Criteria for Boundary Layer Transition

2011 ◽  
Author(s):  
Stefan Becker ◽  
Donald M. McEligot ◽  
Edmond Walsh ◽  
Eckart Laurien
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Boundary Layer Flow ◽  
Leading Edge ◽  
Boundary Layer Transition ◽  
Turbulence Level ◽  
Two Dimensional ◽  
Freestream Turbulence ◽  
Layer Transition ◽  
Layer Flow

New results are deduced to assess the validity of proposed transition indicators when applied to situations other than boundary layers on smooth surfaces. The geometry employed utilizes a two-dimensional square rib to disrupt the boundary layer flow. The objective is to determine whether some available criteria are consistent with the present measurements of laminar recovery and transition for the flow downstream of this rib. For the present data — the proposed values of thresholds for transition in existing literature that are based on the freestream turbulence level at the leading edge are not reached in the recovering laminar run but they are not exceeded in the transitioning run either. Of the pointwise proposals examined, values of the suggested quantity were consistent for three of the criteria; that is, they were less than the threshold in laminar recovery and greater than it in the transitioning case.

Keel Design for Low Viscous Drag

1987 ◽  
Author(s):  
Clifford J. Obara ◽  
C. P. van Dam
Keyword(s):  
Boundary Layer ◽  
Laminar Flow ◽  
Laminar Boundary Layer ◽  
Leading Edge ◽  
Boundary Layer Transition ◽  
Viscous Drag ◽  
Hydrodynamic Characteristics ◽  
Sweep Angle ◽  
Layer Transition ◽  
Layer Flow

In this paper, foil and planform parameters which govern the level of viscous drag produced by the keel of a sailing yacht are discussed. It is shown that the application of laminar boundary-Layer flow offers great potential for increased boat speed resulting from the reduction in viscous drag. Three foil shapes have been designed and it is shown that their hydro­dynamic characteristics are very much dependent on location and mode of boundary-Layer transition. The planform parameter which strongly affects the capabilities of the keel to achieve laminar flow is lea ding-edge sweep angle. The two significant phenomena related to keel sweep angle which can cause premature transition of the laminar boundary layer are crossflow instability and turbulent contamination of the leading-edge attachment line. These flow phenomena and methods to control them are discussed in detail. The remaining factors that affect the maintainability of laminar flow include surface roughness, surface waviness, and freestream turbulence. Recommended limits for these factors are given to insure achievability of laminar flow on the keel. In addition, the application of a simple trailing-edge flap to improve the hydrodynamic characteristics of a foil at moderate-to-high leeway angles is studied.

Keel Design for Low Viscous Drag

1989 ◽  
Vol 33 (02) ◽  
pp. 145-155
Author(s):  
Clifford J. Obara ◽  
C. P. van Dam
Keyword(s):  
Boundary Layer ◽  
Laminar Flow ◽  
Laminar Boundary Layer ◽  
Leading Edge ◽  
Boundary Layer Transition ◽  
Viscous Drag ◽  
Hydrodynamic Characteristics ◽  
Sweep Angle ◽  
Layer Transition ◽  
Layer Flow

Foil and planform parameters which govern the level of viscous drag produced by the keel of a sailing yacht are discussed. It is shown that the application of laminar boundary-layer flow offers great potential for increased boat speed resulting from the reduction in viscous drag. Three foil shapes have been designed and it is shown that their hydrodynamic characteristics are very much dependent on location and mode of boundary-layer transition. The planform parameter which strongly affects the capabilities of the keel to achieve laminar flow is leading-edge sweep angle. The two significant phenomena related to keel sweep angle which can cause premature transition of the laminar boundary layer are crossflow instability and turbulent contamination of the leading-edge attachment line. These flow phenomena and methods to control them are discussed in detail. The remaining factors that affect the maintainability of laminar flow include surface roughness, surface waviness, and freestream turbulence. Recommended limits for these factors are given to insure achievability of laminar flow on the keel. In addition, the application of a simple trailing-edge flap to improve the hydrodynamic characteristics of a foil at moderate-to-high leeway angles is studied.

A Note on the Vortex Patterns in the Boundary Layer Flow of a Swept-back Wing

1952 ◽  
Vol 56 (496) ◽  
pp. 279-285 ◽  
Author(s):  
Joseph Black
Keyword(s):  
Boundary Layer ◽  
Laminar Boundary Layer ◽  
Boundary Layer Flow ◽  
Leading Edge ◽  
Two Dimensional ◽  
Turbulent Layer ◽  
Edge Radius ◽  
Layer Flow ◽  
Vortex Patterns

Detailed investigations of the two-dimensional characteristics of thin aerofoil sections with small leading-edge radius have been made by Gault and McCullough. They found that at angles of incidence well below the stalling angle of the section, the laminar boundary layer separated from the surface near the nose, became transitional away from the surface, and re-attached to the aerofoil as a turbulent layer a little farther downstream. Beneath the separated layer, a “bubble” vortex was formed lying parallel to the leading edge, its rotation being such that the flow adjacent to the surface was towards the leading edge.

Large-eddy simulation of boundary-layer separation and transition at a change of surface curvature

2001 ◽  
Vol 439 ◽  
pp. 305-333 ◽  
Author(s):  
ZHIYIN YANG ◽  
PETER R. VOKE
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Three Dimensional ◽  
Leading Edge ◽  
Boundary Layer Transition ◽  
Two Dimensional ◽  
Layer Transition ◽  
Transition Mechanism ◽  
The Mean ◽  
Dimensional Instability

Transition arising from a separated region of flow is quite common and plays an important role in engineering. It is difficult to predict using conventional models and the transition mechanism is still not fully understood. We report the results of a numerical simulation to study the physics of separated boundary-layer transition induced by a change of curvature of the surface. The geometry is a flat plate with a semicircular leading edge. The Reynolds number based on the uniform inlet velocity and the leading-edge diameter is 3450. The simulated mean and turbulence quantities compare well with the available experimental data.The numerical data have been comprehensively analysed to elucidate the entire transition process leading to breakdown to turbulence. It is evident from the simulation that the primary two-dimensional instability originates from the free shear in the bubble as the free shear layer is inviscidly unstable via the Kelvin–Helmholtz mechanism. These initial two-dimensional instability waves grow downstream with a amplification rate usually larger than that of Tollmien–Schlichting waves. Three-dimensional motions start to develop slowly under any small spanwise disturbance via a secondary instability mechanism associated with distortion of two-dimensional spanwise vortices and the formation of a spanwise peak–valley wave structure. Further downstream the distorted spanwise two-dimensional vortices roll up, leading to streamwise vorticity formation. Significant growth of three-dimensional motions occurs at about half the mean bubble length with hairpin vortices appearing at this stage, leading eventually to full breakdown to turbulence around the mean reattachment point. Vortex shedding from the separated shear layer is also observed and the ‘instantaneous reattachment’ position moves over a distance up to 50% of the mean reattachment length. Following reattachment, a turbulent boundary layer is established very quickly, but it is different from an equilibrium boundary layer.

CONTROLLING BOUNDARY LAYER TRANSITION OVER A SEPARATION BUBBLE: A COMPLEX-LAMELLAR APPROACH

2015 ◽  
Vol 39 (2) ◽  
pp. 153-169
Author(s):  
Maureen L. Kolla ◽  
Jeffrey W. Yokota
Keyword(s):  
Boundary Layer ◽  
Incompressible Flow ◽  
Boundary Layer Flow ◽  
Separation Bubble ◽  
Boundary Layer Transition ◽  
Transition Model ◽  
Layer Model ◽  
Layer Transition ◽  
Fully Developed Turbulent Flow ◽  
Layer Flow

In this paper, we develop a complex-lamellar description of the incompressible flow that exists as a boundary layer transitions from a fully developed laminar to fully developed turbulent flow. This complex-lamellar description is coupled to the shape of the universal intermittency distribution and experimental correlations to obtain a boundary layer model of transition. This transition model is used to analyze the effects of several different freestream turbulence levels on the reattachment location and the length of the resulting separation bubbles. Furthermore, we show that at the separation bubble reattachment location, the resulting boundary layer flow is both turbulent and fully developed. Results obtained from this transition model are compared with, and verified by several different DNS simulations.

Numerical simulation of boundary-layer transition and transition control

1989 ◽  
Vol 199 ◽  
pp. 403-440 ◽  
Author(s):  
E. Laurien ◽  
L. Kleiser
Keyword(s):  
Boundary Layer ◽  
Stokes Equations ◽  
Three Dimensional ◽  
Transition Process ◽  
Numerical Results ◽  
Boundary Layer Transition ◽  
Two Dimensional ◽  
Layer Transition ◽  
Longitudinal Vortices ◽  
Tollmien Schlichting

The laminar-turbulent transition process in a parallel boundary-layer with Blasius profile is simulated by numerical integration of the three-dimensional incompressible Navier-Stokes equations using a spectral method. The model of spatially periodic disturbances developing in time is used. Both the classical Klebanoff-type and the subharmonic type of transition are simulated. Maps of the three-dimensional velocity and vorticity fields and visualizations by integrated fluid markers are obtained. The numerical results are compared with experimental measurements and flow visualizations by other authors. Good qualitative and quantitative agreement is found at corresponding stages of development up to the one-spike stage. After the appearance of two-dimensional Tollmien-Schlichting waves of sufficiently large amplitude an increasing three-dimensionality is observed. In particular, a peak-valley structure of the velocity fluctuations, mean longitudinal vortices and sharp spike-like instantaneous velocity signals are formed. The flow field is dominated by a three-dimensional horseshoe vortex system connected with free high-shear layers. Visualizations by time-lines show the formation of A-structures. Our numerical results connect various observations obtained with different experimental techniques. The initial three-dimensional steps of the transition process are consistent with the linear theory of secondary instability. In the later stages nonlinear interactions of the disturbance modes and the production of higher harmonics are essential.We also study the control of transition by local two-dimensional suction and blowing at the wall. It is shown that transition can be delayed or accelerated by superposing disturbances which are out of phase or in phase with oncoming Tollmien-Schlichting instability waves, respectively. Control is only effective if applied at an early, two-dimensional stage of transition. Mean longitudinal vortices remain even after successful control of the fluctuations.

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