Scaling of the Near-Wall Layer Beneath Reattaching Separated Flow

2004 ◽  
pp. 151-154 ◽  
Author(s):  
Philip E. Hancock
Keyword(s):  
Separated Flow ◽  
Wall Layer ◽  
Near Wall

Large-eddy simulation of separated flow over a swept wing with approximate near-wall modelling

2007 ◽  
Vol 111 (1125) ◽  
pp. 689-697 ◽  
Author(s):  
N. Li ◽  
M. A. Leschziner
Keyword(s):  
Vortex Breakdown ◽  
Separated Flow ◽  
Leading Edge ◽  
Suction Side ◽  
Wall Layer ◽  
Swept Wing ◽  
Validation Data ◽  
Eddy Simulation ◽  
Eddy Viscosity Model ◽  
Near Wall

Abstract The paper investigates, by means of a simulation methodology, the flow separating from a 40 degrees backward-swept wing at 9 degrees incidence and Reynolds number of 210,000, based on the wing-root chord length. The Simulation corresponds to LDA, PIV and suction-side-topology measurements for the same geometry, conducted by other investigators specifically to provide validation data. The finest block-structured mesh contains 23·6 million nodes and is organised in 256 blocks to maximise mesh quality and facilitate parallel solution on multi-processor machines. The near-wall layer is resolved, to a thickness of about y + = 20, by means of parabolised URANS equations that include an algebraic eddy-viscosity model and from which the wall-shear stress is extracted to provide an unsteady boundary condition for the simulation. The numerical solution is in good agreement with the experimental behaviour over the 50-70% inboard portion of the span, but the simulation fails to resolve some complex features close to the wing tip, due to a premature leading-edge vortex breakdown and loss in vortex coherence. The comparisons and their discussion provide useful insight into various physical characteristics of this complex separated wing flow.

scholarly journals Heat Transfer in Highly Turbulent Separated Flows: A Review

Energies ◽  
2021 ◽  
Vol 14 (4) ◽  
pp. 1005
Author(s):  
Viktor I. Terekhov
Keyword(s):  
Heat Transfer ◽  
Turbulent Flow ◽  
Separated Flow ◽  
Separated Flows ◽  
Turbulence Characteristic ◽  
Wall Jets ◽  
Average Heat ◽  
Free Stream Turbulence ◽  
External Turbulence ◽  
Near Wall

The study of flows with a high degree of turbulence in boundary layers, near-wall jets, gas curtains, separated flows behind various obstacles, as well as during combustion is of great importance for increasing energy efficiency of the flow around various elements in the ducts of gas-dynamic installations. This paper gives some general characteristics of experimental work on the study of friction and heat transfer on a smooth surface, in near-wall jets, and gas curtains under conditions of increased free-stream turbulence. Taking into account the significant effect of high external turbulence on dynamics and heat transfer of separated flows, a similar effect on the flow behind various obstacles is analyzed. First of all, the classical cases of flow separation behind a single backward-facing step and a rib are considered. Then, more complex cases of the flow around a rib oriented at different angles to the flow are analyzed, as well as a system of ribs and a transverse trench with straight and inclined walls in a turbulent flow around them. The features of separated flow in a turbulized stream around a cylinder, leading to an increase in the width of the vortex wake, frequency of vortex separation, and increase in the average heat transfer coefficient are analyzed. The experimental results of the author are compared with data of other researchers. The structure of separated flow at high turbulence—characteristic dimensions of the separation region, parameters of the mixing layer, and pressure distribution—are compared with the conditions of low-turbulent flow. Much attention is paid to thermal characteristics: temperature profiles across the shear layer, temperature distributions over the surface, and local and average heat transfer coefficients. It is shown that external turbulence has a much stronger effect on the separated flow than on the boundary layer on a flat surface. For separated flows, its intensifying effect on heat transfer is more pronounced behind a rib than behind a step. The factor of heat transfer intensification by external turbulence is most pronounced in the transverse cavity and in the system of ribs.

Simulating flow separation from continuous surfaces: routes to overcoming the Reynolds number barrier

2009 ◽  
Vol 367 (1899) ◽  
pp. 2885-2903 ◽  
Author(s):  
Michael Leschziner ◽  
Ning Li ◽  
Fabrizio Tessicini
Keyword(s):  
Reynolds Number ◽  
Turbulent Flows ◽  
Major Elements ◽  
Wall Layer ◽  
Navier Stokes ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Rans Models ◽  
High Reynolds ◽  
Near Wall

This paper provides a discussion of several aspects of the construction of approaches that combine statistical (Reynolds-averaged Navier–Stokes, RANS) models with large eddy simulation (LES), with the objective of making LES an economically viable method for predicting complex, high Reynolds number turbulent flows. The first part provides a review of alternative approaches, highlighting their rationale and major elements. Next, two particular methods are introduced in greater detail: one based on coupling near-wall RANS models to the outer LES domain on a single contiguous mesh, and the other involving the application of the RANS and LES procedures on separate zones, the former confined to a thin near-wall layer. Examples for their performance are included for channel flow and, in the case of the zonal strategy, for three separated flows. Finally, a discussion of prospects is given, as viewed from the writer's perspective.

Highly resolved experimental results of the separated flow in a channel with streamwise periodic constrictions

2016 ◽  
Vol 796 ◽  
pp. 257-284 ◽  
Author(s):  
Christian J. Kähler ◽  
Sven Scharnowski ◽  
Christian Cierpka
Keyword(s):  
Turbulent Flow ◽  
Flow Separation ◽  
Flow Direction ◽  
Separated Flow ◽  
Mean Flow ◽  
Complex Flow ◽  
Flow Measurements ◽  
Flow Features ◽  
Wall Flow ◽  
Near Wall

The understanding and accurate prediction of turbulent flow separation on smooth surfaces is still a challenging task because the separation and the reattachment locations are not fixed in space and time. Consequently, reliable experimental data are essential for the validation of numerical flow simulations and the characterization and analysis of the complex flow physics. However, the uncertainty of the existing near-wall flow measurements make a precise analysis of the near-wall flow features, such as separation/reattachment locations and other predicted near-wall flow features which are under debate, often impossible. Therefore, the periodic hill experiment at TU Munich (ERCOFTAC test case 81) was repeated using high resolution particle image velocimetry and particle tracking velocimetry. The results confirm the strong effect of the spatial resolution on the near-wall flow statistics. Furthermore, it is shown that statistically stable values of the turbulent flow variables can only be obtained for averaging times which are challenging to realize with highly resolved large eddy simulation and direct numerical simulation techniques. Additionally, the analysis implies that regions of correlated velocity fluctuations with rather uniform streamwise momentum exist in the flow. Their size in the mean flow direction can be larger than the hill spacing. The possible impact of the correlated turbulent motion on the wake region is discussed, as this interaction might be important for the understanding and control of the flow separation dynamics on smooth bodies.

A Receptivity Based Transition Model

2003 ◽  
Author(s):  
Mark W. Johnson
Keyword(s):  
Boundary Layers ◽  
Reasonable Agreement ◽  
Narrow Band ◽  
Numerical Procedure ◽  
Separated Flow ◽  
Pressure Gradients ◽  
Laminar Boundary Layers ◽  
Wide Range ◽  
Improve Accuracy ◽  
Near Wall

A numerical procedure for predicting the receptivity of laminar boundary layers to freestream turbulence, consisting of vortex arrays with arbitrary orientation, has been developed previously. In the current paper this method is refined to improve accuracy using an unstructured computational grid. Results show that boundary layers only have high receptivity to a narrow band of normal and spanwise frequencies. The computed near wall gains have similar values to those obtained by experiment for zero pressure gradient boundary layers. Near wall gains are also obtained for a wide range of favourable and adverse pressure gradients for both attached and separated boundary layers. The gain values are used to predict start of transition values which are in reasonable agreement with Reθ values which are in reasonable agreement with the Abu-Ghannam and Shaw correlations. The current results extend transition inception prediction into the separated flow regime.

scholarly journals Liposome Drug Delivery System across Endothelial Plasma Membrane: Role of Distance between Endothelial Cells and Blood Flow Rate

Molecules ◽  
2020 ◽  
Vol 25 (8) ◽  
pp. 1875
Author(s):  
Olga E. Glukhova
Keyword(s):  
Drug Delivery ◽  
Endothelial Cells ◽  
Blood Flow ◽  
Drug Release ◽  
Drug Delivery System ◽  
Delivery System ◽  
Small Diameter ◽  
Movement Pattern ◽  
Wall Layer ◽  
Near Wall

This paper discusses specific features of the interactions of small-diameter liposomes with the cytoplasmic membrane of endothelial cells using in silico methods. The movement pattern of the liposomal drug delivery system was modeled in accordance with the conditions of the near-wall layer of blood flow. Our simulation results show that the liposomes can become stuck in the intercellular gaps and even break down when the gap is reduced. Liposomes stuck in the gaps are capable of withstanding a shell deformation of ~15% with an increase in liposome energy by 26%. Critical deformation of the membrane gives an impetus to drug release from the liposome outward. We found that the liposomes moving in the near-wall layer of blood flow inevitably stick to the membrane. Liposome sticking on the membrane is accompanied by its gradual splicing with the membrane bilayer. This leads to a gradual drug release inside the cell.

Instability in a viscous flow driven by streamwise vortices

2001 ◽  
Vol 432 ◽  
pp. 127-166 ◽  
Author(s):  
K. W. BRINCKMAN ◽  
J. D. A. WALKER
Keyword(s):  
Boundary Layer ◽  
Numerical Solutions ◽  
Three Dimensional ◽  
Wall Layer ◽  
External Flow ◽  
Three Dimensional Flow ◽  
Dimensional Flow ◽  
Wall Flow ◽  
Turbulent Wall ◽  
Near Wall

Unsteady separation processes at large finite, Reynolds number, Re, are considered, as well as the possible relation to existing descriptions of boundary-layer separation in the limit Re → ∞. The model problem is a fundamental vortex-driven three-dimensional flow, believed to be relevant to bursting near the wall in a turbulent boundary layer. Bursting is known to be associated with streamwise vortex motion, but the vortex/wall interactions that drive the near-wall flow toward breakdown have not yet been fully identified. Here, a simulation of symmetric counter-rotating vortices is used to assess the influence of sustained pumping action on the development of a viscous wall layer. The calculated solutions describe a three-dimensional flow at finite Re that is independent of the streamwise coordinate and consists of a crossflow plane motion, with a developing streamwise flow. The unsteady problem is constructed to mimic a typical cycle in turbulent wall layers and numerical solutions are obtained over a range of Re. Recirculating eddies develop rapidly in the near-wall flow, but these eddies are eventually bisected by alleyways which open up from the external flow region to the wall. At sufficiently high Re, an oscillation was found to develop in the streamwise vorticity field near the alleyways with a concurrent evolution of a local spiky behaviour in the wall shear. Above a critical value of Re, the oscillation grows rapidly in amplitude and eventually penetrates the external flow field, suggesting the onset of an unstable wall-layer breakdown. Local zones of severely retarded streamwise velocity are computed which are reminiscent of the low-speed streaks commonly observed in turbulent boundary layers. A number of other features also bear a resemblance to observed coherent structure in the turbulent wall layer.

Visualization of crystallization processes in the near-wall layer of a water drop

2013 ◽  
Vol 56 (3) ◽  
pp. 271-274 ◽  
Author(s):  
I. N. Pavlov ◽  
B. S. Rinkevichyus ◽  
A. V. Tolkachev
Keyword(s):  
Water Drop ◽  
Wall Layer ◽  
Near Wall
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