Toshiya Takami

Channel Interface: A Primitive Model for Memory Efficient Communication

Though the size of the system is getting larger towards exa-scale computation, the amount of available memory on computing nodes is expected to remain the same or to decrease. Therefore, memory efficiency is becoming an important issue for achieving scalability. This paper pointed out the problem of memory-inefficiency in the de-facto standard parallel programming model, Message Passing Interface (MPI). To solve this problem, the channel interface was introduced in the paper. This interface enables the programmers to appropriately allocate and de-allocate channels so that the program consumes just-enough amount of memory for communication. In addition to that, by limiting the message transfer supported by a channel as simple as possible, the memory consumption and the overhead for handling messages with this interface can be minimal. This paper showed a sample implementation of this interface. Then, the memory efficiency of the implementation is examined by the models of the memory consumption and the performance.

Breakup and deformation of a droplet falling in a miscible solution.

When a droplet with a higher density falls in a miscible solution, the droplet deforms and breaks up. The instability of a vortex ring, formed by droplet deformation during the falling process, causes the breakup. To determine the origin of the instability, the wavelengths and thicknesses of the vortex rings are investigated at the time when the instability occurs. The experimental results are almost in agreement with the calculated results for the Rayleigh-Taylor instability using the thickness of a higher-density solution. Furthermore, we performed simulations considering the torus shapes and circulations of the vortex ring. The simulations provided patterns similar to those observed experimentally for the breakup process, and showed that the circulations suppress the instability of the vortex ring. These results imply that the Rayleigh-Taylor instability plays a dominant role in the instability of vortex rings.

Theoretical Estimation of the Acoustic Energy Generation and Absorption Caused by Jet Oscillation

We investigate the energy transfer between the fluid field and acoustic field caused by a jet driven by an acoustic particle velocity field across it, which is the key to understanding the aerodynamic sound generation of flue instruments, such as the recorder, flute, and organ pipe. Howe’s energy corollary allows us to estimate the energy transfer between these two fields. For simplicity, we consider the situation such that a free jet is driven by a uniform acoustic particle velocity field across it. We improve the semi-empirical model of the oscillating jet, i.e., exponentially growing jet model, which has been studied in the field of musical acoustics, and introduce a polynomially growing jet model so as to apply Howe’s formula to it. It is found that the relative phase between the acoustic oscillation and jet oscillation, which changes with the distance from the flue exit, determines the quantity of the energy transfer between the two fields. The acoustic energy is mainly generated in the downstream ar...

Numerical study of the influence of the mouth-flue-foot geometry on sounding mechanism of an “air-jet” instrument model

The geometry of flue and foot influences the stability of jet oscillation, which products aerodynamic sound as the sound source of “air-jet instruments. Thus, we numerically study the influence of mouth-flue-foot geometry on the stability of jet oscillation with compressible LES. We introduce a 2D flue organ pipe model terminated with a closed end. We investigate the influence of the length of the flue, the existence of chamfers at the flue exit and the volume of the foot. As a result, eliminating the chamfer makes the jet oscillation unstable, while adding the chamfer stabilizes the jet oscillation and sound generation. A long flue makes the jet motion stable and robust, so that it spends a long time to reach a stable oscillation and cannot response quickly to the change of the air supply. Thus a relatively short flue with the chamfers is suitable for a performance. This result qualitatively agrees with the experimental result reported by Segoufin et al.. We also found that the existence of foot stabiliz...

Effective Calculation with Halo communication using Halo Functions

The issue of halo communication is the decrease of parallel scalability. To overcome the issues, we have introduced Halo thread to our simulation code. However, we have not solved the issue basically in the strong scaling. In this study, we have developed the Halo functions which perform the halo communication effectively. Then we can perform the calculation and communication in a pipeline and obtained good performance.

Acoustic energy generation of “air-jet” instruments: Energy transfer between jet oscillation and acoustic field

In this talk, we discuss how to estimate the acoustic energy generation of “air-jet” instruments with numerical simulation. To attack this problem, we use Howe’s energy corollary, with which we can estimate energy transfer between unsteady flow, i.e., oscillating jet and acoustic field. To calculate Howe’s formula, we need solenoidal velocity of the flow and its vorticity together with acoustic particle velocity separated from the whole velocity of compressible fluid. Recently, a method, which allows us to approximately calculate Howe’s formula, was developed in experiments by Bamberger and Yoshikawa et al., and it can be applied for the numerical calculation. We apply the method for the numerical calculation of a flue organ pipe model. We also introduce a toy model of the oscillating jet to investigate the mechanism of sound generation from the oscillating jet in detail. The acoustic energy is mainly generated in the downstream of the oscillating jet near the edge of the mouth opening, but it is consumed...

Effect of fluid viscosity on surface patterns formed by gravitational instability

When a droplet of a higher-density solution (HDS) is placed on the top of a lower-density solution (LDS), the HDS on the surface of the LDS sinks due to gravitational instability. In the sinking process, the HDS draws a fractal pattern or a hole/cell pattern on the surface of the LDS. It is observed that the surface pattern is determined by an aspect ratio of the container and viscosity of the LDS. In the formation of the surface pattern, a time series of the HDS density is analyzed. It is found that the profile of the series for the fractal pattern is different from that for the hole/cell pattern. In order to clarify the difference, we propose a phenomenological model for the time series to obtain fitting functions for both patterns.

An Identity Parareal Method for Temporal Parallel Computations

A new simplified definition of time-domain parallelism is introduced for explicit time evolution calculations, and is implemented on parallel machines with bucket-brigade type communications. By the use of an identity operator instead of introducing an approximate solver, a recurrence formula for the parareal-in-time algorithm is much simplified. In spite of such a simple definition, it is applicable to many of explicit time-evolution calculations. In addition, this approach overcomes several drawbacks known in the original parareal-in-time method. In order to implement this algorithm on parallel machines, a parallel bucket-brigade interface is introduced, which reduces programming and tuning costs for complicated space-time parallel programs.