Showing papers in "Experiments in Fluids in 1995"

On the accuracy of velocity and vorticity measurements with PIV

Abstract: A number of numerical techniques aimed at improving the accuracy of measurements using the correlation approach in Particle Image Velocimetry, PIV, are proposed and investigated. In this approach the velocity (displacement) is found as the location of a peak in the correlation map. Based on an experimental model the best performing peak finding approaches are selected among different strategies. Second, an algorithm is proposed which minimizes errors on the estimates of vorticity using velocity distributions obtained by means of PIV. The proposed methods are experimentally validated against a flow with known properties.

Image correlation velocimetry

Abstract: This paper focuses on the correlation of two successive scalar images for the purpose of measuring imaged fluid motions. A method is presented for deforming, or transforming, one image to another. Taylor series expansions of the Lagrangian displacement field are used, in conjunction with an integral form of the equations of motion, to approximate this transformation. The proposed method locally correlates images for displacements, rotations, deformations, and higher-order displacement gradient fields, and applies a global minimization procedure to insure a global consistency in the results. An integral form of the equations of motion is employed. No explicit spatial or temporal differentiation of the image data is required in estimating the displacement field. As a consequence, this method is appropriate for both continuous-scalar as well as discrete-particle-image data. Successive two-dimensional digital CCD images of fluid motion marked with dye, are used to verify the capabilities of the method. The utility of the method is also illustrated using a pair of Voyager 2 images of Jupiter.

Reynolds number effect on the skin friction in separated flows behind a backward-facing step

Abstract: Results from the present study show that the skin-friction coefficient decreases as the Reynolds number,Re h , increases in the following manner, C f ,min=−019Re h −1/2 The -1/2 power relationship deduced from the correlationC f,min vsRe h indicates laminar like behavior which is consistent with the findings of Adams et al (1984) Clauser's method, which is frequently used for the determination of the wall shear stress, leads to erroneous results when applied to the velocity measurements obtained in the near field of reattaching flows (and many other wall-bounded nonequilibrium flows) Direct measurements of theC f using the LOI technique give higher values than those obtained by the classical techniques The normalized mean velocity on the wall coordinates violates the universal law-of-the-wall in the near field of reattaching flows

Digital-Particle-Image-Velocimetry (DPIV) in a scanning light-sheet: 3D starting flow around a short cylinder

Abstract: Scanning-Particle-Image-Velocimetry Technique (SPIV), introduced by Brucker (1992) and Brucker and Althaus (1992), offers the quantitative investigation of three-dimensional vortical structures in unsteady flows On principle, this technique combines classical Particle-Image-Velocimetry (PIV) with volume scanning using a scanning light-sheet In our previous studies, single scans obtained from photographic frame series were evaluated to show the instantaneous vortical structure of the respective flow phenomena Here, continuous video recordings are processed to capture also the temporal information for the study of the set-up of 3D effects in the cylinder wake The flow is continuously sampled in depth by the scanning light-sheet and in each of the parallel planes frame-to-frame cross-correlation of the video images (DPIV) is applied to obtain the 2D velocity field Because the scanning frequency and repetition rate is high in comparison with the characteristic time-scale of the flow, the evaluation provides a complete time-record of the 3D flow during the starting process With use of the continuity concept as described by Robinson and Rockwell (1993), we obtained in addition the out-of-plane component of the velocity in spanwise direction This in view, the described technique enabled the reconstruction of the three-dimensional time-dependent velocity and vorticity field The visualization of the dynamical behaviour of these quantities as, eg by video, gave a good impression of the spanwise flow showing the “tornado-like” suction effect of the starting vortices

Flow tagging velocimetry in incompressible flow using photo-activated nonintrusive tracking of molecular motion (PHANTOMM)

Abstract: We report the development of a new optical flow tagging velocimetry technique for hydrodynamic flows. The method utilizes highly water-soluble caged dye Photo-Activated Fluorophores (PAF's) which serve as fluorescent tracers, with essentially indefinite lifetime. Demonstration experiments are presented in a bench-top poiseulle flow and a 5,000 gallon water channel facility. Results of experiments designed to quantify critical optical characteristics of the caged dye PAF's are also presented, as is a comparison with other, similar, optical velocimetry approaches.

New tracking algorithm for particle image velocimetry

Abstract: The cross correlation tracking technique is widely used to analyze image data, in Particle Image Velocimetry (PIV). The technique assumes that the fluid motion, within small regions of the flow field, is parallel over short time intervals. However, actual flow fields may have some distorted motion, such as rotation, shear and expansion. Therefore, if the distortion of the flow field is not negligible, the fluid motion can not be tracked well using the cross correlation technique. In this study, a new algorithm for particle tracking, called the Spring Model technique, has been proposed. The algorithm can be applied to flow fields which exhibit characteristics such as rotation, shear and expansion.

Investigation of the unsteady flow velocity field above an airfoil pitching under deep dynamic stall conditions

Abstract: The unsteady flow field above a NACA 0012 airfoil pitching under deep dynamic stall conditions has been investigated in a low-speed wind tunnel by means of particle image velocimetry. The measurements of the instantaneous flow velocity field show the characteristic features of the dynamic stall process: formation and development of an organized vortex structure for increasing incidences and the subsequent separation. Vorticity and divergence estimated from the measured data give a good insight into the complex flow behaviour during the downstroke motion. Furthermore, small-scale structures could be observed in the separated flow field and even within the dynamic stall vortex.

Visualization of the structure of supersonic turbulent boundary layers

Abstract: A series of flow visualizations has been performed on two flat-plate zero-pressure-gradient supersonic boundary layers. The two different boundary layers had moderate Mach numbers of 2.8 and 2.5 and Reθ's of 82, 000 and 25, 000 respectively. A number of new visualization techniques were applied. One was a variation of conventional schlieren employing “selective cut-off” at the knife edge plane. Motion pictures of the flow were generated with this technique. Droplet seeding was also used to mark the flow, and high speed movies were made to show structure evolution. Still pictures were also taken to show details within the large-scale motions. Finally, Rayleigh scattering was used to construct planar images of the flow. Together, these techniques provide detailed information regarding the character and kinematics of the large-scale motions appearing in boundary layers in supersonic flow. Using these data, in concert with existing hot-wire data, some suggestions are made regarding the characteristics of the “average” large-scale motion.

Analysis of two-point velocity measurements in near-wall flows

Abstract: Two-point measurements of the streamwise velocity in a turbulent channel flow are performed using laser-Doppler anemometry. High spatial and temporal resolutions (about 1 Kolmogorov microscale in space and time) are achieved. Data are obtained at several distances from the wall forRe δ in the range 1500–5000. Results from correlation functions are compared with the hypotheses of Taylor and Tennekes: they reproduce the experimental data even at low Reynolds numbers and small distances from the wall, providing that convection rather than mean velocity is used. Convection velocities are computed from transfer function phase diagrams: the ratio of convection to mean flow velocity is found to decrease with increasing Reynolds number and distance from the wall. Large flow structures are convected with the local mean velocity rather than the test section mean velocity; data at small Reynolds and close to the wall exhibit convection velocities lying between the two. The good agreement between the time evolution of the envelope of space-time-correlation function with the corresponding Lagrangian correlation over one integral time scale confirms the existence of a strict relation between the Eulerian and Lagrangian descriptions of turbulence.

Study of vortex breakdown by particle tracking velocimetry (PTV) Part 3: Time-dependent structure and development of breakdown-modes

Abstract: This third part of the study deals with the time-dependent nature of vortex breakdown. The results show the unsteady velocity and vorticity field of the initiation and development of breakdown and transition between both predominant breakdown modes, the bubble and the spiral. During the development to breakdown, the generated amount of circumferential vorticity follows the theoretical prediction by Brown and Lopez (1990). This confirms the idea of positive feedback as the key-mechanism leading to vortex breakdown. We regard the bubble-type as the fundamental breakdown type, that is stationary and nearly axisymmetric. The circumferential vorticity is distributed in a form of an elliptical vortex-ring-like structure. Starting from this stage, an increase of volume flux to a higher Reynolds number leads to the transition to the spiral-type with an initial stretching of the vortex ring-like structure and a subsequent change to an asymmetric circumferential vorticity distribution. This in combination with the inductive effect causes the front stagnation point to be deflected radially away and later to rotate around the centerline. Consequently the approaching vortex core is radially deflected in opposite direction and evolves in a spiral path. The idea of a second positive feedback-mechanism gives a possible explanation for the transition. Following this theory the asymmetry of circumferential vorticity will trigger itself at a certain degree by the interaction with the inductively affected stagnation point and its influence on the approaching vortex core. This self-enhancing process will finally lead to the spiral-type breakdown in which the radial distance between rotating stagnation point and deflected vortex core is of the order of the characteristic vortex core radius. The reversed transition from the spiral to a stable bubble-type can be regenerated by decreasing the Reynolds number down to the value that corresponds to the stable bubble state. The flow structure evolves nearly in the time-reversed way as during transition from bubble towards the spiral.

Measurements of vaporized and liquid fuel concentration fields in a burning spray jet of acetone using planar laser induced fluorescence

Abstract: Planar LIF of acetone has been performed in the near development field of a burning spray jet. The main difficulty of such investigations comes from the large range of signal levels provided by the vapor and the size dispersed spray, which cannot be covered by the camera dynamics. The key point of the present work lies in a strong compression of the fluorescence signal dynamics as the UV laser radiation is strongly absorbed by the liquid phase. Preliminary experiments were made in homogeneous vapor and with calibrated drops to quantify the acetone fluorescence signals. Analysis of the histograms of signal level in the spray shows that a cut-off signal level can be used to reject the contribution of the liquid phase. The single shot fluorescence profiles have been processed to restore the fields of fluctuating and mean concentration of acetone vapor in the spray. The liquid concentration field was obtained by extracting the individual drops data from the single shot fluorescence images with a reduced gain of the camera. A statistical correction accounting for the discrete extinctions of the laser by the drops has been used. Analysis of the results shows the influence of the input atomization parameters on the structures of the condensed and vaporized concentration fields.

Memory effects in a turbulent plane wake

Abstract: Memory effects in turbulent plane wakes have been investigated for various wake generators (circular, triangular and square cylinders and a screen of 50% solidity) using orthogonal arrays of X-wires, eight in the (x,y) plane and eight in the (x, z) plane. In the far-wake region, discernible differences are observed for different generators, in the measured Reynolds stresses, spectra of v and approximations to the rms spanwise and lateral vorticities. These differences, which reflect variations in various aspects of the organised large-scale structures, are quantified through the contributions these structures make to the Reynolds stresses. The difference between the screen and the solid body wakes is especially pronounced.

Feasibility study of three-dimensional PIV by correlating images of particles within parallel light sheet planes

Abstract: One approach to obtain information about the out-of-plane velocity component from PIV recordings is to analyze the height of the peak in the correlation plane. This value depends on the portion of paired particle images, which itself depends on the out-of-plane velocity component and on other parameters. To circumvent problems with other influences (e.g. background light, amount and size of images), images from another light sheet plane parallel to the first one were also captured for peak height normalization. Our experimental results show the feasibility of an out-of-plane velocity estimation by analyzing images of particles within parallel light sheets by spatial cross-correlation.

Anisotropy of the Reynolds stresses in a turbulent boundary layer on a rough wall

Abstract: The measured anisotropy invariants of the Reynolds stress tensor in a self-preserving rough wall turbulent boundary layer indicate that the anisotropy is significantly smaller than in a smooth wall layer.

Flow of shear-thinning fluids in a concentric annulus

Abstract: Distributions of mean axial velocity, axial and tangential turbulence intensities together with friction factor versus Reynolds number (f-Re) data are presented for three non-Newtonian liquids in fully developed laminar, transitional and turbulent flow in an annular geometry in the absence of centrebody rotation. Each of the non-Newtonian fluids was shear thinning and to some extent elastic and one was also thixotropic in character. For comparison purposes, measurements are also reported for a Newtonian fluid.

Low-Reynolds-number effects in a turbulent boundary layer

Abstract: Low-Reynolds-number effects in a zero pressure gradient turbulent boundary layer have been investigated using a two-component LDV system. The momentum thickness Reynolds number R θ is in the range 400 to 1320. The wall shear stress is determined from the mean velocity gradient close to the wall, allowing scaling on wall variables of the inner region of the layer to be examined unambiguously. The results indicate that, for the present R θ range, this scaling is not appropriate. The effect of R θ on the Reynolds normal and shear stresses is felt within the sublayer. Outside the buffer layer, the mean velocity is more satisfactorily described by a power-law than by a logarithmic distribution.

Particle concentration measurements by phase-doppler anemometry in complex dispersed two-phase flows

Abstract: Particle concentration or mass flux measurements by phase-Doppler anemometry are based on counting the number of particles crossing the probe volume. In complex particulate flows this requires the knowledge of the particle size-dependent cross-section of the measurement volume perpendicular to the instantaneous particle velocity for each sample. A new method is presented which allows to estimate the instantaneous particle velocity using a one-component PDA-system with the aid of the integral value under the envelope of the Doppler signal. The envelope is reliably determined by a recently developed analogue burst detection electronic circuit using narrow band-pass filters and a synchro detector. The particle size-dependent measurement volume is obtained by the mean log-arithmic amplitude method introduced by Qiu and Sommerfeld (1992). The special signal processing requirements for realizing this method were implemented in a novel signal processor which additionally involves a burst detection based on an online estimation of the signal-to-noise ratio (SNR) and hence the data aquisition is only triggered for signals above a pre-set SNR level. The estimation of the signal frequency and phase is based on the calculation of the cross-spectral density using a hardware FFT (Fast Fourier Transform) chip. In order to demonstrate the accuracy of the novel method for particle concentration and mass flux determination, measurements were performed in a liquid spray and a particle laden swirling flow. Especially in the swirling flow, the particles exhibit random trajectories through the measurement volume and the powerfulness of the present method is demonstrated especially for this complex flow.

Lock-in of vortices in the wake of an elevated round turbulent jet in a crossflow

Abstract: An experimental investigation of a round turbulent jet emitted perpendicularly from a pipe (or stack) into a crossflow was conducted by means of multi-point simultaneous velocity measurements. The motivation was to extract and characterize the underlying coherent structures within the near wake region of the flow. The velocity signals were obtained with both normal and X hot-wire anemometer probes and analyzed with correlation, spectral and pattern-recognition techniques. The results establish that Karman-like vortices are shed not only by the stack but also by the jet. These structural features are “locked-in” and are controlled by the stack diameter.

Characterization of near-bed coherent structures in turbulent open channel flow using synchronized high-speed video and hot-film measurements

Abstract: High-speed video recordings (500 Hz) of flow visualizations in the near wall region of a turbulent open channel flow were synchronized with hot-film measurements of flow velocity and bed shear stress. Analysis of the video images provided information about the main characteristics of coherent flow structures associated with the occurrence of low-speed streak ejections near the bed. These structures consisted mainly of oscillating shear layers that were converted in the downstream direction and lifted away from the bed. A visual detection criterion was developed to obtain ensemble averaged profiles of the velocity and shear stress data during ejection events, allowing for the characterization of the associated flow field during the occurrence of coherent structures. Conditional averaging suggests that the occurrence of such coherent patterns affects mainly the turbulence structure in the wall region, and that the observed events reveal a plausible mechanism by which energy is extracted from the mean flow by large scale turbulent fluctuations, and then further transferred towards smaller eddies, while the structures lose their coherence. The intermittent nature of production and dissipation of turbulent energy becomes noticeable, taking place about 21% of the time. The results obtained also provide evidence that seems to link the structures responsible for the turbulent vertical transport of momentum, and for the maintenance of the turbulent state, with the mechanism that triggers the entrainment of sediment into suspension. Comparison of present results with other experiments conducted in different types of flows strongly confirms a universal structure of coherent events in wall bounded flows.

An investigation of the performance of multi layer, neural networks applied to the analysis of PIV images

Abstract: An experimental investigation of freely rising spherical bubbles through a quiescent liquid is presented. The objective of the experiments is to examine the validity of a recently proposed history force expression for clean, spherical bubbles at finite Reynolds number (Mei et al. 1994). Excellent agreement between the measured and predicted bubble trajectory is obtained when using the proposed history force expression; the data presented herein thus provide an indirect validation of the history force expression. The range of Reynolds number and Weber Number based on the terminal velocity are 13 to 212 and 0.03 to 0.69, respectively. The history force expression is only applicable when the quiescent liquid is free of contaminants. Otherwise, both the steady and unsteady forces are difficult to predict because the liquid/ vapor interface is partially immobilized.

Measurements in microscopic flow with a solid-state LDA

Abstract: The application of a solid-state laser doppler anemometer (LDA) to laminar flow occurring in a 175 μm channel is reported. Details of the optical system are described.

Acceleration of a sphere behind planar shock waves

Abstract: The paper presents the results of an investigation on the motion of a spherical particle in a shock tube flow A shock tube facility was used for studying the acceleration of a sphere by an incident shock wave Using different optical methods and performing experiments in two different shock tubes, the trajectory and velocity of a spherical particle were measured Based upon these results and simple one-dimensional calculations, the drag coefficient of a sphere and shading effect of sphere interaction with a shock tube flow were studied

Photobleaching of disodium fluorescein in water

Abstract: Photobleaching of disodium fluorescein dissolved in water is experimentally investigated using laser induced fluorescence (LIF). It is demonstrated that significant photobleaching occurs on the millisecond time scale, resulting in a large decrease in the fluorescence signal emanating from a constant concentration sample. The importance of avoiding photobleaching when using LIF with disodium fluorescein for concentration measurements in water flow experiments is demonstrated. A half-life for photobleaching is introduced and measured for disodium fluorescein and is shown to be a more appropriate measure than the traditional ‘bleaching quantum efficiency’. It is demonstrated that the photobleaching of disodium fluorescein is at least partially reversible.

Non-intrusive measurements of bubble size and velocity

Abstract: A non-intrusive measuring technique based on video-imaging has been developed for the measurement of bubble size, velocity and frequency. Measurements carried out with this method have been compared to those obtained by an optimized phase-Doppler system in standard configuration, for a wide range of bubble sizes produced from single injectors in a quiescent environment. The two measuring techniques have yielded velocities and frequencies that are in very good agreement while the size of spherical bubbles was consistently measured by both methods. The phase-Doppler system was also used to size oblate-spheroidal bubbles moving with their equatorial plane parallel to the scattering plane, yielding measurements reasonably close to the average radius of curvature of the bubbles in the neighborhood of the equatorial plane, as calculated from the video-imaging data. Both methods were used for detailed velocity measurements of the bubble-stream in the neighborhood of the injector tip. The observed bubble-velocity variation with the distance from the injector tip does not always display the usual increasing trend leading into the terminal velocity. When injection conditions are near the transition from discrete to jet injection mode and the bubbles are small, the latter decelerate into a terminal velocity due to direct interaction of successive bubbles at the injector tip. The measured terminal velocities of bubble-chains for a variety of bubble sizes and injection frequencies, are successfully predicted by using a far-field wake approximation to account for the drafting effect which is responsible for bubble-chain velocities higher than those of single bubbles.

Drag measurements at Mach 5 using a stress wave force balance

Abstract: The stress wave force balance, which has been used for measurements of drag on short models in hypersonic impulse facilities, is investigated here for its suitability for drag measurements on a longer, axisymmetric model. The sensitivity of the balance to loading distribution is investigated and results are reported for experiments on a 5° semi-angle cone, 425 mm in length and of 1.71 kg mass. Experimental drag measurements are shown to be in good agreement with theoretical levels. An investigation into the period over which the stress wave force balance can be used is addressed and, for the present model, the balance is shown to be suitable for measurements in flows of durations of one to several milliseconds with an estimated accuracy of ±10%.

Unsteady forces on spherical bubbles

Abstract: An experimental investigation of freely rising spherical bubbles through a quiescent liquid is presented. The objective of the experiments is to examine the validity of a recently proposed history force expression for clean, spherical bubbles at finite Reynolds number (Mei et al. 1994). Excellent agreement between the measured and predicted bubble trajectory is obtained when using the proposed history force expression; the data presented herein thus provide an indirect validation of the history force expression. The range of Reynolds number and Weber Number based on the terminal velocity are 13 to 212 and 0.03 to 0.69, respectively. The history force expression is only applicable when the quiescent liquid is free of contaminants. Otherwise, both the steady and unsteady forces are difficult to predict because the liquid/ vapor interface is partially immobilized.

Drag coefficients of spherical liquid droplets Part 2: Turbulent gaseous fields

Abstract: The drag of non-evaporating, spherical, liquid droplets was measured in turbulent flow fields at parametric ranges relevant to spray combustion, characterized by the droplet Reynolds number, and the intensity and spatial scales of turbulence. The experimental apparatus comprised a wind-tunnel and a piezo-electric droplet generator. The procedure was to inject water droplets of uniform size co-currently and continuously with vertical turbulent air flows while droplet velocity was measured at different elevations using laser-Doppler velocimetry. Turbulence was characterized using hot-wire anemometry prior to droplet injection. Drag coefficients were calculated using these main measurements and the law of conservation of mechanical energy. Reynolds numbers were investigated in the range 10–100, in terms of the equivalent spherical diameter of a droplet, and the mean relative speed between the ambient gaseous field and the droplets. Weber numbers were much less than unity so droplets were effectively spherical. Relative intensities of turbulence were investigated in the range 20–65 percent, in terms of the mean relative speed. Spatial scales of turbulence were large in comparison to the droplets; the ratio between the spatial integral scale and the droplet diameter was in the range 11–38, and the Kolmogorov scale was comparable in size or smaller than the droplet diameter. Experimental data showed that the drag in turbulent fields under these conditions is not significantly different than that of solid spheres in a quiescent field at the same Reynolds number.

Numerical and experimental study of the interaction between a vortex dipole and a circular cylinder

Abstract: This paper describes a study of the centred collision between a dipolar vortex and a solid circular cylinder. The flow was analysed experimentally by using dye visualizations and streak photography. Flow characteristics such as vorticity fields and the transport of passive tracers were compared with numerical simulations. Observations revealed that thin layers of vorticity, created at the cylinder wall are advected by the primary dipole halves, which, while rolling up into compact patches, give rise to the formation of two new asymmetric dipoles that move away along curved trajectories. The structure of the vorticity distribution inside the dipole, before and after the collision, has been investigated. Both the numerical and the experimental results indicate that the vorticity patches originating from the original primary dipole approximately preserve their original functional relationship ω=f(ψ), while the secondary vorticity patches show a tendency to organize into structures attaining a similar relationship.

Uncertainty in calculating vorticity from 2D velocity fields using circulation and least-squares approaches

Abstract: The most common method for determining vorticity from planar velocity information is the circulation method. Its performance has been evaluated using a plane of velocity data obtained from a direct numerical simulation (DNS) of a three dimensional plane shear layer. Both the ability to reproduce the vorticity from the exact velocity field and one perturbed by a 5% random “uncertainty” were assessed. To minimize the sensitivity to velocity uncertainties, a new method was developed using a least-squares approach. The local velocity data is fit to a model velocity field consisting of uniform translation, rigid rotation, a point source, and plane shear. The least-squares method was evaluated in the same manner as the circulation method. The largest differences between the actual and calculated vorticity fields were due to the filter-like nature of the methods. The new method is less sensitive to experimental uncertainty. However the circulation method proved to be slightly better at reproducing the DNS field. The least-squares method provides additional information beyond the circulation method results. Using the correlation $$\overline {P\omega \omega }$$ and a vorticity threshold criteria to identify regions of rigid rotation (or eddies), the rigid rotation component of the least-squares method indicates these same regions.

Experiments to supercritical junction flow

Abstract: Supercritical flow at channel junctions is characterised by a distinct standing wave pattern. This paper aims at describing its features and defining the main dimensions of three waves. Wave C is a junction wave located at the rear of the junction point and may be compared with the wave due to an abrupt wall deflection. Waves B and D are wall waves located downstream of wave C due to its impact on the wall opposite to the lateral branch, and due to the reflection of wave B on the opposite wall, respectively.