The turbulent frictional resistance of dilute polymer solutions can be as low as one-quarter that of the pure solvent. The reduction of drag can be associated with polymer properties and the hydrodynamics of the flow. This review illustrates the key concepts in hydrodynamics, polymer chemistry, and rheology which apply to polymer drag reduction. The effect of parts-per-million polymer concentrations on laminar-turbulent transition, boundary-layer properties and turbulent frictional resistance in pipe flow and along flat plates is emphasized, using experimental data from current literature. Industrial, fire-fighting, marine, and biological applications are discussed.

By the use of high-speed photography, instabilities occurring in high Reynolds number water jets discharging into air have been made visible. These instabilities include the axisymmetric mode accompanying the transition from laminar to turbulent flow at the nozzle exit, spray formation as a culmination of the axisymmetric disturbances, and, further downstream, helical disturbances which result in the entire jet assuming a helical form. The final disruption of the jet is due to amplification of the helical waves. It is further shown that the amplification of the helical disturbances is due in part to aerodynamic form drag, since jets discharging into surrounding air moving at the same speed as the jet remain relatively stable, compared with the case when the jet is discharged into stagnant air.

No abstract

No abstract

No abstract