Mikio Arie

Vortex shedding from a rectangular prism and a circular cylinder placed vertically in a turbulent boundary layer

Measurements of the vortex-shedding frequency behind a vertical rectangular prism and a vertical circular cylinder attached to a plane wall are correlated with the characteristics of the smooth-wall turbulent boundary layer in which they are immersed. Experimental data were collected to investigate the effects of (i) the aspect ratio of these bodies and (ii) the boundary-layer characteristics on the vortex-shedding frequency. The Strouhal number for the rectangular prism and the circular cylinder, defined by S = f c w / U 0 and f c d / U 0 respectively, was found to be expressed by a power function of the aspect ratio h / w (or h / d ). Here f c is the vortex-shedding frequency, U 0 is the free-stream velocity, h is the height, w is the width and d is the diameter. As the aspect ratio is reduced, the type of vortex shedding behind each of the two bodies was found to change from the Karman-type vortex to the arch-type vortex at the aspect ratio of 2·0 for the rectangular prism and 2·5 for the circular cylinder.

Experiments on two-dimensional flow over a normal wall

Measurements of the velocity, pressure, and turbulence behind a series of normal plates in the uniform test section of an air tunnel are described, the oscillation of the wake being prevented in all but one test through use of symmetrically located tail plates. By a combination of experimental and computational techniques, details of the pattern of flow over a wall on a plane boundary in an infinite fluid are closely approximated. A significant difference is indicated between the characteristics of such a flow and those of the flow past an isolated plate with oscillating wake.

Discrete-vortex simulation of a turbulent separation bubble

The discrete-vortex model is applied to simulate the separation bubble over a two- dimensional blunt flat plate with finite thickness and right-angled corners, which is aligned parallel to a uniform approaching stream. This flow situation is chosen because, unlike most previous applications of the model, the separation bubble is supposed to be strongly affected by a nearby solid surface. The major objective of this paper is to examine to what extent the discrete-vortex model is effective for such a flow. A simple procedure is employed to represent the effect of viscosity near the solid surface; in particular, the no-slip condition on the solid surface. A reduction in the circulation of elemental vortices is introduced as a function of their ages in order to represent the three-dimensional deformation of vortex filaments, An experiment was also performed for comparison purposes. The calculation yielded reasonable predictions of the time-mean and r.m.s. values of the velocity and the surface-pressure fluctuations, together with correlations between their fluctuating components, over most of the separation bubble. The interrelation between instantaneous spatial variations of the surface-pressure and velocity fluctuations were also obtained. A comparison between the calculated and measured results suggests that, in the real flow, the three-dimensional deformation of vortex filaments will become more and more dominant as the reattachment point is approached.

Vortex shedding from a circular cylinder in moderate-Reynolds-number shear flow

The frequency of vortex shedding from a circular cylinder in a uniform shear flow and the flow patterns around it were experimentally investigated. The Reynolds number Re , which was defined in terms of the cylinder diameter and the approaching velocity at its centre, ranged from 35 to 1500. The shear parameter, which is the transverse velocity gradient of the shear flow non-dimensionalized by the above two quantities, was varied from 0 to 0·25. The critical Reynolds number beyond which vortex shedding from the cylinder occurred was found to be higher than that for a uniform stream and increased approximately linearly with increasing shear parameter when it was larger than about 0·06. In the Reynolds-number range 43 Re Re

A contribution to an inviscid vortex-shedding model for an inclined flat plate in uniform flow

Unsteady separated flow behind an inclined flat plate is numerically studied through the use of the discrete-vortex approximation, in which the shear layers emanating from the edges of the plate are represented by an array of discrete vortices introduced into the flow field at appropriate time intervals at some fixed points near the edges of the plate. The strengths of the nascent vortices are chosen so as to satisfy the Kutta condition at the edges of the plate. Numerical calculations are performed for a plate at 60° incidence impulsively started from rest in an otherwise stationary incompressible fluid, by systematically changing the distance between the location of the nascent vortices and the edges of the plate. The temporal changes in the drag force, the rate of vorticity transport at both edges of the plate and the velocity of the separated shear layers are given together with the flow patterns behind the plate on the basis of this model. The results of the computation show that the vortex street behind the plate inclines as a whole towards the direction of the time-averaged lift force exerted on the plate. It is also predicted from the calculations that the vortex shedding at one edge of the plate will not occur at the mid-interval of the successive vortex shedding at the other edge. The predicted flow patterns are not inconsistent with a few experimental observations based on the flow-visualization technique.

A contribution to the free-stream turbulence effect on the flow past a circular cylinder

The effect of the free-stream turbulence on the flow past a circular cylinder was studied experimentally in the subcritical and critical regimes. Several grids were used to produce approximately homogeneous turbulent fields with longitudinal integral scales ranging from 0·30 to 3·65 cylinder diameters and with the longitudinal intensities ranging from 1·4 to 18·5%. The critical Reynolds number R c at which the time-mean drag coefficient obtains the value of 0·8 was found to satisfy the relation R c 1·34 T = 1·98 × 10 5 , where T is the Taylor number defined in terms of the longitudinal integral scale. The time-mean drag coefficient, the base-pressure coefficient and the spanwise correlation length of the surface-pressure fluctuations in the vicinity of the separation point were fairly well correlated with the parameter R 1·34 T , R being the Reynolds number. It was argued that the parameter R 1·34 T will control some aspects of the flow past a circular cylinder immersed in turbulent streams.

Time-averaged aerodynamic forces acting on a hemisphere immersed in a turbulent boundary

The time-averaged pressure distribution over the surface of a hemisphere immersed in a fully developed turbulent boundary layer developing along a smooth, plane wall has been measured for a number of hemispheres of differing diameter d in order to establish the relationship between the aerodynamic force acting on such a hemisphere and the characteristics of the boundary layer. It is found that the drag coefficients defined by CDτ = D/(12πuτ2A may be expressed as a function of uτd/ν alone in the range d/δ ⪷ 1.0, where D is the pressure drag, uτ the shear velocity, ν the kinematic viscosity, A denotes the area of the hemisphere projected onto a plane normal to the main flow direction, and δ is the boundary-layer thickness.

Flow around a cubic body immersed in a turbulent boundary layer

Abstract This paper reports the results of an experiment concerning flow in the vicinity of a cubic body immersed in a turbulent boundary layer which is fully developed along a tunnel floor. By changing the flow direction and body height, measurements were made of the pressure and drag acting on the body, and of the flow pattern and pressure distribution on the floor. The results indicate the changes taking place in the pressure distribution over the surface of a cubic body, the pressure drag acting on it, and the flow pattern and pressure distribution over the floor surface as the angle of incidence α is varied.

A free-streamline theory for bluff bodies attached to a plane wall

A free-streamline theory is presented for the separated flow past two-dimensional bluff bodies attached to a long plane wall on which a turbulent boundary layer has developed. The non-uniform velocity profile in the turbulent boundary layer which would be measured if the bluff bodies were absent has been replaced by a hypothetical inviscid parallel shear flow which has a constant vorticity. This model admits analytical solutions and automatically yields closed streamlines in front of the bluff bodies such as the normal plate and the semicircular projection, which are geometrically very similar to observed front separation bubbles. The present theory involves three or four parameters which must be determined on the basis of experimental information, the number of parameters depending upon the shape of bluff bodies. Two typical examples of bluff bodies, i.e. the normal plate and the semicircular projection, are worked out. Pressure distributions around these bodies predicted by the present theory are found to give a good agreement with experimental measurements.

Forces acting on circular cylinders placed in a turbulent plane mixing layer

Abstract Time-averaged pressure distributions along the surface of a circular cylinder placed in a turbulent plane mixing layer were measured in order to clarify the time-averaged aerodynamic forces acting on the cylinders, together with the flow patterns around them. The Reynolds number based on the diameter of the cylinder d and the mainstream velocity U outside the mixing layer was in the range (2.16–4.06) × 10 4 . Both the angular position of the stagnation point and the stagnation-pressure coefficient were well correlated with a parameter d δ , δ being the width of the mixing layer, if the location of the cylinders was expressed in terms of the ratio u c U , where u c is the velocity of the otherwide undisturbed mixing layer at the centre of the cylinders. The drag coefficient was found to be rather insensitive to the parameter d δ , whereas a slight dependence of thedrag coefficient on the cylinder diameter was observed owing to the effect of turbulence in the mixing layer. The lift force was always directed from the high-velocity side to the low-velocity side of the mixing layer, its magnitude being approximately proportional to the parameter d δ .