitting, the growth and shedding of cavitation also lead to the growth and shedding of the vortex structure. The cavitation–vortex interaction is mainly influenced by the vortex stretching term and vortex dilatation term and amplitude of them are larger than 500. The baroclinic torque term may be responsible for generating vorticity during the cloud cavitation collapse and has a lower amplitude about 200. The cavity volume acceleration is the main influencing factor of the low-frequency pressure fluctuation around the cavitating hydrofoil. Moreover, the NACA66 hydrofoil surface-pressure data are collected for dynamic mode decomposition to locate the hydrofoil surface noise sources. The alternate high and low amplitude regions in the mode results overlap highly with the cavitation transformation regions. The cavity transformation and pressure fluctuations are the main reason for the generation of periodic low-frequency noise source regions on the hydrofoil surface. Moreover, the corresponding frequencies of each order mode are linearly correlated with the cavitation shedding frequency of 5.70 Hz. Combined with the results of the multiple mode comparisons, it can be inferred that the hydrofoil suction surface under the cavitation effect will generate quasiperiodic waves starting from upstream and moving downstream.

The low frequency pressure pulsation and control of the open-jet wind tunnel

An open jet wind tunnel has low-frequency pressure pulsation in common wind speed range due to its unique structural form, which seriously damages the quality of flow field in the test section. The low-frequency pressure fluctuation performance and control mechanism of Jilin University open jet and return flow wind tunnel are investigated by experiments and numerical simulation. The results show that the low-frequency pressure fluctuation is a narrow pulse phenomenon that only occurs in certain intervals, and several velocity intervals may be found in the same wind tunnel. The reliability of the numerical simulation is verified by comparing the peak frequency and amplitude of pressure fluctuation in numerical simulation and wind tunnel tests. A simplified model similar to and amplifying the phenomenon is established. The flow structure and vortex evolution are analyzed via detached eddy simulation. In the test section, large-scale shedding vortices are formed at the nozzle exit, introducing periodic pulsating instantaneous velocity and acting with the collector to form an edge-feedback. This acoustic feedback forms resonance with the pipeline circuit, resulting in poor flow field quality. In accordance with the mechanism of nozzle jet, two methods of controlling pulsation are proposed: spoiler and flow-follow device. The study shows that the effects of two methods are abrupt, and the frequency of pressure pulsation is changed. The spoiler destroys the complete structure of vortex ring in free jet and develops into a complementary double vortex ring structure, which is highly sensitive to size factors. The flow-follow device supplements the velocity loss of the free jet at the nozzle and develops into a double vortex ring with master–slave structure in the middle of the test section. Its vibration reduction effect is greatly affected by the flow velocity. It takes effect in an appropriate range where the flow velocity is higher than the nozzle velocity. If the follow velocity is extremely low, the flow-follow device cannot change the original jet structure. If the follow velocity is extremely high, the momentum of the fan will be greatly reduced, the flow field will be unstable, and another order of pulsation may be induced. This work lays a solid foundation for further understanding the aerodynamic characteristics and optimization mechanism of open jet wind tunnel.

Development of a mathematical model of the operation of the swab generator valve

The object of research in the work is a mathematical model of the operation of a hydrodynamic well generator of the spring-valve type. The existing problem lies in the fact that vibrating wave swabbing differs from known methods of vibrating wave influence on the near-breakout zone of the formation in its ease of execution and, as a result, a much smaller range of equipment. At the same time, its effectiveness should not be inferior to known methods because the treatment of the formation is carried out by elastic low-frequency pressure fluctuations against the background of depression on the formation. Since the swab generator valve is, in fact, a hydrodynamic well generator (HWG) of the spring-valve type, the processes of pressure fluctuations in the considered devices will be similar. However, because of the review of HWG designs, the presence of a mathematical model describing the operation of rotor-type devices, and not valve type, was revealed. Despite the fact that the mathematical model takes into account all the main factors that determine the process of the HWG piston movement, this model cannot be used to describe the operation process of the swab generator valve. Due to the absence of a pulsating flow created at the wellhead by pumping units. Taking this fact into account, as well as taking into account the compressibility of the liquid passing through the valve, a new mathematical model was developed. It describes both the process of operation of a spring-valve type HWG operating in a well on pump-compressor pipes, and the operation of a swab generator valve that is lowered into a well on a rope. Thanks to this, it is possible to obtain the parameters of the oscillating system: geometric dimensions, inertial mass of moving parts, stiffness of the spring, coefficient of viscous friction between the piston and the valve body, as well as parameters of the working fluid: pressure, flow and its rheological properties. In comparison with the similarly known technology of vibration wave impact on a formation with known HWG, it provides such advantages as a higher speed of movement and frequency of oscillations of the moving end of the spring from the identified essential parameters of the oscillating system and the working fluid of the spring (100–1000 N/mm), consumption of working liquid (10–130 m3/h), valve opening pressure (1–7 MPa). This will make it possible to calculate a helical cylindrical compression spring made of round steel for its operation in the valve of the swab generator without collision of turns.

Experimental study on flow evolution and pressure fluctuation characteristics of the underwater suction vortex of water jet propulsion pump unit in shallow water

The occurrence of the underwater suction vortex will cause the ship to vibrate violently and even lose thrust. The evolution process of the underwater suction vortex was obtained by high-speed photography. According to the characteristics of vortex in different phases, its diachronic process can be divided into five phases: premonitory, nascent, developing, weakening, and disappearing. The underwater suction vortex is a vertical axis vortex below the intake, and its shape is similar to a tornado, showing a form of thick at the top and thin at the bottom. The pressure fluctuation test studied the time-frequency characteristics of pressure fluctuation induced by the underwater suction vortex. The results show that the underwater suction vortex can cause the pressure fluctuation amplitude to recover after a sudden drop. The frequency of characteristic low frequency induced by the underwater suction vortex is 0.28 Hz, which is easy to be ignored. The corresponding amplitude of this frequency is much larger than that of other frequencies, and the low-frequency pressure fluctuation induced by the underwater suction vortex is dominant. The results can provide a theoretical basis for the mechanism and prevention of suction vortex of water jet propulsion pump unit.

Measurement of infrasound sensor self-noise

Self-noise is a critical performance characteristic of sensors intended for weak-signal detection and localization; however, the lower the self-noise, the more challenging the measurement. A recent international sensor-characterization exercise coordinated by the Provisional Technical Secretariat of the Comprehensive Nuclear-Test-Ban Treaty Organization included self-noise measurement of several infrasound sensors from 0.01 Hz to 10 Hz. Of three common methods for self-noise assessment—isolation of the sensor from external excitation, subtraction of common (coherent) components among several sensors, or de-activation of the sense mechanism—the first two were used in this exercise and the results highlight important measurement issues. In the infrasonic frequency range, isolation from external excitation is challenging. A sealed, thick-walled chamber attenuates ambient noise but heating from dissipation of power in the chamber interior can induce convection with large, low-frequency pressure fluctuations in the chamber. Capping the inlet(s) of a sensor creates a small, closed volume with strong coupling between temperature and pressure fluctuations. Subtraction of coherent components shared by co-located sensors can be effective in reducing the influence of ambient excitation; however, the process may be frustrated by errors in subtraction of large, nearly equal components or by unexpected electrical coupling.

Multi-objective optimization of guide vane closure scheme in clean pumped-storage power plant with emphasis on pressure fluctuations

In this study, multi-objective optimization was conducted for the guide vane closure scheme during the load-rejection process of a pump turbine, considering pressure fluctuation, maximum rotational speed, maximum pressure at the volute, and minimum pressure at the draft tube inlet to decrease the low-frequency high-amplitude pressure fluctuations at conditions near no-load. Two quantitative indicators, minimum discharge (Qmin) and the ratio of rotational speed to hydraulic head (NDH) of the pump turbine, were proposed to represent the backflow-induced pressure fluctuations. Two objective functions were defined to optimize the guide vane closure scheme and two optimized guide vane closure schemes, OPS-Q (based on Qmin) and OPS-NDH (based on NDH), were developed. The enhancements resulting from these vane closure schemes were verified using a one-dimensional–three-dimensional coupling flow simulation method. NDH was revealed to comprehensively represent backflow-induced pressure fluctuations, whereas Qmin partially represented the backflow. Consequently, OPS-NDH suppressed the pressure fluctuations by appropriately increasing the maximum pressure at the volute and the maximum vacuum at the draft tube inlet within the allowance ranges of the pumped-storage power plant. OPS-Q suppressed the pressure fluctuations; however, the minimum pressure at the draft tube inlet could not be controlled to satisfy the design requirements of the pumped-storage power plant. The findings of this study can be used as a theoretical reference for suppressing the pressure fluctuations in a clean pumped-storage power plant under near-no-load conditions.

Optimization design and unsteady flow analysis of stall characteristics in a vertical centrifugal pump

Abstract In this paper, an optimization design of the vertical centrifugal pump with vaned diffuser was accomplished, and analyzed the internal flow pattern of the model before and after optimization under stall conditions to explore the relationship between design parameters and the stall characteristics. The reasons for the improvement of efficiency and hump characteristics curve after optimization were revealed combined with streamline distributions, entropy production theory and pressure fluctuation. The results show that the error between the external characteristic result calculated by Computational Fluid Dynamics (CFD) and the experimental value is less than 5%, which proves the reliability of the numerical simulation method adopted in this paper. The hydraulic efficiency under multiple operating conditions and stall characteristics of the optimized model have been significantly improved. After optimization, the entropy production rate in the vaned diffuser is greatly reduced, and the improvement of the internal flow pattern in the vaned diffuser is the main reason for slowing down the hump characteristic curve. The hydraulic performance and the stall characteristics of the vertical centrifugal pump can be significantly improved by properly increasing the radial distance of the vaneless region. The matching relationship between the vaned diffuser and the impeller is one of the important factors that affect the stall characteristics. The large-scale vortices in the vaned diffuser is the main reason for the low-frequency pressure fluctuation signal when operating in the hump region. The low-frequency signal in the vaned diffuser of the optimized model is obviously weakened.

Application of immersed boundary method to the simulation of transient flow in solid rocket motors

To avoid the cumbersome re-meshing, an easy-to-implement immersed boundary method (IBM) is developed for simulating the coupling of gas flow and burning surface regression in solid rocket motors (SRM). The treatments of the slip wall boundary and the mass flow inlet boundary in the IBM are described. The Euler equations are discretized on a rectangular Cartesian grid based on the HLLC Riemann solver for inviscid fluxes. The interpolated primitive variables on cell boundaries are determined by using the third-order MUSCL method. The solution of the fluid flow is advanced in time by using the second-order TVD Runge–Kutta scheme. The proposed coupling method updates the burning surface and grid classification once when the movement distance is accumulated to half of the grid cell height. The numerical results of Taylor-Culick flow verify that the developed solver has second-order accuracy in space. To conduct the validation of our method, three typical grains are selected for coupling simulations, which are the perforated cylindrical grain with burning on both the ends and the inner surface, the end-slotted end-burning grain, and the perforated cylindrical grain with erosive burning on the inner surface, respectively. The calculation results show that the proposed IBM with the coupling method leads to low-frequency pressure fluctuations within 0.2% of the mean value during the burning surface regression. The accuracy of the calculation results is verified by the comparison with zero-dimensional interior ballistics, the body-fitted grid method, or experiments in literature.

Investigations on Pressure Fluctuations in the S-Shaped Region of a Pump–Turbine

Hydraulic pumped storage is a special power generation and electricity shortage technology, which is usually operated with thermal power and nuclear power units, and plays a key role in ultra-high voltage and smart grid. Pressure fluctuations are the main reasons for the instability of the S-shaped region of pump–turbines, which seriously affects their lifespan and operation stability. To reveal the mechanism and propagation law of pressure fluctuations in the S-shaped region as well as numerical simulations at the turbine, the braking and the reverse pump operating conditions of a pump–turbine were carried out. Numerical results were validated using the performance experiments, and the generation mechanism and propagation law of pressure fluctuation were analyzed in detail. The analyses show that high-amplitude pressure fluctuations mainly occur in the braking and reverse pump operating conditions. Under the braking condition, a 0.49-fn low-frequency pressure fluctuation was captured, which is caused by the rotation of the backflow in the vanes. Under the reverse pump condition, a 0.19-fn low-frequency pressure fluctuation was confirmed, which is caused by the periodic rotation of the vortex between the vaneless space. This study has important guiding significance for practical engineering application.

Spatiotemporal correlation and control of wall pressure fluctuations induced by subsonic cavity flows

Strong wall pressure fluctuations are generated mostly by a high-speed flow passing over a cavity and should be accurately predicted and controlled for the structural design of aircraft. In this work, a spatiotemporal analysis was performed to estimate wall pressure fluctuations and reveal the mechanism of flow control by a sawtooth-like vortex generator installed on the leading edge or by the use of a rounded trailing edge in suppressing wall pressure fluctuations. Both the cavity tones and overall sound pressure level were analyzed under different flow speeds and flow control approaches. To investigate the power transfer in the temporal and spatial domains, power spectral density contours were drawn and discussed. For the mode separation, the phased array technique with an improved beamforming algorithm was adopted to calculate the wavenumber map and then to separate the acoustic modes from the convective modes by utilizing the spatial correlation of the wall pressure fluctuations. The mode separation results show that the convective modes contribute mainly to the low-frequency pressure fluctuations, while the acoustic modes contribute to both the low-frequency and high-frequency pressure fluctuations. Flow control by the rounded trailing edge approach is more effective than that by the serrated structure approach in reducing the cavity acoustic environment, which enables wall pressure fluctuations to transfer from the high-frequency range to the low-frequency range compared with the “no control” case.