Velocity Field Measurement Technique Research Articles

Flow control of circular cylinder with a V-grooved micro-riblet film

Flow structure around a circular cylinder with a V-grooved micro-riblet is investigated experimentally. The results are compared with that of a smooth cylinder having the same diameter. A flexible V-shaped micro-riblet with peak-to-peak spacing of 300 μ m is made using a MEMS fabrication process of PDMS (Polydimethylsiloxane) replica. The flexible micro-riblet is attached on a circular cylinder with which grooves are aligned with the streamwise flow direction. Drag force acting on the cylinder is measured for Reynolds numbers based on a cylinder diameter ( D = 18 mm ) in the range Re D = 2.5 × 10 3 – 3.8 × 10 4 . At Re D = 3.6 × 10 3 ( U 0 = 3 m / s ) , the V-grooved micro-riblet cylinder reduces drag coefficient by 7.6%, compared with a cylinder with smooth PDMS surface. However, it increases drag coefficient about 4.2% at Re D = 3.6 × 10 4 ( U 0 = 30 m / s ) . Flow field around the micro-riblet cylinder is measured by using a 2-frame PIV velocity field measurement technique. Several hundreds instantaneous velocity fields are ensemble-averaged to get the spatial distributions of turbulent statistics including turbulence intensities and turbulent kinetic energy. For the case of drag reduction at Re D = 4.8 × 10 3 , the vortex formation region behind the V-grooved MRF cylinder is reduced about 10%, compared with the smooth cylinder due to enhanced entrainment of ambient inviscid fluid into the wake region. In addition, the total number of secondary vortices located inside the near wake region is decreased about 20%.

초음파 조영제를 애용한 Echo PIV 기법의 개발

The combination of ultrasound echo images with digital particle image velocimetry (DPIV) methods has resulted in a two-dimensional, two-component velocity field measurement technique appropriate for opaque flow conditions including blood flow in clinical applications. Advanced PIV processing algorithms including an iterative scheme and window offsetting were used to increase spatial resolution. The optimum concentration of the ultrasound contrast agent used for seeding was explored. Velocity validation tests in fully developed laminar pipe flow result of echo PIV showed good agreement with both optical PIV measurements and the known analytic solution based on a volume flow measurement.

Development and validation of echo PIV

The combination of ultrasound echo images with digital particle image velocimetry (DPIV) methods has resulted in a two-dimensional, two-component velocity field measurement technique appropriate for opaque flow conditions including blood flow in clinical applications. Advanced PIV processing algorithms including an iterative scheme and window offsetting were used to increase the spatial resolution of the velocity measurement to a maximum of 1.8 mm×3.1 mm. Velocity validation tests in fully developed laminar pipe flow showed good agreement with both optical PIV measurements and the expected parabolic profile. A dynamic range of 1 to 60 cm/s has been obtained to date.

Investigation of the near-field structure of an elliptic jet using stereoscopic particle imagevelocimetry

A stereoscopic particle image velocimetry velocity field measurement technique was used toinvestigate the flow structure and entrainment rate of a turbulent elliptic jet of aspect ratioAR = 2.A 3D calibration procedure was established to compensate for the distortion and refractioneffects embedded in the captured particle images; the results of this procedure were used tocalculate three orthogonal velocity components. The instantaneous velocity fields of threeorthogonal components in several cross sections perpendicular to the jet axis weremeasured and the spatial distributions of the turbulent statistics were obtainedby ensemble averaging 500 instantaneous velocity fields. The variation of theentrainment rate along the jet axis was evaluated from the mean velocity fieldsand compared with that of the round jet of the same equivalent diameter. Atx/De = 2,the jet widths along the major and minor axes are almost identical and axis-switchingoccurs. As the flow goes further downstream, the jet width grows much faster along theminor axis than along the major axis. After axis-switching, the flow structure of theelliptic jet shows two critical points on the minor axis, which are not observed inthe round jet. The total entrainment rate of the elliptic jet is about 1.5% largerthan that of the round jet. In addition, entrainment of the surrounding fluid ismore active on the minor axis as a result of the fast expansion of the jet alongthis axis due to the self-induction of the elliptic vortical structure in the nearfield.

PIV velocity field measurements of flow around a KRISO 3600TEU container ship model

The main purpose of this investigation was to demonstrate a useful application of the particle image velocimetry (PIV) method to analyze the complex flow characteristics around a ship. For a sample illustration, the KRISO 3600TEU container ship model was chosen. The flow structure in the stern and near-wake region of the model has been investigated experimentally in a circulating water channel. Instantaneous velocity fields measured by the PIV velocity field measurement technique have been ensemble-averaged to give details of flow structures such as the spatial distributions of the local mean velocity, vorticity, and turbulent kinetic energy. The free-stream velocity was fixed at Uo = 0.6 m/s, and the corresponding Reynolds number based on the length between perpendiculars was about 9.0 × 105. The container ship model shows a complicated three-dimensional flow structure in the stern and near-wake regions. The PIV results clearly revealed the formation of large-scale bilge vortices in the stern region and their effect on the flow in the near-wake. The results shown here provide valuable information for hull form design and the validation of viscous ship flow codes and of turbulence models.

Quantitative analysis of flow inside the accumulator of a rotary compressor

The flow structure inside the accumulator of a rotary compressor was investigated experimentally using a hybrid particle tracking velocimetry (PTV) velocity field measurement technique. A proto-type accumulator was studied while operating under real working conditions. The flow inside the accumulator was qualitatively visualized using a high-speed camera. The period of one cycle of the rotary compressor was divided into 4 phases and the velocity fields inside the accumulator were measured using a phase-averaging technique. The flow inside the accumulator, especially in the region between the screen holder and tube holder, showed a pulsating periodic flow structure according to the rotational phase of rotary compressor.

Velocity field measurements of entilation flow in a vehicle interior

The ventilation flow in a 1/10-scale model of a vehicle interior was investigated experimentally. Five different ventilation modes with the same total flow rate were tested to study the effect of ventilation mode on the velocity field in the vehicle compartment. For each mode, more than 450 instantaneous velocity fields were measured using a PIV (particle image velocimetry) velocity field measurement technique. These fields were ensemble averaged to obtain the spatial distribution of the mean velocity and vorticity. The panel-vent ventilation mode was more effective than the foot-vent ventilation mode in terms of active momentum transfer and the uniformity of the speed distribution in the rear compartment. The foot-vent mode was characterised by a large-scale vortex structure in the rear compartment, which suppressed the heat and momentum transfer in this region. In contrast to the panel-vent and foot-vent modes, the hybrid-vent modes did not have a large-scale re-circulation flow in the rear passenger compartment; thus, the hybrid-vent modes are expected to efficiently reduce the thermal non-uniformity in the vehicle interior. The present work highlights the usefulness of the PIV velocity field measurement technique for the analysis of HVAC (hearting, ventilation and air conditioning) problems. These experimental results can be used not only to understand the ventilation flow in the passenger compartment in order to improve passenger comfort, but also to validate numerical predictions.

THE STRUCTURE OF TURBULENT SHEAR FLOW AROUND A TWO-DIMENSIONAL POROUS FENCE HAVING A BOTTOM GAP

The flow field behind porous fences of geometric porosity ε=38·5% with various bottom gaps (G) has been investigated using a hybrid PTV velocity field measurement technique. Four gap ratiosG /H=0·0, 0·1, 0·2 and 0·3 were tested in this study. One thousand instantaneous velocity vector fields in the x–y plane were consecutively measured for each gap ratio. The free-stream velocity was fixed at 10cm/s and the corresponding Reynolds number based on the fence height (H) was Re=2985. The results show that the gap ratio G/H=0·1 gives the best shelter effect among the four gap ratios tested in this study, having a small shelter parameter ψ in a large area behind the fence. As the gap ratio increases, the region of mean velocity reduction decreases and the lower shear layer developed from the bottom gap expands upward. From the spatial distributions of turbulence statistics including turbulence intensities, Reynolds shear stress and turbulent kinetic energy, the wake characteristics can be divided into two categories depending on the gap ratio. When the gap ratio is aboveG /H=0·2, the turbulence statistics have large values in the lower shear layer. For the gap ratio G/H≤0·1, however, the lower shear layer displays small turbulence-statistics values and approach those of the no-gap case (G/H=0) with increasing distance downstream. In the upper shear layer separated from the fence top, the turbulence statistics are nearly independent of the gap ratio.

Flow Field Analysis inside a Molten Zn Pot of the Continuous Hot-dip Galvanizing Process.

The flow field inside a molten zinc pot of the continuous hot-dip galvanizing process of steel strips was investigated experimentally. A 1/5-scale transparent water model with induction heaters, scrapers, and baffles was used in this study. Instantaneous velocity fields were measured using a single-frame PIV velocity field measurement technique with varying the strip velocity V s , flow rate Q of the induction heater, scraper location, and baffle type. The general flow pattern inside the strip region is hardly influenced by the strip speed V s , flow rate Q, and the scraper location around the stabilizing roll. When the induction heater is not operated, a pair of vortices is formed in the inner part of the strip: a clockwise rotating flow at the entrance region and a counter-clockwise rotating flow at the exit region. In the exit region outside of the strip, the flow detached from the stabilizing roll divides into two parts: a counter-clockwise rotating flow in the upper region and a clockwise rotating flow in the lower region. For the cases of no scrapper and scrapper, detached from the stabilizing roll, the flow separates from the moving strip and ascends to the free surface. As the flow rate of the induction heater increases, the ascending flow is weakened and the counter-clockwise rotating flow in the upper area of the outside region becomes tranquil. This indicates that the flow in the upper area of the exit region is greatly influenced by the operation of the induction heater. When a scraper is attached onto the stabilizing roll, the separated flow from the strip is guided downward and the up-rising flow around the stabilizing roll becomes slow and tranquil. By attaching baffles near the moving strip in addition to the stabilizing rolls, the flow entrainment into the corner region between the strip and the stabilizing roll is greatly reduced. These flow control devices should be helpful in reducing top drosses in the zinc plating process.

Velocity field measurements of swirl flow around a forward-swept axial fan using a phase-averaged PTV

The flow characteristics around a rotating axial fan were experimentally investigated using a phase-averaged PTV velocity field measurement technique. The axial fan has 5 forward-swept blades with a radius of 25 mm. Measurements were carried out at 4 axial planes and 4 radial planes perpendicular to the axis of rotation. For the axial plane measurements, one fan blade was divided into 4 different phases in order to analyze the flow structure according to blade phase. For each case, 500 instantaneous velocity fields were measured and ensemble averaged to obtain phase-averaged velocity vector fields and vorticity contours. In addition, measurements were carried out at two planes around the blade surface. Phase averaged velocity fields show periodic variations with respect to the blade phase. The periodic shedding of the tip vortex at the blade tip is also observed. The phase averaged velocity fields measured in the radial planes show periodic variations according to the fan phase and the distance from the fan. These experimental results can be used to validate numerical calculations and to understand the flow characteristics of forward-swept axial fans.

Flow field analysis of a turbulent boundary layer over a riblet surface

The near-wall flow structures of a turbulent boundary layer over a riblet surface with semi-circular grooves were investigated experimentally for the cases of drag decreasing (s +=25.2) and drag increasing (s +=40.6). One thousand instantaneous velocity fields over riblets were measured using the velocity field measurement technique and compared with those above a smooth flat plate. The field of view was 6.75 × 6.75 mm2 in physical dimension, containing two grooves. Those instantaneous velocity fields were ensemble averaged to get turbulent statistics including turbulent intensities and turbulent kinetic energy. To see the global flow structure qualitatively, flow visualization was also carried out using the synchronized smoke-wire technique under the same experimental conditions. For the case of drag decreasing (s +=25.2), most of the streamwise vortices stay above the riblets, interacting with the riblet tips frequently. The riblet tips impede the spanwise movement of the streamwise vortices and induce secondary vortices. The normalized rms velocity fluctuations and turbulent kinetic energy are small near the riblet surface, compared with those over a smooth flat plate. Inside the riblet valleys, these are sufficiently small that the increased wetted surface area of the riblets can be compensated. In addition, in the outer region (y + > 30), these values are almost equal to or slightly smaller than those for the smooth plate. For the case of drag increasing (s +=40.6), however, most of the streamwise vortices stay inside the riblet valleys and contact directly with the riblet surface. The high-speed down-wash flow penetrating into the riblet valley interacts actively with the wetted riblet surface and increases the skin friction. The rms velocity fluctuations and turbulent kinetic energy have larger values compared with those over a smooth flat plate.

Improvement of Natural Ventilation in a Factory Building Using a Velocity Field Measurement Technique

Air movement in wokplaces, whether resulting from a forced ventilation system or naturally occurring airflow, has a significant impact on occupational health. In a huge shipbuilding factory building, typical harmful factors such as fume or vaporized gas from welding and cutting of steel plates, and dusts from grinding give unpleasant feeling. From field data survey, the yearly dominant, wind directions for the shipbuilding factory building tested were northwest, northeast and southeast Among the three wind directions, the ventilation improvement was the worst for the northeastern wind. This study was focused on location of the opening vents in order to utilize the natural ventilation effectively. Instantaneous velocity fields inside the 1/1000 scale-down factory building model were measured using a 2-frame PIV system. The factory building model was embedded in an atmospheric boundary layer simulated in a wind tunnel. The modified vents improve the internal Ventilation flow with increasing the flow speed more than two times, compared with that of present vents.

Flow Control Inside a Molten Zn Pot for Improving Surface Quality of Zinc Plated Strips

The flow fields inside a molten Zn pot of continuous hot-chip galvanizing process were investigated experimentally. With varying several parameters including the strip speed Vs, flow rate Q of induction heater. scrapper location and baffle configuration, instantaneous velocity fields were measured using a PIV velocity field measurement technique. Inside the strip region, counter-clockwise rotating flow is dominant. The general flow pattern inside the strip region is nearly not influenced by the strip speed Vs, flow rate Q and the scrapper location. In the exit region, the flow separated from the moving strip due to the existence of a stabilizing roll ascends to the free surface, for the cases of no scrapper and scrapper detached form the roll. On the other hand, the ascending flow to the free surface is decreased, as the flow rate Q of induction heater increases. By installing a baffle around the uprising strip, the flow moving up to the stabilizing roll decreases. In addition, B-type baffle is better than A-type baffle in reducing speed of flow around the stabilizing rolls. However, the flow ascended to the free surface is largely influenced by changing the flow rate Q, and the scrapper location, irrespective of the baffle type.

Velocity Field Measurement of Flow Around an Axial Fan Using a Phase Averaged 2-Frame PTV Technique

The flow structure around a rotating axial-fan was experimentally investigated using a phase averaging velocity field measurement technique. The fan blades were divided into 4 different phases, for which 500 velocity fields were acquired for each phase angle with a 2-frame PTV system. Velocity field measurements were also carried out at two planes parallel to the axis of rotation, with offsets toward the radial direction of the fan. For accurate synchronization of the PTV system with the phase of the axial fan, two synchronization circuits were employed with a photo-detector attached to the rotating shaft. The phase averaged velocity fields show periodic variations with respect to the blade phase. The periodic formation of vortices at the blade tip is also observed in vorticity contour plots. Locations of local maximum turbulence intensities in the axial and radial directions are found to be located in an alternating pattern. These experimental results can be used to validate numerical calculations and to understand the flow characteristics of an axial fan.