Surface Of Cylinder Research Articles

BKT TRANSITION IN PHYLLOTAXIS

We explore a two-parameter renormalization group (RG) within the framework of the «energetic approach» introduced by L. Levitov, for the phyllotaxis model. Our focus lies on an equilibrium distribution of strongly repulsive particles situated on the surface of a finite cylinder. We investigate how these particles redistribute as the cylinder undergoes compression along its axis. Specifically, we construct the modular-invariant β-function for the system, which is explicitly expressed in terms of the Dedekind η-function. Utilizing this β-function, we derive equations that describe the RG flow near the bifurcation points, which mark the boundaries between different lattice configurations. By analyzing the structure of these RG equations, we assert the emergence of Berezinskii – Kosterlitz – Thouless transitions under significant cylinder compression.

Monitoring of a Low-Order Even Radial Vibrational Circumferential Mode in a Round Hollow Cylinder

This paper presents a nondestructive testing method for assessing the structural integrity of cylindrical elements by monitoring a range of radial vibrational modes, with a specific emphasis on the so-called ovalling mode. The method involves exciting the cylinder with a single vibration source in the radial direction and measuring the response using two vibration sensors positioned diametrically on the cylinder's surface. The ovalling mode was extracted from the frequency response by adding the in-phase signals recorded by the sensors. Experiments conducted on a PVC pipe showed good agreement between the measured resonance frequency of the ovalling mode and its predicted value, calculated using the theory for thin cylindrical shells and Finite Element Method simulations. This research is an investigation into the potential and reliability of this nondestructive technique for detecting corrosion and strength-weakening defects in concrete building elements, steel pillars, and columns. The extent of the strength reduction can be determined by analyzing the change in the resonance frequency of the ovalling mode.

Suppressing impact intensity of cavitation collapse by micro-protrusions on the cylinder surface

Under specific cavitating flow conditions, the collapse of cavitation structures near the bluff body surface can trigger intense pressure pulsations, resulting in severe vibration and noise. To effectively control the damage caused by the collapse of cavitation structures, this study investigates the cavitation flow field around a cylinder using a compressible numerical simulation method based on implicit large eddy simulation. This work is the first to examine the cavitation control effects of placing micro-protrusions on the surface of a cylinder. The results show that the wall-attached cavitation collapse near the separation point on the cylinder surface is the main cause of induced vibrations and noise. The height and location of the micro-protrusions are key factors in controlling the wall-attached cavitation in the cylinder's shear layer. It is recommended to position the protrusions approximately 5° upstream of the separation point, with the protrusion height entering at least the logarithmic region of the boundary layer. Appropriately placed micro-protrusions can block the reverse flow, reducing the cavitation volume involved in the collapse and the overall collapse intensity, thereby diminishing the load impact on the cylinder surface.

A hybrid approach portraying the dynamics of free convection condensation around a circular cylinder

We introduce a hybrid framework to model the fluid dynamics of free convection condensation of saturated steam outside a circular cylinder. The liquid film is modeled analytically, and the adjacent vapor flow field is resolved numerically. The model incorporates temperature-dependent thermophysical properties of the condensate. We explore the subtle role of Jakob number (Ja) and Weber number (We) in the two-phase flow and establish the criteria for dynamic similarity of the flow field with two new dimensionless numbers, viz., free-fall Reynolds number and similarity number. For fixed base Prandtl number of the condensate, the increases in the subcooling rate with Ja and the cylinder's surface area to volume ratio with We cause film thickening. We develop a theoretical correlation to predict the average film thickness (δm). We identify entrainment and bypass zones in the vapor flow field demarcated by a separating streamline. The entrainment zone's streamlines converge to the interface, yielding a net condensate drainage. The bypass streamlines never reach the interface and reduce the condensation efficiency. The results show that the tangential velocity is dominant within the liquid film, the radial velocity is dominant within the vapor flow field, and they are of the same order at the interface. We locate the point of flow separation influenced by a surface tension-induced adverse pressure gradient. The location of the flow separation point shifts upstream, and the separating streamlines become steeper with the increase in We. Our investigation reveals the zone of tangential flow reversal near the interface, promoted with increasing We.

Predicting wave run-up on vertical columns based on thermal stereography measurements

Accurate estimation of wave run-up is crucial for the design and safety of marine structures. To facilitate more precise and convenient predictions of wave run-up on vertical columns, including circular and square columns, many empirical formulas have been proposed. However, those derived from wave probe measurements typically underestimate the maximum wave run-up height due to the inability of wire-type wave gauges to closely adhere to the column surface. This study employed the non-contact wave measurement technology based on thermal stereography to measure the wave run-up distributions on vertical columns under regular waves, providing a more accurate measurements of wave run-up closer to the column surface. Utilizing the measurements, a modified formula was proposed to predict the wave run-up on circular columns. Additionally, considering the effects of scattering parameter (kD) and wave steepness (kηmax), a new empirical formula for predicting the wave run-up on square columns was developed based on the velocity stagnation head theory. These formulas were validated using the experimental data from the present study and the literature, demonstrating their ability to provide satisfactory wave run-up predictions. Furthermore, the spatiotemporal wave evolution for circular and square columns were compared. The wave profile in front of the circular cylinder at a half diameter tends to flatten, with water flowing along the cylinder surface to the sides, leading to a significantly lower wave run-up height ratio compared to the square column. This study provides the valuable insights for predicting the wave run-up on vertical columns, contributing to the design and safety assurance of marine structures.

Friction drag model for axial turbulent flow along the surface of a circular cylinder based on the universal characteristics of wall turbulence

The friction drag of the axial flow along the outer surface of a cylinder varies with the cylinder radius and flow conditions. This study included direct numerical simulations of the axial turbulent flow along a circular cylinder under different conditions for obtaining the turbulence statistics and wall friction coefficient. Then the characteristics of velocity streaks were observed from a geometrical perspective of turbulence structures around the circular cylinder, and compared with the characteristics of the turbulence structures in a boundary layer on a flat plate. The results showed that the velocity streak spacing and the distance between the velocity streak and the cylinder surface in the viscous length scale do not vary substantively with the radius of the cylinder, and are the same as those of the turbulent flow along a flat plate. Therefore, they can be considered geometrical characteristics of the turbulence structure independent of the cylinder radius. Moreover, the friction coefficient per pair of high- and low-speed velocity streaks is the same as that of flat-plate turbulent flow, independent of the cylinder radius, and can be regarded as a dynamical characteristic for a pair of velocity streaks. Two equations were derived based on the characteristics of wall turbulence. The characteristics of the turbulence predicted by the two formulae were consistent with the simulation results. Consequently, we showed that the wall friction coefficient and number of the velocity streak pairs, which are statistical and structural characteristics of wall turbulence, can be predicted appropriately by specifying the radius Reynolds number.

Experimental and LES investigations of horseshoe vortex characteristics in wall-mounted rectangular cylinders for various angles of incidence

To examine the flow characteristics of horseshoe vortices (HV) in wall-mounted rectangular cylinders, surface oil-flow visualization and Large Eddy Simulation (LES) were used for experimental and numerical studies, respectively. This research examined various incident angles relative to the approaching flow (LES: α = 0°–45°, oil-flow visualization: α = 0°–90°) and cross-sectional aspect ratios (CR = 1–4) for Reynolds numbers of (12−48)×103. The features of HV have been analyzed, and the critical points (saddle and focal) and their relationships to physical phenomena have been evaluated. The presence of two saddle points (S1 and S2) and their associated nodes (N1 and N2) confirms the formation of two regions in the horseshoe vortices. The region closer to the upstream of the cylinder experiences higher shear stress than that farther away. It was observed that two focal points appear behind the cylinders, which show the positions of arch-shaped vortex legs that meet the bottom surface. Three patterns of horseshoe vortices were identified based on the focal point positions for various α and CR. The focal points in pattern I are symmetric with respect to the streamwise direction, where their spacing is less than the width of the cylinder. In pattern II, the focal points lose symmetry, forming on one side of the cylinder very close to each other. The positions of focal points in pattern III become asymmetric with respect to the streamwise direction. In addition, the width of the HV region and the reattachment point on the cylinder surface were examined for different CR and α. The reattachment point occurs at the junction between cylinders and wall for CR = 4 at α = 0°–15°, CR = 3 at α = 5°–15°, CR = 2 at α = 10°–15°, and CR = 1 only at α = 15°.

ANALYSIS OF SURFACE ROUGHNESS OF REFRIGERATIONCOMPRESSOR FRICTION NODES DUE TO INSUFFICIENTLUBRICATION

The article presents an attempt to evaluate the roughness profiles of the components of the friction nodes inrefrigeration compressors due to insufficient lubrication. The damaging impact on the compressor operationunder various unfavorable operating conditions, including with little or no lubricant at all, was analyzed for areal refrigeration system using a dedicated test stand.The Embraco Aspera NE9213GK refrigeration compressor used was originally filled in by the manufacturerwith a polyester lubricant; for the purpose of the study, the R407C lubricant was applied. An accelerateddamage-controlled procedure was carried out on the compressor by reducing the amount of lubricant insubsequent operating cycles of the system. The amount of oil was adjusted by removing lubricant duringshutdown periods using a dedicated spigot in the body of the semi-hermetic compressor casing. The testingcontinued until the refrigeration compressor seized up.The test stand was used to analyze electrical power measurements at the time of intentional damage tothe compressor caused as a result of forced absence of lubrication. Additionally, the authors analyzed surfaceroughness profiles of the compressor sliding elements prone to wear, including crankshaft journals, pistonsurface, and a cylinder surface.

High-bandwidth pressure field imaging of fin-generated shock wave–boundary layer interactions

The dynamics of a shock-induced separation unit generated by a 20 $^\circ$ sharp fin placed on a cylindrical surface in a Mach 2.5 flow was investigated. Specifically, the present work investigated the mechanisms that govern the mid-frequency range of separation shock unsteadiness in the fin shock wave–boundary layer interaction (SBLI) unit. Two-dimensional pressure fields were obtained over the cylinder surface spanning the entire fin SBLI unit using high-bandwidth pressure-sensitive paint at 40 kHz imaging rate that allowed probing the low- through mid-frequency ranges of the separation shock unsteadiness. The mean pressure field showed a progressive weakening of the separation shock with downstream distance, which is an artifact of the three-dimensional relief offered by the curved mounting surface. The root-mean-square (r.m.s.) pressure field exhibited a banded structure with elevated $p_{r.m.s.}$ levels beneath the intermittent region, separation vortex and adjacent to the fin root. The power spectral density (PSD) of the surface pressure fluctuations obtained beneath the intermittent region revealed that the separation shock oscillations exhibited the mid-frequency content over the majority of its length. Interestingly, neither the PSD nor the length of the intermittent region varied noticeably with downstream distance, revealing a constant separation shock foot velocity along the entire SBLI. The pressure fluctuation PSD beneath the separation vortex also exhibited the broadband peak at the mid-frequency range of the separation shock motions over the majority of its length within the measurement domain. By contrast, the region adjacent to the fin root exhibited pressure oscillations at a substantially lower frequency compared with the separation shock and the separation vortex. Two-point coherence and cross-correlation analysis provided unique insights into the critical sources and mechanisms that drive the separation shock unsteadiness. The separation vortex and separation shock dynamics were found to be driven by a combination of convecting perturbations that originated from the vicinity of the fin leading edge and the local interactions of the separated flow with the incoming boundary layer. The boundary layer locally strengthened or weakened the convecting pressure perturbations depending on the local momentum fluctuations within the boundary layer.

Electrical Resistivity Testing of Concrete Cylinders: Bias, Precision, and Use in Process Control

Electrical resistivity is increasingly used as a test method to assess the transport properties of concrete. This paper describes an interlaboratory study that was conducted to determine precision and bias for resistivity measurements made in surface and bulk configurations of concrete cylinders. Tests were performed following AASHTO T 358-22 and AASHTO T 402-23. A verification device is introduced with a known resistivity (and impedance) to aid in determining the bias associated with resistivity measurements. However, this device can also be used as a training tool and as a tool to evaluate the qualifications of those new to performing the test. Concrete cylinders were prepared, cured, and conditioned using three methods: immersed in simulated pore solution, sealed, and immersed in lime solution. The samples were tested to determine the precision in the resistivity measurements of the samples for the different curing conditions. The paper discusses how resistivity testing can be used for quality control testing as well as quality acceptance. The results were used to develop precision and bias statements for resistivity measurements, which have been adopted in the latest American Association of State Highway and Transportation Officials (AASTHO) standards.

Design Improvement, Thermal Efficiency Optimization and Comparative Study of a Natural Draft Chulha

ABSTRACT This work inscribes the rural-based biomass cookstove (chulha) fabrication with the aim of modification and enhancement of the existing natural draft low smoke heat-efficient stoves and also comparing its thermal efficiency output. This study is important from the perspective of a rural society who cooks their daily meals on biomass-based cookstoves/chulhas. Enhancing its thermal efficiency and reducing smoke emissions from biomass combustion in the cookstove are the primary focuses of this work. Principle used to design the improved chulha is double co-axial cylinder secondary combustion. Mild steel is used for fabricating maximum parts of the stove including the outer cylinder surface, and the inner cylinder surface is made of galvanized steel, and the insulation between both the surfaces of the cylinders is made of fiber glass wool. Addition of a novel insulating material in the chulha for thermal insulation, resizable air inlet, design and incorporation of smoke rings and pot seat in such a way that no smoke passes through the cooking utensil assisted in achieving better thermal efficiency of the cookstove. Pine wood, bamboo, sagun wood, sal wood and neem wood have been selected as per local availability as biomass feedstock for combustion. To calculate the thermal efficiency, a water boiling test (WBT) has been performed. Best thermal efficiency of 47.08% has been achieved for pine wood biomass feedstock and due to proper placements of the grate and the air inlets in the improved chulha design. Also, a very negligible or no smoke has been observed due to the introduction of the advanced design of the smoke ring and pot seat and the chimney for the outlet making a green energy-efficient biomass cookstove.

Numerical simulations for the flow-induced vibrations of tandem dual flexible circular cylinders

The three-dimensional large-eddy-simulation coupled with a mode superposition method was applied to numerically simulate the flow-induced vibrations (FIVs) of tandem dual flexible cylinders at Re = 1000 with three different spacing ratios (Sx/D = 2.5, 3.5, and 5, Sx is center-to-center spacing for tandem cylinders, and D is the diameter of the cylinder), corresponding to the reattachment flow, transition flow, and co-shedding flow regimes in stationary tandem cylinders, respectively. The effects of Sx/D on structural vibrations, flow fields, distributions of the surface pressures, and energy properties were investigated to reveal the mechanism for the FIV. Increasing Sx/D weakens the influence of the upstream cylinder on maximum response amplitudes and lock-in region for downstream cylinder. The wake patterns for tandem flexible cylinders are more complex compared to stationary or vibrated rigid tandem cylinders. The shielding effect reduces surface pressure on the downstream cylinder significantly when its vibrations are smaller, leading to a decrease in mean power as well. Furthermore, different mechanisms contribute to amplified FIV in downstream cylinders depending on Sx/D: when Sx/D = 2.5, the upstream vortices collide with the downstream cylinder's front surface and merge with the vortices generated by the downstream cylinder, increasing negative pressure on both front and rear surfaces of the downstream cylinders and promoting FIV; however when Sx/D =3.5 and 5, a binary vortex street forms behind the downstream cylinder without obvious negative pressures on its front surface, the dominant causes of FIV are primarily attributed to interactions among upstream and downstream vortices.

On magnetohydrodynamics free convective flow within coaxial cylinder due to nonlinear thermal radiation in an isothermal/isoflux condition

A theoretical exploration was reported to highlight the electrically viscous conducting fluid within a coaxial cylinder. Both the outer surface of the inner cylinder and the inner surface of the outer cylinder were porous, heated, and cool, respectively. The action of a simultaneous radial transverse magnetic field and velocity slip were imposed across the coaxial cylinder. For appropriate insight into the physical problem, the thermal Rosseland diffusion approximation was used to indicate the radiative heat flux in the energy equation. The aim is to investigate the effects of isothermal and isoflux conditions on slip-upshot heat transfer in a vertical coaxial cylinder due to nonlinear thermal radiation. The steady-state and unsteady solutions of the physical problem were provided and solved via regular perturbation and implicit finite different domains, respectively. It was noted that the magnetic and suction parameters reduced the fluid velocity and skin friction under both isothermal and isoflux conditions. Conversely, the Grashof number, thermal radiation, and injection parameters increased the fluid velocity, fluid temperature, and skin friction, with a more significant effect observed under isothermal conditions on the surfaces of the coaxial cylinder. Additionally, the sensitivity analysis revealed that the radiation parameter is highly sensitive under isothermal conditions, followed by the temperature difference parameter. In contrast, under isoflux conditions, the injection parameter shows extreme sensitivity, with the radiation parameter being the next most sensitive. The result reveals an excellent agreement between the analytical and numerical results at maximum time.

Numerical simulation of flow around a transversely oscillating square cylinder at Re=22000

The study utilized OpenFOAM software, combined with the k-ω shear stress transport (SST) turbulence model and Arbitrary Lagrangian-Eulerian (ALE) dynamic mesh method, to investigate the influence of the oscillation amplitude on the flow dynamics around a square cylinder at Re = 2.2 × 104. It was found that various oscillation amplitudes affect the non-Gaussian characteristics of fluctuating pressure differently along the cylinder's surface: diminishing at the leading edge while intensifying at the trailing-edge corner. Concurrently, secondary recirculation bubbles near the cylinder's leading and trailing edges gradually diminish and eventually disappear with increasing oscillation amplitudes. Analysis of the amplitude spectrum of transverse velocity indicates the exclusive presence of odd-order harmonics along the central axis, while both odd and even-order harmonics are observed on either side of the central axis. Additionally, heightened oscillation amplitudes result in increased energy transfer from the cylinder to the fluid, with the energy density function predominantly exhibiting negative values and periodic variations occurring at twice the frequency of cylinder displacement or lift coefficient. As the oscillation amplitude increases, the vortex in the wake region transitions from a single-row to a double-row configuration. The research results provide important theoretical guidance for deepening the understanding of the flow characteristics around the oscillating square cylinder.

Influence of gap ratios on wake dynamics of two rectangular 5:1 cylinders placed inline

The present study explores the impact of gap ratio (G) on the wake dynamics of two rectangular 5:1 cylinders placed inline at a fixed Reynolds number (Re) = 250. The lattice Boltzmann method is used for numerical simulations, and the gap ratio is varied in the range of 0.25–10. After validating the code for flow around a single square cylinder, the simulations are further carried out to explore the impact of gap ratio on the fluid flow past two inline cylinders with a similar aspect ratio = 5:1. Both the cylinders are found to face the reattachment of detached shear layers from front edges on to the side surfaces before the final separation. These reattached shear layers generate vortices on the upper and lower surface of cylinders exhibiting the reverse flow. For G < 4.5, the surface vortices appear on the first cylinder only, while for G ≥ 4.5, the surface vortices appear at both cylinders upper and lower sides. The reattachment point on the surface of the first cylinder is found to move downstream till G = 3, but after that, reverse phenomena occur with an increase in the gap ratio. The enhancement of flow induced lift results due to such separating and reattaching flow. Four distinct flow patterns appear at different gap ratios range: (a) single slender body, (b) non-fully developed duplex shedding, (c) fully developed duplex shedding, and (d) unpredictable vortex shedding. The root mean square value of drag coefficient for the first cylinder is mostly found less as compared to the second cylinder for all G. Negative pressure is observed at the upper, lower, and wake region of both bodies. However, the pressure appears to be positive at the foremost surface of second body when the flow pattern changed from fully developed duplex shedding to unpredictable vortex shedding. Strouhal of both cylinders shows distinct values corresponding to dual frequency at small gap ratios but becomes consistent at higher ones.

Optimum Size and Heat Transfer and Velocity Correlations of the Outer and Inner Surfaces of Vertical Hollow Polygonal Cylinders in Natural Convection

Abstract Laminar natural convection heat transfer from vertical hollow polygonal cylinders with a wide range of cross-sectional areas is investigated. The buoyancy-driven three-dimensional (3D) flow around hollow polygonal cylinders immersed in quiescent ambient air with equal outer and inner surface temperatures is analyzed. The governing equations are numerically solved in nondimensional variables using the finite volume method. The numerical solution is validated using available experimental and numerical data. Results of the mean Nusselt number for the outer (Nu¯ho) and inner (Nu¯hi) surfaces are obtained by varying a number of key parameters. These parameters are the Rayleigh number based on the cylinder height (Rah) in the range 103≤ Rah≤ 107, the nondimensional cross-sectional area (AC) in the range 0.006 ≤ AC≤ 0.5, and the number of sides of the polygon (N) in the range 6 ≤ N ≤∞. In all cases, a Prandtl number (Pr) of 0.7 has been assumed. The study shows that at a certain Rayleigh number and a certain number of sides, the heat transfer rate from the inner surface decreases (by as much as 79.8%) as the polygon area decreases (by as much as 83.32%), whereas the heat transfer rate on the outer surface increases (by as much as 133.3%) as the polygon area decreases (by as much as 83.32%). It has also been found that the behavior of the buoyancy-driven flow in the vicinity of the outer surface is fundamentally different than that near the inner surface. Additional details about this fundamental difference are presented in the Results and Discussion section of the paper. New correlations to calculate the average velocity at the exit surface of the cylinder inner core and the mean Nusselt number for both the outer and inner surfaces have also been developed. Also, correlations have been developed for selecting the optimal cross-sectional area for purposes of identifying the regions where the thermal and velocity boundary layers overlap within the inner core of the cylinder.

Characterization of internal defects in cylindrical components using laser ultrasonic method with a modified SAFT algorithm

The health monitoring of cylindrical elements is particularly important for the safe operation of industrial production, because cylinders bear a lot of support and transmission work. Normal non-destructive evaluation techniques need special designs to suit high curvature surfaces of cylinders. Laser ultrasonic (LU) method can provide a remote and non-destructive inspection solution to solid cylinders due to its flexible adaptability to complex structures and strong penetration depth. However, constrained by the common problems of optical detection systems, the detected ultrasonic signals will suffer low signal-to-noise ratio and bad resolution from poor sample surface quality. Thus, a modified synthetic aperture focusing technique (SAFT) optimized for cylindrical components is proposed to improve the detectability of LU on small and buried defects. Mode conversion wave signals obtained by multiple separate excitations are used in SAFT imaging for the reconstruction of the defects under surface profile correction. To reduce the influence of incident waves such as direct surface acoustic wave, besides common difference method, adjacent wave subtraction algorithm based on cross-correlation is used for signal preprocessing, suppressing the incident waves and exaggerating the mode conversion waves. In numerical simulation, internal defects with diameters from 0.6 to 0.2 mm with various buried depths are visualized and accurately located via the optimized SAFT algorithm using mode conversion waves. For validation, a circumferential scanning system is established in LU experiment and internal defects from 0.8 to 0.4 mm in diameter inside solid cylinders are successfully detected with precise location. The results elucidate the reliability of characterizing the location of small internal buried defects in solid cylinder structures through LU-SAFT imaging.

Investigation on the effect of vertical baffle plate on droplet size distribution and mixing efficiency in Taylor Reactor

The Taylor reactor is a dynamic mixer based on the principles of Taylor vortex flow. However, traditional Taylor reactors feature smooth inner and outer cylinder surfaces, presenting challenges in achieving uniform mixing and efficient mass transfer. These limitations do not meet the requirements of modern chemical processes for highly efficient mixing equipment. This article introduces a novel Taylor reactor equipped with vertical baffle plates, designed to overcome these challenges. A comprehensive methodology combining CFD-PBM and experimental techniques was employed to investigate both droplet size distribution and mixing performance in the annular gap. Compared to traditional designs, the new device demonstrates significantly lower COV at 0.018 and Sauter mean diameter at 38.3 μm. Optimal mixing efficiency is achieved at rotating Reynolds number (Re) of 1.17 × 105 and inlet Reynolds number (Rei) of 2.65 × 103. Numerical simulation results suggest that vertical baffles effectively enhance liquid-liquid interactions including collision, interface contact, and droplet residence time within the annular gap. Moreover, increased turbulent stress in the flow field contributes to greater vorticity, turbulence intensity, and turbulent kinetic energy resulting in smaller droplet sizes and improved mixing efficiency. These findings provide a theoretical foundation for hydraulic structural optimization design as well as widespread application of Taylor reactors.

Influence of thermal buoyancy on the wake dynamics of a heated square cylinder

Direct numerical simulation of the three-dimensional (3-D) wake transition of a heated square cylinder subjected to horizontal cross-flow is performed in the presence of buoyancy. In order to capture the effects of large-scale heating, a non-Oberbeck–Boussinesq model is utilized, which includes the governing equations for compressible gas flow. All computations are performed at low free stream Mach number $M=0.1$ using air (free stream Prandtl number, $Pr=0.71$ ) as the working fluid. The 3-D instability modes A and B, which correspond to free stream Reynolds numbers of 180 and 250, are observed with longer and shorter spanwise wavelengths, respectively, and the onset of three-dimensionality is triggered at a Reynolds number of 173. In the presence of buoyancy, baroclinic vorticity production in the near-wake plays an important role for streamwise vorticity generation. The chaotic wake of the Mode-A instability bifurcates into periodic and quasiperiodic wakes at various heating levels, expressed by the overheat ratio, $\varepsilon =(T_w-T_\infty )/T_\infty$ , where $T_w$ and $T_\infty$ are the temperature of the cylinder surface and the ambient air, respectively. At low heating ( $\varepsilon =0.2$ ), the 3-D Mode-A instability is suppressed leading to a two-dimensional wake flow. Further increase in heating, again brings back the three-dimensionality in the wake through Mode-E instability. The variation of thermophysical properties and the effective Reynolds number with increase in heating level around the cylinder is examined. It is shown that the effect of thermophysical properties competes with the baroclinic streamwise vorticity generation at higher levels of heating ( $\varepsilon \geqslant 0.4$ ) to control the 3-D modes and wake dynamics.

Three-Dimensional Probe Mispositioning Errors Compensation: A Feasibility Study in the Non-Redundant Helicoidal Near to Far-Field Transformation Case

A feasibility study on the compensation of 3D mispositioning errors of the probe occurring in the characterization of a long antenna, via a non-redundant (NR) near to far-field (NTFF) transformation with helicoidal scan, is conducted in this article. Such types of errors can result from imperfections in the rail driving the linear motion of the probe and from an imprecise synchronization of the linear and rotational movements of the probe and the antenna when drawing the scan helix. To correct them, an approach, which proceeds through two steps, is proposed. The former step uses a technique called cylindo rical wave (CW) correction for compensating the phase of the near-field (NF) samples, which, owing to the rail imperfections, result in not being acquired over the measurement cylinder surface. The latter exploits an iterative scheme to restore the samples at the sampling points required by the adopted NR representation along the scan helix from those obtained by applying the CW correction technique and impaired by 2D mispositioning errors. The so compensated NF samples are then effectively recovered via a 2D optimal sampling interpolation (OSI) scheme to accurately obtain the input data required to carry out the standard cylindrical NTFF transformation. The OSI representation is determined here by assuming a long antenna under test as enclosed in a prolate ellipsoid or cylinder ending into two hemispheres (cigar) in order to make, depending on the particular geometry of the considered antenna, the representation effectively non-redundant. The reported numerical simulation results show the capability of the proposed approach to compensate even severe 3D mispositioning errors, thus enabling its usage in a real measurement scenario.