Distributed forcing of the flow past a blunt-based axisymmetric bluff body

In this paper, we address the influence of a blowing-/suction-type distributed forcing on the flow past a blunt-based axisymmetric bluff body by means of direct numerical simulations. The forcing is applied via consecutive blowing and suction slots azimuthally distributed along the trailing edge of the bluff body. We examine the impact of the forcing wavelength, amplitude and waveform on the drag experienced by the bluff body and on the occurrence of the reflectional symmetry preserving and reflectional symmetry breaking wake modes, for Reynolds numbers 800 and 1,000. We show that forcing the flow at wavelengths inherent to the unforced flow drastically damps drag oscillations associated with the vortex shedding and vorticity bursts, up to their complete suppression. The overall parameter analysis suggests that this damping results from the surplus of streamwise vorticity provided by the forcing that tends to stabilize the ternary vorticity lobes observed at the aft part of the bluff body. In addition, conversely to a blowing-type or suction-type forcing, the blowing-/suction-type forcing involves strong nonlinear interactions between locally decelerated and accelerated regions, severely affecting both the mean drag and the frequencies representative of the vortex shedding and vorticity bursts.     

Features of the supercritical transition in the wake of a circular cylinder

The features of the wake behind a uniform circular cylinder atRe=200, which is just beyond the critical Reynolds number of 3-D transition, are investigated in detail by direct numerical simulations by solving 3-D incompressible Navier-Stokes equations using mixed spectral-spectral-element method. The high-order splitting algorithm based on the mixed stiffly stable scheme is employed in the time discretization. Due to the nonlinear evolution of the secondary instability of the wake, the spanwise modes with different wavelengths emerge. The spanwise characteristic length determines the transition features and global properties of the wake. The existence of the spanwise phase difference of the primary vortices shedding is confirmed by Fourier analysis of the time series of the spanwise vorticity and attributed to the dominant spanwise mode. The spatial energy distributions of various modes and the velocity profiles in the near wake are obtained. The numerical results indicate that the near wake is in 3-D quasi-periodic laminar state with transitional behaviors at this supercritical Reynolds number. The project supported by the State Key Fundamental Research Project of “Large Scale Scientific Computation Research” (G199903281)     

Experimental estimation of a D-shaped cylinder wake using body-mounted sensors

The effectiveness of a small array of body-mounted sensors, for estimation and eventually feedback flow control of a D-shaped cylinder wake is investigated experimentally. The research is aimed at suppressing unsteady loads resulting from the von-Kármán vortex shedding in the wake of bluff-bodies at a Reynolds number range of 100–1,000. A low-dimensional proper orthogonal decomposition (POD) procedure was applied to the stream-wise and cross-stream velocities in the near wake flow field, with steady-state vortex shedding, obtained using particle image velocimetry (PIV). Data were collected in the unforced condition, which served as a baseline, as well as during influence of forcing within the “lock-in” region. The design of sensor number and placement was based on data from a laminar direct numerical simulation of the Navier-Stokes equations. A linear stochastic estimator (LSE) was employed to map the surface-mounted hot-film sensor signals to the temporal coefficients of the reduced order model of the wake flow field in order to provide accurate yet compact estimates of the low-dimensional states. For a three-sensor configuration, results show that the estimation error of the first two cross-stream modes is within 20–40% of the PIV-generated POD time coefficients. Based on previous investigations, this level of error is acceptable for a moderately robust controller required for feedback flow control.     

Vortex Shedding from a Hemisphere in a Turbulent Boundary Layer

Supercritical turbulent boundary layer flow over a hemisphere with a rough surface (Re= 150000) has been simulated using Large Eddy Simulation (LES) and analyzed using the Karhunen--Loève expansion (“Proper Orthogonal Decomposition,” POD). The time-dependent inflow condition is provided from a separate LES of a boundary layer developing behind a barrier fence and a set of vorticity generators. LES results using significantly different grid resolutions are compared with a corresponding wind tunnel experiment to demonstrate the reliability of the simulation. The separation processes are analyzed by inspecting second-order moments, time spectra, and instantaneous velocity distributions. Applying POD, a detailed study of the spatiotemporal structure of the separation processes has been carried out. From this analysis it can be concluded that the major event in the separated flow behind the obstacle is the shedding of “von Kármán”-type vortices, which can be represented by the first three energetically dominant modes. Received 23 January 1997 and accepted 19 February 1998     

Near wake properties of axisymmetric bluff body flows

Experiments have been made to measure some of the near wake properties of axisymmetric bluff body flows with fixed points of separation, including the detention or residence time of fluid borne scalar entities, base pressure coefficient, wake bubble length parameter and shape parameter. Measurements were made in smooth and turbulent air flow for Reynolds number in the range 2×10³<Re<4×10⁴. The results for a given bluff body were found to be uniquely controlled by a free-stream turbulence parameter. The data for all the shapes of bluff body in the class under consideration were found to collapse into unique inter-relationships by the introduction of the face pressure coefficient as a quantitative measure of “bluffness”. This paper was originally presented at the 14th International Congress of Theoretical and Applied Mechanics, Delft (August–September, 1976).     

Dynamic wind loads and wake characteristics of a wind turbine model in an atmospheric boundary layer wind

An experimental study was conducted to characterize the dynamic wind loads and evolution of the unsteady vortex and turbulent flow structures in the near wake of a horizontal axis wind turbine model placed in an atmospheric boundary layer wind tunnel. In addition to measuring dynamic wind loads (i.e., aerodynamic forces and bending moments) acting on the wind turbine model by using a high-sensitive force-moment sensor unit, a high-resolution digital particle image velocimetry (PIV) system was used to achieve flow field measurements to quantify the characteristics of the turbulent vortex flow in the near wake of the wind turbine model. Besides conducting “free-run” PIV measurements to determine the ensemble-averaged statistics of the flow quantities such as mean velocity, Reynolds stress, and turbulence kinetic energy (TKE) distributions in the wake flow, “phase-locked” PIV measurements were also performed to elucidate further details about evolution of the unsteady vortex structures in the wake flow in relation to the position of the rotating turbine blades. The effects of the tip-speed-ratio of the wind turbine model on the dynamic wind loads and wake flow characteristics were quantified in the terms of the variations of the aerodynamic thrust and bending moment coefficients of the wind turbine model, the evolution of the helical tip vortices and the unsteady vortices shedding from the blade roots and turbine nacelle, the deceleration of the incoming airflows after passing the rotation disk of the turbine blades, the TKE and Reynolds stress distributions in the near wake of the wind turbine model. The detailed flow field measurements were correlated with the dynamic wind load measurements to elucidate underlying physics in order to gain further insight into the characteristics of the dynamic wind loads and turbulent vortex flows in the wakes of wind turbines for the optimal design of the wind turbines operating in atmospheric boundary layer winds.     

A study on the mechanism of high-lift generation by an airfoil in unsteady motion at low reynolds number

The aerodynamic force and flow structure of NACA 0012 airfoil performing an unsteady motion at low Reynolds number (Re=100) are calculated by solving Navier-Stokes equations. The motion consists of three parts: the first translation, rotation and the second translation in the direction opposite to the first. The rotation and the second translation in this motion are expected to represent the rotation and translation of the wing-section of a hovering insect. The flow structure is used in combination with the theory of vorticity dynamics to explain the generation of unsteady aerodynamic force in the motion. During the rotation, due to the creation of strong vortices in short time, large aerodynamic force is produced and the force is almost normal to the airfoil chord. During the second translation, large lift coefficient can be maintained for certain time period and , the lift coefficient averaged over four chord lengths of travel, is larger than 2 (the corresponding steady-state lift coefficient is only 0.9). The large lift coefficient is due to two effects. The first is the delayed shedding of the stall vortex. The second is that the vortices created during the airfoil rotation and in the near wake left by previous translation form a short “vortex street” in front of the airfoil and the “vortex street” induces a “wind”; against this “wind” the airfoil translates, increasing its relative speed. The above results provide insights to the understanding of the mechanism of high-lift generation by a hovering insect. The project supported by the National Natural Science Foundation of China (19725210)     

Vortex stretching and filaments

This paper presents a new technique to produce controlled stretched vortices. Intense elliptical vortices are created by stretching of an initial vorticity sheet. The initial vorticity comes from a laminar boundary layer flow and the stretching is parallel to the vorticity vectors. This low velocity flow enables direct observation of the formation and destabilization of vortices. Visualizations are combined with quasi-instantaneous measurements of a full velocity profile. The velocity profile is obtained with an ultrasonic pulsed Doppler velocimeter. The evolution of the central diameter of the vortices is related to the stretching. It is observed that destabilization occurs by pairing of two vortices, by hairpin deformation, and by breakdown of vortices into a “coil shape”.     

流线型轴对称钝体尾迹特性及其声激励控制

在Ｒｅ＝３．０×１０３～１．０×１０５的范围内实验研究了流线型轴对称钝体尾迹特性，并采用声激励手段对尾迹进行控制．研究表明，自然状态下流线型轴对称钝体尾涡无量纲脱落频率在Ｒｅ＝５．０×１０４～１．０×１０５的范围内具有普适性，适当频率的声激励可以减小回流区，该结果对工程减阻具有实际应用价值．实验中脉冲热线的成功应用为复杂流场的测量提供了新的技术手段．     

Shock-vortex shear-layer interaction in the transonic flow around a supercritical airfoil at high Reynolds number in buffet conditions

This paper provides a conceptual analysis and a computational model for how the unsteady ‘buffeting’ phenomenon develops in transonic, low incidence flow around a supercritical aerofoil, the OAT15A, at Reynolds number of 3.3 million. It is shown how a low-frequency buffet mode is amplified in the shock-wave region and then develops upstream and downstream interaction with the alternating von Kármán eddies in the wake past the trailing-edge as well as with the shear-layer, Kelvin–Helmholtz vortices. These interactions are tracked by wavelet analysis, autoregressive (AR) modelling and by Proper Orthogonal Decomposition. The frequency modulation of the trailing-edge instability modes is shown in the spectra and in the wall-pressure fluctuations. The amplitude modulation of the buffet and von Kármán modes has been also quantified by POD analysis. The thinning of the shear layers, both at the outer edge of the turbulent boundary layers and the wake, caused by an ‘eddy-blocking’ mechanism is modelled by stochastic forcing of the turbulent kinetic energy and dissipation, by small-scale straining of the higher-order POD modes. The benefits from thinning the shear-layers by taking into account the interfacial dynamics are clearly shown in the velocity profiles, and wall pressure distribution in comparison with the experimental data.     

Active Control of a Circular Cylinder Flow at Transitional Reynolds Numbers

Active and passive control of flow around a circular cylinder, at transitional Reynolds numbers was investigated experimentally by measuring cylinder surface pressures and wake velocity profiles. Two- and three-dimensional passive boundary layer tripping was considered and periodic active control using piezo-fluidic actuators was introduced from a two-dimensional slot that was nearly tangential to the cylinder surface. The slot location was varied circumferentially by rotating the cylinder and this facilitated either upstream- or downstream-directed actuation using sinusoidal or modulated wave-forms. Separation was controlled by two distinct methods, namely: by forcing laminar-turbulent transition when applied at relatively small angles (30–60°) from the forward stagnation point; and by directly forcing the separated shear-layer at larger angles. In the latter case, actuation produced the largest load changes when it was introduced at approximately 90° from the forward stagnation point. When the forcing frequency was close to the natural vortex-shedding frequency, the two frequencies “locked-in” creating clear and persistent structures. These were examined and categorized. The “lock-in” effect lowered the base pressure and increased the form-drag whereas delaying separation from the cylinder did the opposite.     

Numerical Simulation of the Flow Around an Infinitely Long Circular Cylinder in the Transition Regime

The development of three-dimensional structures and the succeeding transition to turbulence occurs in the wake of a circular cylinder at Reynolds numbers 190≤Re≤330. This regime is investigated numerically by means of a spectral element method. Earlier numerical works aimed mainly at reproducing characteristic wake patterns observed in experiments. Small sizes of computational domains and short integration times were chosen to save computational resources. Consequently, the quantitative results show a considerable scatter. Within this work, a step by step approach to highly accurate direct numerical simulations is described. Thorough studies of the effect of resolution and blockage are performed in the laminar, two-dimensional regime, resulting in Reynolds number relationships that exactly reproduce experimental data. Based on these results, a stability analysis is performed to obtain wavelengths that are unstable against spanwise perturbations and the critical Reynolds number for the onset of three-dimensionality. The most unstable wavelengths of the “mode A” and “mode B” instabilities and its multiples are used as periodicity length for direct numerical simulations. Effects of integration time, resolution in streamwise as well as spanwise directions, and periodicity length on the flow quantities are studied. Numerically obtained Reynolds number relationships of Strouhal number and base-pressure coefficients that fit accurately within measured results are given for the first time. Curves for drag and lift coefficients are provided and compared with previous numerical studies. Furthermore, physical interpretations of the wake transition are discussed. Since the separation of physical features and effects of experimental arrangements are frequently an open question, our numerical results are able to supply a contribution to the understanding of the physics of cylinder flow. Received 12 September 2000 and accepted 26 June 2001     

Aerodynamic forces and three-dimensional flow structures in the mean wake of a surface-mounted finite-height square prism

Different flow models have been proposed for the flow around surface-mounted finite-height square prisms, but there is still a lack of consensus about the origin and connection of the streamwise tip vortices with the other elements of the wake. This numerical study was performed to address this gap, in addition to clarifying the relationship of the near-wake structures with the far wake and the near-wall flow, which is associated with the fluid forces. A large-eddy simulation approach was adopted to solve the flow around a surface-mounted finite-height square prism with an aspect ratio of AR = 3 and a Reynolds number Re = 500. The mean drag and normal forces and the bending moment for the prism were quantitatively compared in terms of skin-friction and pressure contributions, and related to the near-wall flow. Both three-dimensional visualizations and planar projections of the time-averaged flow field were used to identify, qualitatively, the main structures of the wake, including the horseshoe vortex, corner vortices and regions of high streamwise vorticity in the upper part of the wake. These features showed the same qualitative behavior as reported in high Reynolds number studies. It was found that some regions of high streamwise vorticity magnitude, like the tip vortices, are associated with the three-dimensional bending of the flow, and the tip vortices did not continuously extend to the free end of the prism. The three-dimensional flow analysis, which integrated different observations of the flow field around surface-mounted finite-height square prisms, also revealed that the mean near-wake structure is composed of two sections of different origin and location of dominance.     

Wake modes of a cylinder undergoing free streamwise vortex-induced vibrations

Simultaneous measurements of the response of a circular cylinder experiencing vortex-induced vibrations (VIVs) in the streamwise direction and the resulting wake field were performed for a range of reduced velocities using time-resolved Particle-Image Velocimetry in the Reynolds number range 450–3700. The dominant vortex shedding mode was identified using phase-averaged vorticity fields. The cylinder response amplitude was characterised by two response branches, separated by a low amplitude region at resonance, as has been previously reported in the literature. During the first response branch the wake exhibited not only the symmetric S-I mode, but also the alternate A-II mode at slightly higher reduced velocities. For both modes, the vortices were observed to be shed at the cylinder response frequency, but rearranged downstream into a more stable structure in which the velocity fluctuations were no longer synchronised to the cylinder motion. A special case of the A-II mode, referred to as the SA mode, was found to dominate in the second response branch and the low amplitude region, while the far wake and the cylinder motion were synchronised (lock-in). A change in the timing of the vortex shedding with respect to the cylinder motion was observed between the low amplitude region and the second response branch. This is likely to correspond to a change in the fluid forcing and levels of excitation, and may explain the variation in the cylinder amplitude observed in this region. Lock-in and the second response branch were found to coincide with a contraction of the wake and an increase in strength of the shed vortices. This work reveals the inherent differences between the extensively studied case of transverse-only VIV and the streamwise-only case, which is crucial if the wealth of information available on transverse VIV is to be extended to the more practical two degree-of-freedom case.     

Proper orthogonal decomposition analysis of coherent motions in a turbulent annular jet

A three-dimensional incompressible annular jet is simulated by the large eddy simulation(LES) method at a Reynolds number Re = 8 500. The time-averaged velocity field shows an asymmetric wake behind the central bluff-body although the flow geometry is symmetric. The proper orthogonal decomposition(POD) analysis of the velocity fluctuation vectors is conducted to study the flow dynamics of the wake flow.The distribution of turbulent kinetic energy across the three-dimensional POD modes shows that the first four eigenmodes each capture more than 1% of the turbulent kinetic energy, and hence their impact on the wake dynamics is studied. The results demonstrate that the asymmetric mean flow in the near-field of the annular jet is related to the first two POD modes which correspond to a radial shift of the stagnation point. The modes 3 and 4 involve the stretching or squeezing effects of the recirculation region in the radial direction. In addition, the spatial structure of these four POD eigenmodes also shows the counter-rotating vortices in the streamwise direction downstream of the flow reversal region.     

Local mass/heat transfer from a wall-mounted block in rectangular channel flow

This investigation explores the mass/heat transfer from a wall-mounted block in a rectangular fully developed channel flow. The naphthalene sublimation scheme was used to measure the level of local mass transfer from the block’s surfaces. The heat transfer coefficient can be obtained by analogy between heat and mass transfer. The effects of the Reynolds number on the local mass transfer from the block’s surfaces have been widely discussed. Results showed that, owing to the flow complexity induced by vortices around the block, the block’s surfaces appeared four different spatial Sherwood number distributions, termed “Wave type”, “U type”, “Slant type”, and “Pit type”. A change in the Reynolds number significantly altered the spatial Sherwood number distributions on the block’s surfaces. Besides, four correlations between the Reynolds number and the surface-averaged Sherwood number were presented for the front, top, side, and rear surfaces of the block at a given block’s height, for the purpose of practical applications.     

Wake of a cylinder: a paradigm for energy harvesting with piezoelectric materials

Short-length piezoelectric beams were placed in the wake of a circular cylinder at high Reynolds numbers to evaluate their performance as energy generators. The coherent vortical structures present in this flow generate a periodic forcing on the beam which when tuned to its resonant frequency produces maximum output voltage. There are two mechanisms that contribute to the driving forcing of the beam. The first mechanism is the impingement of induced flow by the passing vortices on one side of the beam, and the second is the low pressure core region of the vortices which is present at the opposite side of the beam. The sequence of these two mechanisms combined with the resonating conditions of the beam generated maximum energy output which was also found to vary with the location in the wake. The maximum power output was measured when the tip of the beam is about two diameters downstream of the cylinder. This power drops off the center line of the wake and decays with downstream distance as (x/D)^−3/2.     

Late-Stage Transitional Boundary-Layer Structures. Direct Numerical Simulation and Experiment

This paper is devoted to direct comparisons of related, detailed experimental and numerical studies of the non-linear, late stages of laminar-turbulenttransition in a boundary layer including flow breakdown and the beginning offlow randomization. Preceding non-linear stages of the transition process arealso well documented and compared with previous studies. The experiments wereconducted with the help of a hot-wire anemometer. The numerical study wascarried out by direct numerical simulation (DNS) of the flow employing theso-called spatial approach. Both the experiments and the DNS were performed atcontrolled disturbance conditions with an excitation of instability waves inthe flat-plate boundary layer. In the two cases, the primary disturbanceconsists of a time-harmonic, two-dimensional Tollmien--Schlichting wave thathas a very weak initial spanwise modulation. Despite somewhat differentinitial disturbance conditions used in the experiment and simulation, thesubsequent flow evolution at late non-linear stages is found to be practicallythe same. Detailed qualitative and quantitative comparisons of theinstantaneous velocity and vorticity fields are performed for twocharacteristic stages of the non-linear flow breakdown: (i) “one-spike stage” and (ii) “three-spike stage.” The twoapproaches clearly show in detail the process of development of the Γ-structure, a periodical formation of ring-like vortices, the evolution of the surrounding flow field, and the beginning of flowrandomization. In particular, it is found experimentally and numerically thatthe ring-like vortices (associated with the well-known spikes) induce somerather intensive positive velocity fluctuations (positive spikes) in thenear-wall region which have the same scales as the ring-like vortices and propagate downstream with the same high (almost free-stream) speed. The positive spikes form a new high-shear layer in the near-wall region. In the experiment the induced near-wall perturbationshave a significant irregular low-frequency component. These non-periodicalmotions play an important role in the process of flow randomization and finaltransition to turbulence that starts under the ring-like vortices in thevicinity of the peak position. Received 13 December 2000 and accepted 30 October 2001     

Comparison of flow structures in the downstream region of a cylinder and sphere

An experimental investigation of flow structures downstream of a circular cylinder and sphere immersed in a free-stream flow is performed for Re = 5000 and 10,000 using qualitative and quantitative flow visualization techniques. The obtained results are presented in terms of time-averaged velocity vectors, patterns of streamlines, vorticity, Reynolds stress correlations and turbulent kinetic energy distributions. Flow data reveal that the size of wake flow region, the location of singular and double points, the peak values of turbulence quantities, such as Reynolds stress correlations, vorticity fluctuations and turbulent kinetic energy vary as a function of models’ geometry and Reynolds Numbers. The concentration of small scale vortices is more dominant in the wake of the sphere than that of the cylinder. The maximum value of turbulent kinetic energy (TKE) occurs close to the saddle point for the cylinder case while two maximum values of TKE occur along shear layers for the sphere one because of the 3-D flow behavior.