DNS Study on Vortex and Vorticity in Late Boundary Layer Transition

2017 ◽  
Vol 22 (2) ◽  
pp. 441-459 ◽  
Author(s):  
Yiqian Wang ◽  
Yong Yang ◽  
Guang Yang ◽  
Chaoqun Liu
Keyword(s):  
Flow Field ◽  
Physical Meaning ◽  
Solid Wall ◽  
Boundary Layer Transition ◽  
Layer Transition ◽  
Clear Physical Meaning ◽  
3 Dimensional ◽  
Mathematical Definition ◽  
Long Time ◽  
Almost All

AbstractVortex and vorticity are two correlated but fundamentally different concepts which have been the central issues in fluid mechanics research. Vorticity has rigorous mathematical definition (curl of velocity), but no clear physical meaning. Vortex has clear physical meaning (rotation) but no rigorous mathematical definition. For a long time, many people treat them as a same thing. However, based on our high-order direct numerical simulation (DNS), we found that first, “vortex” is not “vorticity tube” or “vortex tube” which is widely defined as a bundle of vorticity lines without any vorticity line leak. Actually, vortex is an open area for vorticity line penetration. Second, vortex is not necessarily congregation of vorticity lines, but dispersion in many 3-dimensional cases. Some textbooks say that vortex cannot end inside the flow field but must end on the solid wall (and/or boundaries). Our DNS observation and many other numerical results show almost all vortices are ended inside the flow field. Finally, a more theoretical study shows that neither vortex nor vorticity line can attach to the solid wall and they must be detached from the wall.

scholarly journals New Theories on Boundary Layer Transition and Turbulence Formation

2012 ◽  
Vol 2012 ◽  
pp. 1-22 ◽  
Author(s):  
Chaoqun Liu ◽  
Ping Lu ◽  
Lin Chen ◽  
Yonghua Yan
Keyword(s):  
Boundary Layer ◽  
Flow Field ◽  
Shear Layer ◽  
Boundary Layer Transition ◽  
Length Scale ◽  
Multiple Level ◽  
Shear Layer Instability ◽  
Small Length ◽  
Layer Transition ◽  
Turbulence Generation

This paper is a short review of our recent DNS work on physics of late boundary layer transition and turbulence. Based on our DNS observation, we propose a new theory on boundary layer transition, which has five steps, that is, receptivity, linear instability, large vortex structure formation, small length scale generation, loss of symmetry and randomization to turbulence. For turbulence generation and sustenance, the classical theory, described with Richardson's energy cascade and Kolmogorov length scale, is not observed by our DNS. We proposed a new theory on turbulence generation that all small length scales are generated by “shear layer instability” through multiple level ejections and sweeps and consequent multiple level positive and negative spikes, but not by “vortex breakdown.” We believe “shear layer instability” is the “mother of turbulence.” The energy transferring from large vortices to small vortices is carried out by multiple level sweeps, but does not follow Kolmogorov's theory that large vortices pass energy to small ones through vortex stretch and breakdown. The loss of symmetry starts from the second level ring cycle in the middle of the flow field and spreads to the bottom of the boundary layer and then the whole flow field.

scholarly journals Relation Between Convective Instability and Global Instability on a Rotating Disk

2018 ◽  
Vol 12 (1) ◽  
pp. 37-53
Author(s):  
Lee Keunseob ◽  
Nishio Yu ◽  
Izawa Seiichiro ◽  
Fukunishi Yu
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Flow Field ◽  
Convective Instability ◽  
Unstable Mode ◽  
Rotating Disk ◽  
Boundary Layer Transition ◽  
Global Instability ◽  
Simulation Code ◽  
Layer Transition

Background: The velocity fluctuations grow dominantly by convective instability form 32 spiral vortices which are stationary with respect to the disk. However, recent researches suggest that the global instability plays a role in the boundary layer transition. Objective: The study looks into the relation between convective instability and global instability. Method: A finite difference method is used to carry out numerical simulation. The full Navier-Stokes perturbation equations and the continuity equation solved by simulation code. Results: A disturbance is continuatively introduced to excite the convectively unstable mode, which successfully generates a flow field with 32 spiral and stationary vortices. Next, a short-duration artificial disturbance with an azimuthal wavenumber of 64 is introduced at Reynolds number of 530 in order to introduce a velocity fluctuation of the traveling mode, which is globally unstable. It is shown that the source of vibration for the globally unstable mode exists between Reynolds number of 560 and 670. Finally, the global and traveling wavenumber 64 component is excited in a flow field which is dominated by the convective and stationary wavenumber 32 component. It is shown that the wavenumber 64 component grows by the global instability even when the excitation is very weak. Conclusion: The results suggest that the reason why the globally unstable mode has not been observed in experiments is because the boundary layer transition caused by the convective instability takes place before the globally unstable mode can start to grow by itself.

Estimating aerofoil lift from flow angle

2015 ◽  
Vol 119 (1219) ◽  
pp. 1167-1173
Author(s):  
L. W. Traub
Keyword(s):  
Flow Field ◽  
Field Measurements ◽  
Electronic System ◽  
Wall Pressure ◽  
The Body ◽  
Boundary Layer Transition ◽  
Force Balance ◽  
Layer Transition ◽  
Flow Angle ◽  
Small Surface

Estimation of the lift of an aerofoil is one of the fundamental measurements of fluid mechanics. Lift is commonly measured using a load cell or a force balance. Non-intrusive methods to measure lift are usually pressure based. Aerofoils may be pressure tapped where small surface orifices are connected via tubing to a pressure measurement system, either a multi-tube manometre or an electronic system. Both measurement options add cost and complication, especially in an educational setting. Pressure tapping small aerofoils can also be difficult, especially if the models are rapid prototyped (RP). Low model surface resolution (from RP manufacture) and confined geometry complicate model assembly and finishing. Boundary-layer transition caused by poorly implemented tappings (too large a diametre or poorly aligned, i.e. straight aft) can also alter results. Wall pressure tappings may also be used and have the benefit of being non-intrusive. To implement, the test section roof and floor is tapped with a streamwise row of ports that facilitate measurement of the wall pressure signature. Integration of the pressure differential then relates to the lift produced. This measurement methodology still requires a multi-channel pressure acquisition system and modification of the wind tunnel. In Refs 4,5 methods are presented that facilitate calculation of the instantaneous forces acting on a body through flow field measurements determined using particle image velocimetry. However, the required flow field measurements encompass those surrounding the body, and are not a simple point measurement. In Ref. 6 a method is presented to estimate the lift of an aerofoil using two Pitot-static tubes that are used to measure the velocity above and below the aerofoil’s quarter chord. Wall corrections are required to yield an accurate lift estimate.

EXPERIMENTAL AND NUMERICAL INVESTIGATIONS OF BOUNDARY LAYER TRANSITION ON BLUNTED CONES AT SUPERSONIC FLOW

2010 ◽  
Vol 40 (3) ◽  
pp. 309-319 ◽  
Author(s):  
V. N. Brazhko ◽  
A. V. Vaganov ◽  
N. A. Kovaleva ◽  
N. P. Kolina ◽  
I. I. Lipatov
Keyword(s):  
Boundary Layer ◽  
Supersonic Flow ◽  
Boundary Layer Transition ◽  
Layer Transition ◽  
Numerical Investigations
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