Surface Roughness Impact on Boundary Layer Transition and Loss Mechanisms over a Flat-Plate under a Low-Pressure Turbine Pressure Gradient

2021 ◽  
pp. 1-40
Author(s):  
Heechan Jeong ◽  
Seung Jin Song
Keyword(s):  
Surface Roughness ◽  
Pressure Gradient ◽  
Shear Layer ◽  
Flat Plate ◽  
Turbulence Intensity ◽  
Turbulent Mixing ◽  
Separation Bubble ◽  
Low Pressure Turbine ◽  
Separated Shear Layer ◽  
Profile Loss

Abstract An experimental study has been conducted to investigate the effects of surface roughness on the profile loss of a flat-plate with a contoured wall. All of the measurements have been conducted for the suction side pressure gradient of a high-lift low pressure turbine airfoil at the fixed Reynolds number (Rec) and freestream turbulence intensity (Tu) of 1.2 · 105 and 3.2%, respectively, representing a cruise condition. The time-resolved streamwise and wall-normal velocity fields for three different surface roughness values of Ra/C · 105 = 0.065, 4.417 and 7.428 have been measured with a 2D hot-wire probe. For the smooth surface, a laminar separation bubble forms from about 60% of the chord; and laminar-to-turbulent transition occurs during reattachment. Since the portion of turbulent flow over the flat-plate is relatively small, the overall profile loss is mainly determined by the momentum deficit generated during transition. Increased roughness decreases the maximum height and length of the separation bubble but does not affect the separation bubble onset location. The beneficial effects of increased surface roughness on the profile loss appear in the separated shear layer and reattachment. Increased surface roughness increases turbulent mixing in the separated shear layer. Thus, the shear layer thickness and momentum deficit are reduced. In addition, increased surface roughness reduces the length scale and turbulence intensity of the shed vortices. Consequently, turbulent mixing and momentum deficit during reattachment of boundary layers are decreased, resulting in a lower profile loss.

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.

Experimental and Computational Investigations of Separation and Transition on a Highly Loaded Low-Pressure Turbine Airfoil: Part 2 — High Freestream Turbulence Intensity

2008 ◽  
Author(s):  
Ralph J. Volino ◽  
Olga Kartuzova ◽  
Mounir B. Ibrahim
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Shear Layer ◽  
Separation Bubble ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Transition Model ◽  
Freestream Turbulence ◽  
Low Pressure Turbine ◽  
Separated Shear Layer

Boundary layer separation, transition and reattachment have been studied on a very high lift, low-pressure turbine airfoil. Experiments were done under high (4%) freestream turbulence conditions on a linear cascade in a low speed wind tunnel. Pressure surveys on the airfoil surface and downstream total pressure loss surveys were documented. Velocity profiles were acquired in the suction side boundary layer at several streamwise locations using hot-wire anemometry. Cases were considered at Reynolds numbers (based on the suction surface length and the nominal exit velocity from the cascade) ranging from 25,000 to 300,000. At the lowest Reynolds number the boundary layer separated and did not reattach, in spite of transition in the separated shear layer. At higher Reynolds numbers the boundary layer did reattach, and the separation bubble became smaller as Re increased. High freestream turbulence increased the thickness of the separated shear layer, resulting in a thinner separation bubble. This effect resulted in reattachment at intermediate Reynolds numbers, which was not observed at the same Re under low freestream turbulence conditions. Numerical simulations were performed using an unsteady Reynolds averaged Navier-Stokes (URANS) code with both a shear stress transport k-ω model and a 4 equation shear stress transport Transition model. Both models correctly predicted separation and reattachment (if it occurred) at all Reynolds numbers. The Transition model generally provided better quantitative results, correctly predicting velocities, pressure, and separation and transition locations. The model also correctly predicted the difference between high and low freestream turbulence cases.

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.

Influence of Surface Roughness on the Flat-Plate Boundary Layer Transition Under a High-Lift Airfoil Pressure Gradient and Low Freestream Turbulence

2021 ◽  
Author(s):  
Heechan Jeong ◽  
Seung Jin Song
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Pressure Gradient ◽  
Flat Plate ◽  
Separation Bubble ◽  
Plate Boundary ◽  
Freestream Turbulence ◽  
High Lift ◽  
Displacement Thickness ◽  
Wetted Area

Abstract Effects of surface roughness on the transition of flat-plate boundary layers under a high-lift airfoil pressure gradient with low incoming freestream turbulence level have been investigated. Time-resolved streamwise and wall-normal velocity fields with surface roughness values of Ra/C = 0.065·10−5, 4.417·10−5 and 7.428·10−5 have been measured at a fixed Reynolds number of 5.2·105 and freestream turbulence intensity of 0.2%. For the reference Smooth surface of Ra/C = 0.065·10−5, a laminar separation bubble forms from about 64% to 83% of the chord length. Displacement thickness increases downstream of separation and then decreases during the transition (reattachment), and momentum thickness increases due to the vortices shed from the separation bubble. Increasing surface roughness has little impact on the laminar boundary layer separation onset but reduces the height and length of the separation bubble and induces earlier transition. For Ra/C = 4.417·10−5, displacement thickness during transition is slightly thinner and the overall momentum deficit is slightly lower than those for Ra/C = 0.065·10−5. For Ra/C = 7.428·10−5, the separation bubble becomes hardly visible as the transition mode approaches the attached mode, and turbulent mixing by the wall-bounded turbulence becomes dominant. In addition, the portion of turbulent wetted area increases due to earlier transition, and momentum deficit increases more rapidly in the turbulent wetted area. Thus, the overall momentum deficit for Ra/C = 7.428·10−5 is larger than that for Ra/C = 0.065·10−5.

Aerodynamic Measurements on the Interaction of Secondary Jets and Separation Bubble

2012 ◽  
Author(s):  
A. Samson ◽  
S. Sarkar
Keyword(s):  
Flat Plate ◽  
Large Scale ◽  
Separation Bubble ◽  
Three Dimensional ◽  
Leading Edge ◽  
Bubble Interactions ◽  
Separated Shear Layer ◽  
Bubble Length ◽  
Flow Visualizations ◽  
Turbulence Generation

The dynamics of separation bubble under the influence of continuous jets ejected near the semi-circular leading edge of a flat plate is presented. Two different streamwise injection angles 30° and 60° and velocity ratios 0.5 and 1 for Re = 25000 and 55000 (based on the leading-edge diameter) are considered here. The flow visualizations illustrating jet and separated layer interactions have been carried out with PIV. The objective of this study is to understand the mutual interactions of separation bubble and the injected jets. It is observed that flow separates at the blending point of semi-circular arc and flat plate. The separated shear layer is laminar up to 20% of separation length after which perturbations are amplified and grows in the second-half of the bubble leading to breakdown and reattachment. Blowing has significantly affected the bubble length and thus, turbulence generation. Instantaneous flow visualizations supports the unsteadiness and development of three-dimensional motions leading to formation of Kelvin-Helmholtz rolls and shedding of large-scale vortices due to jet and bubble interactions. In turn, it has been seen that both the spanwise and streamwise dilution of injected air is highly influenced by the separation bubble.

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.

Numerical Study of Turbulent Spot Development in a Separated Shear Layer

2007 ◽  
Author(s):  
B. R. McAuliffe ◽  
M. I. Yaras
Keyword(s):  
Shear Layer ◽  
Internal Structure ◽  
Numerical Study ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Freestream Turbulence ◽  
Helmholtz Instability ◽  
Turbulent Spots ◽  
Separated Shear Layer ◽  
Vortex Loops

The development of turbulent spots in a separation bubble under elevated freestream turbulence levels is examined through direct numerical simulation. The flow Reynolds number, freestream turbulence level, and streamwise pressure distribution are typical of the conditions encountered on the suction side of low-pressure turbine blades of gas-turbine engines. Based on the simulation results, the spreading and propagation rates of the turbulent spots and their internal structure are documented, and comparisons are made to empirical correlations that are used for predicting the transverse growth and streamwise propagation characteristics of turbulent spots. The internal structure of the spots is identified as a series of vortex loops that develop as a result of low-velocity streaks generated in the shear layer. A frequency that is approximately 50% higher than that of the Kelvin-Helmholtz instability is identified in the separated shear layer, which is shown to be associated with convection of these vortex loops through the separated shear layer. While freestream turbulence is noted to promote breakdown of the laminar separated shear layer into turbulence through the generation of turbulent spots, evidence is found to suggest co-existence of the Kelvin-Helmholtz instability, including the possibility of breakdown to turbulence through this mechanism.

The Effects of Wake-Passing Unsteadiness Over a Highly-Loaded Compressor-Like Flat Plate

2002 ◽  
Author(s):  
Xavier Ottavy ◽  
Stephane Vilmin ◽  
Maciek Opoka ◽  
Howard Hodson ◽  
Simon Gallimore
Keyword(s):  
Flat Plate ◽  
Steady Flow ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Axial Compressor ◽  
Suction Side ◽  
Compressor Blade ◽  
Low Pressure Turbine ◽  
Unsteady Effects ◽  
Calming Effect

The present study is concerned with wake-induced unsteady effects in axial-compressor blade rows. The goal is to exploit these effects in order to design high-lift blades without increasing the profile loss, as has been achieved for low-pressure turbine blades. In the first part of this paper, the experimental means and the computational fluid dynamics tools are described. The rig features a flat plate that can be subjected to different velocity distributions representative of the suction side of a real compressor blade. Cylindrical bars mounted on a moving system simulate the incoming wakes from the upstream blade row in the compressor. Results are presented for steady flow and for unsteady compressor-like conditions. In all cases, the separation bubble of the steady flow is suppressed by the turbulence that is induced in the boundary layer by the wakes at approximately 10% of the suction side. Its reappearance is then delayed by a region of stable laminar-like flow and low loss, the so-called calming effect that follows the wake-induced patch. The paper describes these phenomena for an example of configuration. It is then shown that it should be possible to increase the lift by 35% while keeping the same level of loss as the initial conventional pressure distribution of the study.

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