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

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.

Numerical and Experimental Investigation of the Reynolds Number and Reduced Frequency Effects on Low-Pressure Turbine Airfoils

2021 ◽  
Vol 143 (2) ◽  
Author(s):  
M. Bolinches-Gisbert ◽  
David Cadrecha Robles ◽  
Roque Corral ◽  
Fernando Gisbert
Keyword(s):  
Experimental Data ◽  
Numerical Data ◽  
Low Pressure ◽  
Reconstruction Method ◽  
Low Speed ◽  
Low Pressure Turbine ◽  
Reduced Frequency ◽  
Aerodynamic Properties ◽  
Numerical Solver ◽  
Turbine Airfoils

Abstract This article compares experimental and numerical data for a low-speed high-lift low pressure turbine (LPT) cascade under unsteady flow conditions. Three Reynolds numbers representative of LPTs have been tested, namely, 5 × 104, 105, and 2 × 105; at two reduced frequencies, fr = 0.5 and 1, also representative of LPTs. The experimental data were obtained at the low-speed linear cascade wind tunnel at the Polytechnic University of Madrid using hot wire, Laser Doppler Velocimetry (LDV), and pressure tappings. The numerical solver employs a sixth-order compact scheme based on the flux reconstruction method for spatial discretization and a fourth-order Runge–Kutta method to march in time. The longest case ran 550 h on 40 GPUs to reach a statistically periodic state. Pressure coefficients around the profile, boundary layer profiles and exit cross section distributions of velocity, pressure loss defect, shear Reynolds stress, and angle are compared against high-quality experimental data. Cascade loss and exit angle have also been compared against the experimental data. Very good agreement between experimental and numerical data is seen. The results demonstrate the suitability of the present methodology to predict the aerodynamic properties of unsteady flows around LPT linear cascades accurately.

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.

Flutter of Low Pressure Turbine Blades With Cyclic Symmetric Modes: A Preliminary Design Method

2004 ◽  
Vol 126 (2) ◽  
pp. 306-309 ◽  
Author(s):  
Robert Kielb ◽  
Jack Barter ◽  
Olga Chernycheva ◽  
Torsten Fransson
Keyword(s):  
Mode Shape ◽  
Design Method ◽  
Turbine Blades ◽  
Current Method ◽  
Low Pressure ◽  
Mode Shapes ◽  
Preliminary Design ◽  
Complex Mode ◽  
Low Pressure Turbine ◽  
Reduced Frequency

A current preliminary design method for flutter of low pressure turbine blades and vanes only requires knowledge of the reduced frequency and mode shape (real). However, many low pressure turbine (LPT) blade designs include a tip shroud that mechanically connects the blades together in a structure exhibiting cyclic symmetry. A proper vibration analysis produces a frequency and complex mode shape that represents two real modes phase shifted by 90 deg. This paper describes an extension to the current design method to consider these complex mode shapes. As in the current method, baseline unsteady aerodynamic analyses must be performed for the three fundamental motions, two translations and a rotation. Unlike the current method work matrices must be saved for a range of reduced frequencies and interblade phase angles. These work matrices are used to generate the total work for the complex mode shape. Since it still only requires knowledge of the reduced frequency and mode shape (complex), this new method is still very quick and easy to use. Theory and an example application are presented.

CFD Simulations of Unsteady Wakes on a Highly Loaded Low Pressure Turbine Airfoil (L1A)

2012 ◽  
Author(s):  
Mounir B. Ibrahim ◽  
Samuel Vinci ◽  
Olga Kartuzova ◽  
Ralph J. Volino
Keyword(s):  
Computational Study ◽  
Separation Bubble ◽  
Low Pressure ◽  
Flow Coefficient ◽  
Freestream Turbulence ◽  
Suction Surface ◽  
Cfd Simulations ◽  
Ansys Fluent ◽  
Low Pressure Turbine ◽  
Wake Passing

A study of a very high lift, low-pressure turbine airfoil in the presence of unsteady wakes was performed computationally and compared against experimental results. The experiments were conducted in a low speed wind tunnel under high (4.9%) and then low (0.6%) freestream turbulence intensity conditions with a flow coefficient (ζ) of 0.7. The experiments were done on a linear cascade with wakes that were produced from moving rods upstream of the cascade with the rod to blade spacing varied from 1 to 1.6 to 2. In the present study two different Reynolds numbers (25,000 and 50,000, based on the suction surface length and the nominal exit velocity from the cascade) were considered. The experimental and computational data have shown that in cases without wakes, the boundary layer separated and did not reattach. The CFD was performed with Large Eddy Simulation (LES) and Unsteady Reynolds-Averaged Navier-Stokes (URANS), Transition-SST, utilizing the finite-volume code ANSYS FLUENT under the same freestream turbulence and Reynolds number conditions as the experiment but only at a rod to blade spacing of 1. With wakes, separation was largely suppressed, particularly if the wake passing frequency was sufficiently high. Similar effect was predicted by 3D CFD simulations. Computational results for the pressure coefficients and velocity profiles were in a reasonable agreement with experimental ones for all cases examined. The 2D CFD efforts failed to capture the three dimensionality effects of the wake and thus were less consistent with the experimental data. As a further computational study, cases were run to simulate higher wake passing frequencies which were not run experimentally. The results of these computational cases showed that an initial 25% increase from the experimental dimensionless wake passing frequency of F = 0.45 greatly reduced the size of the separation bubble, nearly completely suppressing it, however an additional 33% increase on top of this did not prove to have much of an effect.

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.

The Impact of Blade Loading and Unsteady Pressure Bifurcations on Low-Pressure Turbine Flutter Boundaries

2015 ◽  
Author(s):  
Joshua J. Waite ◽  
Robert E. Kielb
Keyword(s):  
Mode Shape ◽  
Pressure Ratio ◽  
Low Pressure ◽  
Forced Response ◽  
Blade Loading ◽  
Unsteady Pressure ◽  
Aerodynamic Damping ◽  
Low Pressure Turbine ◽  
Reduced Frequency ◽  
The Impact

The three major aeroelastic issues in the turbomachinery blades of jet engines and power turbines are forced response, non-synchronous vibrations, and flutter. Flutter primarily affects high-aspect ratio blades found in the fan, fore high-pressure compressor stages, and aft low-pressure turbine (LPT) stages as low natural frequencies and high axial velocities create smaller reduced frequencies. Often with LPT flutter analyses, physical insights are lost in the exhaustive quest for determining whether the aerodynamic damping is positive or negative. This paper underlines some well known causes of low-pressure turbine flutter in addition to one novel catalyst. In particular, an emphasis is placed on revealing how local aerodynamic damping contributions change as a function of unsteady (e.g. mode shape, reduced frequency) and steady (e.g. blade torque, pressure ratio) parameters. To this end, frequency domain RANS CFD analyses are used as computational wind tunnels to investigate how aerodynamic loading variations affect flutter boundaries. Preliminary results show clear trends between the aerodynamic work influence coefficients and variations in exit Mach number and back pressure, especially for torsional mode shapes affecting the passage throat. Additionally, visualizations of qualitative bifurcations in the unsteady pressure phases around the airfoil shed light on how local damping contributions evolve with steady loading. Final results indicate a sharp drop in aeroelastic stability near specific regions of the pressure ratio indicating a strong correlation between blade loading and flutter. Passage throat shock behavior is shown to be a controlling factor near the trailing edge, and like critical reduced frequency, this phenomenon is shown to be highly dependent on the vibratory mode shape.

scholarly journals On the Identification and Decomposition of the Unsteady Losses in a Turbine Cascade

2018 ◽  
Author(s):  
D. Lengani ◽  
D. Simoni ◽  
R. Pichler ◽  
R. Sandberg ◽  
V. Michelassi ◽  
...  
Keyword(s):  
Flow Region ◽  
Suction Side ◽  
Spatial Location ◽  
Flow Coefficient ◽  
Core Flow ◽  
Low Pressure Turbine ◽  
The Core ◽  
Eddy Simulation ◽  
Reduced Frequency ◽  
Proper Orthogonal

The present paper describes the application of Proper Orthogonal Decomposition (POD) to Large Eddy Simulation (LES) of the T106A low-pressure-turbine profile with unsteady incoming wakes at two different flow conditions. Conventional data analysis applied to time averaged or phase-locked averaged flow fields is not always able to identify and quantify the different sources of losses in the unsteady flow field as they are able to isolate only the deterministic contribution. A newly developed procedure allows such identification of the unsteady loss contribution due to the migration of the incoming wakes, as well as to construct reduced order models able to highlight unsteady losses due to larger and/or smaller flow structures carried by the wakes in the different parts of the blade boundary layers. This enables a designer to identify the dominant modes (i.e. phenomena) responsible for loss, the associated generation mechanism, their dynamics and spatial location. The procedure applied to the two cases shows that losses in the fore part of the blade suction side are basically unaffected by the flow unsteadiness, irrespective of the reduced frequency and the flow coefficient. On the other hand, in the rear part of the suction side the unsteadiness contributes to losses prevalently due to the finer scale (higher order POD modes) embedded into the bulk of the incoming wake. The main difference between the two cases has been identified by the losses produced in the core flow region, where both the largest scale structures and the finer ones produces turbulence during migration. The decomposition into POD modes allows the quantification of this latter extra losses generated in the core flow region, providing further inputs to the designers for future optimization strategies.

Flutter of Low Pressure Turbine Blades With Cyclic Symmetric Modes: A Preliminary Design Method

2003 ◽  
Author(s):  
Robert Kielb ◽  
John Barter ◽  
Olga Chernysheva ◽  
Torsten Fransson
Keyword(s):  
Mode Shape ◽  
Design Method ◽  
Turbine Blades ◽  
Current Method ◽  
Low Pressure ◽  
Mode Shapes ◽  
Preliminary Design ◽  
Complex Mode ◽  
Low Pressure Turbine ◽  
Reduced Frequency

A current preliminary design method for flutter of low pressure turbine blades and vanes only requires knowledge of the reduced frequency and mode shape (real). However, many low pressure turbine (LPT) blade designs include a tip shroud, that mechanically connects the blades together in a structure exhibiting cyclic symmetry. A proper vibration analysis produces a frequency and complex mode shape that represents two real modes phase shifted by 90 degrees. This paper describes an extension to the current design method to consider these complex mode shapes. As in the current method, baseline unsteady aerodynamic analyses must be performed for the 3 fundamental motions, two translations and a rotation. Unlike the current method work matrices must be saved for a range of reduced frequencies and interblade phase angles. These work matrices are used to generate the total work for the complex mode shape. Since it still only requires knowledge of the reduced frequency and mode shape (complex), this new method is still very quick and easy to use. Theory and an example application are presented.

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