Cylinder Wake Flow Research Articles

Generalized framework for PIV-based pressure gradient error field determination and correction

A framework leveraging the governing equations of incompressible flow for the reconstruction and correction of pressure gradient estimation errors from experimental data is extended to incorporate non-zero errors on domain boundaries and Lagrangian pseudo-tracking methods for material acceleration estimation. A second-order system derived from the first-order divergence-curl system governing the pressure gradient error field facilitates handling a variety of error boundary conditions, and the solution can be split into independent Poisson equations, making it straightforward to implement. For known boundary errors, a precise determination of the pressure gradient field is possible, up to the limitations of the numerical method. In practice, the error equations cannot be solved exactly; however, a number of terms may be computed exactly and approximations may be applied on the remaining terms. For the selected test case of a cylinder wake flow in turbulent shedding regime, the analysis of simulated three-dimensional, three-component velocity measurements demonstrates that the errors in pressure estimates can be reduced by up to 50% using a basic finite difference implementation.

Experimental Investigation of Airfoil Performance in the Wake of a Circular Cylinder

Airfoil performance of a NACA 0012 held at static angles of attack from to 20° in the wake of a circular cylinder is investigated experimentally. Time-averaged airloads were measured for varying cylinder–airfoil streamwise distances, (based on cylinder diameter ), along with time- and phase-averaged particle image velocimetry. Changes in the cylinder wake flow structure govern airfoil performance in the cylinder–airfoil interaction. For , the cylinder wake arrangement is a pair of counterrotating vortices in a side-by-side configuration that induce negative drag on the airfoil. As the cylinder–airfoil distance increases to , the cylinder wake arrangement becomes a von Kármán vortex street that induces an oscillating flow field. A leading edge vortex forms over the NACA 0012 as a consequence of the cylinder wake and delays airfoil stall. This work gives fundamental insight into how large turbulent scales affect airfoil performance in the context of ship–aircraft aerodynamics.

Control of cylinder wake flow and noise through a downstream porous treatment

Various passive and active methods have been developed to control flow separation from bluff bodies. However, these methods require adjusting features of the solid surface, such as modifying its geometry or porosity, or applying external force or momentum. This paper develops an off-body-based method, without adjusting any features of the solid surface, for controlling the unsteady flow separation by fixing porous materials downstream of bluff bodies. Numerical study on flow past a circular cylinder at a subcritical Reynolds number is performed, and the result indicates that the added downstream porous material changes flow in the wake and re-laminarizes the turbulent flow around the curved cylinder surface, reducing the wall pressure fluctuation around the cylinder. Therefore, the associated aerodynamic noise is reduced greatly.

Experimental investigation on suppressing circular cylinder VIV by a flow control method based on passive vortex generators

Vortex-induced vibration (VIV) is a typical flow-induced vibration phenomenon, which is usually observed in circular cylindrical structures. This study introduces a flow control method based on passive vortex generators (PVGs) for suppressing the VIV of circular cylindrical structures. A series of wind tunnel experiments were performed using rigid circular cylinder models. Vibration tests were implemented using a spring–mass system for verifying the effectiveness of PVGs, and pressure tests were implemented by using a fixed cylinder model for revealing the aerodynamic mechanisms of PVGs qualitatively. Five geometric parameters of PVGs were tested and the optimal parameters were determined during the wind tunnel experiments. The experiment results prove that the maximum root-mean-square values of the vibration amplitudes can be suppressed by more than 80% by attaching PVGs with optimal parameters on the cylinder surface. According to the pressure test results, PVGs designed suitably can suppress vortex shedding in the cylinder wake flow and weaken the spanwise correlation of the cylinder wake flow. Weakening the wake flow spanwise correlation contributes more to controlling the amplitudes of cylinder VIV. In addition to the control effects on the cylinder VIV, some other aerodynamic phenomena of the cylinder with PVGs were observed and analysed.

Fast 3D flow reconstructions from 2D cross-plane observations

A computationally efficient flow reconstruction technique is proposed, exploiting homogeneity in a given direction, to recreate three-dimensional instantaneous turbulent velocity fields from snapshots of two dimension planar fields. This methodology, termed as ’snapshot optimisation’ or SO, can help to provide 3D data sets for studies which are currently restricted by the limitations of experimental measurement techniques. The SO method aims at optimising the error between an inlet plane with a homogeneous direction and snapshots, obtained over a sufficient period of time, on the observation plane. The observations are carried out on a plane perpendicular to the inlet plane with a shared edge normal to the homogeneity direction. The method is applicable to all flows which display a direction of homogeneity such as cylinder wake flows, channel flow, mixing layer, and jet (axisymmetric). The ability of the method is assessed with two synthetic data sets, and three experimental PIV data sets. A good reconstruction of the large-scale structures is observed for all cases. The small-scale reconstruction ability is partially limited especially for higher dimensional observation systems. POD-based SO method and averaging SO variations of the method are shown to reduce discontinuities created due to temporal mismatch in the homogenous direction providing a smooth velocity reconstruction. The volumetric reconstruction is seen to capture large-scale structures for synthetic and experimental case studies. The algorithm run time is found to be in the order of a minute providing results comparable with the reference. Such a reconstruction methodology can provide important information for data assimilation in the form of initial condition, background condition, and 3D observations.

Suppression characteristics of flow-induced vibration in a test cylinder with varying locations of a control cylinder

This study was designed to identify the suppression characteristics and mechanisms of flow-induced vibration occurring in a test cylinder installed on an elastic support by placing a control cylinder with a smaller diameter behind the test cylinder. To identify the effects of this installation, the diameter and location of the control cylinder were varied systematically to examine the characteristics of flowinduced vibration occurring in the test cylinder. Furthermore, the fluctuating streamwise velocity of the wake flow of the test cylinder was examined as it varied with the installation of the control cylinder. Subsequently, the peripheral flow of the test cylinder installed in front of the control cylinder was examined via a flow visualization test that used a water channel. Results obtained from the experiments are summarized as follows. i) Flow-induced vibrations in the test cylinder, which occur and are varied by the installation of the control cylinder behind the test cylinder, have four types that change according to the different diameters and locations of the control cylinder. ii) The domain of the positions of the control cylinder, which can suppress flow-induced vibration in the test cylinder, increases in proportion with the diameter of the control cylinder. iii) Flow-induced vibration that will be generated in the test cylinder is predicted by examining the distribution of the flow velocity of the wake flow and the intensity of the fluctuating streamwise velocity at the lower flow velocity level before it reaches a flow velocity that can create flow-induced vibration in the test cylinder.

Error reduction for time-resolved PIV data based on Navier–Stokes equations

The post-processing of the measured velocity in particle image velocimetry (PIV) is a critical step in reducing error and predicting missing information of the flow field. In this work, time-resolved PIV data are incorporated with the incompressible Navier–Stokes (N–S) equations to reduce the measurement error and improve the accuracy. A pressure correction scheme (PCS) based on the projection method is adopted to solve the N–S equations, and an optimization algorithm is introduced to balance the fidelity between the PIV data and the numerical solutions. The PCS for PIV data, called PIV–PCS, cannot only reduce the errors in the velocity divergence and the curl of the pressure gradient but also ensure that the flow field satisfies the dynamic constraints imposed by the momentum equation. An important weight coefficient s that balances the level of the velocity modification with the residual of the governing equation is defined and numerically assessed. A method for optimizing the value of s is provided. The new approach is evaluated by two time-resolved PIV experiments: one on the 2D wake flow of a circular cylinder at low Reynolds number and one on tomographic PIV for the 3D wake flow of a hemisphere at high Reynolds number. All the numerical assessments and experimental applications are compared with the divergence-free smoothing (DFS) method. The results indicate that the presented PIV–PCS method is superior to the DFS method in terms of reducing the measurement error and recovering the real physical flow structures.

Data mining of a clean signal from highly noisy data based on compressed data fusion: A fast-responding pressure-sensitive paint application

A data mining approach based on compressed data fusion is developed to extract a clean signal from highly noisy data and it has been successfully applied to flow measurement using fast-responding pressure-sensitive paint (fast PSP). In this approach, spatially resolved but noisy full-field data are fused with clean but scattered data to reconstruct full-field clean data. The fusion process is accomplished based on a compressed sensing algorithm, which has shown significantly improved performance compared with low-dimensional analysis. This is because, in low-dimensional analysis such as proper orthogonal decomposition (POD), the selection criteria of proper POD modes for reconstruction are usually based on subjective observation and the mode coefficients can be severely distorted by noise, which restricts the applications of this method to complicated flow phenomena and leads to a low-quality reconstruction. The solutions to these two problems can be expressed via mathematical optimization by determining the optimal coefficients to reconstruct clean data using the most relevant POD modes. Here, compressed sensing is used as a suitable solution to explore the sparse representation of scattered clean data based on the POD modes obtained from noisy full-field data. A high-quality reconstruction can be obtained using the optimized coefficients. The new method is first demonstrated by using fabricated patterns, demonstrating a reduction of 75% in the reconstruction error compared with POD analysis. It is thereafter successfully applied to recover the unsteady pressure field induced by a cylinder wake flow at low speed. Fast PSP measurement and microphones are used to obtain full-field but noisy pressure field data and scattered but clean data, respectively. In the cases of single and step cylinders, the reconstruction errors are approximately 5% and 25%, respectively, and the accuracy of reconstruction depends on the low dimensionality of the flow phenomena and the total number of microphone sensors. The current technique provides a reliable method to recover clean signals from strong noise, with significant potential for applications to flow measurement, control, and monitoring.

Study on flapping modes of a flexible filament in the wake of an upstream rotating cylinder

Flapping mode of a flexible filament in wake flow behind a rotating circular cylinder has been studied numerically by using of an immersed-boundary-based lattice Boltzmann method. In a parameter space of α =0.1–3.0 and G /D=0.05–3.5, where α is the cylinder rotation rate and G / D is the distance ratio between the filament and cylinder which has a diameter of D , flapping modes of the filament are analyzed in the phase diagram to examine the mode transition features and the mechanism of formation for different modes. Specifically, three kinds of flapping modes have been found which are labelled as reversed, coiled, and normal flapping modes; six sub-modes: reversed one-side, two-side and steady modes, and normal one-side, two-side and steady modes have been identified. Based on the phase diagram in the ( α , G /D) plane, the transition pathways from the reversed to normal modes have been revealed as a consequence of increasing G / D . The critical value of G / D for transitions to the normal flapping modes demonstrates a decreasing trend for a small rotation rate, then becomes unaltered for a large α . The forming mechanism of different modes has been explored by analyzing the vortical wake behind the rotating cylinder and flexible filament. Moreover, great feedback effect of filament flapping on the cylinder wake flow has been revealed. Specifically, at specific parameters, the interaction between the flapping filament and the cylinder wake flow acts to totally suppress or trigger on the unsteady vortex shedding behind the cylinder, which correspondingly leads to great drag reduction even generation of thrust for the rotating cylinder, or significant drag enhancement.

Coarse large-eddy simulations in a transitional wake flow with flow models under location uncertainty

The focus of this paper is to perform coarse-grid large-eddy simulation (LES) using recently developed sub-grid scale (SGS) models of cylinder wake flow at Reynolds number (Re) of 3900. As we approach coarser resolutions, a drop in accuracy is noted for all LES models but more importantly, the numerical stability of classical models is called into question. The objective is to identify a statistically accurate, stable sub-grid scale (SGS) model for this transitional flow at a coarse resolution. The proposed new models under location uncertainty (MULU) are applied in a deterministic coarse LES context and the statistical results are compared with variants of the Smagorinsky model and various reference data-sets (both experimental and Direct Numerical Simulation (DNS)). MULU are shown to better estimate statistics for coarse resolution (at 0.46% the cost of a DNS) while being numerically stable. The performance of the MULU is studied through statistical comparisons, energy spectra, and sub-grid scale (SGS) contributions. The physics behind the MULU are characterised and explored using divergence and curl functions. The additional terms present (velocity bias) in the MULU are shown to improve model performance. The spanwise periodicity observed at low Reynolds is achieved at this moderate Reynolds number through the curl function, in coherence with the birth of streamwise vortices.

Energy separation in transient and steady-state flow across the cylinder

Energy separation is a spontaneous energy redistribution within a fluid flow. As a consequence, there are places with higher and lower values of total temperature in the fluid flow. It is characteristic for many flow geometries. This paper deals with the energy separation in a cylinder wake. Two flow conditions are being considered-transient and steady-state flow in the wake. Two different solvers from the open source package OpenFOAM are used in order to capture the phenomenon of energy separation. One of these solvers is modified for the purpose of calculation in a particular case of the vortex street flow. The energy equation based on the internal energy present in this solver is replaced by the energy equation written in the form of a total enthalpy. The other solver has been previously tested in the vortex tube flow, and can also capture the energy separation in the steady-state wake flow of the cylinder. In both cylinder wake flow conditions, a two-dimensional computational domain is used. Standard ?? ? ?? model is used for computations. It is proved that OpenFOAM is capable of capturing the energy separation phenomenon in a proper way in both of the wake flow cases. Good agreement between the experimental results and the ones from computations is obtained in the case of steady-state flow in the wake. Previous research findings are also confirmed in the case of vortex street flow.

Stochastic modelling and diffusion modes for proper orthogonal decomposition models and small-scale flow analysis

We present here a new stochastic modelling approach in the constitution of fluid flow reduced-order models. This framework introduces a spatially inhomogeneous random field to represent the unresolved small-scale velocity component. Such a decomposition of the velocity in terms of a smooth large-scale velocity component and a rough, highly oscillating component gives rise, without any supplementary assumption, to a large-scale flow dynamics that includes a modified advection term together with an inhomogeneous diffusion term. Both of those terms, related respectively to turbophoresis and mixing effects, depend on the variance of the unresolved small-scale velocity component. They bring an explicit subgrid term to the reduced system which enables us to take into account the action of the truncated modes. Besides, a decomposition of the variance tensor in terms of diffusion modes provides a meaningful statistical representation of the stationary or non-stationary structuration of the small-scale velocity and of its action on the resolved modes. This supplies a useful tool for turbulent fluid flow data analysis. We apply this methodology to circular cylinder wake flow at Reynolds numbers $Re=100$ and $Re=3900$. The finite-dimensional models of the wake flows reveal the energy and the anisotropy distributions of the small-scale diffusion modes. These distributions identify critical regions where corrective advection effects, as well as structured energy dissipation effects, take place. In providing rigorously derived subgrid terms, the proposed approach yields accurate and robust temporal reconstruction of the low-dimensional models.

Identification-Based Closed-Loop Control Strategies for a Cylinder Wake Flow

Four closed-loop control strategies are discussed to reduce the drag of a cylinder wake flow: Linear Quadratic Gaussian (LQG) control, gain-scheduled LQG (GS-LQG) control, gain-scheduled PI control, and multimodel predictive control (M-MPC). The control models are obtained in an input–output framework through ARMAX (for LQG control), multi-ARMAX (for GS-LQG control and M-MPC), and multi-ARX (for gain-scheduled PI control). The use of system identification for the underlying flow control problem gets rid of the difficult task of developing accurate and robust reduced-order models for the Navier–Stokes (NS) equations. The control is introduced through sucking of fluid through the cylinder surface. The drag on the cylinder is reduced for all control methods. The robustness of all control strategies is tested against unmodeled disturbances and/or dynamics through a detailed simulation of an NS equation-based model by varying in time the Reynolds number around its nominal value 200. For the considered cylinder wake, the M-MPC approach is the best solution. The application of the presented closed-loop control algorithms for the cylinder drag control as a benchmark problem constitutes promising solutions for other related flow control problems in industries.

Schlieren technique in soap film flows

We propose the use of the Schlieren technique as a tool to analyse the flows in soap film tunnels. The technique enables to visualize perturbations of the film produced by the interposition of an object in the flow. The variations of intensity of the image are produced as a consequence of the deviations of the light beam traversing the deformed surfaces of the film. The quality of the Schlieren image is compared to images produced by the conventional interferometric technique. The analysis of Schlieren images of a cylinder wake flow indicates that this technique enables an easy visualization of vortex centers. Post-processing of series of two successive images of a grid turbulent flow with a dense motion estimator is used to derive the velocity fields. The results obtained with this self-seeded flow show good agreement with the statistical properties of the 2D turbulent flows reported on the literature.

A triple-exposure color PIV technique for pressure reconstruction

The study developed a triple-exposure color particle image velocimetry (TE-CPIV) technique associated with pressure reconstruction, and validated its feasibility. A light source with the three primary colors of red, green, and blue (R, G, and B) is produced in a time sequence by a liquid crystal display (LCD) projector. Particle images at three different instants under the color illuminations are captured in one snapshot using a color digital single-lens reflex (SLR) camera with a complementary metal-oxide semiconductor (CMOS) sensor. A contamination correction algorithm based on a specific calibration is performed on the different color layers (R layer, G layer, and B layer) of the raw color image to reduce the contaminated intensity of each color illumination on the other two color layers. The corrected intensity generates three new color layers, from which a standard cross-correlation process in the classical PIV method is used to obtain two velocity fields. Eventually, an instantaneous pressure field is reconstructed from the two velocity fields. The feasibility of TE-CPIV was tested by two experiments with a solid body rotation flow and a cylinder wake flow. The results show acceptable accuracy and robustness of the new technique. The idea of the TE-CPIV is believed to provide a simple and effective way of estimating a pressure field with low cost and high convenience.

Flow around a circular cylinder with slit

Characteristics of flow around a modified circular cylinder are experimentally investigated in the present study. The experimental campaign was performed in a wind tunnel at the Reynolds number of Re=2.67×104, based on the cylinder diameter D and the incoming airflow speed. The cylindrical test model is modified with a slit parallel to the incoming airflow to create a flow communicating channel between the windward and leeward stagnation points. The slit width S changes from 0.05D to 0.15D with an increment of 0.025D. Pressure distributions on the cylinder surface were measured to estimate the aerodynamic forces acting on the test model and a particle image velocimetry (PIV) system was employed to quantify the wake flow patterns of the natural and modified cylindrical test models. Experiment results reveal that a slit contributes to reducing the drag and suppressing the fluctuating amplitude of the dynamic wind loads acting on the test model. The changing trend in drag reduction and lift suppression with the increase of slit width are also discussed based on the surface pressure and PIV measurement results. The PIV measurement results demonstrate clearly that the slit generates a self-issuing jet into the cylinder wake and the passive jet is effective in manipulating the wake vortex shedding process from the circular cylinder. As the jet vortices being shifted downstream, they help to detach the shear layers rolled up from both sides of the circular cylinder. Because of this dynamic interaction process, the antisymmetric pattern of the wake vortex shedding from a natural cylinder is found to be converted to a bistable mode flow behind the slotted cylinders. As a result, the dimensionless vortex shedding frequency is observed to be switched to a very low level and a flip-flop phenomenon is found. A linear stability analysis is then performed to suggest that the intrinsic nature of the cylinder wake flow is greatly modified with the implementation of a slit.

Two-dimensional orthogonal wavelet multi-resolution analysis on multi-scale flow structures behind triangle cylinder

This study focuses on the multi-scale turbulent structures of triangle cylinder wake flow measured by high-speed particle image velocimetry (PIV). The measured turbulent structures were spatially extracted into large-, intermediate-, relatively small-, and small-scale components using two-dimensional orthogonal wavelet analysis. Then the features of multi-scale turbulent structures were analyzed in terms of two-point auto correlation, power spectra, turbulent kinetic energy and Reynolds shear stress. It is found that the large-scale structure attributed to coherent motions in the wake flow makes largest contribution to the total fluctuating energy and maximum Reynolds shear stress, accounting for 84% and 92% respectively. The intermediate scale structures, which are unidentifiable by PIV measurement, are found to shed into the separated shear layer due to Kelvin–Helmholtz instability, and the central frequency of intermediate scale vortex is approximately twice of Kármán vortex shedding frequency. Besides, two symmetric regions of intensive velocity fluctuations having relatively small-scale are observed near the cylinder surface and the boundary of separation region, and they are not dominant.

熱線と抵抗線群からなる新型プローブによる２成分乱流速度測定

We propose a novel probe for turbulence measurement of two velocity components. The probe is composed of a hot-wire and a group of cold-wires. The normal hot-wire placed upstream is used not only to measure an instantaneous velocity magnitude but to operate as a heat source. On the other hand, the group of cold-wires placed downstream is used to detect a thermal wake formed behind the upstream hot-wire. By applying the Gaussian interpolation technique to the cold-wire outputs to find the position of a maximum temperature rise, we can also measure an instantaneous vector of fluid velocity. The proposed technique is examined in a cylinder wake flow at x/d = 40 (Red ≃ 2500). The measurement results of the longitudinal and lateral rms velocities and the Reynolds shear stress are quite reasonable. It is found that a response compensation of cold-wires is indispensable for an accurate flow-direction measurement in turbulent flows.

Centrifugal instability of Stokes layers in crossflow: the case of a forced cylinder wake

The wake flow around a circular cylinder at Re ≈100 performing rotatory oscillations has been thoroughly discussed in the literature, mostly focusing on the modifications to the natural Bénard–von Kármán vortex street that result from the forced shedding modes locked to the rotatory oscillation frequency. The usual experimental and theoretical frameworks at these Reynolds numbers are quasi-two-dimensional, because the secondary instabilities bringing a three-dimensional structure to the cylinder wake flow occur only at higher Reynolds numbers. In this paper, we show that a three-dimensional structure can appear below the usual three-dimensionalization threshold, when forcing with frequencies lower than the natural vortex shedding frequency, at high amplitudes, as a result of a previously unreported mechanism: a pulsed centrifugal instability of the oscillating Stokes layer at the wall of the cylinder. The present numerical investigation lets us in this way propose a physical explanation for the turbulence-like features reported in the recent experimental study by the present authors.

Interactions of Vortices of a Square Cylinder and a Rectangular Vortex Generator Under Couette–Poiseuille Flow

This study focuses on interactions of vortices generated by a family of eddy-promoting upstream rectangular cylinders (of different heights a* and widths b*) with the shear layers of a downstream square cylinder (of height A*) placed near a plane in an in-line tandem arrangement under the incidence of Couette–Poiseuille flow based nonuniform linear/nonlinear velocity profile. The dimensionless operational parameters are cylinders spacing distance S, ratio of heights r2=a*/A* (≤1), aspect ratio r1=b*/a* (≤1), Reynolds number Re (based on the velocity at height A* for Couette flow), ReU2 (based on the velocity at height 10A* for Couette–Poiseuille flow), and nondimensional pressure gradient P at the inlet. The governing equations are solved numerically through a pressure-correction-based iterative algorithm (SIMPLE) with the quadratic upwind interpolation for convective kinematics (QUICK) scheme for convective terms. The major issue of appearing multiple peaks in the spectrum of the fluctuating lift coefficient of the downstream cylinder is addressed and justified exhibiting the flow patterns. While considering the rectangular shape (for the upstream cylinder) and nonlinear velocity (at the inlet), the possibility of generating the unsteadiness in the steady wake flow of the downstream cylinder at a Re (based on height a*) less than the critical Re for the downstream cylinder is documented here. The dependence of flow characteristics of the downstream cylinder on the angle of incident linear velocity at specific S and r1 is also demonstrated here. It is observed that the discontinuous jump in the aerodynamic characteristics (due to a sudden change from one distinct flow pattern to the other in the critical spacing distance regime) is directly proportional to the height of the vortex generator. Increasing P under the same characteristic velocity causes the steady flow of cylinder(s) to convert to a periodic flow and reduces the critical spacing distance for the vortex generator.