Particle Image Velocimetry Recordings Research Articles

Simultaneous flow measurement and deformation tracking for passive flow control experiments involving fluid–structure interactions

Fluid–structure interactions (FSI) on highly flexible structures involve large deformations and require specific techniques for a thorough investigation of the flow field and structural deformation. To this purpose, a physics-informed method is introduced that allows for simultaneous determination of the flow fields and the structural deformation by using Particle Image Velocimetry (PIV) raw images. The method combines apriori knowledge of the mechanical characteristics of the flexible structure with classical image processing techniques for segmentation. PIV recordings of an actively pitched, highly deformable hydrodynamic profile experiment in a closed water tunnel serve as an example case. To achieve accurate results, the contour obtained from image segmentation is further defined under the assumption that its flexure can be described with the Euler–Bernoulli beam theory model. This makes it possible to determine the neutral fiber of the structure and the final reconstruction becomes possible from knowledge of the original geometry. The resulting procedure allows for a recognition of the structure itself and is suitable for cross-section deformation measurements and for masking of the structure in the raw images to improve the PIV processing. A test case comprising synthetic data similar to the application with a modeled profile geometry of known shape is used to investigate the accuracy of the method and its validity for deformation measurements. Under conditions of cyclic dynamic stall, a mean absolute error of 0.84° could be reached, with a deterioration up to 2° mean absolute error under static stall. The method has a major advantage compared to other technically more sophisticated and complex methods, such as the combination of Laser interferometers combined with Laser-Doppler Anemometry: the method allows for the usage of a single data source for both, fluid and solid in a unified measurement method. Therefore a direct comparison of instantaneous flow field and deformation is possible. In consequence it is in particular useful for highly dynamic multi-physical processes involving extreme deformations, such as passive flow control and soft actuated or flexible under water robotics.

Experimental Two-Phase Flow Analysis Inside a Laboratory Scale Jet-Engine Combustion Chamber Using Particle Image Velocimetry

Abstract The characterization of the two-phase kerosene/air flow near the nozzle of an aero-engine combustor is important in order to understand the combustion characteristics of the burner. Typically, particle image velocimetry (PIV) or laser Doppler velocimetry is used to measure velocities inside aero-engine combustors. However, these measurement techniques rely on tracer particles to visualize the flow field and are usually only able to measure the velocity field of one phase at a time. In the case of PIV measurements, both the flow tracers and the kerosene droplets scatter the laser light, and thus appear on the PIV recordings. Depending on droplet size and flow velocity, these kerosene droplets do not necessarily follow the airflow leading to errors in the derived velocity field. This work presents a method on how to separate kerosene droplets from flow tracers depending on their optical characteristics in the PIV recording. This phase separation enables the independent measurement of the flow fields of both the gaseous and liquid phase at the same time as well as the instantaneous slip velocity between droplets and gaseous flow using a standard PIV setup. The method is demonstrated on a laboratory scale aero-engine combustor operated at atmospheric conditions. The obtained results show that in the setup under investigation, gaseous and liquid phase can have significantly different flow fields with kerosene droplets moving in the opposite direction of the recirculating airflow.

Near-wall statistics of a turbulent pipe flow at shear Reynolds numbers up to 40 000

This paper reports on near-wall two-component–two-dimensional (2C–2D) particle image velocimetry (PIV) measurements of a turbulent pipe flow at shear Reynolds numbers up to $Re_{\unicode[STIX]{x1D70F}}=40\,000$ acquired in the CICLoPE facility of the University of Bologna. The 111.5 m long pipe of 900 mm diameter offers a well-established turbulent flow with viscous length scales ranging from $85~\unicode[STIX]{x03BC}\text{m}$ at $Re_{\unicode[STIX]{x1D70F}}=5000$ down to $11~\unicode[STIX]{x03BC}\text{m}$ at $Re_{\unicode[STIX]{x1D70F}}=40\,000$. These length scales can be resolved with a high-speed PIV camera at image magnification near unity. Statistically converged velocity profiles were determined using multiple sequences of up to 70 000 PIV recordings acquired at sampling rates of 100 Hz up to 10 kHz. Analysis of the velocity statistics shows a well-resolved inner peak of the streamwise velocity fluctuations that grows with increasing Reynolds number and an outer peak that develops and moves away from the inner peak with increasing Reynolds number.

The Use of Sensor Binning Option in Double-Shutter CCD Based Digital Particle Image Velocimetry

In this work we present an experimental investigation of the bene ts of double-shutter CCD's pixel binning option in double-frame particle image velocimetry experiments. The CCD binning process increases the sensitivity, signal-to-noise ratio and frame rate of the imaging sensor at the cost of spatial resolution. In order to explore the bene ts of the CCD pixel binning option, in low level illuminated particle image velocimetry measurements, we have carried out of series of ow velocity measurements experiments in 30 μm × 300 μm × 50000 μm microchannel using micro particle image velocimetry setup. The system is equipped with dual cavity laser system conjugated with an optical attenuator for volume illumination, a double-shutter CCD camera (1392×1040 quadratic pixels with 6.45 μm size), a high magni cation optical epi uorescent microscope and a syringe pump. The ow images were recorded at normal, 2×1, 1×2 and 2×2 pixel binning modes of a monochrome CCD camera. A comparison of velocity vector patterns obtained in low level illumination experiments for four di erent pixel binning modes shows that pixel binning option signi cantly increases the signal-to-noise ratio in particle image velocimetry recordings. A good agreement of experimental velocity pro les obtained using cross-correlation analysis and sub-pixel interpolation scheme based on a Gaussian regression with theoretical calculated pro les shows the consistency of the experimental results.

PIV Measurement of the Fluidized Particles’ Velocity Field in a Cylindrical Spouted Bed

In this paper, the instantaneous velocity fields of the particles in the spouted bed were measured by the particle image velocimetry (PIV). In the experiment, the spouted particles achieved the visible accumulation effect when the fountain height was 0.85 times of the packed bed height h. Since the particles are dispersed with each other, the PIV recordings were evaluated by the multiple-pass recursive method based on the FFT cross-correlation algorithm. The time-averaged velocity field of the particles obtained from the PIV experiment had similar distribution with the velocity field calculated by the numerical simulation. In the averaged particle velocity field, there was a continuous region which divided the whole flow field into two parts. The particle velocities in the region approximated to zero especially in vertical direction, which means the particles with upwards motion were basically equated to those with downwards motion in the region. Therefore the effects of particles’ face-to-face friction and even abrasion were much stronger in the region. Further, the experimental results also show that the area of the region could be about 11.19% of the whole flow field.

A refractive index-matched facility for fluid–structure interaction studies of pulsatile and oscillating flow in elastic vessels of adjustable compliance

The flow field in the respiratory and vascular system is known to be influenced by the flexibility of the walls. However, up to now, most of the experimental biofluidic investigations have been performed in rigid models due to the complexity and necessity of optical access. In this paper, a facility and measurement techniques for studying oscillating and pulsatile flow in elastic vessels will be described. The investigated vessel models have been adapted such that fluid-mechanical and structure-mechanical characteristics represent realistic blood flows in medium blood vessels. That is, characteristic parameters, i.e., the Reynolds and Womersley number, as well as mechanical properties of the flexible wall, i.e., the Young’s modulus and the material compliance, have been chosen to reasonably represent realistic flow conditions. First, a method to manufacture elastic models, which mimic the structure-mechanical properties of vascular vessels is described. The models possess a tunable compliance and are made of transparent polydimethylsiloxane. Second, the experimental setup of the flow facility will be elucidated. The flow facility allows to mimic pulsatile flow at physiologically relevant Reynolds and Womersley numbers. The precise form of the flow cycle can individually be controlled. Water/glycerine is used as flow medium for refractive index matching particle image velocimetry (PIV) measurements. The PIV recordings not only allow to assess the mean cross-sectional flow field but also further enable to simultaneously detect the movement of the flexible wall. Additionally, the local wall-shear stress can be obtained from the single-pixel line resolved near-wall flow field. To confirm the flow conditions of the oscillatory laminar flow inside the flow facility and to evaluate the ability to assess the flow field, measurements in a straight, uniform diameter, rigid Plexiglas pipe under identical conditions to those of the oscillating flow in the flexible vessel have been performed. The measurements of oscillating flow in the rigid pipe corroborate the experimentally obtained flow field and the wall-shear stress to well confirm Womersley’s analytical solution and thereby evidence the quality of the flow facility and of the measurement techniques. To further study the detectability of the vessel deformation, oscillating flow at Reynolds numbers based on the non-dilated vessel diameter D and peak velocities Re D ranging from 1,000 to 1,750 and at Womersley numbers α ranging from 5 to 17.5 has been investigated in an elastic vessel.

Investigation of aeroacoustic noise generation by simultaneous particle image velocimetry and microphone measurements

A correlation technique is tested, which enables the identification of flow structures that are involved in sound generation processes. At first, the method is applied to the problem of induced noise from flow over a cylinder. The velocity field around a circular cylinder is measured by particle image velocimetry (PIV), while the radiated sound is recorded with a microphone. Both measurements are conducted in a synchronized manner so as to enable the calculation of the cross-correlation between velocity or vorticity fluctuations and the acoustic pressure. The therewith obtained coefficient matrix provides time- and space-resolved information about the statistical dependency between flow structures and the acoustic pressure. Furthermore, a proper orthogonal decomposition (POD) is applied to the velocity field. Then the correlation between dominating modes and the acoustic pressure is computed to identify which modes are mainly involved in the sound generation. Finally, the developed method is applied to the more applied problem of the flow-field inside a leading-edge slat-cove. The results show that, in this case, the signal-to-noise ratio is too low to allow an identification of noise-relevant flow structures, as opposed to the case of the cylinder wake flow, where 5,000 PIV recordings were sufficient to identify the flow structures, which are involved in the noise-generation process. A maximum in spatial distribution of the cross-correlation coefficient is observed 1.6 diameters downstream of the cylinder; its value decreases as one moves further downstream. In this area of maximal correlation, a rapid acceleration of the released vortices takes place. The cross-correlation coefficient fluctuates over time in a sine-type oscillation with maximum values of about \(|R_{v^{\prime}p^{\prime}}| = 0.2.\) \(R_{u^{\prime}p^{\prime}}\) and \(R_{v^{\prime}p^{\prime}}\) show a periodic behavior with a phase shift of π/2 with respect to each other. These regular oscillations can be explained by coherent periodic structures in the flow-field. These structures generate a sound field with the same periodicity, which is perceived as tone. Hence, the correlation between the velocity fluctuations and the acoustic pressure show oscillations identical to those of the input signals. A filtering of uncorrelated noise can be observed; this being caused by the averaging process during the cross-correlation calculation. The correlation with the eigenmodes of a POD gives correlation coefficients, which are no larger than the correlation with a local near-field quantity.

Bubble clustering and trapping in large vortices. Part 2: Time-dependent trapping conditions

In turbulent, periodically excited jets, interactions between bubbles and large coherent vortices are quantitatively studied. Simultaneous, two-phase PIV (particle image velocimetry) and photographic recordings were applied for tracking the large vortices and bubble structures and for investigating trapping phenomena. In order to quantify the interaction between bubbles and the large vortices that are formed in the shear layer, characteristic phase-averaged quantities were determined by PIV. The time-dependent vortex radius, the vorticity at the vortex centre and the time-dependent trapping conditions, obtained from the simulation of the vortex development, were tested against the experimental data.

Advances and applications of the digital mask technique in particle imagevelocimetry experiments

Some advancements of the digital mask technique, which was first described byGui and Merzkirch (1996a ERCOFTAC Bull. 30 45–8) for a phase-separatedevaluation of two-phase flow particle image velocimetry (PIV) recordings, arepresented in this paper. The originally minimum-quadratic-difference- (MQD-)based mask technique is accelerated by using the fast Fourier transformationalgorithm. In order to further increase the evaluation speed and the measurementaccuracy, the digital mask technique is modified so that it can be combined withthe correlation-based central difference image correction (CDIC) method, which isproved to be a more accurate and less peak-locked evaluation algorithm than boththe MQD tracking method and the correlation-based tracking method. Anellipse-shaped interrogation window is constructed with the digital masktechnique to improve the spatial resolution of the evaluation without loss of theevaluation accuracy. Applications in PIV measurements of solid/water two-phaseflow, bubbly water flow, flow around a human blood cell and airflow near the tipof a vibrating cantilever demonstrate that the combination of the digital masktechnique, the CDIC algorithm and the ellipse interrogation window makes apowerful tool for evaluating digital PIV recordings of complex flows.

A correlation-based central difference image correction (CDIC) method and application in a four-roll mill flow PIV measurement

An experiment is conducted in a four-roll mill to verify a novel particle image velocimetry (PIV) recording evaluation method that combines the advantages of central difference interrogation and an image correction technique. Simulations and experiments in the four-roll mill geometry demonstrate that the central difference image correction method described in this paper can not only avoid the bias error resulting from the curvature and high-velocity-gradient flow but also effectively reduce the random error resulting from particle image distortion. Two image correction schemes and two base algorithms are discussed. A four-point image correction scheme is suggested on the basis of the traditional correlation-based interrogation algorithm to enable a fast, high-accuracy evaluation of PIV recordings in complex flows. In addition, the PIV experiment accurately determines the velocity field in the four-roll mill and confirms the linear distributions of the velocity components and the roller speed.

Video systems for PIV recording

CCD video cameras have been used extensively in recent years for image capture in fluid flow measurements with the particle image velocimetry (PIV) technique. Recent advances in CCD imaging technology have improved and extended the PIV technique to new applications, ranging from rotating machinery to 3D measurements. The progressive scan interline transfer CCD allows the time interval between two consecutive frames to be very small. Combined with frame straddling technique and the cross correlation analysis method, video cameras with such CCD design can be used for high-speed flow measurements, without the need for image shifting or image tagging methods to resolve directional ambiguity. Investigation of the pixel fill factor in CCD designs has indicated that the particle image diameter of two pixels or more is required to minimize the error in particle image location, which is important for accurate PIV measurements. Results of the investigation on a commercial video camera with the progressive scan interline transfer CCD design for its capability in high-speed flow measurements are presented. It is shown that this camera can achieve images for a time interval as small as s and for velocities of up to 250 m in the experiment.

The effect of a discrete window offset on the accuracy of cross-correlation analysis of digital PIV recordings

This paper describes how the accuracy for estimating the location of the displacement-correlation peak in (digital) particle image velocimetry (PIV) can be optimized by the use of a window offset equal to the integer-pixel displacement. The method works for both cross-correlation analysis of single-exposure image pairs and multiple-exposure images. The effect is predicted by an analytical model for the statistical properties of estimators for the displacement, and it is observed in the analysis of synthetic PIV images of isotropic turbulence, and in actual measurements of grid-generated turbulence and of fully-developed turbulent pipe flow.

Recent applications of particle image velocimetry in aerodynamic research

Particle image velocimetry (PIV) is increasingly used to investigate unsteady velocity fields instantaneously. For the first time the PIV technique allows the recording of a complete velocity field in a plane of the flow within a few microseconds. The PIV technique thereby provides information about unsteady flow fields which is difficult to obtain with other experimental techniques. The short acquisition times and fast availability of data reduce the operational time, and hence cost, in large scale wind tunnels and test facilities. At DLR a variety of PIV systems for use in industrial wind tunnels has been developed in the past decade. The flexibility of these portable systems is illustrated by presenting several results of recent PIV applications. More recently the original photographic means of PIV image recording has been partially replaced by high resolution electronic imaging which can provide PIV data nearly on-line. Images recorded by either system use the same multiple-pass, cross-correlation analysis software, whose algorithms are briefly described. Several examples of actual applications are given: the flow issuing from a jet nozzle was imaged by a specially developed high-speed video camera at close proximity. A high resolution dual-frame digital camera was applied in the study of helicopter rotor aerodynamics and wake vortex measurements of an airplane model. Further, large image sequences exceeding 100 PIV recordings provided detailed information on the structure of a turbulent boundary layer.

Particle image velocimetry: image labeling using encoding of the point-spread function by application of a polarization-sensitive pupil mask

A method of labeling the first and second exposure particle images of a double-exposed particle image velocimetry (PIV) recording using a single-wavelength, double-pulsed laser is demonstrated. This was achieved by rotating the polarization plane of the second pulse of the illuminating laser beam using a Pockels cell in conjunction with a polarization-sensitive adaptive optic placed close to the iris plane of the photographic camera objective. This system effectively changed the transfer characteristics of the recording optics between exposures, producing two sets of particle images modulated by orthogonally oriented fringes. The particle images that correspond to each exposure were then separated by digital spatial filtering. This permitted the individual particle fields to be cross correlated to yield unambiguous displacement vectors. The technique can be applied to any flow in which the seeding particles retain the incident polarization plane in side scatter. The advantages and disadvantages of such a system compared with conventional PIV recording are discussed.