A novel signal wave pattern is proposed to generate a more efficient synthetic jet, and the unsteady and mean flow characteristics of the synthetic jet generated by different suction duty cycle factors k are examined by numerical simulation. The mean flowfields of the synthetic jet with different k have similar features to that exhibited in previous experiments, but they would have a stronger and larger scale vortex pair with a larger suction duty cycle factor k, when the two key parameters, the stroke length ratio L 0 =D and the Reynolds number Re U 0 (based on the characteristic blowing velocity U 0 and the orifice scale D), are set to be constant. The suction duty cycle factor k affects the vortex strength formed during the blowing cycle only, and nearly does not influence the convection velocity and trajectory of the vortex. During the suction cycle, the entrainment effect of the actuator decreases with the increase of suction duty cycle factor k, so that the vortex pair can propagate farther and coalesce to synthesize a larger scale jet.

No abstract

Experimental studies on the drag reduction of the circular cylinder were conducted by pressure measurement at a Reynolds number of 82 000 (based on the cylinder diameter). A rod was placed upstream of and parallel to the cylinder to control the flow around the cylinder. The upstream rod can reduce the resultant force of the cylinder at various spacing between the rod and the cylinder for α < 5 • (α defined as the staggered angle of the rod and the cylinder). For α > 10 • , the resultant force coefficient has a large value, so the upstream rod cannot reduce the force on the cylinder any more. For α = 0 • and d/D = 0.5 (where d and D are the diameter of the rod and the cylinder, respectively), the maximum drag of the cylinder reduces to 2.34% that of the single cylinder. The mechanism of the drag reduction of the cylinder with an upstream rod in tandem was presented by estimating the local contributions to the drag reduction of the pressure variation. In the staggered arrangement, the flow structures have five flow patterns (they are the cavity mode, the wake splitting mode, the wake merge mode, the weak boundary layer interaction mode and the negligible interaction mode) according to the pressure distribution and the hydrogen bubble flow visualization. The half plane upwind of the cylinder can be divided to four regions, from which one can easily estimates the force acting on the circular cylinder with an upstream rod in staggered arrangement.

No abstract

No abstract