Vortex Formation Region Research Articles

Quantitative measurements of three-dim ensional structures in the wake of a circular cylinder

The fine scale three-dimensional structures usually associated with streamwise vortices in the near wake of a circular cylinder have been studied at Reynolds numbers ranging from 170 to 2200. Spatially continuous velocity measurements along lines parallel to the cylinder axis were obtained with a scanning laser anemometer. To detect the streamwise vortices in the amplitude modulated velocity field, it was necessary to develop a spatial decomposition technique to split the total flow into a primary flow component and a secondary flow component. The primary flow is comprised of the mean flow and Strouhal vortices, while the secondary flow is the result of the three-dimensional streamwise vortices that are the essence of transition to turbulence. The three-dimensional flow amplitude increases in the primary vortex formation region, then saturates shortly after the maximum amplitude in the primary flow is reached. In the near-wake region the wavelength decreases approximately like Re−0.5, but increases with downstream distance. A discontinuous increase in wavelength occurs below Re = 300 suggesting a fundamental change in the character of the three-dimensional flow. At downstream distances (x/D = 10-20), the spanwise wavelength decreases from 1.42D to 1.03D as the Reynolds number increases from 300 to 1200.

Vortex shedding mechanism from prisms having H and I sections

In case of the flow around the H-section prism, a single and double shear layer instabilities coexist. It is found that the fundamental frequency due to the single shear layer instability along the shear layer between the upper (or lower) leading edge and upper (or lower) trailing edge is adjusted so as to coincide with that due to the double shear layer instability behind the prism, and vice versa. On the other hand, in case of the flow around the I-section prism, there exists only a double shear layer instability, and asymmetry of the vortex pair in the vortex formation region and the subsequent transformation into the unstable system of the three vortices, viz., upper main vortex, lower main vortex and third vortex, leads to the alternate vortex shedding. It is realized that the vortex shedding mechanism from the I-section prism is essentially same as that from a circular, triangular or square prism.

Effects of a small airfoil-shaped splitter plate on vortex shedding from a square prism at subsonic Mach numbers

The present study is concerned with effects of a small airfoil-shaped splitter plate (NACA 0018, chord lengthc=20 cm) on the vortex shedding from a single square prism (side lengthD=20 mm) at free-stream Mach numbers between 0.15 and 0.91 and a constant spacing ratioL/D=2.0, whereL is the central distance between the square prism and airfoil arranged in tandem. It is found that while there exist no shock waves in the flow the vortex shedding from a single square prism is retarded by the small airfoil due to the interaction with the upper and lower separating shear layers: The Strouhal number for the square prism and airfoil being arranged in tandem, is almost independent of the Mach number and takes about 0.11. This is smaller than the value of 0.13 known for a single square prism. However, as soon as shock waves appear in the flow, the Strouhal number increases suddenly and then increases with increasing the Mach number. It is inferred that the sudden increase of the Strouhal number is primarily caused by shock waves appearing above and below the vortex formation region, for the shock waves make the vortex formation region small and symmetrical with respect to the common axis of the square prism and airfoil. That means, the small airfoil causes only a secondary effect on the vortex shedding from a single square prism under the presence of shock waves in the flow.

Characteristics of Bubble-Collapse Pressures in a Karman-Vortex Cavity

Flow patterns and bubble-collapse pressures in a Karman-vortex cavity behind a circular cylinder are investigated for several Reynolds numbers and cavitation numbers. The time histories and intensity distributions of the pressures are respectively measured by using a pressure transducer and pressure-sensitive films. The cavitation aspects are also observed by high-speed photography. The bubbles frequently collapse at the instant when the Karman-vortex cavity separates from the cylinder or the cavity itself. High impulsive pressures with spike pulses over 60MPa occur in the separation or the vortex formation region. With a decrease in cavitation number σ, these regions are combined into one. With an increase in Reynolds number Re, however, the distributions of the impulsive pressures show the same tendency, because they have the same Strouhal number St, but the pulse count rate and magnitude of maximum local impulsive pressures increase.

Passive pressure-drag control in a plane wake

We report results of a study of the transition region of a compressible (subsonic) plane wake at moderately high Reynolds numbers, based on the numerical solution of the inviscid time-dependent flow equations. The focus is placed on the flow dynamics in the region of vortex formation and the near wake. A thin interference plate along the wake centreline, which can either be attached or detached from the bluff-body base, is used as a means for passive pressure-drag control by affecting the vortex formation process directly. The results show that the inclusion of an interference plate in the flow configuration can significantly decrease the magnitude of the base pressure coefficient by factors of up to 3, depending on the length of the plate and its separation from the base. The calculated results are in good agreement with the available experimental data and include the detached case for which little or no data exists. The observed self-sustained (global) instabilities in the present simulations were found to be intrinsic features of the flows investigated and are consistent with the local absolute/global instability picture currently favoured.

Heat transfer around a circular cylinder with a single tripping wire.

An experimental study was conducted on the heat transfer under the condition of constant heat flux around a circular cylinder with a single tripping-wire for various diameters, which was affixed at various positions from the forward stagnation point of the cylinder. The testing fluid was air and the Reynolds number Re, based on the cylinder diameter, ranged from 1.2×104 to 6.2×104. Especially investigated are the relations between the heat transfer around the rear face of the cylinder and the flow of the near wake. It is found that the heat transfer around the rear face decreases with increasing of the length of the vortex formation region.

The Near Wake Characteristics of Cavitating Bluff Sources

The influence of cavitation on vortex shedding behind constrained sharp-edged bluff prisms is studied experimentally. At a given blockage, the length of the vortex formation region is found to increase as the cavitation number of the flow is reduced. The vortex appears to be stabilized from breaking up in the partially cavitating regime of flow. Test results indicate that the separation velocity is the proper velocity scale to reduce or eliminate blockage effects.

Discrete vortex simulation for flow around a circular cylinder with a splitter plate

A discrete vortex model was developed to investigate unsteady flow in a wake of a circular cylinder with a splitter plate. In the model the splitter plate was expressed by a series of sources such as to cancel out the flow across it. Various flow features which had been observed experimentally by many researchers were successfully reproduced in the simulation. The vortices suddenly rolled up between the cylinder and the plate when the distance between them was beyond the critical value. The critical distance was hardly affected by the size of the plate when the plate was along the flow, on the other hand it increased proportionally to the size of the plate when the plate was normal to the flow. The critical distance was about a half of the experimental results at Reynold number 10 4–2×10 4, because the length of the vortex formation region in the simulation was shorter than in the experiments. It was also clarified that the various physical quantities such as the base pressure coefficient, the fluctuating lift coefficient, the Strouhal number and the position of the separation point, etc., were systematically varied according to the change of the flow pattern.

Vortex shedding mechanism from a triangular prism in a subsonic flow

Alternate periodic vortices shed from a triangular prism arranged as either a wedge or a reversed wedge are studied experimentally at a Mach number of 0.377 and at a Reynolds number of 1.73 × 105 (based on the prism height). It is found that the flow around the triangular prism arranged as a wedge (referred to as prism 1) separates at the sharp trailing edges, and the main vortices are generated immediately after the separation. No secondary vortex is generated around prism 1 ; however the flow around the other triangular prism arranged as a reversed wedge (prism 2) separates at the sharp leading edges. The main vortices are not formed immediately after the separation; rather their formation is finished either over or under the trailing edge. In this case, strong secondary vortices on and beneath the trailing edge are generated by the upper and lower main vortices, respectively. The vortices behind prism 1 are weaker than those behind prism 2, but the rate of the vortex shedding process behind prism 1 is higher than that behind prism 2. That is, the weaker the vortices are, the higher the rate of the vortex shedding process (or the Strouhal number) is, and vice versa. The vertical spacing between the loci of the upper and lower main vortices behind prism 1 is wider than that behind prism 2, but the width of the wake behind prism 1 is narrower than that behind prism 2. It is suggested that the Strouhal number for a wedge is sensitive to the front shape (i.e. the wedge angle), but that for a reversed wedge is not sensitive to the rear shape. Finally, the vortex shedding mechanism from a triangular prism arranged as either a wedge or reversed wedge has been proposed. It is concluded that, irrespective of the arrangement, vortex shedding is maintained by the unstable system of three vortices generated in the vortex formation region, but the effect of the downstream oscillating wake on vortex shedding is not critical.

Flow similitude and vortex lock-on in bluff body near wakes

Some years ago a universal Strouhal number for bluff body wakes was introduced, which was based upon the Strouhal frequency fs0 of the vortex shedding, the measured wake width d′ at the end of the vortex formation region, and the mean velocity Ub at the edge of the separated boundary layer on the body. This universal parameter was shown to collapse these characteristic scales of bluff body flows onto a single curve for wake Reynolds numbers between 200 and 107. The results of more recent experiments show the concept of universal wake similitude to be even more general than was previously supposed. In this paper these similarity relationships are applied to the case of vortex lock-on in oscillatory flow. A full understanding of the macroscopic averaged properties in the wake in this traditional manner is an important precursor to studying, in detail, the microscale wake properties using the most modern experimental measurements and diagnostic techniques and theoretical concepts such as the absolute-convective stability of shear flows.

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.

Compressible flows in the wakes of a square cylinder and thick symmetrical airfoil arranged in tandem

Compressible flows in the wakes of a two-dimensional square cylinder (side length D = 20 mm) and thick symmetrical airfoil (NACA 0018, chord length 20 mm) arranged in tandem have been examined experi­mentally, at free-stream Mach numbers between 0.15 and 0.91, at free-stream Reynolds numbers (based on the side length) between 7.0 x 10 4 and 4.2 x 10 5 , and with spacing (or central distance) L between the cylinder and airfoil ranging from 22.5 to 110 mm. When the Mach number is smaller than about 0.63, the flow can be divided into three patterns depending upon the spacing. In the first flow pattern, with small spacing, the airfoil is enclosed completely within the vortex formation region of the square cylinder. In the second flow pattern, the separating shear layers from the square cylinder reattach to the airfoil. In the third flow pattern, with large spacing, the separating shear layers roll up upstream of the airfoil. The Strouhal number becomes a minimum at the critical spacing of about 3.3 D and then experiences a sudden jump, practically at the value found for the single square cylinder, which corresponds to the transition from the second flow pattern to the third flow pattern. Once the Mach number becomes larger than about 0.63, the critical spacing disappears. However, although no local flow regions are supersonic, acoustic waves propagating upstream have been observed most clearly when the vortex shed from the square cylinder is incident on the leading edge of the airfoil. Whereas once the local flow regions become supersonic, i. e. the Mach number is larger than about 0.7, the downstream airfoil provides a streamlining effect on the flow behind the square cylinder, and thus lets the alternating vortices form downstream of the trailing edge of the airfoil. The alternating vortices are shed through the gap between the two shock waves formed on the upper and lower separating shear layers. The pressure amplitude in the test section decreases suddenly and various frequency components other than the vortex shedding frequency appear.

Mach number effects on vortex shedding of a square cylinder and thick symmetrical airfoil arranged in tandem

An experimental study was done to elucidate the Mach number effects on vortex shedding of a square cylinder (side length D = 20 mm) and thick symmetrical airfoil (NACA 0018, chord length 20 mm) arranged in tandem , at free stream Mach numbers between 0.1526 and 0.9081, and at free stream Reynolds numbers (based on the side length D ) between 0.702 x 10 5 and 4.188 x 10 5 . The spacing ratio of the central distance, L , between the square cylinder and the airfoil to the side length, D , of the square cylinder was varied from 1.125 to 5.5. It was found that the regular vortex shedding is not suppressed by steady shock waves in the local supersonic flow regions; the periodic vortex shedding is still present, irrespective of the appearance of the shock waves. When the spacing ratio is fixed, the Strouhal number behind the square cylinder is almost constant up to the critical Mach number of about 0.70, but it increases rapidly with further increase of the Mach number. However, once the shock waves are formed on both sides of the vortex formation region, various frequency components, other than the vortex shedding frequency appear; the spectral peaks lower than those of the vortex shedding frequency were identified as frequencies of an acoustic-feedback oscillation and the resonance of the wind tunnel structural system. With increasing the Mach number, the formation region becomes small and asymmetric, and the separating shear layers become wavy. These changes result in an increase of the scale and strength of the vortices and thus enhance the vortex shedding process. However, when the Mach number exceeds the critical value, the streamwise length of the formation region increases suddenly and becomes long enough to enclose the downstream airfoil. Under this circumstance, the formation region is almost symmetrical with respect to the wake axis, and shock waves are formed on the upper and lower separating shear layers. The shock waves are almost normal to the wake axis at M = 0.7512 and 0.8215, but incline to the downstream direction at M = 0.9081. Acoustic waves travelling upstream have been observed most clearly when the vortex shed from the square cylinder hits the leading edge of the airfoil at a Mach number of about 0.63, which is close to, but slightly smaller than the critical value. The mean pressure and the amplitude of the pressure fluctuations in the test section decreases and increases, respectively, with increasing the Mach number. However, the amplitude of the pressure fluctuations decreases suddenly when the steady shock waves are formed on the upper and lower separating shear layers.

Measurements in the wake flow behind finite circular cylinders fitted with slat devices

In this paper the influence of a slat device upon the wake flow behind a finite circular cylinder is investigated. Frequency and cross-correlation (in time) measurements are carried out at a Reynolds number of 31000 and at several streamwise and lateral positions in the near wake of a plain circular cylinder and a circular cylinder fitted with the slat device. The resulting modifications of the wake pattern due to the slat device are discussed. It is shown that the presence of the slat device increases the length of the vortex formation region and thus delays the formation of a periodic vortex wake to relatively long distances behind the body. It is this delay that is largely responsible for the effectiveness of the slat device in suppressing large amplitude vortex-induced oscillations of circular cylindrical bodies.

Airfoil tip vortex formation noise

Spectral data are presented for the noise produced due to the turbulent three-dimensional vortex flow existing near the rounded tip of lifting airfoils. The results are obtained by the comparison of sets of two- and three-dimensional test data for different airfoil model sizes, angles of attack, and tunnel flow velocities. Microphone cross-correlation and cross-spectral methods were used to determine the radiated noise. Corrections were made for tunnel shear layer and source directivity effects. Interpretation of the results are aided by a three-dimensional flow analysis developed for this study which determines open tunnel and finite aspect ratio corrections heretofore neglected in tip vortex studies. Hot wire measurements were made in the tip vortex formation region for the specification of governing flow parameters. The spectral data is normalized in a format considered most useful for subsequent quantitative prediction of this noise mechanism for practical systems such as helicopter rotors. Comparison is made to the analysis of George and Chou. A recommended prediction method is given.

A formation mechanism of alternating vortices behind a circular cylinder at high reynolds number

A formation mechanism of alternating vortices behind a circular cylinder has been examined experimentally at a Reynolds number of 1.7 × 10 5 (based on the diameter of the cylinder) and at a Mach number of 0.391, and has been proposed on the basis of the present results as well as on existing knowledge. Asymmetry of the vortex pair with respect to the wake axis behind the cylinder and the cross-flow through the axis are necessary precursors for alternate vortex shedding. These result in an unstable vortex system consisting of three vortices in the vortex formation region, so that another stable vortex system of a new vortex pair is formed by shedding one of the three vortices downstream. However, the vortex pair is unstable for the asymmetrical disturbances, and thus cross-flow through the wake axis is again induced. It is inferred that this cyclic process maintains the alternate vortex shedding behind the cylinder.

The plane turbulent shear layer with periodic excitation

The influence of periodic excitation on a plane turbulent one-stream shear layer with turbulent separation was investigated. For the qualitative study flow visualization was employed. Quantitative data were obtained with hot-wire anemometry and spectrum analysis. It was found that sinusoidal perturbations with frequencies of order f0 [lsim ] u0/100θ0 (depending on excitation strength), introduced at the trailing edge are always amplified. Maximum amplification factors are observed for the lowest perturbation levels. The frequency and amplitude of excitation determine the downstream location of the amplification maximum in the flow. At sufficient amplitude two-dimensional vortices are formed which subsequently decay without pairing. The development of the periodic r.m.s. values along x follows a universal curve for all frequencies and amplitudes when properly normalized.At high excitation amplitudes the flow development depends strongly on the geometrical conditions of the excitation arrangement at the trailing edge. Thus regular vortex pairing as well as suppression of pairing can be achieved.The excited shear layer has considerably stronger, yet nonlinear, spread than the neutral. The region of vortex formation, irrespective of whether it includes pairing or not, is associated with a step-like increase in width, while after the position of maximum vortex energy, i.e. in the region of decay, the spread is reduced to values below the neutral. There the overall lateral fluctuation energy is increased, while the longitudinal may be decreased as compared with the neutral flow.

Correlation between heat transfer and fluctuating pressure in separated region of a circular cylinder

The wake of a circular cylinder was controlled in two ways: by setting up two small cylinders inside the separated shear layers and by placing a splitter plate behind the cylinder. The former controlled the shake of the shear layers, the latter controlled the vortex formation region. The relationship between the heat transfer in the separated regions and the flow characteristics in connection with the fluctuating components of the flow, was investigated. It was found that the R.M.S. fluctuating pressure at the rear stagnation point was one of the most dominant factors of the heat transfer in the separated regions.

Characteristics of the Flow around a Bluff Body near a Plane Surface

The characteristics of the flow around a normal flat plate and a triangular cylinder near a plane surface has been investigated experimentally in the Reynolds number range of 102to 3×104. Frequency analysis of hot wire signal of the wake flow in a wind tunnel revealed that regular vortex shedding from the normal flat plate and the triangular cylinder was suppressed for spaces less than about 0.6 and 0.37 times the body width respectively. Investigation of the time-mean distance between two shear layers in the vortex formation region demonstrated that the suppression of vortex shedding for both configurations occurred for any space between the inner shear layer and the plane surface less than about 0.3 times the shear Iayer distance. Flow visualization experiments in a water tank revealed the structural differences of the wakes with and without vortex shedding.

Ｂｌｕｆｆ ｂｏｄｙの抵抗減少と後流

This paper describes an experimental study on the near wake behind bluff body in relation to the base drag reduction. The experiments were carried out using the models with two-dimensional blunt trailing edge and with periodically three-dimensional one. REYNOLDS number based on uniform velocity and base thickness is lessthan 8×103.Behind the two-dimensional bluff body, the periodic vortex shedding occurs and the static pressure reduces in the vortex formation region. In the case of the three-dimensional model, it is observed as a general tendency that the static pressure reduction and the concentration of periodic vortices are not notable. This seems to be the essential feature associated with the drag reduction of three-dimensional bluff body.