Multi-plane Particle Image Velocimetry Research Articles

Inexpensive multi-plane particle image velocimetry based on defocusing: Proof of concept on two-component measurement

This paper presents a method for simultaneous particle image velocimetry (PIV) on parallel planes offset in depth. The method places images from two planes onto a different half of a camera sensor by using image splitting optics with variable optical path lengths. A shallow depth of field is achieved to ensure only one plane is in focus on each half of the sensor. Without needing additional lasers, the method is designed as an inexpensive means to increase the number of measurement plane(s) of single/multi-plane PIV setups and can be combined with existing plane discrimination approaches such as polarization and wavelength. The method is useful for studying instantaneous flow correlations on different planes while retaining high in-plane spatial resolution of typical planar PIV measurement. The measurement uncertainty caused by crosstalk from out-of-focus images is discussed. Experimental results from a laminar flow rig test indicate that the average measurement error of each velocity component is lower than 0.1 pixels per time step, with a 20 mm plane separation in depth and a 35 × 54 mm2 field of view. As an application with varying background scatter and out-of-plane flow motions, in-cylinder flow measurements in an optically accessible internal combustion engine were performed on two swirl planes simultaneously. Characteristics of the proposed method performing stereoscopic PIV measurements will be studied in future.

Analysis of the cavitation instabilities with time-resolved stereo and multiplane particle image velocimetry

The present paper is devoted to the analysis of the various instabilities of cavitation attached to a two-dimensional (2D) profile. Time resolved stereo particle image velocimetry was conducted in a small-scale 2D Venturi type section, in different vertical planes in the streamwise direction, located at varying positions in the depth of the channel. These experiments enabled to obtain the time evolution of the three components of the velocity field in the cavitation area and to derive the time-averaged gradients in the spanwise direction. Test cases at various Reynolds numbers were conducted, maintaining either the pressure or the cavitation number constant, to discuss the impact of these parameters on the flow. Then, attention was focused on three distinct flow dynamics, namely, sheet cavitation, where no large-scale instability can be detected, single cloud cavitation, where a large cloud of vapor is shed periodically at the rear of the cavity, and multi-cloud cavitation, where the process is more complex, as more than one clouds are shed downstream. Data reveal that the structure and the structure of the re-entrant jet, which is one of the primary mechanisms of cloud cavitation, are more complex than reported in the previous studies. Although the jet can be detected as an intermittent low speed reverse flow in the streamwise direction, it is actually made of successive vortices about the channel depth, which are convected downstream while expanding in the vertical direction, causing the cavity lift and, thus, contributing to its final split and the cloud shedding.

The influence of wall heating on turbulent boundary layer characteristics during mixed convection

An investigation on the modification of turbulent boundary layer characteristics by wall heating was conducted on the mixed convection turbulent boundary layer flow. The multi-plane particle image velocimetry (PIV) technique was used to characterize turbulent boundary layer dynamics. Experiments were performed over a range of mixed convection flow conditions, quantified by the Richardson number (Ri), from inertia dominant flow (RiL=0.01) to buoyancy-influenced flow (RiL=0.3). The implemented PIV technique produced multiple two-dimensional velocity fields from two orthogonal planes with respect to the mean-flow direction. From the computed velocity fields, the low-order turbulent statistics were computed where a significant modification to the wall normal stress was observed. The cause of this modification was investigated via the probability density function (PDF) of the turbulent wall-normal velocity component (v'). Results show that the PDF of v' is multi-modal in the presence of wall heating, demonstrating that the buoyancy-driven phenomena influence the turbulent boundary layer flow via intermittent bursting and rising of thermals through the boundary layer.

Inspection of structures interaction in laminar separation bubbles with extended proper orthogonal decomposition applied to multi-plane particle image velocimetry data

This work reports the application of an extended proper orthogonal decomposition (E-POD) procedure to multi-plane particle image velocimetry (PIV) measurements describing the evolution of laminar separation bubbles (LSBs). Measurements were performed over a flat plate installed between adjustable end-walls providing a prescribed adverse pressure gradient for two Reynolds numbers (Re = 70 000, 150 000) and free-stream turbulence intensity levels (Tu = 1.5%, 2.5%). A wall-normal and two wall-parallel measuring planes located at different distance from the wall were considered. POD was applied to the entire PIV planes as well as on their sub-domains, showing the main flow features occurring in the different regions of the LSB. Then, the application of E-POD on different plane partitions revealed the existing correlation between the main dynamics observed in the forward part of the bubble and the breakup events occurring in the reattachment region. The E-POD modes computed in the breakup region resemble streaky structures when PIV snapshots are projected onto the POD eigenvectors of the near wall plane. Otherwise, Kelvin–Helmholtz rolls dominate the E-POD modes obtained by projection of the snapshot matrices on the basis computed in the plane located far from the wall. The main scales of the coherent structures highlighted by the E-POD modes were also characterized by means of the streamwise and spanwise autocorrelation functions of E-POD filtered fields. Data in this work clearly highlight the similarity properties of the main flow features observed in LSBs once scaled with the momentum thickness of the boundary layer at the separation position.

Mixed LSE and EPOD based technique for multi-plane PIV measurements synchronization in separated flow condition

A combination of the Linear Stochastic Estimation (LSE) and a modified version of the Extended Proper Orthogonal Decomposition (EPOD) is proposed to link multi-plane particle image velocimetry (PIV) data describing transitional separated boundary layers. Measurements have been obtained in two orthogonal planes, which share a common line providing the spatial consistency between the data sets. A wall-normal plane, capturing the Kelvin–Helmholtz (K-H) rolls shed downstream of the bubble maximum displacement and a wall-parallel plane, capturing the main cross-flow fluctuations occurring during the break-up process, were considered. Data on the wall-normal plane were acquired at high sampling rate to track the formation and propagation of the K-H rolls. On the other hand, the structures observed in the wall-parallel plane are characterized by longer wavelength in the spanwise direction, implying that the PIV frame has to be enlarged reducing the sampling rate due to the actual PIV system constrains. Combining LSE and EPOD allows the estimation of the time-resolved dynamical features of the separated boundary layer in both planes, increasing the temporal resolution of the wall-parallel plane data. Results will clearly show that the reconstructed pseudo 3D time-resolved field preserves the statistical properties of the original (and independent) acquired datasets in the two planes.

Multi-Plane PIV Measurements in a Gasoline Direct Injection Engine

<div class="section abstract"><div class="htmlview paragraph">The flows in-cylinder have a profound effect on the mixture preparation and subsequent combustion in all engines. These flows are highly three-dimensional in nature and information from multiple planes is required to characterise the flow dynamics. The flow measurements reported here are from three orthogonal planes in an optical access engine that is based on the Jaguar Land Rover AJ200 Gasoline Direct Injection (GDI) engine. Particle Image Velocimetry (PIV) measurements have been taken every 5°CA from the start of induction to the end of compression. Data have been obtained from 300 cycles for separate experiments measuring flows in the tumble plane, the swirl plane and the cross-tumble plane. Vector comparison metrics are used to quantitatively compare ensemble averaged PIV flow fields to Computational Fluid Dynamics (CFD) simulations across each plane in terms of both the velocity magnitude and direction. These quantitative metrics enable identification of regions of interest and differences in flow characteristics across a range of engine operating conditions. Combination of the PIV vector fields from all three planes into composite flow fields enables the interaction between the flows in the three planes to be visualised. During induction the intake jet is visible in all three planes, while the swirl plane measurements show a pair of counter-rotating vortices either side of the intake jet and the tumble plane shows the development of the tumble flow. During compression the center of the tumble vortex traverses the cylinder leading to flow reversal in the swirl and cross tumble measurement planes.</div></div>

Multi-plane time-resolved Particle Image Velocimetry (PIV) flow field measurements in an optical Spark-Ignition Direct-Injection (SIDI) engine for Large-Eddy Simulation (LES) model validations

In-cylinder flow characteristics play a significant role in the fuel–air mixing process of Spark-Ignition Direct-Injection (SIDI) engines. Typically, planar Particle Image Velocimetry (PIV) is used to measure a representative velocity field sectioning through the center plane of the engine cylinder. However, a single flow field offers very limited perspective regarding the Three-Dimensional nature of the flow fields. Since the in-cylinder flow is stochastically complex, large datasets of flow field measurements along multiple planes are needed to provide a complete panoramic understanding of the flow dynamics. In this study, a high-speed PIV is applied to measure the crank-angle resolved flow fields inside a single-cylinder four-valve optical SIDI engine. Five flow fields along different tumble planes are captured. These five planes include two orthogonal planes cutting through the spark plug tip, two parallel planes sectioning through middle point of the intake and exhaust valves, and one plane through the centers of two intake valves. In addition, numerical computations are carried out with Large-Eddy Simulation (LES) model in CONVERGE. With the guidance from multi-plane PIV measurements, a novel validation approach is proposed in this study. The quantitative analysis and comparison between LES simulations and PIV experiments are divided in terms of global and local comparison indices. The global comparison indices provide a quantitative single value to quickly check the overall similarity of velocity directions and magnitudes between PIV and LES results of a specific individual plane. The local comparison indices further evaluate the similarity between the flow fields of LES and PIV point by point to identify any dissimilar regions and vortex features, which are likely to indicate the complex flow structures at low-speed regions. In summary, not only can the combined data analysis approach provide a reliable way for LES model validations, it can also reveal the physical quantifications of the complex in-cylinder flow characteristics.

Quasi-4D laser diagnostics using an acousto-optic deflector scanning system

In this paper, a novel scanning system for laser diagnostics was developed and characterized. The system is based on the acousto-optic deflection of a high-speed pulsed laser. Results showed that quasi-volumetric laser illumination with high precision and accuracy can be achieved with a simplified and flexible optical setup. The feasibility of the method for performing high-speed quasi-4D laser diagnostics was demonstrated by the tomographic visualization of a lifted turbulent jet flame using Mie-scattering and multi-plane particle image velocimetry measurements of a turbulent non-reactive mixing case. Three-dimensional flame and flow structures can be detected and tracked with this new scanning system.

Experimental and numerical investigation of the flow characteristics within a Solar Expanding-Vortex Particle Receiver-Reactor

The iso-thermal flow field within several configurations of a Solar Expanding-Vortex Particle Receiver-Reactor (SEVR) was investigated, using a combination of multi-plane particle image velocimetry (PIV) and Reynolds-Averaged Navier-Stokes (RANS) computational modeling, at a constant inlet Reynolds number of 12,300. The experimental and numerical results confirmed that the SEVR generates a well-established vortex flow pattern, which approaches a forced vortex near to the injection plane and a combined (free and forced) vortex in the main body of the cavity. The SEVR also features a reversed flow in the vortex core region and a complex precessing vortex core (PVC) structure, which is stronger in both the inlet and outlet regions of the cavity. The primary mechanism for particle deposition was identified as a primary recirculation zone near to the aperture that transports fluid from the main cavity through the aperture. The presence or absence of this recirculation zone was associated with the relative size of the vortex core dimension at the aperture plane and the aperture size. It was also found that the presence of a PVC at the aperture plane further increases the potential for the particle deposition onto the receiver-reactor window by generating a secondary recirculation zone in the vortex core region that transports fluid through the aperture. The use of a sufficiently long cavity or a sufficiently large cone angle can assist both in stabilising the PVC and increasing the size of the vortex core at the aperture plane. These two effects are postulated to reduce the potential for particle deposition onto the receiver-reactor window.

Analysis of basic flow regimes in a human airway model by stereo-scanning PIV

The detailed understanding of the human lung flow is of high relevance for the optimization of mechanical ventilation. Therefore, the spatial and temporal development of the flow field in a realistic human lung model is investigated for several oscillatory flow regimes using stereo-scanning particle-image velocimetry (PIV). The flow in the right primary bronchus is always measured for a complete sinusoidal ventilation cycle. Three Reynolds and Womersley number sets describing viscous (Re = 10; α = 1.5), unsteady (Re = 40; α = 5), and convective (Re = 150; α = 1.5) regimes are defined to cover various dominating fluid mechanical effects. In addition, multi-plane PIV measurements are performed to analyze steady laminar (Re = 150) and turbulent (Re = 2,650) flow at inspiration and expiration. The steady results show that the maximum velocity is shifted to the outer wall at inspiration and toward the inner wall of the bronchial bend at expiration. At inhalation, a U-shaped high-speed velocity profile develops only inside the left primary bronchus, whereas both primary bronchi contain one vortex pair. During expiration, the vortex pairs from each main bronchus merge into a two-vortex-pair system inside the trachea. From the oscillatory findings, it is evident that an undersupply for the right upper lobe is noticed at low ventilatory frequencies, whereas high-frequency flow leads to a more homogeneous ventilation. The analysis of the temporal development of the absolute velocity in the center plane shows a variable phase lag. Unlike the flow in the unsteady regime, the flow of the viscous flow domain (α = 1.5) is in phase with the applied pressure gradient. Additionally, a premature outflow of the upper right lung lobe can be observed in the unsteady flow regime.

Simultaneous Particle Image Velocimetry and Microphone Array Measurements on a Rod-Airfoil Configuration

The aeroacoustic sound generation process on a rod-airfoil configuration has been investigated by means of simultaneous particle image velocimetry in the near field and phased-microphone-array measurements in the far field. Up to 20,000 particle image velocimetry snapshots per field of view have been recorded. A coplanar multiplane particle image velocimetry system has been used, providing statistically independent samples of the temporal derivative of the velocity field. Both measurements were conducted in a synchronized manner so as to enable the calculation of the cross-correlation between the acoustic pressure and flow quantities derived from the measured velocity fluctuations. The main idea of the concept used in the study presented here was to use the coefficient matrix obtained from the aforementioned correlation to identify regularities in the near-field fluctuations that are related to the radiated sound field. Additionally, a technique to localize flow structures responsible for the far-field sound by means of the calculation of the near-field intensity has been tested.