Statistical characterization of free-stream turbulence induced transition under variable Reynolds number, free-stream turbulence, and pressure gradient

2021 ◽  
Vol 33 (9) ◽  
pp. 094115
Author(s):  
M. Dellacasagrande ◽  
D. Lengani ◽  
D. Simoni ◽  
J. O. Pralits ◽  
K. Durovich ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Pressure Gradient ◽  
Free Stream ◽  
Statistical Characterization ◽  
Free Stream Turbulence

Experimental Investigation of Free Stream Turbulence Effects on Flow Separation

2003 ◽  
Author(s):  
Mahmoud Ardebili ◽  
Yiannis Andreopoulos
Keyword(s):  
Boundary Layer ◽  
Experimental Investigation ◽  
Reynolds Number ◽  
Pressure Gradient ◽  
Flow Separation ◽  
Large Scale ◽  
Free Stream ◽  
Pressure Coefficient ◽  
Free Stream Turbulence ◽  
Vorticity Flux

An experimental investigation of a separated boundary layer flow has been attempted which has been created by perturbing a flat plate flow with a favorable pressure gradient immediately followed by an adverse pressure gradient. The aim of the research program is possible control of flow separation by means of free stream turbulence. The flow is configured in a large-scale low speed wind tunnel where measurements of turbulence can be obtained with high spatial and temporal resolution. A model has been designed by using CFD analysis. Mean wall pressure and vorticity flux measurements are reported in this paper. Twelve experiments with three different mesh size grids at three different Reynolds numbers have been carried out. Three bulk flow parameters seem to characterize the flow: The Reynolds number of the boundary layer, Re+, the Reynolds number of the flow through the grid, ReM, and the solidity of the grid. It was found that the pressure coefficient depends weakly on the solidity of the grids. Vorticity flux also depends on the grid used to generate free stream turbulence. The location of maximum or minimum vorticity flux moves upstream at higher ReM.

scholarly journals Measurements in Separated and Transitional Boundary Layers Under Low-Pressure Turbine Airfoil Conditions

2000 ◽  
Vol 123 (2) ◽  
pp. 189-197 ◽  
Author(s):  
Ralph J. Volino ◽  
Lennart S. Hultgren
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Pressure Gradient ◽  
Boundary Layers ◽  
Free Stream ◽  
Separation Bubble ◽  
Low Pressure ◽  
Mean Velocity ◽  
Free Stream Turbulence ◽  
Low Pressure Turbine

Detailed velocity measurements were made along a flat plate subject to the same dimensionless pressure gradient as the suction side of a modern low-pressure turbine airfoil. Reynolds numbers based on wetted plate length and nominal exit velocity were varied from 50,000 to 300,000, covering cruise to takeoff conditions. Low and high inlet free-stream turbulence intensities (0.2 and 7 percent) were set using passive grids. The location of boundary-layer separation does not depend strongly on the free-stream turbulence level or Reynolds number, as long as the boundary layer remains nonturbulent prior to separation. Strong acceleration prevents transition on the upstream part of the plate in all cases. Both free-stream turbulence and Reynolds number have strong effects on transition in the adverse pressure gradient region. Under low free-stream turbulence conditions, transition is induced by instability waves in the shear layer of the separation bubble. Reattachment generally occurs at the transition start. At Re=50,000 the separation bubble does not close before the trailing edge of the modeled airfoil. At higher Re, transition moves upstream, and the boundary layer reattaches. With high free-stream turbulence levels, transition appears to occur in a bypass mode, similar to that in attached boundary layers. Transition moves upstream, resulting in shorter separation regions. At Re above 200,000, transition begins before separation. Mean velocity, turbulence, and intermittency profiles are presented.

scholarly journals Measurements in Separated and Transitional Boundary Layers Under Low-Pressure Turbine Airfoil Conditions

2000 ◽  
Author(s):  
Ralph J. Volino ◽  
Lennart S. Hultgren
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Pressure Gradient ◽  
Boundary Layers ◽  
Free Stream ◽  
Separation Bubble ◽  
Low Pressure ◽  
Mean Velocity ◽  
Free Stream Turbulence ◽  
Low Pressure Turbine

Detailed velocity measurements were made along a flat plate subject to the same dimensionless pressure gradient as the suction side of a modern low-pressure turbine airfoil. Reynolds numbers based on wetted plate length and nominal exit velocity were varied from 50, 000 to 300, 000, covering cruise to takeoff conditions. Low and high inlet free-stream turbulence intensities (0.2% and 7%) were set using passive grids. The location of boundary-layer separation does not depend strongly on the free-stream turbulence level or Reynolds number, as long as the boundary layer remains non-turbulent prior to separation. Strong acceleration prevents transition on the upstream part of the plate in all cases. Both free-stream turbulence and Reynolds number have strong effects on transition in the adverse pressure gradient region. Under low free-stream turbulence conditions transition is induced by instability waves in the shear layer of the separation bubble. Reattachment generally occurs at the transition start. At Re = 50, 000 the separation bubble does not close before the trailing edge of the modeled airfoil. At higher Re, transition moves upstream, and the boundary layer reattaches. With high free-stream turbulence levels, transition appears to occur in a bypass mode, similar to that in attached boundary layers. Transition moves upstream, resulting in shorter separation regions. At Re above 200,000, transition begins before separation. Mean velocity, turbulence and intermittency profiles are presented.

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.

Effects of Free-Stream Turbulence and Adverse Pressure Gradients on Boundary Layer Transition

1994 ◽  
Vol 116 (3) ◽  
pp. 392-404 ◽  
Author(s):  
J. P. Gostelow ◽  
A. R. Blunden ◽  
G. J. Walker
Keyword(s):  
Pressure Gradient ◽  
Free Stream ◽  
Formation Rate ◽  
Transition Process ◽  
Pressure Gradients ◽  
Turbulence Level ◽  
Free Stream Turbulence ◽  
Exponential Decrease ◽  
Transition Length ◽  
Spot Formation

Boundary layer measurements are presented through transition for six different free-stream turbulence levels and a complete range of adverse pressure gradients for attached laminar flow. Measured intermittency distributions provide an excellent similarity basis for characterizing the transition process under all conditions tested when the Narasimha procedure for determining transition inception is used. This inception location procedure brings consistency to the data. Velocity profiles and integral parameters are influenced by turbulence level and pressure gradient and do not provide a consistent basis. Under strong adverse pressure gradients transition occurs rapidly and the velocity profile has not fully responded before the completion of transition. The starting turbulent layer does not attain an equilibrium velocity profile. A change in pressure gradient from zero to even a modest adverse level is accompanied by a severe reduction in transition length. Under diffusing conditions the physics of the transition process changes and the spot formation rate increases rapidly; instead of the “breakdown in sets” regime experienced in the absence of a pressure gradient, transition under strong adverse pressure gradients is more related to the amplification and subsequent instability of the Tollmien-Schlichting waves. Measurements reveal an exponential decrease in transition length with increasing adverse pressure gradient; a less severe exponential decrease is experienced with increasing turbulence level. Correlations of transition length are provided that facilitate its prediction in the form of suitable length parameters including spot formation rate.

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