Predicting Transitional Separation Bubbles

2004 ◽  
Author(s):  
John A. Redford ◽  
Mark W. Johnson
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Separated Flow ◽  
Leading Edge ◽  
Transition Process ◽  
Reynolds Numbers ◽  
Test Cases ◽  
Transition Model ◽  
Flow Transition ◽  
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 freestream turbulence levels and Reynolds numbers. For the majority of these test cases the measurements were accurately predicted.

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.

Characteristics of a separated flow past a semicircular leading-edge airfoil model under different imposed pressure gradient

2021 ◽  
pp. 095441002110212
Author(s):  
K Anand ◽  
KT Ganesh
Keyword(s):  
Boundary Layer ◽  
Particle Image Velocimetry ◽  
Pressure Gradient ◽  
Particle Image ◽  
Separation Bubble ◽  
Separated Flow ◽  
Leading Edge ◽  
Field Measurements ◽  
Bench Mark ◽  
Image Velocimetry

The effect of pressure gradient on a separated boundary layer past the leading edge of an airfoil model is studied experimentally using electronically scanned pressure (ESP) and particle image velocimetry (PIV) for a Reynolds number ( Re) of 25,000, based on leading-edge diameter ( D). The features of the boundary layer in the region of separation and its development past the reattachment location are examined for three cases of β (−30°, 0°, and +30°). The bubble parameters such as the onset of separation and transition and the reattachment location are identified from the averaged data obtained from pressure and velocity measurements. Surface pressure measurements obtained from ESP show a surge in wall static pressure for β = −30° (flap deflected up), while it goes down for β = +30° (flap deflected down) compared to the fundamental case, β = 0°. Particle image velocimetry results show that the roll up of the shear layer past the onset of separation is early for β = +30°, owing to higher amplification of background disturbances compared to β = 0° and −30°. Downstream to transition location, the instantaneous field measurements reveal a stretched, disoriented, and at instances bigger vortices for β = +30°, whereas a regular, periodically shed vortices, keeping their identity past the reattachment location, is observed for β = 0° and −30°. Above all, this study presents a new insight on the features of a separation bubble receiving a disturbance from the downstream end of the model, and these results may serve as a bench mark for future studies over an airfoil under similar environment.

Modelling Unsteady Transition Effects in an Axial Compressor

2008 ◽  
Author(s):  
Adam D. Beevers ◽  
Joao Amaral-Teixeira ◽  
Roger Wells
Keyword(s):  
Boundary Layer ◽  
Axial Compressor ◽  
Leading Edge ◽  
Transition Process ◽  
Transition Model ◽  
Velocity Defect ◽  
Commercial Code ◽  
Stator Blade ◽  
Transition Effects ◽  
Unsteady Effects

Wake induced transition is simulated at mid-span on a C4 stator blade in a 1.5 stage low speed axial compressor using the CFX γ – θ transition model. IGV and rotor wake inputs were created from a succession of Fourier series produced from experimental data. The purpose of the study was to understand the effectiveness of the γ – θ model implemented in a commercial code to predict the unsteady effects of wake induced transition. The γ – θ transition model was found to predict wake-induced transition and the subsequent calmed region caused by the passing wakes. The wake velocity defect creates conditions within the boundary layer such that the high disturbance energy, which is diffused into the boundary layer at the leading edge from the wake, initiates the transition process. This high disturbance energy travels through the boundary layer directly behind the wake.

Numerical Investigation of the Wake-Boundary Layer Interaction on a Highly Loaded LP Turbine Cascade Blade

2002 ◽  
Author(s):  
Pasquale Cardamone ◽  
Peter Stadtmu¨ller ◽  
Leonhard Fottner
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Transition Process ◽  
Reynolds Numbers ◽  
Numerical Approach ◽  
Experimental Investigations ◽  
Turbine Cascade ◽  
Suction Surface ◽  
Time Resolved ◽  
Highly Loaded

The effects of wake passing on the development of the profile boundary layer of a highly loaded low-pressure turbine cascade are studied using the RANS code TRACE-U. The numerical results are compared with available experimental data to verify the accuracy of the code in predicting the periodic-unsteady transition and separation mechanisms at low Reynolds number conditions. The experimental investigations have been carried out on a turbine cascade called T106D-EIZ subjected to wakes generated by an up-stream moving bar-type generator. The cascade pitch was increased by about 30% with respect to design conditions without modifying the blade geometry in order to obtain a large separation bubble on the suction surface. The extensive database containing time-averaged as well as time-resolved results was presented in a separate paper by Stadtmu¨ller and Fottner (2001) and is discussed only briefly. The time-accurate multistage Navier-Stokes solver TRACE-U developed by the DLR Cologne used for the numerical simulations employs a modified version of the one-equation Spalart-Allmaras turbulence model coupled with a transition correlation based on the work of Abu-Ghannam and Shaw in the formulation of Drela. The objective of this paper is to provide further insight into the aerodynamics of the wake-induced transition process and to rate the application limits of the numerical approach for exit Reynolds numbers as low as 60.000. The CFD predictions for two different flow conditions are compared with the measurements. Plots of wall-shear stress, blade loading, shape factor and loss behaviour are used to verify the reliability of the code. The periodic-unsteady development of the boundary layer as well as the loss behaviour is well reproduced for higher Reynolds numbers. For the case with massive separation, large discrepancies between numerical and experimental results are observed.

Effects of Free Stream Turbulence on Intermittent Boundary Layer Flows

1995 ◽  
Author(s):  
Anestis I. Kalfas ◽  
Robin L. Elder
Keyword(s):  
Boundary Layer ◽  
Stream Flow ◽  
Free Stream ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Flow Transition ◽  
Flow Conditions ◽  
Boundary Layer Flows ◽  
Laminar Separation ◽  
Free Stream Turbulence

This paper considers the effects of free stream turbulence intensity on intermittent boundary layer flows related to turbomachinery. The present experimental investigation has been undertaken under free stream flow conditions dominated by grid generated turbulence and Reynolds numbers appropriate for turbomachinery applications. Unseparated flow transition in the boundary layer has been considered using a flat plate with the C4 leading edge which has been designed to avoid laminar separation. This configuration provided the opportunity to study the effect of a realistic turbomachinery leading edge shape on transition. Boundary layer type hot-wire probes have been used in order to acquire detailed information about the effect of the free stream conditions and the leading edge configuration on the structure of the boundary layer. Furthermore, information about the intermittency distribution throughout the boundary layer has been obtained using statistical analysis of the velocity record of the flow field.

Computation of Laminar-Turbulent Transition in Turbumachinery Using the Correlation Based γ-Reθ Transition Model

2010 ◽  
Author(s):  
M. E. Kelterer ◽  
R. Pecnik ◽  
W. Sanz
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Three Dimensional ◽  
Transition Process ◽  
Boundary Layer Transition ◽  
Correct Prediction ◽  
Test Cases ◽  
Transition Model ◽  
Algebraic Models ◽  
Plate Test

The accurate numerical simulation of the flow through turbomachinery depends on the correct prediction of boundary-layer transition phenomena. Especially heat transfer and skin friction investigations demand a reliable simulation of the transition process. Many models have been developed to simulate the transition process, ranging from simple algebraic models to very sophisticated transport models. But nearly all models suffer from the need to determine boundary layer parameters and from their difficult application in three-dimensional flows. Therefore, in this work the correlation based γ-Reθ transition model developed by Menter and Langtry is implemented into the in-house Reynolds-averaged Navier-Stokes solver. This model avoids the calculation of non-local parameters and is thus very suitable for three-dimensional general flow situations. Two additional transport equations, one for the intermittency and one for the momentum thickness Reynolds number, which is a criterion for the transition onset, are added to the well known SST turbulence model by Menter. Instead of the proprietary model correlations by Menter et al. the authors used correlations by other research groups within the in-house code and tested these correlations for simple flat-plate test cases. The non-satisfying results indicate a strong code dependency of the model. Therefore also in-house correlations are presented and validated. A comprehensive study of the model performance on the well known ERCOFTAC flat plate test cases is performed. After this validation the model is applied to the steady flow in a T106A and a T106 turbine cascade.

Flow Studies of the Leading Edge Stall on a Swept-Back Wing at High Incidence

1956 ◽  
Vol 60 (541) ◽  
pp. 51-60 ◽  
Author(s):  
Joseph Black
Keyword(s):  
Boundary Layer ◽  
Liquid Film ◽  
Boundary Layer Flow ◽  
Separation Bubble ◽  
Flow Direction ◽  
Separated Flow ◽  
Leading Edge ◽  
Outer Edge ◽  
Layer Flow ◽  
Wing Tip

SummaryThe flow separation on a swept-back wing with 44 degrees leading edge sweep at 18 degrees incidence has been investigated by means of detailed pressure distribution measurements over the leading edge, boundary layer flow determination with liquid film technique, and yawmeter traverses. A wool-tuft grid was also used, and a spin detector was developed to search for regions of vorticity. These tests established that the flow separates on the leading edge; over the inboard sections it re-attaches behind a “ short” separation bubble, while outboard it only re-attaches near the trailing edge, thus forming a “ long ” separation bubble, or else fails to attach. The separated flow in what has been commonly called the tip stall does in fact take the form of a “ ram's horn “ vortex with the origin, or “ tip,” located at the junction of the two bubbles on the leading edge. The vortex lies outwards across the wing surface at approximately 20 to 25 degrees to the line-of-flight before curving aft to be shed into the wake, where it extends almost from mid semi-span to the wing tip. This vortex induces considerable changes in flow direction, both on and over the wing, and also in the wake. Thus in the wake a maximum downwash of 23 degrees is induced aft of the mid semi-span, and there is an upwash of 17 degrees at the outer edge of the vortex, almost aft of the tip. A good correlation between yawmeter results and the boundary layer flow direction indications from the liquid film technique was obtained.

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.

Measurements of the Effects of Pressure-Gradient History on Separation-Bubble Transition

2001 ◽  
Author(s):  
M. I. Yaras
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Separation Bubble ◽  
Leading Edge ◽  
Transition Process ◽  
Pressure Gradients ◽  
Local Pressure ◽  
Test Plate ◽  
Transition Prediction ◽  
Current Transition

In many boundary-layer situations, particularly in turbomachinery, separation-bubble transition occurs at a local pressure gradient that differs significantly from the values further upstream. Additionally, this upstream history changes substantially from case to case, with the transitioning separation bubble occurring at streamwise positions along the blade chord varying from close vicinity of the leading edge to mid-chord. In the case of low free-stream disturbances, development of instability waves prior to separation would clearly vary as a result of these differences in the history of the boundary layer prior to separation. Measurements are presented to document the effects of pressure gradients that a laminar boundary layer experiences prior to separation on the transition process that follows in the separated region. The boundary layer development was measured on a smooth, flat plate upon which streamwise pressure gradients were imposed by a flexible, contoured wall opposite to the test plate. Only low freestream-turbulence levels were considered to isolate the effects of pressure-gradient history on the transition process. Two Reynolds numbers were considered for each pressure-gradient setting. Measured quantities consisted of velocity and turbulence intensity obtained with a single hot-wire, and of surface pressures. Observed variations in transition onset location with changes in pressure-gradient history provide the basis for further development of current transition prediction schemes.

Intermittency Transport Modeling of Separated Flow Transition

2004 ◽  
Vol 126 (3) ◽  
pp. 424-431 ◽  
Author(s):  
J. Vicedo ◽  
S. Vilmin ◽  
W. N. Dawes ◽  
A. M. Savill
Keyword(s):  
Transport Model ◽  
Separation Bubble ◽  
Separated Flow ◽  
Leading Edge ◽  
Intermittent Flow ◽  
Experimental Investigations ◽  
Turbulence Characteristic ◽  
Flow Transition ◽  
Bypass Transition ◽  
Freestream 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 freestream 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 freestream turbulence characteristic of turbomachinery blade applications.

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