Active Control of Near-Wall Coherent Structures

2002 ◽  
Author(s):  
P. Konieczny ◽  
A. Bottaro ◽  
V. Monturet ◽  
B. Nogarede
Keyword(s):  
Boundary Layer ◽  
Skin Friction ◽  
Coherent Structures ◽  
Large Scale ◽  
Travelling Wave ◽  
Friction Reduction ◽  
Wall Turbulence ◽  
Integral Control ◽  
Macroscopic Simulation ◽  
Near Wall

This work aims at finding efficient means to reduce skin friction drag in a turbulent boundary layer. The argument on which the study is based is that turbulence exists near a wall because of the presence of an autonomous cycle which is maintained even in the absence of forcing from the free-stream. The central elements of this cycle are the near-wall coherent structures whose dynamics control the turbulence production. It is postulated that an action at the wall capable of disrupting the turbulent wall-cycle can yield a significant skin friction reduction. A model cycle is produced by embedding artificial, large scale streamwise vortices and streaks in a Blasius boundary layer. A control is then conceived, meant to produce an agglomeration of the streaks to hamper the cycle. The action envisaged consists in a movement of the wall, in the form of a spanwise standing or travelling wave of sufficiently long wavelength. The controllers in the present macroscopic simulation are simply cantilever beams whose movement is driven by ceramic piezo-actuators. Piezoelectric fibers realizing the same action (properly rescaled) provide, possibly, the answer to the technological challenge of the integral control of near-wall turbulence.

Towards a Greater Understanding of Turbulent Skin-Friction Reduction

2002 ◽  
Author(s):  
Nick Hutchins ◽  
Kwing-So Choi
Keyword(s):  
Boundary Layer ◽  
Skin Friction ◽  
Outer Layer ◽  
Large Scale ◽  
Convection Velocity ◽  
Friction Reduction ◽  
Horseshoe Vortices ◽  
Layer Region ◽  
Skin Friction Reduction ◽  
Near Wall

Measurements have been made in a turbulent boundary layer modified by flow aligned vertical (sub-boundary layer) elements. Comparisons between the coherent structure (near-wall and outer-layer region) for both modified and canonical cases have been conducted in order to better understand the mechanism of skin friction reduction. Thus far we can report a modified near-wall convection velocity obeying inner scaling and a reduced spread of correlated events away from the wall. The outer-layer appears to be characterised by large-scale arch-like structures which produce a velocity field consistent with the heads of lifted near-wall horseshoe vortices. The modified case shows reduced convection velocity, increased frequency of occurrence and increased entrainment for this type of outer-layer event.

Influence of large-scale motions on the frictional drag in a turbulent boundary layer

2017 ◽  
Vol 829 ◽  
pp. 751-779 ◽  
Author(s):  
Jinyul Hwang ◽  
Hyung Jin Sung
Keyword(s):  
Boundary Layer ◽  
Skin Friction ◽  
Large Scale ◽  
Wall Region ◽  
Local Skin ◽  
Velocity Fluctuations ◽  
Vortical Structures ◽  
Change Of Scale ◽  
Local Skin Friction ◽  
Near Wall

Direct numerical simulation data of a turbulent boundary layer ($Re_{\unicode[STIX]{x1D70F}}=1000$) were used to investigate the large-scale influences on the vortical structures that contribute to the local skin friction. The amplitudes of the streamwise and wall-normal swirling strengths ($\unicode[STIX]{x1D706}_{x}$and$\unicode[STIX]{x1D706}_{y}$) were conditionally sampled by measuring the large-scale streamwise velocity fluctuations ($u_{l}$). In the near-wall region, the amplitudes of$\unicode[STIX]{x1D706}_{x}$and$\unicode[STIX]{x1D706}_{y}$decreased under negative$u_{l}$rather than under positive$u_{l}$. This behaviour arose from the spanwise motions within the footprints of the large-scale low-speed ($u_{l}<0$) and high-speed structures ($u_{l}>0$). The intense spanwise motions under the footprint of positive$u_{l}$noticeably strengthened the small-scale spanwise velocity fluctuations ($w_{s}$) below the centre of the near-wall vortical structures as compared to$w_{s}$within the footprint of negative$u_{l}$. The streamwise and wall-normal components were attenuated or amplified around the modulated vortical motions, which in turn led to the dependence of the swirling strength on the$u_{l}$event. We quantified the contribution of the modulated vortical motions$\langle -w\unicode[STIX]{x1D714}_{y}\rangle$, which were related to a change-of-scale effect due to the vortex-stretching force, to the local skin friction. In the near-wall region, intense values of$\langle -w\unicode[STIX]{x1D714}_{y}\rangle$were observed for positive$u_{l}$. By contrast, these values were low for negative$u_{l}$, in connection with the amplification of$w_{s}$and$\unicode[STIX]{x1D706}_{y}$by the strong spanwise motions of the positive$u_{l}$. The resultant skin friction induced by the amplified vortical motions within$u_{l}^{+}>2$was responsible for 15 % of the total skin friction generated by the change-of-scale effect. Finally, we applied this analysis to a drag-reduced flow and found that the amplified vortical motions within the footprint of positive$u_{l}$were markedly diminished, which ultimately contributed to the total drag reduction.

Turbulent boundary-layer control with plasma actuators

2011 ◽  
Vol 369 (1940) ◽  
pp. 1443-1458 ◽  
Author(s):  
Kwing-So Choi ◽  
Timothy Jukes ◽  
Richard Whalley
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Skin Friction ◽  
Control Strategies ◽  
Travelling Wave ◽  
Friction Reduction ◽  
Plasma Actuators ◽  
Control Technique ◽  
Boundary Layer Control ◽  
Layer Control

This paper reviews turbulent boundary-layer control strategies for skin-friction reduction of aerodynamic bodies. The focus is placed on the drag-reduction mechanisms by two flow control techniques—spanwise oscillation and spanwise travelling wave, which were demonstrated to give up to 45 per cent skin-friction reductions. We show that these techniques can be implemented by dielectric-barrier discharge plasma actuators, which are electric devices that do not require any moving parts or complicated ducting. The experimental results show different modifications to the near-wall structures depending on the control technique.

scholarly journals Large-scale amplitude modulation of the small-scale structures in turbulent boundary layers

2009 ◽  
Vol 628 ◽  
pp. 311-337 ◽  
Author(s):  
ROMAIN MATHIS ◽  
NICHOLAS HUTCHINS ◽  
IVAN MARUSIC
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Amplitude Modulation ◽  
Large Scale ◽  
Wall Turbulence ◽  
Small Scale ◽  
Supporting Evidence ◽  
Scale Structures ◽  
Small Scale Structures ◽  
Near Wall

In this paper we investigate the relationship between the large- and small-scale energy-containing motions in wall turbulence. Recent studies in a high-Reynolds-number turbulent boundary layer (Hutchins & Marusic, Phil. Trans. R. Soc. Lond. A, vol. 365, 2007a, pp. 647–664) have revealed a possible influence of the large-scale boundary-layer motions on the small-scale near-wall cycle, akin to a pure amplitude modulation. In the present study we build upon these observations, using the Hilbert transformation applied to the spectrally filtered small-scale component of fluctuating velocity signals, in order to quantify the interaction. In addition to the large-scale log-region structures superimposing a footprint (or mean shift) on the near-wall fluctuations (Townsend, The Structure of Turbulent Shear Flow, 2nd edn., 1976, Cambridge University Press; Metzger & Klewicki, Phys. Fluids, vol. 13, 2001, pp. 692–701.), we find strong supporting evidence that the small-scale structures are subject to a high degree of amplitude modulation seemingly originating from the much larger scales that inhabit the log region. An analysis of the Reynolds number dependence reveals that the amplitude modulation effect becomes progressively stronger as the Reynolds number increases. This is demonstrated through three orders of magnitude in Reynolds number, from laboratory experiments at Reτ ~ 103–104 to atmospheric surface layer measurements at Reτ ~ 106.

scholarly journals Extracting Coherent Structures in Near-Wall Turbulence Based on Wavelet Analysis

2020 ◽  
Author(s):  
Peng Du ◽  
Haibao Hu ◽  
Xiao Huang
Keyword(s):  
Boundary Layer ◽  
Coherent Structures ◽  
Extraction Method ◽  
Extraction Procedure ◽  
Probability Density Functions ◽  
Wall Turbulence ◽  
Spectrum Density ◽  
Hot Film ◽  
Near Wall Turbulence ◽  
Near Wall

To analyze the properties of the coherent structures in near-wall turbulence, an extraction method based on wavelet transform (WT) and a verification procedure based on correlation analysis are proposed in this work. The flow field of the turbulent boundary layer is measured using the hot-film anemometer in a gravitational low-speed water tunnel. The obtained velocity profile and turbulence intensity are validated with traditional boundary layer theory. The fluctuating velocities at three testing positions are analyzed. Using the power spectrum density (PSD) and WT, coherent and incoherent parts of the near-wall turbulence are extracted and analyzed. The probability density functions (PDFs) of the extracted signals indicate that the incoherent structures of turbulence obey the Gaussian distribution, while the coherent structures deviate from it. The PDFs of coherent structures and original turbulence signals are similar, which means that coherent structures make the most contributions to the turbulence entrainment. A correlation parameter is defined at last to prove the validity of our extraction procedure.

Application of Air Microblowing Through a Porous Wall for Skin Friction Reduction on a Flat Plate

2010 ◽  
Vol 5 (3) ◽  
pp. 38-46
Author(s):  
Vladimir I. Kornilov ◽  
Andrey V. Boiko
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Skin Friction ◽  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Porous Wall ◽  
Friction Reduction ◽  
Local Skin ◽  
Skin Friction Reduction

The effect of air microblowing through a porous wall on the properties of a turbulent boundary layer formed on a flat plate in an incompressible flow is studied experimentally. The Reynolds number based on the momentum thickness of the boundary layer in front of the porous insert is 3 900. The mass flow rate of the blowing air per unit area was varied within Q = 0−0.0488 кg/s/m2 . A consistent decrease in local skin friction, reaching up to 45−47 %, is observed to occur at the maximal blowing air mass flow rate studied.

Experimental Study on Skin Friction Reduction With Micro-Blowing

Volume 4 ◽  
2004 ◽  
Author(s):  
Song Liu ◽  
Hongmin Li ◽  
Minel J. Braun
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Skin Friction ◽  
Full Range ◽  
Experimental System ◽  
Main Flow ◽  
Friction Reduction ◽  
Long Distance ◽  
Full Field ◽  
Set Up

Reducing skin friction, such as friction on a car hood or a plane wing, can significantly reduce the drag force and decrease specific fuel consumption. Many techniques and methods have been tried. The Micro-blowing Technique (MBT) is an innovative way to reduce skin friction. Suggested by early research in boundary layer injection in 1950s, MBT was actually brought to effective use in 1994 by Hwang [1]. The basic idea is that by blowing fluid, same as or different from the mainstream flow, at an angle with that of the main flow, a decrease in the velocity gradient at the wall can be achieved, and thus the shear stress on the surface is reduced. Although the experimental data on boundary layer with micro blowing show a significant friction reduction, the mechanism of MBT is still not well understood and thus its full range of application is not yet established. In this paper, we further the understanding of the MBT mechanism. An experimental system is set up to visualize the flow structure on a plate with and without micro blowing in a tunnel. A long distance microscope is combined with a Full Field Flow Tracking visualization method in order to elucidate the nature of the flow interaction and mixing between the blowing flow and the main flow. The flow above the porous plates is visualized and velocities both in the blowing layer immediately adjacent to the plate and in the main flow are quantified using the PIV procedure. The flow and shear stress analysis shows that MTB has significantly different effects on a flow with a boundary layer and fully developed internal flows.

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