Passive control of the flow-induced noise from a rectangular cylinder using porous walls

A noise reducing technique for the flow-induced noise using a porous material was studied experimentally and numerically. In the experiment, flow-induced noises emitted from three types of rectangular cylinders were measured in a low-noise wind tunnel. One cylinder was made of four aluminum plates and others were made of two or three aluminum plates. Measurement results show that the frequency of the distinct tonal noise was different among three cylinders, that frequency was higher for using porous material. It was also found that the sound pressure lelvel of the noise was also different and that of the cylinder using two porous material plates was 25 dB smaller at maximum. Velocity field of the wake of cylinders were examined by the PIV measurement and that showed that time and space scale of separated vortices around cylinder were smaller for using two porous material plates. It is assumed that the change of aerodynamic sound was caused by that change in velocity field. In the numerical simulation, we could simulate changes of the emitted noise and the wake of the cylinder by applying the slip boundary condition of the velocity to the wall of the cylinder.

THERMODYNAMIC MODEL OF AERODYNAMIC SOUND HARDENING METHOD

As a result of scientific research, a method of aerodynamic sound hardening (ADH) has been developed and patented, which makes it possible to achieve improved properties of hard alloys by reducing their defectiveness, improving the homogeneity of the structure. The physics of the ADH process is that the hardened product is preheated to an acceptable temperature at which the hard alloy does not lose the plasticity and hardness acquired during manufacture. Then the product is exposed to sound frequency waves, reduced in the range of 140...160 Hz into a resonant state, in which the formation of a resonant amplitude increased by several hundred times occurs. The article gives a description of the essence of the created ADH method. The dependence for determining the action energy on a hardened solid by ADH is provided. A thermodynamic model of the ADH method is presented, based on energy thermal and wave effects on the hardened structure. On the basis of the thermodynamic explanation, the ADH method is reduced to a change in the initial structure of the hard alloy under the influence of temperature and wave resonant energy fluxes on it, through which activating and dissipative processes of energy outflow are excited in the hardening object in the mode of an open thermodynamic system. In addition, the quasi-static process of wave energy transfer, carried out in a non-equilibrium medium, significantly exceeds the relaxation time of the strengthening system. When hardening by ADH, the impact toughness increases by 19–23 % in hard alloys, while the values of impact toughness equal to 39.54–42.05 kJ/m2 are achieved, the hardness according to the HRC parameter increases by 3.0–5.2 %.

On the Passive Noise Control of the Flow-Induced Noise Using Porous Materials

A passive noise control technique for the flow-induced noise using a porous material was studied experimentally. The purpose of this study was to decrease the aerodynamic sound using porous material that permeated only sound and clarify that reduction mechanism. In the experiment, flow-induced noises emitted from two types of rectangular cylinders was measured in a low-noise wind tunnel. One cylinder was made of four aluminum plates and the other was two aluminum and porous material plates each. Measurement results show that the frequency of the distinct tonal noise was different between two cylinders, that frequency was higher for using porous material. It was also found that the sound pressure level of the noise was also different and that of the cylinder using porous material plate was 25 dB smaller at maximum. Velocity field of the wake of cylinders were examined by the PIV measurement and that showed that time and space scale of separated vortices around cylinder were smaller for using porous material. It is assumed that the change of aerodynamic sound was caused by that change in velocity field.

Tonal-Noise Assessment of Quadrotor-Type UAV Using Source-Mode Expansions

The present work deals with the modeling of the aerodynamic sound generated by the propellers of small-size drones, taking into account the effects of horizontal forward flight with negative pitch and of installation on supporting struts. Analytical aeroacoustic formulations are used, dedicated to the loading noise. The fluctuating lift forces on the blades are expanded as circular distributions of acoustic dipoles, the radiated field of which is calculated by using the free-space Green’s function. This provides descriptions of the sound field, valid in the entire space. The stationary mean-flow distortions responsible for the lift fluctuations and at the origin of the sound are estimated from existing numerical flow simulations and from ad hoc models. Installation and forward-flight effects are found to generate much more sound than the steady loading on the blades associated with thrust. Therefore, the models are believed reliable fast-running tools that could be used for preliminary low-noise design through repeated parametric calculations, or for noise-impact estimates corresponding to prescribed urban traffic.

Experimental visualization of aerodynamic sound sources using parallel phase-shifting interferometry

Aerodynamic sound is one of the causes of noise in high-speed trains, automobiles, and wind turbines. To investigate the characteristics of aerodynamic sound generation, measurements around the sound sources are required. Aerodynamic sound is typically measured using microphones. However, microphones cannot capture the near-field of aerodynamic sound because they become new noise sources inside the air flow. To observe the aerodynamic sound near-field, we performed two-dimensional visualization of aerodynamic sound using an optical method. The optical method used in this research, parallel phase-shifting interferometry (PPSI), can detect the pressure within the measurement area as variations of the phase of light. PPSI can therefore visualize the pressure fields. We visualized both the sound pressure and flow components of the sound generated by flow around a square cylinder and flat plates. The visualized pressure fields are provided as animations in the online resources. Analysis of the sound and flow component time variations confirmed the correlations between them. Graphic abstract

Study on Effect of Aerodynamic Sound Hardening for Wear of Coated Carbide Metal Plates

To increase the durability of metal-cutting carbide plates operating during in harsh technological conditions with impact load, an aerodynamic sound hardening method has been developed that can increase a life of carbide tools up to 3.7 times with a small added cost. The wear of plates hardened by the aerodynamic sound method, after 100 min of cutting, is 1.12–1.7 times less than their un-strengthened analogues. A coating on metal carbide plates does not have a prevailing value when a tool is working with impact loads. While working with impact loads viscosity of an internal plate structure occurs the greatest influence on increasing resistance. For metal-cutting carbide plates during interrupted cutting with significant impact loads, a method of aerodynamic sound hardening is more effective than a coating method, not only in terms of tool performance, but also in the cost of completion itself. Empirical dependences of wear on the rear surface of carbide plates hardened by a aerodynamic sound method and plates with PVD coatings have been obtained in the form of approximation by polynomials of the 5th and 2nd degrees, which are convenient to use in a production environment. It has been revealed the higher carbide plate strength in bending leads to less influence of the method of aerodynamic sound hardening on the increase in wear resistance. So, taking into account the fact that for ВК8-base the ultimate bending strength is 1666 MPa, and for T5K10 it is 1421 MPa, wear reduction after hardening by the aerodynamic sound method for ВП3115-plates with ВК base is 11.5 %, while for ВП3225 – plates with ТК-base – 27.1 %.

Study on acoustic and flow-induced noise characteristics of L-shaped duct with a shallow cavity

An aerodynamic sound generated by a flow inside a duct is one of the noise pro- blems. Flows in ducts with uneven surfaces such as grooves or cavities can be seen in various industrial devices and industrial products such as air-conditioning equipment in various plants or piping products. In this article, we have performed experiments and simulations to clarify acoustic and flow-induced sound characteris- tics of L-shaped duct with a shallow cavity installed. The experiments and simula- tions were performed under several inflow velocity conditions. The results show that the characteristics of the flow-induced sound in the duct are strongly affected by the acoustic characteristics of the duct interior sound field and the location of the shallow cavity. Especially, it was found that the acoustic characteristics were af- fected by the location of the shallow cavity in the frequency range between 1000 Hz and 1700 Hz.

An Efficiency Study of the Aerodynamic Sound Generators Suitable for Acoustic Particle Agglomeration

The object of this study is the acoustic field generated by aerodynamic acoustic generators of various types and designs. Six types of aerodynamic acoustic generators were studied experimentally and theoretically to determine the parameters of their generated acoustic field. It was established that the aerodynamic Hartmann type sound generator produces the necessary for acoustic particle agglomeration acoustic field and can be used in acoustic air cleaning equipment. It was established that classical theoretical calculation methods underestimate the design features of aerodynamic acoustic generators and cannot be used to calculate their characteristics.