Smoke-wire Flow Visualization Technique Research Articles

The vortex-induced vibrations of a triple-box girder model of a long-span rail-cum-road bridge

Triple-box girders, i.e., three separated box girders are arranged in tandem, are gradually used in super-span bridges because of their excellent flutter stability. However, the gaps between triple boxes can lead to complex vortex dynamics around the structures that result in vortex-induced vibrations (VIVs). In the present study, we experimentally investigate VIVs in a long-span bridge with a triple-box girder cross-section using wind tunnel tests. In the static test, in which the test model is fixed, a ‘dual-frequency’ phenomenon that expands the VIV lock-in region is observed in the global flow field. Furthermore, the results of a dynamic test indicate that the existence of gaps is essential to the occurrence of VIVs in triple-box girders. The smoke-wire flow visualization technique is employed to explore the excitation mechanism by examining the surrounding flow dynamics of the triple-box model. Separated motion-induced vortices (MIVs) and von Karman vortices (KVs) are found to be coupled with structural vibrations to sustain the VIVs. Five stages of VIV regions, along with five types of vortices in the surrounding flow field, are identified for the girder model. Finally, the effects of accessory structures of the bridge model on the VIV responses are also studied. This study contributes to our understanding of the unsteady flow dynamics and VIV mechanisms of a typical triple-box girder.

Flow induced oscillation of two rigid rectangular plates in a side-by-side configuration

The present work is an experimental study of two oscillating rigid plates placed in side-by-side configuration, hinged at their leading edges, subjected to low subsonic flow. This problem is investigated using smoke-wire flow visualization, hot-wire anemometry, and time resolved particle image velocimetry. It is found that beyond a critical Reynolds number, the plates set into oscillatory motion. This critical Reynolds number depends on the gap between the plates. It is also seen that this value of Reynolds number, at lower values of gap to thickness ratio (<7) is significantly higher than that of the single plate configuration value. The frequency and amplitude of the oscillating plates at various gaps and Reynolds numbers have been studied and compared with the characteristics of an oscillating single plate. It is also found that depending on the gap and acceleration of the free-stream, there exist two modes of oscillation - (i) in-phase and (ii) out-of-phase. For gap to thickness ratio less than 10, only in-phase oscillations take place for all values of free-stream velocity considered in the present work, whereas, when this ratio is greater than 10, the mode of oscillation depends on the initial conditions up to a certain free-stream velocity, beyond which the plates switch to in-phase mode. Smoke wire flow visualization technique along with time resolved particle image velocimetry reveal that the vorticity distributions around the plates are responsible for the initiation of the two modes of oscillations.

Effects of single- and dual-blockage disks on swirling coaxial jets at high annulus Reynolds numbers

Using the laser-assisted smoke wire flow visualization technique, particle image velocimetry, and concentration detection method, this study examines the effects of two circular disks, arranged in tandem, on the flow structures, turbulence properties, and mixing characteristics of swirling double-concentric jets at high annulus Reynolds numbers. The smoke flow patterns, velocity vector fields, streamlines, normalized velocity contours, fluctuation intensity contours, vorticity contours, and distributions of carbon dioxide are presented and discussed. When a single-blockage disk is installed at the exit of swirling double-concentric jets, two pairs of counter-rotating vortices are formed in the near wake. The central jet fluids penetrate the recirculation zone so that no stagnation point, and therefore no reverse flow induced by annular jet flow, is found along the central axis. This causes high values of carbon dioxide concentration along the central axis. When a second circular disk is installed a short distance downstream of the disk at the jet exit, the axial momentum of the central jet transfers radially, turns downstream at the edge of the downstream disk, and then merges with the swirling annular flow. For this dual-disk case, a large reverse-flow zone is observed in the near wake in the condition of high annulus Reynolds and swirl numbers. Large turbulence intensity region is therefore induced downstream disks. Consequently, the mixing capability of dual-disk-controlled swirling double-concentric jets is significant improved compared to that of the single-blockage disk.

An improved smoke-wire flow visualization technique using capacitor as power source

ABSTRACT An improved smoke-wire flow visualization technique using a large capacitor as the power source was proposed. Electric current discharged from capacitors was used to heat a fine metal wire suspended in the flow field. The oil droplets attached to the wire were vaporized and smoke filaments followed the flow motions. A digital camera was used to record the images of the smoke filaments. The actions of discharging and camera shutter were triggered by signals from a microcontroller to ensure the accurate timing. Clear images of the streaklines were captured at a free-stream velocity up to 12.9 m/s, much higher than the limits of the existing techniques.

An experimental investigation of laminar separation bubble formation on flexible membrane wing

In this study, fluid–structure interaction over a flexible membrane wing with aspect ratio (AR) of 3 at low Reynolds numbers was investigated experimentally. Smoke wire flow visualization technique was performed for analyzing time-dependent behavior of flow over this flexible membrane wing. Furthermore, time dependent deformation of flexible membrane wing was measured. It was clearly seen that the value of membrane deformation increased with increasing angle of attack. Moreover, it was stated that the size of laminar separation bubble (LSB) changed with time due to the unsteady flow characteristics of membrane wing. The unsteady behavior over the flexible membrane wing caused different deformation modes to form at different angles of attack. For the flexible wings with higher aspect ratios, the LSB was more dominant in the membrane wings at low Re numbers, and caused the membrane vibration to increase based on the angle of attack which the LSB started to be overpowering.

Wind farm characteristics of side-by-side and tandem configurations

This study explores the effects of different arrangements such as side-by-side and tandem in wind turbine towers under various Reynolds numbers. The Reynolds number was changed from 1.2×103 to 4×106. The wake-flow behaviors, velocity characteristics, and wake vortex shedding were studied. The effects of side-by-side gap ratio (G*) and tandem gap ratio (L*) were considered. Smoke-wire flow visualization technique was used to render the flow characteristics. At different heights, the flow characteristics were observed in four sections: the blade at tower top, bluff body of blade and generator, cylinder of wind turbine tower, and junction at the bottom. A hot-wire anemometer was used to measure the vortex-street frequency. The characteristic flow modes for the side-by-side configuration are the anti- and in-phase vortex modes, and those for the tandem arrangement are the single, attached, and bi-vortex modes. For side-by-side wind towers, the wake flow field behaved like that of single one when G*> 1.0. For tandem wind turbine towers, the flow behaviors are independent when L*> 0.85.

Control of flow around a square cylinder at incidence by using a splitter plate

Passive control of vortex shedding behind a square cylinder at incidence has been conducted experimentally by using a stationary splitter plate for the Reynolds numbers of 3.0×104. The splitter plate was located at the center of the rear face of the square cylinder in tandem. The width of the cylinder and the plate were both chosen to be 30mm and the incidence angle of the square cylinder was rotated between 0° to 45°. In this study, the combined effects of the splitter plate and angle of incidence on the pressure distributions and vortex-shedding phenomenon were investigated. Vortex shedding frequency was obtained from velocity measurements and aerodynamic force coefficients acted on the cylinder were calculated from pressure distributions. Characteristics of the vortex formation region and location of the flow attachments, reattachments and separation were observed by using the smoke–wire flow visualization technique. For the case with the plate, there is a sudden jump in the Strouhal number in the vicinity of 13° which corresponds to a minimum value of the drag coefficient. At zero angle of incidence, Strouhal number and a drag coefficient of the square cylinder decreased about 20% by means of the splitter plate. Drag reduction was minimum at about 13° and reached its maximum value at about 20°.

Patterns of electronegative ions past a cylinder

The behavior of the wake of an electronegative gas flow past either a glass or a magnetic circular cylinder was observed using the smoke wire flow visualization technique. Converging or chaotic vortices for the electronegative flow of air and oxygen past a magnetic cylinder were seen in shedding vortices, especially at higher concentrations of electronegative molecules, eventually leading to the disappearance of a Kármán vortex, possibly due to the effect of ‘interactive solenoids’.

Visualization of oscillating flow in a double-inlet pulse tube refrigerator with a diaphragm inserted in a bypass-tube

The double-inlet pulse tube refrigerator that has a diaphragm inserted in a bypass-tube, which enabled it to transmit a pressure oscillation whereas to obstruct a DC gas flow, was manufactured and tested. The oscillating flow behavior inside of the refrigerator was studied by using a smoke-wire flow visualization technique. It was found that if the diaphragm was optimized, the performance would be improved more than that of the refrigerator with a bypass valve due to the increase in the P–V work of the gas and the decrease in the convective heat loss caused by a secondary flow.

Fluidic Oscillation Influences on V-Shaped Bluffbody Flow

The V-shaped fluidic oscillator is developed by inserting a target blockage with a specially designed crescent surface into the downstream cavity of a slit V gutter. Stable, self-sustained, periodic fluidic oscillations can be induced by the dynamic Coanda effect when the geometric parameters of the target blockage and the Reynolds number are properly tuned. The fluidic oscillations are directed through two slit passageways and injected into the near wake of the V-shaped fluidic oscillator like pulsing jets. The oscillation behaviors, frequency selection, streamline patterns, and turbulence properties of the unsteady flows in the near wakes of the V-shaped fluidic oscillator are studied experimentally in a wind tunnel by using the smoke-wire flow visualization technique, hotwire anemometry, and laser Doppler velocimetry. Flowfields of the slit V gutter and the closed-tip V gutter, which are the counterparts of the V-shaped fluidic oscillator, are also measured for comparison. The Strouhal number of the fluidic oscillation based on the slit width of the presently developed oscillator can attain a value of about 0.56 at large Reynolds numbers, which is about 80 times larger than the previous results for the enhancement of heat transfer and about 60 times larger than that for the fluidic flowmeter. The fluidic oscillations act as an excitation source for the wake. The kinetic energy of the fluidic oscillations is transferred to turbulent fluctuations and therefore causes increases in the size of the recirculation bubble and the turbulence intensity in the wake of the V-shaped fluidic oscillator. The turbulence intensity in the near wake can be increased by about 7% when compared with that of the closed-tip V gutter.

Oscillation Frequency in Wake of a Vee Gutter

The oscillating behaviors and frequency selection of the unsteady flows in the wake of a vee gutter are studied experimentally in a wind-tunnel by the use of the smoke-wire flow visualization technique and the hot-wire anemometer. The effective length of the vee gutter is determined by the variation of vortex shedding frequency. The functional relationships of the dominant mechanisms in the limiting ranges of Reynolds number are justified by the arguments made on the Strouhal number and the Roshko number. The oscillating motions in the wake region of the vee gutter possess two types of instabilities: the high-frequency shear-layer instability waves and the low-frequency vortex shedding. Four characteristic modes of the shed vortices: laminar, subcritical, transitional, and supercritical, are identified according to the differences of their physical properties. Variations of the Strouhal numbers of the shear-layer instability waves and the vortex shedding are presented and discussed. Characteristic frequencies of the wakes behind various geometries and sharpness of trailing edges are compared.

Development and Characterization of Jet-Injected Vee-Gutter

ABSTRACTThe vee-gutter, which was conventionally used in a combustor for flame holding, was re-designed by employing the unsteady Coanda effect to inject fluids periodically into near wake of the vee-gutter. Fluidic targets were developed to induce self-sustained transverse oscillation of slit-jet. The self-sustained oscillating jet was conducted through passages and injected into the near wake of the vee-gutter. The behaviors and frequency characteristics of the slit-jet in the oscillation cavity and the turbulence properties in the wake were studied experimentally in a wind-tunnel by using the smoke-wire flow visualization technique and the hot-wire anemometer. The oscillation frequencies of the presently developed jet-injection vee-gutter were about 25 to 40 times higher than that of the conventionally used fluidic flowmeter. By estimating the Lagrangian integral time scale and employing the Taylor's frozen flow hypothesis, the integral length scales of turbulence fluctuations were calculated. The results showed that the integral length scales of turbulences of the jet-injected vee-gutter were significantly smaller than their counter parts of the conventional vee-gutter, which indicated the effects of vortex stretching induced by the periodic jet injection. The modifications of turbulence properties were presented and discussed.

Dependence of convective secondary flow on inclination angle in an inclined pulse tube refrigerator revealed by visualization

Secondary flow in an inclined orifice pulse tube refrigerator at typical inclination angles of 0–180° was studied by using a smoke-wire flow visualization technique. It was revealed that the secondary flow formed a unicellular convective flow in the pulse tube and had two flow patterns depending on the angle. This dependence of flow pattern on the inclination angle is well explained by the superposition of gravity-driven convective flow on acoustic streaming. Even if the cold end was lower than the hot end, the gravity-driven convective flow occurred and the secondary flow was affected by gravity.

Axisymmetric Swirling Vortical Wakes Modulated by a Control Disk

The effects of an above-exit circular disk on the flow structures, turbulence properties, and mixing characteristics of swirling jets separated by a large diameter blockage disk at jet exit are studied by using the smoke-wire flow-visualization technique and a two-component laser Doppler velocimeter. A carbon-dioxide tracer-gas experiment is conducted to examine the mixing capability of the flows modulated by the control disk. The smoke-streak flow patterns, characteristic lengths, velocity vectors, streamlines, velocity distributions, turbulence properties, and concentration distributions of the carbon dioxide are presented and discussed. Topological techniques are applied to analyze and interpret the flow patterns. With the variation of the distance between the control and blockage disks and the central jet Reynolds number, the bubble structures in the near field display complex behaviors of extension, escape, and encapsulation. Under the influence of the control disk, the central jet deflects radially, merges with the swirling annular flow, then turns downstream to form a recirculation bubble in the control-disk wake

Flow field characteristics of swirling double concentric jets

Abstract The flow structures and turbulence properties of double concentric jets with a large separation between the central jet and swirling annular flows are studied using the smoke-wire flow visualization technique and a two-component laser Doppler velocimeter. The smoke-streak flow patterns, characteristic lengths, velocity vectors, streamlines, velocity distributions, and turbulence properties are presented and discussed. The smoke-streak patterns show that a large spatial separation at the exit between the central and swirling annular jets can expedite the formation of a recirculation zone at low swirl and Reynolds numbers. Complex flow structures, single bubble , dual rings , vortex breakdown , and vortex shedding , are found in the recirculation zone. Large central jet velocity induces a large entrainment of the fluids in the recirculation zone and thus reduces the size of the recirculation bubble. The streamline patterns of the dual-ring mode show no stagnation point existing on the central axis, which is different from the non-swirling concentric jets. An off-axis saddle point which fits the topological rule is observed. The turbulence intensities and shear stress around the saddle point are particularly large. A carbon dioxide tracer gas experiment is conducted to examine the mixing capability. Effects of the swirl number and separation between the concentric jets are presented.

熱音響冷凍機 可視化によるパルス管冷凍機内の流動状態の観察

We tried to observe the flow behavior in typical pulse tube refrigerators (i.e., basic, orifice and double-inlet pulse tube refrigerators) using a smoke-wire flow visualization technique, and discuss the fundamental flow behavior in those pulse tube refrigerators. The pulse tube is made of a transparent acrylate tube having the effective length of 280mm, an inner diameter of 16mm and a wall thickness of 4mm. The regenerator is composed of a stainless-steel housing and a bakelite tube, 18mm in diameter and 168mm long packed with 770 discs of 100-mesh stainless-steel screen. The reservoir has a volume of about 3×10-4m3, which is 5.5 times larger than the pulse tube volume. Air is used as the working gas and pressure oscillation is generated by introducing pressurized air of about 0.2MPa into the rotary valve and releasing it to the atmosphere. Visualization is done under the conditions of frequency of 5Hz and compression ratio of about 1.2.The flow behavior throughout the pulse tube during one cycle is cleared. It was found that typical velocity profiles of the viscous oscillating flow are observed for all pulse tube refrigerators, and that the orifice and double-inlet types have larger velocity and displacement compared to the basic one. It was also found that the thickness of velocity boundary layer for oscillating flow is estimated to be about 30% of the radius of the pulse tube. Secondary streaming in the pulse tube, which flows in the direction from cold- to hot-end in the core region and the opposite direction in the boundary region, was recognized for all refrigerators.

Visualized flow patterns of double concentric jets at low annulus velocities

The flow structures in the near wake region of the unducted double concentric jets at low Reynolds numbers are studied by smoke-wire flow visualization technique. Four typical characteristic flow regimes—weak flow, pre-penetration, transition, and penetration—are identified for different jet velocities in the near disk region. Flow patterns and behavior in each characteristic flow regime are investigated. The contours of the separation surfaces and the lengths of the recirculation zone in various flow regimes are correlated with nondimensional parameters. The recirculation length reaches a maximum in the transition region. The toroidal recirculation region exhibits both expelling and shear-layer vortex shedding. The shedding processes are presented, and the frequencies are correlated with Strouhal number and central-to-annular jet velocity ratio. Nomenclature D - diameter of circular disk, 20 mm Da = outer diameter of annular jet, 30 mm Dc = exit diameter of central jet, 3.4 mm / = frequency of vortex shedding, Hz Ja = momentum flux of annular jet, pau2a Jc = momentum flux of central jet, pcwj Lr = axial length of recirculation zone R - radius of circular disk, 10 mm Rea = Reynolds number of annular jet based on the disk diameter Rec - Reynolds number of central jet based on the central jet diameter r = radial coordinate, originated from center of circular disk St = Strouhal number, fD/ua ua = average exit velocity of annular jet uc = average exit velocity of central jet ud = displacement velocity, 03uc- ua wmax = maximum half-width of the recirculation zone Z = axial coordinate, originated from center of circular disk pa = mass density of annular flow pc = mass density of central jet

Smoke-wire flow visualization of the near-wake region behind a circular disc at low Reynolds numbers

The flow structures in the near field of the unducted wake region behind a circular disc for annular flow at low Reynolds numbers were studied by smoke-wire flow visualization technique. A twisted-dual-wire was employed to perform the time evolving visualization. Three typical characteristic flow modes: Q-tip, open-top toroid, and closed toroid, were identified in the near disc region. For Reynolds number between 130 and 390, the Q-tip flow mode which subject to a periodic up-down oscillatory motion was observed. The open-top toroid mode which experiences the expelling vortex shedding was found for Reynolds number between 390 and 455. The free separation surface turns around and merges to the central axisymmetric axis to form the conventionally observed toroidal recirculation bubble for Reynolds number higher than 455. The closed toroid mode exhibits both expelling and shear-layer vortex sheddings. With the identified flow modes at low Reynolds numbers, the recirculation contours, recirculation length, and the shedding frequency in each mode were measured and discussed.

Visualization of a free and impinging round jet

The smoke-wire flow visualization technique was used to gain a deeper physical insight into a regular vortex structure of free and impinging round subsonic jets. The jet issued into ambient air at rest through a bell-shaped nozzle. Toroidal vortex initiation, vortex pairing, and fluid entrainment processes are visualized, with the uniformly distributed smoke filaments providing a cross-sectional view of the jet within the first few diameters. The role of large-scale toroidal vortices in the entrainment process and the separation distance between two neighboring large-scale toroidal vortices in the jet is discussed. Interaction between the ordered large-scale toroidal vortex structure and the normally impinged flat plate is shown. At low nozzle-to-plate separations (below a jet core length), the ring-shaped wall eddies induced between toroidal vortices successively diverge in the radial direction along the plate surface.