PIV analysis of flow around a container ship model with a rotating propeller

The flow characteristics of the propeller wake behind a container ship model with a rotating propeller were investigated using a two-frame PIV (Particle Image Velocimetry) technique. Ensemble-averaged mean velocity fields were measured at four different blade phases and ensemble-averaged to investigate the flow structure in the near-wake region. The mean velocity fields in longitudinal planes show that a velocity deficit is formed in the regions near the blade tips and hub. As the flow develops in the downstream direction, the trailing vortices formed behind the propeller hub move upward slightly due to the presence of the hull wake and free surface. Interaction between the bilge vortices and the incoming flow around the hull causes the flow structure to be asymmetric. Contour plots of the vorticity give information on the radial distribution of the loading on the blades. The radial velocity profiles fluctuate to a greater extent under the heavy (J=0.59) and light loading (J=0.88) conditions than under the design loading condition (J=0.72). The turbulence intensity has large values around the tip and trailing vortices. As the wake develops in the downstream direction, the strength of the vorticity diminishes and the turbulence intensity increases due to turbulent diffusion and active mixing between the tip vortices and the adjacent wake flow.     

Experimental study on wake flow structures of screen cylinders using PIV

Experiments were conducted in a water flume using Particle Image Velocimetry (PIV) to study the evolution of the vortical structures in the wakes of four types of screen cylinders at a Reynolds number of about 3200. The results were compared with that of a bare cylinder. The screen cylinders were made of stainless steel screen meshes of various porosities (37%, 48%, 61% and 67%) rolled into cylindrical shapes. Smoke wire flow visualisations in a wind tunnel were also conducted in support of the PIV tests. Depending on the porosity of the screen mesh, two vortex formation mechanisms for the screen cylinder wakes were identified. One was associated with wake instability and the other was associated with shear-layer (Kelvin-Helmholtz) convective instability which involved merging through pairing and tripling of small-scale vortices within the shear layers. The former was responsible for the formation of large-scale vortices in the bare cylinder and the screen cylinder wakes with 37% and 48% porosities, while the latter was responsible for the screen cylinder wakes with 61% and 67% porosities. The results also showed that with increasing porosity, the vortex formation region was extended farther downstream and the Reynolds shear stress, the Turbulent Kinetic Energy (TKE) and vortex intensity were decreased constantly.     

Particle image velocimetry （PIV） measurements of tip vortex wake structure of wind turbine

Large-view flow field measurements using the particle image velocimetry (PIV) technique with high resolution CCD cameras on a rotating 1/8 scale blade model of the NREL UAE phase VI wind turbine are conducted in the engineering-oriented Φ3.2 m wind tunnel.The motivation is to establish the database of the initiation and development of the tip vortex to study the flow structure and mechanism of the wind turbine.The results show that the tip vortex first moves inward for a very short period and then moves out...     

Measurement of instantaneous velocity and surface topography in the wake of a cylinder at low Reynolds number

A technique capable of simultaneous measurement of free-surface topography and velocity vector field data is presented. This technique offers substantial benefits of both reduced complexity and enhanced accuracy over all other techniques known to offer the same measurements. The flow behind a circular cylinder at low Reynolds numbers is measured using this technique. The velocity and vorticity fields as well as Strouhal number closely match the expected results. The free-surface topography, which can be related to the pressure field, exhibits an intimate relationship to the vorticity field.     

3-D scanning-particle-image-velocimetry: Technique and application to a spherical cap wake flow

A 3-D time-resolving and whole-volume Digital-Particle-Image-Velocimetry (DPIV) technique based on the concept of a scanning light-sheet is presented and applied here to the 3-D transient wake phenomena in the spherical cap wake flow. The technique uses a scanning light-sheet for rapid sampling of the flow in depth and a two-camera recording system for stereoscopic 3-D DPIV. Application of a correlation technique in combination with a calibration yields, aside from the correct in-plane displacement, also the out of plane component and thus the total velocity vectors within the planes of the scanning light-sheet. With a high scanning rate in comparison to the characteristic time scales the method provides the 3-D velocity field in space and time. Through the use of conventional video-techniques the temporal evolution of the complete velocity and vorticity field can be obtained quantitatively from experiments. This is demonstrated for the 3-D starting flow around a spherical cap at Re=300. During the starting process, the flow in the wake evolves into a spherical vortex ring where the velocity distribution is very close to the theoretical solution of the Hill-type vortex. Later on, the Hill-type vortex ring deforms and the flow changes from a rotational symmetric stage to a planar symmetric flow with a double-threaded vortical structure which consists of two counter-rotating streamwise vortices similar to the ones observed in sphere wake flow.Presented at the EUROMECH Coll. 335, Image Techniques and Analysis in Fluid Dynamics, 5–7 June 1995, Roma, Italy. A version of this paper has been published in Proc. 7th Int. Symp. Flow Visualization (ed. J.P. Crowder), Begell House Inc., New York, 1995, pp. 715–720.     

A multiphase,micro-scale PIV measurement technique for liquid film velocity measurements in annular two-phase flow

Prediction methods for two-phase annular flow require accurate knowledge of the velocity profile within the liquid film flowing at its perimeter as the gradients within this film influence to a large extent the overall transport processes within the entire channel. This film, however, is quite thin and variable and traditional velocimetry methods have met with only very limited success in providing velocity data. The present work describes the application of Particle Image Velocimetry (PIV) to the measurement of velocity fields in the annular liquid flow. Because the liquid is constrained to distances on the order of a millimeter or less, the technique employed here borrows strategies from micro-PIV, but micro-PIV studies do not typically encounter the challenges presented by annular flow, including very large velocity gradients, a free surface that varies in position from moment to moment, the presence of droplet impacts and the passage of waves that can be 10 times the average thickness of the base film. This technique combines the seeding and imaging typical to micro-PIV with a unique lighting and image processing approach to deal with the challenges of a continuously varying liquid film thickness and interface. Mean velocity data are presented for air–water in two-phase co-current upward flow in a rectangular duct, which are the first detailed velocity profiles obtained within the liquid film of upward vertical annular flow to the authors’ knowledge. The velocity data presented here do not distinguish between data from waves and data from the base film. The resulting velocity profiles are compared with the classical Law of the Wall turbulent boundary layer model and found to require a decreased turbulent diffusivity for the model to predict well. These results agree with hypotheses previously presented in the literature.     

Influence of unsteady wake on a turbulent separation bubble

 An experimental study was made of turbulent separated and reattaching flow over a blunt body, where unsteady wake was generated by a spoked-wheel type of wake generator with cylindrical rods. The influence of unsteady wake was scrutinized by altering the rotation direction (clockwise and counter-clockwise) and the normalized passing frequency (0 ≤ St _H  ≤ 0.20). The Reynolds number based on the cylindrical rod was Re _d =375. A phase-averaging technique was employed to characterize the unsteady wake. The effect of different rotation directions, which gave a significant reduction of x _R , was examined in detail. The wall pressure fluctuations on the blunt body were analyzed in terms of the spectrum and the coherence. Received: 15 January 2001 / Accepted: 17 July 2001     

Étude d'une ligne de jets impactant une paroi concave par PIV stéréoscopiqueStudy of a row of jets impinging a concave wall by stereoscopic PIV

The aerodynamic study of a row of axisymmetric jets impinging a concave wall is carried out from velocity measurements obtained by the standard and stereoscopic Particle Image Velocimetry. The principle and the specific aspects of the stereoscopic PIV set up, a recent technique of three-dimensional velocimetry, are explained. After a statistical data processing, the three-dimensional structure and the characteristics of multiple jets impinging a concave wall are described with the mean velocity fields and the turbulent values in several planes of the flow. To cite this article: V. Gilard, L.-E. Brizzi, C. R. Mecanique 334 (2006).     

Determination of complete velocity gradient tensor by using cinematographic stereoscopic PIV in a turbulent jet

Cinematographic stereoscopic PIV measurements were performed in the far field of an axisymmetric co-flowing turbulent round jet (Re _T ≈ 150, where Re _T is the Reynolds number based on Taylor micro scale) to resolve small and intermediate scales of turbulence. The time-resolved three-component PIV measurements were performed in a plane normal to the axis of the jet and the data were converted to quasi-instantaneous three-dimensional (volumetric) data by using Taylor’s hypothesis. The availability of the quasi-three-dimensional data enabled the computation of all nine components of the velocity gradient tensor over a volume. The use of Taylor’s hypothesis was validated by performing a separate set of time-resolved two component “side-view” PIV measurements in a plane along the jet axis. Probability density distributions of the velocity gradients computed using Taylor’s hypothesis show good agreement with those computed directly with the spatially resolved data. The overall spatial structure of the gradients computed directly exhibits excellent similarity with that computed using Taylor’s hypothesis. The accuracy of the velocity gradients computed from the pseudo-volume was assessed by computing the divergence error in the flow field. The root mean square (rms) of the divergence error relative to the magnitude of the velocity gradient tensor was found to be 0.25, which is consistent with results based on other gradient measurement techniques. The velocity gradients, vorticity components and mean dissipation in the self-similar far field of the jet were found to satisfy the axisymmetric isotropy conditions. The divergence error present in the data is attributed to the intrinsic uncertainty associated with performing stereoscopic PIV measurements and not to the use of Taylor’s hypothesis. The divergence error in the data is found to affect areas of low gradient values and manifests as nonphysical values for quantities like the normalized eigenvalues of the strain-rate tensor. However, the high gradients are less affected by the divergence error and so it can be inferred that structural features of regions of intense vorticity and dissipation will be faithfully rendered.     

NUMERICAL STUDY OF PARTICLE DISTRIBUTION IN WAKE OF LIQUID-PARTICLE FLOWS PAST A CIRCULAR CYLINDER USING DISCRETE VORTEX METHOD

Particle-laden water flows past a circular cylinder were numerically investigated. The discrete vortex method (DVM) was employed to evaluate the unsteady water flow fields and a Lagrangian approach was applied for tracking individual solid particles. A dispersion function was defined to represent the dispersion scale of the particle. The wake vortex patterns, the distributions and the time series of dispersion functions of particles with different Stokes numbers were obtained. Numerical results show that the particle distribution in the wake of the circular cylinder is closely related to the particle's Stokes number and the structure of wake vortices: (1) the intermediate sized particles with Stokes numbers, St, of 0.25, 1.0 and 4.0 can not enter the vortex cores and concentrate near the peripheries of the vortex structures, (2) in the circular cylinder wake, the dispersion intensity of particles decreases as St is increased from 0.25 to 4.0.     

Three-component micro-PIV using the continuity equation and a comparison of the performance with that of stereoscopic measurements

Recently, a number of techniques have been presented for the determination of the third “out-of-plane” velocity component in micro particle image velocimetry (micro-PIV) data. In particular, the conventional macroscopic stereo-PIV technique has been converted to the micro scale by the use of stereo-microscopy. In this work a different technique is investigated, which uses conventional, two-component micro-PIV to generate velocity data on a number of planes. The in-plane velocity gradients are then calculated, which can be used in the continuity equation to produce the out-of-plane velocity gradients. These, together with the no-penetration boundary condition, can then be used to calculate the out-of-plane velocities. An algorithm is presented that is capable of handling up to one invalid vector per column of data by using a combination of first order and second order projections of the velocity. The advantage of the continuity based technique is that it uses the existing two-component micro-PIV technology, which at present is in a more advanced stage of development then stereo-microscopy based micro-PIV. The technique is investigated using a flow similar to one used previously to assess stereoscopic micro-PIV (Meas Sci Technol 17:2175–2185, 2006). This allows a comparison of the performance of the two techniques. The results show that the continuity based data agrees well with an independent computational fluid dynamics solution and has a smaller experimental uncertainty than the stereoscopic technique at a better spatial resolution. There are, however, potential limitations to the continuity based technique. These include the two-dimensionality of the data, which is limited to the planes on which the original images were taken, and the dependence of the technique on the data close to surfaces, where the experimental errors are often greatest. Stereoscopic micro-PIV does not have these limitations so, whilst at present it appears that continuity based techniques may be more accurate, there is sufficient potential for stereoscopic techniques to justify their further development.     

Measurement of the acoustic velocity field of nonlinear standing waves using the synchronized PIV technique

The motion of gas within an air-filled rigid-walled square channel subjected to acoustic standing waves is experimentally investigated. The synchronized particle image velocimetry (PIV) technique has been used to measure the acoustic velocity fields at different phases over the excitation signal period. The acoustic velocity measurements have been conducted for two different acoustic intensities in the quasi-nonlinear range (in which the nonlinear effects can be neglected in comparison with the dissipation effects), and one acoustic intensity in the finite-amplitude nonlinear range (in which both the nonlinear term and the dissipative term play a role in the wave equation). The experimental velocity fields for the quasi-nonlinear cases are compared with the analytical results obtained from the time-harmonic solution of the wave equation. Good agreement between the experimental and analytical velocity fields proves the ability of the synchronized PIV technique to accurately measure both temporal and spatial variations of the acoustic velocity fields. The verified technique is then used to measure the acoustic velocity fields of the finite-amplitude nonlinear case at different phases.     

A novel architecture for a super-resolution PIV algorithm developed for the improvement of the resolution of large velocity gradient measurements

Three different particle image processing algorithms have been developed for the improvement of PIV velocity measurements characterized by large velocity gradients. The objectives of this study are to point out the limitations of the standard processing methods and to propose a complete algorithm to enhance the measurement accuracy. The heart of the PIV image processing is a direct cross-correlation calculation in order to obtain complete flexibility in the choice of the size and the shape of the interrogation window (IW). An iterative procedure is then applied for the reduction of the size of IW at each measurement location. This procedure allows taking into account the local particle concentration in the image. The results of this first iterative processing, applied to synthetic images, show both a significant improvement of measurement accuracy and an increase of the spatial resolution. Finally, a super-resolution algorithm is developed to further increase the spatial resolution of the measurement by determining the displacement of each particle. The computer time for a complete image processing is optimized by the introduction of original data storage in Binary Space Partitions trees. It is shown that measurement errors for large velocity gradient flows are similar to those obtained in simpler cases with uniform translation displacements. This last result validates the ability of the developed super-resolution algorithm for the aerodynamic characterization of large velocity gradient flows.     

Turbulent kinetic energy balance measurements in the wake of a low-pressure turbine blade

The turbulent kinetic energy budget in the wake generated by a high lift, low-pressure two-dimensional blade cascade of the T106 profile was investigated experimentally using hot-wire anemometry. The purpose of this study is to examine the transport mechanism of the turbulent kinetic energy and provide validation data for turbulence modeling. Point measurements were conducted on a high spatial resolution, two-dimensional grid that allowed precise derivative calculations. Positioning of the probe was achieved using a high accuracy traversing mechanism. The turbulent kinetic energy (TKE) convection, production, viscous diffusion and turbulent diffusion were all obtained directly from experimental measurements. Dissipation and pressure diffusion were calculated indirectly using techniques presented and validated by previous investigators. Results for all terms of the turbulent kinetic energy budget are presented and discussed in detail in the present work.     

Implementation and validation of a slender vortex filament code: Its application to the study of a four‐vortex wake model

A computational code EZ‐vortex is developed for the motion of slender vortex filaments of closed or open shape. The integro‐differential equations governing the motion of the vortex centre lines are either the Callegari and Ting equations, which are the leading order solution of a matched asymptotic analysis, or equivalent forms of these equations. They include large axial velocity and nonsimilar profiles in the vortical cores. The fluid may be viscous or inviscid. This code is validated both against known solutions of these equations and results from linear stability analyses. The linear and non‐linear stages of a perturbed two‐vortex wake and of a four‐vortex wake model are then computed. Copyright © 2004 John Wiley & Sons, Ltd.     

Unsteady velocity field measurements at the outlet of an automotive supercharger using particle image velocimetry (PIV)

Particle image velocimetry (PIV) was used to study air flow characteristics at the outlet of an automotive supercharger. Instantaneous velocity fields were analyzed to yield ensemble-averaged velocities and Reynolds stresses, and the ensemble-averages were used to determine maximum velocity and exit flow angle as a function of blade position for various speeds and pressure ratios. The results show that the flow exits the supercharger as a high-speed jet that not only varies in the parallel plane but also in the perpendicular plane, generating a complex three-dimensional flow. The flow varies in the magnitude and the angle at which it leaves the supercharger with the change in blade position and follows a periodic behavior. The maximum velocity at which the flow exits the supercharger also follows a periodic behavior with a variation of 25–30% observed for all the cases. In the parallel plane, the exit angles are periodic every 60° of blade rotation and vary by as much as 40°, whereas periodic behavior with every 120° of blade rotation and a variation of 60° is observed in the perpendicular plane. Variation in flow with blade position is also observed in the velocity and turbulence profiles, with periodic behavior with every 60° blade rotation. The velocity and velocity fluctuation profiles show that the unsteady nature of the flow is most significant close to the outlet, and these unsteady variations diminish 58 mm downstream of the outlet. An exit flow pattern of a Fig. 8 is generated as the flow leaves the blades with one complete blade rotation of 120° for all the cases, except 4000 rpm, pressure ratio 1.4, where the flow exits in a circular pattern.     

Controlling the near vortex wake of a swept half-wing by means of mini-flaps

The effect of mini-flaps on the flow pattern in the near vortex wake behind a model swept half-wing is investigated. The distributions of the time-average flow velocity were measured in a subsonic wind tunnel, in a section normal to the freestream velocity vector located at a distance of 3.8 wing half-spans from its trailing edge. When mini-flaps are mounted on both upper and lower wing surfaces, two vortices (tip and auxiliary) of the same sign are observable in the above-mentioned flow section; they are separated by an extended region of vorticity of the opposite sign. The model angle-of-attack effect on the intensities of the tip and auxiliary vortices is estimated.