Study of separated shear layer in moderate Reynolds number plane sudden expansion flows

AIAA Journal ◽  
1992 ◽  
Vol 30 (7) ◽  
pp. 1808-1814 ◽  
Author(s):  
M. Volkan Otugen ◽  
George Muckenthaler
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
Sudden Expansion ◽  
Separated Shear Layer ◽  
Moderate Reynolds Number

Experimental Investigation of the Flow Through Axisymmetric Expansions

1989 ◽  
Vol 111 (4) ◽  
pp. 464-471 ◽  
Author(s):  
M. Stieglmeier ◽  
C. Tropea ◽  
N. Weiser ◽  
W. Nitsche
Keyword(s):  
Experimental Investigation ◽  
Reynolds Number ◽  
Shear Layer ◽  
Test Facility ◽  
Recirculation Region ◽  
Velocity Measurements ◽  
Forward Scatter ◽  
Separated Shear Layer ◽  
Flow Through ◽  
Diffusion Rates

This study examines the flow field in three axisymmetric expansions having diffuser half-angles of 14, 18, and 90 deg, respectively. Velocity measurements were performed at a Reynolds number of Re = 1.56 × 104 using a single component LDA operated in forward scatter. The test facility was refractive index matched, allowing measurement of the velocities U, V, W, u2, v2, w2, uv and uw upstream of, and throughout the entire recirculation region. The results indicate that the diffuser geometry influences the separated shear layer appreciably over the entire length of the diffuser section. The production of turbulence immediately after separation is much higher in the case of the 14 and 18 deg diffuser compared to the 90 deg expansion, leading to higher diffusion rates in the separated shear layer, and hence earlier reattachment of the shear layer.

The Application of Flow Control to an Aft-Loaded Low Pressure Turbine Cascade With Unsteady Wakes

2008 ◽  
Author(s):  
Jeffrey P. Bons ◽  
Jon Pluim ◽  
Kyle Gompertz ◽  
Matthew Bloxham ◽  
John P. Clark
Keyword(s):  
Reynolds Number ◽  
Flow Control ◽  
Shear Layer ◽  
Pressure Loss ◽  
Total Pressure ◽  
Separation Zone ◽  
Total Pressure Loss ◽  
Separation Control ◽  
Low Pressure Turbine ◽  
Separated Shear Layer

The synchronous application of flow control in the presence of unsteady wakes was studied on a highly-loaded low pressure turbine blade. The L1A blade has a design Zweifel coefficient of 1.34 and a suction peak at 58% axial chord, making it an aft-loaded pressure distribution. Velocity and pressure data were acquired at Rec = 20,000 with 3% incoming freestream turbulence. Unsteady wakes from an upstream vane row are simulated with a moving row of bars at a flow coefficient of 0.76. At this Reynolds number, the blade exhibits a non-reattaching separation bubble beginning at 57% axial chord under steady flow conditions without upstream wakes. The separation zone is modified substantially by the presence of unsteady wakes, producing a smaller separation zone and reducing the area-averaged wake total pressure loss by more than 50%. The wake disturbance accelerates transition in the separated shear layer but stops short of reattaching the flow. Rather, a new time-averaged equilibrium location is established for the separated shear layer, further downstream than without wakes. The focus of this study was the application of pulsed flow control using two spanwise rows of discrete vortex generator jets (VGJs). The VGJs were located at 59% Cx, approximately the peak cp location, and at 72% Cx. The most effective separation control was achieved at the 59% Cx location. Wake total pressure loss decreased 60% from the wake only level and the cp distribution fully recovered its high Reynolds number (attached flow) performance. The VGJ disturbance dominates the dynamics of the separated shear layer, with the wake disturbance assuming a secondary role only. When the pulsed jet actuation (30% duty cycle) was initiated at the 72% Cx location, synchronization with the wake passing frequency (10.6Hz) was key to producing the most effective separation control. A 25% improvement in effectiveness was obtained by aligning the jet actuation between wake events. Evidence suggests that flow control using VGJs will be effective in the highly unsteady LPT environment of an operating gas turbine, provided the VGJ location and amplitude are adapted for the specific blade profile.

Passive Manipulation of Separation-Bubble Transition Using Surface Modifications

2009 ◽  
Vol 131 (2) ◽  
Author(s):  
Brian R. McAuliffe ◽  
Metin I. Yaras
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Shear Layer ◽  
Separation Bubble ◽  
Surface Modifications ◽  
Small Scale ◽  
Two Dimensional ◽  
Vortex Pairing ◽  
Separated Shear Layer ◽  
Scale Turbulence

Through experiments using two-dimensional particle-image velocimetry (PIV), this paper examines the nature of transition in a separation bubble and manipulations of the resultant breakdown to turbulence through passive means of control. An airfoil was used that provides minimal variation in the separation location over a wide operating range, with various two-dimensional modifications made to the surface for the purpose of manipulating the transition process. The study was conducted under low-freestream-turbulence conditions over a flow Reynolds number range of 28,000–101,000 based on airfoil chord. The spatial nature of the measurements has allowed identification of the dominant flow structures associated with transition in the separated shear layer and the manipulations introduced by the surface modifications. The Kelvin–Helmholtz (K-H) instability is identified as the dominant transition mechanism in the separated shear layer, leading to the roll-up of spanwise vorticity and subsequent breakdown into small-scale turbulence. Similarities with planar free-shear layers are noted, including the frequency of maximum amplification rate for the K-H instability and the vortex-pairing phenomenon initiated by a subharmonic instability. In some cases, secondary pairing events are observed and result in a laminar intervortex region consisting of freestream fluid entrained toward the surface due to the strong circulation of the large-scale vortices. Results of the surface-modification study show that different physical mechanisms can be manipulated to affect the separation, transition, and reattachment processes over the airfoil. These manipulations are also shown to affect the boundary-layer losses observed downstream of reattachment, with all surface-indentation configurations providing decreased losses at the three lowest Reynolds numbers and three of the five configurations providing decreased losses at the highest Reynolds number. The primary mechanisms that provide these manipulations include: suppression of the vortex-pairing phenomenon, which reduces both the shear-layer thickness and the levels of small-scale turbulence; the promotion of smaller-scale turbulence, resulting from the disturbances generated upstream of separation, which provides quicker transition and shorter separation bubbles; the elimination of the separation bubble with transition occurring in an attached boundary layer; and physical disturbance, downstream of separation, of the growing instability waves to manipulate the vortical structures and cause quicker reattachment.

scholarly journals Decomposition of wake dynamics in fluid–structure interaction via low-dimensional models

2019 ◽  
Vol 867 ◽  
pp. 723-764 ◽  
Author(s):  
T. P. Miyanawala ◽  
R. K. Jaiman
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
High Amplitude ◽  
Wake Flow ◽  
High Dimensional ◽  
Near Wake ◽  
Structure Interaction ◽  
Moderate Reynolds Number ◽  
Low Dimensional ◽  
Lock In

We present a dynamic decomposition analysis of the wake flow in fluid–structure interaction (FSI) systems under both laminar and turbulent flow conditions. Of particular interest is to provide the significance of low-dimensional wake flow features and their interaction dynamics to sustain the free vibration of a square cylinder at a relatively low mass ratio. To obtain the high-dimensional data, we employ a body-conforming variational FSI solver based on the recently developed partitioned iterative scheme and the dynamic subgrid-scale turbulence model for a moderate Reynolds number ($Re$). The snapshot data from high-dimensional FSI simulations are projected to a low-dimensional subspace using the proper orthogonal decomposition (POD). We utilize each corresponding POD mode to detect features of the organized motions, namely, the vortex street, the shear layer and the near-wake bubble. We find that the vortex shedding modes contribute solely to the lift force, while the near-wake and shear layer modes play a dominant role in the drag force. We further examine the fundamental mechanism of this dynamical behaviour and propose a force decomposition technique via low-dimensional approximation. To elucidate the frequency lock-in, we systematically analyse the decomposed modes and their dynamical contributions to the force fluctuations for a range of reduced velocity at low Reynolds number laminar flow. These quantitative mode energy contributions demonstrate that the shear layer feeds the vorticity flux to the wake vortices and the near-wake bubble during the wake–body synchronization. Based on the decomposition of wake dynamics, we suggest an interaction cycle for the frequency lock-in during the wake–body interaction, which provides the interrelationship between the high-amplitude motion and the dominating wake features. Through our investigation of wake–body synchronization below critical $Re$ range, we discover that the bluff body can undergo a synchronized high-amplitude vibration due to flexibility-induced unsteadiness. Owing to the wake turbulence at a moderate Reynolds number of $Re=22\,000$, a distorted set of POD modes and the broadband energy distribution are observed, while the interaction cycle for the wake synchronization is found to be valid for the turbulent wake flow.

scholarly journals On vortex shedding from an airfoil in low-Reynolds-number flows

2009 ◽  
Vol 632 ◽  
pp. 245-271 ◽  
Author(s):  
SERHIY YARUSEVYCH ◽  
PIERRE E. SULLIVAN ◽  
JOHN G. KAWALL
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
Vortex Shedding ◽  
Coherent Structures ◽  
Flow Regimes ◽  
Reynolds Numbers ◽  
Frequency Scaling ◽  
Separated Shear Layer ◽  
Low Reynolds Number Flows ◽  
Low Reynolds

Development of coherent structures in the separated shear layer and wake of an airfoil in low-Reynolds-number flows was studied experimentally for a range of airfoil chord Reynolds numbers, 55 × 103 ≤ Rec ≤ 210 × 103, and three angles of attack, α = 0°, 5° and 10°. To illustrate the effect of separated shear layer development on the characteristics of coherent structures, experiments were conducted for two flow regimes common to airfoil operation at low Reynolds numbers: (i) boundary layer separation without reattachment and (ii) separation bubble formation. The results demonstrate that roll-up vortices form in the separated shear layer due to the amplification of natural disturbances, and these structures play a key role in flow transition to turbulence. The final stage of transition in the separated shear layer, associated with the growth of a sub-harmonic component of fundamental disturbances, is linked to the merging of the roll-up vortices. Turbulent wake vortex shedding is shown to occur for both flow regimes investigated. Each of the two flow regimes produces distinctly different characteristics of the roll-up and wake vortices. The study focuses on frequency scaling of the investigated coherent structures and the effect of flow regime on the frequency scaling. Analysis of the results and available data from previous experiments shows that the fundamental frequency of the shear layer vortices exhibits a power law dependency on the Reynolds number for both flow regimes. In contrast, the wake vortex shedding frequency is shown to vary linearly with the Reynolds number. An alternative frequency scaling is proposed, which results in a good collapse of experimental data across the investigated range of Reynolds numbers.

Experiments on Leading-Edge Induced Separated Shear Layer Under Various Imposed Pressure Gradients

2014 ◽  
Author(s):  
K. Anand ◽  
S. Sarkar ◽  
N. Thilakan
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
Free Stream ◽  
Separation Bubble ◽  
Leading Edge ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Pressure Gradients ◽  
Mean Flow ◽  
Separated Shear Layer

The behaviour of a separated shear layer past a semi-circular leading edge flat plate, its transition and reattachment downstream to separation are investigated for different imposed pressure gradients. The experiments are carried out in a blowing tunnel for a Reynolds number of 2.44×105 (based on chord and free-stream velocity). The mean flow characteristics and the instantaneous vector field are documented using a two-component LDA and a planar PIV, whereas, surface pressures are measured with Electronically scanned pressure (ESP). The onset of separation occurs near the blend point for all values of β (flap angle deflection), however, a considerable shift is noticed in the point of reattachment. The dimensions of the separation bubble is highly susceptible to β and plays an important role in the activity of the outer shear layer. Instantaneous results from PIV show a significant unsteadiness in the shear layer at about 30% of the bubble length, which is further amplified in the second half of the bubble leading to three-dimensional motions. The reverse flow velocity is higher for a favourable pressure gradient (β = +30°) and is found to be 21% of the free stream velocity. The Reynolds number calculated based on ll (laminar shear layer length), falls in the range of 0.9×104 to 1.4×104. The numerical values concerning the criterion for separation and reattachment agree well with the available literature.

Görtler vortices in low-Reynolds-number flow over multi-element airfoil

2017 ◽  
Vol 835 ◽  
pp. 898-935 ◽  
Author(s):  
Jiang-Sheng Wang ◽  
Li-Hao Feng ◽  
Jin-jun Wang ◽  
Tian Li
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
Main Element ◽  
Flow Structures ◽  
Low Reynolds Number Flow ◽  
Reynolds Number Flow ◽  
Separated Shear Layer ◽  
Görtler Vortices ◽  
Number Flow ◽  
Gortler Vortices

The low-Reynolds-number flow over a multi-element airfoil (30P30N) is investigated with time-resolved particle image velocimetry (TR-PIV) and flow visualization (FV). Dominant flow structures over the main element of the multi-element airfoil are explored with the variation of angle of attack ($\unicode[STIX]{x1D6FC}$). It is of great importance that Görtler vortices are first observed with this configuration at $\unicode[STIX]{x1D6FC}=2^{\circ }{-}12^{\circ }$, which is quite different from the high-Reynolds-number cases. The characteristics of the Görtler vortices are explored to determine the origin of these unexpected flow structures. It is found that these Görtler vortices travel in the spanwise direction. Secondary counter-rotating vortices are induced beneath the main Görtler vortices. The travelling property of the Görtler vortices is utilized to determine the positions of the main Görtler vortices and the secondary counter-rotating vortices. It is observed that Görtler vortices reside above the separated shear layer originating from the leading-edge separation of the main element. The secondary counter-rotating vortices are located within the separated shear layer, as a result of the interaction between the Görtler vortices and the separated shear layer. The relative positions of the Görtler vortices, the secondary counter-rotating vortices and the separated shear layer result in a special transition scenario within the separated shear layer. The position of Görtler vortices combined with the Rayleigh discriminant indicates the mechanism that the Görtler vortices are generated by a virtual curved boundary. The travelling property of the Görtler vortices, which is different from the classical stationary Görtler vortices, can also be interpreted by this mechanism. Ultimately, modified criteria for generating Görtler vortices with a virtual curved boundary are proposed to provide references for the follow-up works.

Controlling Turbulence in a Rearward-Facing Step Combustor Using Countercurrent Shear

2005 ◽  
Vol 127 (3) ◽  
pp. 438-448 ◽  
Author(s):  
David J. Forliti ◽  
Paul J. Strykowski
Keyword(s):  
Kinetic Energy ◽  
Shear Flow ◽  
Shear Layer ◽  
Turbulent Kinetic Energy ◽  
Near Field ◽  
Sudden Expansion ◽  
Turbulent Fluctuation ◽  
Separated Shear Layer ◽  
Field Control ◽  
Countercurrent Shear

The present work describes the application of countercurrent shear flow control to the nonreacting flow in a novel step combustor. The countercurrent shear control employs a suction based approach, which induces counterflow through a gap at the sudden expansion plane. Peak turbulent fluctuation levels, cross-stream averaged turbulent kinetic energy, and cross-stream momentum diffusion increased with applied suction. The control downstream of the step operates via two mechanisms: enhanced global recirculation and near field control of the separated shear layer. The use of counterflow also enhances three dimensionality, a feature that is expected to be beneficial under burning conditions.

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