scholarly journals Spatial–spectral characteristics of momentum transport in a turbulent boundary layer

2017 ◽  
Vol 836 ◽  
pp. 599-634 ◽  
Author(s):  
D. Fiscaletti ◽  
R. de Kat ◽  
B. Ganapathisubramani
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Spatial Organization ◽  
Spectral Characteristics ◽  
Momentum Transport ◽  
Spectral Content ◽  
Length Scales ◽  
Wall Distance ◽  
Wave Velocities ◽  
Local Average

Spectral content and spatial organization of momentum transport events are investigated in a turbulent boundary layer at the Reynolds number $(Re_{\unicode[STIX]{x1D70F}})=2700$, with time-resolved planar particle image velocimetry. The spectral content of the Reynolds-shear-stress fluctuations reveals that the largest range of time and length scales can be observed in proximity to the wall, while this range becomes progressively more narrow when the wall distance increases. Farther from the wall, longer time and larger length scales exhibit an increasing spectral content. Wave velocities of transport events are estimated from wavenumber–frequency power spectra at different wall-normal locations. Wave velocities associated with ejection events (Q2) are lower than the local average streamwise velocity, while sweep events (Q4) are characterized by wave velocities larger than the local average velocity. These velocity deficits are almost insensitive to the wall distance, which is also confirmed from time tracking the intense transport events. The vertical advection velocities of the intense ejection and sweep events are on average a small fraction of the friction velocity $U_{\unicode[STIX]{x1D70F}}$, different from previous observations in a channel flow. In the range of wall-normal locations $60<y^{+}<600$, sweeps are considerably larger than ejections, which could be because the ejections are preferentially located in between the legs of hairpin packets. Finally, it is observed that negative quadrant events of the same type tend to appear in groups over a large spatial streamwise extent.

Characterization of Visually Detected Coherent Motions in the Turbulent Boundary Layer

2001 ◽  
Author(s):  
Christopher Robin Hirschi
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Turbulent Boundary Layer ◽  
Flow Characteristics ◽  
Viscous Sublayer ◽  
Vertical Transport ◽  
Momentum Transport ◽  
Coherent Motion ◽  
The Past

Abstract Research over the past 40 years indicates that coherent motions within the turbulent boundary layer account for disproportionate contributions to momentum transport (Robinson, 1991). To better understand these motions, low-Reynolds number turbulent boundary layer experiments were conducted to investigate the instantaneous velocity and vorticity fields associated with near-wall coherent motion interactions. The present study identifies and explores the most prevalent flow characteristics associated with the vertical transport of injected passive marker from the viscous sublayer.

On the spatial organization of hairpin packets in a turbulent boundary layer at low-to-moderate Reynolds number

2018 ◽  
Vol 844 ◽  
pp. 635-668 ◽  
Author(s):  
Sichao Deng ◽  
Chong Pan ◽  
Jinjun Wang ◽  
Guosheng He
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Large Scale ◽  
Spatial Organization ◽  
Vortical Structures ◽  
Planar Velocity ◽  
Image Velocimetry ◽  
Moderate Reynolds Number ◽  
Spatio Temporal ◽  
Low Order

The present study is devoted to characterizing the coherent organization of vortical structures, which can be fitted into the paradigm of the hairpin-packet model, in the streamwise–wall-normal plane of a canonical turbulent boundary layer at $Re_{\unicode[STIX]{x1D70F}}=377{-}1093$. Proper orthogonal decomposition (POD) of the planar velocity fields measured via two-dimensional particle image velocimetry, together with a spatio-temporal coherence analysis, shows that the first four leading-order POD modes share both geometric similarity and dynamic coherence and jointly depict the downstream convection of the large-scale Q2/Q4 events, which can be regarded as the low-order imprints of the hairpin packets. A simple low-order indicator is then proposed to extract the inclined interfaces of the hairpin packets, based on which a two-point conditional correlation analysis forms a statistical picture of the spatial organization of multiple prograde vortices aligned along the interface within one packet. A saturation of the self-similar growth of the streamwise gap between two neighbouring vortices is seen. This implies a detachment of the hairpin packets from the inner layer. Both the detachment height and the saturated streamwise spacing are found to scale as $Re_{\unicode[STIX]{x1D70F}}^{1/2}$.

Very-large-scale motions in a turbulent boundary layer

2011 ◽  
Vol 673 ◽  
pp. 80-120 ◽  
Author(s):  
JAE HWA LEE ◽  
HYUNG JIN SUNG
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Turbulent Boundary Layer ◽  
Large Scale ◽  
Spatial Organization ◽  
Reynolds Shear Stress ◽  
Three Dimensional ◽  
Hairpin Vortices ◽  
Extraction Algorithm ◽  
Normal Distance

Direct numerical simulation of a turbulent boundary layer was performed to investigate the spatially coherent structures associated with very-large-scale motions (VLSMs). The Reynolds number was varied in the range Reθ = 570–2560. The main simulation was conducted by using a computational box greater than 50δo in the streamwise domain, where δo is the boundary layer thickness at the inlet, and inflow data was obtained from a separate inflow simulation based on Lund's method. Inspection of the three-dimensional instantaneous fields showed that groups of hairpin vortices are coherently arranged in the streamwise direction and that these groups create significantly elongated low- and high-momentum regions with large amounts of Reynolds shear stress. Adjacent packet-type structures combine to form the VLSMs; this formation process is attributed to continuous stretching of the hairpins coupled with lifting-up and backward curling of the vortices. The growth of the spanwise scale of the hairpin packets occurs continuously, so it increases rapidly to double that of the original width of the packets. We employed the modified feature extraction algorithm developed by Ganapathisubramani, Longmire & Marusic (J. Fluid Mech., vol. 478, 2003, p. 35) to identify the properties of the VLSMs of hairpin vortices. In the log layer, patches with the length greater than 3δ–4δ account for more than 40% of all the patches and these VLSMs contribute approximately 45% of the total Reynolds shear stress included in all the patches. The VLSMs have a statistical streamwise coherence of the order of ~6δ; the spatial organization and coherence decrease away from the wall, but the spanwise width increases monotonically with the wall-normal distance. Finally, the application of linear stochastic estimation demonstrated the presence of packet organization in the form of a train of packets in the log layer.

On the Axisymmetric Turbulent Boundary Layer Growth Along Long Thin Circular Cylinders

2014 ◽  
Vol 136 (5) ◽  
Author(s):  
Stephen A. Jordan
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Flat Plate ◽  
Shape Factor ◽  
Experimental Testing ◽  
Reynolds Numbers ◽  
Layer Growth ◽  
Factor H ◽  
Circular Cylinders ◽  
Length Scales

Even after several decades of experimental and numerical testing, our present-day knowledge of the axisymmetric turbulent boundary layer (TBL) along long thin circular cylinders still lacks a clear picture of many fundamental characteristics. The main issues causing this reside in the experimental testing complexities and the numerical simplifications. An important characteristic that is crucial for routine scaling is the boundary layer length scales, but the downstream growth of these scales (boundary layer, displacement, and momentum thicknesses) is largely unknown from the leading to trailing edges. Herein, we combine pertinent datasets with many complementary numerical computations (large-eddy simulations) to address this shortfall. We are particularly interested in expressing the length scales in terms of the radius-based and axial-based Reynolds numbers (Rea and Rex). Although the composite dataset gave an averaged shape factor H = 1.09 that is substantially lower than the planar value (H = 1.27), the shape factor distribution along the cylinder axis actually begins at the flat plate value then decays logarithmically to near unity. The integral length scales displayed power-law evolutions with variable exponents until high Rea (Rea > 35,000) where both scales then mimic streamwise consistency. Beneath this threshold, their streamwise growth is much slower than the flat plate (especially at low-Rea). The boundary layer thickness grew according to an empirical expression that is dependent on both Rea and Rex where its streamwise growth can far exceed the planar turbulent flow. These unique characteristics rank the thin cylinder axisymmetric TBL as a separate canonical flow, which was well documented by the previous investigations.

scholarly journals Numerical Study of the Influence of the Inlet Turbulence Length Scale on the Turbulent Boundary Layer

2021 ◽  
Vol 11 (11) ◽  
pp. 5177
Author(s):  
Young-Tae Lee ◽  
Lokesh Kalyan Gutti ◽  
Hee-Chang Lim
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Numerical Study ◽  
Flow Simulation ◽  
Temporal Correlation ◽  
Spanwise Direction ◽  
Length Scale ◽  
Inlet Section ◽  
Length Scales ◽  
Turbulent Characteristics

In the past half century, large eddy simulations (LESs) have played an important role in turbulent flow simulation and improving the performance of computing technology. To generate a fully developed turbulent boundary layer in the channel domain using LES, suitable inflow conditions along with turbulent characteristics are required. This study aimed to clarify the effect of the integral length scale on the generation of turbulent boundary layers. To accomplish this, an artificially created boundary layer was imposed on the inlet section, which gradually evolved into a fully developed turbulent boundary layer flow inside the numerical domain. In this study, the synthetic inflow method, which is a commonly employed technique, was used by imposing the spatial and temporal correlation between two different points on the inlet section. In addition, we conducted parametric length scale studies on the inlet section and compared our results with existing data. The results showed that the larger length scales in the spanwise direction were not only effective in achieving the target shape of a fully developed turbulent boundary layer, but also developed it faster than the smaller length scales.

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