Stereo PIV Technique Research Articles

Flow field investigation of Confined Coaxial Swirling Jets by Stereo-PIV

This work analyses the isothermal flow field of a research burner exploiting both swirl and radially staged combustion air. The burner is composed of two coaxial co-rotating swirling jets, and the swirl level of the two jets can be controlled independently of each other. The influence of the swirl levels on the flow field is experimentally investigated under isothermal conditions using the Stereo-PIV technique, and the results evidence significant differences in the mean flow field at the different swirl levels. The vortex breakdown occurs in all the investigated cases, except for the combination of the lowest outer swirl level and the intermediate inner swirl level. Besides, the central recirculating flow region shape and size are affected by the momentum ratio and swirl level of the central jet. When the swirl intensities of the two coaxial jets are at their maximum, LDV measurements evidence on the burner axis tangential velocity fluctuations at a frequency of about 86 Hz; a POD-based phase average of the PIV maps allows recognizing the presence of a Precessing Vortex Core (PVC) which induces a periodic merging of the two jets. Those results can be of significance in the design of double swirl burners and for the control of the combustion process. Nevertheless, further tests are required to extend the current analysis to combustion conditions.

On 3D structure of a circular cylinder wake

The 3D structure of the wake behind a circular cylinder in a cross-flow is studied experimentally using stereo PIV technique, Reynolds number was around 5 thousands. The statistical characteristics along the cylinder axis are effectively constant and thus 2D, however instantaneous flow-field in the wake is fully 3D. Within the wake dynamical flow, the streamwise vortices and spots of streamwise velocity component pulsations are detected.

Stereo PIV measurement of diffuser swirling flow

The swirling flow in a diffuser has been studied using the stereo PIV technique. The DANTEC hardware and software were used for the experiment. The water pipe flow in the diffuser inlet has been set spinning by introducing tangential jet flow. Both piping and diffuser were of circular cross-section, however diffuser casing had rectangular (square) cross-section. The plane of measurement was located perpendicular to the flow within the diffuser. The two cameras with the Scheinpflug lens mounting were used, however the angle between the lenses axes and the diffuser outer surface was far from to be perpendicular, as recommended in similar situations. This configuration has been dictated by technical conditions in the laboratory. The two-level calibration target with dots was used for the cameras calibration. The calibration covers the diffusers central region in a good quality, however regions close to the walls were not well resolved. Many combinations of axial and tangential inflow conditions have been studied. The acquisition covered about 3 seconds in real time with frequency 500 Hz. Both statistics and dynamics of the flow have been studied in details. Results are shown for both statistical characteristics and dynamical analysis of various conditions. During experiments the air-glass-water interface was not normal to the camera viewing axis. The discussion how to solve this problem and of the bias of the calibration will be introduced.

Application of stereo PIV to the 70° delta wing at subsonic speed

A stereo PIV technique is utilized for the analysis of the flow field over a 70° delta wing. Instantaneous velocity vectors are measured at a series of stations in the chordwise direction. The angle of attack from 0° to 45° are investigated with the free stream velocities of 20, 30, 40, 50 and 60 m/s, respectively. The velocity measurements conducted in a small-sized subsonic wind tunnel (1.8m×1.4m) at China Areodynamics Research and Development Center (CARDC) are compared with the previously obtained PSP results. The comparison shows a good agreement with each other, providing the validity of the PIV technique as well as indicating its effectiveness to investigate complex flow fields in a fairly large-scale wind tunnel facility.

Dynamics of the Tip Vortices in the Wake Behind a Circular Cylinder of Finite Length

The dynamics of the tip vortices in the wake behind a wall-mounted finite-length circular cylinder of the aspect ratio 2 was studied experimentally using time resolved stereo PIV technique. The cylinder was mounted normal to a ground plane and it was subjected to a cross-flow with thin boundary layer developed on the wall, the Reynolds number based on inflow velocity and cylinder diameter was 9.7 thousands. The dynamics of tip vortices were analysed using the POD method applied to the plane perpendicular to the flow close to the cylinder. Besides the decaying power spectrum, slower that the Kolmogorov-type one, the two distinct frequencies were detected on Strouhal numbers 0.09 and 0.15. These frequencies could be linked to the vortical structures dynamics in the wake. The frequency Sh = 0.15 corresponds to predominantly spanwise vortices dynamics with anti-symmetrical patterns with respect to the cylinder axis, while the frequency Sh = 0.09 corresponds to mainly streamwise vortical structures dynamics with symmetrical patterns respectively. Thus, the von Kármán vortex street was detected on Strouhal frequency 0.15.

Suppressing tip vortex cavitation by winglets

Despite the numerous remedies prescribed so far, tip vortex cavitation (TVC) remains a major issue in design and operation of diverse applications. In this paper, we experimentally investigate the effectiveness of winglets in suppressing TVC. An elliptical hydrofoil is selected as the baseline geometry and various winglets are realized by bending the last 5 or 10% of the span at ± 45° and ± 90° dihedral angles. To better focus on the physics of the problem, we have intentionally avoided any optimization on the geometries and our winglets are only smooth non-planar extensions of the original cross-section. Modifying no more than 3.7% of the lifting surface, lift-and-drag force measurements demonstrate that the hydrodynamic performances of the winglet-equipped hydrofoils are not substantially different from the baseline. Nevertheless, cavitation inception–desinence tests reveal that undeniable advantages are achieved by the winglets in TVC alleviation. It is found that the 10%-bent 90° winglets are more effective than the 45° cases, with − 90° (bent down toward the pressure side) performing superior to + 90°. For instance, the 90°-bent-downward winglet reduces the TVC inception index from 2.5 for the baseline down to 0.8 (a reduction of 68%) at 15 m/s freestream velocity and 14° incidence angle. In addition, the study on the bending length effect conducted for the 90° configurations shows that the 5%-bent winglets are not as striking as the 10% ones. Employing Stereo-PIV technique, the influence of winglets on non-cavitating flow structures is examined. For the most effective winglet (10%-bent 90°-downward), we observe that the maximum tangential velocity of the tip vortex falls to almost half of the baseline and the vortex core size increases significantly (by almost 70%). These effects are accompanied by a tangible reduction in the axial velocity at the vortex core leading to further mitigation of TVC.

Experimental investigation of CH4-air-O2 turbulent swirling flames by Stereo-PIV

This paper presents an investigation of the dynamic characteristics of non-premixed turbulent swirling flames using the stereo-PIV technique. To finely characterize the flow field, experiments are performed in the longitudinal and transverse planes in non-reacting and reacting conditions. The burner configuration studied in this work is a coaxial swirl stabilized burner used in the turbulent regime. The swirler is placed in an annular part supplying the oxidant flow and the central pipe delivers radially a fuel through eight holes symmetrically distributed on the periphery of the tube. The experiments are conducted using a 25 kW parallelepiped combustion chamber, 1 m high and 0.5 m wide, using circulating water as coolant. In the reacting case, the CH4-air flame is oxygen enriched from 21% to 30% in volume. Effects of the swirl number (0.8–1.4) and the global equivalence ratio (0.8–1) on the flows topology and structure are analyzed. Results of the present work concern the 3D mean velocity fields, the turbulence intensities, the swirl number measurements and the entrainment rate along the flow. SPIV results show the three-dimensional aspect of the flow including the recirculation zone and the annular zone with swirling jet effects. The comparison between reacting and non-reacting flows shows that the presence of the flame induces a larger radial flow expansion, higher mean velocities and higher turbulence intensities. The results show a bifurcation of the tangential velocity towards the jet central axis by crossing the flame front. It is also shown that the fluctuations of the tangential velocity are mainly responsible for the highest turbulence kinetic energy levels at the top of the central recirculation zone. The entrainment rate of the surrounding gases is measured by stereo-PIV as a function of the swirl number and equivalence ratio. The results explain one possible mechanism for the decrease of the NOx emissions when the global equivalence ratio increases via an increase of the entrainment rate at the flame base.

Study on Vortex Structure in the Wake behind Circular Disk by Stereo PIV and Conditional Sampling Technique

Study on Vortex Structure in the Wake behind Circular Disk by Stereo PIV and Conditional Sampling Technique

Measurement on the flow structure of a gas-liquid separator applied in TMSR

The swirl flow in an axial gas-liquid separator is critical for the fission gas separation in the thorium molten salt reactor. Although the two-phase flow pattern was visualized in our previous studies, the liquid flow field is still unknown. To clarify the three dimensional fluid structure, we used the stereo PIV technique to explore the distribution of three velocity components. The velocity information was extracted from five typical planar positions covering the whole swirl chamber. The averaged tangential velocity profile indicates that a quasi-Rankine vortex dominates the swirl flow and the swirl intensity represented by the maximal tangential velocity firstly increases and then decays gradually. For the first time, we found that a secondary counter rotating vortex is contained in the core region of the swirl flow and the downstream orifice configured in the separator probably induces the variation of flow structure. The averaged axial velocity is distributed in a W type manner across the swirl chamber. The averaged radial velocity is more complicated due to the special configuration of the separator. The magnitude of radial velocity component is of the same order compared to other two velocity components and the distribution is asymmetrical in the first two observing fields and gradually becoming symmetrical in the downstream. All velocity distributions presented in the 3D waterfall constitute a complete picture of the swirl flow structure in the gas-liquid separator.

Investigation of turbulent swirling jet-flames by PIV / OH PLIF / HCHO PLIF

The present paper reports on the investigation of fuel-lean and fuel-rich turbulent combustion in a high-swirl jet. Swirl rate of the flow exceeded a critical value for breakdown of the swirling jet’s vortex core and formation of the recirculation zone at the jet axis. The measurements were performed by the stereo PIV, OH PLIF and HCHO PLIF techniques, simultaneously. The Reynolds number based on the flow rate and viscosity of the air was fixed as 5 000 (the bulk velocity was U0 = 5 m/s). Three cases of the equivalence ratio ϕ of the mixture issuing from the nozzle-burner were considered, viz., 0.7, 1.4 and 2.5. The latter case corresponded to a lifted flame of fuel-rich swirling jet flow, partially premixed with the surrounding air. In all cases the flame front was subjected to deformations due to large-scale vortices, which rolled-up in the inner (around the central recirculation zone) and outer (between the annular jet core and surrounding air) mixing layers.

Comparison between premixed and partially premixed combustion in swirling jet from PIV, OH PLIF and HCHO PLIF measurements

The present paper reports on the investigation of fuel-rich and fuel-lean turbulent combustion in a high-swirl jet. The jet flow was featured by a breakdown of the vortex core, presence of the central recirculation zone and intensive precession of the flow. The measurements were performed by the stereo PIV, OH PLIF and HCHO PLIF techniques, simultaneously. Fluorescence of OH* in the flame and combustion products was excited via transition in the (1,0) vibrational band of the A2Σ+ – X2Π electronic system. The fluorescence was detected in the spectral range of 305-320 nm. In the case of HCHO PLIF measurements the A-X transition was excited. The jet Reynolds number was fixed as 5 000 (the bulk velocity was U0 = 5 m/s). Three cases of the equivalence ratio ϕ of methane/air mixture issued from the nozzle were considered 0.7, 1.4 and 2.5. In all cases the flame front was subjected to deformations due to large-scale vortices, which rolled-up in the inner (around the central recirculation zone) and outer (between the annular jet core and surrounding air) mixing layers.

The effect of single-horn glaze ice on the vortex structures in the wake of a horizontal axis wind turbine

The present study experimentally investigated the effect of a simulated single-horn glaze ice accreted on rotor blades on the vortex structures in the wake of a horizontal axis wind turbine by using the stereoscopic particle image velocimetry (Stereo-PIV) technique. During the experiments, four horizontal axis wind turbine models were tested, and both “free-run” and “phase-locked” Stereo-PIV measurements were carried out. Based on the “free-run” measurements, it was found that because of the simulated single-horn glaze ice, the shape, vorticity, and trajectory of tip vortices were changed significantly, and less kinetic energy of the airflow could be harvested by the wind turbine. In addition, the “phase-locked” results indicated that the presence of simulated single-horn glaze ice resulted in a dramatic reduction of the vorticity peak of the tip vortices. Moreover, as the length of the glaze ice increased, both root and tip vortex gaps were found to increase accordingly.

Experimental investigation of the effect of Reynolds number on flow structures in the wake of a circular parachute canopy

In the present study, an experimental study was conducted to characterize the effect of Reynolds number on flow structures in the turbulent wake of a circular parachute canopy by utilizing stereoscopic particle image velocimetry (Stereo-PIV) technique. The parachute model tested in the present study was attached by 28 nylon suspension lines and placed horizontally at the test section center of the wind tunnel. The obtained results showed that with the increase of Reynolds number, the intensities of the vortices near the downstream region of the canopy skirt were found to increase accordingly. However, the increase of Reynolds number did not result in a significant change in ensembleaveraged normalized x-component of the velocity, ensembleaveraged normalized vorticity, normalized Reynolds stress, and normalized turbulent kinetic energy distributions in the turbulent wake of the circular parachute canopy. The obtained results are very useful to further our understanding about the unsteady aerodynamics in the wake of flexible circular parachute canopies and to constitute a reference for CFD computation.

Gas-dynamic disturbances created by surface dielectric barrier discharge in the constricted mode

Three-dimensional structure of the gas-dynamic disturbances, created by surface dielectric barrier discharge in a constricted (saturated) mode, was analyzed simultaneously with the discharge morphology. Discharge was created in the still air under normal conditions. Flow visualization was performed by shadowgraphy and stereo-PIV technique. The wall-normal jets with the origins located in between the positions of the constricted filaments are found. Velocity magnitude in the wall-normal direction is comparable with the tangential component. Flow structure is similar to the one created by the serpentine actuator.

Effect of passive pile on 3D ground deformation and on active pile response.

Using a series of model tests, this study investigated the effect of a passive pile on 3D ground deformation around a laterally loaded pile and on that laterally loaded pile's response in sand. The active pile head was subjected to lateral loads, and the passive pile was arranged in front of the active pile. In the model tests, the distance between the two pile centers was set to zero (i.e., a single pile test), 2.5, 4, and 6 times the pile width (B). The 3D ground surface deformations around the active and passive piles were obtained using a newly developed Stereo-PIV technique. The experimental results showed that the ground surface movements were restrained by the passive pile when the pile spacing was less than 6B. The response of the active pile was affected by the passive pile when the pile spacing was less than 4B. This study combined the response of the active pile and surrounding 3D ground deformation to investigate the effect of the passive pile, which is useful to further understand the pile-soil-pile interactions and to enhance pile foundation design in engineering practice.

Experimental assessment of the aero-thermal performance of rib roughened trailing edge cooling channels for gas turbine blades

Based on the combined analysis of detailed flow field and heat transfer experimental data, the aero-thermal behaviour of different trailing edge cooling channels is reported.The reference geometry (G0) is characterized by a trapezoidal cross section of high aspect-ratio, inlet radial flow, and coolant discharge at both model tip and trailing edge, where seven elongated pedestals are also installed. Two variations of the reference geometry have squared ribs installed inside the channel radial central portion (G1) or inside the trailing edge exit region (G2). The forced convection heat transfer coefficient has been measured by means of a steady state Liquid Crystal Thermography (LCT) technique, while reliable and detailed flow measurements have been performed by means of Particle Image Velocimetry (PIV) or Stereo-PIV techniques. The experimental Reynolds number has been fixed at 20,000.The heat transfer data for the three configurations have been analyzed and compared considering both local and channel-averaged features of the heat transfer fields. In particular, the flow mechanisms responsible for the existence of high or low heat transfer regions have been identified and explained. The effects of the different turbulence promoters on both the flow and heat transfer fields have been put in evidence as well. With the aim to determine the most effective configuration, area averaged heat transfer data have been compared, together with information about the channels pressure losses. Configuration G1 turned out to be the most promising, giving rise to a significant heat transfer enhancement associated to a moderate increase in pressure losses.

Flow field analysis inside a gas turbine trailing edge cooling channel under static and rotating conditions: Effect of ribs

The present work is part of a wider research program which concerns the aero-thermal characterization of cooling channels for the trailing edge of gas turbine blades. The selected passage model is characterized by a trapezoidal cross-section of high aspect-ratio and coolant discharge at the blade tip and along the wedge-shaped trailing edge, where seven elongated pedestals are also installed. In this contribution, a new channel configuration provided with inclined ribs installed inside the radial development region is analyzed, extending the previous results and completing the already available data base, thus providing an overall review of the aero-thermal performance of the considered passage. The velocity field inside the channel was measured by means of 2D and Stereo-PIV techniques in multiple flow planes under static and rotating conditions. The tests were performed under engine similar conditions with respect to both Reynolds (Re=20,000) and Rotation (Ro=0, 0.23) numbers. Time averaged flow fields and velocity fluctuation data inside the stationary and rotating channels are analyzed and also critically compared with the data acquired without ribs. In this way the effects on the flow field induced by both rotation and ribs are clearly described. In particular, the ribs modify substantially both the flow field on the channel walls where they are installed and the 3D separation structures that surround the pedestals. If also rotation is taken into account, the relative flow field is characterized by a considerable guiding effect of the ribs coupled with a stronger flow separation on the obstacles that further enhances the heat transfer performances. This behavior was confirmed exploiting the wide thermal data base already available, obtaining a direct link between the observed flow features and the heat transfer performances.

Stereo particle image velocimetry measurement of 3D soil deformation around laterally loaded pile in sand

A developed stereo particle image velocimetry (stereo-PIV) system was proposed to measure three-dimensional (3D) soil deformation around a laterally loaded pile in sand. The stereo-PIV technique extended 2D measurement to 3D based on a binocular vision model, where two cameras with a well geometrical setting were utilized to image the same object simultaneously. This system utilized two open software packages and some simple programs in MATLAB, which can easily be adjusted to meet user needs at a low cost. The failure planes form an angle with the horizontal line, which are measured at 27°–29°, approximately three-fourths of the frictional angle of soil. The edge of the strain wedge formed in front of the pile is an arc, which is slightly different from the straight line reported in the literature. The active and passive influence zones are about twice and six times of the diameter of the pile, respectively. The test demonstrates the good performance and feasibility of this stereo-PIV system for more advanced geotechnical testing.

1186 STEREO PIV MEASUREMENTS OF VORTEX BEHAVIOR PRODUCED BY A PITCHING OSCILLATING DISCOID AIRFOIL

Flying and aquatic animals release a vortex by pitching a wing or a fin. It is well known that vortex motion plays an important role in generation of unsteady fluid force. We intend to elucidate the relationship between unsteady fluid forces and three-dimensional vortex structure in the case of the greater reduced frequency. In the present study, pitch-oscillating motion was carried out as a basic unsteady motion. The vortex structure and its behaviour of a discoid airfoil are investigated during pitch-oscillating. The vertical flow fields were measured using a stereo PIV technique. The vortex growth is observed during pitch-oscillating motion, and large scale vortex is released into the wake close to the airfoil. It was confirmed that two significant vortices are released during one cycle of pitching oscillating and the fluid force shows a peak value at the instant of the vortex shedding.

Flow over back‐facing step in a narrow channel

AbstractFlow structure behind the backward facing step in a narrow channel was studied in details. The step height was 25% of the channel width. The structure of the region just behind the step forming the back‐flow region is studied in details using stereo PIV technique. Time‐mean 3D structures behind the step are evaluated and shown in the paper. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)