Cylinder In Cross Flow Research Articles

Global and local skin friction diagnostics from TSP surface patterns on an underwater cylinder in crossflow

A systematical method is formulated for extracting skin-friction fields from Temperature Sensitive Paint (TSP) images in the sense of time-averaging and phase-averaging. The method is applied to an underwater cylinder in crossflow at two subcritical regimes (Re = 72 000 and 144 000). TSP maps are decomposed in a time-averaged, a phase-averaged, and a random component. The asymptotic form of the energy equation at the wall provides an Euler-Lagrange equation set that is solved numerically to gain the relative skin friction time- and phase-averaged fields from the TSP surface temperature maps. The comparison of the time averaged relative skin-friction profiles with the literature data shows an excellent agreement on the whole laminar boundary layer up to the laminar separation line. Downstream of separation, time averaged results identify the secondary reattachment/separation events, which are lost in the available literature data. The periodic behavior of the skin-friction is taken, describing how the laminar separation bubble evolves by providing the time history of the laminar separation line and of the secondary reattachment/separation over the entire vortex shedding period. Instantaneous skin friction maps reveal the existence of coherent structures by capturing their footprint on the cylinder’s surface. An array of Π-shaped traces marks the existence of counter-rotating, streamwise-oriented vortices just before the laminar separation line. Their interaction with the laminar boundary layer and with the separation line is briefly described. An example of the intermittent excerpt of their influence through the laminar separation line is reported.

Inverse estimation of heat flux from a hollow cylinder in cross-flow of air

The heat flux at the surface of an initially heated cylinder under the cross-flow of air has been investigated. The aim is to propose an inverse heat conduction method to avoid expensive measuring processes or that eliminates difficulties in direct measurements such as applying the sensors directly to the surface which may either burn it or affects the flow field. In addition, to develop empirical correlations to predict local and average heat transfer from a cylinder under subcritical range of Reynolds number 1.1×104⩽Re⩽6.2×104. The inverse method makes use of Levenberg-Marquardt approach for parametric estimation to determine the surface heat flux. It requires the computation of sensitivity matrix, which has been computed by using the Duhamel’s theorem. An analysis of heat transfer data obtained from the proposed inverse method and the predictions from the developed empirical correlations revealed an average deviation of less than 1% compared to reported literatures.

Scrutinizing URANS in Shedding Flows: The Case of Cylinder in Cross-Flow in the Subcritical Regime

To unravel the widespread perception that the RANS (Reynolds-averaged Navier-Stokes) concept is unreliable in predicting the dynamics of separated flows, we assessed the performance of two RANS closure levels, the linear eddy-viscosity (LEVM) and the second-moment (Reynolds stress, RSM) approaches in a massively separated generic flow over a bluff body. Considered is the canonical, zero-turbulence, cross-flow over an infinite cylinder with reference to our LES and the available DNS and experiments at two Reynolds numbers, Re = 3.9 × 103 and 1.4 × 105, both within the sub-critical regime with laminar separation. Both models capture successfully the vortex shedding frequency, but the low frequency modulations are detected only by the RSM. At high Reynolds numbers the RSM is markedly superior to the LEVM showing very good agreement with the LES and experimental data. The RSM, accounting naturally for the stress anisotropy and phase lag between the stress and strain eigenvectors, is especially successful in reproducing the growth rate of the turbulent kinetic energy in the initial shear layer which proved to be crucial for accurate prediction of the separation-induced transition. A scrutiny of the unsteady RANS (URANS) stress terms based on the conditional phase-averaged LES data shows a remarkable similarity of the normalized coherent and stochastic (modeled) stress components for the two Reynolds numbers considered. The mixed (cross) correlations, while non-negligible at the low Re number, diminish fast relative to the stochastic ones with increasing Reynolds number and, in the whole, are not significant to undermine the URANS concept and its applicability to high Re flows of industrial relevance.

Effect of Microstructure Geometric Form on Surface Shear Stress

Low Reynolds number flow of liquids over micron-sized structures and the control of subsequently induced shear stress are critical for the performance and functionality of many different microfluidic platforms that are extensively used in present day lab-on-a-chip (LOC) domains. However, the role of geometric form in systematically altering surface shear on these microstructures remains poorly understood. In this study, 36 microstructures of diverse geometry were chosen, and the resultant overall and facet shear stresses were systematically characterized as a function of Reynolds number to provide a theoretical basis to design microstructures for a wide array of applications. Through a set of detailed numerical calculations over a broad parametric space, it was found that the top facet (with respect to incident flow) of the noncylindrical microstructures experiences the largest surface shear stress. By systematically studying the variation of the physical dimensions of the microstructures and the angle of incident flow, a comprehensive regime map was developed for low to high surface shear structures and compared against the widely studied right circular cylinder in cross flow.

Identification of the heat transfer frequency response in pulsating laminar and subcritical flow across a cylinder

The steady-state heat transfer from a cylinder in cross-flow is a prototype problem in thermo-fluiddynamics. However, in many applications such as the Rijke tube, the flow may fluctuate. This work analyses the phenomenon combining numerical simulation with system identification. Direct numerical simulation of laminar flow and Large Eddy Simulation at subcritical flow at Reynolds number equal to 3900 are used, respectively. Fluctuations of the inlet velocity in the simulation are excited over a wide range of frequencies. Time series of unsteady heat release and velocity are post-processed to identify dynamic models, which may be represented as transfer functions. They accurately describe the dynamic behavior and can be used for further modeling.

A comparative experimental evaluation of uncertainty estimation methods for two-component PIV

Uncertainty quantification in planar particle image velocimetry (PIV) measurement is critical for proper assessment of the quality and significance of reported results. New uncertainty estimation methods have been recently introduced generating interest about their applicability and utility. The present study compares and contrasts current methods, across two separate experiments and three software packages in order to provide a diversified assessment of the methods. We evaluated the performance of four uncertainty estimation methods, primary peak ratio (PPR), mutual information (MI), image matching (IM) and correlation statistics (CS). The PPR method was implemented and tested in two processing codes, using in-house open source PIV processing software (PRANA, Purdue University) and Insight4G (TSI, Inc.). The MI method was evaluated in PRANA, as was the IM method. The CS method was evaluated using DaVis (LaVision, GmbH). Utilizing two PIV systems for high and low-resolution measurements and a laser doppler velocimetry (LDV) system, data were acquired in a total of three cases: a jet flow and a cylinder in cross flow at two Reynolds numbers. LDV measurements were used to establish a point validation against which the high-resolution PIV measurements were validated. Subsequently, the high-resolution PIV measurements were used as a reference against which the low-resolution PIV data were assessed for error and uncertainty. We compared error and uncertainty distributions, spatially varying RMS error and RMS uncertainty, and standard uncertainty coverages. We observed that qualitatively, each method responded to spatially varying error (i.e. higher error regions resulted in higher uncertainty predictions in that region). However, the PPR and MI methods demonstrated reduced uncertainty dynamic range response. In contrast, the IM and CS methods showed better response, but under-predicted the uncertainty ranges. The standard coverages (68% confidence interval) ranged from approximately 65%–77% for PPR and MI methods, 40%–50% for IM and near 50% for CS. These observations illustrate some of the strengths and weaknesses of the methods considered herein and identify future directions for development and improvement.

Characterizing fluid structure interactions of a helical coil in cross flow

Mechanical vibrations compromise the integrity of key components of thermal power plants. Without careful design, strong resonances during steady state operation can wear these components to the point of failure, leading to an unsafe situation that may force a plant to shut down. The purpose of this research is to further the understanding of the vibrations induced in a helical coil subject to steady fluid flow. A helical coil steam generator, such as that found in most integral pressurized water reactors, appears to eliminate many flow-induced vibration concerns when compared to traditional steam generators; however this has yet to be clearly demonstrated experimentally. The objective of this study is to detail and demonstrate a new method to quantify the motion of a helical coil in an annulus subject to external axial flow of water and further characterizing the influence of pitch-to-diameter ratio on the fluidelastic instability of a helical coil. This is accomplished by observing the motion of a helical coil mounted to an inner opaque cylinder through an outer glass tube using a high speed video camera. A mirrored image-pair is used to observe this structure from two perspectives simultaneously, allowing for three-dimensional characterization of the coil motion. The experimental facility is described in detail. The method developed herein for identifying specific points on the coil from images and mapping them to the coil location using the law of refraction is described. An uncertainty analysis of the coil position measurement is conducted based on geometry and refractive index which can be readily applied to measurements obtained using this method. The outcome of empirical observations shows these helical coils to hold a slightly higher resonance frequency than that of cylinders in cross flow, their mechanical stiffness approximated through analytical means shows to produce relatively accurate natural frequencies when compared to the empirical data – 14.1Hz and 12.5Hz, respectively for first mode vibration. This study”s contributions present a new method for metering fluid structure interactions with high-fidelity, provide new empirical data which has not previously been produced, and make observations to the response of a helical coil which are new to the field of fluid-structure interactions.

Wake of tandem cylinders near a wall

This article reports an experimental investigation on the near wake of two identical circular cylinders in crossflow arranged in tandem configuration in the streamwise direction and with the additional interference of the ground. The Reynolds number based on the cylinders diameter is 4.9×103. The present study analyses the effect of longitudinal pitch-to-diameter ratios spanning the well known flow regimes of tandem cylinders in absence of ground effect, i.e. single bluff body, shear layer reattachment and vortex co-shedding of twin cylinders. Particle Image Velocimetry measurements are performed to extract first and second order statistics of the wake. The wake features have been analyzed and compared with the bluff-body wake models present in literature. Additionally, the flow fields are decomposed in Proper Orthogonal modes to characterize the main coherent structures. The time coefficients of the modes are analyzed to extract phase relations between vortical features.Far enough from the wall the cylinders wake is symmetric, with Von Kármán vortices shed symmetrically in the wake with respect to cylinders centerline. The measured average wake characteristics and vortical structures are consistent with the data reported in the literature for tandem cylinders without ground effect. For a wall-to-cylinder gap equal to 1 diameter, the ground strongly influences the flow field, introducing asymmetry in the typical Von Kármán wake. The ground boundary layer is thickened past the downstream cylinder and a wall-jet appears, enclosed between the wall boundary layer and the cylinders wake. Eventually these flow structures are not distinguishable when the cylinders wake and the wall boundary layer are grown enough to merge. From POD analysis, vorticity blobs appear on the wall, paired with vortex shedding.If the wall gap is decreased to 0.3 diameters, an extended low speed region appears close downstream of the cylinders. A POD mode similar to a vortex shedding is still present; however it cannot be associated to an alternate Von Kármán street. The shed structures show a shorter wavelength than the Von Kármán shedding in far-from-the ground cases and are accompanied by a flapping jet-like structure ejected from the wall gap between the cylinder and the wall. The jet strongly changes the features of the wake on the side opposite to that of the wall, thus suggesting a possible coexistence between jet oscillations and shedding.

Aeroacoustic Scattering Using a Particle Accelerated Computational Fluid Dynamics/Boundary Element Technique

A particle accelerated computational fluid dynamics/boundary element method technique to predict the sound pressure field produced by low Mach number flow past a rigid body is presented. An incompressible computational fluid dynamics solver is used to calculate the transient hydrodynamic flowfield. A near-field formulation based on Lighthill’s analogy is coupled with a particle condensation technique to predict the incident acoustic field and its normal derivative on the body. The near-field formulation involves singular surface and volume integrals, which are regularized via singularity subtraction. A particle condensation technique is applied to accelerate the incident field computations and reduce the amount of data that must be stored during the computational fluid dynamics analysis. The incident field is then combined with a boundary element method model of the body, and the scattered sound pressure field is obtained by solving the Burton–Miller boundary integral equations. The accuracy and computational cost of the particle accelerated computational fluid dynamics/boundary element method approach is demonstrated by calculating the incident acoustic field on a cylinder in crossflow under laminar and turbulent flow conditions as well as predicting the scattered and far-field sound pressures.

Study of Heat Transfer over a Square Cylinder in Cross Flow using Variable Resolution Modeling

In the present study, a method of Partial-Averaged Navier-Stokes (PANS) equations, purported to perform variable resolution modeling, is used to predict the heat transfer over a square cylinder in a cross-flow. The PANS closure is based on the RANS SST k-ω model paradigm. The simulations were carried out using an open source software, namely, Open FOAM, at Reynolds number = 22000. The open source code and the PANS model are validated against the experimental work reported in the literature and it was observed that both the mean flow properties and turbulent statistics were in good agreement with the experimental results. Further the capability of the PANS approach in predicting heat transfer in turbulent flow is also studied. An algebraic wall function is used for the near wall treatment of the energy equation. The computed, average and local Nusselt numbers are compared with the experimental and LES results reported in the literature. The phase-averaged analysis of the shedding phenomenon is studied to understand the heat transfer phenomenon at different faces of the cylinder and turbulent heat fluxes are also considered to understand the effect of turbulence on convection.

Laboratory measurements of vortex-induced sediment pickup rates

In the present study, vortices were generated in open channel flow with a cross-flow cylinder installed horizontally near the bed. Sediment pickup rates were then measured over the channel bed downstream the cylinder using a sediment lift. The experimental data show that the pickup rate increases exponentially in the presence of vortices. Two different relationships can be clearly observed between the pickup rate and the maximum root-mean-square (rms) value of the streamwise velocity fluctuation, one for the cylinder-obstructed flow and the other for the unobstructed flow. The results imply that the vortex-induced sediment pickup cannot be explained based on the traditional boundary layer theory.

External forced convection from circular cylinders with surface protrusions

The application of surface structures to heat exchanging devices can help to enhance their heat transfer performance. In this study, surface protrusions were added as a modification to the circular cylinder in crossflow. The modified cylinder maintained a constant temperature and heated the cool air that flowed over it. We first investigated the influence on the heat transfer performance of the number of protrusions, the non-dimensional protrusion height, and two alignment modes of the modified cylinders. For the modified cylinders with no more than six protrusions, we observed increase in the temporally and spatially averaged Nusselt number. Then, we inspected the local Nusselt number along the modified cylinder surface as well as the temperature and flow fields, and found that the notches between the protrusions caused major deterioration to local heat transfer. Based on the observation, two alternative configurations to the initial protrusion design were proposed to further enhance the heat transfer performance. The first alternative was to apply a pair of protrusions symmetrically on the cylinder surface, in which case we studied the effects of the protrusion height, protrusion width, and two series of protrusion placements. The second alternative was to gradually fill the notches between the protrusions, in which case we studied the effects of the protrusion height, degree of filling, and two modes of alignment. Compared to the circular cylinder, the dual protrusion design and the notch-filling design achieved as much as 7% and 2% increase in the temporally and spatially averaged Nusselt number, while being accompanied by 8% and 16% drag penalty, respectively.

Aerodynamic sound from a square cylinder with a downstream wedge

The effect of placing a wedge in the wake of a square cylinder (side length D) at a Reynolds number of 22,000 is numerically investigated. In particular, the effect of the wedge on aerodynamic noise is observed along with its effect on the flow field. Wedge base height (h) and its gap distance (G) downstream of the cylinder are systematically varied. Flow simulations are carried out using an unsteady RANS model employing the k–ω SST turbulence model, whereas the calculation of aerodynamic noise radiated from the flow is solved using Curle's equation. A special correction technique is applied to consider spanwise effects on noise production and validation is provided using new aeroacoustic data for a square cylinder in cross-flow. It is found that the flow behavior can be divided into two main regimes (regime I and regime II), with a linking transition regime. For regime I, the generated sound is lower than that of the isolated square cylinder case. The thinnest wedge produced the best sound reduction (11.79 dB) when the wedge is placed at G=2D. For regime II, the calculated sound level is higher than the case of an isolated square cylinder. This is because the sound emitted from both bodies have about the same magnitude and are in phase. For this case, the maximum increase of sound pressure is 6.24 dB, when the medium wedge is at G=2.5D.

Mass transfer correlations for dissolution of cylindrical additions in liquid metals with gas agitation

The results of an experimental study of the effect of gas agitation on mass transfer from cylindrical Si specimens to molten Al were used to deduce mean mass transfer coefficients in a two-phase flow. Then the three-dimensional gas-agitated liquid was simulated using the commercial software FLOW-3D, to obtain predictions of bubble distribution and estimates of the velocity field within the opaque liquid metal, to be used to predict mass transfer rates. An existing mass transfer correlation for a gas-agitated liquid without an external bulk flow was used to fit the experimental data within 5%. With a bulk velocity, a correlation for mass transfer from a vertical cylinder in cross flow was modified to incorporate the effect of gas injection, and it predicted the measured mass transfer coefficients within 11%.

Flow characteristics and drag force of a square cylinder in crossflow modulated by a slot jet injected from upstream surface

The flow characteristics and drag force of a square cylinder with a front jet injection were experimentally investigated in a wind tunnel. The evolution process of the characteristic flow patterns was recorded by the laser-assisted smoke flow visualization method. The time-averaged velocity fields measured by particle image velocimetry (PIV) were applied to analyze the velocity vectors, streamline patterns, vorticity contours, velocity distributions, and time histories of instantaneous velocities around the upstream region of the square cylinder. The drag force experienced by the square cylinder was obtained by measuring the surface pressures on the front and rear faces. The results show that the jet emitted from the upstream surface of the square cylinder periodically swings left-and-right in the experimental range of injection ratio ⩽0.9. In the time-averaged velocity field, the jet flow impinges the freestream at a four-way saddle and subsequently bifurcates into two streams; one stream goes toward the left edge of the upstream surface, while the other stream directs toward the right edge of the upstream surface. Two recirculation regions formed above the upstream surface of the square cylinder are enclosed by those two streams. The vorticity contours around the upstream surface of the square cylinder are characterized by two adjacent vorticity-concentrated areas of opposite signs. The time histories of the instantaneous velocities around the four-way saddle, jet exit, and shear layer of the swinging jet represent periodic oscillation. This characteristic oscillation frequency is dominated by the wake instability. The recirculation regions formed above the cylinder’s upstream surface prevent impingement from the freestream. Consequently, the surface pressure coefficients on the upstream surface of the square cylinder are reduced. This reduction in the surface pressure coefficient decreases the drag force acting on the square cylinder.

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Strut arms contribute significantly to the overall drag in VAWT and reduces the overall power output. Four low drag air foil cross section profiles suitable for strut arm construction were simulated and optimized using the XFOIL 1.0 program. The profile with the lowest simulated drag coefficient, UWI-1, was constructed and tested in an open-circuit suction type conventional non-return laboratory wind tunnel. The flow characteristic was examined experimentally using surface pressure measurements. The results showed that an increase in the Reynolds number from 3.32 x 105 to 9.64 x 105 resulted in a decrease in the drag coefficient from 0.01241 to 0.00984. This is a significantly lower drag coefficient when compared to the nominal value of about 0.4 for cylinders in cross flow within the Reynolds number range tested. The results obtained for the experimental drag coefficient values and the simulated XFOIL were within 10.7%.

Heat Transfer to Truncated Cylinder in Cross-flow

The objective of this work is to provide a correlation between the heat transfer coefficient and the Nusselt Number with the flow Reynolds number for a truncated cylindrical probe in cross-flow, a configuration that has not previously been well defined. In the experiment, thermally stabilized, dry air was discharged into the test section across a heated probe that was instrumented with thermocouples and exhausted to atmosphere. Experimental data were recorded for several run conditions. A correlation between the probe heat transfer coefficient and the Reynolds number was developed with a calculated uncertainty of 1.24%.

Heat transfer characteristics of circular and elliptic cylinders in cross flow

This article presents heat transfer characteristics of circular and elliptic cylinders in cross flow for various Reynolds numbers. Numerical investigations as well as wind tunnel experiments are carried out to study the convective heat transfer of in-line cylinder banks. In particular, the heat transfer characteristics of upstream and downstream cylinders are compared in terms of local and average Nusselt numbers. The effects of turbulence induced by upstream cylinders on the heat transfer of downstream cylinders are discussed with the help of detailed numerical results.

U-shaped fairings suppress vortex-induced vibrations for cylinders in cross-flow

We employ three-dimensional direct and large-eddy numerical simulations of the vibrations and flow past cylinders fitted with free-to-rotate U-shaped fairings placed in a cross-flow at Reynolds number $100\leqslant \mathit{Re}\leqslant 10\,000$. Such fairings are nearly neutrally buoyant devices fitted along the axis of long circular risers to suppress vortex-induced vibrations (VIVs). We consider three different geometric configurations: a homogeneous fairing, and two configurations (denoted A and AB) involving a gap between adjacent segments. For the latter two cases, we investigate the effect of the gap on the hydrodynamic force coefficients and the translational and rotational motions of the system. For all configurations, as the Reynolds number increases beyond 500, both the lift and drag coefficients decrease. Compared to a plain cylinder, a homogeneous fairing system (no gaps) can help reduce the drag force coefficient by 15 % for reduced velocity $U^{\ast }=4.65$, while a type A gap system can reduce the drag force coefficient by almost 50 % for reduced velocity $U^{\ast }=3.5,4.65,6$, and, correspondingly, the vibration response of the combined system, as well as the fairing rotation amplitude, are substantially reduced. For a homogeneous fairing, the cross-flow amplitude is reduced by about 80 %, whereas for fairings with a gap longer than half a cylinder diameter, VIVs are completely eliminated, resulting in additional reduction in the drag coefficient. We have related such VIV suppression or elimination to the features of the wake flow structure. We find that a gap causes the generation of strong streamwise vorticity in the gap region that interferes destructively with the vorticity generated by the fairings, hence disorganizing the formation of coherent spanwise cortical patterns. We provide visualization of the incoherent wake flow that leads to total elimination of the vibration and rotation of the fairing–cylinder system. Finally, we investigate the effect of the friction coefficient between cylinder and fairing. The effect overall is small, even when the friction coefficients of adjacent segments are different. In some cases the equilibrium positions of the fairings are rotated by a small angle on either side of the centreline, in a symmetry-breaking bifurcation, which depends strongly on Reynolds number.

Fast-response underwater TSP investigation of subcritical instabilities of a cylinder in crossflow

We investigate the classic cylinder in crossflow case to test the effectiveness of a fast-response underwater temperature-sensitive paint coating (TSP) in providing highly resolved spatial and time observations of the action of a flow over a bluff body surface. The flow is investigated at Reynolds number <190 k, before the onset of the drag-crisis state. The obtained TSP image sequences convey an accurate description of the evolution of the main features in the fluid–cylinder interaction, like the separation line position, the pattern of the large coherent structures acting on the cylinder’s surface and the small-scale intermittent streamwise arrays of vortices. Ad hoc data management and features extraction techniques are proposed which allow extraction of quantitative data, such as separation line position and vortex-shedding frequency, and results are compared to the literature. Use of TSP for water applications introduces an interesting point of view about the fluid–body interactions by focusing directly on the effect of the flow on the model surface.