scholarly journals Edge tracking in spatially developing boundary layer flows

2019 ◽  
Vol 881 ◽  
pp. 164-181 ◽  
Author(s):  
Miguel Beneitez ◽  
Yohann Duguet ◽  
Philipp Schlatter ◽  
Dan S. Henningson
Keyword(s):  
Boundary Layer ◽  
Basins Of Attraction ◽  
Base Flow ◽  
Boundary Layer Transition ◽  
Transition To Turbulence ◽  
Structure Characteristic ◽  
Asymptotic State ◽  
Bypass Transition ◽  
Layer Transition ◽  
Blasius Boundary Layer

Recent progress in understanding subcritical transition to turbulence is based on the concept of the edge, the manifold separating the basins of attraction of the laminar and the turbulent state. Originally developed in numerical studies of parallel shear flows with a linearly stable base flow, this concept is adapted here to the case of a spatially developing Blasius boundary layer. Longer time horizons fundamentally change the nature of the problem due to the loss of stability of the base flow due to Tollmien–Schlichting (TS) waves. We demonstrate, using a moving box technique, that efficient long-time tracking of edge trajectories is possible for the parameter range relevant to bypass transition, even if the asymptotic state itself remains out of reach. The flow along the edge trajectory features streak switching observed for the first time in the Blasius boundary layer. At long enough times, TS waves co-exist with the coherent structure characteristic of edge trajectories. In this situation we suggest a reinterpretation of the edge as a manifold dividing the state space between the two main types of boundary layer transition, i.e. bypass transition and classical transition.

On the stability of a Blasius boundary layer subject to localised suction

2019 ◽  
Vol 871 ◽  
pp. 717-741 ◽  
Author(s):  
Mattias Brynjell-Rahkola ◽  
Ardeshir Hanifi ◽  
Dan S. Henningson
Keyword(s):  
Boundary Layer ◽  
Flow Instability ◽  
Linear Instability ◽  
Boundary Layer Transition ◽  
Transition To Turbulence ◽  
Layer Transition ◽  
Structural Sensitivity ◽  
Blasius Boundary Layer ◽  
Structural Sensitivity Analysis ◽  
The Stability

In this study the origins of premature transition due to oversuction in boundary layers are studied. An infinite row of circular suction pipes that are mounted at right angles to a flat plate subject to a Blasius boundary layer is considered. The interaction between the flow originating from neighbouring holes is weak and for the parameters investigated, the pipe is always found to be unsteady regardless of the state of the flow in the boundary layer. A stability analysis reveals that the appearance of boundary layer transition can be associated with a linear instability in the form of two unstable eigenmodes inside the pipe that have weak tails, which extend into the boundary layer. Through an energy budget and a structural sensitivity analysis, the origin of this flow instability is traced to the structures developing inside the pipe near the pipe junction. Although the amplitudes of the modes in the boundary layer are orders of magnitude smaller than the corresponding amplitudes inside the pipe, a Koopman analysis of the data gathered from a nonlinear direct numerical simulation confirms that it is precisely these disturbances that are responsible for transition to turbulence in the boundary layer due to oversuction.

Application of Constructal Theory to Prediction of Boundary Layer Transition Onset

2006 ◽  
Author(s):  
E. J. Walsh ◽  
D. H. Hernon ◽  
D. M. McEligot ◽  
M. R. D. Davies ◽  
A. Bejan
Keyword(s):  
Boundary Layer ◽  
Transition Process ◽  
Boundary Layer Transition ◽  
Constructal Theory ◽  
Transition To Turbulence ◽  
Bypass Transition ◽  
Layer Transition ◽  
New Approach ◽  
Transition Modeling ◽  
New Perspective

Accurate transition onset modeling is a fundamental part of modern turbomachinery designs, where bypass transition is the dominant mechanism of transition to turbulence. Despite this situation a range of transition onset models exist primarily based upon both integral and local parameters within the boundary layer. All such transition models have empirical origins. To date the relationships between such models has not been forthcoming and hence lack of physical understanding of the transition process is evident. This paper details a new approach to transition modeling and provides a theoretically based approach to transition onset prediction by invoking a single principle developed within constructal theory. We not only present a new model but also demonstrate the equivalence between existing models by implementing the same theory. Such understanding of the transition onset problem may provide a new perspective towards more theoretically based transition onset models rather than empirical ones, although much work remains to be done in understanding the receptivity mechanisms within a laminar boundary layer.

Effects of Reynolds Number and Free-Stream Turbulence on Boundary Layer Transition in a Compressor Cascade

2000 ◽  
Author(s):  
Heinz-Adolf Schreiber ◽  
Wolfgang Steinert ◽  
Bernhard Küsters
Keyword(s):  
Boundary Layer ◽  
Free Stream ◽  
Reynolds Numbers ◽  
Boundary Layer Transition ◽  
Bypass Transition ◽  
Layer Transition ◽  
Free Stream Turbulence ◽  
High Reynolds Numbers ◽  
High Turbulence ◽  
High Reynolds

An experimental and analytical study has been performed on the effect of Reynolds number and free-stream turbulence on boundary layer transition location on the suction surface of a controlled diffusion airfoil (CDA). The experiments were conducted in a rectilinear cascade facility at Reynolds numbers between 0.7 and 3.0×106 and turbulence intensities from about 0.7 to 4%. An oil streak technique and liquid crystal coatings were used to visualize the boundary layer state. For small turbulence levels and all Reynolds numbers tested the accelerated front portion of the blade is laminar and transition occurs within a laminar separation bubble shortly after the maximum velocity near 35–40% of chord. For high turbulence levels (Tu > 3%) and high Reynolds numbers transition propagates upstream into the accelerated front portion of the CDA blade. For those conditions, the sensitivity to surface roughness increases considerably and at Tu = 4% bypass transition is observed near 7–10% of chord. Experimental results are compared to theoretical predictions using the transition model which is implemented in the MISES code of Youngren and Drela. Overall the results indicate that early bypass transition at high turbulence levels must alter the profile velocity distribution for compressor blades that are designed and optimized for high Reynolds numbers.

Large Eddy Simulation of Boundary Layer Transition Mechanisms in a Gas-Turbine Compressor Cascade

2018 ◽  
Author(s):  
Ashley D. Scillitoe ◽  
Paul G. Tucker ◽  
Paolo Adami
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Boundary Layer Transition ◽  
Bypass Transition ◽  
Suction Surface ◽  
Layer Transition ◽  
Turbulent Spots ◽  
Pressure Surface ◽  
Eddy Simulation ◽  
Large Eddy

Large Eddy Simulation (LES) is used to explore the boundary layer transition mechanisms in two rectilinear compressor cascades. To reduce numerical dissipation, a novel locally adaptive smoothing scheme is added to an unstructured finite-volume solver. The performance of a number of Sub-Grid Scale (SGS) models is explored. With the first cascade, numerical results at two different freestream turbulence intensities (Ti’s), 3.25% and 10%, are compared. At both Ti’s, time-averaged skin-friction and pressure coefficient distributions agree well with previous Direct Numerical Simulations (DNS). At Ti = 3.25%, separation induced transition occurs on the suction surface, whilst it is bypassed on the pressure surface. The pressure surface transition is dominated by modes originating from the convection of Tollmien-Schlichting waves by Klebanoff streaks. However, they do not resembled a classical bypass transition. Instead, they display characteristics of the “overlap” and “inner” transition modes observed in the previous DNS. At Ti = 10%, classical bypass transition occurs, with Klebanoff streaks incepting turbulent spots. With the second cascade, the influence of unsteady wakes on transition is examined. Wake-amplified Klebanoff streaks were found to instigate turbulent spots, which periodically shorten the suction surface separation bubble. The celerity line corresponding to 70% of the free-stream velocity, which is associated with the convection speed of the amplified Klebanoff streaks, was found to be important.

Simulation of boundary layer transition induced by periodically passing wakes

1999 ◽  
Vol 398 ◽  
pp. 109-153 ◽  
Author(s):  
XIAOHUA WU ◽  
ROBERT G. JACOBS ◽  
JULIAN C. R. HUNT ◽  
PAUL A. DURBIN
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Free Stream ◽  
Boundary Layer Transition ◽  
Parallel Computer ◽  
Transition To Turbulence ◽  
Transitional Region ◽  
Bypass Transition ◽  
Turbulent Spots ◽  
Good Agreement

The interaction between an initially laminar boundary layer developing spatially on a flat plate and wakes traversing the inlet periodically has been simulated numerically. The three-dimensional, time-dependent Navier–Stokes equations were solved with 5.24×107 grid points using a message passing interface on a scalable parallel computer. The flow bears a close resemblance to the transitional boundary layer on turbomachinery blades and was designed following, in outline, the experiments by Liu & Rodi (1991). The momentum thickness Reynolds number evolves from Reθ = 80 to 1120. Mean and second-order statistics downstream of Reθ = 800 are of canonical flat-plate turbulent boundary layers and are in good agreement with Spalart (1988).In many important aspects the mechanism leading to the inception of turbulence is in agreement with previous fundamental studies on boundary layer bypass transition, as summarized in Alfredsson & Matsubara (1996). Inlet wake disturbances inside the boundary layer evolve rapidly into longitudinal puffs during an initial receptivity phase. In the absence of strong forcing from free-stream vortices, these structures exhibit streamwise elongation with gradual decay in amplitude. Selective intensification of the puffs occurs when certain types of turbulent eddies from the free-stream wake interact with the boundary layer flow through a localized instability. Breakdown of the puffs into young turbulent spots is preceded by a wavy motion in the velocity field in the outer part of the boundary layer.Properties and streamwise evolution of the turbulent spots following breakdown, as well as the process of completion of transition to turbulence, are in agreement with previous engineering turbomachinery flow studies. The overall geometrical characteristics of the matured turbulent spot are in good agreement with those observed in the experiments of Zhong et al. (1998). When breakdown occurs in the outer layer, where local convection speed is large, as in the present case, the spots broaden downstream, having the vague appearance of an arrowhead pointing upstream.The flow has also been studied statistically. Phase-averaged velocity fields and skin-friction coefficients in the transitional region show similar features to previous cascade experiments. Selected results from additional thought experiments and simulations are also presented to illustrate the effects of streamwise pressure gradient and free-stream turbulence.

scholarly journals Predicting Bypass Transition: A Physical Model Versus Empirical Correlations

1997 ◽  
Author(s):  
Mark W. Johnson ◽  
Ali H. Ercan
Keyword(s):  
Boundary Layer ◽  
Adverse Pressure Gradient ◽  
Empirical Correlation ◽  
Boundary Layer Transition ◽  
Bypass Transition ◽  
Empirical Correlations ◽  
Layer Transition ◽  
Model Versus ◽  
Current Modelling ◽  
Modelling Approach

A boundary layer transition model is presented which relates the near wall velocity fluctuations to the formation of turbulent spots. This model is used to determine the turbulent intermittency within a boundary layer integral code. Comparisons are made between the code predictions and established empirical correlations for the adverse pressure gradient transition experiments performed by Gostelow and co-workers. Similarly good accuracy was achieved by both the model and empirical correlation for start of transition. However, empirical correlations were less reliable than the model for predicting end of transition. The model was also able to predict the evolution of the measured intermittency considerably more accurately than the Narasimha empirical correlation. The current modelling approach is thus demonstrated to be more reliable than empirical correlation for the modelling of transitional boundary layers.

Boundary Layer Transition on a Low Pressure Turbine Blade due to Downstream Potential Interaction

2012 ◽  
Author(s):  
Véronique Penin ◽  
Pascale Kulisa ◽  
François Bario
Keyword(s):  
Boundary Layer ◽  
Turbine Blade ◽  
Turbulence Intensity ◽  
Large Scale ◽  
Laser Doppler Anemometry ◽  
Boundary Layer Transition ◽  
Bypass Transition ◽  
Pressure Potential ◽  
Layer Transition ◽  
Transition Mode

Engine manufacturers wish to reduce the size and weight of their engines, and one way of achieving this is by reducing the rotor-stator gap. It follows that rotor-stator interactions become stronger, especially the influence of the pressure potential, which, despite its rapid spatial decay, becomes significant as the inter-row gap is reduced. Here we examine the upstream potential effect generated by downstream moving cylindrical rods on an upstream turbine blade. A large scale rectilinear blade cascade was constructed to improve access to the boundary layer. The Reynolds number was 1.6 × 105. Pressure measurements and two-dimensional Laser Doppler Anemometry around the blade were performed to study the boundary layer behavior. At low turbulence intensity (Tu−in = 1.8%), the laminar boundary layer experiences separation once per rod period. There are two transition modes which alternate during a rod period: separation transition mode and bypass mode. At high turbulence intensity (Tu−in = 4.0%), no boundary layer separation occurs. The boundary layer follows a bypass transition mode during an entire rod period.

Boundary Layer Transition at High Levels of Free Stream Turbulence

1998 ◽  
Author(s):  
Masaharu Matsubara ◽  
P. Henrik Alfredsson ◽  
K. Johan A. Westin
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Free Stream ◽  
Turbine Blades ◽  
Boundary Layer Transition ◽  
Transition To Turbulence ◽  
Flow Visualisation ◽  
Mean Flow ◽  
Layer Transition ◽  
Free Stream Turbulence

Transition to turbulence in laminar boundary layers subjected to high levels of free stream turbulence (FST) can still not be reliably predicted, despite its technical importance, e.g. in the case of boundary layers developing on gas turbine blades. In a series of experiments in the MTL-wind tunnel at KTH the influence of grid-generated FST on boundary layer transition has been studied, with FST-levels up to 6%. It was shown from both flow visualisation and hot-wire measurements that the boundary layer develops unsteady streaky structures with high and low streamwise velocity. This leads to large amplitude low frequency fluctuations inside the boundary layer although the mean flow is still close to the laminar profile. Breakdown to turbulence occurs through an instability of the streaks which leads to the formation of turbulent spots. Accurate physical modelling of these processes seems to be needed in order to obtain a reliable prediction method.

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