A Lagrangian criterion of unsteady flow separation for two-dimensional periodic flows

The present paper proposes a Lagrangian criterion of unsteady flow separation for two-dimensional periodic flows based on the principle of weighted averaging zero skin-friction given by Haller （HALLER, G. Exact theory of unsteady separation for two-dimensional flows. Journal of Fluid Mechanics, 512, 257-311 （2004））. By analyzing the distribution of the finite-time Lyapunov exponent （FTLE） along the no-slip wall, it can be found that the periodic separation takes place at the point of the zero FTLE. This new criterion is verified with an analytical solution of the separation bubble and a numerical simulation of lid-driven cavity flows.

Analysis and experimental study on resistance-increasing behavior of composite high efficiency autonomous inflow control device

Bottom water coning is the main reason to reduce the recovery of horizontal bottom water reservoir. By water coning, we mean the oil-water interface changes from a horizontal state to a mound-shaped cone and breaks through to the wellbore. Autonomous inflow control device(AICD) is an important instrument maintain normal production after bottom water coning, however, the resistance increasing ability of the swirl type AICD is insufficient at present, which seriously affects the water control effect. Aiming this problem, this paper designs a multi-stage resistance-increasing and composite type AICD. The separation mechanism of oil-water two phases in this structure, the resistance form of oil-water single phase and the resistance-increasing principle of water phase are analyzed. Establishing the dual-phase multi-stage separation and resistance-increasing model, and verified by measuring the throttling pressure drop and oil-water volume fraction of the AICD, it is found that the composite type AICD has the effect of ICD and AICD at the same time, which can balance the production rate of each well section at the initial stage of production, delay the occurrence of bottom water coning. In the middle and later stages of production, water-blocking can be effectively increased to achieve water control and stable production.After structural sensitivity analysis, the influence law of various structural parameters on the water control performance of composite AICD was obtained. The simulation calculation results show that,compared with the existing swirl type AICD, composite AICD has higher sensitivity to moisture content,the water phase throttling pressure drop is increased by 4.5 times on average. The composite AICD is suitable for the entire stage of horizontal well production.

Analysis of flow separation control using nanosecond-pulse discharge plasma actuators on a flying wing

Dielectric barrier discharge （DBD） plasma is one of most promising flow control method for its several advantages. The present work investigates the control authority of nanosecond pulse DBD plasma actuators on a flying wing model＇s aerodynamic characteristics. The aerodynamic forces and moments are studied by means of experiment and numerical simulation. The numerical simulation results are in good agreement with experiment results. Both results indicate that the NS-DBD plasma actuators have negligible effect on aerodynamic forces and moment at the angles of attack smaller than 16-. However, significant changes can be achieved with actuation when the model＇s angle of attack is larger than 16° where the flow separation occurs. The spatial flow field structure results from numerical simulation suggest that the volumetric heat produced by NS-DBD plasma actuator changes the local temperature and density and induces several vortex structures, which strengthen the mixing of the shear layer with the main flow and delay separation or even reattach the separated flow.

Comparative assessment of SAS and DES turbulence modeling for massively separated flows

Numerical studies of the flow past a circular cylinder at Reynolds number 1.4 × 105 and NACA0021 airfoil at the angle of attack 60° have been carried out by scale-adaptive simulation （SAS） and detached eddy simu- lation （DES）, in comparison with the existing experimental data. The new version of the model developed by Egorov and Menter is assessed, and advantages and disadvantages of the SAS simulation are analyzed in detail to provide guidance for industrial application in the future. Moreover, the mechanism of the scale-adaptive characteristics in separated regions is discussed, which is obscure in previous analyses. It is con- cluded that： the mean flow properties satisfactorily agree with the experimental results for the SAS simulation, although the prediction of the second order turbulent statistics in the near wake region is just reasonable. The SAS model can produce a larger magnitude of the turbulent kinetic energy in the recir- culation bubble, and, consequently, a smaller recirculation region and a more rapid recovery of the mean velocity out- side the recirculation region than the DES approach with the same grid resolution. The vortex shedding is slightly less irregular with the SAS model than with the DES approach, probably due to the higher dissipation of the SAS simulation under the condition of the coarse mesh.

Flow Separation and Vortex Dynamics in Waves Propagating over A Submerged Quartercircular Breakwater

The interactions of cnoidal waves with a submerged quartercircular breakwater are investigated by a ReynoldsAveraged Navier–Stokes(RANS) flow solver with a Volume of Fluid(VOF) surface capturing scheme(RANSVOF) model. The vertical variation of the instantaneous velocity indicates that flow separation occurs at the boundary layer near the breakwater. The temporal evolution of the velocity and vorticity fields demonstrates vortex generation and shedding around the submerged quartercircular breakwater due to the flow separation. An empirical relationship between the vortex intensity and a few hydrodynamic parameters is proposed based on parametric analysis. In addition, the instantaneous and time-averaged vorticity fields reveal a pair of vortices of opposite signs at the breakwater which are expected to have significant effect on sediment entrainment, suspension, and transportation,therefore, scour on the leeside of the breakwater.

Applications of URANS on predicting unsteady turbulent separated flows

Accurate prediction of unsteady separated turbulent flows remains one of the toughest tasks and a practi cal challenge for turbulence modeling. In this paper, a 2D flow past a circular cylinder at Reynolds number 3,900 is numerically investigated by using the technique of unsteady RANS （URANS）. Some typical linear and nonlinear eddy viscosity turbulence models （LEVM and NLEVM） and a quadratic explicit algebraic stress model （EASM） are evaluated. Numerical results have shown that a high-performance cubic NLEVM, such as CLS, are superior to the others in simulating turbulent separated flows with unsteady vortex shedding.

FLUID FLOW SEPARATION CHARACTER ON NOVEL HYBRID JOURNAL BEARING

The influence of the structure and running parameters of a novel spiral oil wedge hybrid journal bearing on the fluid flow trace is investigated. The governing equation of the flow trace of lubricant is set up, and the simulation is carried out by using finite difference method. The results show that the lubricant flow status and end leakage quantity are greatly influenced by spiral angle,and that the rotating speed has little influence on the flow status. With advisable geometry design, the separation of lubricant between different oil wedges can be obtained, which can decrease the temperature rise effectively.

Flow separation control over an airfoil using continuous alternating current plasma actuator

The flow separation control over an NACA 0015 airfoil using continuous alternating current(AC)dielectric barrier discharge(DBD)plasma actuator is investigated experimentally and numerically.This work is intended to report some observations made from our experiment,to which little attention is paid in the previous studies,but which is thought to be important to the understanding of control of complex flow separation with AC DBD.To this end,the response of separated flow to AC plasma actuation is visualized through the time-resolved particle image velocimetry(PIV)measurement,whereas numerical simulation is carried out to complement the experiment.The flow control process at chord-based Reynolds number(Re)of 3.31×105 is investigated.It is found that the response of external flow to plasma forcing is delayed for up to tens of milliseconds and the delay time increases with angle of attack increasing.Also observed is that at the intermediate angle of attack near stall,the forced flow features a well re-organized flow pattern.However,for airfoil at high post-stall angle of attack,the already well suppressed flow field can recover to the massively separated flow state and then reattach to airfoil surface with the flow pattern fluctuating between the two states in an irregular manner.This is contrary to one’s first thought that the forced flow at any angles of attack will become well organized and regular,and reflects the complexity of flow separation control.

Numerical simulation of the pulsing air separation field based on CFD

The flow field of pulsing air separation is normally in an unsteady turbulence state.With the application of the basic principles of multiphase turbulent flows,we established the dynamical computational model,which shows a remarkable variation of the unstable pulsing air flow field.CFD(computational fluid dynamics) was used to conduct the numerical simulation of the actual geometric model of the classifier.The inside velocity of the flowing fields was analyzed later.The simulation results indicate that the designed structure of the active pulsing air classifier provided a favorable environment for the separation of the particles with different physical characters by density.We shot the movement behaviors of the typical tracer grains in the active pulsing flow field using a high speed dynamic camera.The displacement and velocity curves of the particles in the continuous impulse periods were then analyzed.The experimental results indicate that the effective separation by density of the particles with the same settling velocity and different ranges of the density and particle size can be achieved in the active pulsing airflow field.The experimental results provide an agreement with the simulation results.

Experimental Investigation of Flow Separation Control Using Dielectric Barrier Discharge Plasma Actuators

Influence of plasma actuators as a flow separation control device was investigated experimentally. Hump model was used to demonstrate the effect of plasma actuators on external flow separation, while for internal flow separation a set of compressor cascade was adopted. In order to investigate the modification of the flow structure by the plasma actuator, the flow field was examined non-intrusively by particle image velocimetry measurements in the hump model experiment and by a hot film probe in the compressor cascade experiment. The results showed that the plasma actuator could be effective in controlling the flow separation both over the hump and in the compressor cascade when the incoming velocity was low. As the incoming velocity increased, the plasma actuator was less effective. It is urgent to enhance the intensity of the plasma actuator for its better application. Methods to increase the intensity of plasma actuator were also studied.

Control of flow separation over a wing model with plasma synthetic jets

An array of 30 plasma synthetic jet actuators(PSJAs)is deployed using a modified multichannel discharge circuit to suppress the flow separation over a straight-wing model.The lift and drag of the wing model are measured by a force balance,and the velocity fields over the suction surface are captured by a particle imaging velocimetry system.Results show that the flow separation of the straight wing originates from the middle of the model and expands towards the wingtips as the angle of attack increases.The flow separation can be suppressed effectively by the PSJAs array.The best flow control effect is achieved at a dimensionless discharge frequency of F^+=1,with the peak lift coefficient increased by 10.5%and the stall angle postponed by 2°.To further optimize the power consumption of the PSJAs,the influence of the density of PSJAs on the flow control effect is investigated.A threshold of the density exits(with the spanwise spacing of PSJAs being 0.2 times of the chord length in the current research),below which the flow control effect starts to deteriorate remarkably.In addition,for comparison purposes,a dielectric barrier discharge(DBD)plasma actuator is installed at the same location of the PSJAs.At the same power consumption,4.9%increase of the peak lift coefficient is achieved by DBD,while that achieved by PSJAs reaches 5.6%.

Fluctuating Pressure Inside/Outside the Flow Separation Region in High Speed Flowfield

An experimental study was conducted on the interactions of shock wave/turbulence or laminar boundary layer caused by fin-type protuberance, as the lack of detailed understanding of fluctuating pressure loads inside and outside the laminar or turbulence boundary layer separation region in hypersonic flow. The changes of fluctuating pressure in separation region were focused on in this paper. The study shows that the existence of fin changes flowfiled on the plate significantly. The laminar boundary layer separation occurs earlier and the separation region is more extensive. Similar flow is observed between a couple of measurement points outside the laminar separation region. However, there are significant differences between the flow inside and outside the separation region. The level of fluctuating pressure of laminar boundary layer is smaller than that in turbulent case. Even so, in laminar case, the peak fluctuating pressure still reaches a high level. Therefore, the structural influence （damage and/or early fatigue） of fluctuating pressure loads caused by the laminar boundary layer separation should not be ignored.

INVESTIGATION OF PULSED-WIRE TECHNIQUE FOR WALL PARAMETERS MEASUREMENT IN SEPARATED FLOWS

A pulsed-wire wall probe measurement system was developed in this paper,which can be used for measuring wall Parameters in separated flow- The operating princi-ple was described and the way of probe calibration was given. Wall parameters of back-ward-facing and forward-facing step flow were measured,and the wall nows structure andcharacters were revealed.

Numerical study on flow separation control over NACA0015 aerofoil using electromagnetic fields

In this study, a flow solver was developed based on the governing RANS equations of compressible flows and was further extended to include the effects of electromagnetic forces namely Lorentz forces. Lorentz forces may be added as a source term in the governing fluid flow equations. Numerical studies were carried out for NACA0015 aerofoil at high angles of incidences from 15° to 30° and compared with some available cases of experimental and incompressible numerical solutions. The hydrodynamics performance was improved using a magnetic momentum coefficient of up to 0.048. The size of flow separation zone was decreased or completely eliminated by increasing this coefficient. The overall drag was not changed considerably, however the overall lift was increased up to 80 percent at stall angles.

THE EVOLUTION OF STARTING VORTEX AND SECONDARY SEPARATION ON AN ELLIPTIC CYLINDER

Flow visualization was used to investigate experimentally the evolution process from symmetrical shedding to staggered shedding of the starting vortex and the phenomenon of secondary separation on an elliptic cylinder at moderate Reynolds numbers.The vortex structure of the flow separation was studied.The temporal variation of separation angle and length of wake vortex were given. The photographs and experimental results provided basis for further investigation of the complicated feature of the starting process of unsteady separated flows around an elliptic cylinder.

SEPARATION CONTROL FOR THE OUTWING OF A STRAKE－WING BY ROTATING CONE PLACED NEAR THE LEADING EDGE

Based on the research result on the strake-wing, when the size of a strakeis not large, there is a separation zone near the leading edge of the outwing of thestrake-wing at middle angles of attack. So the idea on separation control by rotating acone placed near the leading edge is presented. The cone surface consists of the part ofthe wing. The effect of rotating the cone on aerodynamic characteristics of thestrake-wing is investigated. The results show that a rotating surface could play an important role in controlling the flow separation for a 3-dimensional wing. For example,the relative increment in maximum lift coefficient attains 30%. The separation zone issuppressed to a certain extent.

EXPERIMENTAL INVESTIGATION ON THE TURBULENT COHERENT STRUCTURES OF LAMINAR SEPARATION FLOW OVER A BACKWARD FACING STEP

The experimental investigation is conducted with LDV and hydrogen bubble technique in water flow. The shear layer thickness. the vorticity thickness. the maximulll value of turbulence intensities. the turbulent coherent structure. the variations of wall shear stress and the boundary layer shape factor are obtained. In the redevelopment region. the detailed analysis is first made for the streak structures in the near wall region and the turbulent boundary layer is formed at (x-xr) / h = 20.

NUMERICAL SOLUTION OF NAVIER-STOKES EQUATION OF UNSTEADY SEPARATED FLOWS DUE TO A SPOILER'S OSCILLATION

The finite difference method (FDM) is applied in the present paper to solve the unsteady NHS equations for incompressible fluids. ADI and SLOR methods are served for the vorticity equation and the Poisson equation for ψ respectively. The upwind scheme is used for the convective terms. The moving boundary conditions are specially treated, and the effects of outlet conditions on the flow field are abo examined. Numerical results obtained show that the spoiler's oscillation induces forming, growing and shedding of the vortices. The shedding frequency of vortices is equal to that of the spoiler's oscillation. The forced unsteady separated flows under the present investigation depend mainly on the reduced frequency. At low reduced frequency, the vortices shed from the spoiler interact weakly with each other, and move downstream at an almost uniform speed of 038 V∞. At high reduced frequency, the interaction between the adjacent vortices strengthens. They close up to and rotate around each other, and eventually, merge into one vortex.

Measurment of gas-liquid two-phase slug flow with a Venturi meter based on blind source separation

We propose a novel flow measurement method for gas–liquid two-phase slug flow by using the blind source separation technique. The flow measurement model is established based on the fluctuation characteristics of differential pressure(DP) signals measured from a Venturi meter. It is demonstrated that DP signals of two-phase flow are a linear mixture of DP signals of single phase fluids. The measurement model is a combination of throttle relationship and blind source separation model. In addition, we estimate the mixture matrix using the independent component analysis(ICA) technique. The mixture matrix could be described using the variances of two DP signals acquired from two Venturi meters. The validity of the proposed model was tested in the gas–liquid twophase flow loop facility. Experimental results showed that for most slug flow the relative error is within 10%.We also find that the mixture matrix is beneficial to investigate the flow mechanism of gas–liquid two-phase flow.

Use of Targeted Orographic Smoothing in Very High Resolution Simulations of a Downslope Windstorm and Rotor in a Sub-tropical Highland Location

Nested simulations of a downslope windstorm over Cangshan mountain,Yunnan,China,have been used to demonstrate a method of topographic smoothing that preserves a relatively large amount of terrain detail compared to typical smoothing procedures required for models with terrain-following grids to run stably.The simulations were carried out using the Met Office Unified Model(MetUM)to investigate downslope winds.The smoothing method seamlessly blends two terrain datasets to which uniform smoothing has been applied—one with a minimum of smoothing,the other smoothed more heavily to remove gradients that would cause model instabilities.The latter dataset dominates the blend where the steepest slopes exist,but this is localised and recedes outside these areas.As a result,increased detail is starkly apparent in depictions of flow simulated using the blend,compared to one using the default approach.This includes qualitative flow details that were absent in the latter,such as narrow shooting flows emerging from roughly 1-2 km wide leeside channels.Flow separation is more common due to steeper lee slopes.The use of targeted smoothing also results in increased lee side temporal variability at a given point during the windstorm,including over flat areas.Low-/high-pass filtering of the wind perturbation field reveals that relative spatial variability above 30 km in scale(reflecting the background flow)is similar whether or not targeting is used.Beneath this scale,when smoothing is targeted,relative flow variability decreases at the larger scales,and increases at lower scales.This seems linked to fast smaller scale flows disturbing more coherent flows(notably an along-valley current over Erhai Lake).Spatial variability of winds in the model is unsurprisingly weaker at key times than is observed across a local network sampling mesoscale variation,but results are compromised due to relatively few observation locations sampling the windstorm.Only when targeted smoothing is applied does the model capture the downslope windstorm's extension over the city of Dali at the mountain's foot,and the peak mean absolute wind.