Study of Turbulent Structure Close to a Wall Using Conditional Sampling Techniques
Lau, Kenneth Kailung
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https://hdl.handle.net/2142/66638
Description
Title
Study of Turbulent Structure Close to a Wall Using Conditional Sampling Techniques
Author(s)
Lau, Kenneth Kailung
Issue Date
1980
Department of Study
Chemical Engineering
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Chemical
Language
eng
Abstract
The principle objective of the present work is to characterize the spatial and temporal velocity variation of the flow close to a wall. Experiments were carried out in which the velocity at a number of locations in the fluid was measured simultaneously with the measurement of the two components of the fluctuating velocity gradient at the wall s(,x) and s(,z). The lateral profile of the variation of s(,z) as well as some specialized procedures were used as indicator functions to conditionally average the velocity measurements at different locations in a flow oriented eddy and at different times in its development. Using multiple-point measurements and conditional sampling methods both the kinematics and the dynamics of the wall eddies could be studied. The axial and spanwise velocity components in the fluid were measured by a hot-film X-probe.
The operation of the measuring sensors was based on a mass transfer controlled electrochemical reaction taking place on the surface of a platinum sensing element. All measurements were carried out in an 8-in. flow loop. The experimental set-up was interfaced with an IBM 1800 computer for digital data acquisition.
The results of this work confirm previous proposed model that counterrotating eddies in the wall region do exist; but rather than being the dominant flow structure as a number of investigators have proposed they exist only for short periods during transition between inflow and outflow.
The lateral wavelength of the eddies was found to be very close to (lamda)(,z) = 100 using the conditional average profiles of s(,x) and s(,z). A significant finding is that these profiles are in excellent agreement with our proposed idealized eddy pattern. The temporal development of these profiles shows that eddy evolution is influenced by the dynamics of flow in the spanwise direction.
From simultaneous measurements using the hot-film probe in the fluid there was indication that the direction of propagation of the wall flow phenomena is from the outer region. This is in contrast to suggestions by a number of authors that secondary flow is initiated by an instability at the wall.
Our results show that there are two types of shear layers--positive and negative. Positive shear layers are frequently encountered. Negative shear layers can be present when fluid near the wall has a higher momentum content than fluid away from the wall.
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