“…The inner dimensions of the anechoic chamber are , with a cutoff frequency of 100 Hz and a free-field radius of 1.9 m. For further details of the aerodynamic performance and acoustic characteristics of the wind tunnel the reader may refer to Yang et al. (2021 a ). Figure 1.Experimental set-up of PIV (not drawn to scale).…”
Section: Experimental Set-up and Methodologymentioning
confidence: 99%
“…The flow enters the test section through seven screens, a honey comb and a 7 : 1 contraction nozzle, which results in a turbulence intensity of less than 0.15 % at the maximum velocity. The inner dimensions of the anechoic chamber are 3.8 m × 5.7 m × 3.0 m, with a cutoff frequency of 100 Hz and a free-field radius of 1.9 m. For further details of the aerodynamic performance and acoustic characteristics of the wind tunnel the reader may refer to Yang et al (2021a).…”
Acoustic emission of a NACA 0012 aerofoil is investigated over a range of free-stream velocities. Acoustic spectra show a dominant tone and two sets of weaker side tones characterised by different frequency intervals. The frequency of the dominant tones in the acoustic spectra varies with velocity in a ladder-type structure. With increasing Reynolds number, the spectrum becomes progressively more broadband in nature. Through synchronised particle image velocimetry and acoustic measurements, the aeroacoustic noise generation mechanisms, resulting in different spectral characteristics and modulation types, are further investigated. A separation bubble and related significant velocity fluctuations are observed on the pressure side. Pressure side velocity spectra show characteristics similar to the acoustic ones, whereas velocity spectra on the suction side feature broadband characteristics. These findings confirm that noise emission is dominated by pressure side events for the Reynolds number range of this study, i.e.
$2 \times 10^{5}$
–
$7 \times 10^{5}$
. As the acoustic emission is defined by coherent flow structures, the proper orthogonal decomposition method is adopted to facilitate the understanding of the relation between the complex flow field and acoustic emission. Side tones in the acoustic spectra are attributed to two different modulation mechanisms in the aeroacoustic source region near the trailing edge. By aligning the sound pressure time history and the time coefficients of the dominant modes, the primary modulation of the dominant tone is found to be related to the amplitude modulation of the high-frequency velocity fluctuations associated with the acoustic feedback loop. A secondary modulation is attributed to periodic variation of the separation bubble and, therefore, variation in the roll-up of the shear layer, which results in a modulation of the amplitude of the velocity fluctuations associated with the convecting vortices at the trailing edge.
“…The inner dimensions of the anechoic chamber are , with a cutoff frequency of 100 Hz and a free-field radius of 1.9 m. For further details of the aerodynamic performance and acoustic characteristics of the wind tunnel the reader may refer to Yang et al. (2021 a ). Figure 1.Experimental set-up of PIV (not drawn to scale).…”
Section: Experimental Set-up and Methodologymentioning
confidence: 99%
“…The flow enters the test section through seven screens, a honey comb and a 7 : 1 contraction nozzle, which results in a turbulence intensity of less than 0.15 % at the maximum velocity. The inner dimensions of the anechoic chamber are 3.8 m × 5.7 m × 3.0 m, with a cutoff frequency of 100 Hz and a free-field radius of 1.9 m. For further details of the aerodynamic performance and acoustic characteristics of the wind tunnel the reader may refer to Yang et al (2021a).…”
Acoustic emission of a NACA 0012 aerofoil is investigated over a range of free-stream velocities. Acoustic spectra show a dominant tone and two sets of weaker side tones characterised by different frequency intervals. The frequency of the dominant tones in the acoustic spectra varies with velocity in a ladder-type structure. With increasing Reynolds number, the spectrum becomes progressively more broadband in nature. Through synchronised particle image velocimetry and acoustic measurements, the aeroacoustic noise generation mechanisms, resulting in different spectral characteristics and modulation types, are further investigated. A separation bubble and related significant velocity fluctuations are observed on the pressure side. Pressure side velocity spectra show characteristics similar to the acoustic ones, whereas velocity spectra on the suction side feature broadband characteristics. These findings confirm that noise emission is dominated by pressure side events for the Reynolds number range of this study, i.e.
$2 \times 10^{5}$
–
$7 \times 10^{5}$
. As the acoustic emission is defined by coherent flow structures, the proper orthogonal decomposition method is adopted to facilitate the understanding of the relation between the complex flow field and acoustic emission. Side tones in the acoustic spectra are attributed to two different modulation mechanisms in the aeroacoustic source region near the trailing edge. By aligning the sound pressure time history and the time coefficients of the dominant modes, the primary modulation of the dominant tone is found to be related to the amplitude modulation of the high-frequency velocity fluctuations associated with the acoustic feedback loop. A secondary modulation is attributed to periodic variation of the separation bubble and, therefore, variation in the roll-up of the shear layer, which results in a modulation of the amplitude of the velocity fluctuations associated with the convecting vortices at the trailing edge.
“…The measurements were carried out in a low-speed, open-jet anechoic wind tunnel at the Southern University of Science and Technology (SUSTech) (Yang et al. 2021), as shown in figure 1( a ). The closed-loop wind tunnel has a rectangular cross-section nozzle with a width of 600 mm and a height of 550 mm.…”
In this experimental study, the impact of symmetric local blowing on suppressing the vortex-induced noise of a circular cylinder was investigated. A highly instrumented cylinder with pressure taps and a series of blowing chambers was used to inject air along the span (seven times the cylinder diameter) at circumferential angles
$\theta _{b}={\pm }41^{\circ }$
,
${\pm }90^{\circ }$
and
${\pm }131^{\circ }$
corresponding to the boundary layer, shear layers on the cylinder and separated shear layers, respectively. The investigation aimed to understand the noise reduction mechanism of local blowing by conducting near-field pressure and far-field noise measurements in synchronisation with flow field velocity measurements. Near-field pressure was measured around the circumference of the cylinder using a remote-sensing technique and planar particle image velocimetry was implemented to measure the velocity of the wake flow field at a diameter-based Reynolds number of
$Re=7\times 10^{4}$
. The results revealed that the interaction of the rolling up separated shear layers, under the influence of high-momentum fluid travelling from the free stream to the wake, induced significant vertical flow movement in the vortex-formation region. This movement led to strong alternating surface pressure fluctuations at the cylinder's shoulders, contributing to the scattering of noise. It was demonstrated that local blowing delayed vortex shedding for all cases, except at
$\theta _{b}={\pm }90^{\circ }$
, which elongated the shear layers and pushed the high-momentum transfer area farther downstream. The application of local blowing at
$\theta _{b}={\pm }41^{\circ }$
was particularly effective in increasing the vortex formation size due to reduced entrainment of fluid-bearing vorticity.
“…The air flow in the wind tunnel consists of the chamber, contraction section, test section, diffuser, and fan successively, with specific sizes, as shown in Table 2. Wire mesh honeycomb is set in the chamber to filter the largescale eddy structures (Yang et al 2021), attain the steady fluid, and reduce the turbulence intensity. The contraction section satisfies the bicubic curve distribution with 6.8 ratios (Fang et al 2001) where ρ and μ are density and dynamic viscosity of air, respectively, u is free-stream velocity, and L is the feature-length of the test section, which is 850 mm in this experiment.…”
Surface structure is used to interfere with the turbulent boundary layer in the groove drag reduction, which is important to the endurance and stability of high-speed and ultrahigh-speed aircraft. The size of the groove structure directly affects the flow in the turbulent boundary layer and changes the drag reduction effect. The drag reduction characteristics of bionic triangular (V-groove) riblets were studied through Particle Image Velocimetry (PIV) experiment and Finite Volume Method (FVM) simulation. Triangular riblets with adjacent height ratios (AHR) of 1.00, 1.74, and 3.02 were considered in this research, and the influence of these groove structures on the flow characteristics of turbulence near the wall is compared with those of the smooth plates. The distribution of time-averaged velocity, turbulence intensity, and coherent structures of turbulent boundary layer on the riblet surface is analyzed to document the effects of the geometric parameters of various groove structures on drag reduction rates. Results showed that the best drag reduction is obtained using the V-groove riblets with adjacent height ratio of 1:1 under the low free-stream velocity. The results can be used as a reference for further optimization of drag reduction structures with surface grooves.
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