Effects Of Free Ends Research Articles

Analysis of the wake flow behind concave curved cylinders with velocity measurements by particle image velocimetry and modal decomposition

The properties of the wake flow behind concave curved cylinders have been investigated analyzing simultaneously the mean velocity profiles extracted along concentric arcs parallel to the cylinder axis and the dynamically most relevant coherent structures obtained by spectral proper orthogonal decomposition. These analyzes have allowed us to determine the position, the extension, and the evolution of the different wake regimes. The velocity measurements have been obtained through the stereo particle image velocimetry (stereo-PIV) technique. The study considers various Reynolds numbers, based on cylinder diameter (240<Re<840), and curvatures. The quarter-of-ring cylinder is first analyzed. Near the cylinder root, the flow exhibits a topology dominated by an oblique vortex shedding, contrary to what is observed in the literature for similar geometric configurations. We attribute this disagreement to differences in the treatment of the free-end conditions that play a role in triggering the shedding regime. We find that the local shedding inclination is driven by the axial velocity, whereas its extension and wavelength depend mainly on the Reynolds number. The near free-end region presents, instead, two counter-rotating standing vortices that induce a cross-wise velocity directed toward the cylinder root and a tip vortex that expands evolving downstream. As the Reynolds number increases, the wake presents irregularities and the shedding spreads becoming nearly normal to the incoming flow. At smaller curvatures, the cylinder free-end becoming more inclined with respect to the incoming flow, and the free-end effects are enhanced. The interaction with the vortex sheets of the standing vortices that develop at the leeward side weakens. As a consequence, these vortices stretch in the stream-wise direction giving rise to the trailing vortices that stem from the cylinder surface and affect the cross-wise velocity distributions.

Mechanisms of wake asymmetry and secondary structures behind low aspect-ratio wall-mounted prisms

The wake of a wall-mounted finite prism is numerically studied and characterized with an aspect ratio (height-to-width) of$1$and varying depth ratios (length to width) of between$0.016$and$4$at Reynolds numbers of 50–500. The prism is immersed in a laminar boundary layer. The minimum depth ratio considered here accounts for the special case of a wall-mounted very thin prism (similar to a flat plate), which is used to establish the mechanism and evolution of the wake associated with free-end effects and the shear-layer dynamics in small aspect-ratio prisms. The onset of an unsteady wake behind a very thin prism at a Reynolds number of$200$is characterized by symmetric shedding of hairpin-like vortices. A unique asymmetric wake pattern appears at lower depth ratios starting at a Reynolds number of 250, which transitions to an symmetric wake with increasing depth ratio. The threshold depth ratio for this symmetric transition increases with Reynolds number. The asymmetric wake results from alternate shear-layer peel-off from either side of the prism, which itself is attributed to the out-of-phase shedding of tip vortices at a lower Strouhal number ($St_{sh}/2$) that interact with the detaching side shear layers. Alternate shedding of tip vortices form secondary vortex structures that are fed by the excess vorticity resulting from shear-layer detachment from either side of the prism. Increasing the depth ratio leads to simultaneous shedding of the tip vortices, which restores the commonly observed wake symmetric patterns. Thus, we identify and characterize the formation and interaction mechanisms of symmetric and asymmetric wakes during the transition process with increasing Reynolds number for different depth-ratio prisms.

Numerical study of free end effect of cylinder with low aspect ratios on vortex induced motion

Numerical study of free end effect of cylinder with low aspect ratios on vortex induced motion

Identification of an initial non-linear transition in reciprocating finite-length pipe flow

We present an experimental study of reciprocating pipe flow with free-end effects, specifically, oscillations of a column of water trapped inside an open vertical cylinder under the action of regular linear waves. The aims were to identify parameters where an initial transition to non-linearity in reciprocating flow might occur and to quantify energy losses in the non-linear regime. Reciprocating flows are a subclass of oscillatory flows that are characterized by a zero-mean flux. The investigation may provide a deeper fundamental understanding of the characteristics of reciprocating flows, which differ significantly from the properties of well-studied unidirectional flows. From measured amplitudes of the incoming and excited oscillations inside the column, we draw the dependence of the amount of the absorbed kinetic power on the amplitude and frequency of the incident waves. Quantifications were made of the balance between the energy absorbed by the column and dissipated due to the radiation of waves and wall friction, which are usually considered linear, as well as non-linear losses of energy due to flow separation at the walls and vortex shedding and turbulence at the bottom free end of the cylinder. It is found that the non-linear effects become significant at incident wave amplitudes much smaller than expected, thus affecting the energy balance. We suggest that the initial transition from linear to non-linear behavior in reciprocating finite pipe flow has been identified and the critical oscillatory Reynolds number at which it occurs has been estimated as Reos=4400±1250.

Experimental investigation of flow around a square cylinder with very small aspect ratios

Flow around a pile-cap of bridge is three-dimensional and complicated. When the influence of pile group and bridge pier is ignored, the current forces on the pile-cap are significantly affected by aspect ratio AR, free surface and free end. Experiments were conducted to investigate the drag force, transverse force, vertical force and flow field of a square cylinder with AR ranging from 0.033 to 1 and Re ranging from 5.4×104 to 23.1×104. Visualization of the flow field shows that two recirculation zones generate below and behind the square cylinder respectively. The interaction of spanwise vortices generated from both sides of the square cylinder is attenuated by the free end effect. For the case with AR = 0330., when Re=15.1×104~23.1×104, the free surface deforms severely and air is sucked into the free end to form an air cavity, then the air cavity expands, breaks up into smaller bubbles and escapes away from the free end. The changing rules of the force coefficients of the cases with AR smaller than 0.167 are different from that with AR larger than 0.167.

Robotic PTV study of the flow around automotive side-view mirror models

The flow around three simplified models of side-view mirrors for automobiles is experimentally investigated by means of Robotic particle tracking velocimetry (PTV). The measurements are performed coupling a coaxial volumetric velocimeter with time-resolved three-dimensional (4D) Lagrangian Particle Tracking (LPT) data analyzed with the Shake-The-Box (STB) algorithm. Helium-filled soap bubbles were used as tracers in the wind tunnel experiment to achieve the measurement volume needed for characterizing the flow structures around and behind the models of automobile side-view mirrors. The aspect ratio of all the models is kept constant to H/W = 2. The effect of a slant angle at the front and the reduction of cross section at the model root are specifically investigated respectively using model 2 and model 3. In model 1 which represents generic geometry having semi-cylinder-spherical shape, the extension of the recirculation zone shrank in the z direction due to the free-end effect. When the flow is developing downstream, a vortex pair inclines towards the ground plane due to the downwash effect. This streamwise vortex shows a dipole distribution. In model 2 the trailing vortex does not fall toward downstream because a base vortex with opposite sign formed under the trailing vortex. This prevents the downwash flow from reaching the ground plane. In model 3, a streamwise vortex is generated from a strong upwash flow, which generates a short trailing vortex up to x/H = 1. This occurs because the vortex that was formed from the top and side end is suppressed and destroyed. The leg of the reverse U-shape structure of model 1 is almost perpendicular to the ground plane. However, in models 2 and 3, vortex shedding near the bottom is delayed, and an arch vortex is slightly slanted toward the model body.

The effect of base column on vortex-induced vibration of a circular cylinder with low aspect ratio

The end condition of a cylinder is known to influence its vortex-induced vibration (VIV) response. The design of a short circular cylinder with a concentric base column attached to its bottom is adopted by some floating structures for improved stability, but how the base column would affect the VIV of the cylinder has been rarely studied. In this paper, the VIV of an elastically mounted rigid cylinder with a base column and a low aspect ratio of 2 is investigated numerically by solving the Navier-Stokes equations. The numerical methods are validated against two existing VIV studies, including a 2D cylinder and a cylinder with a finite length. The impacts of the base column on the cylinder are analysed. It is found that although the free end effects associated with fluid flowing around the cylinder end are still present, attaching the base column leads to the expansion of the lock-in regime of the cylinder response. A relationship between non-dimensional response amplitude and lift coefficient is established, which takes into consideration the geometrical properties of the base column. By analysing the energy transfer from fluid to structure, the base column is also found to have significant damping effects on the cylinder response.

Effects of free patchy ends in ssDNA and dsDNA on gold nanoparticles in a colorimetric gene sensor for Hepatitis C virus RNA.

The presence or absence and nature of the free patchy ends in DNA sequences has a decisive effect on the performance of colorimetric sensors based on the use of gold nanoparticles (Au NPs). The authors have designed two unmodified gene probes (probe 1: a 19-mer; probe 2: an 18-mer). They are complementary to either half of a 37-mer target derived from the conserved region of Hepatitis C Virus (HCV) RNA. Each probe has further been modified with 10-mer poly(A) and thiol-functionalized 10-mer poly(A) at the 5' positions. Nine combinations of probe and HCV RNA were then designed to investigate the effect of free patchy ends on the stability of citrate-modified Au NPs against salt-induced aggregation which lead to color change from red to blue. The aggregation of Au NPs can be monitored by ratiometric spectroscopy at wavelengths of 520 and 700nm. The differentiation between HCV RNA and control has also been studied by varying the concentration of probe and analyte. The particle size and zeta potentials were determined before and after aggregation. It is demonstrated that the change in surface charge density of the Au NPs governs the critical coagulation concentration of NaCl. The method presented here can be used to quantify HCV RNA in the 370nM to 3μM concentration range, and the detection limit is 500nM. The results obtained with Au NPs that are chemically non-conjugated with the oligonucleotides have been found to be valuable in rationally devising the design rules for rapid and efficient colorimetric sensing of oligonucleotides. Graphical abstract Schematic representation of the nine combinatorial pairs of oligonucleotides that vary in the length of patchy ends and their position to unearth their effect in rapid gold nanoparticle-based colorimetric gene sensing without time-consuming and expensive thiol-conjugation step.

Numerical and experimental investigation of flow around a balloon-shaped bluff body

Air accumulator is a critical component in Underwater Compressed Air Energy Storage systems and Buoyancy Energy Storage systems. In order to bridge the gap between the CFD simulations and experiment verification of flow around air accumulator, in this study, the force characteristics and flow structures around a balloon-shaped air accumulator bluff body model have been investigated using a computational-experimental dual prong method for the first time. The three-dimensional Unsteady Reynolds Averaged Navier-Stokes (URANS) closure is achieved through the k-ω SST turbulence model in FLUENT. The force characteristics and flow structures are measured in a low-speed closed-loop wind tunnel to verify the simulation results. The free end and shape effects are investigated by comparing the flow structure of three different but interconnected bluff bodies at the equivalent Reynolds number of 7.0 × 104. The results show that the k-ω SST turbulence model can correctly predict the time-averaged force characteristics of the balloon-shaped bluff body and flow structures in the wake. Besides, the distinct vortex structures are induced due to the free end and tapered shape effects.

Effect of Inclination on Vortex Shedding Frequency Behind a Bent Cylinder: An Experimental Study

This paper presents experimental results on the vortex shedding frequency measured behind a bent cylinder. Experiments were conducted in a wind tunnel covering Reynolds numbers between 50 and 500, a range of interest for flow sensing, flow control, and energy harvesting applications. The bent cylinder comprised a vertical leg always oriented at normal incidence with respect to the free-stream flow, and an inclined leg whose inclination was varied during the tests between 90° and 15°. The bent cylinder was oriented in the wind tunnel with the vertical leg upstream and the inclined leg downstream, and the vortex shedding frequency was measured with hot-wire anemometry at several locations behind the inclined leg. The present bent cylinder design improves upon those previously considered by providing a finer control on the upstream boundary condition acting upon the inclined leg, which in the present design is not affected by the yaw angle of the inclined leg. With the exception of free-end effects, only noticeable for certain inclinations and Reynolds number values, inclination effects were surprisingly not observed, and the frequency of vortex shedding measured behind the inclined leg of the bent cylinder was consistent (within a few percent) with the cross-flow vortex shedding frequency at the same flow velocity. The present results corroborate and significantly extend the limited observations on bent cylinders available in the literature, further highlighting the importance of the upstream boundary condition on the vortex shedding process with inclined cylinders.

Vortex-induced vibration of floating circular cylinders with very low aspect ratio

Experiments regarding vortex-induced vibration (VIV) on floating circular cylinders with low aspect ratio were carried out in a recirculation water channel. The floating circular cylinders were elastically supported by a set of linear springs. Eight different aspect ratios were tested, namely L/D= 0.2, 0.3, 0.4, 0.5, 0.75, 1.0, 1.5 and 2.0. These aspect ratios were selected to cover the aspect ratio range of the main offshore circular platforms, such as spar and monocolumn. The aim was to understand the vortex-induced motions (VIM) of such platforms; due to this, the cylinders were floating, or m∗=1. The range of Reynolds number covered 2800 < Re < 55,400. The amplitude results showed a decrease in amplitude with decreasing aspect ratio in the in-line and in the transverse directions. The frequency results confirm a different behavior for cylinders with L/D≤0.5; in these cases, the cylinder free-end effects were predominant. The resonant behavior was no longer observed for L/D≤0.2. The decrease in Strouhal number with decreasing aspect ratio is also verified, as well as for drag and lift forces. The added mass results help to identify the end of resonance for both directions, in-line and transverse. The amplitude results for the vertical direction, roll, pitch and yaw did not affect the VIV behavior.

Ratchetting damage of railhead material of gapped rail joints with reference to free rail end effects

Free ends of insulated rail joints occur because gaps between the rails and endposts can be created due to pull-apart problems as the rails contract longitudinally in winter and by degradation of railhead material. Dynamic behaviour of gapped rail joints changes adversely compared to that of insulated rail joints. Thus, material degradation and damage of gapped rail joint components such as rail ends, joint bars, etc. are accelerated. Only limited literatures are available addressing the free end of rail effects at rail joints, targeting stress and pressure distributions in the vicinity of the rail joints. To understand clearly the material degradation and delamination process of gapped rail joints, a thorough analysis of failure of both insulated rail joints and gapped rail joints and subsequent damage of the railhead material is necessary to improve the service life of these joints. A new three-dimensional finite element analysis is carried out in this paper to assess damage to railhead material when gapped rail joints form. Both narrow (5 mm) and wide (10 mm) gaps are considered, using a peak vertical pressure load of 2500 MPa applied cyclically at one rail end, forming vertical impacts. Stress distributions and plastic deformations in the vicinity of gapped rail joints are quantified using finite element analysis data and compared with that of the insulated rail joints to show the effects of free rail ends. Residual stress and strain distributions indicate the damage to the railhead material. Equivalent plastic strain (PEEQ) quantifies the progressive damage to the railhead material at the rail ends. The free end of rail effects can be further illustrated by comparing PEEQ for insulated rail joints and gapped rail joints. The railhead material of 5 and 10 mm gapped rail joints is more sensitive to permanent deformation compared to that of the corresponding insulated rail joints. Therefore, free rail end joints pose an increased potential threat to rail operations in relation to crack initiation, damage and premature failure of railhead material.

Flow around circular cylinders with very low aspect ratio

Experiments on the flow around stationary circular cylinders with very low aspect ratio piercing the water free surface were carried out in a recirculating water channel. Eight different aspect ratios were tested, namely L/D=0.1, 0.2, 0.3, 0.5, 0.75, 1.0, 1.5 and 2.0; no end-plates were employed. Forces were measured using a six degree-of-freedom load cell and the Strouhal number was inferred through the transverse force fluctuation frequency. The range of Reynolds number covered 10 000<Re<50 000. PIV measurements were performed in some aspect ratio cases, namely L/D=0.3, 0.5, 1.0 and 2.0 for Reynolds number equal to 43 000. The results showed a decrease in drag force coefficients with decreasing aspect ratio, as well as a decrease in Strouhal number with decreasing aspect ratio. The PIV measurements and the PSD of forces showed different behavior for cylinders with L/D≤0.5, in which cases the free-end effects were predominant. Even without von Kármán street main characteristics around the majority length of the cylinder, in the range of 0.2<L/D≤0.5, the vortex shedding around it is capable of producing alternating forces in the transverse direction. Therefore, alternating forces were not observed in the transverse direction for cylinders with L/D≤0.2.

Welding of gold nanowires with different joining procedures

Welding of single-crystal metallic nanowires is likely to have an important role in the bottom-up fabrication of nanodevices. The welding effects of free ends of two single-crystal gold nanowires (Au–Au) were demonstrated by Monte Carlo simulations in this paper. The quantum corrected Sutton–Chen type many-body potential was used to model the metal–metal interactions. Metallic nanowires were first placed closely with head-to-head, head-to-side and side-to-side joining procedures. Two ends were successfully welded together to form a continuous nanowire by annealing at different temperatures. The welding effects of the different joining procedure with different temperatures were compared. Structures of the welded specimens were characterized by the common neighbor analysis technique. Variable atomic mobility, freedom and contacting angles may result in different bonding strengths in the three different configurations. The results showed that the joint structure welded at low temperature were similar to the cold welding without fusion meanwhile the molten phase was presented in the joint when processing at the high temperatures.

Transition dynamics and selection of the distinct S-DNA and strand unpeeling modes of double helix overstretching

Recent studies have revealed two distinct pathways for the DNA overstretching transition near 65 pN: ‘unpeeling’ of one strand from the other, and a transition from B-DNA to an elongated double-stranded ‘S-DNA’ form. However, basic questions concerning the dynamics of these transitions, relative stability of the two competing overstretched states, and effects of nicks and free DNA ends on overstretching, remain open. In this study we report that: (i) stepwise extension changes caused by sequence-defined barriers occur during the strand-unpeeling transition, whereas rapid, sequence-independent extension fluctuations occur during the B to S transition; (ii) the secondary transition that often occurs following the overstretching transition is strand-unpeeling, during which the extension increases by 0.01–0.02 nm per base pair of S-DNA converted to single-stranded DNA at forces between 75 and 110 pN; (iii) even in the presence of nicks or free ends, S-DNA can be stable under physiological solution conditions; (iv) distribution of small GC-rich islands in a large DNA plays a key role in determining the transition pathways; and (v) in the absence of nicks or free ends, torsion-unconstrained DNA undergoes the overstretching transition via creation of S-DNA. Our study provides a new, high-resolution understanding of the competition between unpeeling and formation of S-DNA.

三次元円柱の渦励振現象に関するLES解析 : 自由端による渦形成の変化とロッキング振動モードの効果

This paper shows LES analysis of the physical mechanism of vortex-induced oscillation of a three dimensional circular cylinder. First, we discuss the accuracy of the computed results of free-end effect on the wake structures and the distributions of aerodynamic coefficients in the axial direction of a three dimensional stationary circular cylinder. Next, the present LES method is applied to the flow around a freely oscillating circular cylinder which has rocking oscillation mode. The effects of rocking mode shape on characteristics of the response and vortex motions around an oscillating circular cylinder are investigated. Consequently, we clarified the difference between the physical mechanism of vortex-induced oscillation of an three dimensional circular cylinder and those in two dimensionnal ones.

Free end effect on the stress and displacement distributions in a cracked composite strip

Taking into account the marked anisotropic character of carbon fibre composite materials, a small parameter related to material properties is introduced. The stress field in a semi-infinite strip with a semi-infinite crack whose tip is close to the strip free end is investigated through a singular perturbation method. For slow crack propagation, the quasi-static character of the stress field is established. An accurate asymptotic solution is derived, which allows the interaction between the strip free end and the stress field around the crack tip to be studied. For the carbon–epoxy material investigated, this interaction becomes negligible when the distance between the crack tip and the strip free end is greater than 1.5 h, where h is the strip half-height.

Free end effects on the near wake flow structure behind a finite circular cylinder

The free end effect on the near wake of a finite circular cylinder in a cross flow has been investigated experimentally. Three finite cylinders with aspect ratios ( L/ D) of 6, 10 and 13 were tested in a subsonic wind tunnel at a Reynolds number of 20 000. A hot-wire anemometer was employed to measure the wake velocity. Mean pressure distributions on the cylinder surface were also measured. The flow near the free end was visualized to observe the flow structure qualitatively in a circulating water channel. The experimental results from these finite cylinder (FC) models were compared with those of a two-dimensional circular cylinder. The flow past the FC free end shows a complicated three-dimensional wake structure. As the FC aspect ratio decreases, the vortex shedding frequency is decreased and the vortex formation region is elongated. The free end effect becomes dominant close to the FC free end. The three-dimensionality of the FC wake may be attributed mainly to the strong entrainment of irrotational fluids, caused by the downwash of counter-rotating vortices separated from the FC free end. The downwash flow is concentrated in the central region of the wake. A peculiar flow structure having a 24 Hz frequency component was observed near the free end using spectral analysis and cross-correlation of the velocity signals. This 24 Hz frequency component is closely related to the counter-rotating twin vortices formed near the FC free end.

Flow structure of wake behind a finite circular cylinder

Flow characteristics of the wake behind a finite circular cylinder(FC) mounted on a flat plate was experimentally investigated. Three finite cylinder models having aspect ratio (length to diameter ratio, L/D) of 6,10 and 13 were tested in this study. Wake velocity was measured by a hot-wire anemometry at Reynolds number of 20,000, and the results were compared with those of two-dimensional circular cylinder. As a result, the free-end effect on the wake structure becomes more dominant with decreasing the aspect ratio(L/D) of the finite cylinder. Invisid flow entrained into the wake region decreases the turbulence intensity and periodicity of the vortex shedding due to existence of the free end. From spectral analysis and cross correlation of the velocity signals, vortices having 24Hz frequency characteristics are found in the down wash flow just behind the free end. There exists very complicated flow near the free end due to interaction between the entrained flow and streamwise vortices. Vortex formation region is destroyed significantly in the near wake and shows quite different wake structures from those of 2-D cylinder.

Association probabilities between the single-chain amphiphiles into a binary mixture in plan monolayers (II)

A test system of homologous binary mixtures of single-chain amphiphiles was considered. The dependence of the association probabilities on the hydrophobic chain length, and on the dipole moment of the shorter chain component when the longer chain component has a low dipole moment, was studied. The free-end effect of each hydrophobic chain on the association probability was also analysed. Generally, both the shorter chain end effect and the longer chain end effect on the association process are independent of their own initial chain length, but are dependent on the chain length of the other component and dipole moment. The changes in every association probability produced by the addition of one methylene group to the shorter chain and to the longer chain are unequal and opposite. For this reason the efficiency ratios were defined and analysed. The thermotropic behaviour of lipid mixtures can be related to association processes.