Towards Understanding of Natural Boundary-Layer Transition for Low-Speed Flows via Random Excitation

2021 ◽  
pp. 391-405
Author(s):  
Shirzad Hosseinverdi ◽  
Hermann F. Fasel
Keyword(s):  
Boundary Layer ◽  
Random Excitation ◽  
Boundary Layer Transition ◽  
Natural Boundary ◽  
Layer Transition ◽  
Low Speed

A FEniCS-based model for prediction of boundary layer transition in low-speed aerodynamic flows

2019 ◽  
Vol 233 (15) ◽  
pp. 5729-5745
Author(s):  
Amir Kolaei ◽  
Götz Bramesfeld
Keyword(s):  
Boundary Layer ◽  
High Performance ◽  
Stokes Equations ◽  
Element Model ◽  
Least Square ◽  
Boundary Layer Transition ◽  
Navier Stokes ◽  
Layer Transition ◽  
Low Speed ◽  
Aerodynamic Flows

In the paper, a finite element model is developed that predicts boundary layer transition in low-speed aerodynamic flows. The model is based on a Reynolds-averaged Navier-Stokes approach, where the incompressible form of the Navier-Stokes equations is solved together with a three-equation eddy-viscosity model utilizing the FEniCS framework. A least-square stabilized Galerkin method is employed in order to prevent numerical oscillations that can arise from dominant advection terms. The proposed FEniCS model is ideal for applications with complex geometries and is tested on high performance computing platforms for parallel processing. The FEniCS model is validated by comparing the skin friction coefficient as well as profiles of velocity and total fluctuation kinetic energy with the benchmark experimental data for transitional boundary layers on a flat plate. The validity of the solver is further examined using experimental measurements reported for a NLF(1)-0416 natural laminar flow airfoil at different angles of attack. The airfoil results are also compared with those obtained using XFOIL, a well-known tool for the design of two-dimensional airfoils. These comparisons suggest that the proposed FEniCS-based model can effectively simulate aerodynamic flow fields that involve laminar-to-turbulent transition.

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.

Experiments on localized secondary instability in bypass boundary layer transition

2017 ◽  
Vol 817 ◽  
pp. 217-263 ◽  
Author(s):  
G. Balamurugan ◽  
A. C. Mandal
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Free Stream ◽  
Secondary Instability ◽  
Boundary Layer Transition ◽  
Convection Velocity ◽  
Layer Transition ◽  
Time Resolved ◽  
Piv Measurements ◽  
Low Speed

An experimental study on localized secondary instability of unsteady streamwise streaks in bypass boundary layer transition under an elevated level of free-stream turbulence has been carried out mainly using the particle image velocimetry (PIV) technique. Simultaneous orthogonal dual-plane PIV measurements were performed for a concurrent examination of the transitional flow features in both wall-normal and spanwise planes. These quantitative and simultaneous visualizations clearly show the wall-normal view of a low-speed streak undergoing sinuous/varicose motion in the spanwise plane. An oscillating shear layer in the wall-normal plane is found to be associated with the sinuous/varicose streak oscillation in the spanwise plane. Further, these measurements indicate that a localized secondary instability wavepacket can originate near the boundary layer edge. The time-resolved PIV measurements in the wall-normal plane clearly show how an instability develops on a lifted-up inclined shear layer and leads to flow breakdown. The estimated wavelength and convection velocity of such instabilities are found to compare well with those calculated from the one-dimensional linear stability analysis of the spatially averaged velocity profiles associated with the lifted-up shear layers. The time-resolved PIV measurements in the spanwise plane also facilitate quantitative visualizations of sinuous and varicose instabilities. These measurements experimentally confirm that a varicose instability at the juncture of an incoming high-speed streak and a downstream low-speed streak can eventually lead to the formation of lambda structures. The estimated convection velocity, wavelength and growth rate of these instabilities are found to be consistent with the numerical results reported in the literature. Moreover, the streak secondary instability is found to be apparent in the velocity contours, while the estimated streak amplitude is approximately 30 % of the free-stream velocity.

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