scholarly journals On Ripples—A Boundary Layer-Theoretical Definition

Water ◽  
2021 ◽  
Vol 13 (7) ◽  
pp. 892
Author(s):  
Ulrich Zanke ◽  
Aron Roland
Keyword(s):  
Boundary Layer ◽  
Viscous Sublayer ◽  
Boundary Layer Theory ◽  
Sediment Properties ◽  
Sand Waves ◽  
Actual Flow ◽  
Physical Background ◽  
The Subject

Once the first initial ripples have developed, they form according to the actual flow forces and sediment properties. In this paper, a semianalytical approach to determine the length of the developed ripples is presented. The theory assumes initial disturbances at the bed surface and corresponding flow separations resulting from an individual respective boundary layer. What causes the initial rhythmic perturbations is not the subject of this paper. Based on boundary layer theory, this approach explains a possible physical background for the existence and length of developed ripples in cohesion-free sediments. At the same time, the approach provides a distinction from dunes: ripples are sand waves affected by a viscous sublayer, and dunes are sand waves where this is not the case. Applications to Earth, Mars, and Titan are shown.

Paper 1: Centrifugal Compressors for Gas Turbines

1968 ◽  
Vol 183 (14) ◽  
pp. 1-10 ◽  
Author(s):  
T. B. Ferguson
Keyword(s):  
Boundary Layer ◽  
Gas Turbines ◽  
Three Dimensional ◽  
Boundary Layer Theory ◽  
Pressure Gradients ◽  
Two Dimensional ◽  
Centrifugal Compressors ◽  
Blade Loading ◽  
Actual Flow ◽  
High Mach

The trends in fluid mechanical development of centrifugal compressors are discussed. The main developments in the impeller are the application of quasi-three-dimensional isentropic methods together with some separation criteria based on two-dimensional turbulent boundary layer theory. Diffusers are sometimes designed on a simplified two-dimensional basis but channel diffusers still appear to be preferred especially at high Mach numbers. Recent visualization studies have shown how far the actual flow in impellers may depart from the actual model and there is a lack of systematic quantitative experimental work on limiting blade loading and pressure gradients both in impellers and diffusers. A summary of gas turbine centrifugal compressors is also made.

Flow field in a downward diverging channel and its application

2016 ◽  
Vol 70 (8) ◽  
Author(s):  
Marek Večeř ◽  
Kamil Wichterle
Keyword(s):  
Boundary Layer ◽  
Image Analysis ◽  
Velocity Field ◽  
Boundary Layer Theory ◽  
Experimental Setup ◽  
Air Bubbles ◽  
Characteristic Parameters ◽  
Diverging Channel ◽  
Divergent Channel ◽  
Actual Flow

AbstractThe flow in a downward divergent channel turns out to be an interesting experimental setup for the observation of upward floating bubbles that appear to be levitating in view of the observer. A more detailed analysis of this flow and its characteristic parameters is necessary for better understanding of this phenomenon. The boundary layer theory was used to derive the velocity field for the experimental setup. The actual flow of a liquid in the presence of a bubble was studied experimentally by measuring the position of the bubble; the data were then statistically processed by an image analysis. Observation of the bubble positions distribution showed that it is reasonable to assume a flat velocity profile of the liquid in the channel and that the bubbles do not tend to move into the boundary layer. In our experiments, volume of the air bubbles floating in water was 200 mm

INVERSE PROBLEMS OF BOUNDARY LAYER THEORY

2018 ◽  
Vol 49 (8) ◽  
pp. 793-807
Author(s):  
Vladimir Efimovich Kovalev
Keyword(s):  
Boundary Layer ◽  
Inverse Problems ◽  
Boundary Layer Theory

‘Inactive’ motion and pressure fluctuations in turbulent boundary layers

1967 ◽  
Vol 30 (2) ◽  
pp. 241-258 ◽  
Author(s):  
P. Bradshaw
Keyword(s):  
Boundary Layer ◽  
Outer Layer ◽  
Pressure Fluctuations ◽  
Turbulent Energy ◽  
Viscous Sublayer ◽  
Wave Number ◽  
Noticeable Effect ◽  
Frequency Spectra ◽  
Active Motion ◽  
Order Of Magnitude

Townsend's (1961) hypothesis that the turbulent motion in the inner region of a boundary layer consists of (i) an ‘active’ part which produces the shear stress τ and whose statistical properties are universal functions of τ and y, and (ii) an ‘inactive’ and effectively irrotational part determined by the turbulence in the outer layer, is supported in the present paper by measurements of frequency spectra in a strongly retarded boundary layer, in which the ‘inactive’ motion is particularly intense. The only noticeable effect of the inactive motion is an increased dissipation of kinetic energy into heat in the viscous sublayer, supplied by turbulent energy diffusion from the outer layer towards the surface. The required diffusion is of the right order of magnitude to explain the non-universal values of the triple products measured near the surface, which can therefore be reconciled with universality of the ‘active’ motion.Dimensional analysis shows that the contribution of the ‘active’ inner layer motion to the one-dimensional wave-number spectrum of the surface pressure fluctuations varies as τ2w/k1 up to a wave-number inversely proportional to the thickness of the viscous sublayer. This result is strongly supported by the recent measurements of Hodgson (1967), made with a much smaller ratio of microphone diameter to boundary-layer thickness than has been achieved previously. The disagreement of the result with most other measurements is attributed to inadequate transducer resolution in the other experiments.

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