School of Mechanical Engineering Seminar
Wednesday, March 25, 2015 at 15:00
Wolfson Building of Mechanical Engineering, Room 206
Evolution of a turbulent patch in dilute polymer solutions
Baevsky Mark
M.Sc. Student of Prof. Alex Liberzon
Drag reduction effect by dilute polymer solutions was discovered in 1946 by Toms, but the basic mechanisms by which polymers modify the turbulent flow have not been understood thoroughly, despite the progress in understanding the drag reduction in pipes or channels. One of the main problems is relatively poor understanding of dilute polymer solutions and inter-scale transfer of energy in turbulent flows. The problem intensifies in the case of turbulent entrainment across turbulent/non-turbulent interfaces on the boundaries of turbulent jets, wakes or mixing layers. The polymer is sought to alter this region of flow significantly due to the large gradients at the interface and strong interaction of multiple scales - large scales that deflect the interface and the small scales that diffuse the vorticity and strain. There is however no detailed experimental studies devoted to the interfaces and the numerical simulations that use polymer models (such as Oldroyd-B or FENE-P) require a solid empirical background for comparison.
An experimental study has been performed to characterize the basic mechanisms of turbulent entrainment in water - poly(ethylene oxide) solutions, alongside the benchmark case of the fresh water. A new experimental setup was developed to create a spherical localized turbulent patch, thus isolating the polymer effect far from the boundaries with negligible wall friction effects, as opposed to the previously utilized 2D space-filling planar oscillating grids. The setup enables a direct comparison of the results with the direct numerical simulations. We performed a large set of particle image velocimetry (PIV) measurements. The patch life cycle comprises of three phases: initial growth, a steady state and the decay phase after the forcing have ceased. The direct polymer effect is in every stage, from a reduced growth rate, to monotonically decreasing energy levels at steady state and a reduced decay rate, with increasing polymer concentration (0 ppm is a freshwater benchmark case).
From enstrophy fields we could deduce the position of the sharp interface between the turbulent patch and its surrounding fluid. We observe a smaller patch, much smoother interface and the depletion of the length scales separation. An algorithm for patch interface detection is proposed and successfully applied to the PIV measurements, revealing the change in energy transfer towards and across the interface, along with additional physical measures of the patch evolution. The results will be used in developing an improved models of turbulent entrainment and possibly implemented in the applications that require a precise control of localized mixing rates.
School of Mechanical Engineering Seminar
Wednesday, March 25, 2015 at 15:00
Wolfson Building of Mechanical Engineering, Room 206
A Hyperelastic Micromechanical Model for Biocomposite Material:
Collagen Fiber in Alginate Matrix
Ofir Gilad
MSc Student of Prof. Rami Haj-Ali and Prof. Yehuda Benayahu
Soft tissues are biocomposite materials formed by biological matrix and fibers. Developing novel Biocomposite that will provide appropriate mimic of native tissues can have many practical uses in medicine. Tailoring the mechanical properties of the developed biocomposite for a certain use requires an accurate material model.
The purpose of this research is to find a material model for a biocomposite system of Alginate hydrogel matrix reinforced with Collagen fibers of coral origin.
The material model is a hyperplasic micromechanical one which describes the strain energy density of the system as a function of the volume fracture of the fibers.
In order to find the model for the system, the mechanical properties of the fibers in tension were characterized by conducting tensile experiments. The stress strain curve from the experiments was used for adjusting a hyperelastic model for the fibers. Tensile experiments for the matrix, taken from other research, were used for adjusting both linear and hyperelastic models for the matrix.
These material models for the fiber and matrix were implemented in final element model of unit cell in order to characterize the behavior of the material as a homorganic material in tension. Such unit cells, in deferent volume fracture of the fiber were modeled and the stress strain curve from each one was recorded
For every curve, a hyperelastic model was adjusted and the constants of the models were generalized in order to get to general models for the bio-composite material as a function of the fibers volume fracture. The two models show good agreement with experimental results of tensile tests conducted on the biocomposite.
