Shiki Okamoto

Vortex shedding from a circular cylinder of finite length placed on a ground plane

This paper describes a study of changes in the vortex formation and the turbulent wake from a circular cylinder with a finite aspect ratio, placed on a ground plane. The experiment was carried out in an N.P.L. blow down type wind-tunnel, with a working section of 500 mm × 500 mm × 2,000 mm, and between the Reynolds number 2.5 × 104 and 4.7 × 104 . The surface-pressure distributions on the circular cylinder were measured and the drag coefficient was determined from these measurements. Vortices of two kinds generated in the flow-field around the cylinder were observed. The power spectrum, auto-correlation, space-correlation, velocity defects, and turbulent intensities in the turbulent wake behind a circular cylinder were also measured. It was found that the flow pattern changed rapidly above aspect ratio H/D = 4, with vortex shedding changing from symmetric “arch” type to antisymmetric “Karman” type.

Turbulent shear flow and heat transfer over the repeated two-dimensional square ribs on ground plane

This paper describes the flow structure over the repeated two-dimensional square ribs of side length D, placed at a pitch S on a ground plane. The value of S/D which most augments the turbulence of the free stream and, hence the heat transfer is calculated. The region of interest in this investigation is far downstream where the velocity and temperature distributions follow similarity rules. The time-mean velocity, static pressure, and the velocity vectors were measured by Pitot- and static pressure tubes and a three hole cylindrical yawmeter. The turbulence intensities and integral scale were obtained using a hot wire anemometer. The mean temperature distribution was measured by thermocouples and the local heat transfer coefficient was then calculated. It is found that at S/D = 9 the turbulence intensity is maximized. As a result of this effect and the fact that for S/D = 9 the flow reattaches within a groove, the heat transfer is also maximized. The measurements show how the location of reattachment depends on S/D and that high local heat transfer coefficient coincides with the reattachment point. The average heat transfer coefficient and the pressure drop correlation is quantified.

Turbulent near-wakes of periodic array of square blocks on a plate

Abstract This paper describes the experimental study of the flow structure past square blocks located on a ground plate with various spacings. The experiment was carried out in a circuit type wind tunnel of a 200 × 200 mm working section and 2000 mm in length at a Reynolds number of 990, which is based on the free-stream velocity and the height of block. The test blocks, side length D = 23 mm and height H = 5 mm, were aligned at a regular spacing S in a square array on the lower wall of the test section, and the spacing was systematically varied to provide S H = 2, 3, 5, 7, 10, and 13 . The mean velocity and turbulence intensities were measured by a laser Doppler velocimeter (LDV). It was found that the spacing ratio S H = 7 is optimum to augment the turbulence intensity near the ground plate when the repeated blocks are aligned in a square array.

Turbulent shear flow behind a single row of bluff obstacles placed on plane boundary

Abstract This paper presents an experimental investigation concerned with methods of artificially thickening turbulent boundary layers on a plane surface by using a row of several bluff obstacles. The thickness of this shear layer, the velocity profile and the turbulence intensity of the shear flow behind a row of obstacles were measured and compared with those of a naturally developed turbulent boundary layer. Experiments showed that the shear layer is significantly thicker than the naturally developed turbulent boundary layer, and the velocity profile and the turbulence intensity profiles for the case of a row of cones and triangular plates with vertical angle 2α = 30° are nearly the same as those of a naturally developed turbulent boundary layer.

Flow around Perforated Plate Placed Perpendicularly on Ground Plane

This paper describes the flow past a two-dimensional perforated plate placed on a ground plane. The experiment was carried out in the 20 cm×20cm circuit-type wind-tunnel having the working section of 2 m length at Reynolds number of 1.32×104. The time mean velocity and turbulence intensity in the flow field behind the perforated plate were measured by use of laser Doppler velocimeter. The results were discussed for the various values of porosity in comparison with those of the existing investigations and the calculated value of the velocity in the mixing zone for Gortler. The flow pattern and vortices were observed by flow-visualization. Consequently, the windbreak is effective for the perforated plate of porosity ratio λ=0.238.

Flow around Three-Dimensional Rectangular Cavity.

In this paper we describe the flow associated with three-dimensional surface-mounted rectangular cavities. The experiment was carried out in two types wind tunnel and water channel. A significant aspect of the study has been an examination of the effects of changing the flow incidence angle from 0 degree to the normal from the cavity major axis to 90 degrees. The cavities investigated have had sectional geometries with width/depth of 1.0 and planform aspect ratio of length/width equal to 9.5. The time mean surface pressure distributions, mean velocity inside cavities, and mean velocity and turbulent intensity in the immediate wake of the cut-out were measured as the flow incidence angle was varied. The flow pattern inside cavities was observed by flow-visualization. Consequently it was found that (i) the greatest disturbance to the cavity flow occurs when the flow incidence angle is approximately 75 degrees to the longest cavity axis, and (ii) this is reflected in a large asymmetric wake associated with an increase in cavity drag.

Behavior of Secondary Flow in Turbulent Boundary Layer along Rows of Streamwise Square Ribs on Plane Surface.

This paper presents an experimental investigation of the turbulent boundary layer flow along rows of streamwise square ribs on a plane surface for various spacings of rows. The spacing between the centers of two adjoining square ribs was varied at S/D=2, 3, 4, 5, 7 and ∞, i.e., a single square rib. The time-mean velocity was measured by the Pitot and static pressure tubes. The turbulence intensities and auto-correlation were obtained using the data processing system and the F.F.T. analyzer connected to a hot wire anemometer. The Velocity vectors of secondary flow were measured by LDV. As a main result, the secondary flow becomes offset as the spacing between two ribs becomes small, since the directions of the secondary flow near the edges between the marked rib and the adjoining rib are opposite

Effect of spacing ratio on flow over repeated two-dimensional square ribs on ground plane

This paper describes a detailed study of the flow structure over repeated two-dimensional square ribs on a ground plane for various values of S/D, and the optimum value of S/D to augment the turbulence intensity near the ground plane. The pitch between the centers of two adjoining square ribs was varied at S/D=2, 3, 5, 7, 9, 11, 13 and 17. The time-mean velocity and turbulence intensity were measured by Laser Doppler Velocimeter. The static pressure was measured by Pitot static tubes. As a result, it is concluded that the pitch ratio S/D=9 is optimum to augment the turbulence intensity in the groove between two adjoining square ribs, when the two-dimensional square ribs are aligned on the wall as a heat transfer promoter.

Effect of Rib Shape on Flow Property and Heat Transfer over Rows of Two-Dimensional Ribs on Ground Plane.

This paper describes the comparison of the flow structure over the rows of two-dimensional ribs of various rib shapes on the ground plane. The pitch between the centers of two adjoining ribs was varied for several rib shapes. Six two-dimensional rib shapes were used in this experiment. The time-mean velocity and turbulence intensity were measured by laser Doppler velocimetry. The auto-correlation was obtained using a data processing system and Fast Fourier Transform analyzer connected to a hot-wire anemometer. The mean temperature was measured with thermocouples and the local heat transfer coefficient was obtained from these data. The turbulence intensity and heat transfer coefficient become largest and the pressure loss is minimum for the semi-elliptic ribs, among all the rib shapes used in this experiment. As a result it is concluded that semi-elliptic ribs aligned at pitch ratio S/D=7 have the optimum shape to promote heat transfer.

Flow in Region between Two Perforated Plates Placed Perpendicularly on a Ground Plane.

This paper describes the flow past two perforated plates placed perpendicularly on a ground plane. The experiment was carried out in the N. P. L.-type wind tunnel with a 40cm×40cm working section of 2m length at the Reynolds number of 6.32×104. The distribution of velocity in the flow field, the recirculation region between the two perforated plates, and the surface pressure on the ground plane were measured. The results were discussed for various values of porosity and spacing between the two perforated plates. As a result, it was found that the windbreak is practically effective for the perforated plate of porosity ratio λ=0.3.