Active Control of a Circular Cylinder Flow at Transitional Reynolds Numbers

Abstract

Active and passive control of flow around a circular cylinder, at transitional Reynolds numbers was investigated experimentally by measuring cylinder surface pressures and wake velocity profiles. Two-and three-dimensional passive boundary layer tripping was considered and periodic active control using piezo-fluidic actuators was introduced from a two-dimensional slot that was nearly tangential to the cylinder surface. The slot location was varied circumferentially by rotating the cylinder and this facilitated either upstream-or downstream-directed actuation using sinusoidal or modulated waveforms. Separation was controlled by two distinct methods, namely: by forcing laminarturbulent transition when applied at relatively small angles (30-60°) from the forward stagnation point; and by directly forcing the separated shear-layer at larger angles. In the latter case, actuation produced the largest load changes when it was introduced at approximately 90°from the forward stagnation point. When the forcing frequency was close to the natural vortex-shedding frequency, the two frequencies "locked-in" creating clear and persistent structures. These were examined and categorized. The "lock-in" effect lowered the base pressure and increased the form-drag whereas delaying separation from the cylinder did the opposite. Nomenclature bslot width C d drag coefficient, d=q 1 D C dp form drag coefficient, dp=q 1 D Flow Turbulence Combust (2007) 78:

A direct numerical simulation investigation of the synthetic jet frequency effects on separation control of low-Re flow past an airfoil

“…He observed that the periodic forcing by the jet breaks the large-scale separated vortices into smaller ones, delaying the separation point and resulting in an improved aerodynamic performance. The experimental investigation by Naim et al 19 concluded that, for flow over a circular cylinder, the control effects are not only determined by the position and forcing frequency of the jet but also by its direction with respect to the oncoming cross-flow. The SJ opposing the cross-flow is more effective in promoting transition at low momentum coefficient, thus reducing the laminar flow separation.…”

Passive control of flow structures around a circular cylinder by using screen

“…The various passive control devices were comprehensively discussed by Zdravkovich (1997). For the latter, heating the cylinder (Lecordier et al, 1991), rotary oscillation of the cylinder at an appropriate frequency (Lee and Lee, 2008;Naim et al, 2007), acoustics excitations (Blevins, 1985), and control of the electromagnetic force (Kim and Lee, 2001) can be remarked as the sample investigations. The passive control methods (Lam et al, 2010;Zhou et al, 2011) are still developed since there is no external energy input and no feedback sensor; furthermore they are easier to carry out than the active control methods which require actuation as well as sensing.…”

Flow-control-induced vibrations for power generation using pulsed plasma actuators

“…The lowest power systems are typically boundary layer trips, flaps or spoilers, because these generally require power input only during deployment. For example, Naim et al (2007) showed that a passive boundary layer trip could produce two-dimensional lift coefficient C l variations of 0.81, while Low et al (1991) used a passive flap to produce C l E1.05 (see Table 2). …”

“…Amitay et al, 1998;Naim et al, 2007) with C l E1 (see Table 2). These lateral loads, however, did not show a significant benefit over the DBD plasma actuators employed here.…”

“…Method Control parameter C L DC D Low et al (1991) Flap/spoiler -1.05 0.95 Naim et al (2007) Passive trip -0.81 0.52 Amitay et al (1998) Normal ZMF blowing C m ¼0.0006 0.95 0.26 Naim et al (2007) Tangential ZMF blowing C m ¼0.092 1.25 0.6 Dunham (1968) Steady slot blowing C m ¼0.56 7.9 - Low et al (1991) Flap plus steady suction C Q ¼ 0.20 5.65 1.3…”