A hydrodynamic model of using quartz tuning forks (QTFs) for density and viscosity sensing, by measuring the resonance frequency and quality factor, has been established based on the cantilever beam theory applied to the atomic force microscope (AFM). Two examples are presented to verify the usability of this model. Then, the Sobol index method is chosen for explaining quantitatively how the resonance frequency and quality factor of the QTFs are affected by the fluid density and viscosity, respectively. The results show that the relative mean square error in viscosity of the eight solutions evaluated by the hydrodynamic model is reduced by an order of magnitude comparing with Butterworth–Van Dyke equivalent circuit method. When the measured resonance frequency and quality factor of the QTFs vary from 25,800–26,100 Hz and 28–41, the sensitivities of the quality factor affected by the fluid density increase. This model provides an idea for improving the accuracy of fluid component recognition in real time, and lays a foundation for the application of miniaturized and cost-effective downhole fluid density and viscosity sensors.

Bubbles are widely used in the medical field due to their strong acoustic scattering properties, and the interaction between bubbles affects the scattering acoustic field caused by the bubble cluster. In this study, the dynamic equations of bubbles oscillating in viscoelastic tissues are solved numerically. The effect of bubble interaction on the scattered acoustic pressure under dual-frequency ultrasound is analyzed. In addition, the frequency spectra of the scattered acoustic waves due to the bubbles with and without considering the interaction are compared. The results show that the suppression or enlargement of the scattered sound pressure caused by the interaction between bubbles is related to the bubble radius and the incident frequency. Moreover, when the incident frequency is equal to the resonant frequency of the bubble with equilibrium radius R₀, the effect of resonant bubbles is stronger than that of non-resonant bubbles. Meanwhile, for the multi-bubble system with a small bubble number density, the total response of the bubble cluster can be approximated as an algebraic sum of the dynamical behavior of individual bubbles.

The unique double-cantilever beam structure and vibration mode of the tuning fork enable the measuring of fluid density and viscosity synchronously in a decoupling manner. Therefore, it is widely employed in oil and gas development and in petrochemical, food, textile, and other industries. In this paper, quality factors are used to characterize the energy losses of lithium niobate tuning forks when vibrating in a fluid, and the influence parameters, such as length, width, and thickness of the tuning fork arm, etc., of different quality factors are examined with a focus on the viscous quality factor of the fluid. The optimized design of lithium niobate tuning fork dimensions is carried out on this premise, and the analytical solution of the optimal dimension of the lithium niobate tuning fork in the air is obtained. Secondly, the optimal dimension of the lithium niobate tuning fork in fluids is given out by finite element simulation, and the sensitivity of the optimized fork to the viscosity of fluids is investigated. The results show that the optimized tuning fork has a higher quality factor, and thus has a larger parameter measurement range as well as being more sensitive to the change in the fluid density and viscosity. Therefore, the results are of great significance for guiding the preparation and practical application of lithium niobate tuning forks.

Bubbles are widely used in industrial production, biomedical engineering, and many other fields. The analysis of the response of bubble clusters under acoustic waves is significant for the application of bubbles. In this paper, different bubble dynamics models are used to calculate the instantaneous radius of multi-bubbles, and the results corresponding to the different models are compared with those by the finite element method. In addition, the effect of bubble interactions on bubble pulsation is analyzed at different distances, and the effect of incident acoustic direction on the bubble's instantaneous radius is also investigated. The results show that time delays, which have often been neglected in previous work, should be considered except when the bubble spacing is minimal. The results calculated by the finite element method indicate that the attraction and repulsion between bubbles are alternating, which is related to the velocity field between bubbles. Moreover, the suppression or promotion of bubble interactions for bubble pulsation is affected by bubble distance under the same acoustic excitation conditions. Finally, for a bubble cluster with large bubble distances, its total scattered field relating to the incident acoustic wave can be approximated as the interference field of multiple secondary acoustic sources with the same waveform and different phases. If these bubbles are in random motion, the total scattered sound intensity of the bubble cluster is proportional to the number of bubbles.

Bubbly liquids are widely present in the natural environment and industrial fields, such as seawater near the ocean bottom, the multiphase flow in petroleum reservoirs, and the blood with bubbles resulting in decompression sickness. Therefore, accurate measurement of the gas content is of great significance for hydroacoustic physics, oil and gas resources exploration, and disease prevention and diagnosis. Trace bubbles in liquids can lead to considerable changes in the acoustic properties of gas–liquid two-phase media. Acoustic measurements can therefore be applied for trace bubble detection. This study derived the reflection coefficient of acoustic waves propagating in a sandwich layering model with liquid, bubbly liquid, and liquid. The influences of gas contents on the reflection coefficient at the layer interface were analyzed based on theoretical calculations. It was revealed that the magnitude of the reflection coefficient and the frequency interval between its valleys have a quantitative correlation with the gas contents. Thus, a novel means to detect the contents of trace bubbles was proposed by evaluating the reflection coefficients. The reflection features of a thin layer with bubbly liquid were then studied through experiments. It was validated by acoustical measurements and theories that the reflection coefficient is considerably sensitive to the change of gas contents as long as the gas content is tiny. With the increasing gas content, the maximum value of the reflection coefficient increases; meanwhile, the frequency intervals between the valleys become smaller. However, when the gas content is extensive enough, e.g., greater than 1%, the effect of the change of gas content on the reflection coefficient becomes inapparent. In that case, it is not easy to measure the gas content by the acoustic reflection signals with satisfying precision. This proposed method has potential applications for the detection of trace gas bubble content in several scenarios, e.g., decompression illness prevention and diagnosis.

In acoustic logging while drilling (ALWD), it is crucial to have an ALWD isolator to reduce collar waves and to meet the requirement of wideband acoustic measurements. In this work, we proposed a new acoustic isolator based on phononic crystal structures for reducing collar waves, and its acoustic insulation performance is numerically studied through the finite difference simulation. For the typical ALWD tool, the optimized acoustic isolator has eight-section graded grooves and each contains 11 small grooves varying from 3 to 5 mm, with an interval of 0.2 m between adjacent sections. Performances of the isolators we designed are verified by numerical results. The attenuation effects of the improved isolator and the traditional one on collar arrivals are compared. The comparison of synthetic waveforms reveals that the newly proposed design with grooves characterized by gradually varying width displays better acoustic isolation performance than the traditional isolator with uniform grooves when operated with various source central frequencies from 13 to 15 kHz. By applying the proposed isolator, the formation longitudinal wave velocities can be separated effectively from the wave group containing the collar waves in the fast formation model. These studies may be useful for the ALWD tool design and data processing.