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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...

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.

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.

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 main interest of today’s research in fluid mechanics is more and more directed to problems where unsteady and separated flows are predominant. For investigations of flow fields with pronounced spatial structures and/or rapid temporal or spatial changes (transition from laminar to turbulent flow, coherent structures, pitching airfoils in transonic flows with shocks, rotors, test facilities with short run time, etc.) optical experimental techniques, such as Particle Image Velocimetry (PIV) and Lagrangian Particle Tracking (LPT) are required which allow to capture the flow velocity of large flow fields instantaneously. An important feature of PIV and LPT is that a reliable basis of experimental flow field data is provided for direct comparison with numerical calculations and hence, for validation of flow simulation codes. During the last years an increasing number of scientists have started to utilize the PIV techniques to investigate the instantaneous structure of velocity fields in various areas of fluidmechanics. A large number of diff erent approaches for the recording and evaluation of PIV images have been described in literature. This course, which is the 27th course on PIV since 1993 organized by DLR, will mainly concentrate on those aspects of the theory of PIV relevant to applications. Besides giving lectures on the fundamental aspects, special emphasis is placed on the presentation of practical and reliable solutions of problems which are faced during the implementation of this technique in wind tunnels and other test facilities. During practice the participants will have the opportunity to carry out the recording and the evaluation of PIV images by themselves in small groups. Matured developments of the PIV technique such as Stereo PIV, Time Resolved PIV, Micro-PIV and recent innovations in 3D(t)-PIV/3D-PTV (tomographic and digital holographic PIV, LPT Shake-TheBox) will be discussed and demonstrated.

Zusammenfassung Die Ausbildung der Strömung fließenden Wassers einer gravimetrischen Kalt- und Warmwasser-Normalmessanlage für große Volumenströme wird anhand von Stereoscopic Particle Image Velocimetry (SPIV)-Messungen an verschiedenen Positionen gezeigt. Dabei wird nachgewiesen, dass an dieser Prüfanlage nahezu optimale Strömungszustände zur Kalibrierung von Durchflusssensoren gewährleistet werden. SPIV eignet sich als schnelles und präzises Messverfahren zur zeitlich und räumlich hochaufgelösten Ermittlung von 3D-Geschwindigkeitsverteilungen in Rohrleitungen.

Particle Image Velocimetry measurements in towing tanks usually require ad-hoc solutions to address the need to protect electronic equipment from water. Pioneering works in this field rely upon fully underwater probe systems to address this issue, as in [1]. In this work we present an approach based on the use of endoscopic equipment (boroscopes) to carry out towing-tank Stereo PIV (SPIV) measurements of the hull wake of a ship model in steady drift. A boroscopic SPIV system provides increased flexibility thanks to the absence of water-proof camera casings. The only underwater components of the presented apparatus are the boroscope tubes and the light-sheet forming device, which are not critical in terms of water protection. The major drawbacks are the reduced light-collecting efficiency of the boroscope devices as well as the geometric deformations and aberrations due to the short focal length, which must be specifically addressed with adequate algorithms. The boroscope-based SPIV system presented in this work features three cameras for redundancy and to increase measurement accuracy. We present the results of towing-tank testing of the hull wake of a twin-screw model at Froude number Fr = 0.24 in straight-ahead and steady drift conditions.

Prosthetic heart valves can be associated to mechanical loading of blood, potentially linked to complications (hemolysis and thrombogenicity) which can be clinically relevant. In order to test such devices in pulsatile mode, pulse duplicators (PDs) have been designed and built according to different concepts. This study was carried out to compare anemometric measurements made on the same prosthetic device, with two widely used PDs.The valve (a 27-mm bileaflet valve) was mounted in the aortic section of the PD. The Sheffield University PD and the RWTH Aachen PD were selected as physical models of the circulation. These two PDs differ mainly in the vertical vs horizontal realization, and in the ventricular section, which in the RWTH PD allows for storage of potential energy in the elastic walls of the ventricle. A glassblown aorta, realized according to the geometric data of the same anatomical district in healthy individuals, was positioned downstream of the valve, obtaining 1:1 geometric similarity conditions. A NaI-glycerol-water solution of suitable kinematic viscosity and, at the same time, the proper refractive index, was selected. The flow field downstream of the valve was measured by means of the stereo-PIV (Particle Image Velocimetry) technique, capable of providing the complete 3D velocity field as well as the entire Reynolds stress tensor. The measurements were carried out at the plane intersecting the valve axis.A three-jet profile was clearly found in the plane crossing the leaflets, with both PDs. The extent of the typical recirculation zone in the Valsalva sinus was much larger in the RWTH PD, on account of the different duration of the swirling motion in the ventricular chamber, caused by the elasticity of the ventricle and its geometry.The comparison of the hemodynamical behaviour of the same bileaflet valve tested in two PDs demonstrated the role of the mock loop in affecting the valve performance.

Zusammenfassung Stereoscopic Particle Image Velocimetry (SPIV) eignet sich als schnelles und präzises Messverfahren zur zeitlich und räumlich hochaufgelösten Ermittlung von 3D-Geschwindigkeitsverteilungen in Rohrleitungen. Die Methode wird mit dem bekannten Laser Doppler Velocimetry (LDV)-Messverfahren und Computational Fluid Dynamics (CFD)-Simulationen am Beispiel einer stark gestörten Strömung hinter einem Drallgenerator bewertet. Erstmalig können so SPIV-Messabweichungen bei der Volumenstrombestimmung von deutlicher kleiner als 2% an einem komplexen Strömungszustand nachgewiesen werden.

A stereo PIV system for investigating complex highly turbulent flows, typical for gas flows in aircraft engines and gas turbines, is developed, and the values obtained for the velocity are compared with the results measured using a laser Doppler anemometer. The features of the use of the PIV system for measurements in three-dimensional highly turbulent flows with high velocity gradients are investigated.

With the dual-plane stereo PIV technique the instantaneous three-dimensional resolved rate-of-strain tensor is directly measured in turbulent premixed flames. Simultaneously, also the instantaneous subgrid scale (SGS) scalar flux is measured with fine resolution, where for the latter term the conditioned particle image velocimetry (CPIV) technique is applied. The subgrid resolution reaches 118 μm, allowing a 9 × 9 resolution of a subgrid filter with width Δ = 1 mm. This combined measurement approach allows the a-priori comparison of models for the SGS scalar flux term with direct measurements which is important for large eddy simulation methods in turbulent premixed flames. Two different flame conditions of a premixed V-shaped turbulent flame are investigated where the turbulence intensity is varied by a factor of nearly three. The instantaneous radial and axial SGS fluxes are compared with the following three models: gradient model with Smagorinsky approach for the turbulent viscosity, Clark model, and extended gradient model with an anisotropy term. None of these models shows a good correlation with the directly measured flux. The anisotropy term alone (being nearly similar to the Clark model) shows, however, a right trend behaviour. An analysis of the data indicates a significant dependency of the experimentally determined SGS flux on the Favre averaged reaction progress (spatially averaged over the SGS area). A relatively simple closure for the SGS flux, which describes the dilatation due to the gasdynamic expansion, and which is a function proportional to ( c ¯ - c ˜ ) , shows a rather good correlation with direct measurement for some of the components. A successful SGS scalar flux model for premixed turbulent flames most likely needs to include at least two different effects.

It is assumed that cerebral homodynamics is an important factor for the cerebrovascular disorders such as subarachnoid hemorrhage. Since a blood vessel has bend and bifurcation with pulsatile blood flow, the flow structure becomes complex and in the three-dimensions. In order to obtain the information on velocity components at an orbital location and time, the stereo PIV technique is applied to measure and visualize flow in the realistic cerebral aneurysm model, which is based on the medical image data. And to discuss the difference of inflow conditions, we measure the velocity distribution at the pulsatile state and steady state.As a result of measurement at the peak systole, the velocity profiles are different. In steady flow, the flow structure along the aneurysm is observed. But in pulsatile flow, the flow that goes to top of aneurysm is observed.

An experimental investigation of a five blade MAU propeller wake behind a Series 60 Cb=0.6 ship model has been performed using Stereo Particle Image Velocimetry (Stereo-PIV) in a large free-surface cavitation tunnel. The investigation of the wake and its evolution during propeller revolution and at different longitudinal stations has pointed out the capability of Stereo-PIV in resolving the complex flow field with great accuracy. As a first step of this investigation, a systematic measurement analysis has been done to find the best angle between the two cameras. The blade viscous wake, developing from the blade surface boundary layers; the trailing vortex sheets, due to the radial gradient of the bound circulation; and the velocity fluctuation distributions are identified and discussed. The complex interaction between the hull wake and propeller is described through the evolution of the mean velocity components and the vorticity fields. In the near field the effects of turbulent diffusion and viscous dissipation, which cause a rapid space-broadening of the velocity gradients in the trailing edge wake, are also examined. Comparison with LDV measurements shows a substantial agreement between the two techniques.