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

Author(s):  
K. Anand ◽  
S. Sarkar ◽  
N. Thilakan

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.

2011 ◽  
Vol 681 ◽  
pp. 370-410 ◽  
Author(s):  
JOHN D. COULL ◽  
HOWARD P. HODSON

This paper examines the transition process in a boundary layer similar to that present over the suction surfaces of aero-engine low-pressure (LP) turbine blades. This transition process is of significant practical interest since the behaviour of this boundary layer largely determines the overall efficiency of the LP turbine. Modern ‘high-lift’ blade designs typically feature a closed laminar separation bubble on the aft portion of the suction surface. The size of this bubble and hence the inefficiency it generates is controlled by the transition between laminar and turbulent flow in the boundary layer and separated shear layer. The transition process is complicated by the inherent unsteadiness of the multi-stage machine: the wakes shed by one blade row convect through the downstream blade passages, periodically disturbing the boundary layers. As a consequence, the transition to turbulence is multi-modal by nature, being promoted by periodic and turbulent fluctuations in the free stream and the inherent instabilities of the boundary layer. Despite many studies examining the flow behaviour, the detailed physics of the unsteady transition phenomena are not yet fully understood. The boundary-layer transition process has been studied experimentally on a flat plate. The opposing test-section wall was curved to impose a streamwise pressure distribution typical of modern high-lift LP turbines over the flat plate. The presence of an upstream blade row has been simulated by a set of moving bars, which shed wakes across the test section inlet. Further upstream, a grid has been installed to elevate the free-stream turbulence to a level believed to be representative of multi-stage LP turbines. Extensive particle imaging velocimetry (PIV) measurements have been performed on the flat-plate boundary layer to examine the flow behaviour. In the absence of the incoming bar wakes, the grid-generated free-stream turbulence induces relatively weak Klebanoff streaks in the boundary layer which are evident as streamwise streaks of low-velocity fluid. Transition is promoted by the streaks and by the inherent inflectional (Kelvin–Helmholtz (KH)) instability of the separation bubble. In unsteady flow, the incoming bar wakes generate stronger Klebanoff streaks as they pass over the leading edge, which convect downstream at a fraction of the free-stream velocity and spread in the streamwise direction. The region of amplified streaks convects in a similar manner to a classical turbulent spot: the leading and trailing edges travel at around 88% and 50% of the free-stream velocity, respectively. The strongest disturbances travel at around 70% of the free-stream velocity. The wakes induce a second type of disturbance as they pass over the separation bubble, in the form of short-span KH structures. Both the streaks and the KH structures contribute to the early wake-induced transition. The KH structures are similar to those observed in the simulation of separated flow transition with high free-stream turbulence by McAuliffe & Yaras (ASME J. Turbomach., vol. 132, no. 1, 2010, 011004), who observed that these structures originated from localised instabilities of the shear layer induced by Klebanoff streaks. In the current measurements, KH structures are frequently observed directly under the path of the wake. The wake-amplified Klebanoff streaks cannot affect the generation of these structures since they do not arrive at the bubble until later in the wake cycle. Rather, the KH structures arise from an interaction between the flow disturbances in the wake and localised instabilities in the shear layer, which are caused by the weak Klebanoff streaks induced by the grid turbulence. The breakdown of the KH structures to small-scale turbulence occurs a short time after the wake has passed over the bubble, and is largely driven by the arrival of the wake-amplified Klebanoff streaks from the leading edge. During this process, the re-attachment location moves rapidly upstream. The minimum length of the bubble occurs when the strongest wake-amplified Klebanoff streaks arrive from the leading edge; these structures travel at around 70% of the free-stream velocity. The bubble remains shorter than its steady-flow length until the trailing edge of the wake-amplified Klebanoff streaks, travelling at 50% of the free-stream velocity, convect past. After this time, the reattachment location moves aft on the surface as a consequence of a calmed flow region which follows behind the wake-induced turbulence.


Author(s):  
K. Anand ◽  
S. Sarkar

Shear layer development over a thick flat plate with a semi-circular leading edge is investigated for a range of angles of attack under different imposed pressure gradients for a Reynolds number of 2.44×105 (based on chord and free-stream velocity). The features of the separated shear layer are very well documented through a combination of surface pressure measurement and flow visualization by particle image velocimetry (PIV). The instability of the separated layer occurs because of enhanced receptivity of perturbations leading to the development of significant unsteadiness and three-dimensional motions in the second-half of the bubble. The onset of separation, transition and the point of reattachment are identified for varying angles of attack and imposed pressure gradients. The reattachment point shifts from 12.5% to 53% of chord resulting in enhancement of bubble length from 5% to 47%, while onset of transition shifts upstream from 14% to 7.5% as α increases. The Reynolds number based on the length of laminar shear layer is found to be in the range of 0.7×104 to 2.0×104. The separated shear layer fails to reattach attributing to bubble bursting at α = 12° for β = −45°, while, it bursts at α = 5° for β = +45°. The bubble falls in the category of short bubble for α < 3°, whereas, it becomes long for α ≥ 3°. The data concerning laminar portion and reattachment points agree well with the literature.


1997 ◽  
Vol 342 ◽  
pp. 119-139 ◽  
Author(s):  
M. KIYA ◽  
M. SHIMIZU ◽  
O. MOCHIZUKI

A turbulent separation bubble is forced by single- and double-frequency sinusoidal disturbances, with the emphasis placed on the reattachment length as a function of the forcing amplitude and frequency. The separation bubble is that formed along the side of a blunt circular cylinder with a square leading edge. In single-frequency forcing, the reattachment length attains a minimum at a particular forcing frequency, F, which scales with the frequency of shedding of vortices from the reattachment region of the separated shear layer. A flow model is presented to interpret the frequency F. Forcing of sufficiently high amplitude eliminates the recirculating region in a range of the forcing frequency. Flow visualization and a survey of the mean flow and turbulence properties demonstrate how the flow in the separated shear layer is modified by the forcing. In double-frequency forcing, the superposition of the F-component on its higher or subharmonic components is considered. A non-resonant combination of the two frequencies is also considered.


Author(s):  
Birinchi K. Hazarika ◽  
Charles Hirsch

An experimental investigation of a separation bubble on a C4 leading edge plate at an incidence in a low turbulence free stream at six Reynolds numbers, is reported. The long separation bubble, formed at the leading edge, has a short laminar and transitional zone followed by a long turbulent zone. The increase in Reynolds number reduced the laminar and transitional part significantly, but its effect on the length of the separation bubble is marginal till the transition starts at the separation point. The peak intermittency factor, which occurs at the centre of the shear layer, follows the universal intermittency distribution curve. The spot production rate for the separated flows are several orders of magnitude higher than that for the attached boundary layers. The transition process is initiated by the amplification of the instability waves in the shear layer similar to the natural mode of transition. At high Reynolds numbers, the onset of transition is likely to take place at the separation point. At lower chord Reynolds numbers, the separation to onset Reynolds number and the spot production rate parameter are functions of the separation momentum thickness Reynolds number. The free stream turbulence intensity has a strong influence on the spot production rate. New correlations for transition in the leading edge separation bubbles are proposed based on all the available intermittency measurements in the leading edge separation bubbles.


Author(s):  
A Samson ◽  
S Sarkar

This paper describes the dynamics of a laminar separation bubble formed on the semi-circular leading edge of constant thickness aerofoil model. Detailed experimental studies are carried out in a low-speed wind tunnel, where surface pressure and time-averaged velocity in the separated region and as well as in the downstream are presented along with flow field visualisations through PIV for various Reynolds numbers ranging from 25,000 to 75,000 (based on the leading edge diameter). The results illustrate that the separated shear layer is laminar up to 20% of separation length and then the perturbations are amplified in the second half attributing to breakdown and reattachment. The bubble length is highly susceptible to change in Reynolds number and plays an important role in outer layer activities. Further, the transition of a separated shear layer is studied through variation of intermittency factor and comparing with existing correlations available in the literature for attached flow and as well as separated flow. Transition of the separated shear layer occurs through formation of K-H rolls, where the intermittency following spot propagation theory appears valid. The predominant shedding frequency when normalised with respect to the momentum thickness at separation remains almost constant with change in Reynolds number. The relaxation is slow after reattachment and the flow takes about five bubble lengths to approach a canonical layer.


Author(s):  
Bryn N. Ubald ◽  
Jiahuan Cui ◽  
Rob Watson ◽  
Paul G. Tucker ◽  
Shahrokh Shahpar

The measurement accuracy of the temperature/pressure probe mounted at the leading edge of a turbine/compressor blade is crucial for estimating the fuel consumption of a turbo-fan engine. Apart from the measurement error itself, the probe also introduces extra losses. This again would compromise the measurement accuracy of the overall engine efficiency. This paper utilizes high-fidelity numerical analysis to understand the complex flow around the probe and quantify the loss sources due to the interaction between the blade and its instrumentation. With the inclusion of leading edge probes, three dimensional flow phenomena develop, with some flow features acting in a similar manner to a jet in cross flow. The separated flow formed at the leading edge of the probe blocks a large area of the probe bleed-hole, which is one of the reasons why the probe accuracy can be sensitive to Mach and Reynolds numbers. The addition of 4% free stream turbulence is shown to have a marginal impact on the jet trajectory originated from the probe bleedhole. However, a slight reduction is observed in the size of the separation bubble formed at the leading edge of the probe, preceding the two bleedhole exits. The free stream turbulence also has a significant impact on the size of the separation bubble near the trailing edge of the blade. With the addition of the free stream turbulence, the loss observed within the trailing edge wake is reduced. More than 50% of the losses at the cascade exit are generated by the leading edge probe. A breakdown of the dissipation terms from the mean flow kinetic energy equation demonstrates that the Reynolds stresses are the key terms in dissipating the counter rotating vortex pairs with the viscous stresses responsible for the boundary layer.


1991 ◽  
Vol 113 (3) ◽  
pp. 405-410 ◽  
Author(s):  
Kyuro Sasaki ◽  
Masaru Kiya

This paper describes the results of a flow visualization study which concerns three-dimensional vortex structures in a leading-edge separation bubble formed along the sides of a blunt flat plate. Dye and hydrogen bubbles were used as tracers. Reynolds number (Re), based on the plate thickness, was varied from 80 to 800. For 80 < Re < 320, the separated shear layer remains laminar up to the reattachment line without significant spanwise distortion of vortex filaments. For 320 < Re < 380, a Λ-shaped deformation of vortex filaments appears shortly downstream of the reattachment and is arranged in-phase in the downstream direction. For Re > 380, hairpin-like structures are formed and arranged in a staggered manner. The longitudinal and spanwise distances of the vortex arrangement are presented as functions of the Reynolds number.


2009 ◽  
Vol 131 (2) ◽  
Author(s):  
Brian R. McAuliffe ◽  
Metin I. Yaras

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.


1996 ◽  
Vol 118 (1) ◽  
pp. 162-171 ◽  
Author(s):  
M. V. Finnis ◽  
A. Brown

Measurements are presented of the streamwise velocity variation within a laminar boundary layer on a concave surface of 4 m radius of curvature for which the free-stream velocity gradient factor (ν/U02)dU0/dx was approximately 1 × 10−6. The stream velocity variation was consistent with the presence of counterrotating vortices resulting from the Go¨rtler instability. The vortices exhibited exponential growth over the streamwise extent of the measurements to a disturbance amplitude of approximately 13 percent of the local free-stream velocity. The vortex growth rates were found to be less than those for a zero velocity gradient factor, indicating that a favorable pressure gradient stabilizes the flow with respect to the Go¨rtler instability. Boundary layer profiles at local upwash and downwash positions are compared with the linear theory for which the mean flow was modeled using the Pohlhausen approximation to the solution of the boundary layer equations. The agreement between the measured and predicted profiles indicates that the linear stability theory can provide a fair approximation to the small amplitude growth of the Go¨rtler instability.


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