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.

Intermittency Transport Modeling of Separated Flow Transition

2003 ◽  
Author(s):  
J. Vicedo ◽  
S. Vilmin ◽  
W. N. Dawes ◽  
A. M. Savill
Keyword(s):  
Free Stream ◽  
Transport Model ◽  
Separation Bubble ◽  
Separated Flow ◽  
Intermittent Flow ◽  
Experimental Investigations ◽  
Turbulence Characteristic ◽  
Flow Transition ◽  
Bypass Transition ◽  
Free Stream Turbulence

An intermittency transport model is proposed for modeling separated-flow transition. The model is based on earlier work on prediction of attached flow bypass transition and is applied for the first time to model transition in a separation bubble at various degrees of free-stream turbulence. The model has been developed so that it takes into account the entrainment of the surrounding fluid. Experimental investigations suggest that it is this phenomena which ultimately determines the extent of the separation bubble. Transition onset is determined via a boundary layer correlation based on momentum thickness at the point of separation. The intermittent flow characteristic of the transition process is modeled via an intermittency transport equation. This accounts for both normal and streamwise variation of intermittency and hence models the entrainment of surrounding flow in a more accurate manner than alternative prescribed intermittency models. The model has been validated against the well established T3L semicircular leading edge flat plate test case for three different degrees of free-stream turbulence characteristic of turbomachinery blade applications.

Measurements and Prediction of Free-Stream Turbulence and Pressure-Gradient Effects on Attached-Flow Boundary-Layer Transition

2003 ◽  
Author(s):  
S. K. Roberts ◽  
M. I. Yaras
Keyword(s):  
Reynolds Number ◽  
Free Stream ◽  
Turbine Blades ◽  
Boundary Layer Transition ◽  
Pressure Gradients ◽  
Layer Transition ◽  
Free Stream Turbulence ◽  
Flow Reynolds Number ◽  
Attached Flow ◽  
Transition Length

This paper presents measurements of free-stream turbulence, streamwise pressure gradients and flow Reynolds number effects on attached-flow transition. The measurements were performed on a flat plate, at free-stream turbulence intensities ranging from 0.5% to 9.0%, four Reynolds numbers, and several streamwise pressure distributions, including ones that are typical of the suction side pressures of axial turbine blades. Based on the results, the extent of upstream movement of transition location with free-stream turbulence, the changes in transition length with variations in the streamwise pressure gradients, and the sensitivity of these trends to flow Reynolds number are quantified. Interpretation of the measurements is based primarily on streamwise and cross-stream intermittency distributions extracted from the velocity traces of hot-wire traverses. The measured transition inception locations and transition lengths are used to evaluate mathematical models available in the published literature. A modification is proposed to a transition length model to improve the prediction of the streamwise intermittency distribution.

Predicting Transition in Turbomachinery—Part I: A Review and New Model Development

2004 ◽  
Vol 129 (1) ◽  
pp. 1-13 ◽  
Author(s):  
T. J. Praisner ◽  
J. P. Clark
Keyword(s):  
Boundary Layers ◽  
Free Stream ◽  
Separation Bubble ◽  
Model Development ◽  
Separated Flow ◽  
Design System ◽  
Critical Ratio ◽  
Flow Transition ◽  
Free Stream Turbulence ◽  
Transition Modeling

Here we report on an effort to include an empirically based transition modeling capability in a Reynolds Averaged Navier-Stokes solver. Well known empirical models for both attached- and separated-flow transition were tested against cascade data and found unsuitable for use in turbomachinery design. Consequently, a program was launched to develop models with sufficient accuracy for use in design. As a first step, accurate prediction of free stream turbulence development was identified as a prerequisite for accurate modeling. Additionally, a demonstrated capability to capture the effects of free stream turbulence on pre-transitional boundary layers became an impetus for the work. A computational fluid dynamics (CFD)-supplemented database of 104 experimental cascade cases was constructed to explore the development of new correlations. Dimensional analyses were performed to guide the work, and appropriate non-dimensional parameters were then extracted from CFD predictions of the laminar boundary layers existing on the airfoil surfaces prior to either transition onset or incipient separation. For attached-flow transition, onset was found to occur at a critical ratio of the boundary-layer diffusion time to a time scale associated with the energy-bearing turbulent eddies. In the case of separated-flow transition, it was found that the length of a separation bubble prior to turbulent reattachment was a simple function of the local momentum thickness at separation and the overall surface length traversed by a fluid element prior to separation. Both the attached- and separated-flow transition models were implemented into the design system as point-like trips.

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.

Correlations for the Prediction of Intermittency and Turbulent Spot Production Rate in Separated Flows

2018 ◽  
Author(s):  
M. Dellacasagrande ◽  
R. Guida ◽  
D. Lengani ◽  
D. Simoni ◽  
M. Ubaldi ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Production Rate ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Turbine Blades ◽  
Transition Process ◽  
Separated Flows ◽  
Intensity Level ◽  
Turbulent Spot ◽  
Free Stream Turbulence

Experimental data describing laminar separation bubbles developing under strong adverse pressure gradients, typical of Ultra-High-Lift turbine blades, have been analyzed to define empirical correlations able to predict the main features of the separated flow transition. Tests have been performed for three different Reynolds numbers and three different free-stream turbulence intensity levels. For each condition, around 4000 Particle Image Velocimetry (PIV) snapshots have been acquired. A wavelet based intermittency detection technique, able to identify the large scale vortices shed as a consequence of the separation, has been applied to the large amount of data to efficiently compute the intermittency function for the different conditions. The transition onset and end positions, as well as the turbulent spot production rate are evaluated. Thanks to the recent advancements in the understanding on the role played by Reynolds number and free-stream turbulence intensity on the dynamics leading to transition in separated flows, guest functions are proposed in the paper to fit the data. The proposed functions are able to mimic the effects of Reynolds number and free-stream turbulence intensity level on the receptivity process of the boundary layer in the attached part, on the disturbance exponential growth rate observed in the linear stability region of the separated shear layer, as well as on the nonlinear later stage of completing transition. Once identified the structure of the correlation functions, a fitting process with own and literature data allowed us to calibrate the unknown constants. Results reported in the paper show the ability of the proposed correlations to adequately predict the transition process in the case of separated flows. The correlation for the spot production rate here proposed extends the correlations proposed in liter-ature for attached (by-pass like) transition process, and could be used in γ–Reϑ codes, where the spot production rate appears as a source term in the intermittency function transport equation.

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.

A CFD Study of Low Reynolds Number Flow in High Lift Cascades

2010 ◽  
Author(s):  
Roberto Pacciani ◽  
Michele Marconcini ◽  
Andrea Arnone ◽  
Francesco Bertini
Keyword(s):  
Reynolds Number ◽  
Separation Bubble ◽  
Structural Characteristics ◽  
Low Reynolds Number ◽  
Separated Flow ◽  
Flow Region ◽  
Suction Side ◽  
Transitional Region ◽  
High Lift ◽  
Low Reynolds

A study of the separated flow in high-lift, low-Reynolds-number cascade, has been carried out using a novel three-equation, transition-sensitive, turbulence model. It is based on the coupling of an additional transport equation for the so-called laminar kinetic energy with the Wilcox k-ω model. Such an approach takes into account the increase of the non-turbulent fluctuations in the pre-transitional and transitional region. Two high-lift cascades (T106C and T108), recently tested at the von Ka´rma´n Institute in the framework of the European project TATMo (Turbulence and Transition Modelling for Special Turbomachinery Applications), were analyzed. The two cascades have different loading distributions and suction side diffusion rates, and therefore also different separation bubble characteristics and loss levels. The analyzed Reynolds number values span the whole range typically encountered in aeroengines low-pressure turbines operations. Several expansion ratios for steady inflow conditions characterized by different freestream turbulence intensities were considered. A detailed comparison between measurements and computations, including bubble structural characteristics, will be presented and discussed. Results with the proposed model show its ability to predict the evolution of the separated flow region, including bubble bursting phenomena, in high-lift cascades operating in LP-turbine conditions.

A Prediction Model for Separated-Flow Transition

1998 ◽  
Author(s):  
Anca Hatman ◽  
Ting Wang
Keyword(s):  
Reynolds Number ◽  
Prediction Model ◽  
Flow Behavior ◽  
Separated Flow ◽  
Separation Point ◽  
Flow Transition ◽  
Laminar Separation ◽  
Free Stream Turbulence ◽  
Displacement Thickness ◽  
Transition Modes

The present study formulates an improved approach for analyzing separated-flow transition that differentiates between the transition process in boundary layers which are laminar at separation and those which are already transitional at separation. The paper introduces new parameters that are necessary in classifying separated-flow transition modes and in accounting for the concomitant evolution of transition in separated shear layer and the average effect of periodic separation bubble build-up and vortex shedding. At least three separated-flow transition modes are positively distinguished: (a) transitional separation, with the transition starting upstream of the separation point and developing mostly as natural transition, (b) laminar separation - short bubble mode, with the onset of transition induced downstream of the separation point by inflexional instability and with a quick transition completion, and (c) laminar separation - long bubble mode, with the onset of transition also induced downstream of the separation point by inflexional instability, and with the transition completion delayed. Passing from one mode to another takes place continuously through a succession of intermediate stages. The location of maximum bubble elevation has been proved to be the controlling parameter for the separated flow behavior. It was found that, downstream of the separation point, the experimental data expressed in terms of distance Reynolds number Rex can be correlated better than momentum or displacement thickness Reynolds number data. For each mode of separated-flow transition, the onset of transition, the transition length, and separated flow general characteristic are determined. This prediction model is developed mainly on low free-stream turbulence flat plate data and limited airfoil data. Extension to airfoils and high turbulence environment requires additional study.

Aerodynamic Performance of a Very High Lift Low Pressure Turbine Airfoil (T106C) at Low Reynolds and High Mach Number With Effect of Free Stream Turbulence Intensity

2012 ◽  
Vol 134 (6) ◽  
Author(s):  
Jan Michálek ◽  
Michelangelo Monaldi ◽  
Tony Arts
Keyword(s):  
Mach Number ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Separated Flow ◽  
Aerodynamic Performance ◽  
Flow Transition ◽  
High Lift ◽  
Free Stream Turbulence ◽  
Low Pressure Turbine ◽  
Very High

A detailed experimental analysis of the effects of the Reynolds number and free-stream turbulence intensity on the aerodynamic performance of a very high-lift, mid-loaded low-pressure turbine blade (T106C) is presented in this paper. The study was carried out on a large scale linear cascade in the VKI S1/C high-speed wind tunnel, operating at high exit Mach number (0.65) with a range of low Reynolds numbers (80,000–160,000) and three levels of free-stream turbulence intensity (0.8–3.2%). In the first part of the paper, the overall aerodynamic performance of the airfoil is presented, based on mid-span measurements performed by means of static pressure taps, hot-film sensors and a five-hole probe traversing downstream of the cascade. Some specific features of separated flow transition are also discussed for selected cases. The second part presents the analysis of the results in terms of correlations derived for the characteristic points of boundary layer separation and transition. A comparison with some previously published prediction models is shown. The large variety of boundary conditions provides a unique database for validating codes dealing with separated flow transition in turbomachinery.

Predicting Transitional Separation Bubbles

2004 ◽  
Vol 127 (3) ◽  
pp. 497-501
Author(s):  
John A. Redford ◽  
Mark W. Johnson
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Separated Flow ◽  
Leading Edge ◽  
Transition Process ◽  
Test Cases ◽  
Transition Model ◽  
Flow Transition ◽  
Free Stream Turbulence ◽  
Attached Flow

This paper describes the modifications made to a successful attached flow transition model to produce a model capable of predicting both attached and separated flow transition. This transition model is used in combination with the Fluent CFD software, which is used to compute the flow around the blade assuming that it remains entirely laminar. The transition model then determines the start of transition location and the development of the intermittency. These intermittency values weight the laminar and turbulent boundary layer profiles to obtain the resulting transitional boundary layer parameters. The ERCOFTAC T3L test cases are used to validate the predictions. The T3L blade is a flat plate with a semi-circular leading edge, which results in the formation of a separation bubble the length of which is strongly dependent on the transition process. Predictions were performed for five T3L test cases for differing free-stream turbulence levels and Reynolds numbers. For the majority of these test cases the measurements were accurately predicted.

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