Vibrations of infinite plates and the mean value vibrations of finite plates excited by turbulent boundary layer flows

2002 ◽  
Vol 111 (5) ◽  
pp. 2424
Author(s):  
Stephen Hambric ◽  
Yun-Fan Hwang
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Mean Value ◽  
Boundary Layer Flows ◽  
The Mean

Experimental Study on Drag-Reducing Turbulent Boundary Layer Flows of Surfactant Solutions

2007 ◽  
Author(s):  
M. Itoh ◽  
S. Tamano ◽  
T. Inoue ◽  
K. Yokota
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Drag Reduction ◽  
Laser Doppler Velocimetry ◽  
Friction Velocity ◽  
Boundary Layer Flows ◽  
Experimental Conditions ◽  
Mean Velocity ◽  
The Mean ◽  
Peak Value

In this study, the influence of a drag-reducing surfactant on the turbulent boundary layer under different solution concentrations and temperatures was extensively investigated using a two-component laser-Doppler velocimetry system. It is found that the drag reduction ratio DR at the temperature T = 20°C becomes larger downstream, and decreases with the increase of concentration from C = 65 to 150 ppm. The DR for C = 100 ppm becomes smaller with the increase of the temperature from T = 25 to 35°C, and the DR at T = 20°C is smaller than DR at T = 25°C. For the different solution concentrations and temperatures, the value of the mean velocity scaled by the friction velocity increases with increasing the amount of drag reduction. For the present experimental conditions tested, the peak value of streamwise turbulence intensity seems to be not related to the amount of DR directly and to be affected by the low Reynolds number effect strongly.

Acoustic Doppler Velocimeter in Measurements of Turbulent Boundary Layer Flows

2012 ◽  
Vol 239-240 ◽  
pp. 100-103 ◽  
Author(s):  
Hao Lu ◽  
Bing Wang ◽  
Hui Qiang Zhang ◽  
Xi Lin Wang
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Acoustic Doppler Velocimeter ◽  
Boundary Layer Flows ◽  
Flow Reynolds Number ◽  
Simple Implementation ◽  
Threshold Filter ◽  
The Mean ◽  
Layer Flow ◽  
Maximum Threshold

Acoustic Doppler Velocimeter (ADV)’s applications in measuring velocity profiles of turbulent boundary layers (TBLs) are investigated at a flow Reynolds number of 36000 in a lab-made open channel. The 3D instantaneous velocity signals at different heights of TBLs are obtained, which present very strong abnormal noises in the near wall regions because of the aliasing of Doppler signal spectra. The minimum/maximum threshold filter is adopted for the signal to remove the noises reflected by walls. The mean streamwise velocities and turbulence intensities profiles are then calculated correctly. The measuring results consist with the law of turbulent boundary layer flow. The ADV has a good performance in measurements of TBLs with a simple implementation of filter to the sampled velocity signals. The present research is guidable for applications of ADV in practice.

A theory of turbulence in stratified fluids

1970 ◽  
Vol 42 (2) ◽  
pp. 349-365 ◽  
Author(s):  
Robert R. Long
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Density Profile ◽  
Richardson Number ◽  
Mixed Volume ◽  
Volume Of Fluid ◽  
Velocity Shear ◽  
Mean Velocity ◽  
Large Wave ◽  
The Mean

An effort is made to understand turbulence in fluid systems like the oceans and atmosphere in which the Richardson number is generally large. Toward this end, a theory is developed for turbulent flow over a flat plate which is moved and cooled in such a way as to produce constant vertical fluxes of momentum and heat. The theory indicates that in a co-ordinate system fixed in the plate the mean velocity increases linearly with heightzabove a turbulent boundary layer and the mean density decreases asz3, so that the Richardson number is large far from the plate. Near the plate, the results reduce to those of Monin & Obukhov.Thecurvatureof the density profile is essential in the formulation of the theory. When the curvature is negative, a volume of fluid, thoroughly mixed by turbulence, will tend to flatten out at a new level well above the original centre of mass, thereby transporting heat downward. When the curvature is positive a mixed volume of fluid will tend to fall a similar distance, again transporting heat downward. A well-mixed volume of fluid will also tend to rise when the density profile is linear, but this rise is negligible on the basis of the Boussinesq approximation. The interchange of fluid of different, mean horizontal speeds in the formation of the turbulent patch transfers momentum. As the mixing in the patch destroys the mean velocity shear locally, kinetic energy is transferred from mean motion to disturbed motion. The turbulence can arise in spite of the high Richardson number because the precise variations of mean density and mean velocity mentioned above permit wave energy to propagate from the turbulent boundary layer to the whole region above the plate. At the levels of reflexion, where the amplitudes become large, wave-breaking and turbulence will tend to develop.The relationship between the curvature of the density profile and the transfer of heat suggests that the density gradient near the level of a point of inflexion of the density curve (in general cases of stratified, shearing flow) will increase locally as time goes on. There will also be a tendency to increase the shear through the action of local wave stresses. If this results in a progressive reduction in Richardson number, an ultimate outbreak of Kelvin–Helmholtz instability will occur. The resulting sporadic turbulence will transfer heat (and momentum) through the level of the inflexion point. This mechanism for the appearance of regions of low Richardson number is offered as a possible explanation for the formation of the surfaces of strong density and velocity differences observed in the oceans and atmosphere, and for the turbulence that appears on these surfaces.

Prediction of compressible turbulent boundary layer via a symmetry-based length model

2018 ◽  
Vol 857 ◽  
pp. 449-468 ◽  
Author(s):  
Zhen-Su She ◽  
Hong-Yue Zou ◽  
Meng-Juan Xiao ◽  
Xi Chen ◽  
Fazle Hussain
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Layer Structure ◽  
Analytic Form ◽  
Algebraic Model ◽  
Length Function ◽  
Turbulent Heat Transfer ◽  
Turbulent Heat ◽  
Reynolds Analogy ◽  
The Mean

A recently developed symmetry-based theory is extended to derive an algebraic model for compressible turbulent boundary layers (CTBL) – predicting mean profiles of velocity, temperature and density – valid from incompressible to hypersonic flow regimes, thus achieving a Mach number ($Ma$) invariant description. The theory leads to a multi-layer analytic form of a stress length function which yields a closure of the mean momentum equation. A generalized Reynolds analogy is then employed to predict the turbulent heat transfer. The mean profiles and the friction coefficient are compared with direct numerical simulations of CTBL for a range of$Ma$from 0 (e.g. incompressible) to 6.0 (e.g. hypersonic), with an accuracy notably superior to popular current models such as Baldwin–Lomax and Spalart–Allmaras models. Further analysis shows that the modification is due to an improved eddy viscosity function compared to competing models. The results confirm the validity of our$Ma$-invariant stress length function and suggest the path for developing turbulent boundary layer models which incorporate the multi-layer structure.

Turbulent Wall-Pressure Fluctuations: A New Model for Off-Axis Cross-Spectral Density

1997 ◽  
Vol 119 (2) ◽  
pp. 277-280 ◽  
Author(s):  
B. A. Singer
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Spectral Density ◽  
Flow Direction ◽  
Wall Pressure ◽  
Mean Flow ◽  
New Approach ◽  
Cross Spectral Density ◽  
The Mean ◽  
The Cross

Models for the distribution of the wall-pressure under a turbulent boundary layer often estimate the coherence of the cross-spectral density in terms of a product of two coherence functions. One such function describes the coherence as a function of separation distance in the mean-flow direction, the other function describes the coherence in the cross-stream direction. Analysis of data from a large-eddy simulation of a turbulent boundary layer reveals that this approximation dramatically underpredicts the coherence for separation directions that are neither aligned with nor perpendicular to the mean-flow direction. These models fail even when the coherence functions in the directions parallel and perpendicular to the mean flow are known exactly. A new approach for combining the parallel and perpendicular coherence functions is presented. The new approach results in vastly improved approximations for the coherence.

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