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.

On the wall structure of the turbulent boundary layer

1976 ◽  
Vol 76 (1) ◽  
pp. 89-112 ◽  
Author(s):  
R. F. Blackwelder ◽  
R. E. Kaplan
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Large Scale ◽  
Reynolds Shear Stress ◽  
Streamwise Velocity ◽  
Mean Value ◽  
Wall Structure ◽  
Spanwise Direction ◽  
Normal Velocity ◽  
Conditional Sampling

The wall structure of the turbulent boundary layer was examined using hot-wire rakes and conditional sampling techniques. Instantaneous velocity measurements indicate a high degree of coherence over a considerable area in the direction normal to the wall. Aty+= 15, there is some evidence of large-scale correlation in the spanwise direction, but almost no indication of the streamwise streaks that exist in the lower regions of the boundary layer. Conditional sampling showed that the normal velocity is directed outwards in regions of strong stream-wise-momentum deficit, and inwards when the streamwise velocity exceeds its mean value. The conditionally averaged Reynolds shear stress was approximately an order of magnitude greater than its conventionally averaged value and decayed slowly downstream.

scholarly journals Active control of multiscale features in wall-bounded turbulence

2019 ◽  
Vol 36 (1) ◽  
pp. 12-21 ◽  
Author(s):  
Xiaotong Cui ◽  
Nan Jiang ◽  
Xiaobo Zheng ◽  
Zhanqi Tang
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Turbulent Energy ◽  
Streamwise Velocity ◽  
Discrete Wavelet ◽  
Wall Region ◽  
Mean Velocity ◽  
Pzt Actuator ◽  
The Impact ◽  
Near Wall

Abstract This study experimentally investigates the impact of a single piezoelectric (PZT) actuator on a turbulent boundary layer from a statistical viewpoint. The working conditions of the actuator include a range of frequencies and amplitudes. The streamwise velocity signals in the turbulent boundary layer flow are measured downstream of the actuator using a hot-wire anemometer. The mean velocity profiles and other basic parameters are reported. Spectra results obtained by discrete wavelet decomposition indicate that the PZT vibration primarily influences the near-wall region. The turbulent intensities at different scales suggest that the actuator redistributes the near-wall turbulent energy. The skewness and flatness distributions show that the actuator effectively alters the sweep events and reduces intermittency at smaller scales. Moreover, under the impact of the PZT actuator, the symmetry of vibration scales’ velocity signals is promoted and the structural composition appears in an orderly manner. Probability distribution function results indicate that perturbation causes the fluctuations in vibration scales and smaller scales with high intensity and low intermittency. Based on the flatness factor, the bursting process is also detected. The vibrations reduce the relative intensities of the burst events, indicating that the streamwise vortices in the buffer layer experience direct interference due to the PZT control.

The turbulent boundary layer over transverse square cavities

1999 ◽  
Vol 395 ◽  
pp. 271-294 ◽  
Author(s):  
L. DJENIDI ◽  
R. ELAVARASAN ◽  
R. A. ANTONIA
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Flow Direction ◽  
Reynolds Shear Stress ◽  
Local Maximum ◽  
Turbulent Energy ◽  
Skin Friction Coefficient ◽  
Reynolds Stresses ◽  
Smooth Wall ◽  
Local Skin

Laser-induced uorescence (LIF) and laser Doppler velocimetry (LDV) are used to explore the structure of a turbulent boundary layer over a wall made up of two-dimensional square cavities placed transversely to the flow direction. There is strong evidence of occurrence of outflows of fluid from the cavities as well as inflows into the cavities. These events occur in a pseudo-random manner and are closely associated with the passage of near-wall quasi-streamwise vortices. These vortices and the associated low-speed streaks are similar to those found in a turbulent boundary layer over a smooth wall. It is conjectured that outflows play an important role in maintaining the level of turbulent energy in the layer and enhancing the approach towards self-preservation. Relative to a smooth wall layer, there is a discernible increase in the magnitudes of all the Reynolds stresses and a smaller streamwise variation of the local skin friction coefficient. A local maximum in the Reynolds shear stress is observed in the shear layers over the cavities.

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.

The Effect of Biofilms on Turbulent Boundary Layers

1999 ◽  
Vol 121 (1) ◽  
pp. 44-51 ◽  
Author(s):  
M. P. Schultz ◽  
G. W. Swain
Keyword(s):  
Boundary Layer ◽  
Friction Coefficient ◽  
Turbulent Boundary Layer ◽  
Skin Friction ◽  
Layer Structure ◽  
Reynolds Shear Stress ◽  
Skin Friction Coefficient ◽  
Laser Doppler Velocimeter ◽  
Reynolds Stresses ◽  
The Mean

Materials exposed in the marine environment, including those protected by antifouling paints, may rapidly become colonized by microfouling. This may affect frictional resistance and turbulent boundary layer structure. This study compares the mean and turbulent boundary layer velocity characteristics of surfaces covered with a marine biofilm with those of a smooth surface. Measurements were made in a nominally zero pressure gradient, boundary layer flow with a two-component laser Doppler velocimeter at momentum thickness Reynolds numbers of 5600 to 19,000 in a recirculating water tunnel. Profiles of the mean and turbulence velocity components, including the Reynolds shear stress, were measured. An average increase in the skin friction coefficient of 33 to 187 percent was measured on the fouled specimens. The skin friction coefficient was found to be dependent on both biofilm thickness and morphology. The biofilms tested showed varying effect on the Reynolds stresses when those quantities were normalized with the friction velocity.

scholarly journals Local transport of passive scalar released from a point source in a turbulent boundary layer

2018 ◽  
Vol 846 ◽  
pp. 292-317 ◽  
Author(s):  
K. M. Talluru ◽  
J. Philip ◽  
K. A. Chauhan
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Large Scale ◽  
Reynolds Shear Stress ◽  
Phase Relationship ◽  
Passive Scalar ◽  
Streamwise Velocity ◽  
Concentration Fluctuations ◽  
Normal Velocity ◽  
Velocity Fluctuations

Simultaneous measurements of streamwise velocity ($\tilde{U}$) and concentration ($\tilde{C}$) for a horizontal plume released at eight different vertical locations within a turbulent boundary layer are discussed in this paper. These are supplemented by limited simultaneous three-component velocity and concentration measurements. Results of the integral time scale ($\unicode[STIX]{x1D70F}_{c}$) of concentration fluctuations across the width of the plume are presented here for the first time. It is found that$\unicode[STIX]{x1D70F}_{c}$has two distinct peaks: one closer to the plume centreline and the other at a vertical distance of plume half-width above the centreline. The time-averaged streamwise concentration flux is found to be positive and negative, respectively, below and above the plume centreline. This behaviour is a resultant of wall-normal velocity fluctuations ($w$) and Reynolds shear stress ($\overline{uw}$). Confirmation of these observations is found in the results of joint probability density functions of$u$(streamwise velocity fluctuations) and$\tilde{C}$as well as that of$w$and$\tilde{C}$. Results of cross-correlation coefficient show that high- and low-momentum regions have a distinctive role in the transport of passive scalar. Above the plume centreline, low-speed structures have a lead over the meandering plume, while high-momentum regions are seen to lag behind the plume below its centreline. Further examination of the phase relationship between time-varying$u$and$c$(concentration fluctuations) via cross-spectrum analysis is consistent with this observation. Based on these observations, a phenomenological model is presented for the relative arrangement of a passive scalar plume with respect to large-scale velocity structures in the flow.

Influence of Wall Proximity on the Wake Dynamics Behind a Square Cylinder

2021 ◽  
Author(s):  
Samuel Addai ◽  
Afua A. Mante ◽  
Sedem Kumahor ◽  
Xingjun Fang ◽  
Mark F. Tachie
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Normal Stress ◽  
Reynolds Shear Stress ◽  
Stream Velocity ◽  
Reynolds Stresses ◽  
Square Cylinder ◽  
Mean Flow ◽  
The Mean ◽  
Wall Proximity

Abstract In the present study, the effects of wall proximity on the wake dynamics behind a square cylinder subjected to a thick upstream turbulent boundary layer were experimentally investigated using particle image velocimetry. The Reynolds number based on the free-stream velocity and the cylinder height (h) was 12750 while the ratio of the turbulent boundary layer thickness to the cylinder height was 3.6. The gap distance (G) between the bottom face of the cylinder and the wall was varied, resulting in gap ratios (G/h) of 0, 0.3, 0.5, 1.0, 2.0 and 8.0. The flow topological differences among the various gap ratios were analyzed in terms of the mean flow and Reynolds stresses. The results show that as the cylinder approaches the wall, the mean flow becomes increasingly asymmetric about the horizontal centerline of the cylinder and the size of the mean separation bubbles in the cylinder wake increases. Also, the magnitudes of the Reynolds stresses decrease with decreasing gap ratio. For G/h > 0, the distributions of the streamwise Reynolds normal stress and Reynolds shear stress are concentrated along the upper and lower separated shear layers, resulting in characteristic double peaks. The distributions of the vertical Reynolds normal stress, however, are concentrated in the wake about the horizontal centerline of the cylinder and reveal only single peaks.

Turbulence measurements around a mild separation bubble and downstream of reattachment

1996 ◽  
Vol 322 ◽  
pp. 297-328 ◽  
Author(s):  
Amy E. Alving ◽  
H. H. Fernholz
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Pressure Gradient ◽  
Boundary Layers ◽  
Outer Layer ◽  
Large Scale ◽  
Reverse Flow ◽  
Separation Bubble ◽  
Reynolds Stresses ◽  
Inner Region

This paper describes the behaviour of a turbulent boundary layer on a smooth, axisymmetric body exposed to an adverse pressure gradient of sufficient strength to cause a short region of mean reverse flow ('separation’). The pressure distribution is tailored such that the boundary layer reattaches and then develops in a nominally zero pressure gradient. Hot-wire and pulsed-wire measurements are presented over the separated region and downstream of reattachment. The response of the turbulence quantities to separation and to reattachment is discussed, with emphasis on the relaxation behaviour after reattachment. Over the separation bubble, the response is characteristic of that seen by other workers: the Reynolds stresses in the inner region are reduced and stress peaks develop away from the wall. At reattachment, the skewness of the fluctuating wall shear stress vanishes, as it is known to do at separation. After reattachment, the outer-layer stresses decay towards levels typical of unperturbed boundary layers. But the inner-layer relaxation is unusual. As the viscous wall stress increases downstream of reattachment, the recovery does not start at the wall and travel outward via the formation of an ‘internal’ layer, the process observed in many other relaxing flows. In fact, the inner layer responds markedly more slowly than the outer layer, even though response times are shortest near the wall. It is concluded that the large-scale, outer structures in the turbulent boundary layer survive the separation process and interfere with the regeneration of Reynolds stresses in the inner region after reattachment. This behaviour continues for at least six bubble lengths (20 boundary-layer thicknesses) after reattachment and is believed to have profound implications for our understanding of the interaction between inner and outer layers in turbulent boundary layers.

Statistical structure of turbulent-boundary-layer velocity–vorticity products at high and low Reynolds numbers

2007 ◽  
Vol 570 ◽  
pp. 307-346 ◽  
Author(s):  
P. J. A. PRIYADARSHANA ◽  
J. C. KLEWICKI ◽  
S. TREAT ◽  
J. F. FOSS
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Shear Stress ◽  
Turbulent Boundary Layer ◽  
Large Scale ◽  
Reynolds Shear Stress ◽  
Rough Wall ◽  
Reynolds Stresses ◽  
Wall Boundary Layer ◽  
Layer Flow

The mean wall-normal gradients of the Reynolds shear stress and the turbulent kinetic energy have direct connections to the transport mechanisms of turbulent-boundary-layer flow. According to the Stokes–Helmholtz decomposition, these gradients can be expressed in terms of velocity–vorticity products. Physical experiments were conducted to explore the statistical properties of some of the relevant velocity–vorticity products. The high-Reynolds-number data (Rθ≃O(106), where θ is the momentum thickness) were acquired in the near neutrally stable atmospheric-surface-layer flow over a salt playa under both smooth- and rough-wall conditions. The low-Rθdata were from a database acquired in a large-scale laboratory facility at 1000 >Rθ> 5000. Corresponding to a companion study of the Reynolds stresses (Priyadarshana & Klewicki,Phys. Fluids, vol. 16, 2004, p. 4586), comparisons of low- and high-Rθas well as smooth- and rough-wall boundary-layer results were made at the approximate wall-normal locationsyp/2 and 2yp, whereypis the wall-normal location of the peak of the Reynolds shear stress, at each Reynolds number. In this paper, the properties of thevωz,wωyanduωzproducts are analysed through their statistics and cospectra over a three-decade variation in Reynolds number. Hereu,vandware the fluctuating streamwise, wall-normal and spanwise velocity components and ωyand ωzare the fluctuating wall-normal and spanwise vorticity components. It is observed thatv–ωzstatistics and spectral behaviours exhibit considerable sensitivity to Reynolds number as well as to wall roughness. More broadly, the correlations between thevand ω fields are seen to arise from a ‘scale selection’ near the peak in the associated vorticity spectra and, in some cases, near the peak in the associated velocity spectra as well.

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