New Scaling Parameter For Turbulent Boundary
Layer With Large Roughness
Author(s)
C. S. Subramanian & M. Lebrun
Abstract
Nikuradse uses the equivalent sand-grain roughness to characterize the effect of
roughness. While this approach works when the roughness is contained in the
inner layer, it does not apply in recent studies with a larger roughness. Various
techniques have been applied in the past to scale the mean velocity and the
Reynolds stress profiles for a zero pressure gradient boundary layer, the classical
scaling using the friction velocity u* to normalize the velocity profiles. However
none of these techniques holds universally.
This study attempts to improve the understanding that we have of the way
roughness affects the inner layer behaviour and aims to find an alternative
scaling parameter for cases where roughness is large compared to the inner layer.
Measured mean and turbulent velocity profiles on a large regular roughness
show a non-zero wall normal pressure is caused which contributes to the velocity
deficit in the near wall rough boundary layer velocity profile. The normal
turbulent stresses are also increased. Hence a pressure gradient velocity rather
than the friction velocity is defined to capture the pressure effects induced by
roughness. The power law seems to give a better representation of the velocity
profiles than the log law in this case.
Keywords: large roughness, boundary layer, friction velocity, turbulent velocity,
pressure gradient velocity, log law, power law.
1 Introduction
Some of the most recent studies of the effects of surface roughness on boundary
layer structure were performed by Perry et al. [2], Bandyopadhyay and Watson
[3], Barenblatt [4], Acharya et al. [5] and Keirsbulk et al. [6]. Roughness may be
Keywords
large roughness, boundary layer, friction velocity, turbulent velocity,
pressure gradient velocity, log law, power law.
Related Book
Other papers in this volume
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