Experimental Investigation of a Flapping Motion Downstream of a Backward Facing Step

2018 ◽  
Author(s):  
Zhuoyue Li ◽  
Nan Gao
Keyword(s):  
Low Frequency ◽  
Reynolds Numbers ◽  
Stereoscopic Particle Image Velocimetry ◽  
Spanwise Direction ◽  
Backward Facing Step ◽  
Freestream Velocity ◽  
Flapping Motion ◽  
Separated Shear Layer ◽  
Image Velocimetry ◽  
Shedding Frequency

The flow field downstream of a backward facing step with Reynolds numbers of 1050–3890 were studied using stereoscopic particle image velocimetry (PIV) with field-of-views perpendicular to the incoming flow. It was found that the separated shear layer underwent a flapping motion with a frequency fH/Uo ≲ 0.04, much smaller than the shedding frequency (fH/Uo ≈ 0.1) for Reynolds number larger than 2000. Here H is the step height and Uo is the freestream velocity. The low frequency flapping motion appeared to be two dimensional, i.e. the motion was in-phased in the spanwise direction. The cause of the flapping motion is still not clear.

scholarly journals Secondary flows and heat transfer in shallow flow around a cylinder: LES, PIV

2019 ◽  
Vol 196 ◽  
pp. 00028 ◽  
Author(s):  
Egor Palkin ◽  
Maxim Shestakov ◽  
Rustam Mullyadzhanov
Keyword(s):  
Heat Transfer ◽  
Skin Friction ◽  
Plane Channel ◽  
Low Frequency ◽  
Reynolds Numbers ◽  
Secondary Flows ◽  
Image Velocimetry ◽  
Large Eddy ◽  
Secondary Motion ◽  
Horseshoe Vortices

We report on Large-eddy simulations (LES) of flow around a short cylinder mounted in a narrow plane channel in a range of Reynolds numbers 1000, 2000, 3750 based on the bulk velocity of the flow and diameter of the cylinder supplemented with Particle image velocimetry (PIV) measurements for the highest considered Re. First two cases appear to be steady, however, for Re=3750 the flow becomes unsteady with the wake dominated by periodic vortex shedding. In front of the cylinder typical horseshoe vortices are identified intensifying the skin friction and heat transfer on the wall, while in the near wake we observe a quasiperiodic low-frequency secondary motion in the form of a pair of counterrotating eddies developing in the transverse direction. The Karman vortex street remains the dominant pattern, but further downstream from the cylinder the transport across the channel is associated with the secondary streamwise vortices, as also previously observed in slot jets. We observe their impact on heat transfer and skin friction on the wall of the channel.

scholarly journals Characteristics of helicopter engine exhaust through scaled experiments using stereoscopic particle image velocimetry

2020 ◽  
pp. 095441002096647
Author(s):  
Zhen Wei Teo ◽  
Wai Hou Wong ◽  
Zhi Wen Lee ◽  
Tze How New ◽  
Bing Feng Ng
Keyword(s):  
Particle Image Velocimetry ◽  
Particle Image ◽  
Vortex Pair ◽  
Stereoscopic Particle Image Velocimetry ◽  
Engine Exhaust ◽  
Freestream Velocity ◽  
Hot Air ◽  
Image Velocimetry ◽  
Air Blower ◽  
Counter Rotating Vortex Pair

Helicopter engines are often mounted atop the fuselage to keep the aircraft footprint small and optimal for operations. As a result, hot gases produced by the engines may inadvertently impinge upon the tail boom or dissipate inefficiently that compromises on operation safety. In this study, a scaled fuselage model with a hot air blower was used to simulate hot exhaust gases. The velocity field immediately outside the exhaust port was measured through stereoscopic particle image velocimetry to capture the trajectory and flow behaviour of the gases. Two cases were considered: freestream to exhaust velocity ratios of 0 (no freestream velocity) and 0.46 (co-flowing free stream), respectively. The formation of a counter-rotating vortex pair was detected for both cases but were opposite in the rotational sense. For the case without freestream, the plume formed into a small “kidney” shape, before expanding and dissipating downstream. For the case with freestream, the plume formed into a slenderer and more elongated “reversed-C” shape as compared to the case without freestream. It also retained its shape further downstream and maintained its relative position. These observations on the trajectory and shape of plume provide basis to understanding the nature and interaction of the plume with its surroundings.

Energetic scales in a bluff body shear layer

2019 ◽  
Vol 875 ◽  
pp. 543-575 ◽  
Author(s):  
D. M. Moore ◽  
C. W. Letchford ◽  
M. Amitay
Keyword(s):  
Shear Layer ◽  
Bluff Body ◽  
Reynolds Numbers ◽  
The Body ◽  
Aspect Ratios ◽  
Separated Shear Layer ◽  
Image Velocimetry ◽  
Highly Nonlinear ◽  
Point Correlation ◽  
Reynolds Number Dependency

A detailed experimental campaign into separated shear layers stemming from rectangular sections (having aspect ratios of 5 : 1, 3 : 1 and 1 : 1) was carried out at Reynolds numbers range between $1.34\times 10^{4}$ and $1.18\times 10^{5}$ based on the body thickness. Particle image velocimetry was used to locate the highest concentration of fluctuations in the velocity field and subsequent hot-wire measurements at those locations provided adequate spectral resolution to follow the evolution of various instabilities that are active within the separated shear layer. Similar to recent findings by this same group, the shear layer behaviour is observed to contain a combination of Reynolds invariant characteristics, including its time-averaged position, while other properties demonstrate clear Reynolds number dependency, including the spatial amplification of turbulent kinetic energy. Additional results here show that the ratio of side lengths of the body is a key parameter in revealing these effects. One reason for this is the level of coupling between modes of instability, which is evaluated using two-point correlation methods. These findings indicate that the separated shear layer on a bluff body is highly nonlinear. A specific set of scales responsible for these unique behaviours is identified and discussed, along with their relationship to other scales in the flow.

Low Reynolds Number Open Channel Flows Over a Backward Facing Step

2012 ◽  
Author(s):  
Afua A. Ampadu-Mintah ◽  
Mark F. Tachie
Keyword(s):  
Reynolds Number ◽  
Open Channel ◽  
Reynolds Shear Stress ◽  
Low Reynolds Number ◽  
Reynolds Numbers ◽  
Backward Facing Step ◽  
Closed Channel ◽  
Freestream Velocity ◽  
Reynolds Number Effects ◽  
Low Reynolds

Low Reynolds number effects on turbulent flows over a backward facing step (BFS) in an open channel were investigated. The Reynolds numbers based on momentum thickness (θ) and step height (h) are in the range 590 ≤ Reθ ≤ 1950 and 950 ≤ Reh ≤ 2900, respectively. The Froude number based on the approach water depth and freestream velocity varied from 0.12 to 0.37. A particle image velocimetry technique was used to measure the velocity field. The flow patterns in the reattachment and redevelopment regions are qualitatively similar for all the three Reynolds numbers studied. The mean velocity profiles in outer coordinates do not exhibit significant Reynolds number effects downstream of the BFS. On the contrary, the turbulence intensities and Reynolds shear stress do not show Reynolds number similarity. As expected, similarity with the upstream profile improves with increasing streamwise distance from the reattachment point. Data obtained in this study were also compared with previous measurements made over backward facing step in a closed channel to study free surface effects. The results showed that deviation of flow over BFS in open channel from flow over BFS in a closed channel is more significant in the immediate vicinity of the step.

A Study on Vortex Shedding From Spheres in a Uniform Flow

1990 ◽  
Vol 112 (4) ◽  
pp. 386-392 ◽  
Author(s):  
H. Sakamoto ◽  
H. Haniu
Keyword(s):  
Reynolds Number ◽  
Strouhal Number ◽  
Vortex Shedding ◽  
Water Channel ◽  
Reynolds Numbers ◽  
Uniform Flow ◽  
Freestream Velocity ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency ◽  
Visualization Experiments

Vortex shedding from spheres at Reynolds numbers from 3 × 102 to 4 × 104 in a uniform flow was investigated experimentally. Standard hot-wire technique were used to measure the vortex shedding frequency from spheres in a low-speed wind tunnel. Flow-visualization experiments were carried out in a water channel. Important results from the investigation were that (i) the variation of the Strouhal number St (=fD/U0, U0: freestream velocity, D: diameter of the sphere, f: vortex shedding frequency) with the Reynolds number (= U0D/v, v: kinematic viscosity) can be classified into four regions, (ii) the Reynolds number at which the hairpinshaped vortices begin to change from laminar to turbulent vortices so that the wake structure behind the sphere is not shown clearly when a Reynolds number of about 800 is reached, and (vi) at Reynolds numbers ranging from 8X102 to 1.5X104, the higher and lower frequency modes of the Strouhal number coexist.

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.

Momentum Distribution in the Wake of a Trapezoidal Pitching Panel

2016 ◽  
Vol 50 (5) ◽  
pp. 9-23 ◽  
Author(s):  
Rajeev Kumar ◽  
Justin T. King ◽  
Melissa A. Green
Keyword(s):  
Strouhal Number ◽  
Particle Image ◽  
Three Dimensional ◽  
Velocity Fields ◽  
Stereoscopic Particle Image Velocimetry ◽  
Vortex Rings ◽  
Vortex Identification ◽  
Freestream Velocity ◽  
Image Velocimetry ◽  
Thrust Production

AbstractThe oscillation of bioinspired fin-like panels in a uniform freestream flow creates chains of vortex rings, including streamwise segments that induce significant three-dimensional effects. With increasing Strouhal number, this wake structure induces flow with increasing nondimensional momentum, defined relative to the freestream velocity, in the downstream direction. This increase in relative momentum with increasing Strouhal number is consistent with greater nondimensional thrust production, which has been shown previously in the literature. These results were obtained via stereoscopic particle image velocimetry water tunnel experiments at Strouhal numbers ranging from 0.17 to 0.56 downstream of a continuously pitching trapezoidal panel. Features of the wake dynamics including spanwise compression, transverse expansion, transverse wake splitting or bifurcation, and wake breakdown are elucidated through analyses of phase-averaged as well as time-averaged velocity fields, in addition to common vortex identification methods.

Mechanisms of instability growth, interaction and breakdown induced by a backward-facing step in a swept-wing flow

2021 ◽  
Vol 931 ◽  
Author(s):  
Jenna L. Eppink
Keyword(s):  
Particle Image ◽  
Low Frequency ◽  
Streamwise Velocity ◽  
Swept Wing ◽  
Backward Facing Step ◽  
Time Resolved ◽  
Image Velocimetry ◽  
Large Velocity ◽  
Instability Growth ◽  
Constructive Interaction

Time-resolved particle image velocimetry measurements were performed downstream of a swept backward-facing step. The measurements allow detailed analysis of the interactions between the unsteady instabilities and the stationary crossflow vortices. Different mechanisms are identified that lead to the modulation of the different families of unsteady instabilities that occur downstream of the step. For the low-frequency spanwise-travelling mode, the modulation occurs due to a redistribution of momentum when the instability encounters regions of large spanwise shear of the wall-normal and streamwise velocity. However, the higher-frequency streamwise-travelling instabilities undergo the familiar ‘lift-up’ mechanism when they encounter the regions of large vertical velocity due to the presence of the stationary crossflow vortices. The process leading to large velocity spikes, and ultimately to a laminar breakdown to turbulence, is identified as a constructive interaction between the different unsteady instabilities, coupled with an interaction with the stationary crossflow vortices when the phases align properly.

Wake Flow Measurements Behind Rotating Smooth Spheres and Baseballs Near Critical Reynolds Numbers

2021 ◽  
Author(s):  
David Rooney ◽  
Patrick Mortimer ◽  
Frank Tricouros ◽  
John Vaccaro
Keyword(s):  
Tangential Velocity ◽  
Reynolds Numbers ◽  
Stereoscopic Particle Image Velocimetry ◽  
Wake Flow ◽  
Mean Velocity ◽  
Flow Measurements ◽  
Freestream Velocity ◽  
Magnus Effect ◽  
Critical Reynolds Numbers ◽  
Smooth Sphere

Abstract The flow field behind spinning baseballs at two different seam orientations was investigated, and compared with a smooth sphere, to isolate effects of seams on the Magnus effect at Reynolds numbers of 5×104 and 1×105. The rotational speed of the three spheres varied from 0-2400 rpm, which are typical of spin rates imparted to a thrown baseball. These spin rates are represented non-dimensionally as a relative spin rate relating the surface tangential velocity to the freestream velocity, and varied between 0-0.94. Mean velocity profiles, streamline patterns, and power spectral density of the velocity signals were taken using hot-wire anemometry and/or stereoscopic particle image velocimetry in the wake region. The sphere wake orientation changed over a range of relative spin rates, indicating an inverse Magnus effect. Vortex shedding at a Strouhal number of 0.25 was present on the sphere at low relative spin rates. However, the seams on the baseball prevented any consequential change in wake orientation and, at most spin rates, suppressed the shedding frequency exhibited by the sphere. Instead, frequencies corresponding to the seam rotation rates were observed in the wake flow. It was concluded that the so-called inverse Magnus effect recorded by previous investigators at specific combinations of Reynolds number and relative spin rate on a sphere exists for a smooth sphere or an axisymmetrically dimpled sphere but not for a baseball near critical Reynolds numbers, where the wake flow pattern is strongly influenced by the raised seams.

Numerical Evidence of Mode Switching in the Flow-Induced Oscillations by a Cavity

2003 ◽  
Vol 2 (2) ◽  
pp. 193-217 ◽  
Author(s):  
Xavier Gloerfelt ◽  
Christophe Bogey ◽  
Christophe Bailly
Keyword(s):  
Acoustic Field ◽  
Computational Cost ◽  
Low Frequency ◽  
Periodic Boundary Conditions ◽  
Frequency Component ◽  
Mode Switching ◽  
Spanwise Direction ◽  
Recirculation Flow ◽  
Separated Shear Layer ◽  
Cavity Modes

A Direct Noise Computation (DNC) has been performed for a turbulent boundary layer past a rectangular cavity, matching one configuration of Karamcheti1 experiments. An LES approach with periodic boundary conditions in the spanwise direction is used to evaluate the solution at a reasonable computational cost. The two components in the pressure spectra found experimentally are well reproduced. The acoustic field appears to be dominated by the low-frequency component whereas the experimental visualization indicates a radiation at the higher frequency. The mechanism giving rise to the lower frequency is investigated providing evidence on the possibility of switching between two cavity modes and that the strong coupling of the separated shear layer with the recirculation flow within the cavity is likely to participate to the low-frequency modulation. Moreover, an extrapolation method is proposed and applied to obtain the far-field from the near acoustic field.

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