Akiyoshi Iida

Prediction of aeroacoustic sound using the flow field obtained by time-resolved particle image velocimetry

Based on vortex theory, we experimentally and directly predict sound sources distributing in the flow field and determine the sound pressure level as a result of the spatial integration of sound sources. In employing this direct evaluation method for the aeroacoustic sound, the problem is that a large integration area is required to minimize errors caused by the sudden truncation of the integration area; we overcome it by adopting and applying a modified formula that neglects the quadrupole sound under the condition that the dipole sound is dominant at a low Mach number. Through the flow field measurement using a time-resolved particle image velocimetry (TR-PIV) technique, we will clearly demonstrate the feasibility of our method and the distribution of dipole sound sources in the vicinity of a body even if a comparatively small integration area must be taken. In this basic study, a circular cylinder with a diameter of 6.0 mm is used; the spatially integrated sound pressure is compared with the actual sound pressure which is measured with a microphone. Further, the sound sources evaluated using only the flow field are determined, which give us detailed information about the amplitude and phase of the sound source structure. This direct evaluation method for the dipole sound is applicable to a more complex body.

Direct aeroacoustic simulation of acoustic radiation in recorders with different windway geometries

To clarify the effects of the windway geometry on the aeroacoustic feedback in the jet fluctuations in recorders, direct aeroacoustic simulations were performed along with experiments. The simulations were based on the compressible Navier-Stokes equations to predict the fluid-acoustic interactions. The volume penalization method was used to reproduce the flow and acoustic fields around the complex shape of the recorders. Two recorders with straight and arch-shaped windway were explored. The occurrence of mode change was observed at the higher velocity for an arch-shaped windway model compared with the straight windway model. The modified formulation of the negative displacement model (N.H. Fletcher et al., 1976, J. Acoust.) was proposed based on the predicted jet fluctuations, where the jet fluctuations were divided into hydrodynamic and acoustic components. The ratio of the hydrodynamic component to the acoustic component near the windway exit was lower in the archshaped windway model than that in the straight windway model, whereas the amplification factor of the jet fluctuations was larger in the arch-shaped windway model.The relevance of these results and the mode change along the jet velocity is to be discussed.To clarify the effects of the windway geometry on the aeroacoustic feedback in the jet fluctuations in recorders, direct aeroacoustic simulations were performed along with experiments. The simulations were based on the compressible Navier-Stokes equations to predict the fluid-acoustic interactions. The volume penalization method was used to reproduce the flow and acoustic fields around the complex shape of the recorders. Two recorders with straight and arch-shaped windway were explored. The occurrence of mode change was observed at the higher velocity for an arch-shaped windway model compared with the straight windway model. The modified formulation of the negative displacement model (N.H. Fletcher et al., 1976, J. Acoust.) was proposed based on the predicted jet fluctuations, where the jet fluctuations were divided into hydrodynamic and acoustic components. The ratio of the hydrodynamic component to the acoustic component near the windway exit was lower in the archshaped windway model than that in the st...

Effects of wake-turbine blade interactions on power production of wind turbines

In offshore wind farms, deterioration in power generation performance due to the mutual interference of flow around the wind turbines is a serious issue. To clarify the effects of wake-turbine blade interactions on the performance of wind farms, we conducted large-scale simulations of the flow around two full-scale wind turbines in a tandem-arrangement with two different spacings. The spacing between the two turbines was L/D = 1.0 and L/D = 2.0, with D being the rotor diameter. The predicted results show that vortices generated in the wake of the first turbine interfere with the blades of the second turbine and the interference becomes more intense for the case of L/D = 1.0. Thus, the power coefficient of the downstream turbine becomes lower by 80% for the case of L/D = 1.0 compared with the case of a single wind turbine.

Measurement of correlation between aerodynamic noise and flow around a door-mirror

The purpose of this investigation is to understand mechanism of aerodynamic noise from door mirrors. In order to clarify velocity fluctuation and radiated noise, velocity fluctuation and aerodynamic sound were measured in the low-noise wind tunnel. The fundamental flow around door mirrors was simulated with half cylindrical model which was proposed by German researchers. The pure tone was observed in the case of bump with door mirror. The instability wave was introduced in the approaching boundary layer by the small bump. The sinusoidal wave was observed both of energy and sound spectra. The coherence function in terms of velocity fluctuation and noise was high level from the bump to edge of mirror. On the other hand, no correlation was observed in the wake of door mirror. The noise was not generated when the approaching boundary layer was turbulence. Therefore, laminar boundary layers and sinusoidal waves were dominant noise source of door mirror. It reveals that the bump on the door mirror surface is extremely important to generate aerodynamic noise from a door mirror.