A mechanism for excitation of coherent structures in wall region of a turbulent boundary layer

Introduction Themechanismforthegenerationofcoherentstructuresinthewallregionofaturbulent boundarylayerhasalwaysbeeninconcernandinvestigated.AccordingtoTsujimotoand Miyake[1],thecharacteristicsofturbulenceinthewallregionweremainlydeterminedbythe generationandevolutionofcoherentstructures,notbythesmall_scaleturbulence.However, excitationsfromregionofy >60werefoundtobenecessary,otherwisethewallregionwould degeneratetolaminarflow.Therefore,theinvestigationofthemechanismthathowcoherent structuresi…     

Numerical prediction of turbulent boundary layer noise from a sharp-edged flat plate

An efficient hybrid uncorrelated wall plane waves–boundary element method (UWPW-BEM) technique is proposed to predict the flow-induced noise from a structure in low Mach number turbulent flow. Reynolds-averaged Navier-Stokes equations are used to estimate the turbulent boundary layer parameters such as convective velocity, boundary layer thickness, and wall shear stress over the surface of the structure. The spectrum of the wall pressure fluctuations is evaluated from the turbulent boundary layer parameters and by using semi-empirical models from literature. The wall pressure field underneath the turbulent boundary layer is synthesized by realizations of uncorrelated wall plane waves (UWPW). An acoustic BEM solver is then employed to compute the acoustic pressure scattered by the structure from the synthesized wall pressure field. Finally, the acoustic response of the structure in turbulent flow is obtained as an ensemble average of the acoustic pressures due to all realizations of uncorrelated plane waves. To demonstrate the hybrid UWPW-BEM approach, the self-noise generated by a flat plate in turbulent flow with Reynolds number based on chord Re_c = 4.9 × 10⁵ is predicted. The results are compared with those obtained from a large eddy simulation (LES)-BEM technique as well as with experimental data from literature.     

A kind of fast changing coherent structure in a turbulent boundary layer

Coherent structures of a turbulent boundary layer were investigated by hydrogen bubble method. A kind of fast changing structure was observed. That is a spot in which all the hydrogen bubbles vanish much faster than in other regions. This investigation verified that dark-spot is formed by a strong sweep from outer layer. Inside a dark-spot the local instantaneous flow speed might be four times of its neighboring high-speed streaks. Comparing with the low/high speed streaks, both dark-spot and the vortical structures around it are changing very fast. Around dark-spot intensive shear layers are formed and indications of the generation of small-scale structures could be observed. The project supported by the National Natural Science Foundation of China (No. 19672004) and the National Climbing Project     

Bubble migration in a turbulent boundary layer

The wall void peaking distribution observed in an upward turbulent bubbly boundary layer along a flat plate is generated by bubbles that move towards the plate, come into contact with the wall and then slide along it. This transverse ‘migration’ has been studied using flow visualization, high speed video and particle tracking techniques to measure the trajectories of mono-disperse air bubbles at very low void fractions. Investigations have been performed at four Reynolds numbers in the range 280 < Re_θ < 3000, covering both the laminar and turbulent regimes, with mono-disperse bubbles of mean equivalent diameter between 2 mm and 6 mm. Lagrangian statistics calculated from hundreds of trajectories show that the migration only occurs in the turbulent regime and for bubble diameters below some critical value: 3.5 mm < d_eqcrit < 4 mm. Above this size (We > 3), the interface deformation is such that bubbles do not remain at the wall, even when they are released at the surface. Also, bubble migration is shown to be non-systematic, to have a non-deterministic character in the sense that trajectories differ significantly, to increase with Reynolds number and to take place on a short time scale. A series of experiments with isolated bubbles demonstrates that these results are not influenced by bubble–bubble interactions and confirm that two-way coupling in the flow is limited. Flow visualizations show that the migration originates with the capture of bubbles inside the large turbulent structures of the boundary layer (‘bulges’). The bubbles begin to move towards the wall as they cross these structures, and the point at which they reach the wall is strongly correlated with the position of the deep ‘valleys’ which separate the turbulent ‘bulges’. The analysis of the mean Lagrangian trajectories of migrating bubbles confirms these observations. Firstly, the average time of migration calculated from these trajectories coincides with the mean transit time of the bubbles across the structures. Secondly, once the trajectories have been scaled by this transit time and the boundary layer thickness δ, they all have the same form in the region y/δ < 0.4, independent of the Reynolds number.     

The structures of turbulent boundary layer in the vicinity of separation

Large coherent structures of turbulent boundary layer in the vicinity of separation were observed in a water channel by the hydrogen bubble method. Motion pictures of the de views were taken. The features of the instantaneous velocity profiles, the large transverse and streamwise vortices were discussed.     

Resonant interactions of Tollmien-Schlichting waves in the boundary layer on a flat plate

Resonant interaction phenomena of Tollmien-Schlichting waves (T.-S. waves) are examined experimentally by spectral analysis method. Results demonstrate that, in the spectra measured in the vicinity of the critical layer of the unstable boundary layer, the energy of T.-S. waves concentrates in a narrow band of frequency with one to three peaks in the power spectra corresponding to the eigenfrequencies of T.-S. waves and the frequency with maximum growth rate located between the upper and lower branches of the neutral curve. The resonant interactions transfer the energy of these eigencomponents to their subsequent subharmonics in a range ofR _δ* where their growth rates increase from zero to a maximum value, and the boundary layer becomes turbulent after the third resonant interaction.     

An experimental research on the instability of natural convection boundary layer around a vertical heated flat plate

Experimental results on the instability of the isothermal naturalconvection boundary layer around a vertical heated flat plate are presented. It is demonstrated that the characteristics of the instability wave in the outer layer is consistent with the calculation of Brewster & Gebhart. After an initial growth of its low frequency components at the downstream side of the turning point of the neutral curve (Gr≈120) its comparatively higher frequency components develop and become turbulent subsequently with a buoyancy subrange in its power spectra. Simultaneously, in the measurement at the inner layer near the wall a viscous instability signal the same as the Tollmien-Schlichting waves in ordinary boundary layer and its subharmonics in a much higher frequency domain is discovered and an inertial subrange can be observed in the spectra atGr≈378.6. The project supported by the National Natural Science Foundation of China (19572004)     

Lagrangian model on the turbulent motion of small solid particle in turbulent boundary layer flows

.Intr0ductionSurfaceerosionofmaterialbysolid-particleimpactisanimportantprobleminmultiphaseflowindustriaIdevicesandthecharacteristicsoftheparticIe'smotioninaturbulentboundarylayerflowisthebaseofthestudyofthematerialsurfaceerosion.Manycalculationmodelshave…     

ON THE MECHANISM OF TURBULENT COHERENT STRUCTURE(II)──A PHYSICAL MODEL OF COHERENT STRUCTURE FOR THE ROUGH BOUNDARY LAYER

In this paper, the differences of turbulent coherent structure between the smooth and rough boundary layers are analysed. Based on the discussing the transient properties from the smooth wall to the rough wall, the physical model of coherent structure for the rough boundary layer are established. The width of slowly-moving turbulent spot and the bursting time are obtained, which are in agreement with experimental results.The project supported by the National Nature Science Foundation of China     

Aerodynamic heating in the region of shock and turbulent boundary layer interaction induced by a cylinder

Detailed distributions of heat flux in the region of shock wave and turbulent boundary layer interaction induced by a cylinder were measured in the shock tunnel. Oil flow patterns and Schlieren photographs were taken. Empirical relations were given for determining separation shock angle, peaks of heat flux and their locations on both cylinder leading edge and flat plate surface, and other characteristic parameters of the interaction region.     

Computation of shock wave/turbulent boundary layer interactions using a two-equation model with compressibility corrections

In this paper computational results for two different types of shock wave / turbulent boundary layer interaction flows are presented. It is shown that upstream effects of the shock induced separation cannot be reproduced by Wilcox's (1991) k--model, whereas downstream of the interaction, predictions of pressure distribution and skin friction are acceptable. The inclusion of the compressible part of the dissipation rate and the pressure dilatation in the model has noticeable, but not dramatic effects on wall pressure and skin friction in the selected flow cases.     

Diffusion of macromolecular solutions in the turbulent boundary layer of a cylindrical pipe — Evolution of wall concentration

Summary In this paper, we are presenting a model of the evolution of the wall concentration of a macromolecular solution (PEO) annularly injected in a cylindrical pipe in a turbulent flow. This model valid for all diffusion zones is based on mathematical and physical considerations and proves to be in good agreement with the experimental data.

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Zusammenfassung Es wird ein Modell der Entwicklung der Wandkonzentration einer makromolekularen Lösung (PÄO) vorgestellt, die in einem wandnahen Ringspalt in die turbulente Strömung durch ein zylindrisches Rohr injiziert worden ist. Dieses für alle Diffusionszonen gültige Modell basiert auf mathematischen und physikalischen Betrachtungen und erweist sich für die Beschreibung der experimentellen Daten als gut geeignet.

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C _w wall concentration - C _i initial concentration before injection - L ₀ distance from the slot at which the wall concentration drops toe ^-1 of its original value - L _IT ,L _IF ,L _F characteristic lengths - L _I length scale of the second region - x downstream distance from the source - n _I ,n _T ,n _F characteristic exponents - K ₀,K _I ,K _F characteristic constants - ln natural logarithm - q _i flow rate of injection - Q _T flow rate - C _j =C _{i · q i} /Q _T concentration in homogeneous medium - A, B, C, m constants - p andq annex variables - Re Reynolds number With 7 figures     

Low- and high-speed structures in the outer region of an adverse-pressure-gradient turbulent boundary layer

Large- and very large-scale structures in the form of elongated regions of low and high streamwise momentum have been studied in the outer region of a turbulent boundary layer subjected to a strong adverse pressure gradient. Large sets of streamwise–spanwise instantaneous velocity fields are acquired by particle image velocimetry at three wall-normal positions (0.2δ, 0.5δ, 0.8δ) at three different streamwise locations and at 0.1δ at the last streamwise location which allows us to study the wall-normal and streamwise variations of the structures. Subsequently, a pattern-recognition method and a classification scheme are employed in order to detect, classify and characterize the structures in an efficient and rigorous manner. Like in the case of zero-pressure-gradient turbulent boundary layers, long meandering streaky regions of low and high momentum are observed in the outer region of the present flow but they appear less frequently; especially in the lower part (at 0.1δ and 0.2δ) of the large-velocity-defect zone, i.e. near detachment. The dimensions of these large structures scale on boundary-layer thickness (δ) and are generally comparable to those previously reported for such structures in the overlap region of zero-pressure-gradient turbulent boundary layers. Interestingly, the adverse pressure gradient does not significantly affect the dimensions and arrangement of the large-scale structures in the upper part (at 0.5δ and 0.8δ) a segment of the outer region where the scaled Reynolds stresses also remain fairly self-similar.     

Multiple distortion of a supersonic turbulent boundary layer

Two experiments were performed to study the response of a supersonic turbulent boundary layer to successive distortions. In the first experiment (Case 1), the flow passed over a forward-facing ramp formed by 20° compression corner followed by a 20° expansion corner located about 4_o downstream, where _o is the incoming boundary layer thickness. In the second experiment (Case 2), the forward-facing ramp was constructed of curved compression and expansion surfaces with the same turning angles and total step height as in Case 1. The radii of curvature for the compression and expansion surfaces were equal to 12_o. In both experiments, the flow relaxation was observed over a distance equal to 12_o. In this relaxation region, the mean and turbulent flow behavior of the boundary layer was measured. The mean velocity profile was found to be altered by the distortion. Recovery of the profile began near the wall and occurred rapidly, but in the outer part of the boundary layer, recovery proceeded slowly. Turbulence measurements revealed a dramatic reduction in the turbulence shear stress and a progressively decaying streamwise Reynolds stress profile.     

壁面温度在多参数下对平板边界层的影响研究

为了得到壁面温度在不同来流速度、不同湍流强度条件下对边界层转捩与减阻的影响规律,本文采用Transitionk-kl-ω模型对低来流速度下无压力梯度的光滑平板进行了数值模拟。结果表明,随着来流速度的升高,壁温升高所起到的减阻效果更好,即高来流速度对壁面温度更为敏感。当来流处于中高湍流强度下时,壁温升高能起到推迟转捩的作用,且随着湍流强度的升高,转捩推迟的效果越好,但减阻效果正好相反;当来流处于低湍流强度下时,壁温升高会使得转捩提前发生。壁温升高抑制了边界层内流体的脉动程度,使得层流的稳态不易被破坏,流动更加稳定;同时,壁温升高使得边界层内流体的速度梯度减小,从而降低了壁面摩擦系数,故壁温升高能起到推迟边界层转捩与减阻的作用。     

A theoretical mode for the instability of turbulent boundary layer over compliant surface

A theoretical model for the instability of turbulent boundary layer over compliant surfaces is described. The investigation of instability is carried out from a time-asymptotic space-time perspective that classifies instabilities as either convective or absolute. Results are compared against experimental observations of surface waves on elastic and viscoelastic compliant layers.     

Some new results of the observation on the coherent structures of turbulent boundary layer in flow with adverse pressure gradient

By a suitable manipulation of hydrogen bubble generation, some new results were obtained: (1) The long-streaks are generated along the interfaces between low and high-speed streaks. The long-streaks are generally stretching and are moving faster than its neighboring high-speed streaks. The hydrogen bubbles in long-streaks have longer life. (2) The stream-wise vortices are also generated along the interfaces. The project is supported by the National Natural Science Foundation of China.     

Transport of particles in an atmospheric turbulent boundary layer

A program incorporating the parallel code of large eddy simulation (LES) and particle transportation model is developed to simulate the motion of particles in an atmospheric turbulent boundary layer (ATBL). A model of particles of 100-micrometer order coupling with large scale ATBL is proposed. Two typical cases are studied, one focuses on the evolution of particle profile in the ATBL and the landing displacement of particles, whereas the other on the motion of particle stream.The English text was polished by Yunming Chen.     

Multi-scale coherent structures in turbulent boundary layer detected by locally averaged velocity structure functions

Introduction Inearlyperiodofstudyofturbulentflow,itwasdeemedrandomanddisorderedmotionsof fluidparticles,sothecharacteristicsofturbulencewerestudiedbystatisticsaveragemethod. Kolmogorov[1]analyzedtherelativemotionoffluidparticlesinfullydeveloped(Reynolds numbertrendstoinfinity)isotropicandhomogeneousturbulentflow,onbasisofrandomfield theory,andpresentedtheconceptofstructurefunctions,whichdescribedtherelativevelocityof twofluidparticlesindistanceofl,toresearchthelawbetweenthemulti_orderstatistic…     

Scale-by-scale energy budget in a turbulent boundary layer over a rough wall

Hot-wire velocity measurements are carried out in a turbulent boundary layer over a rough wall consisting of transverse circular rods, with a ratio of 8 between the spacing (w) of two consecutive rods and the rod height (k). The pressure distribution around the roughness element is used to accurately measure the mean friction velocity ($U_{\tau }$) and the error in the origin. It is found that $U_{\tau }$ remained practically constant in the streamwise direction suggesting that the boundary layer over this surface is evolving in a self-similar manner. This is further corroborated by the similarity observed at all scales of motion, in the region $0.2⩽y/\delta ⩽0.6$, as reflected in the constancy of Reynolds number ($R_{\lambda }$) based on Taylor’s microscale and the collapse of Kolmogorov normalized velocity spectra at all wavenumbers.A scale-by-scale budget for the second-order structure function $〈{(\delta u)}^2〉$ ($\delta u=u(x+r)-u(x)$, where u is the fluctuating streamwise velocity component and r is the longitudinal separation) is carried out to investigate the energy distribution amongst different scales in the boundary layer. It is found that while the small scales are controlled by the viscosity, intermediate scales over which the transfer of energy (or $〈{(\delta u)}^3〉$) is important are affected by mechanisms induced by the large-scale inhomogeneities in the flow, such as production, advection and turbulent diffusion. For example, there are non-negligible contributions from the large-scale inhomogeneity to the budget at scales of the order of $\lambda$, the Taylor microscale, in the region of the boundary layer extending from $y/\delta =0.2$ to 0.6 ($\delta$ is the boundary layer thickness).