Large-Eddy Simulation of the Variable Speed Power Turbine Cascade With Inflow Turbulence

2020 ◽  
Author(s):  
Kenji Miki ◽  
Ali Ameri
Keyword(s):  
Separation Bubble ◽  
Incidence Angle ◽  
Suction Side ◽  
Equation Model ◽  
Separation Mechanism ◽  
Variable Speed ◽  
Free Stream Turbulence ◽  
Transition Mechanism ◽  
Power Turbine ◽  
Inflow Turbulence

Abstract Numerical results are presented from the NASA Glenn Research Center’s in-house turbomachinery code, Glenn-HT applied to the Variable Speed Power Turbine (VSPT) experiment at the NASA Transonic Turbine Blade Cascade Facility. The main goal of this paper is to implement a digital filtering method to generate turbulence upstream and a sub-grid model (Localized dynamic k-equation model (LDKM)) in the framework of LES in order to investigate the effect of inflow turbulence on the transition seen in the VSPT experimental data at the cruise condition (incidence angle of 40° and Tu = 0.5%, 5%, 10%, and 15%). Although the boundary layer on the suction side and pressure side of the blades is initially laminar due to favorable pressure gradient, the laminar flow can transition to turbulent flow past a separation zone on the suction side or by natural or by-pass transition. This process determines the total-pressure losses in the wake. Therefore, it is desirable to develop a reliable prediction tool to accurately capture the transition mechanism in blade rows operated under the conditions of low Reynolds number and at a variety of free stream turbulence conditions. Our numerical studies reveal that the location of separation is rather insensitive to the level of Tu, however the effect of increasing Tu seems to be in reducing the size and ultimately suppressing the separation bubble. In addition, we performed spectral analysis to identify the peak frequencies in the region where the separation bubble is formed, which provides valuable insights into the transition/separation mechanism.

Large-Eddy Simulation of Variable Speed Power Turbine Cascade With Inflow Turbulence

2021 ◽  
pp. 1-40
Author(s):  
Kenji Miki ◽  
Ali Ameri
Keyword(s):  
Separation Bubble ◽  
Incidence Angle ◽  
Equation Model ◽  
Separation Mechanism ◽  
Variable Speed ◽  
Eddy Simulation ◽  
Numerical Studies ◽  
Power Turbine ◽  
Large Eddy ◽  
Inflow Turbulence

Abstract Numerical results are presented from the NASA Glenn Research Center's in-house turbomachinery code, Glenn-HT applied to the Variable Speed Power Turbine (VSPT) experiment at the NASA Transonic Turbine Blade Cascade Facility. The main goal of this paper is to implement a digital filtering method to generate turbulence upstream and a sub-grid model (Localized dynamic k-equation model (LDKM)) in the framework of LES in order to investigate the effect of inflow turbulence on the transition seen in the VSPT experimental data at the cruise condition (incidence angle of 40° and Tu = 0.5%, 5%,10%, and 15%). Our numerical studies reveal that the location of separation is rather insensitive to the level of Tu, however the effect of increasing Tu seems to be in reducing the size and ultimately suppressing the separation bubble. In addition, we performed spectral analysis to identify the peak frequencies in the region where the separation bubble is formed, which provides valuable insights into the transition/separation mechanism.

Implicit-LES Simulation of Variable-Speed Power Turbine Cascade for Low Free-Stream Turbulence Conditions

2018 ◽  
Author(s):  
Ali Ameri
Keyword(s):  
Free Stream ◽  
Reynolds Numbers ◽  
Scale Model ◽  
Variable Speed ◽  
Low Reynolds Numbers ◽  
Free Stream Turbulence ◽  
Pressure Loss Coefficient ◽  
Power Turbine ◽  
Actual Design ◽  
Low Reynolds

It is a challenge to simulate the flow in a Variable Speed Power Turbine (VSPT), or, for that matter, rear stages of low pressure turbines at low Reynolds numbers due to laminar flow separation or laminar/turbulent flow transition on the blades. At low Reynolds numbers, separation induced-transition is more prevalent which can result in efficiency lapse. LES has been used in recent years to simulate these types of flows with a good degree of success. In the present work, very low free stream turbulence flows at exit Reynolds number of 220k were simulated. The geometry was a cascade which was constructed with the midspan section of a VSPT design. Most LES simulations to date, have focused on the midspan region. As the endwall effect was significant in these simulations due to thick incoming boundary layer, full blade span computation was necessitated. Inlet flow angles representative of take-off and cruise conditions, dictated by the rotor speed in an actual design, were analyzed. This was done using a second order finite volume code and a high resolution grid. As is the case with Implicit-LES methods, no sub-grid scale model was used. Blade static pressure data, at various span locations, and downstream probe survey measurements of total pressure loss coefficient were used to verify the results. The comparisons showed good agreement between the simulations and the experimental data.

Effects of Periodic Unsteadiness, Free-Stream Turbulence, and Flow Reynolds Number on Separation-Bubble Transition

2003 ◽  
Author(s):  
S. K. Roberts ◽  
M. I. Yaras
Keyword(s):  
Reynolds Number ◽  
Free Stream ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Separated Flow ◽  
Suction Side ◽  
Streamwise Velocity ◽  
Flow Transition ◽  
Free Stream Turbulence ◽  
Flow Reynolds Number

This paper presents measurements of the combined effects of free-stream turbulence and periodic streamwise velocity variations on separation-bubble transition. The measurements were performed on a flat plate at two values of flow Reynolds number, with a streamwise pressure distribution similar to those encountered on the suction side of axial turbine blades. The experiment was designed to facilitate independent control of turbulence and periodic velocity fluctuations in the free-stream. The free-stream turbulence intensity was varied from 0.4% to 4.5%, and the periodic unsteadiness corresponded to Strouhal numbers of 0.0, 2.4 and 4.0. Based on the results, the relative importance of free-stream turbulence and periodic unsteadiness on the streamwise locations of separation, transition and reattachment points are quantified. Existing mathematical models for predicting separated-flow transition and reattachment are then evaluated in this context.

Separated Flow Transition in a Low Pressure Turbine Cascade: Mean Flow and Turbulence Spectra

2003 ◽  
Author(s):  
Ralph J. Volino ◽  
Christopher G. Murawski
Keyword(s):  
Boundary Layer ◽  
Free Stream ◽  
Separation Bubble ◽  
Suction Side ◽  
Low Pressure ◽  
Turbine Cascade ◽  
Mean Flow ◽  
Turbulence Spectra ◽  
Free Stream Turbulence ◽  
Low Pressure Turbine

Boundary layer separation, transition and reattachment have been studied experimentally in a low-pressure turbine cascade. Cases with Reynolds numbers (Re) ranging from 50,000 to 200,000 (based on suction surface length and exit velocity) have been considered under low free-stream turbulence conditions. Mean and fluctuating velocity profiles and turbulence spectra are presented for streamwise locations along the suction side of one airfoil and in the wake downstream of the airfoils. Hot film gages on the suction side surface of the airfoil are used to measure the fluctuation level and the spectra of the fluctuations on the surface. Higher Re moves transition upstream. Transition is initiated in the shear layer over the separation bubble and leads to boundary layer reattachment. Peak frequencies in the boundary layer spectra match those found in similar cases in the literature, indicating that the important frequencies may be predictable. Spectra in the wake downstream of the airfoils were similar to the spectra in the boundary layer near the trailing edge of the airfoil. Comparisons to the literature indicate that small but measurable differences in the spectra of the low free-stream turbulence can have a significant effect on boundary layer reattachment.

Large-Eddy Simulation and Conjugate Heat Transfer Around a Low-Mach Turbine Blade

2013 ◽  
Vol 136 (5) ◽  
Author(s):  
Florent Duchaine ◽  
Nicolas Maheu ◽  
Vincent Moureau ◽  
Guillaume Balarac ◽  
Stéphane Moreau
Keyword(s):  
Heat Transfer ◽  
Temperature Distribution ◽  
Flow Field ◽  
Gas Turbines ◽  
Conjugate Heat Transfer ◽  
Separation Bubble ◽  
Suction Side ◽  
Free Stream Turbulence ◽  
Moderate Reynolds Number ◽  
Large Eddy

Determination of heat loads is a key issue in the design of gas turbines. In order to optimize the cooling, an exact knowledge of the heat flux and temperature distributions on the airfoils surface is necessary. Heat transfer is influenced by various factors, like pressure distribution, wakes, surface curvature, secondary flow effects, surface roughness, free stream turbulence, and separation. Each of these phenomenons is a challenge for numerical simulations. Among numerical methods, large eddy simulations (LES) offers new design paths to diminish development costs of turbines through important reductions of the number of experimental tests. In this study, LES is coupled with a thermal solver in order to investigate the flow field and heat transfer around a highly loaded low pressure water-cooled turbine vane at moderate Reynolds number (150,000). The meshing strategy (hybrid grid with layers of prisms at the wall and tetrahedra elsewhere) combined with a high fidelity LES solver gives accurate predictions of the wall heat transfer coefficient for isothermal computations. Mesh convergence underlines the known result that wall-resolved LES requires discretizations for which y+ is of the order of one. The analysis of the flow field gives a comprehensive view of the main flow features responsible for heat transfer, mainly the separation bubble on the suction side that triggers transition to a turbulent boundary layer and the massive separation region on the pressure side. Conjugate heat transfer computation gives access to the temperature distribution in the blade, which is in good agreement with experimental measurements. Finally, given the uncertainty on the coolant water temperature provided by experimentalists, uncertainty quantification allows apprehension of the effect of this parameter on the temperature distribution.

Effects of Free-Stream Turbulence on Transition of a Separated Boundary Layer Over the Leading-Edge of a Constant Thickness Airfoil

2015 ◽  
Vol 138 (2) ◽  
Author(s):  
A. Samson ◽  
S. Sarkar
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Free Stream ◽  
Separation Bubble ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Constant Thickness ◽  
Free Stream Turbulence ◽  
Transition Mechanism ◽  
Bubble Length

This paper describes the change in the transition mechanism of a separated boundary layer formed from the semicircular leading-edge of a constant thickness airfoil as the free-stream turbulence (fst) increases. Experiments are carried out in a low-speed wind tunnel for three levels of fst (Tu = 0.65%, 4.6%, and 7.7%) at two Reynolds numbers (Re) 25,000 and 55,000 (based on the leading-edge diameter). Measurements of velocity and surface pressure along with flow field visualizations are carried out using a planar particle image velocimetry (PIV). The flow undergoes separation in the vicinity of leading-edge and reattaches in the downstream forming a separation bubble. The shear layer is laminar up to 20% of separation length, and then, the perturbations are amplified in the second-half attributing to breakdown and reattachment. The bubble length is highly susceptible to change in Tu. At low fst, the primary mode of instability of the shear layer is Kelvin–Helmholtz (K-H), although the local viscous effect may not be neglected. At high fst, the mechanism of shear layer rollup is bypassed with transient growth of perturbations along with evidence of spot formation. The predominant shedding frequency when normalized with respect to the momentum thickness at separation is almost constant and shows a good agreement with the previous studies. After reattachment, the flow takes longer length to approach a canonical boundary layer.

scholarly journals Effect of Different Subgrid-Scale Models and Inflow Turbulence Conditions on the Boundary Layer Transition in a Transonic Linear Turbine Cascade

2021 ◽  
Vol 6 (3) ◽  
pp. 35
Author(s):  
Ettore Bertolini ◽  
Paul Pieringer ◽  
Wolfgang Sanz
Keyword(s):  
Boundary Layer ◽  
Suction Side ◽  
Boundary Layer Transition ◽  
Subgrid Scale ◽  
Turbine Cascade ◽  
Layer Transition ◽  
Free Stream Turbulence ◽  
Inflow Turbulence ◽  
Turbulence Length ◽  
Turbulence Length Scale

The aim of this work is to study the influence of different subgrid-scale (SGS) closure models and inflow turbulence conditions on the boundary layer transition on the suction side of a highly loaded transonic turbine cascade in the presence of high free-stream turbulence using large eddy simulations (LES) of the MUR237 test case. For the numerical simulations, the MUR237 flow case was considered and the incoming free-stream turbulence was reproduced using the synthetic eddy method (SEM). The boundary layer transition on the blade suction side was found to be significantly influenced by the choice of the SGS closure model and the SEM parameters. These two aspects were carefully evaluated in this work. Initially, the influence of three different closure models (Smagorinsky, WALE, and subgrid-scale kinetic energy model) was evaluated. Among them, the WALE SGS closure model performed best compared to the Smagorinsky and KEM models and, for this reason, was used in the following analysis. Finally, different values of the turbulence length scale, eddies density, and inlet turbulence for the SEM were evaluated. As shown by the results, among the different parameters, the choice of the turbulence length scale plays a major role in the transition onset on the blade suction side.

Numerical Study on the Effects of the Upstream Wake Generator on the Aerodynamic Performance of a High-Lift Low Pressure Turbine Blade

2016 ◽  
Author(s):  
Fabio Bigoni ◽  
Stefano Vagnoli ◽  
Tony Arts ◽  
Tom Verstraete
Keyword(s):  
Turbine Blade ◽  
Numerical Study ◽  
Separation Bubble ◽  
Suction Side ◽  
Low Pressure ◽  
High Lift ◽  
Blade Loading ◽  
Free Stream Turbulence ◽  
Low Pressure Turbine ◽  
Sst Turbulence Model

The aim of the present paper is to analyze and discuss in detail the effects of the upstream incoming wakes on both the aerodynamic loading and the evolution of the laminar separation bubble developing along the suction side of the high-lift T106-C low pressure turbine blade at engine similar Reynolds and Mach numbers, but at a low free stream turbulence level. The investigation is carried out numerically by means of steady and unsteady RANS simulations for two different Reynolds numbers (100,000 and 140,000), employing the SST turbulence model coupled to the γ–Re~θt transition model. The numerical results are compared with the experimental data provided by the von Karman Institute in terms of variation of losses and blade loading between steady and unsteady inflow conditions. In general, the incoming wakes have a crucial effect both on the reduction of the separation bubble and on the modification of the blade loading. This is analyzed in detail, in order to separate these contributions.

Large-Eddy Simulation of Transition in a Separation Bubble

2005 ◽  
Author(s):  
Stephen K. Roberts ◽  
Metin I. Yaras
Keyword(s):  
Large Eddy Simulation ◽  
Separation Bubble ◽  
Suction Side ◽  
Transition Process ◽  
Numerical Dissipation ◽  
Free Stream Turbulence ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Scale Turbulence ◽  
Turbine Airfoils

In this paper, large-eddy simulation of the transition process in a separation bubble is compared to experimental results. The measurements and simulations are conducted under low free-stream turbulence conditions over a flat plate with a streamwise pressure distribution typical of those encountered on the suction side of turbine airfoils. The computational grid is sufficiently refined that the effects of sub-grid scale turbulence are adequately represented by the numerical dissipation of the computational algorithm. The large-eddy simulations are shown to accurately capture the transition process in the separated shear layer. The results of these simulations are used to gain further insight into the breakdown mechanisms in transitioning separation bubbles.

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