Determination Of Steady-state Parameters Research Articles

Modelling ripple development under non-uniform flow and sediment supply-limited conditions in a laboratory flume

Results are reported herein of an open channel flow laboratory based study of the development of ripples on a fine silica sand bed, and under non-uniform turbulent subcritical flow conditions. The hydraulic model used included a diverging channel, which resulted in a variation of hydraulic and sediment transport parameters along the channel. Sediment supply limitation occurred during experimentation, impacting bed form development. The overall aim of this study was to improve the understanding and modelling capability of the development of bed forms under limited sediment supply and non-uniform flow conditions. In particular, the applicability of an existing empirical model capable of predicting ripple development was tested for the conditions of this study, using measured ripple dimensions. The ripple height and length results were extracted from detailed bed profile records, obtained using an acoustic Doppler probe traversed longitudinally over the sediment bed, at various experimentation time intervals. It was found that the non-uniform flow conditions affected the development rate of the bed forms, while sediment supply limitation impacted their steady state dimensions. The measured steady state ripple dimensions were lower, on average, than the corresponding equilibrium dimensions predicted using existing empirical equations. Non-uniform flow also caused the simultaneous occurrence of bed forms at different stages of development along the hydraulic model, where 3D and 2D ripples and incipient bed forms were recorded. Such a scenario can occur in estuarine and coastal flows, due to changing hydraulic conditions and/or a limitation of sediment supply. The ripple development model tested was verified for the conditions of this study, with its accuracy being shown to depend on an accurate determination of steady state parameters.

Dislocation theory of steady state deformation and its approach in creep and dynamic tests

A concept of strain hardening in constant strain rate (CSR) tests and creep tests is presented taking into account dislocation production and recovery by slip, climb and concurrent subboundary migration. A master equation is derived to relate the evolution of the dislocation density with strain rate and stress. Transient and steady state equations are developed for various combinations of recovery mechanisms in both types of tests, and are analyzed with respect to material characteristics ( μ, b, D, γ sf ), and test conditions ( T, ⋗3, σ). For steady state equivalent relations between stress and strain rate are obtained for both test modes. The appropriate relations for the approach to steady state and their mutual correlations are elaborated. The derived dependencies are discussed with respect to experimental results in order to elucidate the occurrence and impact of possible recovery mechanisms and to apply proper procedures for a conclusive determination of steady state parameters.