An experimental investigation of screech noise generation

1999 ◽  
Vol 378 ◽  
pp. 71-96 ◽  
Author(s):  
J. PANDA
Keyword(s):  
Shear Layer ◽  
Standing Wave ◽  
Pressure Fluctuations ◽  
Length Scale ◽  
Generation Process ◽  
Noise Generation ◽  
Sound Waves ◽  
Sonic Nozzle ◽  
Hydrodynamic Fluctuations ◽  
Partial Interference

The screech noise generation process from supersonic underexpanded jets, issuing from a sonic nozzle at pressure ratios of 2.4 and 3.3 (fully expanded Mach number, Mj=1.19 and 1.42), was investigated experimentally. The extremely detailed data provide a fresh, new look at the screech generation mechanism. Spark schlieren visualization at different phases of the screech cycle clearly shows the convection of the organized turbulent structures over a train of shock waves. The potential pressure field (hydrodynamic fluctuations) associated with the organized structures is fairly intense and extends outside the shear layer. The time evolution of the near-field pressure fluctuations was obtained from phase-averaged microphone measurements. Phase-matched combined views of schlieren photographs and pressure fluctuations show the sound generation process. The individual compression and rarefaction parts of the sound waves are found to be generated from similar hydrodynamic fluctuations. A partial interference between the upstream-propagating sound waves and the downstream-propagating hydrodynamic waves is found to be present along the jet boundary. The partial interference manifests itself as a standing wave in the root-mean-square pressure fluctuation data. The standing wavelength is found to be close to, but somewhat different from, the shock spacing. An outcome of the interference is a curious ‘pause and go’ motion of the sound waves along the jet periphery. Interestingly, a length scale identical to the standing wavelength is found to be present inside the jet shear layer. The coherent fluctuations and the convective velocity of the organized vortices are found to be modulated periodically, and the periodicity is found to match with the standing wavelength distance rather than the shock spacing. The reason for the appearance of this additional length scale, different from the shock spacing, could not be explained. Nevertheless, it is demonstrated that an exact screech frequency formula can be derived from the simple standing wave relationship. The exact relationship shows that the correct spacing between the sources, for a point source model similar to that of Powell (1953), should be a standing wavelength (not the shock spacing).

Wall-Pressure Fluctuations Inside Attached Cavitation

2021 ◽  
Author(s):  
Changchang Wang ◽  
Guoyu Wang ◽  
Mindi Zhang ◽  
Qin Wu
Keyword(s):  
Shock Wave ◽  
Shear Layer ◽  
High Frequency ◽  
Pressure Fluctuations ◽  
Wall Pressure ◽  
Cloud Cavitation ◽  
Unsteady Pressure ◽  
Wall Pressure Fluctuations ◽  
Kurtosis Factor ◽  
Skewness And Kurtosis

Abstract This study experimentally investigates the statistics of wall-pressure fluctuations and their source inside attached cavitation under different cavity regimes. Experiments were conducted in the divergent section of a convergent-divergent channel at a constant Reynolds number of Re = 7.8 × 105 based on throat height, and different cavitation numbers σ = 1.18, 0.92, 0.82 and 0.78. Four high-frequency unsteady pressure transducers were flushed-mounted in the divergent section downstream the throat where cavitation develops to sample the unsteady pressure signals induced by cavity behaviors. Flow visualization and wall-pressure measurement in high frequency on the order of MHz were employed using a synchronizing sampling technique. Results are presented for sheet/cloud cavitating flows. Specifically, sheet cavitation with both inception shear layer and fully cavitated shear layer and cloud cavitation under re-entrant jet dominated shedding and shock wave dominated shedding are studied. Compared with re-entrant jet, the interactions between shock wave and cavity could induce pressure peaks with high magnitude within cavity, which will collapse the local vapor along its propagating path and reduce local void fraction. Furthermore, statistics analysis shows that within the cavity, wall-pressure fluctuations increase with the distance to cavity leading edge increase in the first half of cavity length, and the moments of the probability density distribution skewness and kurtosis factor decrease, indicating the asymmetry and intermittency of wall-pressure fluctuation signals decrease. In shock wave dominated cavity shedding condition, the skewness and kurtosis factor increase. These results can provide data to improve the accuracy of turbulence modeling in numerical simulation of turbulent cavitating flow.

Analysis of Fluid-Structure Interaction of an Elastic Blade in Cascade

2002 ◽  
Author(s):  
R. C. K. Leung ◽  
Y. L. Lau ◽  
R. M. C. Si
Keyword(s):  
Numerical Technique ◽  
Fluid Structure Interaction ◽  
Uniform Flow ◽  
Frequency Parameter ◽  
Generation Process ◽  
Noise Generation ◽  
Fluid Structure Interactions ◽  
Fluid Structure ◽  
Structure Interaction ◽  
Normalized Frequency

A time-marching numerical model for the analysis of fluid-structure interaction caused by oncoming alternating vortices has been developed by Jadic et al. (1998). Its applicability to analyzing realistic fluid–structure interaction problems has successfully been established in a recent experimental work of a flat plate in a circular cylinder wake (Lau et al. 2002). Using the model, So et al. (1999) have predicted that, under the excitation of oncoming Karman vortex street (KVS) vortices, an elastic airfoil/blade in inviscid uniform flow exhibits two types of fluid–structure resonance, namely aerodynamic and structural resonance. Aerodynamic resonance is of pure aerodynamic origin and occurs with rigid airfoil/blade excited at normalized frequency parameter c/d = 0.5, 1.5, 2.5 etc., where c is the blade chord and d is the streamwise separation between two neighboring vortices. For an elastic airfoil/blade, as a result of coupled fluid–structure interaction, structural resonance occurs at a normalized frequency close to the natural frequency in vacuo of the airfoil/blade. The occurrence of fluid-structure resonance has also been shown critical in noise generation process (Leung & So 2001). The present study extends the scope of the analysis to fluid–structure interactions occurring in axial–flow turbomachine cascade. When the flow is passing through the rotor, it generates wakes containing KVS vortices behind the rotor blades. The convecting wake will induce perturbations on the downstream stator blades at a wake passing frequency (Rao 1991). Such wake–blade interaction is important in determining the fatigue life of the blades and noise generation of the cascade. The cascade analysis starts with modeling the two-dimensional turbine stator by five high–loading blades evenly separated by s in inviscid uniform flow. Oncoming KVS vortices are released upstream to represent the passing wake originating from the rotor, and are allowed to pass through the stator blades. The blade pitch to blade chord ratio s/c and normalized frequency parameter c/d are important parameters of the problems. Fluid–structure interactions are fully resolved by the same numerical technique (Jadic et al. 1998, So et al. 1999). The combined effects of s/c and c/d on the aerodynamic and structural responses of the central blade are studied and discussed.

A DISTRIBUTED ALGORITHM FOR FLOW INDUCED ACOUSTICS

2006 ◽  
Vol 14 (01) ◽  
pp. 131-141
Author(s):  
C.-H. LAI ◽  
Z. K. WANG ◽  
G. S. DJAMBAZOV ◽  
K. A. PERICLEOUS
Keyword(s):  
Distributed Algorithm ◽  
Noise Analysis ◽  
Pressure Fluctuations ◽  
Correction Method ◽  
Navier Stokes ◽  
Sound Waves ◽  
Analysis Software ◽  
Flow Acceleration ◽  
Acoustic Fluctuations ◽  
Flow Induced Noise

Sound waves are propagating pressure fluctuations and are typically several orders of magnitude smaller than the pressure variations in the flow field that account for flow acceleration. On the other hand, these fluctuations travel at the speed of sound in the medium, not as a transported fluid quantity. Due to the above two properties, the Reynolds averaged Navier–Stokes (RANS) equations do not resolve the acoustic fluctuations. Direct numerical simulation of turbulent flow is still a prohibitively expensive tool to perform noise analysis. This paper proposes a distributed algorithm, based on the acoustic correction method, which leads to an efficient method for computational aeroacoustics and noise analysis. Software issues and advantages are discussed. Numerical experiments on a flow induced noise problem are included.

scholarly journals New Theories on Boundary Layer Transition and Turbulence Formation

2012 ◽  
Vol 2012 ◽  
pp. 1-22 ◽  
Author(s):  
Chaoqun Liu ◽  
Ping Lu ◽  
Lin Chen ◽  
Yonghua Yan
Keyword(s):  
Boundary Layer ◽  
Flow Field ◽  
Shear Layer ◽  
Boundary Layer Transition ◽  
Length Scale ◽  
Multiple Level ◽  
Shear Layer Instability ◽  
Small Length ◽  
Layer Transition ◽  
Turbulence Generation

This paper is a short review of our recent DNS work on physics of late boundary layer transition and turbulence. Based on our DNS observation, we propose a new theory on boundary layer transition, which has five steps, that is, receptivity, linear instability, large vortex structure formation, small length scale generation, loss of symmetry and randomization to turbulence. For turbulence generation and sustenance, the classical theory, described with Richardson's energy cascade and Kolmogorov length scale, is not observed by our DNS. We proposed a new theory on turbulence generation that all small length scales are generated by “shear layer instability” through multiple level ejections and sweeps and consequent multiple level positive and negative spikes, but not by “vortex breakdown.” We believe “shear layer instability” is the “mother of turbulence.” The energy transferring from large vortices to small vortices is carried out by multiple level sweeps, but does not follow Kolmogorov's theory that large vortices pass energy to small ones through vortex stretch and breakdown. The loss of symmetry starts from the second level ring cycle in the middle of the flow field and spreads to the bottom of the boundary layer and then the whole flow field.

Direct numerical simulation of a separated turbulent boundary layer

1998 ◽  
Vol 374 ◽  
pp. 379-405 ◽  
Author(s):  
Y. NA ◽  
P. MOIN
Keyword(s):  
Numerical Simulation ◽  
Boundary Layer ◽  
Shear Stress ◽  
Turbulent Boundary Layer ◽  
Direct Numerical Simulation ◽  
Shear Layer ◽  
Stokes Equations ◽  
Pressure Fluctuations ◽  
Shear Stresses ◽  
Computational Domain

A separated turbulent boundary layer over a flat plate was investigated by direct numerical simulation of the incompressible Navier–Stokes equations. A suction-blowing velocity distribution was prescribed along the upper boundary of the computational domain to create an adverse-to-favourable pressure gradient that produces a closed separation bubble. The Reynolds number based on inlet free-stream velocity and momentum thickness is 300. Neither instantaneous detachment nor reattachment points are fixed in space but fluctuate significantly. The mean detachment and reattachment locations determined by three different definitions, i.e. (i) location of 50% forward flow fraction, (ii) mean dividing streamline (ψ=0), (iii) location of zero wall-shear stress (τw=0), are in good agreement. Instantaneous vorticity contours show that the turbulent structures emanating upstream of separation move upwards into the shear layer in the detachment region and then turn around the bubble. The locations of the maximum turbulence intensities as well as Reynolds shear stress occur in the middle of the shear layer. In the detached flow region, Reynolds shear stresses and their gradients are large away from the wall and thus the largest pressure fluctuations are in the middle of the shear layer. Iso-surfaces of negative pressure fluctuations which correspond to the core region of the vortices show that large-scale structures grow in the shear layer and agglomerate. They then impinge on the wall and subsequently convect downstream. The characteristic Strouhal number St=fδ*in/U0 associated with this motion ranges from 0.0025 to 0.01. The kinetic energy budget in the detachment region is very similar to that of a plane mixing layer.

scholarly journals Experimental and numerical studies of sound generated by cavities

2019 ◽  
Vol 137 ◽  
pp. 01011
Author(s):  
Sebastian Rulik ◽  
Włodzimierz Wrόblewski ◽  
Mirosław Majkut ◽  
Michał Strozik ◽  
Krzysztof Rusin
Keyword(s):  
High Frequency ◽  
Measurement Techniques ◽  
Pressure Fluctuations ◽  
Three Dimensional ◽  
High Amplitude ◽  
Noise Generation ◽  
Flow Conditions ◽  
Numerical Studies ◽  
Wide Range ◽  
Specific Frequency

Cavities and gaps are an important element in the construction of many devices and machines, including energy sector applications. This type of flow is usually coupled with strong pressure fluctuations inside the cavity, which are emitted into the far field in the form of a sound wave responsible for the noise generation. This applies to both subsonic and supersonic flows. Pressure fluctuations often have the character of single tones of a specific frequency and high amplitude and their generation is associated with a vortex shedding formed directly above the inlet and its interaction with the walls of the cavity. The presented work include description of developed test stand and applied measurement techniques dedicated to the analysis of high frequency phenomena. In addition, the adopted numerical model will be described, including conducted two-dimensional and three-dimensional analysis. The developed models will be validated based on experimental measurements concerning wide range of flow conditions.

An experimental investigation of turbulent flow over a two-dimensional obstacle on a flat plate

2021 ◽  
Vol 263 (2) ◽  
pp. 4459-4470
Author(s):  
Shivam Sundeep ◽  
Xin Zhang ◽  
Siyang Zhong ◽  
Huanxian Bu
Keyword(s):  
Shear Layer ◽  
Large Scale ◽  
Pressure Fluctuations ◽  
Wall Pressure ◽  
Far Field ◽  
Two Dimensional ◽  
Vortical Structures ◽  
Wall Pressure Fluctuations ◽  
Field Noise ◽  
Velocity Spectra

Aeroacoustic and aerodynamic characteristics of the turbulent boundary layer encountering a large obstacle are experimentally investigated in this paper. Two-dimensional obstacles with a square and a semi-circular cross-section mounted on a flat plate are studied in wind tunnel tests, with particular interests in the shear layer characteristics, wall pressure fluctuations, and far-field noise induced by the obstacles. Synchronized measurements of the far-field noise and the wall pressure fluctuations were conducted using microphone arrays in the far-field and flush-mounted in the plate, respectively. Additionally, the streamwise and wall-normal velocity fluctuations behind the obstacle were measured using the X-wire probe. The measured velocity profiles, spectra, and wall pressure spectra are compared, showing that the rectangular obstacle has a significant impact on both the turbulent flow and far-field noise. The large-scale vortical structures shed from the obstacles can be identified in the wall pressure spectra, the streamwise velocity spectra, and the wall pressure coherence analysis. Within the shear layer, the pairing of vortices occurs and the frequency of the broadband peak in the velocity spectra decreases as the shear layer grows downstream. Further eddy convective velocities of large-scale vortical structures inside the shear layer were analyzed based on the wall pressure fluctuations.

Study on the effect of separation and reattachment flow between blades on fan noise

2021 ◽  
Vol 263 (4) ◽  
pp. 2458-2467
Author(s):  
Sho Kosaka ◽  
Masaharu Sakai ◽  
Hideaki Sato ◽  
Kaori Seki
Keyword(s):  
Shear Layer ◽  
Noise Generation ◽  
Air Conditioner ◽  
Blade Surface ◽  
Fan Noise ◽  
The Past ◽  
Cabin Noise ◽  
Velocity Condition ◽  
Blade Shape ◽  
Separated And Reattached Flow

With the growth of the EV/HV market, the main cause of cabin noise has changed from engine driving sound to air conditioner noise. The blower noise is the largest in the air conditioner noise, and the noise reduction is urgent. Separated and reattached flows between fan blades are considered to be the main sources of blower noise. In the past, we tried to reduce the noise by reducing the separation. This time, the blade shape to further reduce the separation was produced and evaluated. As a result, the noise was greatly reduced, but a new problem was found that there was a flow velocity condition in which the noise increased despite the small separation. Therefore, we visualized the flow between blades by PIV, investigated the state of separated and reattached flow in detail, and investigated the factors related to noise increase and decrease by measuring noise and pressure fluctuation of blade surface simultaneously. As a result, it was found that the noise generation condition in the separation reattachment flow between blades is not only the size of separation but also the distance of separation shear layer from blade surface and the strength of vortex generated in shear layer.

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