High-Fidelity Simulations of Low-Pressure Turbines: Effect of Flow Coefficient and Reduced Frequency on Losses

2016 ◽  
Vol 138 (11) ◽  
Author(s):  
V. Michelassi ◽  
L. Chen ◽  
R. Pichler ◽  
R. Sandberg ◽  
R. Bhaskaran
Keyword(s):  
Boundary Layer ◽  
Low Pressure ◽  
Flow Coefficient ◽  
High Fidelity ◽  
Clear Trend ◽  
Low Pressure Turbine ◽  
Reduced Frequency ◽  
Unsteady Wake ◽  
Large Eddy ◽  
Wake Distortion

Large eddy simulations validated with the aid of direct numerical simulation (DNS) are used to study the concerted action of reduced frequency and flow coefficient on the performance of the T106A low-pressure turbine profile. The simulations are carried out by using a discretization in space and time that allows minimizing the accuracy loss with respect to DNS. The reference Reynolds number is 100,000, while reduced frequency and flow coefficient cover a range wide enough to provide valid qualitative information to designers. The various configurations reveal differences in the loss generation mechanism that blends steady and unsteady boundary layer losses with unsteady wake ingestion losses. Large values of the flow coefficient can alter the pressure side unsteadiness and the consequent loss generation. Low values of the flow coefficient are associated with wake fogging and reduced unsteadiness around the blade. The reduced frequency further modulates these effects. The simulations also reveal a clear trend of losses with the wake path, discussed by conducting a loss-breakdown analysis that distinguishes boundary layer from wake distortion losses.

High-Fidelity Simulations of Low-Pressure Turbines: Effect of Flow Coefficient and Reduced Frequency on Losses

2015 ◽  
Author(s):  
V. Michelassi ◽  
L. Chen ◽  
R. Pichler ◽  
R. Sandberg ◽  
R. Bhaskaran
Keyword(s):  
Numerical Simulation ◽  
Boundary Layer ◽  
Direct Numerical Simulation ◽  
Low Pressure ◽  
Flow Coefficient ◽  
Clear Trend ◽  
Low Pressure Turbine ◽  
Reduced Frequency ◽  
Unsteady Wake ◽  
Wake Distortion

Large Eddy Simulations validated with the aid of Direct Numerical Simulation are used to study the concerted action of reduced frequency and flow coefficient on the performance of the T106A low-pressure-turbine profile. The simulations are carried out by using a discretization in space and time that allows minimizing the accuracy loss with respect to Direct Numerical Simulation. The reference Reynolds number is 100,000, while reduced frequency and flow coefficient cover a range wide enough to provide valid qualitative information to designers. The various configurations reveal differences in the loss generation mechanism that blends steady and unsteady boundary layer losses with unsteady wake ingestion losses. Large values of the flow coefficient can alter the pressure side unsteadiness, and the consequent loss generation. Low values of the flow coefficient are associated with wake fogging and reduced unsteadiness around the blade. The reduced frequency further modulates these effects. The simulations also reveal a clear trend of losses with the wake path, discussed by conducting a loss-breakdown analysis that distinguishes boundary layer from wake distortion losses.

Effects of Periodic Wakes and Freestream Turbulence on Coherent Structures in Low-Pressure Turbine Boundary Layer

2012 ◽  
Author(s):  
Weihao Zhang ◽  
Zhengping Zou ◽  
Kun Zhou ◽  
Huoxing Liu ◽  
Jian Ye
Keyword(s):  
Boundary Layer ◽  
Coherent Structures ◽  
Low Pressure ◽  
Turbine Cascade ◽  
Freestream Turbulence ◽  
High Lift ◽  
Low Pressure Turbine ◽  
Reduced Frequency ◽  
Large Eddy ◽  
Separation Bubbles

The effects of periodic wakes and inlet freestream turbulence intensity (FSTI) on coherent structures in the boundary layer of a high-lift low-pressure turbine cascade are studied in this paper. Large-eddy simulations (LES) are performed on T106D-EIZ profile at Reynolds number (Re) of 60,154 (based on the chord and outflow velocity). Eight cases, considering FSTI of 0, 2.5%, 5% and 10% as well as the wake reduced frequency (fr) of 0.67, 1.34 and 0.335, are conducted and discussed. The results show that the open separation could be compressed by freestream turbulence to a small extent, whereas, it could be replaced by separation bubbles under wake conditions. Stripe structures and turbulence spots appear in shear layer over the separation bubbles. The increments of wake frequency or FSTI can accelerate the transition progress which result in shorter separation bubbles, meanwhile, emphasize the turbulence spots.

Flow Coefficient and Reduced Frequency Effects on Low Pressure Turbine Unsteady Losses

2021 ◽  
pp. 1-12
Author(s):  
Edward Canepa ◽  
Davide Lengani ◽  
Alessandro Nilberto ◽  
Daniele Petronio ◽  
Daniele Simoni ◽  
...  
Keyword(s):  
Low Pressure ◽  
Flow Coefficient ◽  
Frequency Effects ◽  
Low Pressure Turbine ◽  
Reduced Frequency

Recognition of Structures Leading to Transition in a Low Pressure Turbine Cascade: Effect of Reduced Frequency

2019 ◽  
Author(s):  
D. Lengani ◽  
D. Simoni ◽  
M. Ubaldi ◽  
P. Zunino ◽  
F. Bertini
Keyword(s):  
Boundary Layer ◽  
Suction Side ◽  
Low Pressure ◽  
Turbine Cascade ◽  
Turbulent Structures ◽  
Time Resolved ◽  
Low Pressure Turbine ◽  
Reduced Frequency ◽  
Displacement Thickness ◽  
Wake Passing

Abstract The boundary layer developing over the suction side of a low pressure turbine cascade operating under unsteady inflow conditions has been experimentally investigated. Time-resolved Particle Image Velocimetry (PIV) measurements have been performed in two orthogonal planes, the blade to blade and a wall parallel plane embedded within the boundary layer, for two different wake reduced frequencies. Proper Orthogonal Decomposition (POD) has been used to analyze the data and to provide an interpretation of the most significant flow structures for each phase of the wake passing cycle. To this purpose, a POD based procedure that sorts the data synchronizing the measurements of the two planes has been developed. Phase averaged data are then obtained for both cases. Moreover, once properly sorted, POD has been applied to sub-ensembles of data at the same relative phase within the wake passing cycle. Detailed information on the most energetic turbulent structures at a particular phase are obtained with this procedure (called phased POD), overcoming the limit of classical phase average that just provides a statistical representation of the turbulence field. Furthermore, the synchronization of the measurements in the two planes allows the computation of the characteristic dimension of boundary layer structures that are responsible for transition. These structures are often identified as vortical filaments parallel to the wall, typically referred to as boundary layer streaks. The largest and most energetic structures are observed when the wake centerline passes over the rear part of the suction side, and they appear practically the same for both reduced frequencies. The passing wake forces transition leading to the breakdown of the boundary layer streaks. Otherwise, the largest differences between the low and high reduced frequency are observed in the calmed region. The post-processing of these two planes further allowed us to compute the spacing of the streaks and make it non-dimensional by the boundary layer displacement thickness observed for each phase. The non-dimensional value of the streaks spacing is about constant, irrespective of the reduced frequency.

Experimental Study of the Effect of Periodic Unsteady Wake Flow on Boundary Layer Development, Separation, and Reattachment Along the Surface of a Low Pressure Turbine Blade

2004 ◽  
Vol 126 (4) ◽  
pp. 663-676 ◽  
Author(s):  
M. T. Schobeiri ◽  
B. O¨ztu¨rk
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Wake Flow ◽  
Suction Surface ◽  
Low Pressure Turbine ◽  
Reduced Frequency ◽  
Boundary Layer Development ◽  
Unsteady Wake ◽  
Unsteady Wake Flow ◽  
Layer Development

The paper experimentally studies the effects of periodic unsteady wake flow on boundary layer development, separation and reattachment along the suction surface of a low pressure turbine blade. The experimental investigations were performed on a large scale, subsonic unsteady turbine cascade research facility at the Turbomachinery Performance and Flow Research Laboratory (TPFL), Texas A&M University. The experiments were carried out at a Reynolds number of 110,000 (based on suction surface length and exit velocity) with a free-stream turbulence intensity of 1.9%. One steady and two different unsteady inlet flow conditions with the corresponding passing frequencies, wake velocities, and turbulence intensities were investigated. The reduced frequencies cover the entire operating range of LP turbines. In addition to the unsteady boundary layer measurements, blade surface measurements were performed at the same Reynolds number. The surface pressure measurements were also carried out at one steady and two periodic unsteady inlet flow conditions. The results presented in ensemble-averaged, and the contour plot forms help to understand the physics of the separation phenomenon under periodic unsteady wake flow. It was found that the suction surface displayed a strong separation bubble for these three different reduced frequencies. For each condition, the locations and the heights defining the separation bubble were determined by carefully analyzing and examining the pressure and the mean velocity profile data. The location of boundary layer separation was independent of the reduced frequency level. However, the extent of the separation was strongly dependent on the reduced frequency level. Once the unsteady wake started to penetrate into the separation bubble, the turbulent spot produced in the wake paths caused a reduction of the separation bubble height.

Experimental Study of the Effect of Periodic Unsteady Wake Flow on Boundary Layer Development, Separation, and Re-Attachment Along the Surface of a Low Pressure Turbine Blade

2004 ◽  
Author(s):  
M. T. Schobeiri ◽  
B. O¨ztu¨rk
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Wake Flow ◽  
Suction Surface ◽  
Low Pressure Turbine ◽  
Reduced Frequency ◽  
Boundary Layer Development ◽  
Unsteady Wake ◽  
Unsteady Wake Flow ◽  
Layer Development

The paper experimentally studies the effects of periodic unsteady wake flow on boundary layer development, separation and re-attachment along the suction surface of a low pressure turbine blade. The experimental investigations were performed on a large scale, subsonic unsteady turbine cascade research facility at Turbomachinery Performance and Flow Research Laboratory (TPFL), Texas A&M University. The experiments were carried out at a Reynolds number of 110,000 (based on suction surface length and exit velocity) with a free-stream turbulence intensity of 1.9%. One steady and two different unsteady inlet flow conditions with the corresponding passing frequencies, wake velocities, and turbulence intensities were investigated. The reduced frequencies cover the entire operating range of LP turbines. In addition to the unsteady boundary layer measurements, blade surface measurements were performed at the same Reynolds number. The surface pressure measurements were also carried out at one steady and two periodic unsteady inlet flow conditions. The results presented in ensemble-averaged, and the contour plot forms help to understand the physics of the separation phenomenon under periodic unsteady wake flow. It was found that the suction surface displayed a strong separation bubble for these three different reduced frequencies. For each condition, the locations and the heights defining the separation bubble were determined by carefully analyzing and examining the pressure and the mean velocity profile data. The location of boundary layer separation was independent of the reduced frequency level. However, the extent of the separation was strongly dependent on the reduced frequency level. Once the unsteady wake started to penetrate into the separation bubble, the turbulent spot produced in the wake paths caused a reduction of the separation bubble height.

The Effects of Periodic Wakes on Boundary Layer Separation of Low-Pressure Turbine Using Large Eddy Simulation

2016 ◽  
Author(s):  
Xiaodi Wu ◽  
Fu Chen ◽  
Yunfei Wang
Keyword(s):  
Boundary Layer ◽  
Flow Field ◽  
Boundary Layer Separation ◽  
Low Pressure ◽  
Layer Separation ◽  
Low Pressure Turbine ◽  
Eddy Simulation ◽  
Instantaneous Flow Field ◽  
Large Eddy ◽  
Scale Structures

For low-pressure turbine, the unsteady disturbances are dominated by relative motions between rotors and stators and the unsteady flow is closely associated with aerodynamic efficiency of low-pressure turbine and engine performance. One of its most important manifestations is the boundary layer separation on the turbine blades by the passing wakes produced by upstream rows of blades. Hence, accurate prediction of the flow physics at low Reynolds number conditions is required to effectively implement flow control techniques which can help mitigate separation induced losses. The present paper concentrates on simulations for boundary layer separation of low-pressure turbine cascade under periodic wakes. In this paper, a multiblock computational fluid dynamics (CFD) code of compressible N-S equations is developed for predicting the phenomenon of boundary layer separation, transition and reattachment using large eddy simulation (LES) in the field of turbomachinery. The large-scale structures can be directly obtained from the solution of the filtered Naiver-Strokes equations and the small-scale structures are modeled by dynamic subgrid-scale model of turbulence. Firstly, unsteady boundary layer separation on a flat plate with adverse pressure gradient is simulated under periodic inflow. The time-averaged field, the phase-averaged field and the instantaneous flow field are presented and analyzed. The separation bubble becomes unstable and the location of transition moves back and forth due to vortex shedding. Secondly, a stator of turbomachinery which is influenced by wakes periodically passing is simulated. The results of the numerical simulations are discussed and compared with experimental data. For the instantaneous flow field, it seems that the spanwise vortices induced by upstream wakes are the primary reason of the initial roll-up of the shear layer and the Kelvin-Helmholtz instability plays an important role in the transition to turbulence which is observed in the separated flow.

Effects of periodic wakes on boundary layer development on an ultra-high-lift low pressure turbine airfoil

2016 ◽  
Vol 231 (1) ◽  
pp. 25-38 ◽  
Author(s):  
Xingen Lu ◽  
Yanfeng Zhang ◽  
Wei Li ◽  
Shuzhen Hu ◽  
Junqiang Zhu
Keyword(s):  
Boundary Layer ◽  
Transition Process ◽  
Low Pressure ◽  
Suction Surface ◽  
High Lift ◽  
Low Pressure Turbine ◽  
Turbulent Transition ◽  
Boundary Layer Development ◽  
Unsteady Wake ◽  
Layer Development

The laminar-turbulent transition process in the boundary layer is of significant practical interest because the behavior of this boundary layer largely determines the overall efficiency of a low pressure turbine. This article presents complementary experimental and computational studies of the boundary layer development on an ultra-high-lift low pressure turbine airfoil under periodically unsteady incoming flow conditions. Particular emphasis is placed on the influence of the periodic wake on the laminar-turbulent transition process on the blade suction surface. The measurements were distinctive in that a closely spaced array of hot-film sensors allowed a very detailed examination of the suction surface boundary layer behavior. Measurements were made in a low-speed linear cascade facility at a freestream turbulence intensity level of 1.5%, a reduced frequency of 1.28, a flow coefficient of 0.70, and Reynolds numbers of 50,000 and 100,000, based on the cascade inlet velocity and the airfoil axial chord length. Experimental data were supplemented with numerical predictions from a commercially available Computational Fluid Dynamics code. The wake had a significant influence on the boundary layer of the ultra-high-lift low pressure turbine blade. Both the wake’s high turbulence and the negative jet behavior of the wake dominated the interaction between the unsteady wake and the separated boundary layer on the suction surface of the ultra-high-lift low pressure turbine airfoil. The upstream unsteady wake segments convecting through the blade passage behaved as a negative jet, with the highest turbulence occurring above the suction surface around the wake center. Transition of the unsteady boundary layer on the blade suction surface was initiated by the wake turbulence. The incoming wakes promoted transition onset upstream, which led to a periodic suppression of the separation bubble. The loss reduction was a compromise between the positive effect of the separation reduction and the negative effect of the larger turbulent-wetted area after reattachment due to the earlier boundary layer transition caused by the unsteady wakes. It appeared that the successful application of ultra-high-lift low pressure turbine blades required additional loss reduction mechanisms other than “simple” wake-blade interaction.

Low Pressure Turbine Relaminarization Bubble Characterization using Massively-Parallel Large Eddy Simulations

2012 ◽  
Vol 134 (2) ◽  
Author(s):  
Shriram Jagannathan ◽  
Markus Schwänen ◽  
Andrew Duggleby
Keyword(s):  
Boundary Layer ◽  
Reverse Flow ◽  
Leading Edge ◽  
Suction Side ◽  
Low Pressure ◽  
Surface Boundary ◽  
Suction Surface ◽  
Hairpin Vortices ◽  
Low Pressure Turbine ◽  
Large Eddy

The separation and reattachment of suction surface boundary layer in a low pressure turbine is characterized using large-eddy simulation at Ress = 69000 based on inlet velocity and suction surface length. Favorable comparisons are drawn with experiments using a high pass filtered Smagorinsky model for sub-grid scales. The onset of time mean separation is at s/so = 0.61 and reattachment at s/so = 0.81, extending over 20% of the suction surface. The boundary layer is convectively unstable with a maximum reverse flow velocity of about 13% of freestream. The breakdown to turbulence occurs over a very short distance of suction surface and is followed by reattachment. Turbulence near the bubble is further characterized using anisotropy invariant mapping and time orthogonal decomposition diagnostics. Particularly the vortex shedding and shear layer flapping phenomena are addressed. On the suction side, dominant hairpin structures near the transitional and turbulent flow regime are observed. The hairpin vortices are carried by the freestream even downstream of the trailing edge of the blade with a possibility of reaching the next stage. Longitudinal streaks that evolve from the breakdown of hairpin vortices formed near the leading edge are observed on the pressure surface.

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