Highly Resolved Large Eddy Simulation Study of Gap Size Effect on Low-Pressure Turbine Stage

2017 ◽  
Vol 140 (2) ◽  
Author(s):  
R. Pichler ◽  
V. Michelassi ◽  
R. Sandberg ◽  
J. Ong
Keyword(s):  
Kinetic Energy ◽  
Cross Sections ◽  
Control Volume ◽  
Leading Edge ◽  
Suction Side ◽  
Low Pressure ◽  
Small Time ◽  
Gap Size ◽  
Low Pressure Turbine ◽  
Large Eddy

Blade-to-blade interactions in a low-pressure turbine (LPT) were investigated using highly resolved compressible large eddy simulations (LESs). For a realistic setup, a stator and rotor configuration with profiles typical of LPTs was used. Simulations were conducted with an in-house solver varying the gap size between stator and rotor from 21.5% to 43% rotor chord. To investigate the effect of the gap size on the prevailing loss mechanisms, a loss breakdown was conducted. It was found that in the large gap (LG) size case, the turbulence kinetic energy (TKE) levels of the stator wake close to the rotor leading edge were only one third of those in the small gap (SG) case, due to the longer distance of constant area mixing. The small time-averaged suction side separation on the blade, found in the LG case, disappeared in the SG calculations, confirming how stronger wakes can keep the boundary layer attached. The higher intensity wake impinging on the blade, however, did not affect the time-averaged losses calculated using the control volume approach of Denton. On the other hand, losses computed by taking cross sections upstream and downstream of the blade revealed a greater distortion loss generated by the stator wakes in the SG case. Despite the suction side separation suppression, the SG case gave higher losses overall due to the incoming wake turbulent kinetic energy amplification along the blade passage.

Highly Resolved LES Study of Gap Size Effect on Low-Pressure Turbine Stage

2017 ◽  
Author(s):  
R. Pichler ◽  
V. Michelassi ◽  
R. Sandberg ◽  
J. Ong
Keyword(s):  
Kinetic Energy ◽  
Cross Sections ◽  
Control Volume ◽  
Energy Levels ◽  
Leading Edge ◽  
Suction Side ◽  
Low Pressure ◽  
Small Time ◽  
Gap Size ◽  
Low Pressure Turbine

Blade-to-blade interactions in a low-pressure turbine were investigated using highly resolved compressible large eddy simulations. For a realistic setup, a stator and rotor configuration with profiles typical of low-pressure turbines was used. Simulations were conducted with an in-house solver varying the gap size between stator and rotor from 21.5% to 43% rotor chord. To investigate the effect of the gap size on the prevailing loss mechanisms, a loss breakdown was conducted. It was found that in the large gap size case, the turbulence kinetic energy levels of the stator wake close to the rotor leading edge were only one third of those in the small gap case, due to the longer distance of constant area mixing. The small time averaged suction side separation on the blade, found in the large gap case, disappeared in the small gap calculations, confirming how stronger wakes can keep the boundary layer attached. The higher intensity wake impinging on the blade, however, did not affect the time averaged losses calculated using the control volume approach of Denton. On the other hand, losses computed by taking cross sections upstream and downstream of the blade revealed a greater distortion loss generated by the stator wakes in the small gap case. Despite the suction side separation suppression, the small gap case gave higher losses overall due to the incoming wake turbulent kinetic energy amplification along the blade passage.

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.

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.

scholarly journals Turbulent Kinetic Energy Production in the Vane of a Low-Pressure Linear Turbine Cascade with Incoming Wakes

2015 ◽  
Vol 2015 ◽  
pp. 1-15 ◽  
Author(s):  
V. Michelassi ◽  
J. G. Wissink
Keyword(s):  
Boundary Layer ◽  
Kinetic Energy ◽  
Large Eddy Simulation ◽  
Turbulent Kinetic Energy ◽  
Suction Side ◽  
Low Pressure ◽  
Eddy Simulation ◽  
Transition Mechanism ◽  
Principal Axes ◽  
Large Eddy

Incompressible large eddy simulation and direct numerical simulation of a low-pressure turbine atRe=5.18×104and1.48×105with discrete incoming wakes are analyzed to identify the turbulent kinetic energy generation mechanism outside of the blade boundary layer. The results highlight the growth of turbulent kinetic energy at the bow apex of the wake and correlate it to the stress-strain tensors relative orientation. The production rate is analytically split according to the principal axes, and then terms are computed by using the simulation results. The analysis of the turbulent kinetic energy is followed both along the discrete incoming wakes and in the stationary frame of reference. Both direct numerical and large eddy simulation concur in identifying the same production mechanism that is driven by both a growth of strain rate in the wake, first, followed by the growth of turbulent shear stress after. The peak of turbulent kinetic energy diffuses and can eventually reach the suction side boundary layer for the largest Reynolds number investigated here with higher incidence angle. As a consequence, the local turbulence intensity outside the boundary layer can grow significantly above the free-stream level with a potential impact on the suction side boundary layer transition mechanism.

scholarly journals Large-Eddy Simulation of Low-Pressure Turbine Cascade with Unsteady Wakes

Aerospace ◽  
2021 ◽  
Vol 8 (7) ◽  
pp. 184
Author(s):  
Zachary Robison ◽  
Andreas Gross
Keyword(s):  
Flow Field ◽  
Flow Structure ◽  
Three Dimensional ◽  
Suction Side ◽  
Low Pressure ◽  
Flow Fields ◽  
Mean Flow ◽  
Reaction Stage ◽  
Low Pressure Turbine ◽  
Large Eddy

To better understand the wake effects at low Reynolds numbers, large-eddy simulations of a 50% reaction low-pressure turbine stage and a linear cascade with two different bar wake generators were carried out for a chord Reynolds number of 50,000. For the chosen front-loaded high-lift airfoil, the endwall structures are stronger than for more traditional mid-loaded moderate-lift airfoils. By comparing the 50% reaction stage results with the bar wake generator results, insight is gained into the effect of the three-dimensional wake components on the downstream flow field.For the cases with bar wake generator, the endwall boundary layer is growing faster because of the relative motion of the endwall with respect to the freestream. The half-width of the wake is approximately matched for the larger one of the two considered bar wake generators. To improve the quality of the phase-averaged flow fields, the proper orthogonal decomposition was employed as a filter to remove the low-energy unsteady flow field content. Both the mean flow and filtered phase-averaged flow fields were analyzed in detail. Visualizations of the phase-averaged flow field reveal a periodic suppression of the laminar suction side separation from the downstream airfoil even for the smaller bar wake generator. The passage vortex is entirely suppressed for the 50% reaction stage and for the larger bar wake generator. Furthermore, the phase-averaged data for the 50% reaction stage reveal a new longitudinal flow structure that is traced back to near-wall wake vorticity. This flow structure is missing for the bar wake generator cases.

Large-Eddy Simulations of Low Pressure Turbine Endwall Flow and Heat Transfer

2018 ◽  
Author(s):  
Stephen Lynch
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Secondary Flow ◽  
Suction Side ◽  
Horseshoe Vortex ◽  
Low Pressure ◽  
Secondary Flows ◽  
Reynolds Analogy ◽  
Low Pressure Turbine ◽  
Large Eddy

Turbine airfoils are subject to strong secondary flows that produce total pressure loss and high surface heat transfer in the airfoil passage. The secondary flows arise from the high overall flow turning acting on the incoming boundary layer, as well as the generation of a horseshoe vortex at the leading edge of the airfoil. Prediction of the effects of secondary flows on endwall heat transfer using steady Reynolds-averaged Navier-Stokes (RANS) approaches has so far been somewhat unsatisfactory, but it is unclear whether this is due to unsteadiness of the secondary flow, modeling assumptions (such as the Boussinesq approximation and Reynolds analogy), strongly non-equilibrium boundary layer behavior in the highly skewed endwall flow, or some combination of all. To address some of these questions, and to determine the efficacy of higher-fidelity computational approaches to predict endwall heat transfer, a low pressure turbine cascade was modeled using a wall-modeled Large Eddy Simulation (LES) approach. The result was compared to a steady Reynolds-stress modeling (RSM) approach, and to experimental data. Results indicate that the effect of the unsteadiness of the pressure side leg of the horseshoe vortex results in a broad distribution of heat transfer in the front of the passage, and high heat transfer on the aft suction side corner, which is not predicted by steady RANS. However, the time-mean heat transfer is still not well predicted due to slight differences in the secondary flow pattern. Turbulence quantities in the blade passage agree fairly well to prior measurements and highlight the effect of the strong passage curvature on the endwall boundary layer, but the LES approach here overpredicts turbulence in the secondary flow at the cascade outlet due to a thick airfoil suction side boundary layer. Overall, more work remains to identify the specific model deficiencies in RSM or wall-modeled LES approaches.

scholarly journals Investigation of Separation Phenomena in a Radial Pump at Reduced Flow Rate by Large-Eddy Simulation

2016 ◽  
Vol 138 (12) ◽  
Author(s):  
Antonio Posa ◽  
Antonio Lippolis ◽  
Elias Balaras
Keyword(s):  
Reverse Flow ◽  
Leading Edge ◽  
Suction Side ◽  
Secondary Flows ◽  
Vaned Diffuser ◽  
Flow Rates ◽  
Turbulent Statistics ◽  
Large Eddy ◽  
Radial Pump ◽  
Reduced Flow

Turbopumps operating at reduced flow rates experience significant separation and backflow phenomena. Although Reynolds-Averaged Navier–Stokes (RANS) approaches proved to be usually able to capture the main flow features at design working conditions, previous numerical studies in the literature verified that eddy-resolving techniques are required in order to simulate the strong secondary flows generated at reduced loads. Here, highly resolved large-eddy simulations (LES) of a radial pump with a vaned diffuser are reported. The results are compared to particle image velocimetry (PIV) experiments in the literature. The main focus of the present work is to investigate the separation and backflow phenomena occurring at reduced flow rates. Our results indicate that the effect of these phenomena extends up to the impeller inflow: they involve the outer radii of the impeller vanes, influencing significantly the turbulent statistics of the flow. Also in the diffuser vanes, a strong spanwise evolution of the flow has been observed at the reduced load, with reverse flow, located mainly on the shroud side and on the suction side (SS) of the stationary channels, especially near the leading edge of the diffuser blades.

The Effect of Pulsed Blowing on the Boundary Layer of a Highly Loaded Low Pressure Turbine Blade

2013 ◽  
Author(s):  
Marion Mack ◽  
Roland Brachmanski ◽  
Reinhard Niehuis
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Mach Number ◽  
Suction Side ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Trailing Edge ◽  
Low Pressure Turbine ◽  
Wide Range ◽  
Highly Loaded

The performance of the low pressure turbine (LPT) can vary appreciably, because this component operates under a wide range of Reynolds numbers. At higher Reynolds numbers, mid and aft loaded profiles have the advantage that transition of suction side boundary layer happens further downstream than at front loaded profiles, resulting in lower profile loss. At lower Reynolds numbers, aft loading of the blade can mean that if a suction side separation exists, it may remain open up to the trailing edge. This is especially the case when blade lift is increased via increased pitch to chord ratio. There is a trend in research towards exploring the effect of coupling boundary layer control with highly loaded turbine blades, in order to maximize performance over the full relevant Reynolds number range. In an earlier work, pulsed blowing with fluidic oscillators was shown to be effective in reducing the extent of the separated flow region and to significantly decrease the profile losses caused by separation over a wide range of Reynolds numbers. These experiments were carried out in the High-Speed Cascade Wind Tunnel of the German Federal Armed Forces University Munich, Germany, which allows to capture the effects of pulsed blowing at engine relevant conditions. The assumed control mechanism was the triggering of boundary layer transition by excitation of the Tollmien-Schlichting waves. The current work aims to gain further insight into the effects of pulsed blowing. It investigates the effect of a highly efficient configuration of pulsed blowing at a frequency of 9.5 kHz on the boundary layer at a Reynolds number of 70000 and exit Mach number of 0.6. The boundary layer profiles were measured at five positions between peak Mach number and the trailing edge with hot wire anemometry and pneumatic probes. Experiments were conducted with and without actuation under steady as well as periodically unsteady inflow conditions. The results show the development of the boundary layer and its interaction with incoming wakes. It is shown that pulsed blowing accelerates transition over the separation bubble and drastically reduces the boundary layer thickness.

Profiled End-Wall Design Using an Adjoint Navier-Stokes Solver

2006 ◽  
Author(s):  
Roque Corral ◽  
Fernando Gisbert
Keyword(s):  
Kinetic Energy ◽  
Cost Function ◽  
Unstructured Mesh ◽  
Low Pressure ◽  
Navier Stokes ◽  
Low Pressure Turbine ◽  
Gradient Based ◽  
Parallel Multigrid ◽  
End Wall ◽  
Discrete Formulation

A methodology to minimize blade secondary losses by modifying turbine end-walls is presented. The optimization is addressed using a gradient-based method, where the computation of the gradient is performed using an adjoint code and the secondary kinetic energy is used as a cost function. The adjoint code is implemented on the basis of the discrete formulation of a parallel multigrid unstructured mesh Navier-Stokes solver. The results of the optimization of two end-walls of a low pressure turbine row are shown.

Endwall Boundary Layer Development in an Engine Representative Four-Stage Low Pressure Turbine Rig

2008 ◽  
Vol 131 (1) ◽  
Author(s):  
Maria Vera ◽  
Elena de la Rosa Blanco ◽  
Howard Hodson ◽  
Raul Vazquez
Keyword(s):  
Boundary Layer ◽  
Leading Edge ◽  
Low Pressure ◽  
The State ◽  
Linear Cascade ◽  
Low Speed ◽  
Cold Flow ◽  
Low Pressure Turbine ◽  
Transitional Boundary Layer ◽  
Passage Vortex

Research by de la Rosa Blanco et al. (“Influence of the State of the Inlet Endwall Boundary Layer on the Interaction Between the Pressure Surface Separation and the Endwall Flows,” Proc. Inst. Mech. Eng., Part A, 217, pp. 433–441) in a linear cascade of low pressure turbine (LPT) blades has shown that the position and strength of the vortices forming the endwall flows depend on the state of the inlet endwall boundary layer, i.e., whether it is laminar or turbulent. This determines, amongst other effects, the location where the inlet boundary layer rolls up into a passage vortex, the amount of fluid that is entrained into the passage vortex, and the interaction of the vortex with the pressure side separation bubble. As a consequence, the mass-averaged stagnation pressure loss and therefore the design of a LPT depend on the state of the inlet endwall boundary layer. Unfortunately, the state of the boundary layer along the hub and casing under realistic engine conditions is not known. The results presented in this paper are taken from hot-film measurements performed on the casing of the fourth stage of the nozzle guide vanes of the cold flow affordable near term low emission (ANTLE) LPT rig. These results are compared with those from a low speed linear cascade of similar LPT blades. In the four-stage LPT rig, a transitional boundary layer has been found on the platforms upstream of the leading edge of the blades. The boundary layer is more turbulent near the leading edge of the blade and for higher Reynolds numbers. Within the passage, for both the cold flow four-stage rig and the low speed linear cascade, the new inlet boundary layer formed behind the pressure leg of the horseshoe vortex is a transitional boundary layer. The transition process progresses from the pressure to the suction surface of the passage in the direction of the secondary flow.

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