Concentration Profiles of End-Grafted, Diblock and Triblock Polymers in the Melt: Near-Wall Structure and Effects of Segment-Wall Interaction

1995 ◽  
Vol 32 (3) ◽  
pp. 211-216 ◽  
Author(s):  
Y Liu ◽  
S. A Schwarz ◽  
W Zhao ◽  
J Quinn ◽  
J Sokolov ◽  
...  
Keyword(s):  
Wall Structure ◽  
Concentration Profiles ◽  
Wall Interaction ◽  
Triblock Polymers ◽  
Near Wall

Characterization of a Newly Designed Test Bench for Investigations of Flame-Wall-Interaction

2021 ◽  
Author(s):  
Rahand Dalshad ◽  
Tobias Sander ◽  
Michael Pfitzner
Keyword(s):  
Test Bench ◽  
Turbine Blades ◽  
Thermal Design ◽  
Optical Methods ◽  
Detailed Knowledge ◽  
Combustion Chambers ◽  
First Results ◽  
Wall Interaction ◽  
Set Up ◽  
Near Wall

Abstract For the thermal design of combustion chambers and turbine blades in jet engines, a detailed knowledge of the combustion and of the heat loads to the walls is necessary. In general, high operating temperatures and reduced combustor size are striven for in order to increase engine efficiency and reduce weight. Consequently, the components are exposed to temperatures above the melting point of the materials and there is a growing risk of incomplete combustion within the combustion chambers. To study these effects, we set up a new test bench for fundamental investigation of chemical near-wall reactions at atmospheric pressure. First results of gaseous, non-premixed near-wall CH4/air and H2/air flames are presented. Optical methods such as two-line laser-induced fluorescence thermometry and OH* chemiluminescence were applied. Further, the heat release to the wall was determined by means of inverse heat conduction calculation using the data of implemented thermocouples.

Modification of Near-Wall Structure in a Shear-Driven 3-D Turbulent Boundary Layer

2001 ◽  
Vol 124 (1) ◽  
pp. 118-126 ◽  
Author(s):  
Robert O. Kiesow ◽  
Michael W. Plesniak
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Reynolds Shear Stress ◽  
Power Spectra ◽  
Streamwise Velocity ◽  
Wall Structure ◽  
Reynolds Stresses ◽  
Planar Shear ◽  
Inner Region ◽  
Near Wall

The near-wall physics of a planar, shear-driven, 3-D turbulent boundary layer with varying strengths of crossflow are examined. Flow visualization data reveals a reduction of mean streak length by as much as 50% with increasing spanwise shear. Power spectra of velocity confirm this shift towards higher temporal frequencies, corresponding to decreased streamwise length scales. PIV measurements indicate a significant modification of the inner region of the boundary layer with increasing spanwise shear. Streamwise velocity profiles exhibit an increasing velocity deficit with increased crossflow. Increased levels of the normal Reynolds stresses u′2¯ and v′2¯ and an increase in the −u′v′¯ Reynolds shear stress are also observed. Modifications in the spanwise and transverse vorticity were also observed at higher shear rates.

scholarly journals Turbulent boundary layers over permeable walls: scaling and near-wall structure

2011 ◽  
Vol 687 ◽  
pp. 141-170 ◽  
Author(s):  
C. Manes ◽  
D. Poggi ◽  
L. Ridolfi
Keyword(s):  
Turbulent Flows ◽  
Laser Doppler Anemometry ◽  
Wall Turbulence ◽  
Shear Instability ◽  
Wall Structure ◽  
Wall Region ◽  
Mean Velocity ◽  
Wide Range ◽  
Permeable Walls ◽  
Near Wall

AbstractThis paper presents an experimental study devoted to investigating the effects of permeability on wall turbulence. Velocity measurements were performed by means of laser Doppler anemometry in open channel flows over walls characterized by a wide range of permeability. Previous studies proposed that the von Kármán coefficient associated with mean velocity profiles over permeable walls is significantly lower than the standard values reported for flows over smooth and rough walls. Furthermore, it was observed that turbulent flows over permeable walls do not fully respect the widely accepted paradigm of outer-layer similarity. Our data suggest that both anomalies can be explained as an effect of poor inner–outer scale separation if the depth of shear penetration within the permeable wall is considered as the representative length scale of the inner layer. We observed that with increasing permeability, the near-wall structure progressively evolves towards a more organized state until it reaches the condition of a perturbed mixing layer where the shear instability of the inflectional mean velocity profile dictates the scale of the dominant eddies. In our experiments such shear instability eddies were detected only over the wall with the highest permeability. In contrast attached eddies were present over all the other wall conditions. On the basis of these findings, we argue that the near-wall structure of turbulent flows over permeable walls is regulated by a competing mechanism between attached and shear instability eddies. We also argue that the ratio between the shear penetration depth and the boundary layer thickness quantifies the ratio between such eddy scales and, therefore, can be used as a diagnostic parameter to assess which eddy structure dominates the near-wall region for different wall permeability and flow conditions.

The Effect of an Active Travelling Wave Wall Motion on the Structure of a Turbulent Boundary Layer

2006 ◽  
Author(s):  
Pavlos P. Vlachos ◽  
Ali Etebari ◽  
Olga Pierrakos
Keyword(s):  
Turbulent Flows ◽  
Travelling Wave ◽  
Viscous Drag ◽  
Wall Structure ◽  
Order Estimate ◽  
Reynolds Stresses ◽  
Actuation Frequency ◽  
Compliant Surface ◽  
Wall Flow ◽  
Near Wall

The topic of drag reduction (DR) in wall-bounded turbulent flows has received great attention over the past thirty years. This effort is motivated by the notion that the internal near wall structure of a turbulent flow can be substantially affected if a force is exerted on the fluid by a traveling wave propagating transversely on an active compliant surface. The work presented here supports this notion by demonstrating that the active skin results in organizing the structures that populate the near wall flow. For the first time, this mechanism is shown to significantly reduce the contribution of the Reynolds stresses to the viscous drag. The modification of the turbulent near wall flow as a function of actuation frequency is investigated and the persistence of the flow control is explored. In addition, the performance of the mechanical active skin actuator is characterized and a first order estimate of its energetics is achieved.

Near-wall structure of a turbulent boundary layer with riblets

1989 ◽  
Vol 208 ◽  
pp. 417-458 ◽  
Author(s):  
Kwing-So Choi
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Flow Visualization ◽  
Extensive Study ◽  
Wall Turbulence ◽  
Wall Structure ◽  
Turbulence Structure ◽  
Turbulent Drag ◽  
Wind Tunnel Study ◽  
Near Wall

A detailed wind tunnel study has been carried out on the near-wall turbulence structure over smooth and riblet wall surfaces under zero pressure gradient. Time-average quantities as ‘well as conditionally sampled profiles were obtained using hotwire/film anemometry, along with a simultaneous flow visualization using the smoke-wire technique and a sheet of laser light. The experimental results indicated a significant change of the structure in the turbulent boundary layer near the riblet surface. The change was confined within a small volume of the flow close to the wall surface. A conceptual model for the sequence of the bursts was then proposed based on an extensive study of the flow visualization, and was supported by the results of conditionally sampled velocity fields. A possible mechanism of turbulent drag reduction by riblets is discussed.

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