Control of vortex shedding on a circular cylinder using self-adaptive hairy-flaps

Abstract

Experiments on separation control using flexible self-adaptive hairy-flaps are presented herein. The wake-flow behind a circular cylinder is investigated without and with flexible hairy-flaps at the aft-part of the cylinder. Flow dynamics and hair motion were measured by particle image velocimetry and image processing in a range of Reynolds number 5000 < Re < 31 000. The experiments and POD analysis show, that the hairy-flaps alter the natural vortex separation cycle in such a way that the vortices do not shed in a zigzag like arrangement as in the classical von Kármán vortex street but in line in a row with the cylinder wake axis. Thus, the wake-deficit is largely reduced. Furthermore, flow fluctuations are considerably reduced about 42% in streamwise and 35% in transversal direction compared to the reference case without hairy-flaps, too. The condition for this mode change is the lock-in of the vortex shedding with a traveling wave running through the flexible hair bundles in transversal direction at the aft-part of the cylinder. As a consequence, the vortex shedding frequency is increased, the length of the separation bubble is decreased and drag force is decreased, too. The lock-in appears as a jump-like change of the shedding frequency and a jump in the Strouhal-Reynolds number diagram. However, when the characteristic length for the normalized frequency is chosen as the length of the separation bubble instead of the cylinder diameter, the Str-Re dependence is regular again. This hints on the relevance of the resonator model as proposed by Sigurdson and Roshko (1988) [16] on vortex shedding mechanism when boundary conditions are changed such as in our case, where the hairy-flap bundle imposes a flexible wall with visco-elastic coupling in transversal direction.

The PELskin project: part IV—control of bluff body wakes using hairy filaments

“…In particular, their device consisting of a couple of flaps made from the combination of a rigid plastic skeleton coated with a porous fabric mimicking the shaft and the vane of the birds feathers, led to a net drag reduction of about 22 %. At the same time, experimental studies on circular cylinders with hairy flaps on it proved that these flaps are able to modify the shedding cycle [9]. The study showed a characteristic jump in the shedding frequency at a critical Reynolds number of Re c % 14;000 when comparing to the classical behaviour of a plain cylinder wake flow.…”

“…The analysis of the motions of the hairy-flaps showed that for Re ¼ Re c the amplitude of the flap motion is considerably increased and a characteristic travelling wave-like motion pattern could be observed along the row of flaps. As a consequence the presence of the hairy flaps alter the phase within the vortex shedding cycle such that the transversal dislocation (transversal distance from the centerline) of the shed vortices is reduced [9]. Accordingly, the vortices are not arranged in a classical zig-zag pattern of the Kármán vortexstreet, rather they are shed in a row along the centerline.…”

“…The focus is again the wake of a cylinder where flexible structures are attached to the aft part. The details of the experiments are described in [9]. All flow measurements were carried out in a range of Reynolds number 5000 Re D 31;000 which covers the range where the shedding frequency passes the resonance frequency of the first bending mode.…”

Influence of self-adaptive hairy flaps on the stall delay of an airfoil in ramp-up motion

“…Schatz et al (2004) have shown that a self-activated spanwise flap near the trailing edge can enhance lift by more than 10% at a Reynolds number of Re=1-2×10 6 . Schlüter experiments involving a cylinder with flexible flaps similar to those used in the present study (Kunze and Brücker, 2012). A numerical study of the effect of hairy coatings on an NACA0012 airfoil was performed by Venkataraman and Bottaro (2012).…”

Altıgen Delikli Silindirin Geçirgenlik Oranının Silindir Arkasındaki Akış Bölgesine Etkisi

“…Gu ve arkadaşları [9] silindir arkasındaki ayırıcı plakanın serbest bir şekilde hareket edebildiği sistemde akış yapısını gözlemlemişlerdir. Kunze ve Brücker [10] çalışmalarında, dairesel silindir arkasında esnek ve hareketli plakalar kullanarak akış dinamiklerini PIV yöntemiyle 5000<Re<31000 aralığında incelemişlerdir. Plakaların girdap kopmalarını değiştirmede etkili olduğunu ve plakasız duruma göre akış yapısının salınımın akış yönünde %42 ve akış eninde %35 oranlarında azaldığını belirtmişlerdir.…”