Aerodynamic Investigation on the Excitation of a Laminar Separation Bubble by Secondary Jets

2014 ◽  
Author(s):  
Samson AnnapuReddy ◽  
S. Sarkar
Keyword(s):  
Shear Layer ◽  
Laser Doppler Anemometry ◽  
Separation Bubble ◽  
Sudden Change ◽  
Separated Flow ◽  
Leading Edge ◽  
Constant Thickness ◽  
Laminar Separation ◽  
Separated Shear Layer ◽  
Transition Criteria

Aerodynamic interactions of a separated shear layer from the semi-circular leading edge of a constant thickness aerofoil model with jets ejecting in the vicinity of reattachment from a row of discrete holes at an angle of 30° in the streamwise direction are elucidated. Experiments are carried out for two Reynolds numbers (ReD) 25000 and 55000 (based on the leading-edge diameter) and two velocity ratios (V.R) 0.5 and 1. The time-averaged velocity and turbulence quantities are measured using Laser-Doppler Anemometry (LDA) at different streamwise locations along the centre line of the jet, while the instantaneous flow filed is obtained using Particle Image Velocimetry (PIV). Measurements reveal that the flow undergoes separation at the blending point of semi-circle and flat plate owing to sudden change in geometry. It is observed that in the absence of the jet, the separated shear layer undergoes transition with formation Kelvin-Helmholtz (K-H) rolls and a significant growth of Reynolds stress in the second-half of the bubble is evident. With injection, the separation bubble length in the upstream of jet has decreased with increased growth rate of velocity fluctuations. However, the characteristics of the flow in the separated region remains almost unchanged and the transition criteria even follows the universal intermittency characteristics of Dhawan and Narasimha [35]. The instantaneous results elucidate that K-H rolls from the separated shear layer interact with the injected jet resulting in its oscillation. In the downstream of injection, the presence of the jet is felt with enhanced turbulence activities in the outer layer, particularly for high V.R. Further, the onset and end of transition of the separated flow are compared with correlations available in the literature.

An experimental investigation of a laminar separation bubble on the leading-edge of a modelled aerofoil for different Reynolds numbers

2015 ◽  
Vol 230 (13) ◽  
pp. 2208-2224 ◽  
Author(s):  
A Samson ◽  
S Sarkar
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
Separation Bubble ◽  
Experimental Studies ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Constant Thickness ◽  
Laminar Separation Bubble ◽  
Laminar Separation ◽  
Separated Shear Layer

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.

Exploitation of Subharmonics for Separated Shear Layer Control on a High-Lift Low-Pressure Turbine Using Acoustic Forcing

2013 ◽  
Vol 136 (5) ◽  
Author(s):  
Chiara Bernardini ◽  
Stuart I. Benton ◽  
Jen-Ping Chen ◽  
Jeffrey P. Bons
Keyword(s):  
Shear Layer ◽  
Separation Bubble ◽  
Boundary Layer Separation ◽  
Low Pressure ◽  
Linear Instability ◽  
Laminar Separation ◽  
Significant Control ◽  
Low Pressure Turbine ◽  
Separated Shear Layer ◽  
Sound Control

The mechanism of separation control by sound excitation is investigated on the aft-loaded low-pressure turbine (LPT) blade profile, the L1A, which experiences a large boundary layer separation at low Reynolds numbers. Previous work by the authors has shown that on a laminar separation bubble such as that experienced by the front-loaded L2F profile, sound excitation control has its best performance at the most unstable frequency of the shear layer due to the exploitation of the linear instability mechanism. The different loading distribution on the L1A increases the distance of the separated shear layer from the wall and the exploitation of the same linear mechanism is no longer effective in these conditions. However, significant control authority is found in the range of the first subharmonic of the natural unstable frequency. The amplitude of forced excitation required for significant wake loss reduction is higher than that needed when exploiting linear instability, but unlike the latter case, no threshold amplitude is found. The fluid-dynamics mechanisms under these conditions are investigated by particle image velocimetry (PIV) measurements. Phase-locked PIV data gives insight into the growth and development of structures as they are shed from the shear layer and merge to lock into the excited frequency. Unlike near-wall laminar separation sound control, it is found that when such large separated shear layers occur, sound excitation at subharmonics of the fundamental frequency is still effective with high-Tu levels.

The Effect of Reynolds Number and Laminar Separation on Axial Cascade Performance

1975 ◽  
Vol 97 (2) ◽  
pp. 261-273 ◽  
Author(s):  
W. B. Roberts
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
Separation Bubble ◽  
Axial Compressor ◽  
Leading Edge ◽  
Practical Method ◽  
Reynolds Numbers ◽  
Compressor Cascade ◽  
Laminar Separation ◽  
Semiempirical Theory

Testing over a range of Reynolds numbers was done for three NACA 65 Profiles in cascade. The testing was carried out in the VKI C-1 Low Speed Cascade Wind Tunnel; blade chord Reynolds number was varied from 250,000 to 40,000. A semiempirical theory is developed which will predict the behavior of the shear layer across a laminar separation bubble. The method is proposed for two-dimensional incompressible flow and is applicable down to short bubble bursting. The method can be used to predict the length of the laminar bubble, the bursting Reynolds number, and the development of the shear layer through the separated region. As such it is a practical method for calculating the profile losses of axial compressor and turbine cascades in the presence of laminar separation bubbles. It can also be used to predict the abrupt leading edge stall associated with thin airfoil sections. The predictions made by the method are compared with the available experimental data. The agreement could be considered good. The method was also used to predict regions of laminar separation in converging flows through axial compressor cascades (exterior to the corner vortices) with good results. For Reynolds numbers below bursting the semiempirical theory no longer applies. For this situation the performance of an axial compressor cascade can be computed using an empirical correlation proposed by the author. Comparison of performance prediction with experiment shows satisfactory agreement. Finally, a tentative correlation, based on the NACA Diffusion Factor, is presented that allows a rapid estimation of the bursting Reynolds number of an axial compressor cascade.

Effects of Free-Stream Turbulence on Transition of a Separated Boundary Layer Over the Leading-Edge of a Constant Thickness Airfoil

2015 ◽  
Vol 138 (2) ◽  
Author(s):  
A. Samson ◽  
S. Sarkar
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Free Stream ◽  
Separation Bubble ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Constant Thickness ◽  
Free Stream Turbulence ◽  
Transition Mechanism ◽  
Bubble Length

This paper describes the change in the transition mechanism of a separated boundary layer formed from the semicircular leading-edge of a constant thickness airfoil as the free-stream turbulence (fst) increases. Experiments are carried out in a low-speed wind tunnel for three levels of fst (Tu = 0.65%, 4.6%, and 7.7%) at two Reynolds numbers (Re) 25,000 and 55,000 (based on the leading-edge diameter). Measurements of velocity and surface pressure along with flow field visualizations are carried out using a planar particle image velocimetry (PIV). The flow undergoes separation in the vicinity of leading-edge and reattaches in the downstream forming a separation bubble. The 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 Tu. At low fst, the primary mode of instability of the shear layer is Kelvin–Helmholtz (K-H), although the local viscous effect may not be neglected. At high fst, the mechanism of shear layer rollup is bypassed with transient growth of perturbations along with evidence of spot formation. The predominant shedding frequency when normalized with respect to the momentum thickness at separation is almost constant and shows a good agreement with the previous studies. After reattachment, the flow takes longer length to approach a canonical boundary layer.

Leading Edge Separation From a Blunt Plate at Low Reynolds Number

1980 ◽  
Vol 102 (4) ◽  
pp. 494-496 ◽  
Author(s):  
J. C. Lane ◽  
R. I. Loehrke
Keyword(s):  
Reynolds Number ◽  
Separation Bubble ◽  
Plate Thickness ◽  
Leading Edge ◽  
Laminar Separation Bubble ◽  
Laminar Separation ◽  
Separated Shear Layer ◽  
Separation Zones ◽  
Edge Separation ◽  
Steady Laminar

The flow over a blunt plate aligned parallel to the stream was visualized using dye tracers. A leading edge separation bubble was observed to form at a Reynolds number based on plate thickness of 100. The steady, laminar separation bubble on a long plate, L/t ≥ 8, grows in size with increasing Reynolds number reaching a maximum streamwise length at Ret = 325. The separated shear layer becomes unsteady and the bubble shrinks in size with further increases in Reynolds number. The leading and trailing edge separation zones on short plates, L/t ≤ 4, may combine to form a large recirculation pocket.

Sinusoidal forcing of a turbulent separation bubble

1997 ◽  
Vol 342 ◽  
pp. 119-139 ◽  
Author(s):  
M. KIYA ◽  
M. SHIMIZU ◽  
O. MOCHIZUKI
Keyword(s):  
Shear Layer ◽  
Separation Bubble ◽  
Leading Edge ◽  
High Amplitude ◽  
Single Frequency ◽  
Reattachment Length ◽  
Mean Flow ◽  
Double Frequency ◽  
Separated Shear Layer ◽  
Turbulent Separation

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.

Exploitation of Subharmonics for Separated Shear Layer Control on a High-Lift LPT Using Acoustic Forcing

2013 ◽  
Author(s):  
Chiara Bernardini ◽  
Stuart Benton ◽  
Jen-Ping Chen ◽  
Jeffrey P. Bons
Keyword(s):  
Shear Layer ◽  
Separation Bubble ◽  
Reynolds Numbers ◽  
Boundary Layer Separation ◽  
Linear Instability ◽  
Laminar Separation ◽  
Significant Control ◽  
Separated Shear Layer ◽  
Sound Control ◽  
Threshold Amplitude

The mechanism of separation control by sound excitation is investigated on the aft loaded LPT blade profile, the L1A, which experiences a large boundary layer separation at low Reynolds numbers. Previous work by the authors has shown that on a laminar separation bubble such as that experienced by the front-loaded L2F profile, sound excitation control has its best performance at the most unstable frequency of the shear layer due to the exploitation of the linear instability mechanism. The different loading distribution on the L1A increases the distance of the separated shear layer from the wall and the exploitation of the same linear mechanism is no longer effective in these conditions. However, significant control authority is found in the range of the first subharmonic of the natural unstable frequency. The amplitude of forced excitation required for significant wake loss reduction is higher than that needed when exploiting linear instability, but unlike the latter case, no threshold amplitude is found. The fluid-dynamics mechanisms under these conditions are investigated by PIV measurements. Phase-locked PIV data gives insight into the growth and development of structures as they are shed from the shear layer and merge to lock into the excited frequency. Unlike near-wall laminar separation sound control, it is found that when such large separated shear layers occur, sound excitation at subharmonics of the fundamental frequency is still effective with high Tu levels.

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.

Computational Study of Flow Characteristics of Thick and Thin Airfoil With Implicit Large-Eddy Simulation at Low Reynolds Number

2011 ◽  
Author(s):  
Ryoji Kojima ◽  
Taku Nonomura ◽  
Akira Oyama ◽  
Kozo Fujii
Keyword(s):  
Reynolds Number ◽  
Separation Bubble ◽  
Separated Flow ◽  
Leading Edge ◽  
Laminar Separation Bubble ◽  
Laminar Separation ◽  
Eddy Simulation ◽  
Naca0012 Airfoil ◽  
Large Eddy ◽  
Implicit Large Eddy Simulation

The flow fields around NACA0012 and NACA0002 at Reynolds number of 23,000, and their aerodynamic characteristics are analyzed. Computations are conducted with implicit large-eddy simulation solver and Reynolds-averaged-Navier-Stokes solver. Around this Reynolds number, the flow over an airfoil separates, transits and reattaches, resulting in generation of a laminar separation bubble at angle of attack in the range of certain degrees. Over a NACA0012 airfoil a separation point moves toward its leading edge with increasing angle of attack, and a separated flow may transit to create a short bubble. On the other hand, over a NACA0002 airfoil a separation point is kept at its leading edge, and a separated flow may transit to create a long bubble. Moreover, there appears nonlinearity in lift curve for NACA0012 airfoil, but does not appear in that for NACA0002 in spite of existence of a laminar separation bubble.

On the origin of the inflectional instability of a laminar separation bubble

2009 ◽  
Vol 629 ◽  
pp. 263-298 ◽  
Author(s):  
SOURABH S. DIWAN ◽  
O. N. RAMESH
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Shear Layer ◽  
Separation Bubble ◽  
Adverse Pressure Gradient ◽  
Maximum Height ◽  
Laminar Separation Bubble ◽  
Laminar Separation ◽  
Separated Shear Layer ◽  
The Mean

This is an experimental and theoretical study of a laminar separation bubble and the associated linear stability mechanisms. Experiments were performed over a flat plate kept in a wind tunnel, with an imposed pressure gradient typical of an aerofoil that would involve a laminar separation bubble. The separation bubble was characterized by measurement of surface-pressure distribution and streamwise velocity using hot-wire anemometry. Single component hot-wire anemometry was also used for a detailed study of the transition dynamics. It was found that the so-called dead-air region in the front portion of the bubble corresponded to a region of small disturbance amplitudes, with the amplitude reaching a maximum value close to the reattachment point. An exponential growth rate of the disturbance was seen in the region upstream of the mean maximum height of the bubble, and this was indicative of a linear instability mechanism at work. An infinitesimal disturbance was impulsively introduced into the boundary layer upstream of separation location, and the wave packet was tracked (in an ensemble-averaged sense) while it was getting advected downstream. The disturbance was found to be convective in nature. Linear stability analyses (both the Orr–Sommerfeld and Rayleigh calculations) were performed for mean velocity profiles, starting from an attached adverse-pressure-gradient boundary layer all the way up to the front portion of the separation-bubble region (i.e. up to the end of the dead-air region in which linear evolution of the disturbance could be expected). The conclusion from the present work is that the primary instability mechanism in a separation bubble is inflectional in nature, and its origin can be traced back to upstream of the separation location. In other words, the inviscid inflectional instability of the separated shear layer should be logically seen as an extension of the instability of the upstream attached adverse-pressure-gradient boundary layer. This modifies the traditional view that pegs the origin of the instability in a separation bubble to the detached shear layer outside the bubble, with its associated Kelvin–Helmholtz mechanism. We contend that only when the separated shear layer has moved considerably away from the wall (and this happens near the maximum-height location of the mean bubble), a description by the Kelvin–Helmholtz instability paradigm, with its associated scaling principles, could become relevant. We also propose a new scaling for the most amplified frequency for a wall-bounded shear layer in terms of the inflection-point height and the vorticity thickness and show it to be universal.

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