On the Simulation and Spectral Analysis of Unsteady Turbulence and Transition Effects in a Multistage Low Pressure Turbine

2019 ◽  
Vol 141 (5) ◽  
Author(s):  
Georg Geiser ◽  
Jens Wellner ◽  
Edmund Kügeler ◽  
Anton Weber ◽  
Anselm Moors
Keyword(s):  
Frequency Domain ◽  
Large Scale ◽  
Separation Bubble ◽  
Pressure Fluctuations ◽  
Low Pressure ◽  
Transition Flow ◽  
Low Pressure Turbine ◽  
Wall Pressure Fluctuations ◽  
Frequency Domain Methods ◽  
Transition Effects

A nonlinear full-wheel time-domain simulation of a two-stage low pressure turbine is presented, analyzed, and compared with the available experimental data. Recent improvements to the computational fluid dynamics (CFD) solver TRACE that lead to significantly reduced wall-clock times for such large scale simulations are described in brief. Since the configuration is characterized by significant unsteady turbulence and transition effects, it is well suited for the validation and benchmarking of frequency-domain methods. Transition, flow separation and wall pressure fluctuations on the stator blades of the second stage are analyzed in detail. A strong azimuthal π-periodicity is observed, manifesting in a significantly varying stability of the midspan trailing edge flow with a quasi-steady closed separation bubble on certain blades and highly dynamic partially open separation bubbles with recurring transition and turbulent reattachment on other blades. The energy spectrum of fluctuating wall quantities in that regime shows a high bandwidth and considerable disharmonic content, which is challenging for frequency-domain-based simulation methods.

On the Simulation of Unsteady Turbulence and Transition Effects in a Multistage Low Pressure Turbine: Part II — Full-Wheel Simulation

2018 ◽  
Author(s):  
Georg Geiser ◽  
Jens Wellner ◽  
Edmund Kügeler ◽  
Anton Weber ◽  
Anselm Moors
Keyword(s):  
Large Scale ◽  
Separation Bubble ◽  
Pressure Fluctuations ◽  
Low Pressure ◽  
Transition Flow ◽  
Low Pressure Turbine ◽  
Wall Pressure Fluctuations ◽  
High Bandwidth ◽  
Transition Effects ◽  
Large Scale Simulations

This is the second part of a series of three papers on the simulation of turbulence and transition effects in a multistage low pressure turbine. In this second part, the investigated two-stage low pressure turbine is described and results of a nonlinear full-wheel time-domain simulation are presented, analyzed and compared with the available experimental data. Furthermore, recent improvements to the CFD solver TRACE are described in brief that lead to significantly reduced wall-clock times for such large scale simulations. The utilized models match those used in the Harmonic Balance (HB) based simulations that are presented in the third paper of this series, such that the full-wheel result can be utilized to validate the HB result. Transition, flow separation and wall pressure fluctuations on the stator blades of the second stage are analyzed in detail. A strong azimuthal π-periodicity is observed, manifesting in a significantly varying stability of the midspan trailing edge flow with a quasi-steady closed separation bubble on certain blades and highly dynamic partially open separation bubbles with recurring transition and turbulent reattachment on other blades. The energy spectrum of fluctuating wall quantities in that regime shows a high bandwidth and considerable disharmonic content, which is challenging for the HB based simulations.

scholarly journals Prediction of Transient Pressure Fluctuations within a Low-Pressure Turbine Cascade Using a Lanczos-Filtered Harmonic Balance Method

2021 ◽  
Vol 6 (3) ◽  
pp. 25
Author(s):  
Jan Philipp Heners ◽  
Stephan Stotz ◽  
Annette Krosse ◽  
Detlef Korte ◽  
Maximilian Beck ◽  
...  
Keyword(s):  
Turbulence Model ◽  
Harmonic Balance ◽  
Separation Bubble ◽  
Pressure Fluctuations ◽  
Harmonic Balance Method ◽  
Low Pressure ◽  
Filter Method ◽  
Turbine Cascade ◽  
Transient Pressure ◽  
Low Pressure Turbine

Unsteady pressure fluctuations measured by fast-response pressure transducers mounted in a low-pressure turbine cascade are compared to unsteady simulation results. Three differing simulation approaches are considered, one time-integration method and two harmonic balance methods either resolving or averaging the time-dependent components within the turbulence model. The observations are used to evaluate the capability of the harmonic balance solver to predict the transient pressure fluctuations acting on the investigated stator surface. Wakes of an upstream rotor are generated by moving cylindrical bars at a prescribed rotational speed that refers to a frequency of f∼500 Hz. The excitation at the rear part of the suction side is essentially driven by the presence of a separation bubble and is therefore highly dependent on the unsteady behavior of turbulence. In order to increase the stability of the investigated harmonic balance solver, a developed Lanczos-type filter method is applied if the turbulence model is considered in an unsteady fashion.

Reynolds-number dependence of wall-pressure fluctuations in a pressure-induced turbulent separation bubble

2017 ◽  
Vol 833 ◽  
pp. 563-598 ◽  
Author(s):  
Hiroyuki Abe
Keyword(s):  
Reynolds Number ◽  
Pressure Gradient ◽  
Large Scale ◽  
Separation Bubble ◽  
Pressure Fluctuations ◽  
Local Maximum ◽  
Wall Pressure ◽  
Mean Flow ◽  
Frequency Spectra ◽  
Wall Pressure Fluctuations

Direct numerical simulations are used to examine the behaviour of wall-pressure fluctuations $p_{w}$ in a flat-plate turbulent boundary layer with large adverse and favourable pressure gradients, involving separation and reattachment. The Reynolds number $Re_{\unicode[STIX]{x1D703}}$ based on momentum thickness is equal to 300, 600 and 900. Particular attention is given to effects of Reynolds number on root-mean-square (r.m.s.) values, frequency/power spectra and instantaneous fields. The possible scaling laws are also examined as compared with the existing direct numerical simulation and experimental data. The r.m.s. value of $p_{w}$ normalized by the local maximum Reynolds shear stress $-\unicode[STIX]{x1D70C}\overline{uv}_{max}$ (Simpson et al. J. Fluid Mech. vol. 177, 1987, pp. 167–186; Na & Moin J. Fluid Mech. vol. 377, 1998b, pp. 347–373) leads to near plateau (i.e. $p_{w\,rms}/-\unicode[STIX]{x1D70C}\overline{uv}_{max}=2.5\sim 3$) in the adverse pressure gradient and separated regions in which the frequency spectra exhibit good collapse at low frequencies. The magnitude of $p_{w\,rms}/-\unicode[STIX]{x1D70C}\overline{uv}_{max}$ is however reduced down to 1.8 near reattachment where good collapse is also obtained with normalization by the local maximum wall-normal Reynolds stress $\unicode[STIX]{x1D70C}\overline{vv}_{max}$. Near reattachment, $p_{w\,rms}/-\unicode[STIX]{x1D70C}\overline{vv}_{max}=1.2$ is attained unambiguously independently of the Reynolds number and pressure gradient. The present magnitude (1.2) is smaller than (1.35) obtained for step-induced separation by Ji & Wang (J. Fluid Mech. vol. 712, 2012, pp. 471–504). The reason for this difference is intrinsically associated with convective nature of a pressure-induced separation bubble near reattachment where the magnitude of $p_{w\,rms}$ depends essentially on the favourable pressure gradient. The resulting mean flow acceleration leads to delay of the r.m.s. peak after reattachment. Attention is also given to structures of $p_{w}$. It is shown that large-scale spanwise rollers of low pressure fluctuations are formed above the bubble, whilst changing to large-scale streamwise elongated structures after reattachment. These large-scale structures become more prominent with increasing $Re_{\unicode[STIX]{x1D703}}$ and affect $p_{w}$ significantly.

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.

On the Simulation of Unsteady Turbulence and Transition Effects in a Multistage Low Pressure Turbine: Part III — Comparison of Harmonic Balance and Full Wheel Simulation

2018 ◽  
Author(s):  
Edmund Kügeler ◽  
Georg Geiser ◽  
Jens Wellner ◽  
Anton Weber ◽  
Anselm Moors
Keyword(s):  
Harmonic Balance ◽  
Leading Edge ◽  
Suction Side ◽  
Low Pressure ◽  
Transition Model ◽  
Higher Harmonics ◽  
Low Pressure Turbine ◽  
The Third ◽  
Transition Effects ◽  
Transition Models

This is the third part of a series of three papers on the simulation of turbulence and transition effects in a multistage low pressure turbine. The third part of the series deals with the detailed comparison of the Harmonic Balance calculations with the full wheel simulations and measurements for the two-stage low-pressure turbine. The Harmonic Balance simulations were carried out in two confingurations, either using only the 0th harmonic in the turbulence and transition model or additional in all harmonics. The same Menter SST two-equation k–ω turbulence model along with Menter and Langtrys two-equation γ–Reθ transition model is used in the Harmonic Balance simulation as in the full wheel simulations. The measurements on the second stator ofthe low-pressure turbine have been carried out separately for downstream and upstream influences. Thus, a dedicated comparison of the downstream and upstream influences of the flow to the second stator is possible. In the Harmonic Balance calculations, the influences of the not directly adjacent blade, i.e. the first stator, were also included in the second stator In the first analysis, however, it was shown that the consistency with the full wheel configuration and the measurement in this case was not as good as expected. From the analysis ofthe full wheel simulation, we found that there is a considerable variation in the order ofmagnitude ofthe unsteady values in the second stator. In a further deeper consideration of the configuration, it is found that modes are reflected in upstream rows and influences the flow in the second stator. After the integration of these modes into the Harmonic Balance calculations, a much better agreement was reached with results ofthe full wheel simulation and the measurements. The second stator has a laminar region on the suction side starting at the leading edge and then transition takes place via a separation or in bypass mode, depending on the particular blade viewed in the circumferential direction. In the area oftransition, the clear difference between the calculations without and with consideration ofthe higher harmonics in the turbulence and transition models can be clearly seen. The consideration ofthe higher harmonics in the turbulence and transition models results an improvement in the consistency.

Transition Mechanisms in Laminar Separated Flow Under Simulated Low Pressure Turbine Aerofoil Conditions

2012 ◽  
Vol 135 (1) ◽  
Author(s):  
Jerrit Dähnert ◽  
Christoph Lyko ◽  
Dieter Peitsch
Keyword(s):  
Reynolds Number ◽  
Separation Bubble ◽  
Separated Flow ◽  
Low Pressure ◽  
Main Flow ◽  
Test Facility ◽  
Flow Conditions ◽  
Laminar Separation ◽  
Free Stream Turbulence ◽  
Low Pressure Turbine

Based on detailed experimental work conducted at a low speed test facility, this paper describes the transition process in the presence of a separation bubble with low Reynolds number, low free-stream turbulence, and steady main flow conditions. A pressure distribution has been created on a long flat plate by means of a contoured wall opposite of the plate, matching the suction side of a modern low-pressure turbine aerofoil. The main flow conditions for four Reynolds numbers, based on suction surface length and nominal exit velocity, were varied from 80,000 to 300,000, which covers the typical range of flight conditions. Velocity profiles and the overall flow field were acquired in the boundary layer at several streamwise locations using hot-wire anemometry. The data given is in the form of contours for velocity, turbulence intensity, and turbulent intermittency. The results highlight the effects of Reynolds number, the mechanisms of separation, transition, and reattachment, which feature laminar separation-long bubble and laminar separation-short bubble modes. For each Reynolds number, the onset of transition, the transition length, and the general characteristics of separated flow are determined. These findings are compared to the measurement results found in the literature. Furthermore, the experimental data is compared with two categories of correlation functions also given in the literature: (1) correlations predicting the onset of transition and (2) correlations predicting the mode of separated flow transition. Moreover, it is shown that the type of instability involved corresponds to the inviscid Kelvin-Helmholtz instability mode at a dominant frequency that is in agreement with the typical ranges occurring in published studies of separated and free-shear layers.

The Effect of Wake Passing on a Flow Separation in a Low-Pressure Turbine Cascade

2004 ◽  
Vol 126 (2) ◽  
pp. 250-256 ◽  
Author(s):  
Michael J. Brear ◽  
Howard P. Hodson
Keyword(s):  
Separation Bubble ◽  
Phase Locking ◽  
Experimental Studies ◽  
Leading Edge ◽  
Low Pressure ◽  
Pressure Surface ◽  
Low Pressure Turbine ◽  
Numerical Predictions ◽  
Unsteady Interaction ◽  
Wake Passing

This paper describes an investigation into the effect that passing wakes have on a separation bubble that exists on the pressure surface and near the leading edge of a low-pressure turbine blade. Previous experimental studies have shown that the behavior of this separation is strongly incidence dependent and that it responds to its disturbance environment. The results presented in this paper examine the effect of wake passing in greater detail. Two-dimensional, Reynolds averaged, numerical predictions are first used to examine qualitatively the unsteady interaction between the wakes and the separation bubble. The separation is predicted to consist of spanwise vortices whose development is in phase with the wake passing. However, comparison with experiments shows that the numerical predictions exaggerate the coherence of these vortices and also overpredict the time-averaged length of the separation. Nonetheless, experiments strongly suggest that the predicted phase locking of the vortices in the separation onto the wake passing is physical.

Transition Mechanisms in Laminar Separated Flow Under Simulated Low Pressure Turbine Airfoil Conditions

2011 ◽  
Author(s):  
Jerrit Da¨hnert ◽  
Christoph Lyko ◽  
Dieter Peitsch
Keyword(s):  
Reynolds Number ◽  
Separation Bubble ◽  
Separated Flow ◽  
Low Pressure ◽  
Main Flow ◽  
Test Facility ◽  
Flow Conditions ◽  
Laminar Separation ◽  
Free Stream Turbulence ◽  
Low Pressure Turbine

Based on detailed experimental work conducted at a low speed test facility, this paper describes the transition process in the presence of a separation bubble with low Reynolds number, low free-stream turbulence, and steady main flow conditions. A pressure distribution has been created on a long flat plate by means of a contoured wall opposite of the plate, matching the suction side of a modern low-pressure turbine aerofoil. The main flow conditions for four Reynolds numbers, based on suction surface length and nominal exit velocity, were varied from 80,000 to 300,000, which covers the typical range of flight conditions. Velocity profiles and the overall flow field were acquired in the boundary layer at several streamwise locations using hot-wire anemometry. The data given is in the form of contours for velocity, turbulence intensity, and turbulent intermittency. The results highlight the effects of Reynolds number, the mechanisms of separation, transition, and reattachment, which feature laminar separation-long bubble and laminar separation-short bubble modes. For each Reynolds number, the onset of transition, the transition length, and the general characteristics of separated flow are determined. These findings are compared to the measurement results found in the literature. Furthermore, the experimental data is compared with two categories of correlation functions also given in the open literature: (1) correlations predicting the onset of transition and (2) correlations predicting the mode of separated flow transition. Moreover, it is shown that the type of instability involved corresponds to the inviscid Kelvin-Helmholtz instability mode at a dominant frequency that is in agreement with the typical ranges occurring in published studies of separated and free-shear layers.

Multiple-arrayed pressure measurement for investigation of the unsteady flow structure of a reattaching shear layer

2002 ◽  
Vol 463 ◽  
pp. 377-402 ◽  
Author(s):  
INWON LEE ◽  
HYUNG JIN SUNG
Keyword(s):  
Flow Field ◽  
Large Scale ◽  
Vortical Structure ◽  
Single Point ◽  
Pressure Fluctuations ◽  
Vortical Flow ◽  
Recirculation Region ◽  
Stream Velocity ◽  
Vortical Structures ◽  
Wall Pressure Fluctuations

Spatio-temporal characteristics of wall pressure fluctuations in separated and reattaching flows over a backward-facing step were investigated through an extensive pressure-velocity joint measurement with an array of microphones. The experiment was performed in a wind tunnel with a Reynolds number of 33 000 based on the step height and the free-stream velocity. Synchronized wavelet maps showed the evolutionary behaviour of pressure fluctuations and gave further insight into the modulated nature of large-scale vortical structures. To see the relationship between the flow field and the relevant spatial mode of the pressure field, a new kind of wavenumber filtering, termed ‘spatial box filtering’ (SBF), was introduced and examined. The vortical flow field was reconstructed using every single-point velocity measurement by means of the conditional average based on the SBF second mode of pressure fluctuations. The flow field showed a well-organized spanwise vortical structure convected with a speed of 0.6U0 and a characteristic ‘sawtooth’ pattern of the unsteady trace of reattachment length. In addition to the coherent vortical structures, the periodic enlargement/shrinkage process of the recirculation region owing to apping motion was analysed. The recirculation region was found to undergo an enlargement/shrinkage cycle in accordance with the lowpass-filtered component of pressure fluctuations. In addition, such modulatory behaviour of the vortical structure as the global oscillation phase was discussed in connection with the conditionally averaged flow field.

Aerodynamic Performance of a Very High Lift Low Pressure Turbine Blade With Emphasis on Separation Prediction

2004 ◽  
Vol 126 (3) ◽  
pp. 406-413 ◽  
Author(s):  
Re´gis Houtermans ◽  
Thomas Coton ◽  
Tony Arts
Keyword(s):  
Turbine Blade ◽  
Separation Bubble ◽  
Separated Flow ◽  
Low Pressure ◽  
Secondary Flows ◽  
Flow Mode ◽  
High Lift ◽  
Flow Angle ◽  
Low Pressure Turbine ◽  
Very High

The present paper is based on an experimental study of a front-loaded very high lift, low pressure turbine blade designed at the VKI. The experiments have been carried out in a low-speed wind tunnel over a wide operating range of incidence and Reynolds number. The aim of the study is to characterize the flow through the cascade in terms of losses, mean outlet flow angle, and secondary flows. At low inlet freestream turbulence intensity, a laminar separation bubble is present, and a prediction model for a separated flow mode of transition has been developed.

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