Precursors of backflow events and their relationship with the near-wall self-sustaining process

2021 ◽  
Vol 933 ◽  
Author(s):  
Byron Guerrero ◽  
Martin F. Lambert ◽  
Rey C. Chin
Keyword(s):  
Vortical Structure ◽  
Reverse Flow ◽  
Streamwise Vortex ◽  
Turbulent Pipe Flow ◽  
Wall Region ◽  
Vortex Generation ◽  
Spatiotemporal Resolution ◽  
Nonlinear Mechanism ◽  
First Time ◽  
Near Wall

This study examines the precursors and consequences of rare backflow events at the wall using direct numerical simulation of turbulent pipe flow with a high spatiotemporal resolution. The results obtained from conditionally averaged fields reveal that the precursor of a backflow event is the asymmetric collision between a high- and a low-speed streak (LSS) associated with the sinuous mode of the streaks. As the collision occurs, a lifted shear layer with high local azimuthal enstrophy is formed at the trailing end of the LSS. Subsequently, a spanwise or an oblique vortex spontaneously arises. The dominant nonlinear mechanism by which this vortex is engendered is enstrophy intensification due to direct stretching of the lifted vorticity lines in the azimuthal direction. As time progresses, this vortex tilts and orientates towards the streamwise direction and, as its enstrophy increases, it induces the breakdown of the LSS located below it. Subsequently, this vortical structure advects as a quasi-streamwise vortex, as it tilts and stretches with time. As a result, it is shown that reverse flow events at the wall are the signature of the nonlinear mechanism of the self-sustaining process occurring at the near-wall region. Additionally, each backflow event has been tracked in space and time, showing that approximately 50 % of these events are followed by at least one additional vortex generation that gives rise to new backflow events. It is also found that up to a maximum of seven regenerations occur after a backflow event has appeared for the first time.

Wall Suction Effects on the Structure of Fully Developed Turbulent Pipe Flow

1996 ◽  
Vol 118 (1) ◽  
pp. 33-39 ◽  
Author(s):  
D. Sofialidis ◽  
P. Prinos
Keyword(s):  
Shear Stress ◽  
Pipe Flow ◽  
Stokes Equations ◽  
Turbulent Pipe Flow ◽  
Turbulent Shear ◽  
Experimental Measurements ◽  
Wall Region ◽  
Wall Suction ◽  
Law Of The Wall ◽  
Near Wall

The effects of wall suction on the structure of fully developed pipe flow are studied numerically by solving the Reynolds averaged Navier-Stokes equations. Linear and nonlinear k-ε or k-ω low-Re models of turbulence are used for “closing” the system of the governing equations. Computed results are compared satisfactorily against experimental measurements. Analytical results, based on boundary layer assumptions and the mixing length concept, provide a law of the wall for pipe flow under the influence of low suction rates. The analytical solution is found in satisfactory agreement with computed and experimental data for a suction rate of A = 0.46 percent. For the much higher rate of A = 2.53 percent the above assumptions are not valid and analytical velocities do not follow the computed and experimental profiles, especially in the near-wall region. Near-wall velocities, as well as the boundary shear stress, are increased with increasing suction rates. The excess wall shear stress, resulting from suction, is found to be 1.5 to 5.5 times the respective one with no suction. The turbulence levels are reduced with the presence of the wall suction. Computed results of the turbulent shear stress uv are in close agreement with experimental measurements. The distribution of the turbulent kinetic energy k is predicted better by the k-ω model of Wilcox (1993). Nonlinear models of the k-ε and k-ω type predict the reduction of the turbulence intensities u’, v’, w’, and the correct levels of v’ and w’ but they underpredict the level of u’.

scholarly journals Effect of reverse flow on the pattern of wall shear stress near arterial branches

2011 ◽  
Vol 8 (64) ◽  
pp. 1594-1603 ◽  
Author(s):  
A. Kazakidi ◽  
A. M. Plata ◽  
S. J. Sherwin ◽  
P. D. Weinberg
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Steady Flow ◽  
Stokes Equations ◽  
Reverse Flow ◽  
Side Branch ◽  
Navier Stokes ◽  
Wall Region ◽  
Wall Shear ◽  
Near Wall

Atherosclerotic lesions have a patchy distribution within arteries that suggests a controlling influence of haemodynamic stresses on their development. The distribution near aortic branches varies with age and species, perhaps reflecting differences in these stresses. Our previous work, which assumed steady flow, revealed a dependence of wall shear stress (WSS) patterns on Reynolds number and side-branch flow rate. Here, we examine effects of pulsatile flow. Flow and WSS patterns were computed by applying high-order unstructured spectral/hp element methods to the Newtonian incompressible Navier–Stokes equations in a geometrically simplified model of an aorto-intercostal junction. The effect of pulsatile but non-reversing side-branch flow was small; the aortic WSS pattern resembled that obtained under steady flow conditions, with high WSS upstream and downstream of the branch. When flow in the side branch or in the aortic near-wall region reversed during part of the cycle, significantly different instantaneous patterns were generated, with low WSS appearing upstream and downstream. Time-averaged WSS was similar to the steady flow case, reflecting the short duration of these events, but patterns of the oscillatory shear index for reversing aortic near-wall flow were profoundly altered. Effects of reverse flow may help explain the different distributions of lesions.

The structure of Reynolds stress in the near-wall region of a fully developed turbulent pipe flow

1992 ◽  
Vol 13 (6) ◽  
pp. 405-413 ◽  
Author(s):  
P. -A. Chevrin ◽  
H. L. Petrie ◽  
S. Deutsch
Keyword(s):  
Reynolds Stress ◽  
Pipe Flow ◽  
Turbulent Pipe Flow ◽  
Wall Region ◽  
Near Wall

Effects of an axisymmetric contraction on a turbulent pipe flow

2011 ◽  
Vol 687 ◽  
pp. 376-403 ◽  
Author(s):  
Seong Jae Jang ◽  
Hyung Jin Sung ◽  
Per-Åge Krogstad
Keyword(s):  
Pipe Flow ◽  
Core Region ◽  
Turbulent Pipe Flow ◽  
Wall Region ◽  
Mean Flow ◽  
The Core ◽  
Thin Region ◽  
Local Mean ◽  
The Mean ◽  
Near Wall

AbstractThe flow in an axisymmetric contraction fitted to a fully developed pipe flow is experimentally and numerically studied. The reduction in turbulence intensity in the core region of the flow is discussed on the basis of the budgets for the various turbulent stresses as they develop downstream. The contraction generates a corresponding increase in energy in the near-wall region, where the sources for energy production are quite different and of opposite sign compared to the core region, where these effects are caused primarily by vortex stretching. The vortices in the pipe become aligned with the flow as the stretching develops through the contraction. Vortices which originally have a spanwise component in the pipe are stretched into pairs of counter-rotating vortices which become disconnected and aligned with the mean flow. The structures originating in the pipe which are inclined at an angle with respect to the wall are rotated towards the local mean streamlines. In the very near-wall region and the central part of the contraction the flow tends towards two-component turbulence, but these structures are different. The streamwise and azimuthal stresses are dominant in the near-wall region, while the lateral components dominate in the central part of the flow. The two regions are separated by a rather thin region where the flow is almost isotropic.

A Thermal Law of the Wall for Separating and Recirculating Flows

1999 ◽  
Author(s):  
Daniel O. A. Cruz ◽  
Atila P. Silva Freire
Keyword(s):  
Reverse Flow ◽  
Flow Region ◽  
Separation Point ◽  
Law Of The Wall ◽  
Previous Formulation ◽  
Half Power ◽  
First Time ◽  
Analytical Expressions ◽  
Trans Asme ◽  
Near Wall

Abstract The characteristics of the turbulent boundary layer near a separation point are studied here. The emphasis is on providing analytical expressions for the velocity and temperature near wall solutions which are also valid in the region of reverse flow. These expressions can then be used as boundary conditions in numerical schemes that use two-equation differential models. The paper proposes a new expression for the description of the near wall characteristic length which is shown to hold also in the reverse flow region. The specified velocity profiles are based on a previous formulation of the problem by Cruz and Silva Freire (IJHMT, 41, 2097 2111, 1998) but, as written here, they are presented for the first time. The temperature profiles near the wall are also described by a newly proposed expression that reduces to the logarithmic profile in the attached region, and assumes a minus half power law profile at the separation point. In the separated region, the logarithmic profile is recovered. All results are validated through the data of Vogel and Eaton (Trans. ASME JHT, 107, 922 929, 1985).

Resistance Reduction in Pulsating Turbulent Pipe Flows

2003 ◽  
Author(s):  
Marcello Manna ◽  
Andrea Vacca
Keyword(s):  
Large Scale ◽  
Stokes Equations ◽  
Three Dimensional ◽  
Turbulent Pipe Flow ◽  
Navier Stokes ◽  
Recent Experimental Data ◽  
Wall Region ◽  
Navier Stokes Equations ◽  
Oscillating Motion ◽  
Near Wall

The paper describes the effects of a forced harmonic oscillations of fixed frequency and amplitudes in the range Λ = Um/Ub = 1 ÷ 11 on the characteristics of a turbulent pipe flow with a bulk Reynolds number of 5900. The resulting Stokes layer δ is a fraction of the pipe radius (χ = R/δ = 53) so that the vorticity associated to the oscillating motion is generated in a small near wall region. The analysis is carried out processing a set of statistically independent samples obtained from wall resolved Large Eddy Simulations; time and space averaged global quantities, extracted for the sake of comparison with recent experimental data, confirm the presence of a non negligible drag reduction phenomenon. Phase averaged profiles of the Reynolds stress tensor components provide valuable material for the comprehension of the effects of the time varying mean shear upon the near wall turbulent flow structures. The large scale of motion are directly computed through numerical integration of the space filtered three dimensional Navier-Stokes equations with a spectrally accurate code; the subgrid scale terms are parametrized with a dynamic procedure.

LDA measurements in the near-wall region of a turbulent pipe flow

1995 ◽  
Vol 295 (-1) ◽  
pp. 305 ◽  
Author(s):  
F. Durst ◽  
J. Jovanovic ◽  
J. Sender
Keyword(s):  
Pipe Flow ◽  
Turbulent Pipe Flow ◽  
Wall Region ◽  
Near Wall
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