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.

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.

Effects of incidence angle on a low-pressure turbine blade boundary layer evolution through large eddy simulation

2018 ◽  
Vol 232 (6) ◽  
pp. 722-734 ◽  
Author(s):  
Yunfei Wang ◽  
Xiuming Sui ◽  
Kai Zhang ◽  
Xiaorong Xiang ◽  
Qingjun Zhao
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Coherent Structures ◽  
Separation Bubble ◽  
Incidence Angle ◽  
Leading Edge ◽  
Pressure Side ◽  
Low Pressure Turbine ◽  
Eddy Simulation ◽  
Large Eddy

The evolution mechanism of the boundary layer and coherent structures in a low-pressure turbine blade is discussed. Five different incidence angles over the T106A blade for a Mach number Ma = 0.404 and Reynolds number Re = 0.6 × 105 (based on the axial chord and outlet velocity) are performed using large eddy simulation method. The calculation results at +7.8 incidence angle are agreed well with the experimental and direct numerical simulation data. The influence of the incidence angle on the flow field is mainly shown at the front of the suction side and pressure side. As the incidence angle changes from positive to negative, the separation bubble near the leading edge disappears and the blade loading decreases gradually. When the incidence angle reduces to −5°, separation bubble appears near the leading edge of the pressure side. At the case of incidence angle equaling −10°, the length of time-averaged separation bubble on the pressure side grows to 39% axial chord and the evolution process of the coherent structures is extremely complex. The spanwise vortexes roll up near the leading edge and gradually evolve into streamwise vortexes. High-energy fluid in the main flow was driven to near-wall zone by the rotating effect of streamwise vortexes, which increases the fluid momentum inside the boundary layer. The streamwise vortexes are stretched by the strong acceleration of the flow until they transport to the trailing edge.

Large Eddy Simulation of Boundary Layer Separation and Reattachment in a LPT Blade at Different Incidence Angles

2015 ◽  
Author(s):  
Yunfei Wang ◽  
Huaping Liu ◽  
Yanping Song ◽  
Fu Chen
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Separation Bubble ◽  
Incidence Angle ◽  
Leading Edge ◽  
Suction Side ◽  
Boundary Layer Separation ◽  
Reattachment Point ◽  
Eddy Simulation ◽  
Large Eddy

In order to predict the phenomenon of laminar flow separation, transition and reattachment in a high-lift low-pressure turbine (LPT), a self-developed large eddy simulation program to solve three dimensional compressible N-S equations was used to simulate the flow structures in T106A LPT blade passage. The outlet Mach number is 0.4 and the Reynolds number is 1.1×105 based on the exit isentropic velocity and the axial chord. The distributions of the time-averaged static pressure coefficient, kinetic loss coefficient and wall shear stress on the blade surface at +7.8° incidence angle agree well with the results of experiment and direct numerical simulation (DNS). The locations of laminar separation and reattachment point occur around 83.6% and 97% axial chord respectively. The evolutionary process of spanwise vorticity and large-scale coherent structure near the trailing edge on the suction side in one period indicates that the two-dimensional shear layer is gradually unstable as a result of spanwise fluctuation and Kelvin-Helmholtz (K-H) instability. The boundary layer separates from the suction surface and the hairpin vortex appears in succession, which leads to transition to turbulence. Analysis of the incidence angle effect on the boundary layer separation point as well as separation bubble scale was also performed. A small scale separation bubble exists around the leading edge at positive incidences. As the incidence angle changes from positive to negative, the separation bubble near the leading edge disappears and the boundary layer thickness reduces gradually. The separation point at the rear part of suction side moves downstream, yet the reattachment point barely changes. The Reynolds stress and turbulent kinetic energy profiles change dramatically at zero and positive incidence. This illustrates that the incidence angle has great influence on the development of the boundary layer and the flow field structures.

scholarly journals Large-eddy simulation of laminar transonic buffet

2018 ◽  
Vol 850 ◽  
pp. 156-178 ◽  
Author(s):  
Julien Dandois ◽  
Ivan Mary ◽  
Vincent Brion
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Separation Bubble ◽  
Oscillation Amplitude ◽  
Boundary Layer Separation ◽  
Stream Velocity ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Shock Oscillation ◽  
Transonic Buffet

A large-eddy simulation of laminar transonic buffet on an airfoil at a Mach number $M=0.735$, an angle of attack $\unicode[STIX]{x1D6FC}=4^{\circ }$, a Reynolds number $Re_{c}=3\times 10^{6}$ has been carried out. The boundary layer is laminar up to the shock foot and laminar/turbulent transition occurs in the separation bubble at the shock foot. Contrary to the turbulent case for which wall pressure spectra are characterised by well-marked peaks at low frequencies ($St=f\cdot c/U_{\infty }\simeq 0.06{-}0.07$, where $St$ is the Strouhal number, $f$ the shock oscillation frequency, $c$ the chord length and $U_{\infty }$ the free-stream velocity), in the laminar case, there are also well-marked peaks but at a much higher frequency ($St=1.2$). The shock oscillation amplitude is also lower: 6 % of chord and limited to the shock foot area in the laminar case instead of 20 % with a whole shock oscillation and intermittent boundary layer separation and reattachment in the turbulent case. The analysis of the phase-averaged fields allowed linking of the frequency of the laminar transonic buffet to a separation bubble breathing phenomenon associated with a vortex shedding mechanism. These vortices are convected at $U_{c}/U_{\infty }\simeq 0.4$ (where $U_{c}$ is the convection velocity). The main finding of the present paper is that the higher frequency of the shock oscillation in the laminar regime is due to a different mechanism than in the turbulent one: laminar transonic buffet is due to a separation bubble breathing phenomenon occurring at the shock foot.

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.

Improved Prediction of Losses with Large Eddy Simulation in a Low- Pressure Turbine

2021 ◽  
pp. 1-38
Author(s):  
Kenji Miki ◽  
Ali Ameri
Keyword(s):  
Experimental Data ◽  
Gas Turbines ◽  
Suction Side ◽  
Transition Process ◽  
Low Pressure ◽  
Validation Data ◽  
Eddy Simulation ◽  
Power Turbine ◽  
Large Eddy ◽  
Characteristic Features

Abstract There is a need to improve predictions of losses resulting from large eddy simulations (LES) of low-pressure turbines (LPT) in gas turbines. This may be done by assessing the accuracy of predictions against validation data and understanding the source of any inaccuracies. LES is a promising approach for capturing the laminar/turbulent transition process in a LPT. In previous studies, the authors utilized LES to model the flow field over a Variable Speed Power Turbine (VSPT) blade and successfully captured characteristic features of separation/reattachment and transition on the suction side at both the cruise (positive incidence) and take-off conditions (negative incidence) and as well, simulated the effect of freestream turbulence (FST) on those phenomena. The predicted pressure loading profiles agreed well with the experimental data for both a high and a low FST case at a Reynolds number of Reex = 220,000. In this paper, we present wake profiles resulting from computations for a range of FST values. Although the predicted wake profiles for the lowest FST case (Tu = 0.5%) matched the experimental data, at higher FST (Tu = 10-15%,) the wake was wider than the experimentally measured wake and for both cases were displaced laterally when compared to the experimental measurements. In our investigation of the causes of the said discrepancies we have identified important effects which could strongly influence the predicted wake profile. Predicted losses were improved by assuring the validity of the flow solution.

scholarly journals Role of Inflow Turbulence and Surrounding Buildings on Large Eddy Simulations of Urban Wind Energy

Energies ◽  
2020 ◽  
Vol 13 (19) ◽  
pp. 5208 ◽  
Author(s):  
Giulio Vita ◽  
Syeda Anam Hashmi ◽  
Simone Salvadori ◽  
Hassan Hemida ◽  
Charalampos Baniotopoulos
Keyword(s):  
Wind Energy ◽  
Geometric Model ◽  
Tall Buildings ◽  
Flow Patterns ◽  
Eddy Simulation ◽  
Turbulent Inflow ◽  
Positioning Strategy ◽  
Large Eddy ◽  
Inflow Turbulence ◽  
Mean Wind Speed

Predicting flow patterns that develop on the roof of high-rise buildings is critical for the development of urban wind energy. In particular, the performance and reliability of devices largely depends on the positioning strategy, a major unresolved challenge. This work aims at investigating the effect of variations in the turbulent inflow and the geometric model on the flow patterns that develop on the roof of tall buildings in the realistic configuration of the University of Birmingham’s campus in the United Kingdom (UK). Results confirm that the accuracy of Large Eddy Simulation (LES) predictions is only marginally affected by differences in the inflow mean wind speed and turbulence intensity, provided that turbulence is not absent. The effect of the presence of surrounding buildings is also investigated and found to be marginal to the results if the inflow is turbulent. The integral length scale is the parameter most affected by the turbulence characteristics of the inflow, while gustiness is only marginally influenced. This work will contribute to LES applications on the urban wind resource and their computational setup simplification.

Large Eddy Simulation of Bypass Transition in Vane Passage With Freestream Turbulence

2020 ◽  
Vol 142 (6) ◽  
Author(s):  
Yousef Kanani ◽  
Sumanta Acharya ◽  
Forrest Ames
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Bypass Transition ◽  
Freestream Turbulence ◽  
Turbulent Spot ◽  
Suction Surface ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Inflow Turbulence

Abstract High Reynolds flow over a nozzle guide-vane with elevated inflow turbulence was simulated using wall-resolved large eddy simulation (LES). The simulations were undertaken at an exit Reynolds number of 0.5 × 106 and inflow turbulence levels of 0.7% and 7.9% and for uniform heat-flux boundary conditions corresponding to the measurements of Varty and Ames (2016, “Experimental Heat Transfer Distributions Over an Aft Loaded Vane With a Large Leading Edge at Very High Turbulence Levels,” ASME Paper No. IMECE2016-67029). The predicted heat transfer distribution over the vane is in excellent agreement with measurements. At higher freestream turbulence, the simulations accurately capture the laminar heat transfer augmentation on the pressure surface and the transition to turbulence on the suction surface. The bypass transition on the suction surface is preceded by boundary layer streaks formed under the external forcing of freestream disturbances which breakdown to turbulence through inner-mode secondary instabilities. Underneath the locally formed turbulent spot, heat transfer coefficient spikes and generally follows the same pattern as the turbulent spot. The details of the flow and temperature fields on the suction side are characterized, and first- and second-order statistics are documented. The turbulent Prandtl number in the boundary layer is generally in the range of 0.7–1, but decays rapidly near the wall.

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