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.

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.

Experimental Study of Flow Separation over NACA633018 Wing with Synthetic Jet Control at Low Reynolds Numbers

2012 ◽  
Vol 29 (1) ◽  
pp. 45-52 ◽  
Author(s):  
C.-Y. Lin ◽  
F.-B. Hsiao
Keyword(s):  
Reynolds Number ◽  
Flow Separation ◽  
Separation Bubble ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Laminar Separation ◽  
Low Reynolds Numbers ◽  
Jet Control

AbstractThis paper experimentally studies flow separation and aerodynamic performance of a NACA633018 wing using a series of piezoelectric-driven disks, which are located at 12% chord length from the leading edge to generate a spanwise-distributed synthetic jets to excite the passing flow. The experiment is conducted in an open-type wind tunnel with Reynolds numbers (Re) of 8 × 104 and 1.2 × 105, respectively, based on the wing chord. The oscillations of the synthetic jet actuators (SJAs) disturb the neighboring passage flow on the upper surface of the wing before the laminar separation takes place. The disturbances of energy influence the downstream development of boundary layers to eliminate or reduce the separation bubble on the upper surface of the wing. Significant lift increase and drag decrease are found at the tested Reynolds number of 8 × 104 due to the actuators excitation. Furthermore, the effect of drag also reduces dominant with increasing Reynolds number, but the increase on lift is reduced with the Reynolds number increased.

scholarly journals The Effect of Reynolds Number on the Behaviour of a Leading Edge Separation Bubble

1996 ◽  
Author(s):  
Birinchi K. Hazarika ◽  
Charles Hirsch
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
Production Rate ◽  
Free Stream ◽  
Separation Bubble ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Separation Point ◽  
Separation Bubbles ◽  
Edge Separation

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.

Effect of Leading Edge Roughness and Reynolds Number on Compressor Profile Loss

2013 ◽  
Author(s):  
Ju Hyun Im ◽  
Ju Hyun Shin ◽  
Garth V. Hobson ◽  
Seung Jin Song ◽  
Knox T. Millsaps
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Separation Bubble ◽  
Leading Edge ◽  
Suction Side ◽  
Reynolds Numbers ◽  
Laser Doppler Velocimeter ◽  
Compressor Cascade ◽  
Edge Roughness ◽  
Profile Loss

An experimental investigation has been conducted to characterize the influence of leading edge roughness and Reynolds number on compressor cascade profile loss. Tests have been conducted in a low-speed linear compressor cascade at Reynolds numbers between 210,000 and 640,000. Blade loading and loss have been measured with pressure taps and pneumatic probes. In addition, a two-component laser-doppler velocimeter (LDV) has been used to measure the boundary layer velocity profiles and turbulence levels at various chordwise locations near the blade suction surface. The “smooth” blade has a centerline-averaged roughness (Ra) of 0.62 μm. The “rough” blade is roughened by covering the leading edge of the “smooth” blade, including 2% of the pressure side and 2% of the suction side, with a 100 μm-thick tape with a roughness Ra of 4.97 μm. At Reynolds numbers ranging from 210,000 to 380,000, the leading edge roughness decreases loss slightly. At Reynolds number of 210,000, the leading edge roughness reduces the size of the suction side laminar separation bubble and turbulence level in the turbulent boundary layer after reattachment. Thus, the leading edge roughness reduces displacement and momentum thicknesses as well as profile loss at Reynolds number of 210,000. However, the same leading edge roughness increases loss significantly for Re = 450,000 ∼ 640,000. At Reynolds number of 640,000, the leading edge roughness decreases the magnitude of the favorable pressure gradient for axial chordwise locations less than 0.41 and induces turbulent separation for axial chordwise locations greater than 0.63, drastically increasing loss. Thus, roughness limited to the leading edge still has a profound effect on the compressor flow field.

Acoustic and phase portrait analysis of leading-edge roughness element on laminar separation bubbles at low Reynolds number flow

2021 ◽  
pp. 095441002110443
Author(s):  
Hossein Jabbari ◽  
Esmaeili Ali ◽  
Mohammad Hasan Djavareshkian
Keyword(s):  
Reynolds Number ◽  
Phase Portrait ◽  
Separation Bubble ◽  
Roughness Element ◽  
Low Reynolds Number ◽  
Leading Edge ◽  
Suction Side ◽  
Laminar Separation ◽  
Roughness Elements ◽  
Edge Roughness

Since laminar separation bubbles are neutrally shaped on the suction side of full-span wings in low Reynolds number flows, a roughness element can be used to improve the performance of micro aerial vehicles. The purpose of this article was to investigate the leading-edge roughness element’s effect and its location on upstream of the laminar separation bubble from phase portrait point of view. Therefore, passive control might have an acoustic side effect, especially when the bubble might burst and increase noise. Consequently, the effect of the leading-edge roughness element features on the bubble’s behavior is considered on the acoustic pressure field and the vortices behind the NASA-LS0417 cross-section. The consequences express that the distribution of roughness in the appropriate dimensions and location could contribute to increasing the performance of the airfoil and the interaction of vortices produced by roughness elements with shear layers on the suction side has increased the sound frequency in the relevant sound pressure level (SPL). The results have demonstrated that vortex shedding frequency was increased in the presence of roughness compared to the smooth airfoil. Also, more complexity of the phase portrait circuits was found, retrieved from velocity gradient limitation. Likewise, the highest SPL is related to the state where the separation bubble phenomenon is on the surface versus placing roughness elements on the leading edge leads to a negative amount of SPL.

Shear Layer Development, Separation, and Stability Over a Low-Reynolds Number Airfoil

2018 ◽  
Vol 140 (7) ◽  
Author(s):  
Paul Ziadé ◽  
Mark A. Feero ◽  
Philippe Lavoie ◽  
Pierre E. Sullivan
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Shear Layer ◽  
Separation Bubble ◽  
Low Reynolds Number ◽  
Base Flow ◽  
Mean Velocity ◽  
Laminar Separation ◽  
Low Reynolds ◽  
Layer Development

The shear layer development for a NACA 0025 airfoil at a low Reynolds number was investigated experimentally and numerically using large eddy simulation (LES). Two angles of attack (AOAs) were considered: 5 deg and 12 deg. Experiments and numerics confirm that two flow regimes are present. The first regime, present for an angle-of-attack of 5 deg, exhibits boundary layer reattachment with formation of a laminar separation bubble. The second regime consists of boundary layer separation without reattachment. Linear stability analysis (LSA) of mean velocity profiles is shown to provide adequate agreement between measured and computed growth rates. The stability equations exhibit significant sensitivity to variations in the base flow. This highlights that caution must be applied when experimental or computational uncertainties are present, particularly when performing comparisons. LSA suggests that the first regime is characterized by high frequency instabilities with low spatial growth, whereas the second regime experiences low frequency instabilities with more rapid growth. Spectral analysis confirms the dominance of a central frequency in the laminar separation region of the shear layer, and the importance of nonlinear interactions with harmonics in the transition process.

scholarly journals Turbulence Amplification with Incidence at the Leading Edge of a Compressor Cascade

1999 ◽  
Vol 5 (2) ◽  
pp. 89-98 ◽  
Author(s):  
Garth V. Hobson ◽  
Bryce E. Wakefield ◽  
William B. Roberts
Keyword(s):  
Separation Bubble ◽  
Leading Edge ◽  
Laser Doppler Velocimeter ◽  
Free Shear Layer ◽  
Compressor Cascade ◽  
Suction Surface ◽  
Flow Angle ◽  
Laminar Separation ◽  
High Incidence ◽  
High Turbulence

Detailed measurements, with a two-component laser-Doppler velocimeter and a thermal anemometer were made near the suction surface leading edge of controlled-diffusion airfoils in cascade. The Reynolds number was near 700,000, Mach number equal to 0.25, and freestream turbulence was at 1.5% ahead of the cascade.It was found that there was a localized region of high turbulence near the suction surface leading edge at high incidence. This turbulence amplification is thought to be due to the interaction of the free-shear layer with the freestream inlet turbulence. The presence of the local high turbulence affects the development of the short laminar separation bubble that forms very near the suction side leading edge of these blades. Calculations indicate that the local high levels of turbulence can cause rapid transition in the laminar bubble allowing it to reattach as a short “non-burst” type.The high turbulence, which can reach point values greater than 25% at high incidence, is the reason that leading edge laminar separation bubbles can reattach in the high pressure gradient regions near the leading edge. Two variations for inlet turbulence intensity were measured for this cascade. The first is the variation ofmaximum inlet turbulence with respect to inlet-flow angle; and the second is the variation of leading edge turbulence with respect to upstream distance from the leading edge of the blades.

Three-Dimensional Vortex Structure in a Leading-Edge Separation Bubble at Moderate Reynolds Numbers

1991 ◽  
Vol 113 (3) ◽  
pp. 405-410 ◽  
Author(s):  
Kyuro Sasaki ◽  
Masaru Kiya
Keyword(s):  
Reynolds Number ◽  
Separation Bubble ◽  
Plate Thickness ◽  
Three Dimensional ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Vortex Filaments ◽  
Separated Shear Layer ◽  
Hydrogen Bubbles ◽  
Edge Separation

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.

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.

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