Abstract: Non-Newtonian fluid properties can change the flow
behaviour significantly, its prediction is more difficult when thermal
effects come into play. Hence, the focal point of this work is the
wake flow behind a heated circular cylinder in the laminar vortex
shedding regime for thermo-viscous shear thinning fluids. In the case
of isothermal flows of Newtonian fluids the vortex shedding regime
is characterised by a distinct Reynolds number and an associated
Strouhal number. In the case of thermo-viscous shear thinning
fluids the flow regime can significantly change in dependence of
the temperature of the viscous wall of the cylinder. The Reynolds
number alters locally and, consequentially, the Strouhal number
globally. In the present CFD study the temperature dependence of
the Reynolds and Strouhal number is investigated for the flow of a
Carreau fluid around a heated cylinder. The temperature dependence
of the fluid viscosity has been modelled by applying the standard
Williams-Landel-Ferry (WLF) equation. In the present simulation
campaign thermal boundary conditions have been varied over a
wide range in order to derive a relation between dimensionless heat
transfer, Reynolds and Strouhal number. Together with the shear
thinning due to the high shear rates close to the cylinder wall
this leads to a significant decrease of viscosity of three orders of
magnitude in the nearfield of the cylinder and a reduction of two
orders of magnitude in the wake field. Yet the shear thinning effect
is able to change the flow topology: a complex K´arm´an vortex street
occurs, also revealing distinct characteristic frequencies associated
with the dominant and sub-dominant vortices. Heating up the cylinder
wall leads to a delayed flow separation and narrower wake flow,
giving lesser space for the sequence of counter-rotating vortices. This
spatial limitation does not only reduce the amplitude of the oscillating
wake flow it also shifts the dominant frequency to higher frequencies,
furthermore it damps higher harmonics. Eventually the locally heated
wake flow smears out. Eventually, the CFD simulation results of the
systematically varied thermal flow parameter study have been used
to describe a relation for the main characteristic order parameters.
Abstract: For a bluff body, roughness elements in simulating a turbulent boundary layer, leading to delayed flow separation, a smaller wake, and lower form drag. In the present work, flow past a circular cylinder with using tripping wires is studied experimentally. The wind tunnel used for modeling free stream is open blow circuit (maximum speed = 30m/s and maximum turbulence of free stream = 0.1%). The selected Reynolds number for all tests was constant (Re = 25000). The circular cylinder selected for this experiment is 20 and 400mm in diameter and length, respectively. The aim of this research is to find the optimal operation mode. In this study installed some tripping wires 1mm in diameter, with a different number of wires on the circular cylinder and the wake characteristics of the circular cylinder is studied. Results showed that by increasing number of tripping wires attached to the circular cylinder (6, 8, and 10, respectively), The optimal angle for the tripping wires with 1mm in diameter to be installed on the cylinder is 60̊ (or 6 wires required at angle difference of 60̊). Strouhal number for the cylinder with tripping wires 1mm in diameter at angular position 60̊ showed the maximum value.
Abstract: A two-dimensional numerical study for flow past a square cylinder in presence of flat plate both at upstream and downstream position is carried out using the single-relaxation-time lattice Boltzmann method for gap spacing 0.5 and 1. We select Reynolds numbers from 80 to 200. The wake structure mechanism within gap spacing and near wake region, vortex structures around and behind the main square cylinder in presence of flat plate are studied and compared with flow pattern around a single square cylinder. The results are obtained in form of vorticity contour, streamlines, power spectra analysis, time trace analysis of drag and lift coefficients. Four different types of flow patterns were observed in both configurations, named as (i) Quasi steady flow (QSF), (ii) steady flow (SF), (iii) shear layer reattachment (SLR), (iv) single bluff body (SBB). It is observed that upstream flat plate plays a vital role in significant drag reduction. On the other hand, rate of suppression of vortex shedding is high for downstream flat plate case at low Reynolds numbers. The reduction in mean drag force and root mean square value of drag force for upstream flat plate case are89.1% and 86.3% at (Re, g) = (80, 0.5d) and (120, 1d) and reduction for downstream flat plate case for mean drag force and root mean square value of drag force are 11.10% and 97.6% obtained at (180, 1d) and (180, 0.5d).