Fluctuating Fluid Forces Research Articles

An Investigation of the Wake Structure and Aerodynamic Characteristics of a Finite Circular Cylinder. 2nd Report. Organized Fluctuating Surface Pressures and Fluctuating Fluid Forces acting on Finite Cylinders with Various Aspect Ratios.

Many components of organized fluctuating pressures with different frequencies, which may be induced by the turbulent organized structures in the wake, have been extracted from the surface pressures on the finite circular cylinders with aspect rations from 1 to 12 at a Reynolds number of 3.7×104 using phase-locked averaging technique. The organized local and global fluctuating fluid forces acting on finite cylinders have been determined from these results. For cylinders with high aspect ratios, as large as more than 8, three different components of organized local fluctuating fluid forces act, i.e., tip component which appears near the tip exhibiting the largest amplitude, the midspan component as well as the root component which is similar to that for a two-dimensional cylinder. The lateral force and the rolling moment dominate in the organized global fluctuating fluid forces for many aspect ratios, but only a little organized fluctuating fluid force acts on the cylinders with aspect ratios from 5 to 6.

Characteristics of Fluid Forces Acting on a Rotary Oscillating Rectangular Cylinder. Rectangular Cylinder with Impinging Leading-Edge Vortices.

Measurements are made of the time-averaged and fluctuating fluid forces, the vortex-shedding frequency and the behavior of the impinging leading-edge vortices of a rectangular cylinder forced into rotary oscillation. The rectangular cylinder having the width-to-height ratio B/H of 3 (H : height of cylinder, B : width of cylinder) in which vortices are formed at the leading edge is adopted. The cylinder is forced into rotary oscillation with angular displacements up to α=±5°and the nondimensional frequency f*H/U0 (f* : frequency of oscillating cylinder, U0 : free-stream velocity)is varied from 0.01 to 0.27, in which the lock-in region is included. It is found that the fluid forces acting on the oscillating rectangular cylinder are strongly dependent on the impinging leading-edge vortices rather than the wake vortices. Also, it is recognized that the fluctuating fluid forces are mainly generated by the phase shift of surface pressure on the basis of the movement of the impinging leading-edge vortices along the side surface.

Flow around Two-Dimensional Circular Cylinder Rows Supported by Elastic Wires. 2nd Report. Fluctuating Pressure and Fluctuating Fluid Force.

In this paper we describe the flow around two-dimensional circular cylinder bundles supported by wires. The experiment was carried out in an N. P. L. blow-down wind-tunnel with a 500mm X 500mm X 2000mm working section, and with Reynolds number 9.4 X 103. The displacement of a vibrating circular cylinder, and the fluctuating pressure distributions on the surface of the circular cylinder were measured. It was found that (i) the critical value of the fluid elastic vibration occuring is obtained as the spacing ratio D/S for a single row, two rows and three rows, (ii) the fluctuating pressure distribution on the surface, fluctuating drag and fluctuating lift by natural frequency increase rapidly beyond the critical value mentioned in (i), and (iii) the variation of fluctuating drag and fluctuating lift corresponds to that of the displacement of a vibrating cylinder.

Transitional Characteristics of Separated Shear Layer in the Vortex-Generating Region Behind a Body. Correspondence Between Image of Flow Visualization and Velocity Field Obtained by LDV Measurement at the Same Phase.

Recently, there have been some studies on reduction of fluctuating fluid forces acting on a bluffbody by controlling a small portion of the separated shear layer from the body. However, the characteristics of the separated shear layer in the vortex-generating region, which are key information to determine an effective way to control the fluid forces, have not been clarified yet. In this study, downstream behavior of small vortices generated in the separated shear layer near the separation point is investigated by computer image processing of flow visualization. In particular, coalescence of the small vortices at the phase when a new vortex is starting to generate and at the phase when the growth of the vortex is in the intermediate stage are investigated in detail. Also the velocity field mapped by LDV measurement is compared with the visualized image at the same phase to clarify the mechanism of the generation of new vortices.

Optimum Suppression of Fluid Forces Acting on a Circular Cylinder and Its Effectiveness. Controlled by a Fine Flat Plate.

The objective of the present study was to investigate the suppression of the fluid forces acting on a circular cylinder by controlling the flow around it. Flow control was established by introducing a fine flat plate in the neighborhood of the main cylinder. Measurements were carried out with variation of the position and attack angle of the flat plate at a Reynolds number of 6.5 × 104. The time-averaged and fluctuating fluid forces were examined to estimate the magnitude of the reduction and to identify the optimum position of the control flat plate in reduction the fluid forces. Furthermore, the mechanism of the flow control, the nature of the controlled wake, the stability of the flow control and the relationship between the characteristics of the controlled fluid forces and behavior of the wake were discussed in detail. Then, the applicability of the present method reduction of the fluid forces and suppression the vortex shedding was evaluated

Characteristics of Fluid Forces Acting on Oscillating Rectangular Cylinders. In the Case of Cross-Stream Oscillation.

Measurements were made of the time-averaged and fluctuating fluid forces, vortex-shedding frequency and wake structures of rectangular cylinders forced to oscillate transversely at a maximum amplitude of 10% of the length of the front face. The ratio of depth (streamwise dimension) to height (cross-stream dimension) of the cross section ranged from 0.3 to 1.0. The nondimensional frequency FvH/U0 (Fν : frequency of forced oscillating cylinder, H : height of cylinder, U0 : free-stream velocity) was varied from about 0.08 to 0.20, in which the lock-in region was included. It has been found that a critical depth where the fluid forces attain a peak in the oscillating cylinder is smaller than that of the stationary cylinder. Also, within the lock-in region in which the vortex shedding is perfectly synchronized with the force frequency Fν of the cylinder only, the fluid forces are found to attain a maximum. Furthermore, it is recognized that the work done by the fluctuating lift changes to positive work, which induces the generation of negative aerodynamic damping from negative work when the ratio of depth to height is smaller than 0.5.

Three-Dimensional Flow Analysis around a Circular Cylinder. 1st Report, In the Case of a Stationary Circular Cylinder.

In general, three-dimensional flow structures are observed in the Karman vortex street behind a circular cylinder when Reynolds number ≥ 200, and it is pointed out that three-dimensional analysis is necessary to simulate such flow structures. In this study, incompressible flow past a stationary circular cylinder at Reynolds number 1100 is computed by finite difference method, and the three dimensional computation shows good agreement with experimental values. Furthermore, a phenomenon that periodic lift and drag forces acting on a cylinder cause a beat frequency can be simulated. This phenomenon cannot be observed in the two-dimensional calculation, and the mechanism is explained on the basis of the relationship between the fluctuating fluid forces and the flow structures in the three-dimensional calculation.

Characteristics of Fluctuating Fluid Forces Acting on an Oscillating Square-Section Cylinder. Forced Rotary Oscillation with Constant Amplitude Angle.

Characteristics of Fluctuating Fluid Forces Acting on an Oscillating Square-Section Cylinder. Forced Rotary Oscillation with Constant Amplitude Angle.

Aerodynamic behaviour of a square cylinder placed in a turbulent plane mixing layer

This paper describes experimental results for the aerodynamic behaviour of a square cylinder placed near and in a turbulent plane mixing layer. The time-mean fluid forces have close similarities irrespective of the change of the mixing layer thickness. The flow patterns are classified into two types at the non-dimensional boundary position η B = − 0.05, namely perfectly separated flow and reattachment flow. The fluctuating fluid force has a conspicuous high rise when the cylinder is installed near a mixing layer thinner than the width of the cylinder, which is induced by the oscillation of the mixing layer upstream of the cylinder. However, such a high rise of the fluctuating forces disappears and there is a fairly good similarity with respect to the non-dimensional position η when the mixing layer is more than 1.3 times thicker than the width of the cylinder.

Fluctuating fluid forces acting on a circular cylinder and interference with a plane wall

The flow around a circular cylinder placed close to a horizontal plane wall was investigated experimentally. Fluctuations of lift and drag of the cylinder and wall interference effects were studied in terms of the gap height between the cylinder and wall and the thickness of the turbulent wall boundary layer. The fluctuating fluid forces acting on the cylinder sharply increased, and the regular vortex shedding, i.e. Karman vortex streets, started to form beyond a critical gap height. The formation of Karman vortex streets was abruptly interrupted when the bottom of the cylinder came in contact with the outer layer of the boundary layer developed on the wall. This critical gap height correlated well with the thickness of the boundary layer.

An optimum suppression of fluid forces by controlling the separated shear layer. Control of one-sided shear layer separated from square prism.

This paper deals with the suppression of the fluid forces by controlling a one-sided shear layer separated from a square prism. The control of the separated shear layer was established by setting up a small circular cylinder (the 'control' cylinder) in a one-sided shear layer near the square prism. Experimental date were collected to examine the effects on the fluid forces and vortex shedding frequency due to variation of the position and diameter of the control cylinder. The results show that (i) the position of the control cylinder corresponding to the minimum of he time-mean drag and fluctuating lift and drag was near what would ordinarily be considered the outer boundary of the shear layer ; (ii) the control of the separated shear layer by means of a small cylinder appeared to be effective in suppressing the fluctuating lift and drag rather than the time-mean drag ; (iii) the fluctuating fluid forces were reduced markedly below the plain prism value. For example with the control cylinder of 5 mm diameter, the fluctuating lift and drag were reduced to approximately 90% and 70% respectively ; (iv) the effectiveness of the control cylinder was attributed to the change of the position of the vortex formation region and suppression of vortex shedding.

Aerodynamics of a circular cylinder in oscillatory flow.

Experimental investigations of the aerodynamics of a circular cylinder in oscillating flow have been made at Reynolds numbers of (1.1∼1.7)×104. The vortex-shedding frequencies, time mean and fluctuating pressure distributions on a cylinder in sinusoidal gusts with various frequencies and amplitudes have been obtained. Fluctuating fluid forces acting on a cylinder have been separated into two parts, i. e., and oscillating fluid force due to vortex shedding and that due to the flow oscillation, using a computer-aided flow visualization technique. The lock-on phenomena occur in the frequency ranges near the natural vortex-shedding frequency and at twice this value. The alternative nature of vortex shedding is maintained in both locked and unlocked conditions, but an appreciable increase in an oscillating lift force appears in locked-on ranges. The oscillating lift and drag forces due to flow oscillation are proportional to the gust velocity components in their individual directions.

Fluctuating fluid forces of downstream circular cylinder in staggered arrangement. (Effect of oscillating upstream cylinder)

Fluctuating fluid forces of downstream circular cylinder in staggered arrangement. (Effect of oscillating upstream cylinder)

Dynamics and Stability of Coaxial Cylindrical Shells Conveying Viscous Fluid

In this paper the dynamics and stability characteristics of coaxial cylindrical shells containing incompressible, viscous fluid flow are examined in contrast to previous studies where the fluid has been considered to be inviscid. Specifically, upstream pressurization of the flow (to overcome frictional pressure drop) and skin friction on the shell surfaces are taken into account, generating time-mean normal and tangential loading on the shells. Shell motions are described by Flu¨gge’s thin shell equations, suitably modified to incorporate the time-mean stress resultants arising from viscous effects. The fluctuating fluid forces, coupled to shell vibration, are determined entirely by means of linearized potential flow theory and formulated with the aid of generalized-force Fourier-transform techniques. It is found that the effect of viscosity in the annular flow generally tends to destabilize the system, vis-a`-vis inviscid flow, whereas viscous effects in the inner flow stabilize the system. These effects can be quantitatively very important, so that, generally, neglect of viscous effects cannot be justified.

Characteristics of fluid forces acting on a square cylinder in a free turbulent mixing layer.

The characteristics of the fluid forces acting on a square cylinder and flow patterns around the cylinder experimentally investigated when the cylinder was placed near and in a free turbulent mixing layer. The flow patterns near the under side of the cylinder, which faces to the high velocity side of the mixing layer, were found to be divided into the perfect separated flow (η< - 0.05) and the reattachment flow (η> - 0.05) . At this boundary ηM&cong;, the time-mean lift force attained its minimum value. It became also clear that the fluctuating fluid forces were very small over ηM and their distributions with respect to η had similarities when h / δ&lsim; 0.617, h being the side length of the cylinder and δ the width of mixing layer.

Interference between Plane Wall and Two-Dimensional Rectangular Cylinder : Fluid Force when the Angle of Attack is 0 Deg.

The pressure distributions around two-dimensional cylinders and the fluctuating fluid forces were measured which are induced by the interference between the plane wall and the cylinder. The pressure distributions were measured with respect to the various width-to-height ratios, and the fluctuating force with the width-to-height ratio=1.0. The time-averaged drags of the cylinders rapidly increase at the spacing ratio s/h&ap;0.5 and 0.8 when the width-to-height ratios are 1.0 and 0.5, respectively. The characteristics of the fluctuating fluid force is that its magnitude increases rapidly over s/h&ap;0.5, though it is almost negligibly small when s/h&lap;0.5.

Sound from the Action of Vorticity

A formulation is suggested which shows how the sound from an aerodynamic flow can be primarily attributed to a volume distribution of dipole generators when a contiguous rigid body sustains a fluctuating fluid force. This can be interpreted in terms of the local fluid acceleration but, more significantly, can be attributed to the deformation of rings of vorticity in the flow (analogous to varying magnetic shells in electromagnetic theory). This leads to a new model especially applicable to aeolian- and edgetones. In the absence of a net fluctuating force, the sound from the vortex ring action degenerates from dipole to quadrupole nature. This situation is illustrated by considering the formation of a single vortex (of length l) from a length b of a thin layer of shear rate U, the distortion of the layer producing sound energy of the order of U7l2ba−5× (2.10)−4. (This is also accountable for by a longitudinal quadrupole generation according to Lighthill's theory.) This must be supplemented by a comparable amount of energy resulting from the dilatational effect due to the appearance of the low pressure core of the vortex. The acoustic power emanating from a succession of such vortices is roughly U8l2a−5× (2.10)−4. This point of view corresponds to the classical hydrodynamic result that the velocity (and hence pressure) at a point is derivable from a vector field (due to vorticity) together with a scalar field (associated with dilatation), except that time retardation effects must be recognized.

On the Edgetone

The feedback mechanism of classical low-speed edgetones in which the action at the edge is interpreted as an acoustical source is developed in detail. A theoretical development indicating that the acoustic field is primarily due to the dipole associated with the fluctuating fluid force on the edge has been verified. It is the hydrodynamic field of the dipole which disturbs the jet, whose instability characteristics are shown to depend acutely on the Reynolds and Strouhal numbers, and the orifice-edge distance. The gain criterion is developed in detail, it being shown how the eigenfrequencies (which can form no algebraic sequence) arise; the lower limit to the orifice-edge distance is discussed, yielding an estimate of the “linear” instability of the stream. The amplitude of the established edgetone depends on the nonlinear behavior of large-amplitude stream disturbances and the corresponding upper limit to the edge force proves to be in satisfactory agreement with measurements, thus yielding acceptable expressions for the sound pressure. Multiple tones and the circumstances of the hysteretic frequency jumps are discussed. The basic action depends only on Reynolds number for geometrically similar systems, while the sound power depends on the cube of the Mach number also.