Velocity Streaks Research Articles

Unsteady Flow and Heat Transfer in a Plate-Fin and Tube Heat Exchanger

Fluid flow and heat transfer in a plate-fin and tube heat exchanger were studied, using a fundamental model consisting of a pair of parallel plates and a square cylinder passing perpendicularly through the plates, which simulated plate-fins and a tube, respectively. The analytical results are reported about the unsteady flow and heat transfer at the Reynolds number Re=1 000 and the Prandtl number Pr=0.71. The unsteady flow around a cylinder is shown in terms of time histories, velocity flucutuation intensity conoturs and the Strouhal numbers. Complicated aspects of the fluid flow are shown in terms of velocity vectors, streamlines, streak lines, pressure contours and wall shear stress contours. Furthermore, heat transfer characteristics under a constant wall heat flux condition are shown in terms of distributions of temperatures and local Nusselt numbers, and the mechanisms of fluid flow and heat transfer are systemtically examined in detail.

Bedforms produced by fine, cohesionless, granular and flakey sediments under subcritical water flows

ABSTRACTExperiments have been conducted in a 10 m long laboratory flume to investigate the bedforms which develop from fine, cohesionless sediment beds. Two grades of near uniformly sized silica grains (of median nominal diameters 15 and 66 μm) and six grades of micaceous flakes (ranging in median nominal diameter from 15.5 to 76 μm) were used. A steady subcritical water discharge, which was increased in steps after several hours, was applied to a flat bed of each grade. The developing bedform sequence for fine granular beds was identified as many small‐sized primary ripples, isolated primary, transverse primary, secondary ripples and then possibly dunes; this development was almost the same as that observed for coarser grains. The sequence for fine flake beds differed from grains. Only the single bedform type of parting lineations was observed; with increased discharge, the lineations began to oscillate and eventually enter into fluid suspension. The low discharge parallel lineations were thought to be generated by ‘streaks’ or lanes of transversely alternate high and low velocity fluid which have been reported to exist in the viscous sub‐layer of a turbulent‐smooth boundary, whilst the higher discharge wandering lineations were attributed to low velocity streak ‘bursts’.

Time-dependent viscous flow in a straight channel

An infinite straight channel, filled with an incompressible viscous fluid, is closed at one end by a piston. This is set in motion with finite acceleration and then maintained at constant velocity until the flow pattern in the fluid reaches a steady state. The development of velocity profiles, stream lines, and streak lines is investigated by direct numerical solution of the complete Navier Stokes equations. It is found that the nonconvex velocity profiles found in previous work on steady-state problems appear from the beginning, and their development is studied. In the downstream region alternative methods can be used which serve as a check on the accuracy of the numerical procedures. The asymptotic behaviour downstream is studied in some detail.