Showing papers in "Experiments in Fluids in 2001"

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

Abstract: 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.

Measurement of flows around modern commercial ship models

Abstract: To document the details of flow characteristics around modern commercial ships, global force, wave pattern, and local mean velocity components were measured in the towing tank. Three modern commercial hull models of a container ship (KRISO container ship = KCS) and of two very large crude-oil carriers (VLCCs) with the same forebody and slightly different afterbody (KVLCC and KVLCC2) having bow and stern bulbs were selected for the test. Uncertainty analysis was performed for the measured data using the procedure recommended by the ITTC. Obtained experimental data will provide a good opportunity to explore integrated flow phenomena around practical hull forms of today. Those can be also used as the validation data for the computational fluid dynamics (CFD) code of both inviscid and viscous flow calculations.

Second-order accurate particle image velocimetry

Abstract: An adaptive, second-order accurate particle image velocimetry (PIV) technique is presented. The technique uses two singly exposed images that are interrogated using a modified cross-correlation algorithm. Consequently, any of the equipment commonly available for conventional PIV (such as dual head Nd: YAG lasers, interline transfer CCD cameras, etc.) can be used with this more accurate algorithm. At the heart of the algorithm is a central difference approximation to the flow velocity (accurate to order Δt 2) versus the forward difference approximation (accurate to order Δt) common in PIV. An adaptive interrogation region-shifting algorithm is used to implement the central difference approximation. Adaptive shifting algorithms have been gaining popularity in recent years because they allow the spatial resolution of the PIV technique to be maximized. Adaptive shifting algorithms also have the virtue of helping to eliminate velocity bias errors. The second- order accuracy resulting from the central difference approximation can be obtained with relatively little additional computational effort compared to that required for a standard first-order accurate forward difference approximation. The adaptive central difference interrogation (CDI) algorithm has two main advantages over adaptive forward difference interrogation (FDI) algorithms: it is more accurate, especially at large time delays between camera exposures; and it provides a temporally symmetric view of the flow. By comparing measurements of flow around a single red blood cell made using both algorithms, the CDI technique is shown to perform better than conventional FDI-PIV interrogation algorithms near flow boundaries. Cylindrical Taylor–Couette flow images, both experimental and simulated, are used to demonstrate that the CDI algorithm is significantly more accurate than conventional PIV algorithms, especially as the time delay between exposures is increased. The results of the interrogations are shown to agree quite well with analytical predictions and confirm that the CDI algorithm is indeed second-order accurate while the conventional FDI algorithm is only first-order accurate.

Temperature sensing with thermochromic liquid crystals

Abstract: A review of the most recent developments in the application of thermochromic liquid crystals to fluid flow temperature measurement is presented. The experimental aspects including application, illumination, recording, and calibration of liquid crystals on solid surfaces, as well as in fluid suspensions, are discussed. Because of the anisotropic optical properties of liquid crystals, on-axis lighting/viewing arrangements, combined with in-situ calibration techniques, generally provide the most accurate temperature assessments. However, where on-axis viewing is not possible, calibration techniques can be employed, which reduce the uncertainty associated with off-axis viewing and lighting arrangements. It has been determined that the use of hue definitions that display a linear trend across the color spectrum yield the most accurate correlation with temperature. The uncertainty of both wide-band and narrow-band thermochromic liquid crystal calibration techniques can be increased due to hysteresis effects, which occur when the temperature of the liquid crystals exceeds their maximum activation temperature. Although liquid crystals are commonly used to provide time-mean temperature measurements, techniques are available which allow the monitoring of temporal changes. Selected examples illustrating the use of thermochromic liquid crystals are shown, and a survey of reported temperature measurement uncertainties is presented.

High-resolution measurements of the spatial and temporal scalar structure of a turbulent plume

Abstract: Two techniques are described for measuring the scalar structure of turbulent flows. A planar laser-induced fluorescence technique is used to make highly resolved measurements of scalar spatial structure, and a single-point laser-induced fluorescence probe is used to make highly resolved measurements of scalar temporal structure. The techniques are used to measure the spatial and temporal structure of an odor plume released from a low-momentum, bed-level source in a turbulent boundary layer. For the experimental setup used in this study, a spatial resolution of 150 μm and a temporal resolution of 1,000 Hz are obtained. The results show a wide range of turbulent structures in rich detail; the nature of the structure varies significantly in different regions of the plume.

Evaporating and combusting droplet temperature measurements using two-color laser-induced fluorescence

Abstract: The paper presents a new technique based on laser-induced fluorescence, allowing droplet temperature measurement of evaporating and combusting droplets to be performed. The liquid spray is seeded with a low concentration of rhodamine B. The fluorescence, induced by the green line of an argon laser, is measured on two separated color bands. It is demonstrated that two color bands can be selected for their strong difference in the temperature sensitivity of the fluorescence quantum yield. The determination of the fluorescence ratio between the fluorescence intensity corresponding to each color band allows the tracer concentration and the droplet size dependences to be eliminated. The technique was applied on a monodisperse spray: the effect of a thermal impulse on the distribution of the droplet temperature is studied and, the temperature of combusting droplets is investigated.

Ultrasonic Doppler velocimetry in liquid gallium

Abstract: For the first time, flow velocity is measured in a vortex of liquid gallium, using the pulsed Doppler shift ultrasonic method. At the top of a copper cylinder filled with liquid gallium, we spin a disk and create a turbulent vortex with a dominant nearly axisymmetric velocity field with little variation in the axial direction. The velocity profiles are shown to be well resolved and in quantitative agreement with earlier observations. Reliable velocity measurements in liquid gallium could be obtained only after serious problems due to the formation of oxides were solved. This work opens the way to performing accurate velocity measurements in other liquid metals; preliminary results for liquid sodium are shown.

Characteristics of the reaction zone in a combustor operating at mild combustion

Abstract: The aerodynamic, chemical and thermal aspects of the mild combustion process have been studied with emphasis on mixing rates, flue gas recirculation and strong shear produced by reactants supplied from discrete jets. Time-averaged and instantaneous structures of turbulent flow were examined by visualization and local measurements within a 5400 W burner operating with methane with an overall equivalence ratio varying from 0.8 to 1.2 and at non-premixed and premixed modes. The results showed that the entrainment of the flue gases into the fresh mixture was very important for the initiation and progress of the reaction, and occurred in two successive mechanisms. Initially, the flue gases were driven with the reverse flow towards the annular exit where, by Biot–Savart induction, they acquired some momentum from the supply streams provided at the center. The resulting mixing process in the close vicinity of the burner was less intermittent and this was evident in relatively lower values of the second order moments of the residence time distribution. Slightly downstream, the second order moments were, however, increased by large-scale turbulent fluctuations and this led to the enhancement of the mixing process and introduced some further intermittency. The latter entrainment mechanism caused the flue gases to partially encapsulate the discrete jets, which resulted in islands of flammable mixture surrounded by the inert gases. Hence, as the instantaneous OH radical visualizations revealed, the reaction was only initiated away from the burner and in disconnected regions where the Rayleigh pictures showed strong temperature gradients. As the distance from the nozzle increased further, the reaction seemed to follow local flow patterns in that it progressed radially outwards with large structures, which resulted in an increased space-averaged temperature. Furthermore, the residence time decreased away from the burner and the flame came close to extinction due to the high stretching rates of the large structures. However, the flue gases entrained up to this point increased the inert content of the fresh mixture with chemical time scales comparable to the time scales of the flow. This allowed the reactants to attain temperatures near to those of the flue gases and to ignite with a small temperature rise, which led to a much lower thermal NO formation. The results also showed that when the equivalence ratio of the non- premixed mixtures was increased, the region where the combustion took place was shifted away from the burner and extended further downstream towards the roof. In the case of premixed combustion, however, the reaction started and terminated earlier and was confined to regions in close proximity to the axis. The emissions of OH radical occurred rather patchily and in relatively high concentrations.

Unsteady behavior of back-facing step flow

Abstract: In this paper, a contribution to the study of the unsteady behavior of a back-facing step flow is reported. The main interest is devoted to low-frequency motions. A probe for skin friction measurements is employed to acquire time histories from the step to downstream the reattachment region. Signals are analyzed in the physical space and in the frequency domain. The obtained results, associated with flow visualization in a companion experiment in water, support a model of cyclic motion of growing and successive breakdown of the secondary recirculating bubble. The frequency of this quasi-periodic motion is comparable to the flapping frequency of the whole separated region, reported in the literature.

A study with particle-image velocimetry of the influence of drag-reducing polymers on the structure of turbulence

Abstract: Particle-image velocimetry has been used to study the effect of drag-reducing polymers on the structure of turbulence in a channel flow, under conditions of 41% and 55% drag reduction. The fluctuating velocity fields in the x-y plane and in one x-z plane were measured. The striking features of these results are the damping of small scales and the repression of fluctuations of the velocity component normal to the wall. The role of the wall in creating turbulence diminishes greatly at large drag reductions; Warholic et al. (1999) have shown that a turbulent flow with zero Reynolds stress exists at maximum drag reduction. Velocity fields presented for conditions approaching this critical behavior are of particular interest.

Simultaneous imaging of temperature and mole fraction using acetone planar laser-induced fluorescence

Abstract: Imaging of concentration with acetone PLIF has become popular in mixing investigations. More recently, studies of the temperature dependences of acetone fluorescence have enabled quantitative imaging of temperature using single- or dual-wavelength excitation strategies. We present here the first demonstration of simultaneous imaging of temperature and mole fraction with acetone PLIF. Laser excitation is at 248 and 308 nm; the resulting fluorescence images are captured by an interline transfer CCD camera capable of acquiring two frames with a separation in time of as little as 500 ns. In addition to adding temperature imaging capability, this dual-wavelength approach enables mole fraction to be accurately measured in non-isothermal flows. Tests in a heated turbulent jet demonstrate the ability to record instantaneous mole fraction and temperature structure. The expected correspondence of the temperature and concentration fields is observed, and mean values of these quantities derived from image averaging show the expected radial and centerline profiles as the jet becomes fully developed.

Characteristics of wall pressure fluctuations in separated and reattaching flows over a backward-facing step: Part I. Time-mean statistics and cross-spectral analyses

Abstract: Laboratory measurements were made of wall pressure fluctuations in separated and reattaching flows over a backward-facing step. An array of 32 microphones in the streamwise as well as the spanwise directions was utilized. The statistical properties of pressure fluctations were scrutinized. Emphasis was placed on the flow inhomogeneity in the streamwise direction. One-point statistics such as the streamwise distribution of rms pressure and autospectra were shown to be generally consistent with the prior results. The peak frequency and the fall-off rate of autospectra demonstrated the shear layer-originated nature of pressure fluctuations. The coherences and wavenumber spectra in the streamwise and spanwise directions were indicative of the presence of dual modes in pressure; one is associated with the large-scale vortical structure in the low-frequency region and the other is the boundary-layer-like decaying mode in the high-frequency region.

Simultaneous DPTV/PLIF measurements of a turbulent jet

Abstract: Simultaneous measurements of instantaneous velocity and concentration fields were performed using digital particle tracking velocimetry and planar laser- induced fluorescence for a turbulent jet at a Reynolds number of 3000. The measurements of mean velocity, turbulent stresses, mean concentration, concentration variance, and turbulent flux of tracer all collapse onto self-similar profiles in the far field of the jet. The measurements showed excellent agreement with previous point velocity and concentration measurements. It is concluded that the system is an effective means of measuring the velocity and concentration distributions and turbulent characteristics.

High-speed imaging of supercavitating underwater projectiles

Abstract: High-speed images of supercavitating underwater projectiles traveling up to and exceeding the speed of sound in water were captured using a variety of methods. These images reveal information on projectile flight behavior, stability mechanisms, cavity shape, and in-barrel launch characteristics. This information was used to understand the physics of supercavitating bodies, verify computer models, aid failure analysis, and produce projectile launch package design modifications. In the supersonic tests, projectile shock waves were revealed. Imaging consisted of standard video, 16-mm high-speed, laser illuminated motion pictures, high-speed gated intensified video, and stroboscope illuminated 35-mm still photography. Both front-lit and shadowgraph configurations were used.

A turbulent plane jet impinging nearby and far from a flat plate

Abstract: Plane air jets presenting an impact find applications in many industrial devices. They can be found in installations of heating, cooling or drying, cleaning, pulverization, or containment of polluted environments. Other applications can be found in the ventilation of buildings. The correct design of these kinds of installations requires thorough knowledge of the structure of the jet from the cinematic point of view. With this intention a test bench with variable geometry was developed. Then, using laser Doppler velocimetry (LDV) and particle image velocimetry (PIV), it is possible to analyze the development of the jet for various geometrical and cinematic configurations. It appears that the development of the jet is independent of the Reynolds number, and the velocity decrease in the developed and impinging zones can be characterized by using very simple laws. Furthermore, by PIV visualization of the impinging zone, it has been possible to highlight the causes of mass transfer through the jet.

Passive acoustic bubble sizing in sparged systems

Abstract: Passive acoustic bubble sizing was investigated in both controlled tests and in a stirred, sparged tank typical of the biotechnology or minerals processing industries. Acoustic techniques have promise for industrial systems where other bubble analysis methods are impractical. Acoustic signals were studied for bubbles precisely formed at higher airflow rates. Acoustic pulses varied with bubble production rate as well as with bubble size. A technique of windowing pulses is proposed. Two alternative versions of this windowing technique were applied to a stirred, sparged tank, giving good agreement. It was shown that, in some cases, it may also be possible to acoustically estimate the spatial distribution of void fraction.

Towing tank PIV measurement system, data and uncertainty assessment for DTMB Model 5512

Abstract: A towed PIV system designed by DANTEC Measurement Technology for the Iowa Institute of Hydraulic Research towing tank is commissioned by measuring the mean velocity and Reynolds stresses at the nominal-wake plane of a model-scale ship. The mean velocities are compared with previous 5-hole pitot probe data. Uncertainty assessment following standard procedures is used to quantify the comparisons and reach conclusions regarding the quality of the data. The PIV results are analyzed with regard to axial velocity defects, axial vorticity, and level, pattern and anisotropy of turbulence. Quantitative comparisons with 5-hole pitot data shows that PIV uncertainties are about 1% lower than those for 5-hole pitot. However, data differences are larger than RSS of PIV and 5-hole pitot uncertainties for the axial and vertical velocity components, indicating unaccounted for bias and precision limits. The bias error of the pitot probe in a shear flow is discussed, and a method for reducing the bias error is suggested. The data is being used for verification and validation of RANS simulation of DTMB Model 5512.

Effect of initial conditions on a circular jet

Abstract: Two circular jets are considered. One issues from a contraction with a laminar top-hat velocity profile. The other exits from a pipe with a fully developed turbulent mean velocity profile. In spite of the significantly different initial conditions, spectra of axial and radial velocity fluctuations in the far field (x≳30d, where d is the jet diameter) collapse over scales ranging from the Kolmogorov length scale to a length scale characterizing the organized motion. This agreement is consistent with the observation that, at the measurement station, the large-scale anisotropy is unchanged between the two flows.

Dual-plane stereoscopic particle image velocimetry: system set-up and its application on a lobed jet mixing flow

Abstract: The technical basis and system set-up of a dual-plane stereoscopic particle image velocimetry (PIV) system, which can obtain the flow velocity (all three components) fields at two spatially separated planes simultaneously, is summarized. The simultaneous measurements were achieved by using two sets of double-pulsed Nd:Yag lasers with additional optics to illuminate the objective fluid flow with two orthogonally linearly polarized laser sheets at two spatially separated planes, as proposed by Kaehler and Kompenhans in 1999. The light scattered by the tracer particles illuminated by laser sheets with orthogonal linear polarization were separated by using polarizing beam-splitter cubes, then recorded by high-resolution CCD cameras. A three-dimensional in-situ calibration procedure was used to determine the relationships between the 2-D image planes and three-dimensional object fields for both position mapping and velocity three-component reconstruction. Unlike conventional two-component PIV systems or single-plane stereoscopic PIV systems, which can only get one-component of vorticity vectors, the present dual-plane stereoscopic PIV system can provide all the three components of the vorticity vectors and various auto-correlation and cross-correlation coefficients of flow variables instantaneously and simultaneously. The present dual-plane stereoscopic PIV system was applied to measure an air jet mixing flow exhausted from a lobed nozzle. Various vortex structures in the lobed jet mixing flow were revealed quantitatively and instantaneously. In order to evaluate the measurement accuracy of the present dual-plane stereoscopic PIV system, the measurement results were compared with the simultaneous measurement results of a laser Doppler velocimetry (LDV) system. It was found that both the instantaneous data and ensemble-averaged values of the stereoscopic PIV measurement results and the LDV measurement results agree well. For the ensemble-averaged values of the out-of-plane velocity component at comparison points, the differences between the stereoscopic PIV and LDV measurement results were found to be less than 2%.

Refractive index matching in large-scale stratified experiments

Abstract: The measurement of dilution in jets and plumes by planar laser-induced fluorescence in large-scale stratified environments with refractive index matching is discussed. The density and refractive index of various salt (NaCl) and ethanol solutions were measured to high precision, and it is shown that linear stratifications with density differences at least up to 20 σ t with acceptably small refractive index variations can be achieved at reasonable cost. Equations are presented to facilitate the design of experiments with arbitrary density differences. The presence of ethanol significantly increases the laser attenuation, however. The rate of attenuation was measured for various solute concentrations and equations presented to predict the attenuation for arbitrary salt and ethanol concentrations. The rapid attenuation due to ethanol may limit the maximum density differences attainable with long laser traverse distances. Examples are given of discharges into linearly stratified fluids including a jet in a stationary environment, and a buoyant jet in a cross flow.

Digital particle image velocimetry/thermometry and application to the wake of a heated circular cylinder

Abstract: Digital particle image velocimetry/thermometry (DPIV/T) is a technique whereby the velocity and temperature fields are obtained using thermochromic liquid crystal (TLC) seeding particles in water. In this paper, the uncertainty levels associated with temperature and velocity measurements using DPIV/T are studied. The study shows that large uncertainties are encountered when the temperature is measured from individual TLC particles. Therefore, an averaging procedure is presented which can reduce the temperature uncertainties. The uncertainty is reduced by computing the average temperature of the particles within the common specified sampling window used for standard DPIV. Using this procedure, the velocity and temperature distributions of an unsteady wake behind a heated circular cylinder are measured experimentally at Re=610. The instantaneous DPIV/T measurements are shown to be useful for computing statistical flow quantities, such as mean and velocity-temperature correlations.

The surface hot wire as a means of measuring mean and fluctuating wall shear stress

Abstract: The paper describes some applications of a wall shear stress sensor technique which is based on hot-wire anemometry. The “surface hot wire” is a flush-mounted thermal resistive wire with a tiny slot underneath. The arrangement of this sensor guarantees an improved signal-to-noise ratio compared to a common surface hot film. The setup and the application of single sensors and of surface hot-wire arrays are shown. Some results are presented that were acquired in several experiments in the field of laminar-turbulent transition.

Identification of a new source of peak locking, analysis and its removal in conventional and super-resolution PIV techniques

Abstract: One of the key factors that limit accuracy of particle image velocimetry (PIV) is the peak-locking effect In this paper, a previously uncharacterised source of peak locking is presented This source is neither related to the sensor geometry nor the subpixel resolution peak-fitting algorithms It is present even when the particles are well described in terms of sensor spatial resolution (ie for particle diameters larger than 2 pixels) If no specific actions to avoid it are taken, its effect is especially important in those super-resolution systems that are based on iteratively reducing the size of the interrogation window In this work, the mentioned source and its effects are studied and modelled Based on this study, the actions required to avoid this type of peak locking are described This includes the most usual correlation-based PIV systems, as well as super-resolution ones Once this source of inaccuracy is avoided, it is possible to discriminate the performance of different types of correlation algorithms As a consequence, specific proposals for the algorithms in the last steps of multigrid super-resolution PIV systems are given The performances of the proposed solutions are verified using both synthetic and real PIV images

A single-camera coupled PTV–LIF technique

Abstract: A single-camera coupled particle tracking velocimetry–laser-induced fluorescence (PTV–LIF) technique and validation results from an experiment in a neutrally buoyant turbulent round jet are presented The single-camera implementation allows the use of a 12-bit 60 frame-per-second 1024 × 1024 pixel digital CCD camera capable of streaming images in real time to hard disk resulting in very accurate PTV and LIF with excellent spatial and temporal resolution The technique is capable of determining the turbulent scalar flux, \(\) as well as the Reynolds stress and mean and fluctuating velocity and concentration fields Details of dye choice, corrections for attenuation due to dye, particles and water, photobleaching, vignetting, CCD calibration, and illumination power and geometry corrections are presented Detailed results from the validation experiment confirm the accuracy and resolution of the technique, and in particular, the ability to measure \(\) Bootstrap 95% uncertainty intervals are presented for the calculated statistics

Application of particle image velocimetry to combusting flows: Design considerations and uncertainty assessment

Abstract: Methodological aspects concerning the application of the PIV technique to the study of turbulent flames are discussed in this paper. The physical features of the flow, which have implications for the experimental set-up, image processing and measurement accuracy are identified. Design considerations are developed focusing on several factors: spatial resolution, particle performance, seeding technique, image formation and recording, and image post-processing for the evaluation of the displacement. Relevant uncertainty concerns are related to the effect of the thermophoretic force, acting on a seeding particle while crossing the flame front, and to the non-homogeneity and time-dependence of the refractive index field. The uncertainty due to thermophoresis is assessed by numerically studying the motion of a particle crossing a reference temperature profile. The effect of the refractive index variation is evaluated by means of theoretical analysis of light propagation and image formation, supported by experimental tests designed for this special purpose.

Quantitative schlieren measurements of coherent structures in a cavity shear layer

Abstract: Quantitative flow-field data were obtained in a planar shear layer spanning an open cavity with an extension of the schlieren method. The technique is based on the measurement of light-intensity fluctuations in a real-time schlieren image. Data were collected using a fiber-optic sensor embedded in the imaging screen coupled to a photodetector. Time-resolved measurements of the instantaneous density gradient at a point in the two dimensional flow cross section were thus obtained. Detailed surveys were carried out with both the optical instrument as well as a hot wire at a Mach number of 0.25 and with the optical instrument alone at a Mach number of 0.6. A comparison of the results shows that the non-intrusive technique can accurately measure the growth rates of instability waves in the initial “linear” region of the shear layer. The density-gradient fluctuations measured at different locations (and times) were synchronized by using a microphone inside the cavity as a reference and integrated to yield profiles of the density fluctuations associated with the dominant large-scale structures in the shear layer. Such quantitative visualization is expected to clarify the mechanism of sound generation by shear-layer impingement at the cavity trailing edge and elucidate the nature of this sound source.

Near-wall investigation of backward-facing step flows

Abstract: The electrodiffusion technique has been used to investigate reattaching and recirculating flows behind a backward-facing step. The instantaneous wall shear rate vectors were determined using the current signals provided by a three-segment electrodiffusion probe. The near-wall extents of two counter-rotating recirculation zones located behind the step were determined under turbulent flow conditions in a water channel. The near-wall flow inside these recirculation zones was found to be very unsteady, with strong low-frequency fluctuations. The streamwise profiles of the wall shear stress were measured at several values of the Reynolds number and a high level of skin friction was obtained in the reverse-flow region. The strong dependence of the peak value of skin friction on the Reynolds number confirms the viscous-dominated character of the reverse flow appearing inside the recirculation zone.

Flow analysis of two-dimensional pulsed jets by particle image velocimetry

Abstract: A purely alternating jet without mean mass flux and a mixed pulsed jet containing an additional blowing component were investigated by particle image velocimetry (PIV). The jets issued from a two-dimensional slit connected to a converging nozzle, opening normally from a flat wall. The pulsation was driven by a loudspeaker. The mean velocity fields were characterized by the combination of downstream directional blowing and omni-directional suction. The velocity fluctuations were dominated by contra-rotating eddy pairs synchronized with the pulsation and formed at the jet edges during blowing. Phase-synchronized measurements permit the investigation of the averaged patterns and the cycle-to-cycle fluctuations of these vortices. The mean trajectories of vortex centers during a whole injection cycle show how large lateral jet expansions are achieved. For a purely alternating jet, the expansion takes place close to the slit. For a mixed pulsed jet, the vortices develop farther from the orifice. In addition, proper orthogonal mode decomposition demonstrates that only a few modes are required to represent the main events of the flow dynamics.

X-ray-based assessment of the three-dimensional velocity of the liquid phase in a bubble column

Abstract: Information regarding the local liquid velocity in bubble columns is of great interest for research into its performance. Common optical methods fail in bubble flows that have large void fractions because of the different refraction indices of the liquid and gaseous phases. The new X-ray particle tracking velocimetry (XPTV) described here solves the problem by the use of X-rays instead of light. X-rays penetrate a gas/liquid flow in straight lines. XPTV enables us to measure the three-dimensional velocity of the liquid phase. This method was applied and validated in two bubble columns. The same method is also applicable to opaque liquids.

Accelerations in water waves by extended particle image velocimetry

Abstract: Particle image velocimetry (PIV) measures instantaneous velocity across an extended area of flow by recording the motion of tracers suspended in and moving with the fluid. This principle is extended to the measurement of higher moments of the velocity field (acceleration) by recording the velocity field at two separate time instants using two cameras, viewing the same region of flow. Planar illumination of large areas within a hydrodynamic flow is achieved using a scanned argon ion laser beam and individual velocity measurements are made by cross-correlating image pairs acquired with a cooled, frame-straddling camera. A high-speed acousto-optic modulator is used to shut off the CW laser after two scans of the flow have been captured by the first camera. The modulator switches the beam back on for the second velocity measurement after a programmed delay. Synchronization of the cameras and beam modulator with the scanning beam system is achieved with a purpose-built multi-channel synchronizer device and operated from an integrated modular tree-based acquisition and processing software system. The extended PIV system is employed to measure the velocities and accelerations in periodic waves in a precise laboratory wave tank. A complementary theoretical description of Stokes waves provides a comparison with the measurements. The theoretical model is very precise, with an error term being less than 0.5% relative to the primary wave for the conditions of the experiments. The purpose is to test the measurement system and to judge the accuracy of the wave experiments under realistic and controllable conditions in the laboratory. Good agreement between the experiments and theory is found. The relative accuracy of the present experiments and measurements may be quantified in terms of the standard deviation due to an ensemble of measurements. In the best case, we find a relative standard deviation of 0.6% for the velocity measurements and 2% for the accelerations. It is indicated that such an accuracy may be generally achieved by appropriately choosing the size of the field of view.