Shizuko Adachi

Numerical Study on Acoustic Emission from Flow Past an Airfoil

The sound pressure level is computed using the Lighthill-Curle theory for the far-field acoustic pressure excited by flow around solid bodies. Time evolution of the flow field is obtained by solving the three-dimensional compressible Navier-Stokes equation with a block pentadiagonal matrix scheme based on the approximate factorization method. The far-field sound pressure level is evaluated from the fluctuation of the pressure on a wing surface. Numerical simulation is done for a NACA-0015 airfoil at the attack angle of 8° in a subsonic flow. The computed results are compared successfully with the experimental data. The surface dipole strength density is estimated to show which regions in space dominantly contribute to prominent peaks in the sound pressure level. We discuss the three-dimensional effect by comparing the results from the three- and two-dimensional computation.

Sound Generation in Oblique Collision of Two Vortex Rings

A theory of vortex sound formulated in the form of multipole expansions is applied to the oblique collision of two vortex rings at right angles. Using the theoretical formula for the far-field acoustic pressure excited by a time-dependent localized vorticity distribution, the coefficients of the multipole modes of the wave pressure are estimated by the numerical data of computer simulation. Time evolution of the vorticity field is obtained by solving a viscous incompressible vorticity equation with a vorticity-potential method developed for a three-dimensional vorticity field in a bounded domain. Numerical simulations are carried out for two kinds of vortex rings with the same core parameters but different ring radii, and the details of the vortex motion during the oblique collision are studied numerically. Computed main-mode amplitudes of the wave pressure excited by the vortex motion are found to be consistent with those of the experimentally observed acoustic wave not only qualitatively but also quanti...

Numerical analysis of thermoacoustic spontaneous oscillations in a 2D rectangular and an axisymmetric closed tube

We analyze stability of thermoacoustic spontaneous oscillations of helium gas (Taconis oscillations) using the numerical simulation. The flow fields in a 2D rectangular closed tube and a circular cylindrical closed tube are simulated by solving the 2D and axisymmetric compressible Navier-Stokes equations, respectively. In the 2D rectangular tube, two steady states (oscillation state and quiescent state) are obtained at the same temperature ratios. On the other hand, in the axisymmetric cylindrical tube three different oscillation modes are observed: the fundamental mode and the second mode of a standing wave, and the oscillation with a shock wave.

Stability of thermo-acoustic oscillation in a closed tube by numerical simulation

The thermo-acoustic oscillation (Taconis oscillation) in a two-dimensional (2D) closed tube with a large thermal gradient was studied by numerical simulation of the 2D compressible Navier–Stokes equations. Both ends of the tube were hot (T=TH), and the center of the side wall was cold (T=TC). We analyzed the stability of thermo-acoustic oscillations in the closed tube at various values of the initial pressure p0 and temperature ratio θ=TH/TC. Hysteresis phenomena were observed when the boundary layers are thin at the initial pressures 25, 35, 50, 75 and 100 kPa. On the other hand, we could not observe the hysteresis phenomenon when the boundary layer is thick at the initial pressure of 17.5 kPa. We analyzed the temperature distributions in the cold region, and a heat pumping from the cold region toward the hot region was observed in the cold region. These heat pumping phenomena are related to the hysteresis phenomena.

Vortical flow structure of thermoacoustic oscillations in a closed tube

Spontaneous thermoacoustic oscillations of a gas in a closed cylindrical tube are studied. Numerical simulations of the flow field in the tube on which a temperature gradient along the axis is imposed are performed by solving the axisymmetric compressible Navier–Stokes equations. The wall temperature of the hot part near both ends (300 K) and that of the cold part near the center (20 K) are fixed. The computations are done for various values of the length ratio ξ of the hot part to the cold part between 0.3 and 1.0, and steady oscillatory states are obtained. These states are divided into three groups according to the magnitude of the pressure amplitude. The state in each group has distinguished features of the flow field. We analyze the effect of vortices on the structure of the temperature distribution. The difference of the boundary layer thickness between the hot part and the cold part is shown to play an important role.

Numerical Analysis of Stability of Thermo-acoustic Oscillation in a 2D Closed Tube

‡The thermo-acoustic oscillation (Taconis oscillation) in a two dimensional closed tube with large thermal gradient is studied by numerical simulation of the two-dimensional compressible Navier-Stokes equations. Both ends of the tube are hot (T = T H ), and the center of side wall is cold (T = T C ). We analyze the stability of thermo-acoustic oscillations in the closed tube with different initial pressures and temperature ratios θ = T H /T C . We observe two steady states between the temperature ratio 5.88 ≤ θ ≤ 6.49 at the initial pressure p 0 = 1 x 10 5 Pa. On the other hand, we do not observe the hysteresis phenomenon at the initial pressure p 0 = 0.175 x 10 5 Pa. We analyze the energy flow in the tube length direction, and the heat pumping is observed in the cold region.