scholarly journals A Complex-Lamellar Description Of Boundary Layer Transition

2021 ◽  
Author(s):  
Maureen L. Kolla
Keyword(s):  
Boundary Layer ◽  
Velocity Field ◽  
Power Series ◽  
Boundary Conditions ◽  
Density Function ◽  
Boundary Layer Transition ◽  
Flow Transition ◽  
Natural Boundary ◽  
Layer Transition ◽  
The Difference

Flow transition is important, in both practical and phenomenological terms. However, there is currently no method for identifying the spatial locations associated with transition, such as the start and end of intermittency. The concept of flow stability and experimental correlations have been used, however, flow stability only identifies the location where disturbances begin to grow in the laminar flow and experimental correlations can only give approximations as measuring the start and end of intermittency is diffcult. Therefore, the focus of this work is to construct a method to identify the start and end of intermittency, for a natural boundary layer transition and a separated flow transition. We obtain these locations by deriving a complex-lamellar description of the velocity field that exists between a fully laminar and fully turbulent boundary condition. Mathematically, this complex-lamellar decomposition, which is constructed from the classical Darwin-Lighthill-Hawthorne drift function and the transport of enstrophy, describes the flow that exists between the fully laminar Pohlhausen equations and Prandtl's fully turbulent one seventh power law. We approximate the difference in enstrophy density between the boundary conditions using a power series. The slope of the power series is scaled by using the shape of the universal intermittency distribution within the intermittency region. We solve the complex-lamellar decomposition of the velocity field along with the slope of the difference in enstrophy density function to determine the location of the laminar and turbulent boundary conditions. Then from the difference in enstrophy density function we calculate the start and end of intermittency. We perform this calculation on a natural boundary layer transition over a flat plate for zero pressure gradient flow and for separated shear flow over a separation bubble. We compare these results to existing experimental results and verify the accuracy of our transition model.

scholarly journals A Complex-Lamellar Description Of Boundary Layer Transition

2021 ◽  
Author(s):  
Maureen L. Kolla
Keyword(s):  
Boundary Layer ◽  
Velocity Field ◽  
Power Series ◽  
Boundary Conditions ◽  
Density Function ◽  
Boundary Layer Transition ◽  
Flow Transition ◽  
Natural Boundary ◽  
Layer Transition ◽  
The Difference

Flow transition is important, in both practical and phenomenological terms. However, there is currently no method for identifying the spatial locations associated with transition, such as the start and end of intermittency. The concept of flow stability and experimental correlations have been used, however, flow stability only identifies the location where disturbances begin to grow in the laminar flow and experimental correlations can only give approximations as measuring the start and end of intermittency is diffcult. Therefore, the focus of this work is to construct a method to identify the start and end of intermittency, for a natural boundary layer transition and a separated flow transition. We obtain these locations by deriving a complex-lamellar description of the velocity field that exists between a fully laminar and fully turbulent boundary condition. Mathematically, this complex-lamellar decomposition, which is constructed from the classical Darwin-Lighthill-Hawthorne drift function and the transport of enstrophy, describes the flow that exists between the fully laminar Pohlhausen equations and Prandtl's fully turbulent one seventh power law. We approximate the difference in enstrophy density between the boundary conditions using a power series. The slope of the power series is scaled by using the shape of the universal intermittency distribution within the intermittency region. We solve the complex-lamellar decomposition of the velocity field along with the slope of the difference in enstrophy density function to determine the location of the laminar and turbulent boundary conditions. Then from the difference in enstrophy density function we calculate the start and end of intermittency. We perform this calculation on a natural boundary layer transition over a flat plate for zero pressure gradient flow and for separated shear flow over a separation bubble. We compare these results to existing experimental results and verify the accuracy of our transition model.

Studies on Flow Transition Under Simulated Low Pressure Turbine Conditions Based on a High Order LES Model

2011 ◽  
Author(s):  
Debasish Biswas ◽  
Tomohiko Jimbo
Keyword(s):  
Boundary Layer ◽  
Gas Turbine ◽  
Separated Flow ◽  
High Order ◽  
Boundary Layer Transition ◽  
Flow Transition ◽  
Layer Transition ◽  
Free Stream Turbulence ◽  
Transition Mechanism ◽  
Flow Through

Boundary layer transition is an important phenomenon experienced by the flow through gas turbine engines. A substantial fraction of the boundary layer on both sides of a gas turbine airfoil may be transitional. The extended transition zone exist due to strong favorable pressure gradients, found on both near the leading edge portion of the suction side and the pressure side, which serve to stabilize the boundary layer and consequently delay the transition process, even under high free-stream turbulence intensity (FSTI) in practical gas turbine. It is very important to properly model and predict the high FSTI transition mechanism, since boundary layer transition leads to substantial increase in friction coefficients and heat transfer rate. Near wall turbulence production is thought to be largely absent in the non-turbulent zone. The intermittent nature of transition need to be taken into account in developing improved transition model. Much has been learned from the to date, but the nature of separated flow transition is still not completely clear, and existing models are still not robust as needed for accurate prediction. Therefore, in the present work a high order LES turbulent model proposed by the author is used to predict the separated flow transition. The experimental data of Volino is chosen for this comparison purpose. In his experimental work, the flow through a single-passage cascade simulator is documented under both high and low FSTI conditions at several different Reynolds numbers. The geometry of the passage (in Volino’s work) corresponds to that of the “Pak-B” airfoil, which is an industry supplied research airfoil that is representative of a modern, aggressive LP turbine design. Volino’s data included a complete documentation of cases with Re as low as 25,000 and also the documentation of turbulent shear stress in the boundary layer under both high and low FSTI.

Modeling of Boundary-Layer Transition

2004 ◽  
Author(s):  
S. K. Roberts ◽  
M. I. Yaras
Keyword(s):  
Boundary Layer ◽  
Mathematical Model ◽  
Surface Roughness ◽  
Production Rate ◽  
Shape Factor ◽  
Separated Flow ◽  
Boundary Layer Transition ◽  
Flow Transition ◽  
Turbulent Spot ◽  
Layer Transition

This paper presents a mathematical model for predicting the rate of turbulent spot production. In this model, attached- and separated-flow transition are treated in a unified manner, and the boundary layer shape factor is identified as the parameter with which the spot production rate correlates. The model is supplemented by several correlations to allow for its practical use in the prediction of the length of the transition zone. Secondly, the paper presents a model for the prediction of the location of transition inception in separation-bubbles. The model improves on the accuracy of existing alternatives, and is the first to account for the effects of surface roughness.

Helicopter Rotor Boundary Layer Transition Measurement in Forward Flight Using an Infrared Camera

2020 ◽  
Vol 65 (1) ◽  
pp. 2-14 ◽  
Author(s):  
A. D. Gardner ◽  
C. C. Wolf ◽  
J. T. Heineck ◽  
M. Barnett ◽  
M. Raffel
Keyword(s):  
Boundary Layer ◽  
Infrared Camera ◽  
Ground Effect ◽  
Boundary Layer Transition ◽  
Helicopter Rotor ◽  
Forward Flight ◽  
Layer Transition ◽  
Pitching Airfoil ◽  
Transition Position ◽  
The Difference

Boundary layer transition measurement was demonstrated using differential infrared thermography (DIT) on the top side of a helicopter rotor in forward flight, which detects the difference in the convective heat transfer at the boundary layer transition position. The tests used a FLIR X8500xc SLS long wave infrared camera to observe the DLR EC135 test helicopter rotor. The boundary layer transition was detected for hover out of ground effect (150 ft) and for forward flight at 80 kt (1700 ft). The measured boundary layer transition positions are consistent with previous measurements of the EC135 hovering in ground effect, and with predicted boundary layer transition positions. A method for the analysis of DIT images for a rotor in forward flight is shown, based on computational analysis of a pitching airfoil with varying inflow.

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