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There is a need for new numerical tools to capture the physics of floating offshore wind turbines (FOWTs) more accurately to refine engineering designs and reduce costs. The conventional measurement apparatuses in tank tests, including wave probes, velocity and current profilers, and Doppler sensors, are unable to provide a full 3D picture of velocity, pressure, turbulence, and vorticity profile. In tank tests, use of the underwater stereoscopic particle image velocimetry (SPIV) method to fully characterise the 3D flow field around floating wind platforms can overcome some of the limitations associated with classical measurement techniques and provide a rich source of validation data to advance high-fidelity numerical tools. The underwater SPIV method has been widely used for marine and offshore applications, including ship and propeller wakes, wave dynamics, and tidal stream turbines; however, to date, this technology has not seen widespread use for the hydrodynamic study of FOWTs. This paper provides a critical review of the suitability of underwater SPIV for analysing the hydrodynamics of FOWTs, reviews the challenges of using the method for FOWT tank test applications, and discusses the contributions the method can make to mitigating current research gaps in FOWT tank tests. [ABSTRACT FROM AUTHOR]

Casing pressure measurements and Stereoscopic Particle-Image Velocimetry (SPIV) measurements are used together to characterize the behavior of the rotor tip leakage flow at both the design and near-stall conditions in a low-speed multistage axial compressor. A three-dimensional Navier-Stokes solver is also performed for the multistage compressor and the prediction of tip leakage flow is compared with SPIV data and casing dynamic static pressure data. During the experiment 10 high-frequency Kulite transducers are mounted in the outer casing of the rotor 3 to investigate the complex flow near the compressor casing and Fourier analyses of the dynamic static pressure on the casing of the rotor 3 are carried out to investigate the tip leakage flow characteristics. At the same time, the two CCD cameras are arranged at the same side of the laser light sheet, which is suitable for investigating unsteady tip leakage flow in the multistage axial compressor. The SPIV measurements identify that the tip leakage flow exists in the rotor passage. The influence of tip leakage flow leads to the existence of low axial velocity region in the rotor passage and the alternating regions of positive and negative radial velocity indicates the emergence of tip leakage vortex (TLV). The trajectory of the tip leakage vortex moves from the suction surface toward the pressure surface of adjacent blade, which is aligned close to the rotor at the design point (DP). However, the tip leakage vortex becomes unstable and breaks down at the near-stall point (NS), making the vortex trajectory move upstream in the rotor passage and causing a large blockage in the middle of the passage. [ABSTRACT FROM AUTHOR]

In this work, the effect of duct cross-section area and geometry, square and rectangular, of a windexchanger (WE) on the room airflow distribution is studied for a room with a window, at windward or leeward. Airflow distribution inside a room produced by natural ventilation is an important bioclimatic architecture strategy for thermal comfort in hot climates. Ten different WE types at different wind directions, resulting in 18 cases, are experimentally evaluated using scale models in a water channel and velocity measurements at the vertical central plane obtained by stereo-particle image velocimetry. The homogeneity index, the area percentage with a significant flow speed, the normalized turbulent kinetic energy and the normalized ventilation flow rate are used as evaluation parameters. It is demonstrated that in general by increasing the duct cross-section area of a given windexchanger, the windexchanger performance improves, the exceptions are when the openings of the WE have mixed functions, i.e. injection and extraction. Nevertheless, more important than the areas ratios are the configuration of the WE and the location of the window. The WEs show a better performance when all their openings have an opposite function to that of the window. [ABSTRACT FROM AUTHOR]

A curved wall-jet (CWJ) burner was employed to stabilize turbulent spray flames that utilized a Coanda effect by supplying air as annular-inward jet over a curved surface, surrounding an axisymmetric solid cone fuel spray. The stabilization characteristics and structure of n-heptane/air turbulent flames were investigated with varying fuel and air flow rates and the position of pressure atomizer ( L ). High-speed planar laser-induced fluorescence (PLIF) of OH radicals delineated reaction zone contours and simultaneously stereoscopic particle image velocimetry (SPIV) quantified the flow field features, involving turbulent mixing within spray, ambient air entrainment and flame-turbulence interaction. High turbulent rms velocities were generated within the recirculation zone, which improved the flame stabilization. OH fluorescence signals revealed a double flame structure near the stabilization edge of lifted flame that consisted of inner partially premixed flame and outer diffusion flame front. The inner reaction zone is highly wrinkled and folded due to significant turbulent mixing between the annular-air jet and the fuel vapor generated from droplets along the contact interface of this air jet with the fuel spray. Larger droplets, having higher momentum are able to penetrate the inner reaction zone and then vaporized in the low-speed hot region bounded by these reaction zones; this supports the outer diffusion flame. Frequent local extinctions in the inner reaction zone were observed at low air flow rate. As flow rate increases, the inner zone is more resistant to local extinction despite of its high wrinkling and corrugation degree. However, the outer reaction zone exhibits stable and mildly wrinkled features irrespective of air flow rate. The liftoff height increases with the air mass flow rate but decreases with L . [ABSTRACT FROM AUTHOR]

Semi-submersible floating wind turbines are generally equipped with heave plates to introduce additional hydrodynamic damping and decrease the platform's natural frequency. Scale effects are one of the most critical problems concerning the accuracy of model-scale experiments on these damping-augmenting devices. The current research aimed to assess the impact of the scale factor on the flow physics of oscillating circular solid plates. Forced oscillation flow visualisation model-scale experiments were performed on three geometrically similar one-leg columns of a semi-submersible platform equipped with a circular solid flat heave plate. The forced oscillation tests were performed in the range of extreme and operational Keulegan–Carpenter numbers (KCs) for two different frequencies of the heave plate's oscillatory motion. A submersible two-dimensional (2D) three-component (3C) Stereoscopic Particle Image Velocimetry (Stereo-PIV) system was employed to measure the flow field velocity distribution. The Stereo-PIV images were obtained at four positions in the oscillation cycle. The investigation focused on detecting scale effects on the velocity and vorticity for the three models of different scales. Similarities in centre positions, shape, and contours between the three models were visualised for the measured velocity field. The computed vorticity field also revealed similarities in centre positions, shape, and contours among the three different scales. Following up on previous works focusing on the hydrodynamic forces on oscillating circular solid plates, the outcomes of this paper also confirm that, for the considered case study, the correct choice of the motion amplitude (KC) impacts the flow physics of the model tests more than the scale factor. • Experimental investigation of the impact of the scale factor on the flow physics of semi-submersible platforms. • Similarities in centre positions, shape, and contours for the measured velocity field between three models of different scales. • Similarities in centre positions, shape, and contours for the computed vorticity field between three models of different scales. • The correct choice of the motion amplitude (KC) impacts the flow physics of the model tests more than the scale factor. [ABSTRACT FROM AUTHOR]

This paper presents the behaviour of three very different and unique flame and flow structures within an atmospheric swirl-stabilised dump combustor supplied with a lean premixed mixture of methane and air. The reactant flow was artificially perturbed with frequencies of 100 Hz, 200 Hz, and 400 Hz. Phase average behaviour and temporal dynamics were characterised using phase locked high speed CH chemiluminescence and high speed stereo particle imaging. The interaction between the flame and flow field, in particular the internal recirculation zone of the vortex breakdown, was determined to be responsible for differences observed in behaviour at the three forcing frequencies. The 100 Hz perturbation frequency displayed simple oscillatory motion. Higher perturbation frequencies of 200 Hz and 400 Hz gave rise to a second toroidal vortex ring which formed within the internal recirculation zone adjacent to the inner shear layer. This caused additional out of phase modulation of the heat release rate and flame area. Twin counter rotating vorticity structures attached to the annulus were formed as a result of the chamber geometry. The oscillating inlet flow and oscillating reversed flow region of the inner recirculation zone caused oscillations in vorticity magnitude which were responsible for flame wrinkling and stretch effects upon the flame front. Vorticity within the shear layers was found to be the source of harmonic frequency generation of the imposed perturbation frequencies. The data is presented in detail to facilitate CFD model comparisons, particularly LES. [ABSTRACT FROM AUTHOR]

Field simulation results of a fire, centered in a small scale enclosure (1.04 m 3 ) with an open doorway, performed with the CFD computer code ISIS, are presented. Three heat release rates of 10.6, 15.5 and 21.7 kW, provided with a propane gas burner, are numerically studied and compared to experimental results. Turbulence and soot modeling are first validated by simulating the gas burner fire in an open atmosphere for the three fire powers. Due to the complex role of buoyancy in production of turbulence inside a pool fire plume, anisotropic modeling, through the generalized gradient diffusion hypothesis, is considered in the standard k – ε model. The comparisons between experimental measurements and numerical simulation results, for the enclosure fire, concern temperature and velocity profiles at the doorway and temperature profiles inside the enclosure. Velocity measurements at the open doorway are performed using a stereoscopic particle image velocimetry (SPIV) technique, allowing a full comparison with computational fluid dynamics (CFD) results. For the three heat release rates, the simulation results agree well with experimental measurements. General flow patterns, provided by CFD simulations, are reported for the highest fire power and supply useful information for understanding enclosure fires. [ABSTRACT FROM AUTHOR]

A windexchanger is a small structure placed on the roof of a building with the aim of increasing the wind driven ventilation. In the literature, this structure is usually named as a windcatcher. Depending on its orientation to the wind direction, the same structure can perform as a windcatcher or a windextractor. Six windexchanger configurations with different wind orientations, placed in a room with a window at windward, are experimentally evaluated. The room without a windexchanger is used as a reference. The experiments were carried out in a water channel using scale models. Flow velocity measurements were performed using stereoscopic particle image velocimetry (SPIV) on the central plane. The velocity vertical profile in the test section corresponding to a small town was generated. The flow rate into the building and the area percentage of the central plane with flow speed greater than a reference value are used as evaluation parameters. It was found that, the performance of a windexchanger strongly depends on the orientation of its openings to the wind. The ventilation performance of the room with a windexchanger ranges from a significant to a negligible improvement. This stresses the importance of the adequate design of this kind of structure. [ABSTRACT FROM AUTHOR]

This paper presents time-averaged and temporally evolving mean flow and turbulence statistics for a turbulent zero-net-mass-flux (ZNMF) jet at high Reynolds and Strouhal numbers in the far-field in a plane perpendicular to the jet axis. The measurements have been obtained using time-resolved stereo particle image velocimetry (TR-SPIV). The jet is generated by the oscillation of a piston, which discharges filtered water to the quiescent fluid in a tank through a round orifice. A multigrid cross correlation digital particle image velocimetry algorithm (MCCDPIV) has been used to compute the images from each camera that subsequently have been combined to obtain the three components of the velocity. Velocity and turbulence statistics are in good agreement with results obtained in previous work. A comparative study has been performed to determine the contribution to the spreading rate due to the displacement of the center of the jet as rigid body. Different criteria to define the instantaneous jet center have been considered. For the different cases carried out for this study it has been observed that subtracting the movement as a rigid body the width of the jet becomes similar to that of a continuous jet. Velocity fluctuation statistics are also influenced by this restriction. The image series have also been analyzed to determine the temporal evolution of the velocity field and its fluctuations. The fast Fourier transforms (FFT) have been used to calculate the power spectra of these variables. [ABSTRACT FROM AUTHOR]

Highlights: [•] The design of the current diffusers in a fuel oil power plant has been modified. [•] Interaction between primary air and liquid spray has been analyzed. [•] Droplet velocity field has been measured using SPIV. [•] Horizontal toroidal structures of the flows have been found at exit of the burner. [•] Optimized design increases swirl and decreases axial velocity. [Copyright &y& Elsevier]

The wake dynamics of the ducted propeller is complicated due to the interaction between the nozzle and the propeller. The development of the tip and hub vortices and the contraction of the slipstream in the near wake are analyzed using the time-averaged velocities measured by the towed underwater stereo particle image velocimetry (SPIV) system. The PIV measurement results are in a two-dimensional plane which contains three velocity components (2D-3C), and the representative features of the ducted propeller wake are captured: The decelerated axial velocity area appears near the interface between the accelerated flow and the free stream. The duct accelerates the evolution process of the propeller wake as a faster decrease of the axial velocity in the downstream is observed. The vorticity magnitude the tip vortex structures are smaller than that of the open propeller at the same downstream positions. The wake energy of the ducted propeller diffuses faster owing to the earlier break down of wake vortex structures. • The decelerated velocity area is captured near the turbulent boundary between the accelerated flow and the free stream. • Faster energy transfer from hub vortex to tip vortices for ducted configurations is noted. • The radial diffusion of AKE shows the earlier breakdown of the coherent structure of the ducted propeller. [ABSTRACT FROM AUTHOR]

Highlights: [•] Studied flow field of a compact inlet both experimentally and numerically. [•] Massive flow separations found on both turns of a compact inlet. [•] Secondary flow structures were identified along sidewalls and lower surface. [•] Spectral analysis associated pressure fluctuations with flow instabilities. [•] Identification of separation was demonstrated with non-intrusive sensor. [ABSTRACT FROM AUTHOR]

Highlights: [•] The design of the current diffusers in a fuel oil power plant has been modified. [•] The new design adds a finned outer swirler ring. [•] The flow field has been visualized and the 3C velocity field has been measured. [•] The new design improves air flow behavior and recirculation zone characteristics. [Copyright &y& Elsevier]

Abstract: This paper investigated the 3-D motion of saltation sand by high-speed photography and stereo particle image velocimetry (SPIV). By the high-speed camera, the sand particle trajectories in the transverse plane near bed surface have been obtained. It could be found that the collision between the particle and the bed surface would in principle cause the transverse motion of the particle regardless of the change of the wind direction. Based on SPIV, The three-dimensional velocities of the sand particles in the wind–sand flow have been obtained by combining the velocity data from double CCD cameras. The three-dimensional velocity of the sand particle was resolved into three component velocities in the paper, i.e. the streamwise velocity u, the vertical velocity v and the transverse velocity w. The distribution of the transverse velocities w of the sand particles approximated symmetrical. The peak value of the PDF (probability density function) of velocity w fell down obviously with the increase of the wind speed when the sand sizes were equal to or less than 125μm. When the sand sizes were larger than 125μm, the peak value of the PDF of w was almost constant with the change of the wind speed. Although the increments of v with the increase of the wind speed were larger than that of w, the velocity w of the sand particle appeared to be much larger than its velocity v in general. Moreover, it was near one fourth probability that the quantity of the particle velocity w was one order higher than that of its velocity v. The inclination angle between w and u of the sand would be less than 60° and the inclination angle between the v and u would be less than 20° integrally. [Copyright &y& Elsevier]

Abstract: Effective control of vortex breakdown in a cylinder with a rotating lid was achieved with small rotating rods positioned on the stationary lid. After validation with accurate measurements using a novel stereoscopic particle image velocimetry (SPIV) technique, analysis of numerical simulations using a high-order spectral element method has been undertaken. The effect of a finite length rod creates additional source terms of vorticity as the rod rotates. These additional source terms and their spatial locations influence the occurrence of the vortex breakdown. [Copyright &y& Elsevier]

The present work focuses on the measurements of instantaneous concentration fields of a passive scalar due to an impinging round jet injection into a liquid filled rectangular tank. Simultaneous measurements of velocity and passive scalar concentration fields have been conducted by using Particle Image Velocimetry (planar 2C and 3C PIV) and Planar Laser Induced Fluorescence (PLIF) techniques. The mixing injection behavior is analyzed for several injection values of depth and flow rate. Results showed the classical developing and self-similar regions of the jet, the mixing layer and the coupled concentration and velocity fields due to impingement. Finally, 3C PIV reveals a 3D flow jet structure which seems to be a swirl that does not disturb 2D analysis. [ABSTRACT FROM AUTHOR]

• POD is used to capture the dominant structures of the vortex. • Significantly improved identification of the vortex centre and vortex meandering. • Different local vortex identification criteria were used and compared. • The vortex meandering has significant Impacts on the fluctuating component. The meandering of a vortex exists in a broad range of engineering applications and can lead to flow instability and other undesirable characteristics. Compared to a static vortex, measurement of a meandering vortex can result in a 'smeared' mean-flow field and increased levels of turbulence at the centre of the vortex. A case study was performed here on the meandering nature of a fin-tip vortex generated by a manoeuvring submarine. From stereoscopic particle image velocimetry (SPIV) measurements, it is possible to remove the meandering by shifting each instantaneous velocity field so as to produce a common centre for the vortex. In this paper, a snapshot Proper Orthogonal Decomposition (POD) technique is used to capture the dominant large-scale coherent structures (from inspection of eigenvalue or energy distributions) and to improve vortex centre identification. The POD reconstructed velocity field using only the most energetic modes enabled the coherent structures of the flow to be clearly visualised, providing improved identification of the vortex centre and subsequent evaluation of the meandering effect on the turbulent statistics. The present findings suggest that the vortex meandering only has a small impact on the ensemble-averaged resultant velocity, while contributing up to a maximum of 28% for the fluctuating component. The meandering correction also leads to an overall decrease of turbulence intensity in the peak fluctuating region of the vortex core. [ABSTRACT FROM AUTHOR]

Though the proper orthogonal decomposition (POD) method has been widely adopted in flow analysis, few publications have systematically studied the influence of different POD processing methods on the POD results. This paper investigates the effects of different decomposition regions and decomposition dimensionalities on POD decomposition and reconstruction concerning the tip flow in the compressor cascade. Stereoscopic particle image velocimetry (SPIV) measurements in the blade channel are addressed to obtain the original flow field. Through vortex core identification, development of the tip leakage vortex along the chord is described. Afterwards, each plane is energetically decomposed by POD. Using the identified vortex core center as the geometric center, the effects of different decomposition regions with respect to the vortex core are analyzed. Furthermore, the effects of different single velocity-components as well as their combination are compared. The effect of different decomposition regions on the mode 1 energy fraction mainly impacts the streamwise velocity component. Though the addition of W velocity component in the decomposition does change the spatial structures of high-order modes, it does not change the dynamic results of reconstruction using a finite number of POD modes. UV global analysis is better for capturing the kinetic physics of the tip leakage vortex (TLV) wandering. [ABSTRACT FROM AUTHOR]