Smooth Cylinder Research Articles

Heave Plates with Holes for Floating Offshore Wind Turbines

Abstract The paper presents an innovative solution which is heave plates with holes. The long-known heave plates are designed to damp the heave motion of platforms. They are most often used for Spar platforms. The growing interest in this type of platform as supporting structures for offshore wind turbines makes it necessary to look for new solutions. Based on the available literature and the authors’ own research, it was concluded that the main element responsible for the damping of heave plates is not so much the surface of the plate, but its edge. Therefore, it was decided to investigate the effect of the holes in heave plates on their damping coefficient. Model tests and CFD calculations were performed for three different structures: a smooth cylinder, a cylinder with heave plates with a diameter of 1.4 times the diameter of the cylinder, and a cylinder with the same plate, in which 24 holes were cut (Fig. 1). Free Decay Tests (FDT) were used to determine the damping coefficient and the natural period of heave, and then the values obtained were compared. The full and punched heave-plate designs were also tested with regular waves of different periods to obtain amplitude characteristics. The results obtained are not unequivocal, as a complex motion appears here; however, it is possible to clearly define the area in which the damping of a plate with holes is greater than that of a full plate.

Vortex-induced vibration of a full-diamond textured cylinder at subcritical Reynolds numbers

Textured pipe has been proposed to improve the propagation buckling capacity of subsea pipelines recently. Due to its special geometry, textured pipe may have the potential to mitigate the vortex induced vibration (VIV) by altering the wake vortex street formation. In the present study, the effectiveness of using a full-diamond textured pipe for VIV suppression is numerically investigated in a coupled fluid-structure interaction (FSI) framework. Three-dimensional (3D) Computational Fluid Dynamics (CFD) analyses are performed by using the Reynolds-Averaged Navier-Stokes (RANS) turbulence model equipped with shear stress transport (SST)K−ω model at the subcritical Reynolds numbers (Re) with Re∈[2000,12000]. The results are compared in detail with an equivalent conventional smooth cylinder subjected to the same flow conditions. Numerical results show that the textured cylinder can significantly mitigate the undesired VIV and the associated hydrodynamic forces. It eliminates the upper excitation regime in the conventional smooth cylinder and the width of the synchronization regime is also remarkably reduced.

Numerical Solutions of Two-Dimensional Navier–Stokes Equations Based on a Generalized Harmonic Polynomial Cell Method With Non-Uniform Grid

Abstract It is essential for a Navier–Stokes equations solver based on a projection method to be able to solve the resulting Poisson equation accurately and efficiently. In this paper, we present numerical solutions of the 2D Navier–Stokes equations using the fourth-order generalized harmonic polynomial cell (GHPC) method as the Poisson equation solver. Particular focus is on the local and global accuracy of the GHPC method on non-uniform grids. Our study reveals that the GHPC method enables the use of more stretched grids than the original HPC method. Compared with a second-order central finite difference method (FDM), global accuracy analysis also demonstrates the advantage of applying the GHPC method on stretched non-uniform grids. An immersed-boundary method is used to deal with general geometries involving the fluid–structure interaction problems. The Taylor–Green vortex and flow around a smooth circular cylinder and square are studied for the purpose of verification and validation. Good agreement with reference results in the literature confirms the accuracy and efficiency of the new 2D Navier–Stokes equation solver based on the present immersed-boundary GHPC method utilizing non-uniform grids. The present Navier–Stokes equations solver uses second-order central FDM and Quadratic Upstream Interpolation for Convective Kinematics scheme for the discretization of the diffusion term and advection term, respectively, which may be replaced by other higher-order schemes to further improve the accuracy.

Experimental studies on wake structures of a circular cylinder with different surface roughness heights

In the present study, the effects of surface roughness height (k/D) on wake characteristics of a circular cylinder were examined experimentally in a water flume at a Reynolds number of 2520 using particle image velocimetry (PIV). The surface roughness was simulated by using 3D printed hemispheres uniformly distributed over the cylinder surface. Four relatively large roughness heights with k/D = 5%, 10%, 15% and 20%, and a coverage ratio of 79%, which were not explored systematically in the literature, were considered, and the results were compared with that of a smooth cylinder (k/D = 0%). It is found that the surface roughness disturbs the shear layers of the cylinder, affecting the vortex shedding frequency and the wake formation region. The Strouhal number decreases with the increase of k/D. This can be attributed to the forward movement of the shear layer separation points. This phenomenon is also confirmed by smoke-wire flow visualisation. Besides, the regularity of the vortex shedding and the magnitudes of the Reynolds stresses are enhanced with the existence of roughness. The proper orthogonal decomposition (POD) analysis shows that with the existence of surface roughness, the large-scale Kármán vortex structures is intensified. This study extends the understanding of the influence of k/D on flow characteristics of a circular cylinder.

Experimental study on the wake characteristics of the circular cylinder with the V-groove surface

In order to understand the wake characteristics for the circular cylinder with the V-groove surface, the Particle Image Velocimetry (PIV) and Proper Orthogonal Decomposition (POD) technology are used to measure and analyze the flow field velocity and the flow characteristic of the wake flow of the circular cylinder with the V-groove surface. This study focuses on the length of the velocity gradient, the Reynolds stress, the turbulent kinetic energy, the vortex shedding process, and the velocity fluctuating characteristics of flow field with POD technology. Compared with the smooth cylinder, the velocity gradient of the circular cylinder with the V-groove surface are larger. The peak values of Reynolds stress of the circular cylinders with the V-groove surface (the peak values are 0.33, 0.28 and 0.24) is lower than that of the smooth cylinder (0.39), and the peak values of turbulent kinetic energy are also lower. The recirculation region of the circular cylinders with the V-groove surface is larger, the length of the recirculation region are increased by 5.9%, 13.0% and 16.1% respectively, and it implies the suppression of vortex by the V-groove structure. Otherwise, the correlation of POD mode coefficients is disturbed for the circular cylinders with the V-groove surface, further proving that the effect of V-groove structure on vortex shedding.

Numerical simulation of surface roughness effects on the vortex-induced vibration of a circular cylinder at a subcritical Reynolds number

The Vortex-Induced Vibration (VIV) of a circular cylinder with different surface roughness is numerically simulated at a subcritical Reynolds number of 3900. The effects of surface roughness on the vibration response amplitude, hydrodynamic coefficients and wake vortex are analyzed. The results show that as surface roughness increases, four branches (initial, upper, super-upper and lower branches) appear in the VIV response for a smooth cylinder, and for a cylinder with a small surface roughness of K S / D = 2.5 × 10 − 3 , this response changes to three branches (initial, upper and lower branches). The vortex shedding for the initial and lower branches is 2S mode, and that for the upper and super-upper branches (lock-in range) is P + S − , P+S + and 2P modes. With increasing surface roughness, the maximum amplitude has little difference, and the width of the lock-in range increases. A large jump in the vortex phase at the transition of the initial and upper branches and a large jump in the total lift phase at the transition of the upper and lower branches are found, and these jumps are associated with a switch in the timing of vortex shedding (jump of the vortex phase: 2S mode to P + S − mode; jump of the lift phase: other modes to 2S mode). The results are significant for the development of flow and vibration control technology.

ВЛИЯНИЕ СВОЙСТВ ПОКРЫТИЙ НА ТЕПЛООБМЕН ПРИ ОХЛАЖДЕНИИ ВЫСОКОТЕМПЕРАТУРНЫХ ЦИЛИНДРИЧЕСКИХ ТЕЛ

The process of cooling high-temperature bodies in liquids is an important physical process, in particular, for the safety of nuclear plants. After the accident at the Fukushima-1 nuclear power plant in 2011, large-scale research was launched to find a new accident tolerant fuel. Studying the effect of the properties of new materials on heat transfer during cooling in the case of repeated flooding of the active zone and the possibility of accurately predicting the transition temperature to the intensive cooling regime will allow substantiating the choice of a new type of tolerant fuel from the standpoint of thermophysics. The aim of this work was an experimental study of the cooling processes of high-temperature cylindrical bodies made of various metals in liquids, as well as determining the effect of coating properties on heat transfer during cooling. A large experimental data array was obtained on the cooling of cylindrical samples; experiments were carried out on smooth cylinders made of various metals, as well as on copper cylinders with various coatings and different degrees of roughness (gold coating, stainless steel coatings).

ПОЛЕВЫЕ ИССЛЕДОВАНИЯ ПОЧВООБРАБАТЫВАЮЩЕГО КАТКА ВИБРАЦИОННОГО ДЕЙСТВИЯ

A completely new design of a vibrating soil tillage roller was developed, as a result of which the quality of soil compaction was increased, while energy costs are reduced due to a decrease of metal consumption by at least 3 times in comparison with commercially available rollers. The vibrations created by the offered roller make it possible to destroy large soil fractions more effectively and ensure high-quality soil compaction. To determine suitable parameters of the soil-tilling roller, a set of studies was carried out in field conditions, by means of which suitable parameters of the proposed design of the vibrating roller were identified. Ballast weight and diameters of the pulleys on the shaft of the hollow cylinder were changed during the experiment, and the influence of these parameters on improvement criterion was also evaluated. Measurements of soil moisture, its density and structure were carried out in the course of the experiment. The experimental results were processed with modern software: Microsoft Excel, Statistica, etc. As a result of the experiment, it was revealed that the quality of soil compaction by the proposed vibrating roller is 24.7% better than that of KKZ-6 roller, which is widely used in agriculture. In this case, the improvement criterion kpl on the non-rolled section was 0.5, which is 82% lower than that of the proposed roller. The structure of the soil on the area rolled by the proposed roller fully meets the agrotechnical requirements. The proposed tillage roller of vibrating action is universal and can be used on different types of soils, provided that the ballast weight and the ratio of the pulley diameters on the axis of the hollow cylinder and on the smooth cylinder are regulated.

A Study of Drag Reduction on Cylinders with Different V-Groove Depths on the Surface

In this study, the drag reduction effect is studied for a cylinder with different V-groove depths on its surface using a k-ω/SST (Shear Stress Transport) turbulence model of computational fluid dynamics (CFD), while a particle image velocimetry (PIV) system is employed to analyze the wake characteristics for a smooth cylinder and a cylinder with different V-groove depths on its surface at different Reynolds numbers. The study focuses on the characteristics of the different V-groove depths on lift coefficient, drag coefficient, the velocity distribution of flow field, pressure coefficient, vortex shedding, and vortex structure. In comparison with a smooth cylinder, the lift coefficient and drag coefficient can be reduced for a cylinder with different V-groove depths on its surface, and the maximum reduction rates of lift coefficient and drag coefficient are about 34.4% and 16%, respectively. Otherwise, the vortex structure presents a complete symmetry for the smooth cylinder, however, the symmetry of the vortex structure becomes insignificant for the V-shaped groove structure with different depths. This is also an important reason for the drag reduction effect of a cylinder with a V-groove surface.

Al2O3/WS2 Surface Layers Produced on the Basis of Aluminum Alloys for Applications in Oil-Free Kinematic Systems.

The article presents the results of an aluminum oxide layer doped with monolayer 2H tungsten disulphide (Al2O3/WS2) for applications in oil-free kinematic systems. The results concern the test carried out on the pneumatic actuator operational test stand, which is the actual pneumatic system with electromagnetic control. The cylinders of actuators are made of Ø 40 mm aluminum tube of EN-AW-6063 aluminum alloy which is used in the manufacture of commercial air cylinder actuators. The inner surfaces of the cylinder surfaces were covered with an Al2O3/WS2 oxide layer obtained by anodic oxidation in a three-component electrolyte and in the same electrolyte with the addition of tungsten disulfide 2H-WS2. The layers of Al2O3 and Al2O3/WS2 obtained on the inner surface of the pneumatic actuators were combined with a piston ring made of polytetrafluoroethylene with carbon (T5W) material and piston seals made of polyurethane (PU). The cooperation occurred in the conditions of technically dry friction. After the test was carried out, the scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) analysis of the surface of the cylinder bearing surfaces and piston seals of the pneumatic cylinders was performed. The analysis revealed the formation of a sliding film on the cylinder surface modified with tungsten disulfide, as well as on the surface of wiper seals. Based on the SEM/EDSM tests, it was also found that the modification of the Al2O3 layer with tungsten disulfide contributed to the formation of a sliding film with the presence of WS2 lubricant, which translated into smooth cylinder operation during 180 h of actuator operation. The cylinder with the unmodified layer showed irregular operation after approximately 70 h thereof.

Magnus Antirolling System for Ships at Zero Speed

This study investigated the performance of an antirolling system based on the Magnus effect at zero speed. The test scheme and control method were designed based on a proportional integration differentiation controller. A scaled ship model equipped with an attitude monitoring and control implementation feedback system was developed. The effects of different swing speeds and angles, cylinder surface shapes, number of effective cylinders, and target ship model rolling angles on the performance of the antirolling system were studied using a shipborne gravity slider and normal transverse regular wave. The Magnus antirolling system exhibited a better swing speed range compared to conventional methods, and its performance improved as the swing angle increased. During the zero-speed antirolling test, the velocity loop control provided a better antiroll damping effect on the ship model than the position loop control, and the bionic cylinder improved the performance more than a smooth cylinder. Moreover, the system retained good antirolling capabilities even in partial failure cases (i.e., operating with fewer cylinders). This study can contribute to the research of ship seakeeping strategies.

Incipient sliding of adhesive contacts

A model is proposed herein to investigate the incipient sliding of contacts in the presence of both friction and adhesion, where the interfacial response is modeled based on traction-separation laws. A Maugis-like parameter is defined to characterize the response in the tangential direction. Subsequently, the model is used to investigate the contact between a smooth cylinder and a flat body, where adhesion-friction interactions are strong. A range of behaviors are observed when a tangential displacement is imposed: When the parameter is low, the contact pressure exhibits a relatively constant profile; when it is high, a pressure spike is observed at the edge of the contact. This difference is caused by a significant interface compliance in the former case, which limits the amount of slip. The results for the mid-range values of the Maugis-like parameter can qualitatively replicate various experiments performed using polydimethylsiloxane (PDMS) balls.

Forced convective heat tranfer in a metallic foam cylinder cooled by a slot jet flow and comparison with a smooth cylinder and a full flow

The aim of the present paper is to study experimentally the enhancement of heat transfer, due to the presence of a layer of aluminum foam covering a smooth circular cylinder. The measurements are carried on with the impingement of a rectangular jet of air, which has a transversal dimension equal or smaller than the cylinder diameter, or jet flow, and full flow (or uniform flow, which has a greater dimension than the cylinder diameter). Literature does not report any paper dealing with this multiple problem. Experimental heat transfer and fluid dynamics measurements are carried out at Reynolds number ranging from ReD=2200 to ReD=24,750. The layer of the aluminum foam, 8 mm thick, is glued onto the smooth cylinder, with an external diameter of D = 10 mm. The height of the slot jet is H = 10 mm, with a width of W = 135 mm. Jet flow visualizations are performed for the smooth and the foam-covered cylinder to explain the heat transfer results from the fluid dynamics point of view. The flow visualizations show that the jet flow always penetrates into the foam, wetting it completely, while the reattachment never occurs downstream. In the smooth cylinder, the jet completely envelops the cylinder, at greater Reynolds numbers and a distance of x/H = 7, with a small recirculation zone, showing that the rear surface also participates in the heat transfer. The heat transfer on the foam-covered cylinder increases, mainly, because of the large increase in the exchange surface, while it is somewhat penalized, at the higher Reynolds values and at the greater distance from the slot exit, because the rear surface is little involved in the heat transfer.

Numerical investigations on the flow control over bumped surface circular cylinders

External flow-induced forces on bumped surface circular cylinders in laminar flow regimes are studied at a range of Reynolds numbers up to 200. This study examines three types of bumped surface cylinders, including square, triangular, and dimpled surfaces. The bumped surfaces are set at the top surface of the cylinder. The bumps are arranged between θ = 0° and 90° from the front stagnation point of the cylinder. It is shown that the presence of a bump on the leading edge of the cylinder manipulates the vortex dynamics and leads to significant changes in flow-induced forces. The results show that due to asymmetrical imposed pressure on the cylinder, the generated negative vorticity of the square bumped surface is approximately 28% stronger than those of triangular and dimpled surfaces once θ = 90°. Comparing to a smooth cylinder, significant reductions in drag and lift coefficients are obtained at Re = 150 once a bump is created on the surface of the cylinder apart from the type of geometry. It is found that the maximum time-mean lift coefficient is produced once the bump is located at θ≥ 45°. Furthermore, the minimum drag force is generated for the bumped surface cylinder with θ = 0° and Re = 150.

Incorporation of subcritical Reynolds number into an aerodynamic damping model for vortex-induced vibration of a smooth circular cylinder

Vortex-induced vibration (VIV) of a circular cylinder is known to be affected by the Reynolds number, while this effect has rarely been considered in VIV modeling. This paper attempts to incorporate the Reynolds number into an aerodynamic damping model for simulating the VIV of a circular cylinder with a smooth surface immersed in a smooth flow. Aerodynamic damping parameters within the Reynolds number range of 500 ∼ 33000 (i.e., the TrSL1 and TrSL2 regimes) are identified according to the griffin plots and existing forced vibration experimental data. The model can simulate the peak VIV amplitudes of an elastically supported rigid circular cylinder at different mass-damping parameters in the TrSL1 and TrSL2 regimes. Both experimental results and numerical simulations demonstrate that the peak VIV amplitude of a low-damping cylinder is highly dependent on the Reynolds number, while the dependency is decreased by increasing the mass-damping parameter. The results suggest that the Reynolds number effect is significant in the VIV analysis of a lightly damped cylindrical structure, while the role of Reynolds number seems insignificant for a moderate-damping or high-damping cylindrical structure. For a flexible circular cylinder, the proposed model is expected to predict slightly higher peak VIV amplitudes because the aerodynamic damping effects are overestimated at some segments along its length.

Noise reduction of sinusoidal wavy cylinder in subcritical flow regime

The sinusoidal wavy cylinder of circular cross section is able to substantially reduce the fluid forces by effectively stabilizing the near wake in the subcritical flow regime. Based on the anechoic wind tunnel measurements and large eddy simulations (LESs), we investigate the capability of the sinusoidal wavy cylinder to reduce aeroacoustic noise as well as underlying flow physics. The wavy cylinder studied in this work covers a range of spanwise wavelength λz = 1.8–6.0Dm and a range of wave amplitude a = 0.15–0.25Dm, where Dm is the mean diameter of the wavy cylinder. The wind tunnel measurements are conducted at Reynolds number ReDm = 2.9–8.0 × 104, while LESs are conducted at ReDm = 3.0 × 104. It is observed that the wavy cylinder's configuration, determined by λz and a, has a profound impact on the far-field sound pressure level (SPL) of both tonal and broadband noise. Compared with the baseline smooth cylinder, the wavy cylinder with λz = 1.8Dm and a = 0.25Dm can reduce the peak value of SPL at the tonal frequency by up to 36.7 dB. The reductions in the overall SPL of the tonal and broadband noise are also tremendous, by up to 31.0 and 7.5 dB, respectively, by the wavy cylinder with the optimum wavelength. Consistent with observations on noise reduction are the significantly weakened near-wake structures and largely attenuated spanwise coherence, as well as substantially suppressed pressure fluctuations in the near wake and over the cylinder surface, based on the LES results. Dependence of the noise reduction on Reynolds number is discussed as well.

Systematic investigation of the tip effects on vortex-induced vibrations for circular cylinders

This paper presents an investigation into tip effects on vortex induced vibrations (VIV) for circular cylinders. Within this context; the results of the tests, which were carried out in Istanbul Technical University Ata Nutku Circulation Channel, were validated with different studies from the literature. Then, a series of VIV tests were carried out for six different smooth circular cylinders with different aspect ratios at TrSL2 and TrSL3 flow regimes having one degree-of-freedom. In the VIV tests, the mass ratios, the aspect ratios and the spring stiffness were changed systematically and the tip effects on these parameters were investigated with respect to the VIV responses of the cylinders. An uncertainty analysis was carried out to obtain the margin of bias and precision errors on the test results of oscillation amplitude for two cylinders in initial, upper and lower branches. As the aspect ratio was increased and the edges of the cylinder got closer to the walls, the synchronization range of VIV motion initiated earlier leading to a wider synchronization range. The transitions from the upper to the lower branch encountered at almost the same reduced velocities. The tip effects were found to be more dominant when the mass ratio was smaller. After the experiments validated and verified, the cylinder was subjected to CFD simulations of forced oscillations using the experimental data of amplitudes and frequencies for two different aspect ratios. The CFD study showed reduced amplitude responses when the cross-flows were higher. Spanwise flow visualization along the cylinder revealed that cross-flows are dominant at cylinder tips with lower aspect ratios.

Effects of passive and combined aerodynamic control on the aerodynamic characteristics of an elliptical cylinder

The flow separation during the process of flows around elliptical cylinders causes unsteady vortex shedding in the wake, which leads to the vortex-induced vibration of structures that can pose serious challenges across different engineering disciplines. Therefore, investigation on the suppression of the flow separation and the vortex shedding of elliptical cylinders so as to improve their aerodynamic characteristics has been one of the most active topics in recent decades. In this study, to investigate the effects of the passive and combined aerodynamic control on the aerodynamic characteristics of an elliptical cylinder immersed in uniform flow, wind tunnel tests of two 2D smooth elliptical cylinder models whose major axes are parallel or perpendicular to the incoming wind direction (i.e. the models S∥ and S⊥), two corresponding rough models with different surface roughness (i.e. the models R∥ and R⊥ with the relative roughness Sr of 5.3 × 10−4 and 1.15 × 10−3) and four corresponding combined aerodynamic controlled models consists of the surface roughness and air suction (i.e. the models RS1∼RS4) have been carried out. The Reynolds numbers for the test models are about 5.52 × 104–8.625 × 104. The surface roughness, suction flux coefficient CQ, azimuthal locations of suction holes and placement of the major axes of the elliptical cylinders are considered as the main experimental variables, and the aerodynamic characteristics in terms of both the wind pressure coefficient distributions and the aerodynamic force coefficients of the smooth models, rough models and combined aerodynamic controlled models are compared to analyze the effect of the passive and combined aerodynamic control. The results show that the aerodynamic characteristics improvements generated by the mere surface roughness are limited but accompanying with some negative effects, especially for the models with their major axes being parallel to the incoming wind direction. As the air suction combined with the surface roughness is implemented, it is found that considerable and steady control effects on the aerodynamic characteristics can be generated as CQ reaches 0.02. And the changes of the aerodynamic characteristics are gradually insignificant as CQ is further increased for most cases. Among all the cases with different combinations of aerodynamic measures, quite effective aerodynamic characteristics improvements are acquired for the combined aerodynamic controlled model RS2 (its suction holes are located at the radial angles of 130° and 230°, and its major axis is parallel to the incoming wind direction) with Sr = 1.15 × 10−3 and the model RS4 (its suction holes are located at the radial angles of 90° and 270°, and its major axis is perpendicular to the incoming wind direction) with Sr = 5.3 × 10−4. The conspicuous reductions of the mean drag coefficient CD for the two models mentioned above under CQ = 0.02 are 87% and 59% respectively. Even so, the drag force coefficients of the model RS4 are much larger than those of the model RS2. The objective of the present study is to better understand the aerodynamic control effects and inherent control mechanism due to the changes of surface roughness and air suction, and to explore a proper combination mode of the passive and active aerodynamic control measures to achieve more aerodynamic characteristics improvement for elliptical cylinders.

Numerical study of cross-flow around a circular cylinder with differently shaped span-wise surface grooves at low Reynolds number

Laminar cross-flow over a circular cylinder with smooth as well as longitudinally grooved surfaces was investigated numerically for low freestream Reynolds number ranging from 50 to 300. Most of the currently published works on similar flow configurations considered high Reynolds number in the turbulent regime. The computations were performed by the ANSYS Fluent code. The numerical results were validated against the experimental results. The triangular V-shaped, the U-shaped, and Rectangular shaped grooves, with unit aspect ratio; along with the smooth cylinder, are considered. The predicted flow-field for the grooved cylinder cases are analyzed and compared with that for the smooth cylinder. The aerodynamic loads, time-averaged velocities, rms velocity, vorticity, and pressure coefficient plots around the smooth and grooved cylinders were compared to explore the effect of the shape of the grooves on flow dynamics over the cylinder. The flow physics around the cylinder is significantly affected by the shape and size of the grooves, and their circumferential distribution. It is found that the U-grooves significantly decrease the mean drag coefficient (around 13% for Re= 200 and 10% for Re= 300 compared to the smooth cylinder case). The grooves decreased viscous drag significantly (up to 30%) compared to pressure drag. The grooves on the cylinder also delayed the separation and reduced the extent of the recirculation zone in the cylinder wake due to the presence of recirculating flow within the individual grooves. The conclusion is that the properly designed longitudinal grooves on the cylinder in a cross-flow can be used advantageously for reducing aerodynamic loads like drag and lift force in laminar flow as well.

Lagrangian coherent track initialization

Advances in time-resolved three-dimensional Particle Tracking Velocimetry (4D-PTV) techniques have consistently revealed more accurate Lagrangian particle motions. A novel track initialization technique as a complementary part of 4D-PTV, based on local temporal and spatial coherency of neighbor trajectories, is proposed. The proposed Lagrangian Coherent Track Initialization (LCTI) applies physics-based Finite Time Lyapunov Exponent (FTLE) to build four frame coherent tracks. We locally determine Lagrangian coherent structures among neighbor trajectories by using the FTLE boundaries (i.e., ridges) to distinguish the clusters of coherent motions. To evaluate the proposed technique, we created an open-access synthetic Lagrangian and Eulerian dataset of the wake downstream of a smooth cylinder at a Reynolds number equal to 3900 obtained from three-dimensional direct numerical simulation. Performance of the proposed method based on three characteristic parameters, temporal scale, particle concentration (i.e., density), and noise ratio, showed robust behavior in finding true tracks compared to the recent initialization algorithms. Sensitivity of LCTI to the number of untracked and wrong tracks is also discussed. We address the capability of using the proposed method as a function of a 4D-PTV scheme in the Lagrangian particle tracking challenge. We showed that LCTI prevents 4D-PTV divergence in flows with high particle concentrations. Finally, the LCTI behavior was demonstrated in a jet impingement experiment. LCTI was found to be a reliable tracking tool in complex flow motions, with a strength revealed for flows with high velocity and acceleration gradients.