Splitting of the resonance frequencies of acoustic waves in a rotating compressible fluid

2008 ◽  
Vol 34 (2) ◽  
pp. 126-128
Author(s):  
A. A. Sokolsky ◽  
A. N. Tarasenko
Keyword(s):  
Compressible Fluid ◽  
Acoustic Waves ◽  
Resonance Frequencies

Convective Scaling of Intrinsic Thermo-Acoustic Eigenfrequencies of a Premixed Swirl Combustor

2017 ◽  
Author(s):  
Alp Albayrak ◽  
Thomas Steinbacher ◽  
Thomas Komarek ◽  
Wolfgang Polifke
Keyword(s):  
Flow Velocity ◽  
Acoustic Waves ◽  
Feedback Loop ◽  
Feedback Mechanism ◽  
Bulk Flow ◽  
Axial Position ◽  
Low Frequencies ◽  
Swirl Combustor ◽  
Acoustic Modes ◽  
Resonance Frequencies

For velocity sensitive premixed flames, intrinsic thermoacoustic (ITA) feedback results from flow-flame-acoustic interactions as follows: perturbations of velocity upstream of the flame result in modulations of the heat release rate, which in turn generate acoustic waves that travel in the downstream as well as the upstream direction. The latter perturb again the upstream velocity, and thus close the ITA feedback loop. This feedback mechanism exhibits resonance frequencies that are not related to acoustic eigenfrequencies of a combustor and generates — in additional to acoustic modes — so-called ITA modes. In this work spectral distributions of the sound pressure level (SPL) observed in a perfectly premixed, swirl stabilized combustion test rig are analyzed. Various burner configurations and operating points are investigated. Spectral peaks in the SPL data for stable as well as for unstable cases are interpreted with the help of a newly developed simple criterion for the prediction of burner intrinsic ITA modes. This criterion extends the known −π measure for the flame transfer function (FTF) by including the burner acoustic. This way, the peaks in the SPL spectra are identified to correspond to either ITA or acoustic modes. It is found that ITA modes are prevalent in this particular combustor. Their frequencies change significantly with the power rating (bulk flow velocity) and the axial position of the swirler, but are insensitive to changes in the length of the combustion chamber. It is argued that the resonance frequencies of the ITA feedback loop are governed by convective time scales. For that reason, they arise at rather low frequencies, which scale with the bulk flow velocity.

On over-reflexion

1976 ◽  
Vol 77 (3) ◽  
pp. 433-472 ◽  
Author(s):  
D. J. Acheson
Keyword(s):  
Gravity Waves ◽  
Incident Wave ◽  
Compressible Fluid ◽  
Acoustic Waves ◽  
Vortex Sheet ◽  
Internal Gravity Waves ◽  
Negative Energy ◽  
Helmholtz Instability ◽  
The Mean ◽  
Parameter Values

Reflexion coefficients greater than unity have now been predicted for a variety of different systems involving waves propagating towards a shear layer, but almost invariably only in regions of parameter space for which the layer exhibits Kelvin-Helmholtz instability. This paper contains a study of two examples in which, for appropriate parameter values, there are no such instabilities to obscure (or even prevent) the ‘over-reflexion’ of an incident wave, namely(a)hydro-magnetic internal gravity waves meeting a vortex-current sheet in a stratified fluid and(b)magneto-acoustic waves meeting a vortex sheet in a compressible fluid. In the former case the energetic aspects of over-reflexion are examined in detail, thus displaying the way in which the excess reflected energy is extracted from the mean motion and the sense in which the transmitted wave may be viewed, by analogy with certain concepts employed in plasma physics, as a carrier of so-called ‘negative energy’.

scholarly journals Experimental and Numerical Assessment of Local Resonance Phenomena in 3D-Printed Acoustic Metamaterials

2020 ◽  
Vol 142 (2) ◽  
Author(s):  
D. Roca ◽  
T. Pàmies ◽  
J. Cante ◽  
O. Lloberas-Valls ◽  
J. Oliver
Keyword(s):  
Acoustic Waves ◽  
Unit Cell ◽  
Material Structure ◽  
Cell Design ◽  
Acoustic Metamaterials ◽  
Local Resonance ◽  
Resonance Phenomena ◽  
Resonance Frequencies ◽  
Characteristic Wavelength ◽  
3D Printed

Abstract The so-called locally resonant acoustic metamaterials (LRAMs) are a new kind of artificially engineered materials capable of attenuating acoustic waves. As the name suggests, this phenomenon occurs in the vicinity of internal frequencies of the material structure and can give rise to acoustic bandgaps. One possible way to achieve this is by considering periodic arrangements of a certain topology (unit cell), smaller in size than the characteristic wavelength. In this context, a computational model based on a homogenization framework has been developed from which one can obtain the aforementioned resonance frequencies for a given LRAM unit cell design in the sub-wavelength regime, which is suitable for low-frequency applications. Aiming at validating both the proposed numerical model and the local resonance phenomena responsible for the attenuation capabilities of such materials, a 3D-printed prototype consisting of a plate with a well selected LRAM unit cell design has been built and its acoustic response to normal incident waves in the range between 500 and 2000 Hz has been tested in an impedance tube. The results demonstrate the attenuating capabilities of the proposed design in the targeted frequency range for normal incident sound pressure waves and also establish the proposed formulation as the fundamental base for the computational design of 3D-printed LRAM-based structures.

scholarly journals Acoustic Biosensor for Discrimination of Pathogens according to the Gram Principle

Proceedings ◽  
2020 ◽  
Vol 60 (1) ◽  
pp. 57
Author(s):  
Vladislav Lemozerskii ◽  
Tatiana Zimina ◽  
Alena Gagarina
Keyword(s):  
Acoustic Waves ◽  
Sharp Decrease ◽  
Resonance Curve ◽  
Bifidobacterium Bifidum ◽  
Staining Procedure ◽  
Biomedical Analysis ◽  
Resonance Effects ◽  
Gram Staining ◽  
Resonance Frequencies ◽  
External Acoustic Field

The microacoustic methods of biomedical analysis, implemented on piezoelectric crystals and ceramics, are becoming increasingly popular due to the fact of their potential for integration into laboratories-on-a-chip, biochips, and biosensors as functional elements of biosensors. An important stage in diagnostics of infectious diseases is the identification of pathogens. One possible applications of such a sensor is an alternative to the time- and labor-consuming Gram method of discriminating bacteria according to the composition of their cell walls. Thus, bacteria, which in a Gram staining procedure do not decolor after application of the dye solution, are classified as Gram-positive (G(+)). They are surrounded with a thick peptidoglycan layer that is pulpy and dampens acoustic waves. While Gram-negative (G(–)) bacteria, which acquire a red color in a Gram procedure, are covered with a thin and springy layer, demonstrating resonance effects when interacting with acoustic fields. Thus, G(+) and G(–), which are differently colored in Gram procedures, also react differently to an external acoustic field: for G(–) bacteria, this was a sharp decrease in the Q-factor of the “resonator–suspension” system and a shift of the resonance curve to lower frequencies. While for G(+) bacteria, although a certain shift of the resonance curve was also observed, the bandwidth of the resonance curve practically did not change. This effect was studied for L. acidophilus (G(+)) and Escherichia coli (G(–)) bacilli with quarts resonators of 4 MHz, 5 MHz, and 10 MHz. The biosensor was tested using Lactobacillus fermentum, E. coli M-17, Bifidobacterium bifidum, Burkholderia cepacian, and Staphylococcus aureus. At this stage, it has been demonstrated that the method is particularly effective for discriminating bacteria of a similar shape, such as, for example, cocci. The discrimination of the Gram factor for cocci and bacilli was less accurate and needs further studies for selection of precise resonance frequencies.

scholarly journals Microfluidic Jetting Deformation and Pinching-off Mechanism in Capillary Tubes by Using Traveling Surface Acoustic Waves

Actuators ◽  
2020 ◽  
Vol 9 (1) ◽  
pp. 5 ◽  
Author(s):  
Yulin Lei ◽  
Hong Hu ◽  
Jian Chen ◽  
Peng Zhang
Keyword(s):  
Acoustic Waves ◽  
Capillary Tube ◽  
Surface Acoustic Waves ◽  
Liquid Volume ◽  
Rf Power ◽  
Single Droplet ◽  
Research Attention ◽  
Number Range ◽  
Drop On Demand ◽  
Resonance Frequencies

To date, there has been little research attention paid to jetting deformation and pinching-off of microfluidic flows induced by the surface acoustic wave (SAW) mechanism. Further, such studies were almost limited to one sessile drop actuation without any confinement mechanisms. Such a scenario is likely attributable to the mechanism’s relatively poor controllability, the difficulty of maintaining the fluid loading position and issues related to stability and repeatability. In this paper, a novel SAW-microfluidic jetting system with a vertical capillary tube was designed, accompanied by a large number of experiments investigating the single droplet jetting mechanism with different device dimensions, resonance frequencies and radio frequency (RF) power capabilities. The study began with the whole jetting deformation and droplet pinching off through the use of a microscope with a high-speed camera, after which the results were discussed to explain the droplet jetting mechanism in a vertical capillary tube. After that, the study continued with experimental and theoretical examinations for high-quality single droplet jetting conditions. Jetting characterization parameters, including threshold RF power, resonance frequency, liquid volume, pinching off droplet dimensions, were thoroughly analyzed. Lastly, the Weber number range, a significant parameter in SAW-microfluidic jetting, was verified, and the pinching off microdroplet dimension was analyzed and compared via experiments. The significance of this study lies in the realization of microfluidic drop-on-demand based on SAW technology.

Nonlinear evolution of spiral density waves generated by the instability of the shear layer in a rotating compressible fluid

1991 ◽  
Vol 233 ◽  
pp. 587-612 ◽  
Author(s):  
I. G. Shukhman
Keyword(s):  
Shear Layer ◽  
Compressible Fluid ◽  
Acoustic Waves ◽  
Evolution Equations ◽  
Nonlinear Evolution ◽  
Density Wave ◽  
Mixing Layer ◽  
Mean Flow ◽  
Spiral Density Waves ◽  
Corotation Radius

In a previous paper we considered the nonlinear stability of a cylindrical mixing layer in an incompressible fluid at large Reynolds numbers. Nonlinear evolution results in the formation of vortex structures in the vicinity of the corotation radius rc. This paper considers the same model but in a compressible fluid. A fundamental difference implied by the presence of compressibility is the possibility of the generation of disturbances which are no longer localized near the shear layer but embrace the entire region. These are acoustic waves generated in the region of corotation resonance and emitted into the periphery. In the r > rc region lines of equal density are trailing spirals. The nonlinear evolution of such disturbances is determined by redistribution of the mean flow inside the critical layer (CL). It is shown that only two possible types of CL, viscous and unsteady, can be realized here. For both types of these regimes, evolution equations describing the dynamics of a spiral density wave amplitude are obtained and their solutions analysed. It appears that at any values (provided that they are small enough) of initial supercriticality of the flow, an explosive growth of amplitude occurs which continues as long as values comparable with background ones are reached.

Multiple-Transducer Scheme for Scanning Tomographic Acoustic Microscopy Using Transverse Waves

1997 ◽  
Vol 19 (4) ◽  
pp. 294-304
Author(s):  
Dae-Sik Ko
Keyword(s):  
Transverse Wave ◽  
Acoustic Waves ◽  
Mode Conversion ◽  
Tomographic Reconstruction ◽  
Wave Mode ◽  
Acoustic Microscopy ◽  
Good Resolution ◽  
Multiple Frequency ◽  
Transverse Waves ◽  
Resonance Frequencies

We propose a new type of multiple-transducer scheme with functions of multiple-angle and multiple-frequency tomography for scanning tomographic acoustic microscopy (STAM) using transverse waves. We review the data acquisition system and mode conversion of the acoustic waves for STAM and the multiple-angle and multiple-frequency tomography. Our multiple-transducer scheme has three insonification angles and three resonance frequencies in order to operate, in the transverse wave mode, multiple-angle and multiple frequency tomography for STAM. In order to evaluate the performance of our transducer scheme, we have simulated tomographic reconstruction with a back-and-forth propagation algorithm. Simulation results show that our multiple-transducer scheme is capable of obtaining good resolution with transverse wave mode and multiple-frequency tomography. We also show that our multiple-transducer scheme is an efficient rotation tool for a number of projections.

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