Smoke-wire Flow Visualization Research Articles

INVESTIGATION OF AERODYNAMIC PERFORMANCE OF A NACA 0021 AIRFOIL MODIFIED WITH CAVITY

In this paper, an experimental investigation was conducted to investigate the effect of cavity on the NACA 0021 airfoil at low Reynolds number. The force measurement and smoke-wire flow visualization experiments were conducted in a low-speed open suction wind tunnel with a test section of 57cm×57cm×100 cm at a Reynolds number of 5.0×104 and 1.0×105. Two different airfoil models produced for the experiments such as base and cavity models. The diameter and location of the cavity was chosen as 0.08C and 80% x/C, respectively. The maximum lift coefficient of the cavity modified airfoil was increased about 6% for both positive and negative angles of attack compared to base airfoil at Reynolds number of 1.0×105. The cavity modified airfoil showed higher lift coefficient values at negative attack angles when compared to base airfoil at Reynolds number of 5.0×104.

Aerodynamics and surrounding flow patterns of a long-span bridge girder model with triple-separated boxes

The triple-box girder is finding broader applications in super long-span bridges for its excellent performance of flutter stability and traffic adaptability. In the present study, a bridge girder model with three separated boxes is experimentally investigated in detail via wind tunnel tests. In consideration of web modifications and accessory structures, the multi-point surface-pressure measurement and smoke-wire flow visualization are applied to investigate the aerostatic characteristics and flow structures. The “dual-frequency” phenomenon is found and explained for the bare girder with curved webs. For the present girder with linear webs, the global field is dominated by the separated-shear-layer instability. For the bare girder with curved webs, the upstream gap flow is dominated by the separated-shear-layer instability while the downstream gap flow is dominated by the double-shear-layer instability. The downstream gap flow exhibits obvious intermittent characteristics. In addition, non-Gaussian characteristic analysis of the surface wind pressures is introduced and improved for engineering practice. The distinguishing principle based on the skewness and kurtosis has high adaptability to both the bare girder and the completed bridge. In addition, semi-linear webs show better drag reduction. Combinations of accessory structures are also studied. The results show that during the construction process, prioritizing the installation of highway wind barriers can optimize the aerostatic performance of bridges. The present study embraces the booming era of long-span bridges with a triple-box girder and enriches the studies of bridge aerostatics.

Flow over a forward-facing step with a flexible membrane at its leading edge

The effects of a flapping membrane at the leading edge of a forward-facing step (FFS) on the flow over and pressure fluctuation on it were experimentally investigated. The FFS was immersed in the oncoming boundary layer (δ/h = 0.4, where δ and h are the oncoming boundary layer thickness and step height, respectively) and the Reynolds number, based on the step height and oncoming flow velocity (U∞) was 3.4×104. Both the length and width of the membrane were identical to h, the height of the FFS. Two-dimensional particle image velocimetry (PIV) and smoke-wire flow visualization techniques were used to investigate the flow over the FFS. Pressure scanners have also been used to study the pressure distribution on an FFS. It was found that the membrane flapped periodically under the effects of the separated shear flow from the leading edge of the FFS, which significantly suppressed the separation bubble on the FFS and prompts an early recovery of the time-averaged pressure. The phase-averaged results revealed that the flapping membrane generated alternating vortices over the FFS. These flapping-induced vortices (FIVs) enhance the Reynolds stresses over the FFS, increasing momentum transport in the wall-normal direction. Moreover, these FIVs synchronize with the pressure fluctuation on the FFS.

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.

Experimental flow control investigation over suction surface of turbine blade with local surface passive oscillation

Impact of the local flexible membrane (LFM) on aerodynamic phenomena including the formation of a laminar separation bubble (LSB) and transition to turbulence was experimentally investigated over the suction surface of a Clark-Y airfoil first time in literature. The experiments such as aerodynamic force measurement, smoke-wire flow visualization and hot-film tests were carried out at the free-stream velocity of U∞ = 3.2 m/s, U∞ = 6.4 m/s, U∞ = 9.6 m/s, U∞ = 12.8 m/s, and Reynolds number based upon on the chord length was Rec = 3.5 × 104, Rec = 7.0 × 104, Rec = 1.05 × 105 and Rec = 1.4 × 105, respectively. The experimental angle of attack was set at 0° = α ≤ 20°. In detailed intermittency analysis by the hot-film sensor over the uncontrolled airfoil, it was seen that the LSB and transition to turbulence formed close to the trailing edge at a lower angle of attack, and it moved towards the leading edge when increasing the angle of attack simultaneously. Employing LFM on the suction surface obviously affected the progress of these flow phenomena. In the results of smoke-wire flow visualization, either the size of the laminar separation bubble (LSB) was reduced or its presence was suppressed at lower incidences. The aerodynamic force measurement results also supported those behaviors. In particular, at lower incidences, the negative effects of LSB were mitigated, resulting in the presence of a more stable lift curve. Additionally, it was clearly observed that utilizing LFM ensured positive effects, especially at the pre- and the post-stall regions in terms of fewer fluctuations at the CL curve, meaning that less aerodynamic vibration and noise on wind/hydro turbine could be obtained.

Gap effects on the aerodynamic characteristics around three rectangular boxes in tandem arrangement

Multiple-box structures have become increasingly important in practical engineering; for example, some recent large-span bridges were creatively designed using triple-box decks. The flow characteristics and flow-induced responses of multiple-box structures are notably complex due to the existence of gaps. Herein, we conducted a detailed investigation on the effects of gap width on the aerodynamics and flow characteristics of three boxes in a tandem arrangement, that is, a triple-box model. The test model comprised three rectangular cylinders with a side ratio of 3.7 (i.e., SR = 3.7), and it was arranged in line with the incoming airflow. The gap ratio (L/D = the ratio of the gap width L to the height of the box D) was varied from 0 to 10.260. Surface pressure measurement and smoke-wire flow visualization were conducted in the wind tunnel tests. For the pressure measurements, the Reynolds number (Re) was varied from 1.01 × 104 to 2.20 × 104. The smoke-wire flow visualization was performed at a relatively low Re of 6767. The results showed that the gap ratio significantly influenced the pressure distributions, aerodynamic forces, and surrounding flow patterns. A “dual-frequency” phenomenon was observed at low and moderate gap ratios; that is, a dominant frequency and secondary frequency were found. Moreover, the secondary frequency was closely related to the secondary vortices. Furthermore, the dual-frequency phenomenon disappeared at large gap ratios, which was different from the tandem circular cylinders. Based on the experimental results, the flow patterns around the triple-box model were categorized into four basic types, depending on the gap ratio.

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.

Experimental Study of Rotor Flow Separation Control using a New Type of Dielectric Barrier Discharge Plasma Actuator

Flow separation occurring on rotor blades is an important limiting factor for helicopter performance. This paper presents a new type of dielectric barrier discharge (DBD) plasma actuator for rotor blade flow separation control called bipolar DBD plasma actuator. The bipolar DBD plasma actuator’s connection mode differs from traditional plasma actuators and eliminates reverse discharge between electrodes. The experiments in this work were carried out by smoke-wire flow visualization and PIV technology in the open test section of a low speed wind tunnel, and solved the problem of high voltage electricity supplied to the plasma actuator while the rotor rotated. In this experiment, the rotor, camera, and laser were synchronized to obtain results. Smoke-wire results and PIV results illustrated that the flow separation weakened with increasing rotor speed; the separated flow area was large at a rotating speed of 300 r/min and gradually became smaller at the rotating speed of 600 r/min, the flow separation disappeared at the rotating speed of 1200 r/min. When the plasma was active, both smoke-wire results and PIV results showed the flow separation area was greatly reduced at the rotating speed of 300 r/min and disappeared at the rotating speeds of 600 r/min and 1200 r/min, the rotor flow separation at high angles of attack could be effectively controlled by the new DBD plasma actuator.

Unsteadiness control of laminar junction flows on pressure fluctuations

Smoke-wire flow visualization is conducted carefully in a laminar junction to explore the physical behavior of laminar junction flows. The two-dimensional (2D) velocity fields in the 30° plane of a laminar junction flow are acquired by a time-resolved particle image velocimetry (PIV) system at a frame rate of 1 kHz, based on which the unsteady fluctuating pressure fields can be calculated by the multi-path integration method proposed in the literature (GAND, F., DECK, S., BRUNET,V., and SAGAUT, P. Flow dynamics past a simplified wing body junction. Physics of Fluids, 22(11), 115111 (2010)). A novel control strategy is utilized to attenuate the unsteadiness of the horseshoe vortices of the laminar junction flow, and the consequent effect on pressure fields is analyzed.

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.

Performance Assessment of Transition Models for Three-Dimensional Flow Over NACA4412 Wings at Low Reynolds Numbers

The performance of the transition models on three-dimensional (3D) flow of wings with aspect ratios (AR) of 1 and 3 at low Reynolds number was assessed in this study. For experimental work; force measurements, surface oil and smoke-wire flow visualizations were performed over the wings with NACA4412 section at Reynolds numbers of 2.5 × 104, 5 × 104, and 7.5 × 104 and the angles of attack of 8 deg, 12 deg, and 20 deg. Results showed that the AR had significant effects on the 3D flow structure over the wing. According to the experimental and numerical results, the flow over the wing having lower ARs can be defined with wingtip vortices, axial flow, and secondary flow including spiral vortex inside the separated flow. When the angle of attack and Reynolds number was increased, wing-tip vortices were enlarged and interacted with the axial flow. At higher AR, flow separation was dominant, whereas wing-tip vortices suppressed the flow separation over the wing with lower AR. In the numerical results, while there were some inconsistencies in the prediction of lift coefficients, the predictions of drag coefficients for two transition models were noticeably better. The performance of the transition models judged from surface patterns was good, but the k–kL–ω was preferable. Secondary flow including spiral vortices near the surface was predicted accurately by the k–kL–ω. Consequently, in comparison with experiments, the predictions of the k–kL–ω were better than those of the shear stress transport (SST) transition.

Experimental Investigation of the Flow Structure over a Delta Wing Via Flow Visualization Methods

It is well known that the flow field over a delta wing is dominated by a pair of counter rotating leading edge vortices (LEV). However, their mechanism is not well understood. The flow visualization technique is a promising non-intrusive method to illustrate the complex flow field spatially and temporally. A basic flow visualization setup consists of a high-powered laser and optic lenses to generate the laser sheet, a camera, a tracer particle generator, and a data processor. The wind tunnel setup, the specifications of devices involved, and the corresponding parameter settings are dependent on the flow features to be obtained. Normal smoke wire flow visualization uses a smoke wire to demonstrate the flow streaklines. However, the performance of this method is limited by poor spatial resolution when it is conducted in a complex flow field. Therefore, an improved smoke flow visualization technique has been developed. This technique illustrates the large-scale global LEV flow field and the small-scale shear layer flow structure at the same time, providing a valuable reference for later detailed particle image velocimetry (PIV) measurement. In this paper, the application of the improved smoke flow visualization and PIV measurement to study the unsteady flow phenomena over a delta wing is demonstrated. The procedure and cautions for conducting the experiment are listed, including wind tunnel setup, data acquisition, and data processing. The representative results show that these two flow visualization methods are effective techniques for investigating the three-dimensional flow field qualitatively and quantitatively.

Experimental Investigation of the Flow Structure over a Delta Wing Via Flow Visualization Methods.

It is well known that the flow field over a delta wing is dominated by a pair of counter rotating leading edge vortices (LEV). However, their mechanism is not well understood. The flow visualization technique is a promising non-intrusive method to illustrate the complex flow field spatially and temporally. A basic flow visualization setup consists of a high-powered laser and optic lenses to generate the laser sheet, a camera, a tracer particle generator, and a data processor. The wind tunnel setup, the specifications of devices involved, and the corresponding parameter settings are dependent on the flow features to be obtained. Normal smoke wire flow visualization uses a smoke wire to demonstrate the flow streaklines. However, the performance of this method is limited by poor spatial resolution when it is conducted in a complex flow field. Therefore, an improved smoke flow visualization technique has been developed. This technique illustrates the large-scale global LEV flow field and the small-scale shear layer flow structure at the same time, providing a valuable reference for later detailed particle image velocimetry (PIV) measurement. In this paper, the application of the improved smoke flow visualization and PIV measurement to study the unsteady flow phenomena over a delta wing is demonstrated. The procedure and cautions for conducting the experiment are listed, including wind tunnel setup, data acquisition, and data processing. The representative results show that these two flow visualization methods are effective techniques for investigating the three-dimensional flow field qualitatively and quantitatively.

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.

Efficient suction control of unsteadiness of turbulent wing-plate junction flows

A wing-body junction flow of a navigating underwater vehicle is considered to be a crucial source of the flow radiating acoustic noise, which attracts much research interest. In this paper, wing-plate junction flows are experimentally investigated in a low-speed wind tunnel by smoke-wire flow visualizations and time-resolved PIV measurements. To reveal the physical behavior of such flows, smoke-wire flow visualizations are conducted for a laminar wing-plate junction. A novel control strategy is proposed, to accurately locate the suction openings where the streamline is about to roll up to form a vortex in the turbulent junction flows. The control effect is discussed in perspectives of both the time-averaged and instantaneous flow fields.

Unsteady Aerodynamic Characteristics depending on Reduced Frequency for a Pitching NACA0012 Airfoil at Rec=2.3×10⁴

Most of small air vehicles with moving wing fly at low Reynolds number condition and the reduced frequency of the moving wing ranges from 0.0 to 1.0. The physical phenomena over the wing dramatically vary with the reduced frequency. This study examines experimentally the effect of the reduced frequency at low Reynolds number. The NACA0012 airfoil performs sinusoidal pitching motion with respect to the quarter chord with the four reduced frequencies of 0.1, 0.2, 0.4 and 0.76 at the Reynolds number 2.3×10⁴. Smoke-wire flow visualization, unsteady surface pressure measurement, and unsteady force calculation are conducted. At the reduced frequency of 0.1 and 0.2, various boundary layer events such as reverse flow, discrete vortices, separation and reattachment change the amplitude and the rotation direction of the unsteady force hysteresis. However, the boundary layer events abruptly disappear at the reduced frequency of 0.4 and 0.76. Especially at the reduced frequency of 0.76, the local variation of the unsteady force with respect to the angle of attack completely vanishes. These results lead us to the conclusion that the unsteady aerodynamic characteristics of the reduced frequency of 0.2 and 0.4 are clearly distinguishable and the unsteady aerodynamic characteristics below the reduced frequency of 0.2 are governed by the boundary layer events.

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.

Flow and mixing characteristics of swirling double-concentric jets influenced by a control disc at low central jet Reynolds numbers

The flow characteristics and mixing properties of the swirling double-concentric jets subject to the influences of a control disk installed at a short distance above the primary disk were studied. The instantaneous flow patterns were illustrated using the laser-light sheet-assisted smoke wire flow visualization method. The velocity characteristics were measured by a particle image velocimeter (PIV). The axial and radial mean velocities and turbulence intensities in the symmetry plane of the flow field were obtained from the PIV measurements. The dispersion properties of the central jet fluid were measured by a gas analyzer using carbon dioxide as the tracer gas. The control disk installed at a short distance above the primary disk caused the central jet to impinge upon the bottom face of the control disk, deflect radially outward, turn downstream, and interact with the swirling annular jet to form four characteristic near-field flow structures (close-top toroidal vortical wake, open-top toroidal vortical wake, turbulent forward-flow wake, and turbulent reverse-flow wake) in different regimes of annular and central jets’ Reynolds numbers. The reverse flow on the central axis of the turbulent reverse-flow wake caused dramatically large turbulence intensities to occur in the near wake-region when compared to the open-top toroidal vortical wake, which contained no stagnation point or reverse flow on the central axis. Therefore, this induced a drastic large radial dispersion of the central jet fluid. The mixing index of the turbulent reverse-flow wake might be enhanced by approximately 90% when compared to that of the swirling double-concentric jets without the influence of a control disk.

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°.