Numerical Investigation of Three-Dimensional Separation in an Axial Flow Compressor: The Influence of Free-Stream Turbulence Intensity and Endwall Boundary Layer State

2016 ◽  
Author(s):  
Ashley D. Scillitoe ◽  
Paul G. Tucker ◽  
Paolo Adami
Keyword(s):  
Boundary Layer ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Separation Bubble ◽  
Three Dimensional ◽  
Boundary Layer Transition ◽  
Surface Boundary ◽  
Suction Surface ◽  
Corner Separation ◽  
Free Stream Turbulence

Regions of three-dimensional separations are an inherent flow feature of the suction surface - endwall corner in axial compressors. These corner separations can cause a significant total pressure loss and reduce the compressor’s efficiency. This paper uses wall-resolved LES to investigate the loss sources in a corner separation, and examines the influence of the inflow turbulence on these sources. Different subgrid scale (SGS) models are tested and the choice of model is found to be important. The σ SGS model, which performed well, is then used to perform LES of a compressor endwall flow. The time-averaged data is in good agreement with measurements. The viscous and turbulent dissipation are used to highlight the sources of loss, with the latter being dominant. The key loss sources are seen to be the 2D laminar separation bubble and trailing edge wake, and the 3D flow region near the endwall. Increasing the free-stream turbulence intensity (FST) changes the suction surface boundary layer transition mode from separation induced to bypass. However, it doesn’t significantly alter the transition location and therefore the corner separation size. Additionally, the FST doesn’t noticeably interact with the corner separation itself, meaning that in this case the corner separation is relatively insensitive to the FST. The endwall boundary layer state is found to be significant. A laminar endwall boundary layer separates much earlier leading to a larger passage vortex. This significantly alters the endwall flow and loss. Hence, the need for accurate boundary measurements is clear.

Numerical Investigation of Three-Dimensional Separation in an Axial Flow Compressor: The Influence of Freestream Turbulence Intensity and Endwall Boundary Layer State

2016 ◽  
Vol 139 (2) ◽  
Author(s):  
Ashley D. Scillitoe ◽  
Paul G. Tucker ◽  
Paolo Adami
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Three Dimensional ◽  
Axial Flow ◽  
Boundary Layer Transition ◽  
Surface Boundary ◽  
Freestream Turbulence ◽  
Suction Surface ◽  
Layer Transition ◽  
Corner Separation

Regions of three-dimensional separations are an inherent flow feature of the suction surface-endwall corner in axial compressors. These corner separations can cause a significant total pressure loss and reduce the compressor's efficiency. This paper uses wall-resolved LES to investigate the loss sources in a corner separation, and examines the influence of the inflow turbulence on these sources. Different subgrid scale (SGS) models are tested and the choice of model is found to be important. The σ SGS model, which performed well, is then used to perform LES of a compressor endwall flow. The time-averaged data are in good agreement with measurements. The viscous and turbulent dissipation are used to highlight the sources of loss, with the latter being dominant. The key loss sources are seen to be the 2D laminar separation bubble and trailing edge wake, and the 3D flow region near the endwall. Increasing the freestream turbulence (FST) intensity changes the suction surface boundary layer transition mode from separation induced to bypass. However, it does not significantly alter the transition location and therefore the corner separation size. Additionally, the FST does not noticeably interact with the corner separation itself, meaning that in this case the corner separation is relatively insensitive to the FST. The endwall boundary layer state is found to be significant. A laminar endwall boundary layer separates much earlier leading to a larger passage vortex. This significantly alters the endwall flow and loss. Hence, the need for accurate boundary measurements is clear.

The Influence of Large-Scale, High-Intensity Turbulence on Vane Aerodynamic Losses, Wake Growth, and the Exit Turbulence Parameters

1997 ◽  
Vol 119 (2) ◽  
pp. 182-192 ◽  
Author(s):  
F. E. Ames ◽  
M. W. Plesniak
Keyword(s):  
Boundary Layer ◽  
High Intensity ◽  
Total Pressure ◽  
Large Scale ◽  
Free Stream ◽  
Turbulent Shear ◽  
Surface Boundary ◽  
Suction Surface ◽  
Free Stream Turbulence ◽  
Eddy Diffusivities

An experimental research program was undertaken to examine the influence of large-scale high-intensity turbulence on vane exit losses, wake growth, and exit turbulence characteristics. The experiment was conducted in a four-vane linear cascade at an exit Reynolds number of 800,000 based on chord length and an exit Mach number of 0.27. Exit measurements were made for four inlet turbulence conditions including a low-turbulence case (Tu ≈ 1 percent), a grid-generated turbulence case (Tu ≈ 7.5. percent) and two levels of large-scale turbulence generated with a mock combustor (Tu ≈ 12 and 8 percent). Exit total pressure surveys were taken at two locations to quantify total pressure losses. The suction surface boundary layer was also traversed to determine losses due to boundary layer growth. Losses occurred in the core of the flow for the elevated turbulence cases. The elevated free-stream turbulence was found to have a significant effect on wake growth. Generally, the wakes subjected to elevated free-stream turbulence were broader and had smaller peak velocity deficits. Reynolds stress profiles exhibited asymmetry in peak amplitudes about the wake centerline, which are attributable to differences in the evolution of the boundary layers on the pressure and suction surfaces of the vanes. The overall level of turbulence and dissipation inside the wakes and in the free stream was determined to document the rotor inlet boundary conditions. This is useful information for assessing rotor heat transfer and aerodynamics. Eddy diffusivities and mixing lengths were estimated using X-wire measurements of turbulent shear stress. The free-stream turbulence was found to strongly affect eddy diffusivities, and thus wake mixing. At the last measuring position, the average eddy diffusivity in the wake of the high-turbulence close combustor configuration (Tu ≈ 12) was three times that of the low turbulence wake.

scholarly journals Direct numerical simulations of transition in a compressor cascade: the influence of free-stream turbulence

2010 ◽  
Vol 665 ◽  
pp. 57-98 ◽  
Author(s):  
TAMER A. ZAKI ◽  
JAN G. WISSINK ◽  
WOLFGANG RODI ◽  
PAUL A. DURBIN
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Free Stream ◽  
Transition To Turbulence ◽  
Surface Boundary ◽  
Bypass Transition ◽  
Suction Surface ◽  
Surface Boundary Layer ◽  
Laminar Separation ◽  
Free Stream Turbulence

The flow through a compressor passage without and with incoming free-stream grid turbulence is simulated. At moderate Reynolds number, laminar-to-turbulence transition can take place on both sides of the aerofoil, but proceeds in distinctly different manners. The direct numerical simulations (DNS) of this flow reveal the mechanics of breakdown to turbulence on both surfaces of the blade. The pressure surface boundary layer undergoes laminar separation in the absence of free-stream disturbances. When exposed to free-stream forcing, the boundary layer remains attached due to transition to turbulence upstream of the laminar separation point. Three types of breakdowns are observed; they combine characteristics of natural and bypass transition. In particular, instability waves, which trace back to discrete modes of the base flow, can be observed, but their development is not independent of the Klebanoff distortions that are caused by free-stream turbulent forcing. At a higher turbulence intensity, the transition mechanism shifts to a purely bypass scenario. Unlike the pressure side, the suction surface boundary layer separates independent of the free-stream condition, be it laminar or a moderate free-stream turbulence of intensityTu~ 3%. Upstream of the separation, the amplification of the Klebanoff distortions is suppressed in the favourable pressure gradient (FPG) region. This suppression is in agreement with simulations of constant pressure gradient boundary layers. FPG is normally stabilizing with respect to bypass transition to turbulence, but is, thereby, unfavourable with respect to separation. Downstream of the FPG section, a strong adverse pressure gradient (APG) on the suction surface of the blade causes the laminar boundary layer to separate. The separation surface is modulated in the instantaneous fields of the Klebanoff distortion inside the shear layer, which consists of forward and backward jet-like perturbations. Separation is followed by breakdown to turbulence and reattachment. As the free-stream turbulence intensity is increased,Tu~ 6.5%, transitional turbulent patches are initiated, and interact with the downstream separated flow, causing local attachment. The calming effect, or delayed re-establishment of the boundary layer separation, is observed in the wake of the turbulent events.

The Influence of Large Scale, High Intensity Turbulence on Vane Aerodynamic Losses, Wake Growth, and the Exit Turbulence Parameters

1995 ◽  
Author(s):  
Forrest E. Ames ◽  
Michael W. Plesniak
Keyword(s):  
Boundary Layer ◽  
High Intensity ◽  
Total Pressure ◽  
Large Scale ◽  
Free Stream ◽  
Turbulent Shear ◽  
Surface Boundary ◽  
Suction Surface ◽  
Free Stream Turbulence ◽  
Eddy Diffusivities

An experimental research program was undertaken to examine the influence of large-scale high, intensity turbulence on vane exit losses, wake growth, and exit turbulence characteristics. The experiment was conducted in a four vane linear cascade at an exit Reynolds number of 800, 000 based on chord length and an exit Mach number of 0.27. Exit measurements were made for four inlet turbulence conditions including a low turbulence case (Tu ≈ 1%), a grid-generated turbulence case (Tu ≈ 7.5%), and two levels of large-scale turbulence generated with a mock combustor (Tu ≈ 12% & Tu ≈ 8%). Exit total pressure surveys were taken at two locations to quantify total pressure losses. The suction surface boundary layer was also traversed to determine losses due boundary layer growth. Losses were also found in the core of the flow for the elevated turbulence cases. The elevated free stream turbulence was found to have a significant effect on wake growth. Generally, the wakes subjected to elevated free stream turbulence were broader and had smaller peak velocity deficits. Reynolds stress profiles exhibited asymmetry in peak amplitudes about the wake centerline, which are attributable to differences in the evolution of the boundary layers on the pressure and suction surfaces of the vanes. The overall level of turbulence and dissipation inside the wakes and in the free stream was determined to document the rotor inlet boundary conditions. This is useful information for assessing rotor heat transfer and aerodynamics. Eddy diffusivities and mixing lengths were estimated using X-wire measurements of turbulent shear stress. The free stream turbulence was found to strongly affect eddy diffusivities, and thus wake mixing. At the last measuring position, the average eddy diffusivity in the wake of the high turbulence close combustor configuration (Tu ≈ 12) was three times that of the low turbulence wake.

Bypass Transition Modelling: A New Method Which Accounts for Free-Stream Turbulence Intensity and Length Scale

2000 ◽  
Author(s):  
Paul E. Roach ◽  
David H. Brierley
Keyword(s):  
Boundary Layer ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Leading Edge ◽  
Boundary Layer Transition ◽  
Test Surface ◽  
Length Scale ◽  
Pressure Gradients ◽  
Layer Transition ◽  
Free Stream Turbulence

The publication of the present authors’ boundary layer transition data in 1992 (now widely known as the ERCOFTAC test case T3) has led to a spate of new experimental and modelling efforts aimed at improving our understanding of this problem. This paper describes a new method of determining boundary layer transition with zero mean pressure gradient. The approach examines the development of a laminar boundary layer to the start of transition, accounting for the influences of free-stream turbulence and test surface geometry. It is presented as a “proof of concept”, requiring a significant amount of work before it can be considered as a practically applicable model for transition prediction. The method is based upon one first put forward by G.I. Taylor in the 1930’s, and accounts for the action of local, instantaneous pressure gradients on the developing laminar boundary layer. These pressure gradients are related to the intensity and length scale of turbulence in the free-stream using Taylor’s simple isotropic model. The findings demonstrate the need to account for the separate influences of free-stream turbulence intensity and length scale when considering the transition process. Although the length scale has less of an effect than the intensity, its influence is, nevertheless, significant and must not be overlooked. This fact goes a long way towards explaining the large scatter to be found in simple correlations which involve only the turbulence intensity. Intriguingly, it is demonstrated that it is the free-stream turbulence at the leading edge of the test surface which is important, not that found locally outside the boundary layer. The additional influence of leading edge geometry is also shown to play a major role in fixing the point at which transition begins. It is suggested that the leading edge geometry will distort the incident turbulent eddies, modifying the effective “free-stream” turbulence properties. Consequently, it is shown that the scale of the eddies relative to the leading edge thickness is a further important parameter, and helps bring together a large number of test cases.

Boundary-Layer Transition Affected by Surface Roughness and Free-Stream Turbulence

2005 ◽  
Vol 127 (3) ◽  
pp. 449-457 ◽  
Author(s):  
S. K. Roberts ◽  
M. I. Yaras
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Free Stream ◽  
Separation Bubble ◽  
Boundary Layer Transition ◽  
Test Cases ◽  
Layer Transition ◽  
Test Matrix ◽  
Free Stream Turbulence ◽  
Attached Flow

This paper presents experimental results documenting the effects of surface roughness and free-stream turbulence on boundary-layer transition. The experiments were conducted on a flat surface, upon which a pressure distribution similar to those prevailing on the suction side of low-pressure turbine blades was imposed. The test matrix consists of five variations in the roughness conditions, at each of three free-stream turbulence intensities (approximately 0.5%, 2.5%, and 4.5%), and two flow Reynolds numbers of 350,000 and 470,000. The ranges of these parameters considered in the study, which are typical of low-pressure turbines, resulted in both attached-flow and separation-bubble transition. The focus of the paper is on separation-bubble transition, but the few attached-flow test cases that occurred under high roughness and free-stream turbulence conditions are also presented for completeness of the test matrix. Based on the experimental results, the effects of surface roughness on the location of transition onset and the rate of transition are quantified, and the sensitivity of these effects to free-stream turbulence is established. The Tollmien–Schlichting instability mechanism is shown to be responsible for transition in each of the test cases presented. The root-mean-square height of the surface roughness elements, their planform size and spacing, and the skewness (bias towards depression or protrusion roughness) of the roughness distribution are shown to be relevant to quantifying the effects of roughness on the transition process.

Boundary-Layer Transition Over Rough Surfaces With Elevated Free-Stream Turbulence

2004 ◽  
Author(s):  
S. K. Roberts ◽  
M. I. Yaras
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Free Stream ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Boundary Layer Transition ◽  
Experimental Results ◽  
Layer Transition ◽  
Free Stream Turbulence ◽  
Attached Flow

This paper presents experimental results documenting the combined effects of surface roughness and free-stream turbulence level on boundary-layer transition. The experiments were conducted on a flat surface, upon which a pressure distribution similar to those prevailing on the suction side of turbine blades was imposed. The test matrix consists of four variations in the roughness conditions, at each of three free-stream turbulence levels and two flow Reynolds numbers. The ranges of these parameters considered in the study, which are typical of low-pressure turbines, resulted in both attached-flow and separation-bubble transition. The experimental results show that the transition inception location remains sensitive to surface roughness with increasing free-stream turbulence. Through spectral analysis of the velocity signals, this is shown to be due to earlier appearance and larger amplitude of Tollmien-Schlichting instability waves in both attached-flow and separation-bubble transition. In the test cases in which a separation-bubble is present, the rate of transition is seen to be insensitive to surface roughness, and only mildly sensitive to free-stream turbulence.

Investigation of Boundary Layer Transition and Separation in an Axial Turbine Cascade Using Glue-On Hot-Film Gages

1988 ◽  
Author(s):  
S. B. Vijayaraghavan ◽  
P. Kavanagh
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Boundary Layer Transition ◽  
Surface Boundary ◽  
Turbine Cascade ◽  
Suction Surface ◽  
Layer Transition ◽  
Axial Turbine ◽  
Free Stream Turbulence ◽  
Hot Film

Experiments were conducted with glue-on hot-film gages in a large-scale axial turbine cascade to identify transition and/or separation on the suction surface of the blade. Standard strain-gage type temperature sensors were adapted and used as the gages and Transition and separation were identified by examining the mean and RMS voltage output. To assist with interpreting the output of the gages, surface oil-flow visualizations were used. Results of this study showed that transition and separation could be easily identified with the hot-film gages. Depending upon the Reynolds number and free stream turbulence level, the suction surface boundary layer was found to undergo bubble-induced transition, natural transition, or a combination of both; i.e, a transition which started naturally but ended abruptly with a bubble.

An Experimental Study of Skewed Turbulent Boundary Layers in Low Speed Flows

1967 ◽  
Vol 89 (3) ◽  
pp. 597-607 ◽  
Author(s):  
G. P. Francis ◽  
F. J. Pierce
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Three Dimensional ◽  
Turbulent Boundary Layers ◽  
Free Stream Turbulence ◽  
Wire Probe ◽  
Curved Channels ◽  
Flow Directions

A detailed experimental investigation is described for the flow of air in skewed turbulent boundary layers on the floor of various curved channels. Measurements of time average velocities, flow directions, turbulence intensities, and growth are made for both the development and decay of the skewed boundary layer. All measurements were made with a unique hot wire probe arrangement. Tests were run with a free stream Reynolds number of approximately one million per ft and a free stream turbulence intensity of approximately 0.0016. The boundary layer was of the order of one inch in thickness. Both the turbulence intensity and velocity profiles indicate that the inner and outer regions, characteristic of two-dimensional profiles, are not necessarily appropriate to skewed three-dimensional flows.

Influence of Free-Stream Turbulence on Boundary Layer Transition in Favorable Pressure Gradients

1982 ◽  
Vol 104 (4) ◽  
pp. 743-750 ◽  
Author(s):  
M. F. Blair
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Pressure Gradient ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Boundary Layer Transition ◽  
Wall Heat Transfer ◽  
Profile Data ◽  
Mean Velocity ◽  
Free Stream Turbulence

Results from an experimental study of large-scale, two-dimensional incompressible transitional boundary layer flows are presented. Tests were conducted on a heated flat wall with a zero pressure gradient and for two levels of “sink” streamwise acceleration; k = ν/U2 ∂U/∂x = 0.2 or 0.75 × 10−6. Free-stream turbulence intensity levels ranged from approximately 0.7 to 5 percent with limited data obtained outside these values. Convective heat-transfer distributions, laminar, transitional, and fully turbulent boundary layer mean velocity and temperature profile data, and free-stream turbulence intensity distributions are presented. Boundary layer integral quantities and shape factors are also given. Transition onset Reynolds number data obtained for this program agreed well with the results of other experimental and theoretical studies for both zero pressure gradient and accelerating flows. Comparisons of the profile data and wall heat-transfer distribution data indicated that fully turbulent mean velocity profiles were achieved upstream of fully turbulent wall heat-transfer rates.

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