Makine Mühendisliği Bölümü Koleksiyonu

Permanent URI for this collectionhttps://hdl.handle.net/20.500.11779/1944

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Citation - WoS: 1
Citation - Scopus: 1
Acoustic Cavitation Model Based on a Novel Reduced Order Gas Pressure Law
(2021) Pasinlioğlu, Şenay; Delale, Can Fuad
The thermal behavior of a spherical gas bubble in a liquid excited by an acoustic pressure signal is investigated by constructing an iterative solution of the energy balance equations between the gas bubble and the surrounding liquid in the uniform pressure approximation. This iterative solution leads to hierarchy equations for the radial partial derivatives of the temperature at the bubble wall, which control the temporal rate of change of the gas pressure and gas temperature within the bubble. In particular, a closure relation for the hierarchy equations is introduced based on the ansatz that approximates the rapid change of state during the collapse of the bubble from almost isothermal to almost adiabatic behavior by time averaging the complex dynamics of change of state over a relatively short characteristic time. This, in turn, leads to the desired reduced order gas pressure law exhibiting power law dependence on the bubble wall temperature and on the bubble radius, with the polytropic index depending on the isentropic exponent of the gas and on a parameter that is a function of the Péclet number and a characteristic time scale. Results of the linear theory for gas bubbles are recovered by identifying this parameter as a function of the Péclet number based on the Minnaert frequency. The novel gas pressure law is then validated against the near-isothermal solution and against the results of the numerical simulations of the original energy balance equations for large amplitude oscillations using spectral methods. Consequently, an acoustic cavitation model that accounts for phase change but that neglects mass diffusion is constructed by employing the reduced order gas pressure law together with the Plesset–Zwick solution for the bubble wall temperature and the Keller–Miksis equation for spherical bubble dynamics. Results obtained using variable interface properties for acoustically driven cavitation bubbles in water show that the time variations of the bubble radius and the bubble wall temperature lie between those obtained by the isothermal and adiabatic laws depending on the value of the Péclet number and the characteristic time scale.
Citation - WoS: 54
Citation - Scopus: 63
An Assessment of the Performance of Impedance Tube Method
(Institute of Noise Control Engineering, 2014) Hasan Körük
The impedance tube method is widely used for measuring sound absorption (or reflection) coefficients of acoustic materials as a function of frequency. However, the sound absorption coefficients obtained using the impedance tube method may have some variations due to the dimensions (limits) of an impedance tube, sample preparation and sample mounting. This paper assesses the performance of the two-microphone impedance tube method as a function of frequency for different tube dimensions and materials and presents suggestions for increasing the reliability and repeatability of impedance tube measurements. First, after summarizing a systematic way for measuring acoustic transfer functions, sound absorption coefficients of a variety of materials ranging from conventional absorbing acoustic materials to samples with thin films are measured using two tubes with different tube diameter and microphone spacing. Uncertainty of sound absorption coefficients for various materials is discussed, and the frequency limits of impedance tubes are assessed. Then, a method for minimizing uncertainty due to sample mounting is proposed and the main findings are discussed.
Citation - WoS: 3
Citation - Scopus: 3
Characterization of Viscoelastic Materials Using Free-Layered and Sandwiched Samples: Assessment and Recommendations
(Polish Physical Society, 2015) Özer, Mehmet Sait; Körük, Hasan; Şanlıtürk, Kenan Yüce
Viscoelastic materials are widely used in many applications in practice. However, determination of the elastic and damping properties of these materials is quite difficult in the sense that the identified results may contain high degree of uncertainty. The characterization of viscoelastic materials using the Oberst beam method, based on non-contact excitation and response measurements, is revisited in this paper. The effects of signal processing parameters such as frequency resolution in Frequency Response Function (FRF) measurements, as well as the effects of various single-degree-of-freedom modal analysis methods, including circle-fit, half-power and line-fit are investigated first. Then, the modal loss factors, Young's modulus and shear modulus of some sample viscoelastic materials are identified using both the free-layered and sandwiched samples. The results obtained from different tests are compared, discussed and some recommendations are made so as to identify the damping and elastic properties of typical viscoelastic materials with better accuracy. Analyses of a large number of FRF measurements show that the selection of the appropriate signal processing parameters and the use of appropriate modal analysis method can be very significant during the identification of viscoelastic materials. By following the approach presented in this paper, the damping and elastic properties of viscoelastic materials can be identified with better accuracy using either free-layered or sandwiched samples. The material properties obtained by this approach can be used for developing valid structural models and/or for damping optimization purposes.
Citation - WoS: 1
Citation - Scopus: 4
Identification of Uncertainty Levels of Acoustic Properties of Biocomposites Under Different Mounting Conditions in Impedance Tube Tests
(Institute of Noise Control Engineering, 2021) Garip Genç; Şanlıtürk, Kenan Y.; Yusuf Saygılı; Hasan Körük
Impedance tube method is widely used to measure acoustic properties of materials. Although this method yields reliable acoustic properties for soft textured materials, uncertainty levels of measured acoustic properties for hard materials, including biocomposites, can be quite large, mainly due to uncertain mounting conditions. Here, the effects of mounting conditions on the acoustic properties of biocomposites in an impedance tube are investigated. First, nominally identical biocomposite samples with a diameter equal to the inner diameter of impedance tube are manufactured and their acoustic properties are determined. As hard materials practically cause fitting problems in the impedance tube, the diameters of samples are reduced, as in practice, by small amounts and acoustic properties of modified samples are determined. Furthermore, in order to match the diameters of samples to the inner diameter of impedance tube, different materials such as tape, petroleum jelly and cotton are applied around samples to close the air gap between the samples and the tube's inner wall. All the results are compared, and the uncertainty levels caused by different mounting conditions on the acoustic properties of biocomposites are identified. The results show that the transmission loss (TL) measurements are dramatically affected by the mounting conditions while the sound absorption conditions are less sensitive to the mounting conditions. The deviations in the measured TL levels are highest for the samples with tape and wax (10–15 dB). On the other hand, the deviations in the measured sound absorption coefficients are highest for the samples with cotton and tape (1–2%).
Citation - WoS: 11
Citation - Scopus: 14
Modelling Small and Large Displacements of a Sphere on an Elastic Half-Space Exposed To a Dynamic Force
(IOP Publishing, 2021) Hasan Körük
Spheres at medium interfaces are encountered in many applications, including in atomic force microscopy or indentation tests. Although the Hertz theory describes the contact mechanics between an elastic sphere and an elastic half-space for static loading and small deformations very well, there is a need to consider the density of the medium, the mass of the sphere and the radiation damping for dynamic loading to obtain reliable results. In this study, an analytical model for predicting the small and large displacements of a sphere on an elastic half-space exposed to a dynamic force is developed. For this purpose, after summarizing a mathematical model that has recently been proposed for the sphere at a medium interface, a finite element model for the sphere at an elastic interface is developed. Based on the comparison of the mathematical and finite element models, an improved analytical model for the sphere at an elastic interface is developed. In addition to considering the elastic properties of the medium and the size of the sphere, the model developed here takes into account the density of the medium, the mass of the sphere, and the radiation damping, and the model is valid for small and large sphere displacements. The developed model can be used to understand the dynamic responses of spherical objects at medium interfaces in practical applications. Furthermore, the proposed model is a remarkable tool for undergraduate and graduate students and researchers in the fields of engineering, materials science and physics to gain insight into the dynamic responses of spheres at medium interfaces. © 2021 European Physical Society.

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