Takehiko Segawa

Evidence against 'ultrahard' thermal turbulence at very high Rayleigh numbers

Several theories predict that a limiting and universal turbulent regime — ‘ultrahard’ turbulence — should occur at large Rayleigh numbers (Ra, the ratio between thermal driving and viscous dissipative forces) in Rayleigh–Bénard thermal convection in a closed, rigid-walled cell. In this regime, viscosity becomes negligible, gravitationally driven buoyant plumes transport the heat and the thermal boundary layer, where the temperature profile is linear, controls the rate of thermal transport. The ultrahard state is predicted to support more efficient thermal transport than ‘hard’ (fully developed) turbulence: transport efficiency in the ultrahard state grows as Ra1/2, as opposed to Ra2/7 in the hard state. The detection of a transition to the ultrahard state has been claimed in recent experiments using mercury and gaseous helium. Here we report experiments on Rayleigh–Bénard convection in mercury at high effective Rayleigh numbers, in which we see no evidence of a transition to an ultrahard state. Our results suggest that the limiting state of thermal turbulence at high Rayleigh numbers is ordinary hard turbulence.

Experimental study on drag-reducing channel flow with surfactant additives-Spatial structure of turbulence investigated by PIV system

Abstract The turbulent frictional drag of water can be reduced dramatically by adding small amounts of drag-reducing materials, such as polymers or surfactants. As a percentage drag reduction of 80% can easily be achieved, this technique is thought to be the most practical method of reducing turbulent frictional drag. In this work, a double pulse particle image velocimetry (PIV) system was used to clarify the spatial velocity distribution of surfactant solution flow in a two-dimensional channel. A type of cationic surfactant cetyltrimethyl ammonium chloride (C16H33N(CH3)3Cl) mixed with the same weight of counter-ion material NaSal (HOC6H4COONa) was used as a drag-reducing additive to water at a mass concentration of 40 ppm. Instantaneous velocity distribution taken by PIV was examined to clarify the effect of surfactant. It was found that the instantaneous velocity distribution taken in water flow exhibits penetration from the low-speed fluid region into the high-speed region, which is one of the important events of turbulence energy production and turbulent mixing. Although this structure is commonly observed in water flow, it was not found in drag-reducing flow under the same Reynolds number. The strong vorticity fluctuation near the wall also disappeared and the integral length scale in streamwise direction of turbulent fluctuation had a smaller value in surfactant solution flow.

Reynolds-number dependence of turbulence structures in a drag-reducing surfactant solution channel flow investigated by particle image velocimetry

The Reynolds-number dependence of turbulence structures in a drag-reducing surfactant solution flow is attributed to the Reynolds-number effect on rheology of the solution flowing in the passage and the rheological properties are associated with the shear-induced structure (SIS) formed in a sheared surfactant solution. Through analysis of turbulence statistics of the two-dimensional velocity field, measured by particle image velocimetry, for a drag-reducing flow with addition of cetyltrimethyl ammonium chloride (CTAC) in different flow regimes characterized by the drag-reducing effectiveness dependent on the Reynolds number, we studied the characteristics of turbulence structures for different Reynolds-number-dependent flow states and indirectly revealed the dynamic processes of SIS across the flow passage. A 25ppm (ppm—parts per million) CTAC solution at 30°C was tested. Based on the relationship between the Reynolds number and the drag-reduction levels, the CTAC solution flow was categorized into four r...

Jet flow induced by a surface plasma actuator

The induced flow field in the vicinity of a single, symmetric, AC plasma actuator has been studied in initially static air at atmospheric pressure. Hot-wire and cold-wire anemometry, together with flow visualization, were used to observe the temporal and spatial structure of the induced flow. The plasma discharge is pulsed on a millisecond scale and the flow forms a series of pulsed wall jets, which can be maintained indefinitely, similar to a synthetic jet. It was observed that the plasma actuator initiates a pair of vortices at the instant of plasma creation, moving at 25° to the surface. After an initiation period, the plasma develops into a laminar wall jet. Thermal imagery has been used to estimate the surface temperature of the dielectric sheet during plasma operation.

Effects of Input Voltage on Flow Separation Control for Low-Pressure Turbine at Low Reynolds Number by Plasma Actuators

Active flow control using dielectric barrier discharge (DBD) plasma actuators was investigated to reattach the simulated boundary layer separation on the suction surface of a turbine blade at low Reynolds number, Re = 1.7 × 104. The flow separation is induced on a curved plate installed in the test section of a low-speed wind tunnel. Particle image velocimetry (PIV) was used to obtain instantaneous and time-averaged two-dimensional velocity measurements. The amplitude of input voltage for the plasma actuator was varied from ±2.0 kV to ±2.8 kV. The separated flow reattached on the curved wall when the input voltage was ±2.4 kV and above. The displacement thickness of the boundary layer near the trailing edge decreased by 20% at ±2.0 kV. The displacement thickness was suddenly reduced as much as 56% at ±2.2 kV, and it was reduced gradually from ±2.4 kV to ±2.8 kV (77% reduction). The total pressure loss coefficient, estimated from the boundary layer displacement thickness and momentum thickness, was 0.172 at the baseline (actuator off) condition. The total pressure loss was reduced to 0.107 (38% reduction) at ±2.2 kV and 0.078 (55% reduction) at ±2.8 kV.

Active control of streak structures in wall turbulence using an actuator array producing inclined wavy disturbances

By using a piezo-ceramic actuator array, turbulent flow control in a two-dimensional water channel was attempted at the flow Reynolds number of 7500. Eight piezo-ceramic actuators were installed sp...

Wave-turbulence interaction of a low-speed plane liquid wall-jet investigated by particle image velocimetry

The surface-wave amplitude (free-surface level) and the turbulent velocity field of the liquid phase of a plane wall-jet flow have been simultaneously measured by means of particle image velocimetry, which allows for the investigation of surface waves and wave-turbulence interaction. The Reynolds number, Weber number, and Ohnesorge number of the tested flow, based on the bulk velocity, height of the closed channel, and physical properties of water, were 3.6×104, 1.2×103, and 9.5×10−4, respectively. Based on the measured datasets of the velocity field and free-surface level, the characteristics of the wave-turbulence interaction as well as the statistics of surface waves and turbulent velocity field were studied. The characteristics of the turbulent velocity field near the wavy free surface obtained in this study were different from those obtained previously for open-channel flows with no shear at the interface or negligible deformation of the free surface. It was found that reverse vortex motion was predo...

Feedback Control of Flow Separation on NACA0024 Airfoil under Periodic Wall Oscillation by Means of DBD Plasma Actuator and FBG Sensor

Feedback control of periodic flow separation on a NACA0024 airfoil has been investigated by using dielectric barrier discharge (DBD) plasma actuators and fiber Bragg grating (FBG) sensors under oscillation of the test section wall. A tangential jet is generated from sheet-type DBD plasma actuators placed at the leading edge of the airfoil. An FBG flow sensor is mounted at the root of a cantilever beam, which detects the vibrations of the cantilever tip by wake flow around the trailing edge. Flow separation can be detected by the standard deviation of Bragg wavelength (B’). A demonstration of flow separation control was conducted successfully under periodic flow separation by a feedback algorithm using a threshold level of B’ = 0.006 at Re = 5.3×10 4 and showed that the flow separation can be mitigated by more than 20%. In addition, string-type DBD plasma actuators were developed to apply DBD plasma actuators on metallic and three dimensional surfaces and their induced flow properties were investigated by PIV.

Wall Normal Jet Produced by DBD Plasma Actuator With Doughnut-Shaped Electrode

Properties of coaxial annular jets produced by a dielectric barrier discharge (DBD) plasma actuator with a doughnut shaped electrodes were investigated under atmospheric pressure and room temperature. The actuator consists of two circular electrodes sandwiching a thin dielectric layer. By applying 0 – ±3.3 kV between the electrodes at radio frequencies, the plasma jet is formed near the inner edge of the top electrode. The radial jet runs toward the center of the electrode and then impinges at the center to generate a wall normal annular jet. The evolution of the wall normal jet was observed precisely using particle image velocimetry (PIV) system. It was found that characteristic velocities increase in proportion to the bursting frequency and inversely proportional to the inner diameter of the electrode at the surging time of the voltage at 5.0 × 10−6 sec.Copyright

Influence of drag-reducing surfactant additives on vortex structures and turbulent events in a channel flow

The characteristics of vortex structures and turbulent events of drag-reducing surfactant (CTAC) solution flows in a two-dimensional channel have been studied using particle image velocimetry (PIV) to measure the instantaneous velocity fields in a streamwise-wall-normal plane. Through visualizing the instantaneous velocity field, contour maps of the swirling strength and instantaneous value of uv, the characteristic angle of vortex packets was quantified, and it was shown that the drag-reducing CTAC additive reduced both the strength and frequency of turbulent bursts near the wall, and that the characteristics of vortex structures and bursts were not only dependent on drag-reduction level but also on concentration of additive. Based on the quantified parameters characterizing turbulent events in a wall-bounded turbulent flow, a model of turbulent contribution to the friction factor, fT , was proposed. It was obtained that fT was linearly proportional to the product of frequency and strength of turbulent events.Copyright