scholarly journals EFFECT OF REYNOLDS NUMBER ON THE SUCTION INFLUENCE ON THE TURBULENT BOUNDARY LAYER STRUCTURES

2007 ◽  
Vol 6 (2) ◽  
pp. 62
Author(s):  
M. O. Oyewola
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Turbulent Boundary Layer ◽  
Large Scale ◽  
Turbulent Transport ◽  
Turbulence Statistics ◽  
Third Order ◽  
Actual Mechanism ◽  
Layer Structures ◽  
Turbulent Statistics

The effect of Reynolds number on the influence of suction on the turbulentboundary layer structures has been quantified through the measurements ofthird- and fourth-order turbulence statistics for several suction rates and streamwise locations downstream of the suction strip. Third- and fourthorderturbulence statistics are more sensitive to a change in boundary condition than second-order moments. The data of the third-order turbulent statistics reveal an alteration in the turbulent transport as a result of the manipulation of the organised motion by suction. The alteration is increased as the suction rate is increased but reduces as the Reynolds number is increased. The results support that, relative to no suction case, Reynolds number modulates the behaviour of higher-order turbulent statistics without changing the actual mechanism of suction on the boundary layer. In general, it is proposed that Reynolds number effect on suction influence in a turbulent boundary layer is universal for the large-scale quantities. This argument is supported by the behaviour of root mean square fluctuating spanwise vorticity.

Generation mechanism of a hierarchy of vortices in a turbulent boundary layer

2019 ◽  
Vol 865 ◽  
pp. 1085-1109 ◽  
Author(s):  
Yutaro Motoori ◽  
Susumu Goto
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Energy Spectrum ◽  
Turbulent Boundary Layer ◽  
Large Scale ◽  
Generation Mechanism ◽  
Small Scale ◽  
Generation Process ◽  
Mean Flow ◽  
The Mean

To understand the generation mechanism of a hierarchy of multiscale vortices in a high-Reynolds-number turbulent boundary layer, we conduct direct numerical simulations and educe the hierarchy of vortices by applying a coarse-graining method to the simulated turbulent velocity field. When the Reynolds number is high enough for the premultiplied energy spectrum of the streamwise velocity component to show the second peak and for the energy spectrum to obey the$-5/3$power law, small-scale vortices, that is, vortices sufficiently smaller than the height from the wall, in the log layer are generated predominantly by the stretching in strain-rate fields at larger scales rather than by the mean-flow stretching. In such a case, the twice-larger scale contributes most to the stretching of smaller-scale vortices. This generation mechanism of small-scale vortices is similar to the one observed in fully developed turbulence in a periodic cube and consistent with the picture of the energy cascade. On the other hand, large-scale vortices, that is, vortices as large as the height, are stretched and amplified directly by the mean flow. We show quantitative evidence of these scale-dependent generation mechanisms of vortices on the basis of numerical analyses of the scale-dependent enstrophy production rate. We also demonstrate concrete examples of the generation process of the hierarchy of multiscale vortices.

scholarly journals The amplification of large-scale motion in a supersonic concave turbulent boundary layer and its impact on the mean and statistical properties

2019 ◽  
Vol 863 ◽  
pp. 454-493 ◽  
Author(s):  
Qian-Cheng Wang ◽  
Zhen-Guo Wang ◽  
Ming-Bo Sun ◽  
Rui Yang ◽  
Yu-Xin Zhao ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Strain Rate ◽  
Turbulence Intensity ◽  
Large Scale ◽  
Principal Strain ◽  
Mean Flow ◽  
Turbulence Statistics ◽  
Concave Boundary ◽  
The Mean

Direct numerical simulation is conducted to uncover the response of a supersonic turbulent boundary layer to streamwise concave curvature and the related physical mechanisms at a Mach number of 2.95. Streamwise variations of mean flow properties, turbulence statistics and turbulent structures are analysed. A method to define the boundary layer thickness based on the principal strain rate is proposed, which is applicable for boundary layers subjected to wall-normal pressure and velocity gradients. While the wall friction grows with the wall turning, the friction velocity decreases. A logarithmic region with constant slope exists in the concave boundary layer. However, with smaller slope, it is located lower than that of the flat boundary layer. Streamwise varying trends of the velocity and the principal strain rate within different wall-normal regions are different. The turbulence level is promoted by the concave curvature. Due to the increased turbulence generation in the outer layer, secondary bumps are noted in the profiles of streamwise and spanwise turbulence intensity. Peak positions in profiles of wall-normal turbulence intensity and Reynolds shear stress are pushed outward because of the same reason. Attributed to the Görtler instability, the streamwise extended vortices within the hairpin packets are intensified and more vortices are generated. Through accumulations of these vortices with a similar sense of rotation, large-scale streamwise roll cells are formed. Originated from the very large-scale motions and by promoting the ejection, sweep and spanwise events, the formation of large-scale streamwise roll cells is the physical cause of the alterations of the mean properties and turbulence statistics. The roll cells further give rise to the vortex generation. The large number of hairpin vortices formed in the near-wall region lead to the improved wall-normal correlation of turbulence in the concave boundary layer.

Phase-Locked PIV Measurement in the Wake of Model Wind Turbines Under Various Inflow Conditions

2012 ◽  
Author(s):  
David J. Green ◽  
Leonardo P. Chamorro ◽  
Roger E. Arndt ◽  
Fotis Sotiropoulos ◽  
Jian Sheng
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Large Scale ◽  
Flow Structures ◽  
Speed Ratio ◽  
Reynolds Stresses ◽  
Turbulence Statistics ◽  
Horizontal Axis Wind Turbine ◽  
Tip Vortices ◽  
Piv Measurement

This paper focuses on understanding correlative interactions between boundary layer flow structures and the resultant unsteady wake of a Horizontal Axis Wind Turbine (HAWT) model. Phase-locked Particle Image Velocimetry (PIV) is employed to measure turbulence statistics such as velocity, turbulence intensity, shear stress, vorticity, and to subsequently identify large-scale coherent flow structures. In the first stage, phase-lock experiments were performed under free-stream flow conditions. Ten consecutive downstream locations up to six rotor diameters from the turbine are captured. Ensemble averaged velocity and vorticity fields reveal that while the identity of tip vortices are maintained over five rotor diameters downstream of the turbine, their strength decays exponentially. When the turbine is placed in the wake of other units, the vortical structures exhibit a rapid decay in both coherence and strength and substantially suppress the wake-vortex and vortex-vortex interactions, playing an important role in the wake recovery. These observations inspire the current investigation using low-speed phase-locked PIV Interactions among the near wall flow structures in a turbulent boundary layer, hub and tip vortices will be investigated in this paper. The model turbine has a 0.108 m hub height, rotor diameter of 0.128 m and tip speed ratio of 4. It is located in a wind tunnel under nearly zero-pressure-gradient and thermally neutrally stratified conditions. A tripped turbulent boundary layer generated by a picket fence located at the inlet has a boundary layer thickness, δ, of 0.55∼0.6 m. Measurements are performed at Re = 3×105, 4×105, and 12 × 105.. To achieve sufficient spatial resolution, two measurement fields are taken at each stream-wise location to cover upper and lower half of the turbines. Measurements locations extend ten diameters downstream. Robust turbulence statistics, such as velocity fluctuations, Reynolds stresses, full budget of turbulent kinetic energy, are computed from large dataset, totaling 400 GBytes.

A coherent structure model of the turbulent boundary layer and its ability to predict Reynolds number dependence

1991 ◽  
Vol 336 (1641) ◽  
pp. 103-129 ◽  
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Turbulent Boundary Layer ◽  
Reynolds Stress ◽  
Coherent Structure ◽  
Large Scale ◽  
Outer Region ◽  
Structure Model ◽  
Hairpin Vortices ◽  
Reynolds Number Dependence

The coherent motions identified in passively marked turbulent boundary-layer experiments are reviewed. Data obtained in our laboratory using simultaneous hot-wire anemometry and flow visualization are analysed to provide measures of the percent contribution of the coherent motions to the total Reynolds stress. A coherent structure model is then developed. In the outer region the model incorporates the large-scale motions, the typical eddies and their interactions. In the wall region the model is characterized by the long streaks, their associated hairpin vortices, and the pockets with their associated pocket and hairpin vortices. The motions in both regions have unique phase relations which play an important role in their evolution and the resulting intensity of their interactions. In addition, the inner-outer region interactions are seen to be strong because typical eddies, microscale motions which can directly initiate the bursting process near a wall, are convected towards the wall by the response of the high speed outer region fluid to the presence of the large-scale motions. This interaction establishes a phasing between the inner and outer regions. The length and velocity scales of the typical eddy are used to remove the Reynolds number dependence of the stream wise fluctuations and the Reynolds stress in the fully turbulent portion of turbulent boundary layers over a wide range of Reynolds numbers

scholarly journals Measurements of higher-order turbulent statistics in a turbulent boundary layer subjected to a short roughness strip

2007 ◽  
Vol 11 (4) ◽  
pp. 41-48
Author(s):  
Olanrewaju Oyewola
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Outer Region ◽  
Higher Order ◽  
Turbulence Structure ◽  
Internal Layers ◽  
Smooth Wall ◽  
Third Order ◽  
Turbulent Statistics ◽  
Roughness Strip

Hot-wire measurements have been undertaken in a turbulent boundary layer which is subjected to an impulse in form of a short roughness strip with the aim of determining its effect on turbulence structure. The quantifications were made through the measurements of higher-order turbulent statistics. The changes observed in the distributions of correlation coefficient, third-order moments, skew ness and flatness factor relative to the smooth wall suggests that the turbulence structure is modified downstream of the short roughness strip. Relative to the undisturbed smooth wall, the third-order moments were increased in the region between the two internal layers. This increased extends to significant portion of the outer region of the boundary layer. While a gain in turbulent kinetic energy by diffusion occurs throughout the boundary layer for a flow over the short roughness strip, those of the smooth wall occur near the wall.

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.

scholarly journals Three-dimensional vortex organization in a high-Reynolds-number supersonic turbulent boundary layer

2010 ◽  
Vol 644 ◽  
pp. 35-60 ◽  
Author(s):  
G. E. ELSINGA ◽  
R. J. ADRIAN ◽  
B. W. VAN OUDHEUSDEN ◽  
F. SCARANO
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Velocity Field ◽  
Turbulent Boundary Layer ◽  
Coherent Structures ◽  
Large Scale ◽  
Three Dimensional ◽  
Velocity Fields ◽  
Hairpin Structure ◽  
Streamwise Direction

Tomographic particle image velocimetry was used to quantitatively visualize the three-dimensional coherent structures in a supersonic (Mach 2) turbulent boundary layer in the region between y/δ = 0.15 and 0.89. The Reynolds number based on momentum thickness Reθ = 34000. The instantaneous velocity fields give evidence of hairpin vortices aligned in the streamwise direction forming very long zones of low-speed fluid, consistent with Tomkins & Adrian (J. Fluid Mech., vol. 490, 2003, p. 37). The observed hairpin structure is also a statistically relevant structure as is shown by the conditional average flow field associated to spanwise swirling motion. Spatial low-pass filtering of the velocity field reveals streamwise vortices and signatures of large-scale hairpins (height > 0.5δ), which are weaker than the smaller scale hairpins in the unfiltered velocity field. The large-scale hairpin structures in the instantaneous velocity fields are observed to be aligned in the streamwise direction and spanwise organized along diagonal lines. Additionally the autocorrelation function of the wall-normal swirling motion representing the large-scale hairpin structure returns positive correlation peaks in the streamwise direction (at 1.5δ distance from the DC peak) and along the 45° diagonals, which also suggest a periodic arrangement in those directions. This is evidence for the existence of a spanwise–streamwise organization of the coherent structures in a fully turbulent boundary layer.

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