Large-Eddy Simulation of Unsteady Surface Pressure Over a LP Turbine Blade Due to Interactions of Passing Wakes and Inflexional Boundary Layer

2005 ◽  
Author(s):  
S. Sarkar ◽  
Peter R. Voke
Keyword(s):  
Large Eddy Simulation ◽  
Turbine Blade ◽  
Coherent Structures ◽  
Pressure Fluctuations ◽  
Time Dependent ◽  
Suction Surface ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Grid Points ◽  
Lp Turbine

The unsteady pressure over the suction surface of a modern low-pressure (LP) turbine blade subjected to periodically passing wakes from a moving bar wake generator is described. The results presented are a part of detailed Large-Eddy Simulation (LES) following earlier experiments over the T106 profile for a Reynolds number of 1.6×105 (based on the chord and exit velocity) and the cascade pitch to chord ratio of 0.8. The present LES uses coupled simulations of cylinder for wake, providing four-dimensional inflow conditions for successor simulations of wake interactions with the blade. The three-dimensional, time-dependent, incompressible Navier-Stokes equations in fully covariant form are solved with 2.4×106 grid points for the cascade and 3.05×106 grid points for the cylinder using a symmetry-preserving finite difference scheme of second-order spatial and temporal accuracy. A separation bubble on the suction surface of the blade was found to form under the steady state condition. Pressure fluctuations of large amplitude appear on the suction surface as the wake passes over the separation region. Enhanced receptivity of perturbations associated with the inflexional velocity profile is the cause of instability and coherent vortices appear over the rear half of the suction surface by the rollup of shear layer via Kelvin-Helmholtz (K-H) mechanism. Once these vortices are formed, the steady-flow separation changes remarkably. These coherent structures embedded in the boundary amplify before breakdown while traveling downstream with a convective speed of about 37 percent of the local free-stream speed. The vortices play an important role in the generation of turbulence and thus to decide the transitional length, which becomes time-dependent. The source of the pressure fluctuations on the rear part of the suction surface is also identified as the formation of these coherent structures. When compared with experiments, it reveals that LES is worth pursuing as an understanding of the eddy motions and interactions is of vital importance for the problem.

Large-Eddy Simulation of Unsteady Surface Pressure Over a Low-Pressure Turbine Blade due to Interactions of Passing Wakes and Inflexional Boundary Layer

2005 ◽  
Vol 128 (2) ◽  
pp. 221-231 ◽  
Author(s):  
S. Sarkar ◽  
Peter R. Voke
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Turbine Blade ◽  
Coherent Structures ◽  
Pressure Fluctuations ◽  
Time Dependent ◽  
Suction Surface ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Grid Points

The unsteady pressure over the suction surface of a modern low-pressure (LP) turbine blade subjected to periodically passing wakes from a moving bar wake generator is described. The results presented are a part of detailed large-eddy simulation (LES) following earlier experiments over the T106 profile for a Reynolds number of 1.6×105 (based on the chord and exit velocity) and the cascade pitch to chord ratio of 0.8. The present LES uses coupled simulations of cylinder for wake, providing four-dimensional inflow conditions for successor simulations of wake interactions with the blade. The three-dimensional, time-dependent, incompressible Navier-Stokes equations in fully covariant form are solved with 2.4×106 grid points for the cascade and 3.05×106 grid points for the cylinder using a symmetry-preserving finite difference scheme of second-order spatial and temporal accuracy. A separation bubble on the suction surface of the blade was found to form under the steady state condition. Pressure fluctuations of large amplitude appear on the suction surface as the wake passes over the separation region. Enhanced receptivity of perturbations associated with the inflexional velocity profile is the cause of instability and coherent vortices appear over the rear half of the suction surface by the rollup of shear layer via Kelvin-Helmholtz (KH) mechanism. Once these vortices are formed, the steady-flow separation changes remarkably. These coherent structures embedded in the boundary layer amplify before breakdown while traveling downstream with a convective speed of about 37% of the local free-stream speed. The vortices play an important role in the generation of turbulence and thus to decide the transitional length, which becomes time dependent. The source of the pressure fluctuations on the rear part of the suction surface is also identified as the formation of these coherent structures. When compared with experiments, it reveals that LES is worth pursuing as an understanding of the eddy motions and interactions is of vital importance for the problem.

Wake-Induced Transitional Flow Over a Highly-Loaded LP Turbine Blade Through Large-Eddy Simulation

2005 ◽  
Author(s):  
S. Sarkar
Keyword(s):  
Boundary Layer ◽  
Turbine Blade ◽  
Separation Bubble ◽  
Time Dependent ◽  
Suction Surface ◽  
Transition Mechanism ◽  
Large Eddy ◽  
Coherent Vortices ◽  
Lp Turbine ◽  
Wake Passing

An attempt is made to describe the physical mechanism of transition of an inflexional boundary layer over the suction surface of a highly cambered low-pressure (LP) turbine blade influenced by the periodic passing wakes. Large-eddy simulations (LES) of wake passing over the T106 profile for a Reynolds number of 1.6×105 (based on the chord and exit velocity) are performed using wake data extracted from precursor simulations of cylinder replacing a moving bar in front of the cascade. The three-dimensional, time-dependent, incompressible Navier-Stokes equations in fully covariant form are solved using a symmetry-preserving finite difference scheme of second-order spatial and temporal accuracy. The present LES results are compared with experiments and DNS. The operating condition of a high-lift LP turbine blade leads to the formation of a separation bubble on the suction side. The interactions of incoming wake with this separation bubble complicate the transition process. Enhanced receptivity of inflexional boundary layer causes amplification of the perturbations produced by the passing wake leading to the formation of coherent vortices within the boundary layer. The transition mechanism during the wake-induced path is highly influenced by the convection and breakdown of these coherent vortices. Streamwise evolution of turbulent kinetic energy and production illustrates that these vortices play an important role in generation of turbulence and thus to decide the transitional length, which becomes time-dependent. LES results resolve a multimoded transition on the suction surface and the calmed region. The calmed region is nothing but an attached flow with low production as the boundary layer tends to relax after wake passing; the level of turbulent intensity suggests that the boundary layer is in a state of transition rather than laminarized.

Large Eddy Simulation of a Compressor Stage

2017 ◽  
Author(s):  
Pranab Mondal ◽  
Joseph Mathew
Keyword(s):  
Large Eddy Simulation ◽  
Tangential Velocity ◽  
Surface Boundary ◽  
Blade Surface ◽  
Suction Surface ◽  
Time Step ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Scale Modelling ◽  
Grid Points

A methodology for large eddy simulation (LES) of a turbomachine stage is presented. Computations of mean fields (RANS) of stages may be performed separately of rotor and stator rows by providing an averaged solution as input to the down-stream row. In unsteady simulations, unsteady field information must be exchanged in both directions after every time step. Here a procedure for linear cascade simulations of a stage has been implemented in a high-resolution compressible flow solver for LES. The LES uses an explicit filtering method for sub-grid-scale modelling. Grids overlap at the interface between blade rows. Field data is transferred in both directions. Rotor velocity is added or subtracted as needed to tangential velocity component during this transfer. The relative movement of the rotor and stator grids is accounted for by suitable periodic tangential shifting of the paired grid points in the overlap for the transfer. The method has been tested against a published DNS of a statorrotor stage. The Reynolds number based on blade chord and mean axial velocity at inflow was 40000. Solution fields show the wake vortex street of the upstream blade row impinging on downstream blades and being convected through the downstream blade passage. The LES captured transition on rotor blade surface boundary layers. Blade surface pressure distributions agree closely on pressure surfaces. Separation and transition on downstream blade suction surface is delayed slightly at the present resolution, but this will improve with grid refinement, monotonically, for this LES method.

Large Eddy Simulation of Wake-Shear Layer Interactions Over a Multi-Element Aerofoil

2020 ◽  
Author(s):  
Souvik Naskar ◽  
S. Sarkar
Keyword(s):  
Large Eddy Simulation ◽  
Shear Layer ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Leading Edge ◽  
Aerodynamic Characteristics ◽  
Suction Surface ◽  
Free Stream Turbulence ◽  
Eddy Simulation ◽  
Large Eddy

Abstract Modern commercial airliners use multi-element aerofoils to enhance take-off and landing performance. Further, multielement aerofoil configurations have been shown to improve the aerodynamic characteristics of wind turbines. In the present study, high resolution Large Eddy Simulation (LES) is used to explore the low Reynolds Number (Re = 0.832 × 104) aerodynamics of a 30P30N multi-element aerofoil at an angle of attack, α = 4°. In the present simulation, wake shed from a leading edge element or slat is found to interact with the separated shear layer developing over the suction surface of the main wing. High receptivity of shear layer via amplification of free-stream turbulence leads to rollup and breakdown, forming a large separation bubble. A transient growth of fluctuations is observed in the first half of the separation bubble, where levels of turbulence becomes maximum near the reattachment and then decay depicting saturation of turbulence. Results of the present LES are found to be in close agreement with the experiment depicting high vortical activity in the outer layer. Some features of the flow field here are similar to those occur due to interactions of passing wake and the separated boundary layer on the suction surface of high lift low pressure turbine blades.

Loss Predictions in the High-Pressure Film-Cooled Turbine Blade Cascade T120D by Mean of Wall-Resolved Large Eddy Simulation

2020 ◽  
Author(s):  
Mael Harnieh ◽  
Nicolas Odier ◽  
Jérôme Dombard ◽  
Florent Duchaine ◽  
Laurent Gicquel
Keyword(s):  
High Pressure ◽  
Large Eddy Simulation ◽  
Turbine Blade ◽  
Film Cooling ◽  
Load Prediction ◽  
Loss Assessment ◽  
Film Cooling Effectiveness ◽  
Eddy Simulation ◽  
Blade Cascade ◽  
Large Eddy

Abstract Film cooling is commonly used to protect turbine vanes and blades from the hot gases produced in the combustion chamber. The design and optimization of these systems can however only be achieved if a precise prediction of the fluid mechanics and film efficiency is guaranteed at a level where induced losses are fully mastered. Such a prerequisite induces at the numerical level to be able to identify and assess losses. In this context, the present study addresses loss assessment in a wall-resolved Large Eddy Simulation (LES) of the film-cooled high-pressure turbine blade cascade T120D from the European project AITEB II. The objectives are twofolds: (1) to evaluate the capacity of LES to predict adiabatic film cooling effectiveness in a mastered academic case; and (2) to investigate loss generation mechanisms in a fully anisothermal configuration. When it comes to LES predictions of T120D, the flow structure around the blade and the coolant jet organization are coherent with literature findings. Satisfactory agreements are furthermore retrieved for the pressure load prediction as well as the adiabatic film effectiveness if compared to the experiment. Loss generation is then investigated illustrating the fact that aerodynamics losses dominate mixing losses which are mainly located in the coolant film. This is in line with the temperature difference between the hot and coolant flows that is low for this experimental condition. Distinct contributions can however be made available by studying the local loss generation maps by means of Second Law Analysis if recast in the specific context of anisothermal flows when simulated by LES.

Large Eddy Simulation of Boundary Layer Transition Mechanisms in a Gas-Turbine Compressor Cascade

2018 ◽  
Author(s):  
Ashley D. Scillitoe ◽  
Paul G. Tucker ◽  
Paolo Adami
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Boundary Layer Transition ◽  
Bypass Transition ◽  
Suction Surface ◽  
Layer Transition ◽  
Turbulent Spots ◽  
Pressure Surface ◽  
Eddy Simulation ◽  
Large Eddy

Large Eddy Simulation (LES) is used to explore the boundary layer transition mechanisms in two rectilinear compressor cascades. To reduce numerical dissipation, a novel locally adaptive smoothing scheme is added to an unstructured finite-volume solver. The performance of a number of Sub-Grid Scale (SGS) models is explored. With the first cascade, numerical results at two different freestream turbulence intensities (Ti’s), 3.25% and 10%, are compared. At both Ti’s, time-averaged skin-friction and pressure coefficient distributions agree well with previous Direct Numerical Simulations (DNS). At Ti = 3.25%, separation induced transition occurs on the suction surface, whilst it is bypassed on the pressure surface. The pressure surface transition is dominated by modes originating from the convection of Tollmien-Schlichting waves by Klebanoff streaks. However, they do not resembled a classical bypass transition. Instead, they display characteristics of the “overlap” and “inner” transition modes observed in the previous DNS. At Ti = 10%, classical bypass transition occurs, with Klebanoff streaks incepting turbulent spots. With the second cascade, the influence of unsteady wakes on transition is examined. Wake-amplified Klebanoff streaks were found to instigate turbulent spots, which periodically shorten the suction surface separation bubble. The celerity line corresponding to 70% of the free-stream velocity, which is associated with the convection speed of the amplified Klebanoff streaks, was found to be important.

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