Combined Particle Image Velocimetry Research Articles

Investigation of passive scalar mixing in a confined rectangular wake using simultaneous PIV and PLIF

A combined particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) system was employed to investigate the turbulent mixing in a confined liquid-phase rectangular-wake flow with a Reynolds number based on hydraulic diameter of 37,500 and a Schmidt number of 1250. The simultaneous velocity and concentration field data were analyzed for flow statistics such as turbulent fluxes, turbulent viscosity and diffusivity, and turbulent Schimdt number. The streamwise and transverse turbulent fluxes were found to be of the same magnitude. The turbulent flux vector was not aligned with the mean concentration gradient. The turbulent Schimdt number was about 0.8. The spatial correlations of turbulent fluxes and concentration fluctuations were evaluated. In the R u ′ ϕ ′ correlation field, there were a positive and a negative vertically oriented elliptical correlation region, which were symmetric around the basis point. The R v ′ ϕ ′ correlation region was a horizontally oriented ellipse with negative values of the correlation coefficient. The correlation field of R ϕ ′ ϕ ′ was also an ellipse with a horizontal major axis. The behavior of large-scale structures in both the velocity and concentration field was studied using linear stochastic estimation with a defined event of concentration fluctuation. Vortex streets were observed in the estimated velocity fields. The streamwise growth of the structure size increased linearly initially but then grew more slowly.

Unsteady aspects of an incident shock wave/turbulent boundary layer interaction

An incident shock wave/turbulent boundary layer interaction at Mach 2.1 is investigated using particle image velocimetry in combination with data processing using the proper orthogonal decomposition, to obtain an instantaneous and statistical description of the unsteady flow organization. The global structure of the interaction is observed to vary considerably in time. Although reversed flow is often measured instantaneously, on average no reversed flow is observed. On an instantaneous basis, the interaction exhibits a multi-layered structure, characterized by a relatively high-velocity outer region and low-velocity inner region. Discrete vortical structures are prevalent along their interface, which create an intermittent fluid exchange as they propagate downstream. A statistical analysis suggests that the instantaneous fullness of the incoming boundary layer velocity profile is (weakly) correlated with the size of the separation bubble and position of the reflected shock wave. The eigenmodes show an energetic association between velocity fluctuations within the incoming boundary layer, separated flow region and across the reflected shock wave, and portray subspace features that represent the phenomenology observed within the instantaneous realizations.

Shear-Induced Unfolding of Lysozyme Monitored In Situ

Conformational changes due to externally applied physiochemical parameters, including pH, temperature, solvent composition, and mechanical forces, have been extensively reported for numerous proteins. However, investigations on the effect of fluid shear flow on protein conformation remain inconclusive despite its importance not only in the research of protein dynamics but also for biotechnology applications where processes such as pumping, filtration, and mixing may expose protein solutions to changes in protein structure. By combining particle image velocimetry and Raman spectroscopy, we have successfully monitored reversible, shear-induced structural changes of lysozyme in well-characterized flows. Shearing of lysozyme in water altered the protein's backbone structure, whereas similar shear rates in glycerol solution affected the solvent exposure of side-chain residues located toward the exterior of the lysozyme α-domain. The results demonstrate the importance of measuring conformational changes in situ and of quantifying fluid stresses by the three-dimensional shear tensor to establish reversible unfolding or misfolding transitions occurring due to flow exposure.

Novel Approach to Characterize Fluidized Bed Dynamics Combining Particle Image Velocimetry and Finite Element Method

This work presents a new experimental−numerical method combining PIV (particle image velocimetry), digital image analysis, and FEM (finite element method) to obtain gas and particle motion around b...

Experimental characterization of solid particle transport by slug flow using Particle Image Velocimetry

This paper presents an experimental study of gas-liquid slug flow on solid particle transport inside a horizontal pipe with two types of experiments conducted. The influence of slug length on solid particle transportation is characterized using high speed photography. Using combined Particle Image Velocimetry (PIV) with Refractive Index Matching (RIM) and fluorescent tracers (two-phase oil-air loop) the velocity distribution inside the slug body is measured. Combining these experimental analyses, an insight is provided into the physical mechanism of solid particle transportation due to slug flow. It was observed that the slug body significantly influences solid particle mobility. The physical mechanism of solid particle transportation was found to be discontinuous. The inactive region (in terms of solid particle transport) upstream of the slug nose was quantified as a function of gas-liquid composition and solid particle size. Measured velocity distributions showed a significant drop in velocity magnitude immediately upstream of the slug nose and therefore the critical velocity for solid particle lifting is reached further upstream.

Turbulent mass and momentum transport of a circular offset dense jet

Turbulence characteristics of a circular dense jet offset by a small distance from a parallel horizontal boundary have been studied using the combined particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) method. The cross-sectional profiles of the turbulence intensity, Reynolds shear stress and Reynolds flux reveal the characteristics of the flow transition from a dense-jet regime to a wall-dense-jet regime. In the dense-jet regime, the profiles at different downstream locations collapse after normalization, and the turbulent. Self-similarity has not been reached in the covered transitional regime. Turbulent mass and momentum transport are found to be weaker than in the unbounded vertical buoyant jet, but are growing with the transition to the wall jet. In the transitional regime, the turbulence characteristics grow stronger but self-similarity was not observed. The associated turbulent viscosity, ν t , turbulent diffusivity, D t as well as the turbulent Schmidt number, Sc t , have been derived from the presented experimental data.

Combined PIV/PLIF measurements of a steady density current front

A novel method for combined particle image velocimetry and laser induced fluorescence is described and results from an experiment in a stratified flow are presented. A standard two-dimensional, one camera particle image velocimetry configuration is used, acquiring images of the seeding particles and the dye marking the current simultaneously and separating the two fields digitally. The implementation of the postprocessing method, its capabilities and the necessary conditions for its use are discussed in detail. The proposed method is applied to an arrested density current front. The front is made stationary by opposing a uniform velocity profile, obtained from the combination of a moving floor and the recirculation of fresh water in the channel. To improve the quality of the images, the current is made optically homogeneous by matching the refractivity index throughout the domain. Instantaneous and time averaged fields are obtained for both velocity and density. Simultaneous measurements of these fields provide insight in the mixing processes at the front of the density current. In particular persistent billow generation, similar to that found in shear layers and associated with Kelvin–Helmholtz instabilities, is observed.

Visualization of turbulence structure in unsteady non-penetrative thermal convection using liquid crystal thermometry and stereo velocimetry

Temperature and velocity fields in unsteady non-penetrative turbulent thermal convection of a horizontal fluid layer are measured in horizontal and vertical planes simultaneously using the combined liquid-crystal thermometry and stereo particle image velocimetry (PIV) with high spatial and temporal resolution. The result shows the formation of convection pattern across the fluid layer, which originates from the spoke structure over the heated surface. The upward fluid motion is generated from the intersection of the bursting lines of the spoke structure, while the downward motion is induced by the low temperature fluid directing toward the center of the spoke structure. Thus, the large-scale convective motion is produced in the fluid layer through the motion of spoke structure. The POD analysis of the temperature and velocity eigenfunctions shows the existence of large-scale motion in the fluid layer, which supports the observed convection pattern near the heated boundary and in the middle of the fluid layer.

The influence of wavy walls on the transport of a passive scalar in turbulent flows

We carry out an experimental study to investigate the turbulent transport of a passive scalar in turbulent flows between a flat top wall and two different complex surfaces and describe the influence of the different surface geometries on the mixing properties.In order to achieve a homogeneous and inhomogeneous reference flow situation two different types of surface geometries are considered: a sinusoidal bottom wall profile and a profile consisting of two superimposed sinusoidal waves.The laboratory investigations were performed by the use of a combined digital particle image velocimetry (DPIV) and planar laser-induced fluorescence (PLIF) technique to examine the spatial variation of the streamwise, spanwise and wall-normal velocity components and to assess the concentration field of the scalar. Statistically sufficient image sequences (frame rate 4 Hz) are obtained from both DPIV and PLIF to calculate time-averaged statistics of the velocity field and the scalar field. We discuss the influence of the complex surfaces on large-scale structures present in the flow field and the transport of the passive scalar. A higher spanwise spreading of the scalar plume is observed compared to plane channel flows. Due to the influence of the complex surfaces a higher transport normal to the mean flow direction is found. The mixing properties of the turbulent flow are enhanced for the surface composed of two superimposed waves compared to the sinusoidal wavy surface. This is consistent with the scaling of large-scale structures found for this surface geometry.

Experiments on gas transfer at the air–water interface induced by oscillating grid turbulence

The gas transfer process across the air–water interface in a turbulent flow environment, with the turbulence generated in the water phase far away from the surface, was experimentally investigated for varying turbulent Reynolds numbersReTranging between 260 and 780. The experiments were performed in a grid-stirred tank using a combined particle image velocimetry – laser induced fluorescence (PIV-LIF) technique, which enables synoptic measurements of two-dimensional velocity and dissolved gas concentration fields. The visualization of the velocity and concentration fields provided direct insight into the gas transfer mechanisms. The high data resolution allowed detailed quantification of the gas concentration distribution (i.e. mean and turbulent fluctuation characteristics) within the thin aqueous boundary layer as well as revealing the near-surface hydrodynamics. The normalized concentration profiles show that asReTincreases, the rate of concentration decay into the bulk becomes slower. Independent benchmark data for the transfer velocityKLwere obtained and their normalized values (KLSc0.5/uHT) depend onReTwith exponent −0.25. The spectra of the covariance termc′w′ indicate that the contribution ofc′w′ is larger in the lower-frequency region for cases with smallReT, whereas for the other cases with higherReT, the contribution ofc′w′ appears to be larger in the higher-frequency region (small eddies). These interrelated facts suggest that the gas transfer process is controlled by a spectrum of different eddy sizes and the gas transfer at different turbulence levels can be associated with certain dominant eddy sizes. The normalized mean turbulent flux$\overline{c^\prime w^\prime}$profiles increase from around 0 at the interface to about 1 within a depth of approximately 2δe, where δeis the thickness of the gas boundary layer. The measured turbulent flux (c′w′) is of the same order as the total flux (j), which shows that the contribution ofc′w′ to the total flux is significant.

Experimental investigation of mixed convective flow over a wavy wall

An experimental study is carried out to address mixed convection from a heated wavy surface. The channel flow between the sinusoidal surface and a flat top wall is investigated by means of a combined digital particle image velocimetry (DPIV) and planar laser-induced fluorescence (PLIF) technique to examine the spatial variation of the streamwise and wall-normal velocity components, and to assess the concentration field of a tracer dye injected into the fluid. We discuss the influence of mixed convection on turbulence quantities and scalar transport properties. Due to the influence of mixed convection we find asymmetric mean velocity profiles and increased momentum transport in the vicinity of the heated surface. The transport of the tracer dye is characterized by enhanced vertical transport due to buoyancy effects and enhanced spanwise transport due to the presence of longitudinal flow structures induced by the mixed convection. We identify two dominant scalar structures with the most influence on vertical transport. The first one is induced by buoyancy effects, the second one by a combination of buoyancy effects and local wall curvature. Thus the transport properties are additionally enhanced compared to mixed convection from a flat plate by the presence of the wavy surface.

Simultaneous velocity and concentration field measurements of passive-scalar mixing in a confined rectangular jet

Simultaneous velocity and concentration fields in a confined liquid-phase rectangular jet with a Reynolds number based on the hydraulic diameter of 50,000 (or 10,000 based on the velocity difference between streams and the jet exit dimension) and a Schmidt number of 1,250 were obtained by means of a combined particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) system. Data were collected at the jet exit and six further downstream locations. The velocity and concentration field data were analyzed for flow statistics such as turbulent fluxes, turbulent viscosity and diffusivity, and turbulent Schmidt number (Sc T ). The streamwise turbulent flux was found to be larger than the transverse turbulent flux, and the mean concentration gradient was not aligned with the turbulent flux vector. The average Sc T was found to vary both in streamwise and in cross stream directions and had a mean value around 0.8, a value consistent with the literature. Spatial correlation fields of turbulent fluxes and concentration were then determined. The R u′ϕ′ correlation was elliptical in shape with a major axis tilted downward with respect to the streamwise axis, whereas the R v′ϕ′ correlation was an ellipse with a major axis aligned with the cross-stream direction. Negative regions of R u′ϕ′ were observed in the outer streams, and these negatively correlated regions decayed with downstream distance and finally disappeared altogether. The R ϕ′ϕ′ correlation field was found to be an ellipse with the major axis inclined at about 45° with respect to the streamwise direction. Linear stochastic estimation was used to interpret spatial correlation data and to determine conditional flow structures. It is believed that a vortex street formed near the splitter plate is responsible for the negatively correlated region observed in the R u′ϕ′ spatial correlations of turbulent fluxes. A positive concentration fluctuation event was observed to correspond to a finger of nearly uniform concentration fluid reaching out into the outer stream, whereas a negative event corresponds to a pocket of nearly uniform fluid being entrained from the outer stream into the center jet region. Large-scale vortical structures were observed in the conditional velocity fields with an elliptical shape and a streamwise major axis. The growth of the structure size increased linearly initially but then grew more slowly as the flow transitioned toward channel flow.

On Boussinesq and non-Boussinesq starting forced plumes

The characteristics of Boussinesq and non-Boussinesq starting forced plumes were investigated in this study. Two distinct periods in the transient plume penetration are identified, namely, the period of flow development (PFD) and period of developed flow (PDF). Similarity solutions are developed in PDF by incorporating the behaviour of an isolated buoyant vortex ring and recent laboratory results on the trailing forced plume, and the temporal variation of the penetration rate is derived during the different phases of jet-like, transitional and plume-like flow. To verify the similarity solutions, experiments were conducted on vertical starting forced plumes using combined particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) with refractive index matching. The discharge Reynolds number was varied from 3773 to 7403 and the range of excess densities ( are initial plume and ambient density, respectively) from 2.77% to 25.07%. The experimental results revealed distinct differences between plumes having an initial density difference of larger or smaller than 15% due to the non-Boussinesq effects. Thus, the value of 15% was employed as an approximate criterion to divide the plumes into Boussinesq versus non-Boussinesq cases. The measured penetration rates and the mean centreline axial velocity of the Boussinesq starting forced plumes agreed well with the analytical predictions at the fully developed stage. However, the behaviour was substantially more complex for the non-Boussinesq plumes. In the transient records, it was noted that the time scales for the penetration of the starting plumes and the velocity development in the trailing forced plume were similar, but the time scale for the Gaussian profile to become self-similar was somewhat longer.

Combined PIV and DGV applied to a pressurized gas turbine combustion facility

This paper provides an overview of flow field measurements on a pressurized generic combustor that shares typical features of realistic gas turbine combustors. Both Doppler global velocimetry (DGV) and particle image velocimetry (PIV) were applied in parallel to achieve volumetric, three-component velocity data sets of the reacting flow field at pressures of 2 and 10 bar with 700 K pre-heating. Limited optical access to the mixing zone required a combination of PIV and DGV to obtain averaged three-component velocity data from a single viewing direction. The acquired volume data sets of the time-averaged flow in the mixing zone contain about 40 parallel planes spaced at 2 mm with a spatial resolution of 1.2 × 1.2 mm2 each. Difficulties encountered in the application of stereoscopic PIV to a simple atmospheric generic combustor illustrate the advantage of the combined PIV–DGV technique.

Flow structure of microbubble-laden turbulent channel flow measured by PIV combined with the shadow image technique

The turbulence structure of a horizontal channel flow with microbubbles is experimentally investigated using combined particle image velocimetry (PIV) in order to clarify the mechanism of drag reduction caused by microbubbles. A new system which simultaneously measures the liquid phase and the dispersed bubbles is proposed, based on a combination of particle tracking velocimetry (PTV), laser-induced fluorescence (LIF) and the shadow image technique (SIT). To accurately obtain the velocity of the liquid phase, tracer particles which overlap with the bubble shadow images are almost entirely eliminated in the post-processing. Finally, the turbulence characteristics of the flow field are presented, including measurements for both phases, and the bubble effect on the turbulence is quantified.

Combined Particle Image Velocimetry of Turbulence Modification in a Microbubble Channel Flow

The turbulence structure of flow field including microbubbles in a horizontal channel is experimentally investigated using an image processing technique in order to clarify the mechanism for drag reduction caused by microbubbles. A new system for the simultaneous measurement of liquid and dispersed gas phases is proposed, which is based on the combination of Particle Tracking Velocimetry, Laser Induced Fluorescence and Shadow Image Technique (PTV/LIF/SIT). To accurately detect the velocity vectors of the liquid phase in the two coexistent phases, the tracer particles overlapped with bubble shadow images are almost entirely eliminated in the post-processing stage. Finally, turbulence characteristics of the objective flow field are presented with the measurements for both phases obtained by the proposed system. The local interaction between bubbles and surrounding liquid is clarified.

Heat transfer mechanism of a swirling impinging jet in a stagnation region

AbstractHeat transfer characteristics of a swirling impinging jet have been experimentally examined using a combined particle image velocimetry (PIV) and laser‐induced fluorescence (LIF) technique for simultaneous measurement of velocity and temperature fields. The present study shows that the radial width of the jet stretches with increasing swirl intensity, and that the stretching phenomenon contributes to the maximum local heat transfer coefficient. At the stagnation region, the flow near the heated surface is mixed intermittently by reverse flows toward upstream, and spatial distributions of temperature are correlated with instantaneous velocity vector maps. The dynamic behavior of recirculation zones, attributed to swirl number Sw and impinging distance, mainly determines the turbulent heat transfer at the stagnation region. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(8): 663–673, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.10120

Combined Particle Image Velocimetry/Planar Laser Induced Fluorescence for Integral Modeling of Buoyant Jets

Combined particle image velocimetry and planar laser induced fluorescence is an efficient measurement approach for laboratory studies in environmental hydraulics. The coupling of the two well-known techniques enables synchronized planar measurements of flow velocity and concentration in an area yielding both their mean distribution as well as turbulence covariance. In this paper, the merits and limitations of the combination are first discussed. An example of experimental setup is then briefly described. Finally, an application is demonstrated for the integral modeling of an inclined round buoyant jet that takes full advantage of the capability of the combined approach.

The turbulent/non-turbulent interface at the outer boundary of a self-similar turbulent jet

The flow at the outer boundary of a submerged self-similar turbulent jet at Re=2×103 is investigated experimentally by means of combined particle image velocimetry (PIV) laser-induced fluorescence (LIF) measurements. The jet fluid contains a fluorescent dye so that the LIF data can be used to discriminate between the jet fluid and the ambient fluid. The axial velocity, Reynolds stress, and vorticity are determined relative to the jet boundary. The results are compared against the conventional profiles, and the results of a direct numerical simulation of the turbulent far-wake behind a flat plate. The results show a sharp transition between rotational and irrotational fluid at the fluid interface, and the existence of a layer of irrotational velocity fluctuations outside the turbulent region.

Measurements of turbulent mass transport of a circular wall jet

The results of a laboratory investigation on the turbulence characteristics of a circular three-dimensional turbulent wall jet are presented. Measurements were taken up to 50 nozzle diameters using combined particle image velocimetry and planar laser induced fluorescence. The results showed that the induced turbulence was still evolving in the present range and had not achieved similarity. While the turbulent intensity for both velocity and concentration increased downstream, the turbulent mass transport showed a decline over distance for both the streamwise and spanwise directions, implying weakening dispersion from the jet core.