Thick Turbulent Boundary Layer Research Articles

Coexistence of different mechanisms underlying the dynamics of supersonic turbulent flow over a compression ramp

Supersonic turbulent flow over a compression ramp is studied using wall-resolved large eddy simulation with a freestream Mach number of 2.95 and a Reynolds number [based on δ0: the thickness of incoming turbulent boundary layer (TBL)] of 63 560. The unsteady dynamics of the present shock wave/turbulent boundary layer interaction (STBLI) flow are investigated by using dynamic mode decomposition techniques, linear and nonlinear disambiguation optimization, local stability analysis (LSA), and global stability analysis (GSA). By analyzing the dynamic system for the STBLI flow, three dynamically important modes with characteristic spanwise wavelengths of 2δ0, 3δ0, and 6δ0 are captured. The 2δ0 mode approximates the spanwise scale of the Görtler-like vortices and Görtler mode of LSA, suggesting the presence of Görtler instability, which is believed to be related to the unsteady motion of streaks downstream of reattachment in the flow. The features of the 3δ0 mode are also observed in large-scale motions of the incoming TBL, implying the existence of a convective mechanism that is excited and maintained by such motions. Additionally, the GSA results show the most unstable mode features a spanwise wavelength of around 6δ0, indicating the existence of global instability that is believed to be related to the oscillating motion of separation shock. The coexistence of these three mechanisms is confirmed. Discussions on the above findings provide an interpretation for low-frequency unsteadiness that the unsteadiness of surface streaks results from the combined effects of the Görtler instability near flow reattachment and the convection of large-scale motions in the incoming boundary layer, while the low-frequency shock motion may be related to a global mode driven by upstream disturbances.

Unsteady wake interference of unequal-height tandem cylinders mounted in a turbulent boundary layer

The unsteady wake interference of unequal-height tandem finite wall-mounted cylinders (FWMCs) fully submerged in a turbulent boundary layer (TBL) was investigated using time-resolved particle image velocimetry. The aspect ratios of the cylinders were fixed at $h/d = 5.3$ for the upstream cylinder (UC) and $H/d = 7.0$ for the downstream cylinder (DC) to achieve a height ratio of $h/H = 0.75$ , where d is the diameter of the cylinders. The Reynolds number based on the cylinder diameter was $Re = 5540$ and the submergence ratio was $\delta /H = 1.2$ , where $\delta $ is the TBL thickness. Three main flow regimes of tandem FWMCs were examined by varying the centre-to-centre spacing ( $s$ ) between the cylinders: extended-body ( $s/d = 2$ ), reattachment ( $s/d = 4$ ) and co-shedding ( $s/d = 6$ ) regimes. These test cases denoted as SR2, SR4 and SR6, respectively, were compared with a reference isolated cylinder (SC) with an aspect ratio similar to that of the DC. Spatio-temporal analysis of the flow field showed that the gap region of SR2 is characterized by a strong downwash of alternating low- and high-momentum fluid induced by the approach flow that is deflected from the unsheltered portion of the DC. In contrast, the gap region of SR4 and SR6 exhibited both downwash and upwash flow with a saddle point that moves closer to the mid-height of the UC as the spacing ratio increases. The upwash and downwash shear layers were associated with small-scale vortices with Strouhal numbers larger than that of the Kármán vortex shedding in the spanwise shear layers. The wake structure behind the DC was significantly altered compared with the SC due to sheltering effects, and the spacing ratio had a significant impact on the spatio-temporal evolution of the vortices.

Turbulent transport mechanism in the roughness sublayers over idealized urban areas and its implication to street-level ventilation

Turbulence in the roughness sublayer (RSL) is inhomogeneous compared with that in the inertial sublayer (ISL) of the atmospheric surface layer (ASL) over urban areas. Drag coefficient Cd, which measures aerodynamic roughness, is employed in this paper to examine how (idealized) urban morphology influences ASL dynamics and transport. Wind tunnel experiments are conducted to study the flows and turbulence in response to different configurations of (identical) roughness elements. Statistics, quadrant analysis, and tilt angle evidence the more efficient RSL transport over rougher surfaces even the winds are slower. Although the power spectra of streamwise u’’ and vertical w’’ fluctuating velocities are rather insensitive to Cd, their cospectrum shows a secondary peak at small motion scales λx (≤ 0.1δ where δ is the thickness of turbulent boundary layer) along with the primary peak at integral length scale Λx (≈ δ). It is thus suggested that, regardless of the turbulence intensity, RSL streamwise and vertical winds are more correlated, enhancing the transport. Amplitude (AM) and frequency (FM) modulations signify the positive correlation between RSL large and small motion scales which is amplified over rougher surfaces. Furthermore, RSL turbulence kinetic energy (TKE) production (entrainment) increases with decreasing (increasing) Cd, fostering the basic mechanism of street-level ventilation.

High-speed shock–boundary-layer interaction over deformed surfaces

This paper concerns the interaction of an oblique shock wave with a supersonic turbulent boundary layer over a thin panel surface, leading to shock–boundary layer interaction and panel buckling. We have performed high-order numerical simulations featuring various static two-dimensional surface deformations typically encountered in experiments. The deformation amplitudes we examined were at least half the height of the incoming turbulent boundary layer thickness. The results show that along the panel midspan, where the maximum deformation amplitude is located, the mean and root mean square pressure are affected by about 10%. Cases for which the pressure at the shock–boundary layer interaction was increased relative to the planar case showed to decrease downstream, and vice versa. Despite the weak response to the mean pressure amplitude, the mean pressure surface contour plots reveal that the streamwise, particularly the spanwise distribution, is affected more noticeably. For example, the surface deformation modes are shown to disrupt the spanwise constant mean pressure, forming higher (or lower) values at either the panel's midspan or edges, depending on the mode. Moreover, the surface curvature leads to a characteristic bending of the spanwise distribution, which can be concave or convex depending on the deformation mode. Analysis of the Reynolds stress anisotropy componentality at different heights from the buckled surface reveals a similar spanwise response of the turbulent velocity fluctuations. The results suggest that the deformation rate plays an important role alongside the deformation amplitude in the turbulent layer and shock–boundary layer interaction.

Response of a supersonic turbulent boundary layer to different streamwise adverse pressure gradients

An adverse pressure gradient (APG) has an impact on the boundary layer, increasing the turbulent intensity of the layer. The mean and turbulent properties of the turbulent boundary layer on a flat plate with different APGs were investigated at Mach 2.7 in the present work utilizing the particle image velocimetry and nanoparticle-based planar laser scattering techniques. According to analysis, the changing trends of boundary layer parameters are different depending on whether the local mainstream velocity or freestream velocity of the wind tunnel is used to normalize. Using the former might make the enhanced effect of the rising APG more visible. With the rise in APG, the principal strain rate, turbulent fluctuation, Reynolds stress, and turbulence production in the boundary layer all increased, while the turbulent boundary layer's thickness dropped. Furthermore, the heightened upward ejection and downward sweep events caused the streamwise turbulence intensity to reach its outer peak under the influence of strong APG. The characteristics of the spanwise vortex in the boundary layer are investigated in conjunction with the probability density function analysis. The growing APG, which primarily promote negative vorticity, can strengthen the rotational strength of spanwise vortices, which are a component of hairpin vortices. As APG rises, the number of small-scale vortices in the boundary layer increases and the fractal dimension grows. The increase in small-scale vortices tends to induce strong transportation and promotes turbulence intensity. Further investigation reveals that the increased volume change caused by the enhanced compression effect with increasing APG exacerbated the vorticity.

Gap ratio effects on the coherent structures surrounding a near-wall square cylinder

Turbulent flow around a square cylinder offset at different heights from an upstream rough wall are investigated using time-resolved particle image velocimetry. The Reynolds number based on the freestream velocity and cylinder height was 12750 and the incoming turbulent boundary layer thickness was 7.2 times the cylinder height. The examined gaps beneath the cylinder were 0.0, 0.3, 0.5, 1.0, 2.0, 4.0, and 8.0 times the cylinder height. The results show that the mean recirculation length remains relatively constant for cases with a gap size greater than or equal to 1.0 times the cylinder height. Flow acceleration through the gap under the cylinder is strongest with a gap size of 2.0 times cylinder height. Downstream of the cylinder, the vertical velocity fluctuations contribute strongly to the production of turbulent kinetic energy by interacting with mean wall-normal velocity gradient in the vertical direction. The dominant frequency embedded in the incoming turbulent boundary layer persists in the wake of the cylinder when the gap size is less than 1.0 cylinder height. Kelvin-Helmholtz vortices originating from the top leading edge of cylinder occur at Strouhal numbers of 2.77-3.11 for the cases with gap ratio ranging from 0.3-8.0 cylinder heights, while spectral migration towards lower frequencies is observed for cases with gap sizes equal to 0.3 and 0.5 cylinder heights. The spatial trajectory of top Kelvin-Helmholtz vortices extends further away from the cylinder as the gap size increases. Spectral proper orthogonal decomposition analysis shows that the von Kármán vortex shedding structures possess the highest spectral energy for all offset cases despite regular vortex shedding being disrupted when the gap size is 0.3 times the cylinder height.

Effects of wall disturbances on the statistics of supersonic turbulent boundary layers

In the present study, we perform direct numerical simulations to investigate the spatial development and basic flow statistics in the supersonic turbulent boundary layers at the free-stream Mach number of 2.0 over smooth and disturbed walls, the latter of which enforces extra Reynolds shear stress in the streamwise direction to emulate the drag increment and mean streamline curvature effects of rough walls. Such disturbances escalate the growth rate of turbulent boundary layer thickness and the shape factor. It is found that under the rescaled global coordinate, the mean velocity, Reynolds stress, and pressure fluctuation variance manifest outer-layer similarity, whereas the average and fluctuation variances of temperature and density do not share such a property. Compressibility effects are enhanced by the wall disturbances, yet not sufficiently strong to directly impact the turbulent kinetic energy transport under the presently considered flow parameters. The generalized Reynolds analogy that relates the mean velocity and temperature can be satisfied by incorporating the refinement in modifying the generalized recovery coefficient, and that associates the fluctuating velocity and temperature work reasonably well, indicating the passive transport of temperature fluctuations. The dispersive motions are dominant and decay exponentially below the equivalent sand grain roughness height ks, above which the wall disturbances are distorted to form unsteady motions responsible for the intensified density and pressure fluctuations in the free-stream traveling isentropically as the acoustic radiations.

Vortex evolution of flow past the near-wall circular cylinder immersed in a flat-plate turbulent boundary layer

The vortex evolutions and the turbulent characteristics in flow past the near-wall circular cylinder immersed in a flat-plate turbulent boundary layer are investigated with particle image velocimetry (PIV), with the objective of clarifying the interaction between the turbulent boundary layer flow and the near-wall cylinder wake. The Reynolds number based on the cylinder diameter D is 1.0×103, and three gap ratios (i.e., G/D=0.5, 1 and 2) are chosen for comparison at δ/D=6.7 where G is the gap size and δ is turbulent boundary layer thickness without the presence of cylinder. The proper orthogonal decomposition (POD) analysis is used to obtain phase-averaged flow fields, combining with the vortex identification method λci, which can capture effectively the evolution process of vortex. For three gap ratios, the cylinder is placed at the logarithmic-law layer of incoming flow, and the Karman-vortex sheets can be generated from the curling of the cylinder shear layers. At G/D=0.5 and G/D=1, the secondary vortex can be formed periodically in the wall due to the induction of the lower wake vortex. The vortex evolutions and turbulent characteristics are significantly affected by incoming turbulent boundary layer flow. The swirling strength of the wake vortices shed from the cylinder is enhanced with increasing G/D, and the swirling strength of the upper wake vortex is slightly bigger than that of the lower wake vortex due to velocity differences of incoming flow. The variation of the Strouhal number (St) is dependent on the gap ratio, and there is nearly 10.4% increase from St=0.211 at G/D=2 to St=0.233 at G/D=0.5, which is caused by the combined effects of the deflected gap flow and the velocity gradient. At large gap ratios, especially at G/D=2, the upstream high-velocity fluids interact with the cylinder wake, leading to strong velocity fluctuations behind the cylinder, and the magnitudes of production and turbulent diffusion in turbulent kinetic energy are increased, which also enhances turbulent transports behind the cylinder.

Effect of exit geometry of blowing air on friction drag of an underwater plate

Air layer drag reduction (ALDR) is one of the important techniques for reducing the frictional resistance of ships and underwater vehicles. The air injection configuration directly influences air layer coverage on the surface and hence drag reduction. Several shapes of air exit have been investigated in the past, including narrow slots, regular-shaped holes, porous media, and arrays/combinations of them. A new exit shape, i.e. modified Nekomimi, is proposed and investigated numerically for ALDR of an underwater flat plate. The work is inspired by air film cooling technology where it is desirable to have an air layer segregating wall from main gas flow. The Reynolds number examined is 6970–22890, based on local turbulent boundary layer thickness, for the incoming water flow or 700–5550, based on the equivalent exit diameter of blowing air. Results show that the Nekomimi exit produces a larger and more even air layer coverage, as indicated by the air volume fraction on the plate, accounting for a more pronounced drag reduction and net energy saving than a circular exit with the same area or spanwise dimension. Multi-Nekomimi-exit arrangement is also investigated to obtain a larger air-layer coverage on the plate, showing good promise in engineering applications.

Numerical investigation on the generation mechanism of aero-heating of rudder shaft from three-dimensional flow separation and vortices

The hypersonic flow past a controlled rudder mounted at a gap to the aircraft fuselage is numerically investigated by solving three-dimensional (3D) Reynolds-averaged Navier–Stokes equations. This paper aims to explain the mechanism of production of extreme thermal environment faced by a rudder shaft from the view of physics of flow in the mounting gap. Simulations are conducted at Mach number of 10, and the gap ratio h/δ varies from 0 to 1.42, where h denotes the gap height and δ represents the thickness of the incoming turbulent boundary layer. Topological theory is utilized to identify the separation types. The formation of vortices is traced by extracting volume streamlines in the 3D space. The results indicate that the types of 3D separation appear in the gap shifts from the coexistence of horseshoe-type and tornado-type separations to only horseshoe-type separation that persists with the increase of h/δ. It is found that high heat flux is generated by the high-momentum fluid transported toward the surface by the horseshoe vortices. The tornado-type vortex prevents the incoming flow from arriving at the rudder shaft, which avoids the generation of high heat flux at the center of the rudder shaft. The rate of local heat transfer increases with h/δ as a result of the shrink and disappearance of the tornado-type vortices, which means that the region of low-speed backflow in front of the rudder is reduced and vanished. This study contributes to a clearer understanding of the flow physics in the complex disturbance area.

Experimental investigation of time-invariant eddy viscosity in wave-current interaction

An experimental study of the turbulent boundary layer, where waves propagate with a current, is presented in this paper. A wide range of test conditions have been covered, namely, flows over a rough bed and a smooth bed, combined flows in a large-scale oscillatory water tunnel and combined waves and currents in two flumes with different scales. Particle Image Velocimetry and Laser Doppler Velocimetry were employed to obtain the velocity field. Detailed analysis of eddy viscosity profiles calculated from the experiments leads to the conclusion that previous assumed profiles do not always accurately describe eddy viscosity distributions in a combined wave-current flow. The distributions of eddy viscosity are categorised into two types (two-layer or three-layer), based on the influence of wave motions superimposed. For those cases in the current-dominated regime, eddy viscosity profiles are similar to unidirectional turbulent currents. When combined flows are in the wave-dominated regime, three-layer eddy viscosity distributions are observed. For both types, a linear eddy viscosity profile is found to be present in the bottom 10 per cent of the turbulent boundary layer. Above this, the classic parabolic profile is observed, over the whole turbulent boundary layer for the first type and over 40 per cent of the turbulent boundary layer thickness for the second type. An empirical eddy viscosity distribution in the outer region is proposed for the second type. This newly developed eddy viscosity distribution provides guidance for numerical modellers in the field of wave-current interaction and for coastal engineers wishing to predict sediment transport.

The Wake Dynamics Behind a Near-Wall Square Cylinder

Abstract Particle image velocimetry is used to experimentally study the wake dynamics behind a near-wall square cylinder subjected to a thick oncoming turbulent boundary layer. The turbulent boundary layer thickness was 3.6 times the cylinder height (h) while the Reynolds number based on the freestream velocity and the cylinder height was 12,750. The gap distance (G) between the bottom face of the cylinder and the wall was varied, resulting in gap ratios (G/h) of 0, 0.3, 0.5, 1.0, 2.0, 4.0, and 8.0. The effects of varying the gap ratio on the mean flow, Reynolds stresses, triple velocity correlation, two-point autocorrelation, and the unsteady wake characteristics were examined. The results indicate that as gap ratio decreases, asymmetry in the wake flow becomes more pronounced, and the size of the mean separation bubbles increases. The magnitudes of the Reynolds stresses and triple velocity correlations generally decrease with the decreasing gap ratio. Moreover, the size of the large-scale structures increases with decreasing gap ratio, and the critical gap ratio, below which Kármán vortex shedding is suppressed, is found to be 0.3. The dominant Strouhal number in the wake flow expressed in terms of the streamwise mean velocity at the cylinder vertical midpoint increases as gap ratio decreases while that based on the freestream velocity is less sensitive to gap ratio for the offset cases (G/h > 0).

PROPELLER CAVITATION AND INDUCED VIBRATION

The demand to increase the efficiency of propellers has led to optimized propeller blade designs finding their way into the construction of high-powered commercial vessels, such as containers or LNG carriers and certain categories of passenger vessels, to mention but a few. It has become increasingly common to see the propeller tip rotate closer to the hull surface, sweeping the thick turbulent boundary layer attached to the hull, causing fluid structure interaction. At the same time, increasing the loading on marine propellers can lead to problems, such as noise, hull vibration, and cavitation. The degree above which, such phenomena as propeller cavitation can be the main perpetrators for intensive vibration during operation, their diagnosis and the solutions to mitigate this risk, such as the use of vortex generators, are discussed here, taking into account cost and longevity of the vessel as well as the involvement of classification rules.

Numerical Study of Effect of Sawtooth Riblets on Low-Reynolds-Number Airfoil Flow Characteristic and Aerodynamic Performance

Riblets with an appropriate size can effectively restrain turbulent boundary layer thickness and reduce viscous drag, but the effects of riblets strongly depend on the appearance of the fabric that is to be applied and its operating conditions. In this study, in order to improve the aerodynamic performance of a low-pressure fan by using riblet technology, sawtooth riblets on NACA4412 airfoil are examined at the low Reynolds number of 1 × 105, and the airfoil is operated at angles of attack (AOAs) ranging from approximately 0° to 12°. The numerical simulation is carried out by employing the SST k–ω turbulence model through the Ansys Fluent, and the effects of the riblets’ length and height on aerodynamic performance and flow characteristics of the airfoil are investigated. The results indicate that the amount of drag reduction varies greatly with riblet length and height and the AOA of airfoil flow. By contrast, the riblets are detrimental to the airfoil in some cases. The most effective riblet length is found to be a length of 0.8 chord, which increases the lift and reduces the drag under whole AOA conditions, and the maximum improvements in both are 17.46% and 15.04%, respectively. The most effective height for the riblet with the length of 0.5 chord is 0.6 mm. This also improves the aerodynamic performance and achieves a change rate of 12.67% and 14.8% in the lift and drag coefficients, respectively. In addition, the riblets facilitate a greater improvement in airfoil at larger AOAs. The flow fields demonstrate that the riblets with a drag reduction effect form “the antifriction-bearing” structure near the airfoil surface and effectively restrain the trailing separation vortex. The ultimate cause of the riblet drag reduction effect is the velocity gradient at the bottom of the boundary layers being increased by the riblets, which results in a decrease in boundary thickness and energy loss.

Shock wave and turbulent boundary layer interaction in a double compression ramp

Direct numerical simulations of shock wave and supersonic turbulent boundary layer interaction in a double compression ramp with fixed ramp angles of 12o and 24o at Mach 2.9 are conducted. The characteristics of the shock interactions are investigated for four different length between the two ramp kinks, corresponding to Lc = 0.9δref, 1.8δref, 2.7δref, and 3.6δref (δref being the upstream turbulent boundary layer thickness). The influence of increasing Lc on flow structures, unsteadiness, Reynolds stress, turbulence kinetic energy, and Reynolds stress anisotropy tensor is assessed. The size of the separation region is significantly decreased and reattached flow appears between the two ramp kinks. Streamwise vorticity contours and streamline curvature show the decreased spanwise width and increased spanwise coherency of Görtler-like vortices. Analysis of fluctuating wall pressure indicates that the low-frequency unsteadiness is strongly suppressed in the interaction region. Profiles of Reynolds stress components and turbulence kinetic energy exhibit different turbulence evolution across the interaction, leading to substantial differences observed in the anisotropy invariant map. It is found that the near wall region is characterized by decreased anisotropy, becoming closer to the axisymmetric compression state, while a significant increase of turbulence is identified in the outer region, following the axisymmetric expansion limit. Moreover, downstream of the interaction, turbulence in the near-wall region experiences a faster recovery and the influence of Lc is found to be marginal. The main effect of Lc is observed in the outer region, an increase of Lc resulting in a monotonic decay of turbulence intensities and an inward movement of turbulent structures.

Direct numerical simulation of a supersonic turbulent boundary layer over a compression–decompression corner

A direct numerical simulation of the interaction between a shock wave and the supersonic turbulent boundary layer in a compression–decompression corner with a fixed 24° deflection angle at Mach 2.9 is conducted. The characteristics of the shock interactions are investigated for two heights between the compression and decompression corners, corresponding to H/δref=4.25, 1.22, where δref denotes the reference turbulent boundary layer thickness. A classic shock wave/turbulent boundary layer interaction flow is reproduced in the higher case. For the lower case, the size of the separation region is significantly decreased, and the low-frequency unsteadiness is slightly suppressed in the interaction region, as assessed by analyzing the mean and fluctuating wall pressure. Flow patterns near the reattachment line show the existence of the Görtler vortices. By analyzing the curvature radius and Görtler number distribution, it was found that a strong centrifuge instability is reserved in the compression corner region and reversed in the decompression corner region due to the convex streamline curvature. The downstream flow of the decompression corner is relatively complex where the additional shocklet and new streamwise vortices are observed. A negative response mechanism is found regarding fluctuating wall-pressure signatures between the upstream and downstream of the decompression corner.

Flat plate drag reduction using plasma-generated streamwise vortices

Abstract

Effects of approach flow conditions on the unsteady three-dimensional wake structure of a square-back Ahmed body

We investigate the effects of approach flow conditions (uniform flow (UF): Case A and thick turbulent boundary layer (TBL): Case B) on the unsteady three-dimensional wake characteristics and bimodality of a square back Ahmed body. Using improved delayed detached eddy simulations we show that TBL induces a much higher ground clearance momentum deficit than UF, which significantly alters wake asymmetry orientation in the wall-normal plane and completely suppresses bimodal wake behavior in the spanwise plane. Both time-averaged and time-resolved turbulence statistics are used to explore the differences between the wake structure of the Ahmed body subject to the two approach flow conditions.

Multi-resolution analysis of aero-optical effects in a supersonic turbulent boundary layer.

Studying the aero-optical effects induced by turbulent structures with different scales helps determine the capture scale of turbulent structures in experiments/calculations and improve the turbulence breakup device. In this paper, the density field of a supersonic turbulent boundary layer at Ma=3.0 was measured based on the nano-tracer plane laser scattering technique. Two-dimensional orthogonal wavelet multi-resolution analysis was applied to obtain information about different flow scales. The ray-tracing method simulates the propagation of a Gaussian plane beam through the nonuniform flow field at different resolutions. The results show that the turbulent boundary layer thickness and its calculation method lead to the difference in scaling calculation results among the existing experiments. The turbulent structures about 0.7δ contribute most to aero-optical effects. With the reduction of the resolution, the contribution of small turbulent structures to aero-optical effects reduces obviously. When the minimum scale of turbulent structures captured is larger than 0.072δ, the resolution can no longer reflect the real aero-optics results of turbulent structures. The smallest optically active scale predicted is 0.017δ in Mani's theory. The turbulent structures smaller than 0.018δ have little effect on optical path difference (OPD), and the higher-order quantities change significantly around 0.009δ∼0.018δ. According to experimental results, it is promising to improve the aero-optical suppression effects by breaking the large eddy into the turbulent structures smaller than 0.018δ, or even 0.009δ.

Experimental and numerical investigation of turbulent wake flow around wall-mounted square cylinder of aspect ratio 2

The turbulent wake velocity fields around a short wall-mounted square cylinder of aspect ratio 2 fully immersing into a thick turbulent boundary layer are obtained experimentally with PIV and numerically with LES. Comparison between the two sets of data is made on the time-averaged mean velocities, turbulent fluctuating velocities, and the detailed characteristics of energetic large-scale coherent structures. It is noted that a good comparison of the low-order statistics of velocity fields does not guarantee a satisfactory reproduction of the coherent structures. These flow structures show great similarities in the spatial patterns and dynamic performance between PIV and LES, but apparent differences in turbulent kinetic energy contribution. On the horizontal plane parallel to the wall, the dominant antisymmetric spatial mode pair illustrates the evolution process of Karman vortex shedding with a clear spectral peak around the characteristic frequency while three extra frequency peaks are found in the symmetric spatial modes. Further investigation on the velocity field on the central vertical plane reveals the connection between the unsteadiness of shear layer flow at the free end and the spanwise symmetric vortex shedding activities. It is confirmed that the flapping motion and vortex convection mechanisms observed for the present cylinder could modulate the symmetric wake dynamics. The three-dimensional extreme wake patterns, available from LES results, are related to the most energetic Karman vortex shedding demonstrate upward flow in the vortex weakening side. Meanwhile, the downwash flow at the extreme time instants is found to become stronger always for this kind of short cylinder with a dipole wake pattern.