Dr Zaman Kamruzzaman

Hot-wire velocity measurements are carried out in a turbulent boundary layer over a rough wall consisting of transverse circular rods, with a ratio of 8 between the spacing (w) of two consecutive rods and the rod height (k). The pressure distribution around the roughness element is used to accurately measure the mean friction velocity (Ut) and the error in the origin. It is found that Ut remained practically constant in the streamwise direction suggesting that the boundary layer over this surface is evolving in a self-similar manner. This is further corroborated by the similarity observed at all scales of motion, in the region 0.2=y/d=0.6, as reflected in the constancy of Reynolds number (R¿) based on Taylor's microscale and the collapse of Kolmogorov normalized velocity spectra at all wavenumbers.A scale-by-scale budget for the second-order structure function <(du)2> (du=u(x+r)-u(x), where u is the fluctuating streamwise velocity component and r is the longitudinal separation) is carried out to investigate the energy distribution amongst different scales in the boundary layer. It is found that while the small scales are controlled by the viscosity, intermediate scales over which the transfer of energy (or <(du)3>) is important are affected by mechanisms induced by the large-scale inhomogeneities in the flow, such as production, advection and turbulent diffusion. For example, there are non-negligible contributions from the large-scale inhomogeneity to the budget at scales of the order of ¿, the Taylor microscale, in the region of the boundary layer extending from y/d=0.2 to 0.6 (d is the boundary layer thickness).

This paper presents a numerical study of an investigation of a fluid flow through a rotating rectangular straight duct in the presence of magnetic field. The straight duct of rectangular cross-section rotates at a constant angular velocity about the centre of the duct cross-section is same as the axis of the magnetic field along the positive direction in the stream wise direction of the flows. Numerical calculation is based on the Magneto hydrodynamics incompressible viscous steady fluid model whereas Spectral method is applied as a main tool. Flow depends on the Magnetic parameter, Dean number and Taylor number. One of the interesting phenomena of the fluid flow is the solution curve and the flow structures in case of rotation of the duct axis. The calculation are carried out for 5 = Mg = 50000, 50 = Tr 100000, Dn 500, 1000, 1500 and 2000 where the aspect ratio ¿ 3.0. The maximum axial flow will be shifted to the centre from the wall and turn into the ring shape under the effects of high magnetic parameter and large Taylor number whereas the fluid particles strength is weak. © 2013 The Authors. Published by Elsevier Ltd.

The decay of turbulence in a shearless mixing layer generated from the junction of side by side grids with different mesh sizes but identical solidity is being investigated using hot wire anemometry. It is observed that turbulence decays according to a power-law, albeit, with a different power-law exponent (n) for each grid. The measurements suggest the existence of turbulent energy transfer from the larger mesh region to the smaller mesh region at distances as large as 75 ML from the grid, where ML is the mesh size of the larger mesh grid. It is further observed that the Reynolds number R¿ remains constant along the centreline of the flow (i.e. the junction of the two grids), confirming that self-preservation is satisfied in this region of the flow. This is supported by the one dimensional velocity spectra Eu(k1). On the centreline, the measured energy spectra at positions x/ML = 45 collapse onto a single curve at all wavenumbers when scaled by either the Kolmogorov velocity and length scales or the rms velocity (u!) and Taylor microscale (X). Away from the centreline the spectra do not present such collapse.

Starting with the Navier-Stokes Equation (NSE), we derived the conditions for self-preservation (SP) in a zeropressure gradient (ZPG) turbulent boundary layer. The analysis showed that it is strictly not possible to obtain SP in a ZPG turbulent boundary layer, unless the viscous term is eliminated from the NSE. This can be achieved in a smooth wall boundary layer only when the Reynolds number (Re) approaches infinity. In the case of rough walls, it is noted that the viscous effects can be compensated by surface roughness and therefore, SP is achievable, irrespective of Re. In this case, SP analysis showed that velocity scale (u*) must be constant and the length scale (l) should vary linearly with streamwise distance (x). These SP conditions are tested using experimental data taken over a similar streamwise fetch on a smooth wall and several types of rough walls. It is observed that complete SP in a ZPG turbulent boundary layer is possible when the roughness height (¿) increases linearly with x, where both the SP constraints (u* =UT = constant and l = d ¿ x) are met. In the present rough wall study, UT is observed to remain practically constant in x and d ~ x and appears to be the next best candidate for achieving SP.

This present work is to investigate on the decay exponent (n) of decay power law (q' 2~(t - To)n , q'2 is the total turbulent kinetic energy, t is the decay time, t0 is the virtual origin) at low Reynolds numbers based on Taylor microscale R¿(= u '¿/ v) = 64 . Hot wire measurements are carried out in a grid turbulence subjected to a 1.36:1 contraction. The grid consists in large square holes (mesh size 43.75 mm and solidity 43%); small square holes (mesh size 14.15mm and solidity 43%) and woven mesh grid (mesh size 5mm and solidity 36%). The decay exponent (n) is determined using three different methods: (i) decay of q'2, (ii) transport equation for s , the mean dissipation of the turbulent kinetic energy and (iii) ¿ method (Taylor microscale ¿ = v5( q2)/ (ed)} , angular bracket denotes the ensemble). Preliminary results indicate that the magnitude n increases while R¿ (= u'¿/v)decreases, in accordance with the turbulence theory.

In this paper, we present the results from a turbulent boundary layer developing over a rough surface. The surface consists of transverse cylindrical rods (k, the rod diameter) that are periodically arranged in the streamwise direction with a spacing of ¿/k = 8 (¿ is the distance between two adjacent roughness elements), that results in maximum form drag. Particular attention is paid to the measurement of the friction velocity (Ut) that plays a major role in the assessment of the roughness effects on the flow. Hot-wire anemometry is used to measure the mean and fluctuating velocity components and pressure tap measurements are carried out to obtain the drag. Two methods are used to determine Ut. One is based on the momentum integral equation. The second relies on measuring the pressure distribution around one roughness element. Results show that both methods give consistent values for Ut to within 3%. Further, the drag coefficient (CD) is observed to be independent of the Reynolds number.