Yutaka Miyake's research while affiliated with Osaka University and other places

To investigate the role of large-scale structures in the layer away from the wall, DNSs (Direct Numerical Simulations) of turbulent channel flow having small size of the computational box are conducted ; i. e., the size of spanwise direction is set narrower than the half channel width to prevent the formation of streamwise roll-mode structures : the axis of rollers are directed to the nearly streamwise direction and their diameter is the order of channel half width. It is turned out that the mean streamwise velocity remarkably increases in the region away from the wall, while the nearwall turbulence retains their universal property independent of the modulation of large-scale flow. POD (Proper Orthogonal Decomposition) method is applied to extract the large-scale flow. Although the large-scale structures are drastically altered from streamwise roll mode to spanwise one, the large-scale flow, regardless of the type of rotation mode, plays a crucially important role in the distribution of turbulent shear stresses and the formation of coherent elementary vortices.

Turbulent channel flows where there are small two-dimensional rods of square cross-section regularly arranged along one wall, normal to the average flow, are simulated without using any model. The other wall is assumed to be flat because of the limitations on computer memory. Sparsely distributed ribs modulate the flow near the wall substantially, but when the ribs are densely distributed the property of flat-wall flow is retained to a large extent. The scales of time and length used to normalize near-wall turbulence are normally wall scales based on total wall drag and viscosity. However, it is thought that these scales may not be adequate, in view of the modification of the turbulence generation mechanism. On the other hand, the flow in the layer away from the wall has been confirmed to be little affected by roughness, and scaled by total wall drag and the distance normalized by the channel half-width. It has also been confirmed that the property of turbulence away from the wall in the case of a flat wall comes closer to that of the wall in rough-wall flow. In addition, the growth process of hairpin vortices is only slightly modified by wall roughness.

Turbulent channel flows having small two-dimensional rods of square cross section that are regularly arranged on one wall normal to the mean flow, are simulated without using any model. The other wall is assumed to be smooth to meet the limit of computer memory. Sparsely distributed ribs modulate the flow near the wall strongly but densely distributed ribs retain property of smooth-wall flow largely. Scales of time and length to normalize near-wall turbulence are normally wall scales based on total wall drag and viscosity. However these scales are found to be not necessarily adequate, in view of modification of generation mechanism of turbulence. On the other hand, the flow in the layer away from the wall has been confirmed to be little affected by roughness and scaled by total wall drag and the distance normalized by channel half width. It has been confirmed also that the property of turbulence away from the wall in smooth-wall case comes closer to the wall in rough wall flow. Formation process of hair-pin vortices is little modified by wall roughness as well.

Comparatively high Reynolds number channel flows up to Retau=640 are simulated by DNS. Noticeable property of the high Reynolds number flow is large-scale streaks which are larger not only in scale but also in their spanwise separation than those well-known in the buffer layer. It is revealed that the low-speed streaks are the region of high turbulent activity and accordingly of high turbulent shear stress with densely populated elementary vortices. These properties are little influenced by wall condition, such as drag controlled or not. LSE (Linear Stochastic Estimation) technique is applied to extract elementary vortex pairs. Time evolution of these vortices submerged in a laminar flow but having velocity distribution of turbulent one reveals the process of generation of large-scale streaks and accompanying densely populated elementary vortices. The formation of large-scale streaks by the model simulation is also little influenced by the wall condition and this modeled process is expected to mimic that taking place in the near-wall layer of practical channel flow. The existence of the low-speed streaks itself is the key for the formation of the structure, thus suggesting the self-sustenance of turbulence by both large-scale streaks and vortices concentrated area.

In order to investigate the diversity of the development of near-field of a round jet found in experiments and in numerical simulations by different authors, we applied a global instability analysis to a secondary instability of the near-field region of a round jet. The secondary instability takes place in a braid region between neighbouring vortex rings after the primary instability generating vortex rings, and streamwise vortices due to the secondary instability is a key issue in a decay process of a spatially developing jet. They consist of various modes with a non-uniform concentration in the azimuthal direction in a spatially devoloping jet. The analysis demonstrates that the small difference in receptivity of each unstable mode causes locally different speed of the growth of disturbances. This non-uniform receptivity in the secondary instability causes diverse process of decay of vortex rings in spatially developing jets.

A Direct numerical simulation(DNS) is conducted for a turbulent channel flow having a rough wall using spectral method. On the rough wall, rib elements are placed for spanwise direction with some extent streamwisely, which are realized by virtual forcing to satisfy adhesive condition on the rib's surface, based on the numerical treatment proposed by D.Goldstein et al. (1994). It is confirmed that the flow in the layer away from the wall is not so much affected by roughness. On the contrary, in near wall region sparcely distributed ribs strongly modulates the flow, but densely distributed one retains property of smooth-wall flow to some extent.

Turbulence modulation in particle-laden flow, especially the influence of vortex shedding, was investigated by means of the direct numerical simulation. To this end, we developed a finite-difference scheme to resolve the flow around each particle moving in turbulence. The method was applied to the flow around a sphere and the accuracy was confirmed up to the Reynolds number range with vortex shedding. The agreement between our 4th-order central finite-difference method and spectral method for turbulent channel flow without particles was also fine. Then, we simulated upward flow in a vertical channel including solid particles. The velocity and vorticity fluctuations as well as Reynolds shear stress were strongly affected by wakes from particles. The shed vortices were elongated in the mainstream direction by the velocity gradient and resulted in the hairpin vortices. They increased the energy production rate in couple with production due to particle-turbulence correlation.

Heat transfer in a channel of rough wall is simulated in this work. Two kinds of roughness are considered, i.e., one assumes sand-grain roughness based on roughness model and the other regularly arranged two-dimensional ribs. No models other than sand-grain roughness implemented in the simulation. For the sake of computational load, one wall is assumed to be smooth and the global Reynolds number Reτ based on mean friction velocity and half channel width is 150. Mean flow property of velocity and thermal fields are found to be little influenced by the property of roughness elements but depends on total drag, except for in the layer close to the wall where direct interference with roughness elements manifests itself. Similarity of thermal field with velocity one is confirmed for the low Prandtl number flows considered in the work. The mixing is found to be controlled by large scale motion which is inherent to the logarithmic layer, getting closer to the wall.

Our objective is to identify the key element of sound source in a turbulent flow near a wall and to realize the noise reduction emitted from wall turbulence. The acoustic field is simulated numerically by DNS (Direct Numerical Simulation) basically based on Hardin & Pope scheme. Acoustic field of coarse spatial resolution is calculated emigrating necessary data from the flow in the source field which requires fine spatial resolution. The source field considered in this simulation is of low Mach number flow, in which compressibility effect is neglected. Periodic condition is found to be inappropriate in simulating acoustic field, interference of sound waves being too serious in contaminating the field. It is revealed that coherent structure, particularly head of hairpin vortex is responsible for turbulent noise. This is confirmed by a numerical experiment documenting evolution of a small single vortex pair which grows to a hairpin vortex. These simulations give the global-and micro-mechanisms of sound generation in wall turbulence in detail. © 2001 by the American Institute of Aeronautics and Astronautics, Inc. All right reserved.