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Citation - WoS: 103
Effects of Cylinder Reynolds Number on the Turbulent Horseshoe Vortex System and Near Wake of a Surface-Mounted Circular Cylinder
(Amer Inst Physics, 2015) Kirkil, Gökhan; Constantinescu, George
The turbulent horseshoe vortex (HV) system and the near-wake flow past a circular cylinder mounted on a flat bed in an open channel are investigated based on the results of eddy-resolving simulations and supporting flow visualizations. Of particular interest are the changes in the mean flow and turbulence statistics within the HV region as the necklace vortices wrap around the cylinder's base and the variation of the mean flow and turbulence statistics in the near wake in between the channel bed and the free surface. While it is well known that the drag crisis induces important changes in the flow past infinitely long circular cylinders the changes are less understood and more complex for the case of flow past a surface-mounted cylinder. This is because even at very high cylinder Reynolds numbers Re-D the flow regime remains subcritical in the vicinity of the bed surface due to the reduction of the incoming flow velocity within the bottom boundary layer. The paper provides a detailed discussion of the changes in the flow physics between cylinder Reynolds numbers at which the flow in the upstream part of the separated shear layers (SSLs) is laminar (Re-D = 16 000 subcritical flow regime) and Reynolds numbers at which the transition occurs inside the attached boundary layers away from the bed and the flow within the SSLs is turbulent (Re-D = 5 * 10(5) supercritical flow regime). The changes between the two regimes in the dynamics and level of coherence of the large-scale coherent structures (necklace vortices vortex tubes shed in the SSLs and roller vortices shed in the wake) and their capacity to induce high-magnitude bed friction velocities in the mean and instantaneous flow fields and to amplify the near-bed turbulence are analyzed. Being able to quantitatively and qualitatively describe these changes is critical to understand Reynolds-number-induced scale effects on sediment erosion mechanisms around cylinders mounted on a loose bed which is a problem of great practical relevance (e.g. for pier scour studies). (C) 2015 AIP Publishing LLC.
Citation - WoS: 4
Citation - Scopus: 4
Modelling of Swelling by the Fluorescence Technique in Kappa Carrageenan Gels
(Amer Inst Physics, 2011) Tari, Özlem; Pekcan, Önder
Kappa (-kappa) carrageenan gels prepared with various carrageenan concentrations in pure water were completely dried and then swelled in water vapor. Steady state fluorescence measurements were performed using a spectrometer equipped with temperature controller. Pyranine was embedded in kappa-carrageenan gels as a fluorescence probe during gel preparation. The fluorescence intensity I increased exponentially as swelling time is increased for all gel samples. The increase in I was modelled using Li-Tanaka equation from which swelling time constants tau(c) and cooperative diffusion coefficients D-c were determined. It was observed that D-c increased as the swelling temperature was increased. On the other hand at each temperature it was seen that D-c decreased as kappa carrageenan concentration was increased. Activation energies for swelling were obtained and found to be 57.4 58.3 and 62.73kJ mol(-1) for the gels with increasing amount of kappa-carrageenan content.
Citation - WoS: 50
Citation - Scopus: 58
A Numerical Study of the Laminar Necklace Vortex System and Its Effect on the Wake for a Circular Cylinder
(Amer Inst Physics, 2012) Kirkil, Gökhan; Constantinescu, George
Large eddy simulation (LES) is used to investigate the structure of the laminar horseshoe vortex (HV) system and the dynamics of the necklace vortices as they fold around the base of a circular cylinder mounted on the flat bed of an open channel for Reynolds numbers defined with the cylinder diameter D smaller than 4460. The study concentrates on the analysis of the structure of the HV system in the periodic breakaway sub-regime which is characterized by the formation of three main necklace vortices. Over one oscillation cycle of the previously observed breakaway sub-regime the corner vortex and the primary vortex merge (amalgamate) and a developing vortex separates from the incoming laminar boundary layer (BL) to become the new primary vortex. Results show that while the classical breakaway sub-regime in which one amalgamation event occurs per oscillation cycle is present when the nondimensional displacement thickness of the incoming BL at the location of the cylinder is relatively large (delta*/D > 0.1) a new type of breakaway sub-regime is present for low values of delta*/D. This sub-regime which we call the double-breakaway sub-regime is characterized by the occurrence of two amalgamation events over one full oscillation cycle. LES results show that when the HV system is in one of the breakaway sub-regimes the interactions between the highly coherent necklace vortices and the eddies shed inside the separated shear layers (SSLs) are very strong. For the relatively shallow flow conditions considered in this study (H/D congruent to 1 H is the channel depth) at times the disturbances induced by the legs of the necklace vortices do not allow the SSLs on the two sides of the cylinder to interact in a way that allows the vorticity redistribution mechanism to lead to the formation of a new wake roller. As a result the shedding of large-scale rollers in the turbulent wake is suppressed for relatively large periods of time. Simulation results show that the wake structure changes randomly between time intervals when large-scale rollers are forming and are convected in the wake (von Karman regime) and time intervals when the rollers do not form. When the wake is in the von Karman regime the shedding frequency of the rollers is close to that observed for flow past infinitely long cylinders.
Signatures of Phase Transitions in Nuclei at Finite Excitation Energies
(Amer Inst Physics, 2012) Alhassid, Yoram; Özen, Cem; Nakada, Hitoshi
The mean-field approximation predicts pairing and shape phase transitions in nuclei as a function of temperature or excitation energy. However in the finite nucleus the singularities of these phase transitions are smoothed out by quantal and thermal fluctuations. An interesting question is whether signatures of these transitions survive despite the large fluctuations. The shell model Monte Carlo (SMMC) approach enables us to calculate the statistical properties of nuclei beyond the mean-field approximation in model spaces that are many orders of magnitude larger than spaces that can be treated by conventional diagonalization methods. We have extended the SMMC method to heavy nuclei and used it to study the transition from vibrational (spherical) to rotational (deformed) nuclei in families of rare-earth isotopes. We have calculated collective enhancement factors of level densities as a function of excitation energy and found that the decay of the vibrational and rotational enhancements is well correlated with the pairing and shape phase transitions respectively.
Citation - WoS: 1
Citation - Scopus: 1
Sparsity Preserving Computation for Spectral Projectors
(Amer Inst Physics, 2011) Yetkin, E. Fatih; Dağ, Hasan
Several areas of applications such as model order reduction preconditioner design and eigenvalue problems for spectral projectors can be found in the literature. In this paper a fast and sparsity preserving approach for computing the spectral projectors is proposed. The suggested approach can be used in both Newton iteration and integral representation based methods. A comparison of the original and the suggested approaches in terms of computation time sparsity preservation and accuracy is presented in this paper.

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