Flows in the atmosphere surrounding us especially attract social concern as a phenomenon; weather forecast, air pollution, global warming. Recent advances in computer performance enable us to perform turbulence simulations based on fundamental physical principles, that is, the direct numerical simulation (DNS) has attracted attention as a powerful tool to study the turbulent flow.
In our laboratory, the numerical simulations of the turbulent flows through DNS are performed using the large parallel vector computer. One of the simulations has achieved the highest Reynolds number turbulent flow in the world.
Turbulence in a channel flow
With an aid of the recent development of the high speed computers, the direct numerical simulation(DNS, hereafter) plays a prominent role in the research of the turbulence and turbulent heat transfer. A large number of DNSs are performed nowadays for various configurations and parameters. Among them, the DNS of the turbulent flow and heat transfer in a channel are of special importance because of its fundamental nature in understanding the convective heat transfer between the fluid and solid wall.
As is known well, the turbulent heat transfer is governed by the Reynolds and Prandtl numbers. In addition, the boundary conditions for the flow and temperature fields also affect the turbulent heat transfer. In spite of its geometrical simplicity, the combination of these factors causes a large variation in the turbulent heat transfer of the channel flow.
The turbulent channel flow can be categorized into Poiseuille(PF) and Couette flows(CF).
The investigation of our laboratory is to study these DNSs of the turbulent heat transfer for both PF and CF with two thermal boundary conditions; that is,
- Poiseuille flow with Uniform heat flux heating
- Poiseuille flow with Constant wall temperature difference
- Couette flow with Uniform heat flux heating
- Couette flow with Constant wall temperature difference
The present databases are made available at this web site.
Turbulent Ekman boundary layer
In this flow field, 3-dimensional velocity profile is observed.
The angle between the geostrophic wind and the mean velocity at the ground is 45 degree in the case of laminar Ekman layer.
In the turbulent layer, the angle decreases because of the enhanced vertical momentum transfer.
Therefore combination of the sweep and ejection has great influence on the flow direction in the vicinity of the ground.
The relation between the flow direction and the vertical velocity fluctuation is discussed quantitatively using the sampling average method.
The horizontal directions of the mean velocity, the Reynolds stress and the turbulent heat flux are compared.
It is found that the horizontal turbulent heat flux is not aligned with the mean velocity and that it represents a similar profile to the Reynolds stress.
The similarity and difference of these double correlation vectors are shown in detail.
The effects of the stable stratification upon the direction of the mean velocity,
the turbulent heat flux and the Reynolds stress are also discussed based on the obtained DNS data.
Under a microgravity in space, we encounter various strange phenomena which cannot be seen in the normal gravity. The Marangoni convection is the flow induced by the surface tension difference due to the temperature gradient over a fluid surface. It becomes predominant in the microgravity because of the absence of the buoyant force. In our laboratory, various experiments and numerical simulations on the thermocapillary convection are being performed.
Large amount of experimental and numerical works have been made on thermocapillary convection in a Half-Zone liquid bridge. It is widely known that the flow field in the liquid bridge exhibits a transition from two-dimensional steady to three-dimensional oscillatory flow at a critical temperature difference. Several types of induced flows are characterized into several regimes mainly according to the suspended particle motion in the liquid bridge and the surface temperature variation. Chaotic and turbulent flows are observed far beyond the critical condition.
PAS: Particle Accumulation Structures
A large amount of experimental and numerical works have been made on thermocapillary convection in a Half-Zone liquid bridge. In these experiments, tracer particles have been used to visualize the flow field. With increasing temperature-difference between the rods sustaining the liquid bridge, the flow exhibited transition from steady to oscillatory convection. Several types of induced flows are characterized mainly by the suspended particle motion in the bridge and the surface temperature variation. Special attention has been paid to the distribution of the dispersed particles in this study. Particles are found to accumulate in some regimes of the flow. This is called Particle Accumulation Structure (PAS). We are investigating the PAS phenomenon, both experimentally and numerically.
PTV: Particle Tracking Velocimetry
The present study aims to reconstruct the three dimensional velocity field of the thermocapillary convection in a small-size half-zone liquid bridge. By placing a cubic beam splitter above a transparent top rod, simultaneous observation of the flow field by use of two CCD cameras is realized. The particles are traced in the successive captured 2D images by applying the triple pattern matching algorithm. The 3-D structures of the steady and oscillatory flows are reconstructed successfully.
The experiments of thermocapillary convection in a half-zone liquid bridge are usually for the purpose of analyzing the structures, critical points. We are performing experiments aiming to control the 3D oscillatory thermocapillary convection.
Simple feedback control is applied to the oscillatory flow. As the result, good suppression of the oscillatory flow to the 2D steady flow on the high Marangoni number has achieved.
Using the computer simulation, the temperature and velocity field under the control is also being analyzed.
Dynamic free surface deformation
A large number of researches have been conducted on the transition of the flow field. Physical mechanism of the transition, however, has not been fully understood. Some experiments indicated that the free surface vibration is observed on the oscillatory flow. We are carrying out three-dimensional numerical simulation taking with the dynamic free surface deformation into account to evaluate the effect of the surface deformation upon the flow field.
It is known that the onset of oscillations of thermocapillary convection in cylindrical liquid bridge is influenced by heat transfer from free surface. In order to understand the influence of heat loss in detail, we visualize not only thermocapillary convection in liquid bridge but also surrounding air motion experimentally. In addition, the air motion is simulated numerically to compute the heat transfer rate from the liquid free surface to the environment.
The present study aims to grasp a relation between the hydrothermal wave (HW) in a thin liquid layer confined in a finite region and the oscillatory flow field in a half-zone liquid bridge. The front of the HW is bent near the rigid hot wall, because there exists sharp temperature gradient due to the existence of the rigid wall and a rolling up of a cold fluid returning from the bottom surface. The traveling wave of the surface temperature over the oscillatory thermocapillary flow in the half-zone liquid bridge can be regarded as the bent HW, because the length of the liquid bridge is usually the same order as that of the bent region.
The instability of Marangoni convection has been
studied as the problem of material processing under a microgravity.
It is further considered that a process of solidification determine
the final character of material.
Accordingly, we try to observe the growth of dendrite structure by instability
of liquid-solid interface, and to investigate the effect of
a convection upon the crystal growth through numerical simulation and experiments.
In a micro gravity environment, natural convection is reduced, on the other hand, influences of minor forces on the ground are relatively enhanced. We had expected to produce ideal material, but the perfect null-gravity condition cannot be achieved in an orbiting spacecraft or in the space station. Actually there exists a residual acceleration in quasi-steady, oscillatory, and transient states. The quasi-steady accellation is caused by atmospheric drag and Keplerian effects. The oscillatory one arises from various natural frequencies of the spacecraft structure. Internal disturbances are raised primarily by crew activities, and external ones by thruster fringes. The transient accellation is caused by orbital control, docking, and others. We try to analyse their effects upon the natural convection through the numerical study.
Flow visualization analysis for artificial heart
Compact blood pumps have been developed as an implantable artificial heart. These pumps have many advantages to increase a quality of life of patients. In the pump, we must prevent both low shear region like stagnation and high shear region like separation, because they cause thrombus and hemolysis. These regions can be estimated by measuring the velocity gradient in the flow field. In this laboratory, we do the experiments using PIV(Particle Image Velocimetry) to analyze these phenomena from hydrodynamic viewpoint.
For promoting the accuracy of ultra-precision machine tool with nano-meter order precision, the temperature regulation of machine tool within 100mK is required. As a new coolant for regulating the temperature, we are testing clathrate hydrate with Tetrabutylammonium fluoride(TBAF) and water.This clathrate hydrate is composed of the micro crystals of 100$B&L(Bm order, which is flowable has a meeting point at the room temperature of about 25$B!n(B. The methods regulating a temperature with the latent heat of this clathrate hydrate have been investigated and 40% density clathrate was experimentally demonstrated to be able to control the temperature within 100mK.
Feeding behavior of newborn infants
Recently we recognize an importance of nursing with breast feeding again. Especially nipple confusion causes an favourable influence upon sucking of infants. It is desirable for mothers to nurse their babies only with the breast feeding; but they must use the bottle feeding together in the cases when infants stay at a NICU(Neonatal Intensive Care Unit) or their mother return early to their work. The existing breast feeding and the bottle feeding are not suitable to some cases such as Down's syndrome and cleft lip and palate. Development of artificial nipples for various needs is a great importance for nursing infants. In this study, we investigates the sucking behavior of different babies to examine their sucking performance.