Takehiro Kushida

Simultaneous measurements of concentration and velocity in a CO2 jet issuing into a grid turbulence by two-sensor hot-wire probe

A new simple method has been developed to measure the concentration and velocity simultaneously for the gas mixture (CO2–air) flow by two hot-wire probes. By using a calibration map, the values of the instantaneous concentration and velocity can be determined from the voltages of two hot-wire probes with different overheat ratios (OHR) by digital data processing. This method does not require subtle adjustment of the OHR to achieve insensitivity to the CO2 concentration, as in the past work by Chassaing (Melange turbulent de gaz inertes dans un jet tube libre, These Doc. Sciences 42, Inst. Nat. Poly. de Toulouse, 1979). The measurements have been performed in a CO2 jet in grid turbulence. The experimental results for the centerline and radial distributions of the mean and r.m.s. values of mass fraction concentration and velocity, and axial scalar flux are consistent with previous data.

An Experimental Study on the Development of Coherent Structure in a Plane Jet (2nd Report, Spatio-temporal Velocity Structure by the Combined Analysis of KL Expnasion and Fourier Transform)

The simulaneous measurement of the velocities at two points with X-type hot wire probes has been performed in three different downstream regions of a plane jet (i.e., the potential core region, the interaction region and the self-preserving region). Mainly by the combined analysis of KL expansion and Fourier transform, the structure development of the plane jet is investigated from a viewpoint of both space and frequency. From the downstream variation of the eigenfunctions, it is found that in the early stage of the interaction region the profiles of the 1st u- and v-POD mode in the low frequency range become self-similar, but in high frequency range these continue to change until the self-preserving region. The characteristics of coherent structure can be extracted efficiently by the spatial correlation profiles reconstructed from the 1st POD mode.

A Study on the Interaction between a Grid-generated Turbulence and a Circular Cylinder. Influence of the Main Stream Turbulence on the Wake-Induced Velocity Field and the Pressure Fluctuations on the Cylinder Surface.

The interaction between the grid-generated turbulent flow and a 2 dimensional circular cylinder is investigated experimentally. Particular attention has been devoted to the effect of the main stream turbulence (intensity and scale of the turbulence) on the wake-induced velocity field in the upstream region and the pressure fluctuation on the cylinder surface. The velocity in the upstream region and the pressure on the cylinder surface have been measured simultaneously with the velocity in the near wake, using the I-type hot wire probes and the pressure transducer. The velocity signals measured in the upstream region and the pressure signals on the cylinder surface are analyzed by the phase average technique on the basis of the reference band-passed velocity signals in the near wake. It is found that in the case that the intensity of main stream turbulence is large and the rate of the scale of turbulence to the cylinder diameter is small, the upstream region where the periodic velocity fluctuations appear becomes large. In this case, the fluctuation of the lift force is also large.

A Study on the Interaction between a Grid-Generated Turbulence and a Circular Cylinder. Phase Average Estimation of the Wake Induced Velocity in the Upstream Region of the Cylinder.

The interaction between the grid-generated turbulent flow and a 2-dimensional circular cylinder is investigated experimentally. Particular attention has been devoted to the effect of the velocity fluctuations induced by the cylinder wake vortices on the upstream turbulent field. The simultaneous velocity measurements at three different points around the cylinder (one in the upstream region and two in the near wake) have been performed by the I-type hot-wire probes. The velocity signals are analyzed by the phase average technique on the basis of the reference velocity signals in the near wake. It is found that the phase averaged velocity in the upstream region of the cylinder shows the large value when the vortices are growing up behind the cylinder. The fluctuation of the phase averaged velocity increases as the cylinder surface is approached and the largest value appears along the line of θ=90° near the cylinder surface. Some kinematic vortex-shedding model is suggested to estimate the intensity of the wake-induced velocity fluctuation. The spatial distribution of the calculated velocity r.m.s. fluctuation shows qualitatively good agreements with the experimental results.