Self-Sustained Oscillations of Turbulent Flow in an Open Cavity

Lee, Sang Bong; Seena, Abu; Sung, Hyung Jin

doi:10.2514/1.44823

Cited by 21 publications

(4 citation statements)

References 23 publications

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“…Interactions of the shear layer with the cavity trailing corner is of particular interest since they are presumably central to the dynamic loading on the surface and the associated noise generation (Tang & Rockwell 1983). Representative studies on this topic include, for example, Rockwell & Knisely (1979), Rockwell (1983Rockwell ( , 1998, Tang & Rockwell (1983), Gharib & Roshko (1987), Huang & Weaver (1991), Najm & Ghoniem (1991), Pereira & Sousa (1995), Lin & Rockwell (2001), Chang, Constantinescu & Park (2006), Bres & Colonius (2008), Haigermoser et al (2008), Kang, Lee & Sung (2008), Kang & Sung (2009) and Lee, Seena & Sung (2010). They all reveal that interactions of the shear layer vortices with the flow around the corner play a major role in the local unsteadiness, turbulence and noise.…”

Section: Introductionmentioning

confidence: 99%

Vortex-corner interactions in a cavity shear layer elucidated by time-resolved measurements of the pressure field

Liu

Katz

2013

J. Fluid Mech.

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The flow structure and turbulence in an open cavity shear layer has been investigated experimentally at a Reynolds number of $4. 0\times 1{0}^{4} $, with an emphasis on interactions of the unsteady pressure field with the cavity corners. A large database of time-resolved two-dimensional PIV measurements has been used to obtain the velocity distributions and calculate the pressure by spatially integrating the material acceleration at a series of sample areas covering the entire shear layer and the flow surrounding the corners. Conditional sampling, low-pass filtering and time correlations among variables enable us to elucidate several processes, which have distinctly different frequency ranges, that dominate the shear layer interactions with the corners. Kelvin–Helmholtz shear layer eddies have the expected Strouhal number range of 0.5–3.2. Their interactions with the trailing corner introduce two sources of vorticity fluctuations above the corner. The first is caused by the expected advection of remnants of the shear layer eddies. The second source involves fluctuations in local viscous vorticity flux away from the wall caused by periodic variations in the streamwise pressure gradients. This local production peaks when the shear layer vortices are located away from the corner, creating a lingering region with peak vorticity just above the corner. The associated periodic pressure minima there are lower than any other point in the entire flow field, making the region above the corner most prone to cavitation inception. Flapping of the shear layer and boundary layer upstream of the leading corner occurs at very low Strouhal numbers of ∼0.05, affecting all the flow and turbulence quantities around both corners. Time-dependent correlations of the shear layer elevation show that the flapping starts in the boundary layer upstream of the leading corner and propagates downstream at the free stream speed. Near the trailing corner, when the shear layer elevation is low, the stagnation pressure in front of the wall, the downward jetting flow along this wall, the fraction of shear layer vorticity entrained back into the cavity, and the magnitude of the pressure minimum above the corner are higher than those measured when the shear layer is high. However, the variations in downward jetting decay rapidly with increasing distance from the trailing corner, indicating that it does not play a direct role in a feedback mechanism that sustains the flapping. There is also low correlation between the boundary/shear layer elevation and the returning flow along the upstream vertical wall, providing little evidence that this returning flow affects the flapping directly. However, the characteristic period of flapping, ∼0.6 s, is consistent with recirculation time of the fluid within the cavity away from boundaries. The high negative correlations of shear/boundary layer elevation with the streamwise pressure gradient above the leading corner introduce a plausible mechanism that sustains the flapping: when the shear layer is low, the boundary layer is subjected to high adverse streamwise pressure gradients that force it to widen, and when the shear layer is high, the pressure gradients decrease, allowing the boundary layer to thin down. Flow mechanisms that would cause the flapping-induced pressure changes, and their relations to the flow within the cavity are discussed.

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Section: Introductionmentioning

confidence: 99%

Vortex-corner interactions in a cavity shear layer elucidated by time-resolved measurements of the pressure field

Liu

Katz

2013

J. Fluid Mech.

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“…After that, the flows moved upwards along the wall face. The flow phenomenon is similar to that the air flows encountered a sunken step at first in the motion direction and then encountered a rising step, so self-oscillation was formed [29][30][31][32][33]. Schematic diagram of oscillation is shown in Fig.…”

Section: Numerical Optimization Of Flow Noises Of Rear View Mirrorsmentioning

confidence: 98%

Numerical optimization for aerodynamic noises of rear view mirrors of vehicles based on rectangular cavity structures

Zhu¹,

Liu²

2018

J. vibroeng.

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Abstract. The rectangular cavity structure was applied to edges of the rear view mirror, numerical computation was conducted for aerodynamic noises of the rear view mirror, and the results of optimized structure were compared with those of the original structure to verify optimized effects. Wind tunnel test was then conducted on the rear view mirror to verify correctness of the computational model. Sound pressure levels of each observation point between experimental test and numerical simulation were basically consistent, where only parts of the peak frequency points were different. The computational accuracy was very high. Two large-size vortexes were in this region behind the common rear view mirror. Vortexes above the rear part of the rear view mirror rotated anticlockwise, while vortexes under it rotated clockwise. There were two vortexes in the region behind the optimized rear view mirror, but the energy intensity of this vortex near the side window panel was very weak. Pressures on the front part of the optimized rear view mirror were also smaller than those of the original structure. In addition, it could be found that radiation noise distribution of the rear view mirror on the lateral window presented symmetry. When lateral window effects were considered, the aerodynamic noise was more than results without considering the lateral window effects. Total noises of each observation point without considering the lateral window effects were 56.7 dB, 59.2 dB, 58.6 dB, 58.9 dB, 62.3 dB and 63.1 dB, respectively. Total noises at each observation point with consideration of the lateral window effects were 65.3 dB, 66.5 dB, 68.7 dB, 69.2 dB, 70.1 dB and 70.8 dB, respectively. Therefore, when aerodynamic noises of the rear view mirror were computed, impacts of lateral window effects must be considered, otherwise the computation would be seriously deviated from actual situations. The frequency corresponding to maximum peaks of the aerodynamic noise after optimization approached the self-oscillation frequency of the rectangular cavity. The result indicates that aerodynamic noises in the rectangular cavity were fluid self-oscillation caused by the cavity structure. In addition, within the analyzed frequency band, the aerodynamic noises of the optimized rear view mirror were smaller than those of the original structure. The maximum decrease rate of total noises of the optimized rear view mirror was 15.62 %, and minimum decrease rate was 8.90 %. Optimized effects were very significant, especially in the low frequency bands.

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“…Interactions of the shear layer with the cavity trailing corner is of particular interest since they are presumably central to the dynamic loading on the surface and the associated noise generation [7]. Representative studies on this topic include, e.g., Tang and Rockwell [7], Lin and Rockwell [8], Rockwell and Knisely [9], Rockwell [5,10], Najm and Ghoniem [11], Pereira and Sousa [12], Chang et al [13], Haigermoser et al [14] and Lee et al [15]. They all reveal that interactions of the shear layer vortices with the flow around the corner play a major role in the local unsteadiness, turbulence and noise.…”

Section: Introductionmentioning

confidence: 99%

Time Resolved Measurements of the Pressure Field Generated by Vortex-Corner Interactions in a Cavity Shear Layer

Liu

Katz

2011

ASME-JSME-KSME 2011 Joint Fluids Engineering Conference: Volume 1, Symposia – Parts A, B, C, and D

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A 2D open cavity shear layer flow, especially its interaction with the trailing corner of the cavity, is investigated experimentally in a water tunnel, at a Reynolds number of 4.0×104, based on cavity length. Time-resolved PIV, at an image sampling rate of 4500 fps is used to simultaneously measure the instantaneous velocity, material acceleration and pressure distribution. The pressure is obtained by spatially integrating the material acceleration (Liu and Katz [19]). A large database of instantaneous realizations enables detailed visualization of the dynamic changes to the shear layer vortices, including convection, deformation and breakup as they impinge on the cavity trailing corner, as well as their interactions with the freshly generated boundary layer around the corner walls. The vorticity on top of the corner is originated from advected shear layer vortices and locally generated vorticity associated with the local pressure gradients. The resulting periodic recirculating flow above the corner generates the lowest mean pressure in the entire flow field. Two mechanisms with distinct characteristic frequencies affect the periodic variations in vorticity and pressure around the corner. The first is caused by streamwise transport of the large shear layer vortices. For example, when these vortices are located just upstream of the corner, they induce a downwash, which periodically eliminates the recirculating flow there. This recirculation and the associated low pressure reappear after the vortex climbs over the corner. The second mechanism involves low frequency undulations of the entire shear layer, which affect its interaction with the corner, and causes substantial variations in the pressure maximum along the vertical wall of the corner, and pressure minimum above it. These undulations are self sustained since the corner pressure fluctuations alter the recirculation speed in the cavity, which in turn change the shear layer elevation during initial rollup, and even modify the boundary layer upstream of the cavity. The time resolved pressure distribution can also be used for estimating the dipole noise radiated from the corner. The characteristic frequencies of the hydraulic and acoustic fields can be traced back to specific flow phenomena. Analysis shows that the unsteady surface pressure along the vertical wall of the trailing corner is a major dominant source of low frequency noise.

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Self-Sustained Oscillations of Turbulent Flow in an Open Cavity

Cited by 21 publications

References 23 publications

Vortex-corner interactions in a cavity shear layer elucidated by time-resolved measurements of the pressure field

Vortex-corner interactions in a cavity shear layer elucidated by time-resolved measurements of the pressure field

Numerical optimization for aerodynamic noises of rear view mirrors of vehicles based on rectangular cavity structures

Time Resolved Measurements of the Pressure Field Generated by Vortex-Corner Interactions in a Cavity Shear Layer

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