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Closure to “Experimental study on the 3D-flow field of a free-surface vortex using stereo PIV” by ALEX DUINMEIJER, GOSSE OLDENZIEL and FRANCOIS CLEMENS, J. Hydraulic Res. 58(1), 2020, 105–119, http...

This paper examines free-surface vortices and the vorticity distribution with time-dependent evolution within a closed pump intake under five pressure conditions. The location and the strength of t...

This study presents time-resolved volumetric measurements in the wake of a peregrine falcon model. The experiments were performed in a water flume with a freestream velocity of 10 cm/s and at an angle of 3.25°. The TSI volumetric PIV system, using Insight V3V-4G software, was used to capture the time-resolved volumetric flow field. The results compare well with previous Stereo PIV measurements; however, the present results also provide true 3-dimensional flow field information which helps decode the reason for the superior maneuverability. This is attributable to the vortex dominated flow field promoted by its morphology.

The influence of aspect ratio (AR) on dynamic stall was studied by comparing experimental performance data and three-component velocity measurements acquired for wings of AR=3, 4, and 5, and an air...

Interactions between turbulent structures (eddies) and premixed flame fronts were characterized with simultaneous CH2O-/OH-PLIF and stereo-PIV at high framing rate (20 kHz). The high temporal resolution of these measurements permitted the tracking and subsequent characterization of the evolution of turbulence–flame interactions. Common events were identified and their effects on the flame structure were assessed, the first of these being that the flame front was locally broadened by large eddies (i.e. those larger than twice the laminar flame thickness). The results suggest that large eddies likely have a two-fold effect on the overall propagation rate, one linked to the increase in burning area (consistent with classical theories) and the other stemming from an enhancement in the local diffusivity experienced by the flame. While this latter point has not been predicted, it has been postulated that large eddies could convect preheated fluid upstream of the flame. To our knowledge the results presented here provide the first conclusive experimental evidence of this phenomenon. The second common event was the persistence of locally thin preheat layers through extremely turbulent flow fields (i.e. those possessing scales much smaller than the laminar flame thickness). Comparisons between sequences of such events and those where broadening occurred indicate that the latter is typically associated with elevated turbulent diffusivity, which supports arguments in favor of using a criterion based on a ratio of diffusivities rather than length scales to predict the onset of preheat-zone broadening.

The Authors have investigated and analysed comprehensive information on the vortex flow characteristics and specifically have examined the performance of the Burger model. They have incorporated st...

The aerodynamic and thermal behaviour of multiperforated zones in combustors is essential to the development of future combustion chambers. Detailed databases are therefore crucial for the validation of RANS/LES solvers, but also regarding the derivation of heat transfer correlations used in 0D/1D in-house codes developed by engine manufacturers. In the framework of FP7 EU SOPRANO Program, the test-rig used in a previous study is modified to be compatible with anisothermal conditions. The plate studied is a 12:1 model with a 90∘ compound angle injection. A heating system is used to generate a moderate temperature gradient of about 20 K between the secondary hot flow and the main cold flow. The aerodynamic field is acquired by a PIV 2D-3C (Stereo Particle Image Velocimetry) system. The surface heat transfer coefficient is derived based on surface temperature distribution acquisitions. Several heating power levels are tested, which allows evaluating the convective heat transfer coefficient and reference temperature through a linear regression. Measurements are conducted on both sides of the plate, which also gives access to those quantities on the injection/suction sides. From a numerical point of view, the configuration is studied using the unstructured ONERA in-house CEDRE solver with an advanced Reynolds Stress Model. A systematic comparison is presented between the experimental and numerical database. Due to the high blowing ratio, the film protection is low in the first rows, with a convective heat transfer coefficient enhancement around three, and freestream cold air brought close to the wall by vortices created at injection. After four rows, the film is building up, leading gradually to a better insulation of the wall. The comparison with the numerical simulation exhibits a qualitative agreement on the main flow structures. However, the mixing between the jets, the film and the freestream is underestimated by the calculation.

The aerodynamic and thermal behaviour of multiperforated zones in combustors is essential to the development of future combustion chambers. Detailed databases are therefore crucial for the validation of RANS/LES solvers, but also regarding the derivation of heat transfer correlations used in 0D/1D in-house codes developed by engine manufacturers. In the framework of FP7 EU SOPRANO Program, the test-rig used in a previous study is modified to be compatible with anisothermal conditions. The plate studied is a 12:1 model with a 90∘ compound angle injection. A heating system is used to generate a moderate temperature gradient of about 20 K between the secondary hot flow and the main cold flow. The aerodynamic field is acquired by a PIV 2D-3C (Stereo Particle Image Velocimetry) system. The surface heat transfer coefficient is derived based on surface temperature distribution acquisitions. Several heating power levels are tested, which allows evaluating the convective heat transfer coefficient and reference temperature through a linear regression. Measurements are conducted on both sides of the plate, which also gives access to those quantities on the injection/suction sides. From a numerical point of view, the configuration is studied using the unstructured ONERA in-house CEDRE solver with an advanced Reynolds Stress Model. A systematic comparison is presented between the experimental and numerical database. Due to the high blowing ratio, the film protection is low in the first rows, with a convective heat transfer coefficient enhancement around three, and freestream cold air brought close to the wall by vortices created at injection. After four rows, the film is building up, leading gradually to a better insulation of the wall. The comparison with the numerical simulation exhibits a qualitative agreement on the main flow structures. However, the mixing between the jets, the film and the freestream is underestimated by the calculation.

Stereo-PIV is used to map turbulent strongly swirling flow inside a pipe connected to a closed recirculating system with a transparent test section of 0.6 m in length and a pipe diameter of 0.041 m. The Perspex pipe was immersed inside a water trough to reduce the effects of refraction. The working fluid was water and the Reynolds number based on the bulk average velocity inside the pipe and pipe diameter was equal to 14,450. The turbulent flow proceeds in the downstream direction and interacts with a circular disk. The measurements include instantaneous velocity vector fields and radial profiles of the mean axial, radial and tangential components of the velocity in the regions between the swirler exit and circular disk and around this later. The results for mean axial velocity show a symmetric behavior with a minimum reverse flow velocity along the centerline. As the flow developed along the pipe’s length, the intensity of the reversed flow was reduced and the intensity of the swirl decays. The mean tangential velocity exhibits a Rankine-vortex distribution and reached its maximum around half of the pipe’s radius. As the flow approaches the disk, the flow reaches stagnation and a complex flow pattern of vortices is formed. The PIV results are contrasted with LDV measurements of mean axial and tangential velocity. Good agreement is shown over the mean velocity profiles.

This is an experimental and theoretical research on the motion of buoyant particles in the flow of a free-surface vortex at moderate to high particle Reynolds numbers.

In order to analyse the flow characteristics of free-surface vortexes and to validate the Burgers vortex model by using stereo particle image velocimetry, experiments are conducted in a 600 mm diameter vortex tank. Measured axial velocities indicate that 10–25% of the flow is transported through the vortex core. The velocity profiles show that the axial flow is concentrated in a domain bounded by two times the core radius. Despite Burgers’ assumption of radially independent axial velocity profiles, the model quantifies the tangential velocity profile within a relative uncertainty of circa 10%. The measurements show that it seems valid to use Burgers’ model to obtain an estimate for the core radius by taking the average axial velocity over a radial domain of approximately 2.2 times the core radius. The Burgers model quantifies the air core depth with an uncertainty of 20% relative to the measurements. When compared with the magnitude of vorticity diffusion by molecular viscosity, the experiments show that there is no significant diffusion by radial turbulence.

A stereo PIV technique is utilized for the analysis of the flow field over a 70° delta wing. Instantaneous velocity vectors are measured at a series of stations in the chordwise direction. The angle of attack from 0° to 45° are investigated with the free stream velocities of 20, 30, 40, 50 and 60 m/s, respectively. The velocity measurements conducted in a small-sized subsonic wind tunnel (1.8m×1.4m) at China Areodynamics Research and Development Center (CARDC) are compared with the previously obtained PSP results. The comparison shows a good agreement with each other, providing the validity of the PIV technique as well as indicating its effectiveness to investigate complex flow fields in a fairly large-scale wind tunnel facility.

Niobium superconductive radio frequency cavities (SRF cavities) are commonly employed in linear accelerator; very pure, clean, and smooth internal walls are required to make the cavity working properly and with a high Q factor. Typically, to achieve a suitable surface quality the SRF cavities are subjected to a surface treatment called Buffered Chemical Polish (BCP), the latter uses a strong concentrated acids mixture flowing through the cavity to chemically etch a thin niobium layer (∼250 μm). A viable way to understand and optimize the BCP process is by using computational fluid dynamic (CFD); however, the experimental validation of the numerical and physical models is required to confidently use CFD results. The aim of our work is to investigate the fluid dynamics of the BCP process by using the Particle Image Velocimetry (PIV) and the refractive index matching (RIM) techniques, and to provide data to validate CFD simulations. The paper describes and briefly analyse the experimental setup and the preliminary results achieved so far.

Swirl flow with the formation of a precessing vortex core (PVC) in a hydro turbine draft tube model was studied using Stereo-PIV and four acoustic sensors. Experiments were performed on an aerodynamic setup in a wide range of operating conditions of the hydro turbine. Using a spatial Fourier decomposition of pressure pulsation data obtained from four acoustic sensors, a PVC was observed for part-load operating regimes (0.3-0.7Qc ). Mean flow features were shown for the range of operating regimes from a deep part-load regime (0.3Qc ) to an overload regime (1.5Qc ). Based on phase-averaged velocity distributions, the PVC spatial structure was identified for maximum pressure pulsations on the cone walls of the draft tube.

A joint measurement campaign has been performed by the MARS project partners NUAA, CNRS (Poitiers) and DLR. The objective was to apply advanced measurement techniques to the actuator types which turned out to be most promising in the previous investigations. The selected devices were synthetic jet actuators from NUAA and plasma actuators from CNRS. Both actuator types were investigated in a backward facing step (BFS) geometry. Additional measurements took place using the synthetic jet actuator in a NACA-0015 profile from NUAA. For the measurements, a tomographic PIV and a long-distance microscopic stereo PIV measurement system—both capable to provide time-resolved data—have been set up by DLR in the one meter wind tunnel (1MG) at DLR Gottingen. The successful completion of this intercontinental experimental cooperation provides results which are a valuable data base for comparison with and validation of numerical methods.

We present an experimental study focusing on the large- and small-scale structures in turbulent mixed convection. Measurements are conducted at stable conditions and during a reconfiguration event using temperature probes and triggered stereoscopic particle image velocimetry. It is found that the amplitude of the temperature distribution is higher for a state with three Large-Scale Circulations (LSC) than for a state with four LSCs. Furthermore, a correlation analysis of velocity fields measured during a reconfiguration process reveals the change of the dominant LSC mode. However, the mean spatial power distribution computed via a fast Fourier transformation reflects no changes in small-scale flow structures.

The unsteady flowfields generated by convoluted aero engine intakes are major sources of instabilities that can compromise the performance of the downstream turbomachinery components. This highlights theneed for high spatial and temporal resolution measurements that will allow a greater understanding of the aerodynamics but also improvements in our current predictive capability for such complex flows. This paper presents the validation of a modern Lattice Boltzmann method (LBM)to predict the unsteady flow and swirl distortion characteristics within a representative S-duct intake.The numerical results are compared against high spatial and temporal resolutionParticle Image Velocimetry(PIV)data for the same S-duct configuration at an inlet Mach number of0.27.The work demonstrates that LBM is broadly able to capture the flow topologies and temporal characteristics with the exception of the magnitude of the unsteady fluctuations which were found to be notably under-predicted compared to the PIV data. Proper Orthogonal Decomposition analysis shows that LBM is able to provide the key flow modes and their spectral distributions which were found broadly in alignment with the PIV data. A statistical assessment of the unsteady distortionhistoryhighlights that LBM can also provide representative distributions of the main swirl distortion descriptors. Overall the work demonstrates that LBM shows promising potential for S-duct unsteady flow predictions which combined with the minimum computational grid requirements, robustness and fast convergence make it an attractive solution for wider use in thearea of unsteady propulsion system aerodynamics.

This paper describes a set of measurements taken at the Caltech Lucas Wind Tunnel, along with Computational Fluid Dynamic analyses, on a wing-body configuration. The configuration is designed to produce regions of separation in the juncture where the wing meets the body. Detailed near- and off-body stereo PIV measurements in the juncture flow are compared with predictions from computational methods based on the Reynolds Average Navier Stokes formulation. The results from the experimental measurements will become part of a database of high fidelity measurements, to be used for calibration of turbulence models and validation of emerging computational techniques.

We report on a study of the impact of coherent helical vortex structures on the shape of the reaction zone and heat release in swirling methane/air flames in regimes with a vortex breakdown. Three kinds of atmospheric flames are considered, viz., fuel-lean and fuel-rich premixed flames and a partially premixed fuel-rich lifted flame. Based on the measurements of the velocity fields by a stereo PIV in combination with the OH PLIF and HCHO PLIF, the impact of the coherent flow structures on large-scale corrugations of the reaction zone is evaluated. Helical vortex structures, detected in both the non-reacting and reacting high-swirl flows by using proper orthogonal decomposition, are found to promote combustion both in the lean premixed and fuel-rich partially premixed flames. In the first case, based on the phase-averaged intensity of the HCHO×OH signal and the location of the helical vortex structure in the inner mixing layer, it is concluded that the vortex locally increases the heat release rate by enlarging the flame front and enhancing the mass exchange between the combustion products inside the recirculation zone and the fresh gases. The events of the local flame extinctions are detected in the instantaneous PLIF snapshots for the lean mixture, but they do not cause extinction of the entire flame or a blow-off. In case of the lifted flame, the outer helical vortex structure promotes combustion by locally intensifying the mass exchange between the fuel-rich jet with the surrounding air.

We observe that decreasing the inner nozzle surface by adding longitudinal ribs increases the jet stability in terms of the amount of turbulent kinetic energy in the near shear layer. We try to explain our observation as a stabilization effect of secondary flow vortices emerging in the corners of the ribs. These stream-wise vortices damage the development of larger-scale structures in the near shear layer. This explanation is supported by autocorrelation function of the stream-wise velocity component, which displays slightly smaller integral length-scale in the case with ribs than in the case of smooth nozzle. The experiment is performed at Reynolds number 2.2 × 105 (based on the nozzle diameter 50 mm); the Stereo-PIV (Particle Image Velocimetry) measurement takes place at the plane perpendicular to the jet axis one diameter past the nozzle exit. Optical 3D scanner controls the real nozzle geometry. This article presents preliminary measurement at single position and single velocity only; further exploration of this problem is needed.

Dual-plane stereoscopic particle image velocimetry (PIV) is capable of quantifying the flow field in terms of three-component (3C) flow vectors and 3C vorticity vectors simultaneously. Here, we present a test rig to carry out the 20 kHz dual-plane stereo PIV measurements on a premixed swirling flame by using a two-legged burst-mode laser. Other than the traditional methods employing the laser polarization direction and the two-color separation methods, two same-color laser sheets with a 100 ns delay were adopted to separate the imaging processes for the two pairs of cameras using the image straddling method. Each laser sheet with the same wavelength of 532 nm has a pulse cyclic frequency of 20 kHz within each burst generated by the high-repetition-rate burst-mode laser. 3C velocity vectors of a swirling flame were obtained based on the sequential particle images for each laser sheet. In spite of non-perfect simultaneous flow measurements on the two spatially separated laser sheets, the velocity error caused by the 100 ns delay on top of a 50 μs duration, which was used for the velocity vector calculation, is negligible. This short-delay separation method significantly simplifies the experimental setup for dual-plane stereo PIV measurements, especially for low-speed flows.

Experimental study of a swirl flow using 3-D Stereo-PIV (Particle Image Velocimetry) that models a gas turbine blade internal cooling configuration is presented. The work is intended to provide an understanding of the advancements of swirl cooling flow methodology utilizing 3-D Stereo-PIV. The study aims at determining the critical swirl number that has the potential to deliver the maximum heat transfer results. In the swirl cooling flow methodology, cooling air is routed to the turbine blades where it passes through the blade’s internal passages lowering the temperature. An experimental setup with seven discrete tangential jets at three different Reynolds numbers is designed to allow detail measurements of the flow. To provide the particles for velocity measurements an oil particle seeder (LAVision) is used. The circular chamber is made of clear acrylic to allow visualization of the flow phenomena. Data is post-processed in DaVis, velocity calculations are conducted in MATLAB, and TechPlot is used for data visualization. This investigation focuses on the continuous swirl flow that must be maintained via continuous injection of tangential flow, where swirl flow is generated with seven inlets and decays with downstream distance. It was also determined that the critical swirl number, Sn, depends greatly on the location and size of the tangential slots.

This paper reports on the investigation of coherent structures in a high-swirl jet with vortex breakdown (Re = 5 000) and combustion of a methane/air mixture (with equivalence ratio equal to 0.7 and 1.4). The measurements were performed by combined application of the stereo PIV, OH PLIF and HCHO PLIF techniques. The velocity data were processed by the proper orthogonal decomposition (POD) and the PLIF data was conditionally sampled according to the POD correlation coefficients. For the non-reacting jet and lean flame, the POD has revealed presence of a coherent structure, consisted of helical vortex structures in the inner and outer mixing layers. Based on the PLIF data it was found that the inner spiraling vortex structure was correlated with positive fluctuations of the local heat release.

The stereo PIV measurement were performed behind a wing in the plane perpendicular to the flow to study the vortical structures oriented in the streamwise direction, which take place both in suction and pressure sides of the wing. The Reynolds numbers during the experiments in the 3 m wind tunnel range from 0.5 million up to 1.5 million.