scholarly journals Experimental Investigation of Coherent Vortex Structures in a Backward-Facing Step Flow

Water ◽  
2019 ◽  
Vol 11 (12) ◽  
pp. 2629
Author(s):  
Fangfang Wang ◽  
Ang Gao ◽  
Shiqiang Wu ◽  
Senlin Zhu ◽  
Jiangyu Dai ◽  
...  
Keyword(s):  
Shear Layer ◽  
Shape Change ◽  
Flow Direction ◽  
Critical Distance ◽  
Vortex Structures ◽  
Free Shear Layer ◽  
Backward Facing Step ◽  
Step Flow ◽  
Image Velocimetry ◽  
Coherent Vortex

Coherent vortex structures (CVS) are discovered for more than half a century, and they are believed to play a significant role in turbulence especially for separated flows. An experimental study is conducted for a pressured backward-facing step flow with Reynolds number (Reh) being 4400 and 9000. A synchronized particle image velocimetry (PIV) system is developed for measurement of a wider range of velocity fields with high resolution. The CVS are proved to exist in the separation-reattachment process. For their temporal evolution, a life cycle is proposed that vortices form in the free shear layer, develop with pairings and divisions and finally shed at the reattachment zone, and sometimes new vortical structures are restructured with recovery of flow pattern. The CVS favor the free shear layer with frequent pairings and divisions particularly at the developing stage around x/h = 2~5 (x: distance from the step in flow direction, h: step height), which may contribute to the high turbulent intensity and shear stress there. A critical distance is believed to exist among CVS, which affects their amalgamation (pairing) and division events. Statistics show that the CVS are well organized in spatial distribution and show specific local features with the flow structures distinguished. The streamwise and vertical diameters (Dx and Dy) and width to height ratio (Dx/Dy) all obey to the lognormal distribution. With increase of Reh from 4400 to 9000, Dx decreases and Dy increases, but the mean diameter (D=0.5 × (Dx + Dy)) keeps around (0.28~0.29) h. As the increase of Reh, the vortical shape change toward a uniform condition, which may be contributed by enhancement of the shear intensity.

Active Control of a Backward Facing Step Flow With Plasma Actuators

2014 ◽  
Vol 136 (12) ◽  
Author(s):  
Juan D'Adamo ◽  
Roberto Sosa ◽  
Guillermo Artana
Keyword(s):  
Shear Layer ◽  
Active Control ◽  
Flow Behavior ◽  
Plasma Actuators ◽  
Backward Facing Step ◽  
Step Flow ◽  
Separated Shear Layer ◽  
Image Velocimetry ◽  
Layer Flow ◽  
Electrohydrodynamic Actuator

Active control over a backward facing step flow is studied experimentally by means of plasma based devices. The Reynolds number based on the step height h is 1520. An electrohydrodynamic actuator (EHD), dielectric barrier discharge (DBD) type, is flush mounted to the step wall. The DBD configuration adds momentum locally, normal to the separated shear layer, thus producing strong modifications downstream. The actuation is periodic and its frequency and amplitude are scrutinized to characterize the flow behavior under forcing. Measures of velocity fields for these flows are obtained from particle image velocimetry (PIV). As reported by previous works, the reattachment length shows an important reduction for an optimum forcing frequency. This value closely matches the shear layer flow natural frequency. On the other hand, the flow is less sensitive to the forcing amplitude though the analysis allows us to optimize the actuation in order to save power consumption.

scholarly journals Instability patterns between counter-rotating disks

2003 ◽  
Vol 10 (3) ◽  
pp. 281-288 ◽  
Author(s):  
F. Moisy ◽  
T. Pasutto ◽  
M. Rabaud
Keyword(s):  
Aspect Ratio ◽  
Shear Layer ◽  
Particle Image ◽  
Shear Layer Instability ◽  
Free Shear Layer ◽  
Rotating Disks ◽  
Height Ratio ◽  
Aspect Ratios ◽  
Image Velocimetry ◽  
Local Reynolds Number

Abstract. The instability patterns in the flow between counter-rotating disks (radius to height ratio R/h from 3.8 to 20.9) are investigated experimentally by means of visualization and Particle Image Velocimetry. We restrict ourselves to the situation where the boundary layers remain stable, focusing on the shear layer instability that occurs only in the counter-rotating regime. The associated pattern is a combination of a circular chain of vortices, as observed by Lopez et al. (2002) at low aspect ratio, surrounded by a set of spiral arms, first described by Gauthier et al. (2002) in the case of high aspect ratio. Stability curve and critical modes are measured for the whole range of aspect ratios. From the measurement of a local Reynolds number based on the shear layer thickness, evidence is given that a free shear layer instability, with only weak curvature effect, is responsible for the observed patterns. Accordingly, the number of vortices is shown to scale as the shear layer radius, which results from the competition between the centrifugal effects of each disk.

scholarly journals Effect of Piezo-Embedded Inverted Flag in Free Shear Layer Wake

Aerospace ◽  
2019 ◽  
Vol 6 (3) ◽  
pp. 33
Author(s):  
Sidaard Gunasekaran ◽  
Grant Ross
Keyword(s):  
Shear Layer ◽  
Oscillation Frequency ◽  
Energy Harvester ◽  
Oscillation Amplitude ◽  
Angle Of Attack ◽  
Lower Surface ◽  
Free Shear Layer ◽  
Polyvinylidene Difluoride ◽  
Image Velocimetry ◽  
Naca 0012

The use of flexible inverted piezo embedded Polyvinylidene Difluoride (PVDF) as a simultaneous energy harvester and as a wake sensor is explored. The oscillation amplitude (characterized by voltage output) and oscillation frequency of the piezo-embedded PDVF was quantified in the wake of a 2D NACA 0012 model and SD7003 model at a Reynolds number of 100,000 and 67,000, respectively. The performance of the sensor was also quantified in the freestream without the presence of the wing. In order to quantify the sensor response to angle of attack and downstream distance, the amplitude and frequency of oscillations were recorded in the wing wake. Increase in angle of attack of the wing resulted in increase in oscillation frequency and amplitude of the PVDF. The results also indicated that the inverted flag configuration performed better in the wake under unsteady conditions when compared to freestream conditions. The results from Particle Image Velocimetry indicated that the wake signature was not affected by the presence of the PVDF in the wake. The root mean square voltage contours in the wake of SD7003 airfoil show remarkable free shear layer wake features such as upper and lower surface stratification and downwash angle which shows the sensitivity of the sensor to the unsteadiness in the wake. The capability of this device to act as a potential energy harvester and as a sensor has serious implications in extending the mission capabilities of small UAVs.

The Response of Injection of Vortex Generator Jets to a Backward Facing Step Flow

2003 ◽  
Author(s):  
Koichi Yamagata ◽  
Manabu Saito ◽  
Tadashi Morioka ◽  
Shinji Honami
Keyword(s):  
Shear Layer ◽  
Large Scale ◽  
Flow Behavior ◽  
Velocity Ratio ◽  
Vortex Generator ◽  
Step Response ◽  
Backward Facing Step ◽  
Longitudinal Vortices ◽  
Step Flow ◽  
Vortex Generator Jets

In this paper, the flow behavior of a reattachment process over a backward facing step flow is reported. The reattachment process is controlled by injection of vortex generator jets. The injection of jets upstream of the step produces the co-rotating longitudinal vortices in a separating shear layer. The experiment of the step response of the injection jet is also conducted in order to investigate the evolution process of the longitudinal vortices. A large scale of primary and counter vortices are observed, when the velocity ratio of the free stream to injected jet is 6. The detailed structure of the longitudinal vortices is clarified. The remarkable effect of the vortices on the separating shear layer downstream of the step is observed.

scholarly journals Direct numerical simulation of turbulent flow over a backward-facing step

1997 ◽  
Vol 330 ◽  
pp. 349-374 ◽  
Author(s):  
HUNG LE ◽  
PARVIZ MOIN ◽  
JOHN KIM
Keyword(s):  
Reynolds Number ◽  
Turbulent Flow ◽  
Shear Layer ◽  
Stokes Equations ◽  
Recirculation Region ◽  
Wall Region ◽  
Free Shear Layer ◽  
Backward Facing Step ◽  
Temporal Behaviour ◽  
Log Law

Turbulent flow over a backward-facing step is studied by direct numerical solution of the Navier–Stokes equations. The simulation was conducted at a Reynolds number of 5100 based on the step height h and inlet free-stream velocity, and an expansion ratio of 1.20. Temporal behaviour of spanwise-averaged pressure fluctuation contours and reattachment length show evidence of an approximate periodic behaviour of the free shear layer with a Strouhal number of 0.06. The instantaneous velocity fields indicate that the reattachment location varies in the spanwise direction, and oscillates about a mean value of 6.28h. Statistical results show excellent agreement with experimental data by Jovic & Driver (1994). Of interest are two observations not previously reported for the backward-facing step flow: (a) at the relatively low Reynolds number considered, large negative skin friction is seen in the recirculation region; the peak |Cf| is about 2.5 times the value measured in experiments at high Reynolds numbers; (b) the velocity profiles in the recovery region fall below the universal log-law. The deviation of the velocity profile from the log-law indicates that the turbulent boundary layer is not fully recovered at 20 step heights behind the separation.The budgets of all Reynolds stress components have been computed. The turbulent kinetic energy budget in the recirculation region is similar to that of a turbulent mixing layer. The turbulent transport term makes a significant contribution to the budget and the peak dissipation is about 60% of the peak production. The velocity–pressure gradient correlation and viscous diffusion are negligible in the shear layer, but both are significant in the near-wall region. This trend is seen throughout the recirculation and reattachment region. In the recovery region, the budgets show that effects of the free shear layer are still present.

Starting Flow and Heat Transfer Downstream of a Backward-Facing Step

1991 ◽  
Vol 113 (3) ◽  
pp. 583-589 ◽  
Author(s):  
F. K. Tsou ◽  
Shih-Jiun Chen ◽  
Win Aung
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Steady State ◽  
Shear Layer ◽  
Characteristic Time ◽  
Free Shear Layer ◽  
Hot Wire ◽  
Backward Facing Step ◽  
Flow And Heat Transfer ◽  
Hot Wire Anemometry

Experiments are performed to study the starting process of heat transfer downstream of a backward-facing step. A Ludwieg tube wind tunnel is employed to produce the incompressible flow, which accelerates from a zero velocity to a steady state value with an accelerating period of 7 ms and a steady-state period of 12 ms. Hot-wire anemometry and heat flux gages are used to measure the flow and heat transfer history, respectively. The onset of transition in the free shear layer shows that the disturbance originates from the top corner of the step, then propagating to the free stream. The velocity and turbulence profiles in the free shear layer reach steady-state values after the leading edge disturbance traverses to the measurement locations. In regions upstream and far downstream of the step, heat flux history data suggest the transformation of the flow from laminar to transitional and finally to turbulent flow. Hot-wire anemometry measurements indicate high-frequency turbulence with a short characteristic time. In the recirculating region, however, a longer characteristic time is observed because of the existence of large-scale eddies. The dimensionless reattachment length (xr/H) is shown to increase with time from the bottom corner (xr/H = 0) in the laminar regime to a maximum value of 13.6 in the transitional regime, and decreases to a constant values of 7.6 in the turbulent regime. The steady-state flow field and heat transfer compare favorably with existing data obtained using steady-state techniques.

Investigation of Unsteady Wake-Separated Boundary Layer Interaction Using Particle-Image-Velocimetry

2007 ◽  
Author(s):  
Og˘uz Uzol ◽  
Xue Feng Zhang ◽  
Alex Cranstone ◽  
Howard Hodson
Keyword(s):  
Boundary Layer ◽  
Particle Image Velocimetry ◽  
Shear Layer ◽  
Particle Image ◽  
Separation Bubble ◽  
Free Shear Layer ◽  
Unsteady Boundary Layer ◽  
Layer Interaction ◽  
Image Velocimetry ◽  
Boundary Layer Interaction

The current paper presents an experimental investigation of the interaction between unsteady wakes and the separated boundary layer on the suction side of an ultra-high-lift low-pressure turbine airfoil. Two-dimensional Particle Image Velocimetry (PIV) measurements of the unsteady boundary layer over the T106C LP turbine profile were performed in a low speed linear cascade facility, at selected phases of passing wakes. The wakes are created by moving cylindrical bars across the inlet of the test section. Various phenomena were investigated such as separation and transition characteristics, vortex structures within the unsteady boundary layer, their interaction and effects on the transition process, the corresponding vortex shedding mechanisms and the unsteady behaviour of the separation bubble due to the wake- boundary layer interaction. The current measurements suggest that rollup vortices are generated as the wake approaches the separated shear layer on the suction surface before the wake centerline starts impinging on the blade. At this instant, the bubble is sufficiently high for the free shear layer to roll up into a vortex and the incoming wake is highly distorted (strained) due to the velocity field within the blade passage, and the turbulence distribution within the wake is not symmetrical. Vortices within the boundary layer, identified using the swirl strength distributions calculated from the eigenvalues of the velocity gradient tensor, seem to be coalescing and forming bigger scale structures, which in turn break up into smaller but higher swirl strength eddies. In between the passing wakes, the separation bubble grows in both in height and length, trying to return to its steady state shape.

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