Shear Layer Reattachment Research Articles

Analysis of flapping motion of reattaching shear layer behind a two-dimensional backward-facing step

An experimental investigation was carried out on the flapping motion of a turbulent reattaching shear layer downstream of a two-dimensional backward-facing step. The Reynolds number was 2.0 × 104, based on the free-stream velocity and the step height. The aim of this study is to analyze the flapping motion, which is featured unsteadiness of the reattaching shear layer, and its interaction with the recirculation region. High-resolution planar particle image velocimetry was used to measure the separated and reattaching shear layer in a horizontal-vertical plane. The velocity vector fields have shown the reattaching shear layer considerably flaps upwards and downwards as much in scale as approximately one step height from the middle part of recirculation region to the reattachment area. As a result, the recirculation region varies in size and the reattachment point shifts upstream and downstream. By applying singular value decomposition and proper orthogonal decomposition, the flapping motion is decomposed into multiple spatial modes, each of which represents interactions between the reattaching shear layer and recirculation region. In particular, the unsteady movement of the reattachment point is highly correlated with the flapping motion, and so is the maximum reverse flow. As a result, the flapping motion contributes substantial parts of the Reynolds shear stress and turbulent kinetic energy within the shear layer in the latter half of the reattachment length.

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This study presented an extensive analysis of the influence of chord-to-thickness ratios on unsteady behaviors of the separated flow and their coupling with wall-pressure fluctuations at the Reynolds number ReD=1.58×104. Three systems with different chord-to-thickness ratios were chosen for comparison: L/D = 3.0, 6.0, and 9.0. The separated flow fields and wall-pressure fluctuations were measured using planar Particle Image Velocimetry (PIV) operating at 1 Hz and a microphone array sampling at 1 kHz, respectively. The distributions of the turbulent kinetic energy and the reverse-flow intermittency indicated that the shortest plate (L/D = 3.0) was subjected to full coverage of the separated flow. In the moderate system (L/D = 6.0), the time-averaged reattachment station was determined to be x/D = 4.75, while in 7% of all realizations the fluid close to the trailing edge surface remained slightly reversed due to the unsteady reattachment of the separated shear layer to the trailing edge and to irregular trailing edge vortex shedding. However, the separated flow always reattached on the plate’s surface in the longest system L/D = 9.0. The measured wall-pressure fluctuations demonstrated combinative influences of hydrodynamic pressure of the large-scale vortical structures above the plate and acoustic excitation propagated by the energetic unsteady wake. This resulted in the salient phase changeover characteristics of the correlative patterns near x/D = 1.5 in the shortest system, L/D = 3.0. Close to the leading edge, the wall-pressure fluctuations in the L/D = 6.0 system were over-exposed to acoustic excitation propagated by the unsteady wake behind the trailing edge, while in the longest system, L/D = 9.0, such influences were relatively attenuated. Proper Orthogonal Decomposition (POD) analysis was performed on the down-sampled wall-pressure fluctuations paired with the flow fields, resulting in a series of energetic flow structures and their corresponding wall-pressure fluctuation profiles. Subsequently, the conditional averaging technique was applied to determine the relationship between the spatial characteristics of the large-scale vortical structures and profiles of the wall-pressure fluctuations.

Drag Reduction on a Square Cylinder using Multiple Detached Control Cylinders

A two-dimensional numerical simulation for flow over a main cylinder with detached controlling cylinders placed at different position is performed through Lattice Boltzmann Method (LBM) to reduce fluid forces. For this study the Reynolds number (Re) is fixed at 160, while gap ratio between these cylinders are taken in the range from 0.5 to 8. The results yield in terms of vorticity, time-series analysis of drag and lift coefficients, power spectra of lift coefficients and force statistics. Depending on gap spacing, flow is classified into four different patterns, called as (i) single blender body, (ii) shear layer reattachment, (iii) fully developed flow and (iv) vortex suppression fully developed flow patterns. In comparison of all three configurations, it is examined that the maximum value of Cdmean and Strouhal number occurs for downstream configuration. Whereas, upstream and dual configuration play a vital role to reduce forces and to suppress vortex shedding. The maximum reduction found in mean drag coefficient for downstream, upstream and for dual configuration is 8.3%, 51% and 50.8%, respectively. Whereas, the reduction in Clrms values for all three configurations is 84.4%, 58.2% and 86.4%, respectively.

Detached-Eddy Simulation of a Supersonic Reattaching Shear Layer

A numerical investigation of low-frequency unsteadiness in a Mach 2.9 turbulent, reattaching shear layer was carried out with delayed detached-eddy simulation. The results were compared to data from a series of experiments performed at Princeton University. The true experimental conditions were matched, and calculations were run for 0.5 s of physical time, capturing a frequency range of about 10 Hz to 1 MHz and about 65 cycles of the Strouhal number 0.03 scale. Because of the approximations used to reduce computational cost, the computations predicted a greater flow turning angle at separation than was measured experimentally. Accounting for the consequent shift in reattachment location, the mean flow properties were predicted fairly accurately. The computations overpredicted the intensity of pressure fluctuation near reattachment, but they captured the form of the pressure fluctuation spectrum accurately. The discrepancies between the computational results and the experimental data may be the result of mean flow three-dimensionality and large-scale fluctuations in the incoming boundary layer that were not accounted for in the numerical model. Few computational studies have captured such long timescales for a high-Reynolds-number flow, and the results indicate the feasibility of studying very low-frequency unsteadiness in reattaching flows with a detached-eddy simulation. The results may also provide guidance for the total simulated time required to fully capture flows with large, slow recirculation zones.

A study of flow patterns for staggered cylinders at low Reynolds number by spectral element method

This study investigates the pattern of flow past two staggered array cylinders using the spectral element method by varying the distance between the cylinders and the angle of incidence (α) at low Reynolds numbers (Re = 100-800). Six flow patterns are identified as Shear layer reattachment (SLR), Induced separation (IS), Vortex impingement (VI), Synchronized vortex shedding (SVS), Vortex pairing and enveloping (VPE), and Vortex pairing splitting and enveloping (VPSE). These flow patterns can be transformed from one to another by changing the distance between the cylinders, the angle of incidence, or Re. SLR, IS and VI flow patterns appear in regimes with small angles of incidence (i.e., α ≤ 30° ) and hold only a single von Karman vortex shedding in a wake with one shedding frequency. SVS, VPE and VPSE flow patterns appear in regimes with large angles of incidence (i.e., 30° ≤ α ≤ 50° ) and present two synchronized von Karman vortices. Quantitative analyses and physical interpretation are also conducted to determine the generation mechanisms of the said flow patterns.

Response of a laminar separation bubble to impulsive forcing

The spatial and temporal response characteristics of a laminar separation bubble to impulsive forcing are investigated by means of time-resolved particle image velocimetry and linear stability theory. A two-dimensional impulsive disturbance is introduced with an alternating current dielectric barrier discharge plasma actuator, exciting pertinent instability modes and ensuring flow development under environmental disturbances. Phase-averaged velocity measurements are employed to analyse the effect of imposed disturbances at different amplitudes on the laminar separation bubble. The impulsive disturbance develops into a wave packet that causes rapid shrinkage of the bubble in both upstream and downstream directions. This is followed by bubble bursting, during which the bubble elongates significantly, while vortex shedding in the aft part ceases. Duration of recovery of the bubble to its unforced state is independent of the forcing amplitude. Quasi-steady linear stability analysis is performed at each individual phase, demonstrating reduction of growth rate and frequency of the most unstable modes with increasing forcing amplitude. Throughout the recovery, amplification rates are directly proportional to the shape factor. This indicates that bursting and flapping mechanisms are driven by altered stability characteristics due to variations in incoming disturbances. The emerging wave packet is characterised in terms of frequency, convective speed and growth rate, with remarkable agreement between linear stability theory predictions and measurements. The wave packet assumes a frequency close to the natural shedding frequency, while its convective speed remains invariant for all forcing amplitudes. The stability of the flow changes only when disturbances interact with the shear layer breakdown and reattachment processes, supporting the notion of a closed feedback loop. The results of this study shed light on the response of laminar separation bubbles to impulsive forcing, providing insight into the attendant changes of flow dynamics and the underlying stability mechanisms.

Wake stabilization mechanism of low-drag suppression devices for vortex-induced vibration

Inspired by the drag reduction mechanism in a tandem cylinder arrangement, the objective of this study is to numerically investigate the characteristic response of shear layer reattachment for the suppression of vortex-induced vibration (VIV). A new device termed as connected-C is introduced, whereby its geometry can be constructed by attaching a C-shaped foil at the end of a splitter plate behind a circular cylinder. We investigate the significance of each part of the connected-C device and compare the proposed device with conventional fairings, the splitter plate, and one of its variant disconnected-C device without the connector plate between the main cylinder and the C-shaped foil. To simplify the numerical study and to concentrate on the wake stabilization mechanism, the cylinder-device systems are only allowed to move in the transverse direction. There is no relative rotation between the cylinder and the attached devices. Two- and three-dimensional studies are carried out at low Reynolds number Re=100 and subcritical Reynolds numbers Re∈[6150,7400]. We observe the counter-rotating pair of recirculations and the shear layer reattachment for the vibrating connected-C device. These flow characteristics of the connected-C device result into a similar VIV performance as the fairing with respect to the suppression of VIV. Similar to fairings, there is a net reduction in the total drag force exerted over the combined cylinder-device system. Based on the results from the low Reynolds number study, while the presence of C-shaped device is essential to prevent the occurrence of galloping at high reduced velocity, the presence of connector plate is not necessary to suppress VIV.

Separated flow over blunt plates with different chord-to-thickness ratios: Unsteady behaviors and wall-pressure fluctuations

This study presented an extensive analysis of the influence of chord-to-thickness ratios on unsteady behaviors of the separated flow and their coupling with wall-pressure fluctuations at the Reynolds number ReD=1.58×104. Three systems with different chord-to-thickness ratios were chosen for comparison: L/D=3.0, 6.0, and 9.0. The separated flow fields and wall-pressure fluctuations were measured using planar Particle Image Velocimetry (PIV) operating at 1Hz and a microphone array sampling at 1kHz, respectively. The distributions of the turbulent kinetic energy and the reverse-flow intermittency indicated that the shortest plate (L/D=3.0) was subjected to full coverage of the separated flow. In the moderate system (L/D=6.0), the time-averaged reattachment station was determined to be x/D=4.75, while in 7% of all realizations the fluid close to the trailing edge surface remained slightly reversed due to the unsteady reattachment of the separated shear layer to the trailing edge and to irregular trailing edge vortex shedding. However, the separated flow always reattached on the plate’s surface in the longest system L/D=9.0. The measured wall-pressure fluctuations demonstrated combinative influences of hydrodynamic pressure of the large-scale vortical structures above the plate and acoustic excitation propagated by the energetic unsteady wake. This resulted in the salient phase changeover characteristics of the correlative patterns near x/D=1.5 in the shortest system, L/D=3.0. Close to the leading edge, the wall-pressure fluctuations in the L/D=6.0 system were over-exposed to acoustic excitation propagated by the unsteady wake behind the trailing edge, while in the longest system, L/D=9.0, such influences were relatively attenuated. Proper Orthogonal Decomposition (POD) analysis was performed on the down-sampled wall-pressure fluctuations paired with the flow fields, resulting in a series of energetic flow structures and their corresponding wall-pressure fluctuation profiles. Subsequently, the conditional averaging technique was applied to determine the relationship between the spatial characteristics of the large-scale vortical structures and profiles of the wall-pressure fluctuations.

Flow around two non-parallel tandem cylinders

An experimental investigation at a Reynolds number (Re) of 5.6 × 104 is carried out to understand fluid dynamics associated with two nonparallel cylinders in a tandem configuration. The upstream and downstream cylinders of identical diameter D are oppositely rotated by 7.5° measured from the normal to free stream direction, which leads to an included angle of 15° between the cylinders. Measurements of Strouhal number (St) and time-mean and instantaneous flow fields are conducted to explore the flow structure variation in the spanwise direction at different spacing ratio L* (=L/D = 1–4.05, where L is the cylinder center-to-center spacing). Three distinct flows are identified, namely, alternating reattachment flow (regime I, 1 ≤ L* < 2.15), bi-stable flow (regime II, 2.15 ≤ L* ≤ 3.1), and coshedding flow (regime III, 3.1 < L* ≤ 4.05). Based on shear layer reattachment and its influence on quasi-steady vortex in the gap and wake regions, regime I is further divided into two subregimes, IA and IB. The three flow regimes are totally different from those for parallel cylinders. A spiral vortex forming in the gap that varies along the cylinder span is responsible for making the difference. Due to the intermittent presence of reattachment and coshedding flows, jump in St prevails for 3.9 < L* < 4.1 for the parallel cylinders and 2.15 ≤ L* ≤ 3.1 for nonparallel cylinders, the range being remarkably long for the latter. Besides this, another jump in St associated with the coshedding flow at L* = 2.5 in regime II for nonparallel cylinders is revealed, which does not occur for parallel cylinders. A wake-flow bifurcation at L* = 2.5 is responsible for the jump, separating the wake flow turning up (L* < 2.5) and turning down (L* > 2.5). The independence principle for nonparallel cylinders is not valid for the flows in regimes I and II but is valid for the flow in regime III, while that for parallel cylinders is valid for all regimes.

Numerical investigation of flow around square cylinder with an upstream control plate at low Reynolds numbers in tandem

The study focuses on the effect of gap spacing (g) ranging g = 1–7, Reynolds numbers (Re) ranging Re = 80–200 and the size of the control plate (w) varied from 0.1d to 1d (where d is the size of the main cylinder) on the flow around a square cylinder with an upstream control plate. Two-dimensional multiple-relaxation-time lattice Boltzmann method is used to find the optimum condition, where the maximum reduction in drag force and suppression of vortex shedding occurs. It is observed that the drag is reduced significantly and the fluctuating lift is also suppressed. The detailed wake structure mechanism within gap spacing and near wake vortex structures around and behind the main square cylinder in the presence of the control plate are studied and compared with a plain square cylinder. In this study, the optimum conditions for maximum drag reduction in terms of control plate width, gap spacings and Reynolds numbers are found. The single bluff body, shear layer reattachment, critical flow, two-row single bluff body, fully developed two-row vortex shedding and quasi-steady-flow patterns are found. The time-trace analysis of drag and lift coefficients, power spectra analysis of lift coefficients, variation in force statistics and inclination angles are discussed in detail for all observed flow patterns. The maximum reductions on the drag force are 99.82, 99.8, 99.8, 99.9, 99.98, 100.4, 100.1, 100.05 and 100.1% for Re = 80, 100, 120, 140, 150, 160, 180, 190 and 200, respectively.

Influence of the bluff body shear layers on the wake of a square prism in a turbulent flow

The influence of freestream turbulence on the transitional characteristics of a 2D square prism shear layer was considered experimentally. Bypass transition was observed to cause intermittent shear layer reattachment; this event has a cascading influence downstream, altering the formation of the von-K\'arm\'an street.

The wake of two staggered square cylinders

This work aims to provide a systematic experimental study of the wake behind two staggered square cylinders at a Reynolds number$Re=1.3\times 10^{4}$. Four distinct flow regimes, i.e. two single-street modes S-I and S-II and two double-street modes T-I and T-II, are identified based on extensive data, including Strouhal numbers$(St)$, flow structures and their downstream evolution. S-I, S-II and T-II are each further subdivided into two types. The flow characteristics in each regime are presented in terms of shear layer reattachment and impingement, vortex impingement, gap flow behaviour, interaction between cylinders and downstream evolution of the wake. A detailed discussion is made regarding how the physical aspects of the flow are connected to the initial conditions and the$St$number.

Numerical characterization of three-dimensional bluff body shear layer behaviour

Three-dimensional bluff body aerodynamics are pertinent across a broad range of engineering disciplines. In three-dimensional bluff body flows, shear layer behaviour has a primary influence on the surface pressure distributions and, therefore, the integrated forces and moments. There currently exists a significant gap in understanding of the flow around canonical three-dimensional bluff bodies such as rectangular prisms and short circular cylinders. High-fidelity numerical experiments using a hybrid turbulence closure that resolves large eddies in separated wakes close this gap and provide new insights into the unsteady behaviour of these bodies. A time-averaging technique that captures the mean shear layer behaviours in these unsteady turbulent flows is developed, and empirical characterizations are developed for important quantities, including the shear layer reattachment distance, the separation bubble pressure, the maximum reattachment pressure, and the stagnation point location. Many of these quantities are found to exhibit a universal behaviour that varies only with the incidence angle and face shape (flat or curved) when an appropriate normalization is applied.

A proper orthogonal decomposition study on the unsteady flow behaviour of a hydrofoil with leading-edge tubercles

Flow behaviour of a hydrofoil with leading-edge tubercles was experimentally studied using particle image velocimetry technique at a Reynolds number of Re=1.4×104. Four angles-of-attack of α=0°, 10°, 15° and 20° were considered. Proper orthogonal decomposition (POD) technique was used to analyse the instantaneous velocity fields and derive information on the variation of corresponding coefficients and flow energy content. Results show that tubercles can alter the vortical behaviour and lead to increased flow unsteadiness. At lower angles-of-attack, the flow behaviour for the baseline hydrofoil is dominated by wake vortex-shedding, which represents a significant percentage of flow energy content. In contrast, separated shear layer and flow reattachment become the dominant flow behaviour for the hydrofoil with tubercles. Higher order POD modes 4 and 5 are associated with coherent vortex convection in the shear layer, which is similar to that of the baseline hydrofoil at higher angles-of-attack. At α=15°, POD modes 2 and 3 along the tubercle trough and peak show coherent vortex-shedding in the flow separation region. However, unstable flow separations are observed along the Mid-plane between the peaks and troughs, as a result of the separated shear layers associated with POD modes 1 and 2.

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.

Reattachment heating upstream of short compression ramps in hypersonic flow

Hypersonic shock-wave/boundary-layer interactions with separation induce unsteady thermal loads of particularly high intensity in flow reattachment regions. Building on earlier semi-empirical correlations, the maximum heat transfer rates upstream of short compression ramp obstacles of angles $$15^{\circ }\leqslant \theta \leqslant 135^{\circ }$$ are here discretised based on time-dependent experimental measurements to develop insight into their transient nature ( $$M_{e}$$ = 8.2–12.3, $$Re_h= 0.17\times 10^{5}$$ – $$0.47\times 10^{5}$$ ). Interactions with an incoming laminar boundary layer experience transition at separation, with heat transfer oscillating between laminar and turbulent levels exceeding slightly those in fully turbulent interactions. Peak heat transfer rates are strongly influenced by the stagnation of the flow upon reattachment close ahead of obstacles and increase with ramp angle all the way up to $$\theta =135^{\circ }$$ , whereby rates well over two orders of magnitude above the undisturbed laminar levels are intermittently measured ( $$q'_\mathrm{max}>10^2q_{u,L}$$ ). Bearing in mind the varying degrees of strength in the competing effect between the inviscid and viscous terms—namely the square of the hypersonic similarity parameter $$(M\theta )^2$$ for strong interactions and the viscous interaction parameter $$\bar{\chi }$$ (primarily a function of Re and M)—the two physical factors that appear to most globally encompass the effects of peak heating for blunt ramps ( $$\theta \geqslant 45^{\circ }$$ ) are deflection angle and stagnation heat transfer, so that this may be fundamentally expressed as $$q'_\mathrm{max}\propto {q_{o,2D}}$$ $$\theta ^2$$ with further parameters in turn influencing the interaction to a lesser extent. The dominant effect of deflection angle is restricted to short obstacle heights, where the rapid expansion at the top edge of the obstacle influences the relaxation region just downstream of reattachment and leads to an upstream displacement of the separation front. The extreme heating rates result from the strengthening of the reattaching shear layer with the increase in separation length for higher deflection angle.

Experimental investigation of flowfield over an iced aerofoil

ABSTRACTWind-tunnel measurements were used to study the characteristics of the unsteady separation bubbles on a NACA 0015 aerofoil with simulated two-dimensional leading-edge glaze ice accretions. The unsteadiness present in the iced-aerofoil flowfield was determined using measurements of the time-dependent aerofoil surface pressure distribution at Reynolds number of 1.0 × 106. Additionally, the unsteady flow features were investigated through the power spectrum of the stream-wise velocity fluctuations using a hot-wire anemometry. The results showed that the highest value of root-mean-square fluctuation of the surface pressure consistently occurred upstream of the mean shear-layer reattachment location. Spectral analysis of stream-wise velocity fluctuations near reattachment location revealed evidence of the regular frequency at Strouhal numbers of 0.5-0.63. Moreover, the low-frequency oscillations associated with shear-layer flapping was also identified in the wake velocity spectra on the order of 10 Hz that resulted in Strouhal numbers of 0.0186-021.

Assessment of 3D-RANS models for the simulation of topographically forced shallow flows

Abstract In this work the performance of Reynolds Averaged Navier-Stokes (RANS) simulations to predict the flow structure developed by the presence of a sidewall obstacle in a uniform open-channel shallow flow is discussed. The tested geometry was selected due to its important role in several fluvial applications, such as the control of riverbank erosion and the creation of improved ecological conditions in river restoration applications. The results are compared against experimental laboratory velocity fields obtained after Large Scale Particle Image Velocimetry (LSPIV) measurements. It is shown that the length of reattachment of the separated shear layer generated by the obstacle is well predicted by a Reynolds Stress Model, while classical two-equation models show important limitations. All the performed RANS simulations are unable to properly predict the formation of a secondary gyre region, which develops immediately downstream the obstacle.

Experimental Study on Vortex Shedding and Sound Radiation from a Rectangular Cylinder at Low Mach Numbers

Sound radiation from a rectangular cylinder, the cross-sectional aspect ratio (AR) of which varied from 0.3 to 4, was studied experimentally at Reynolds numbers of 7.5 × 103 to 1.4 × 105 and Mach numbers below 0.16. In addition to measurements of the lift-dipole sound, detailed flow fields around the rectangular cylinder were examined by means of PIV to better understand the dependence of sound radiation on the AR and Reynolds number as well as the vortex-cylinder interaction associated with sound generation. The mean square sound pressure was proportional to U6∞ St2, with almost the same factor of proportionality for aspect ratios larger than 0.5 in the Reynolds number range examined in spite of the fact that the spanwise correlation length and magnitude of the lift fluctuations were highly dependent on the AR. When vortex shedding at the trailing-edge was locked in phase with the leading-edge-generated vortices, which occurred for AR > 3, the shed vortices were quite two-dimensional, and correspondingly, the radiated sound exhibited a very sharp spectral peak like a line spectrum; the SPL was as strong as that for AR = 0.75, for which the maximum SPL was observed. Reynolds number effects on both SPL and the Strouhal number were weak except around the aspect ratio where reattachment of the separated leading-edge shear layer occurred.

Review of backward-facing step flow and separation reduction

This article reviews the development of research work on backward-facing step flow and separation reduction techniques. Firstly, the effects of inflow conditions and geometry parameters on the bubble length, pressure coefficient, skin friction coefficient, velocity profiles and profiles of Reynolds stresses of the backward-facing step flow are identified. Secondly, we find that the unsteadiness of backward-facing step flow is dominated by shear layer mode and shedding mode, the former including Kelvin-Helmholtz instability, vortex pairing and step mode. Furthermore, the lower-frequency characteristics are consist of shear layer flapping and reattachment oscillating, resulting from the interaction between the coherent structures in the shear layer and the wall. Finally, the flow control practice on the backward-facing step flow has also been discussed. It is found that the recirculating bubble length can be significantly reduced with appropriate excitation frequency in the harmonic excitation practice. The optimum excitation frequency, Sth, is of about 0.2, and a physical explanation is given by the authors.