Mechanism of reduction of aeroacoustic sound by porous material: comparative study of microscopic and macroscopic models

The effects of porous material on the aeroacoustic sound generated in a two-dimensional low-Reynolds-number flow ( $Re=150$ ) past a circular cylinder are studied by direct numerical simulation in which the acoustic waves of small amplitudes are obtained directly as a solution to the compressible Navier–Stokes equations. Two models are introduced for the porous material: the microscopic model, in which the porous material is a collection of small cylinders, and the macroscopic model, in which the porous material is continuum characterized by permeability. The corrected volume penalization method is used to deal with the core cylinder, the small cylinders and the porous material. In the microscopic model, significant reduction of the aeroacoustic sound is found depending on the parameters; the maximum reduction of $24.4$ dB from the case of a bare cylinder is obtained. The results obtained for the modified macroscopic model are in good agreement with those obtained for the microscopic model converted by the theory of homogenization, which establishes that the microscopic and macroscopic models are consistent and valid. The detailed mechanism of sound reduction is elucidated. The presence of a fluid region between the porous material and the core cylinder is important for sound reduction. When the sound is strongly reduced, the pressure field behind the cylinder becomes nearly uniform with a high value to stabilize the shear layer in the wake; as a result, the vortex shedding behind the cylinder is delayed to the far wake to suppress the unsteady vortex motion near the cylinder, which is responsible for the aeroacoustic sound.

Microscopic Model to Take into Account Complex Configurations for Pygmy and Giant Resonances

A microscopic model for taking into account quasiparticle–phonon interaction in magic nuclei is considered within nuclear quantum many-body theory. This model is of interest for constructing a microscopic theory of pygmy and giant multipole resonances—first of all, a description of their fine structure. This article reports on a continuation and development of our earlier study [1]. Basic physics results of that study are confirmed here, and new results are obtained: (i) exact (not approximate, as in [1]) expressions for the first and second variations of the vertex in the phonon field are found and employed; (ii) a new equation involving, in addition to the known effective interaction, the total amplitude for particle–hole interaction is derived for the vertex, which is the main ingredient in the theory of finite Fermi systems; (iii) the required two-phonon configurations are obtained owing to the last result. The new equation for the vertex now contains complex configurations such as $$1p1h\otimes\textrm{phonon}$$ and two-phonon ones, along with numerous ground-state correlations.

Development of a Microscopic Fuzzy Inference Based Cellular Automata Model for Pedestrian Flow

In this work a microscopic model on pedestrian flow has been proposed. Observed data is used to calibrate and validate the proposed model. The model developed here uses a fuzzy inference system to represent the rules and a force–field structure to represent the repulsive and attractive impacts of objects and goals, respectively in the flow space. The flow space and time are discretized and viewed as discrete quantities. This microscopic model of pedestrian behavior, which essentially models how each pedestrian behaves over time in the flow space, is embedded in a simulation model which is used to simulate situations similar to the ones for which experiments have been done. The results show that the model performs reasonably well.