Decreasing pressure losses in microtubes is an important subject, especially when an increase in the flow capacity is required. In the same pumping power, decreasing the pressure drops coincides with increasing the fluid flow rate passing through a microtube. The current study aims to examine how the water pressure drop across a microtube can be affected by the addition of small concentrations of different Drag-reducing polymeric agents. A kind of the laminarization behaviour was found by the addition of the selected polymer to the turbulent flow. The effect of the concentration of the Drag-reducing agent (DRA) on the observed viscosity and Drag values are reported. Moreover, the effects of different types of Drag-reducing agents on the friction factor are obtained at the best concentration in the range of examined concentrations. Under this condition the percentage of the Drag reduction and increase in the capacity of the flow were respectively 33 % and 36 %. The obtained results show that more Drag reduction occurs at a higher flow rate for 20 ppm of the polyacrylamide with a high molecular weight.

The objective of this paper is to examine the effectiveness of wall oscillation as a control scheme of Drag reduction. Two flow configurations are considered: constant flow rate and constant mean pressure gradient. The Navier-Stokes equations are solved using Fourier-Chebyshev spectral methods and the oscillation in sinusoidal form is enforced on the walls through boundary conditions for the spanwise and streamwise (for the case of inclination cycle) velocity components. Results include the effects of oscillation frequency, amplitude, oscillation orientation, and peak wall speed on Drag reduction at a Reynolds number of 180 based on wall-shear velocity and channel half-width as well as the Reynolds number dependency in both flow configurations. Drag reduction as a function of peak wall speed is compared with both experimental and numerical data and the agreement is good in the trend and in the quantity. Comparison between these two flow configurations in the transient response to the sudden start of wall oscillation, turbulence statistics, and instantaneous flow fields is detailed and differences are clearly shown. This analysis and comparison have allowed some light to be shed on the way that oscillations interact with wall turbulence.

The paper presents the results of an experimental investigation wherein the bullet form Drag force as a function of oscillating actuator frequency, various voltage and for different orifice/slot configuration are studied. In order to perform the experiment, an axisymmetric bullet shape model with ellipsoidal nose was used in wind tunnel. The synthetic jet actuator was used to flow control at sharp cut end. The experiment was conducted in a wind tunnel with a working diameter of 1000 mm and a maximum velocity of 45 m/s. The measurements were carried out for the Reynolds number from 88000 to 352000 and for relatively large Strouhal numbers up to St = 4. 5 based on model external diameter and free stream velocity. While synthetic jet was switched on, Drag coefficient has been reduced by-6% and increased by +22% in relation to the case with the synthetic jet was switched off. The synthetic jet has more impact for relatively low free stream velocity and for single axisymmetric orifice.