Effect of vortex generators to the flow around circular cylinder

Abstract

Flow around a circular cylinder has been one of the basic research subjects in hydro-aerodynamics area in almost a hundred years. Related to its complex flow characteristics there are many aspects that make this topic most interesting including surface curvature of the cylinder, severe adverse pressure gradient, massive flow separation, oscillation of the separation point, unstable wake and hence vortex shedding, high sensitivity to free stream properties, aspect ratio, blockage, vortex street formation and completely variable flow pattern between flow regimes. Besides, boundary layer separation causes large energy losses especially by changing the flow pattern around bluff bodies. Accordingly, the performance of many hydro-aerodynamic vehicles is controlled by the location of the separation point. Thus, besides the importance of the separation control as a basic research subject, it can be regarded as an important engineering problem. One of the most effective methods used for preventing and delaying flow separation is the use of passive vortex generators. The macro scale motions of the vortices embedded in the boundary layer caused by the vortex generators provide over tuning of the flow in the vicinity of the wall. Thus, two important subjects of the fluid mechanics are considered in this study. The aim of the study is to experimentally investigate the effect of the streamwise vortex generators on the flow around circular cylinder and vortex formation mechanism. Wake oscillations of the circular cylinder and vortex shedding is visualized with digital PIV system which is the most suitable measurement system for determination of the general flow topology such as vortex structures and stress distribution. The experiments are conducted at Emerson Cavitation Tunnel, School of Marine Science and Technology, Newcastle University. The study consists of five cases including bare cylinder and four cases with vortex generators in which the generators are located at 50, 60, 65, and 70 degrees angles from the first stagnation point. The bare cylinder and vortex generators cases are coded as VG0000, VG1650, VG1660, VG1665, and VG1670, respectively. Vane type vortex generators which are 1.6 mm in height are paced with ±10 degrees sweep angles on the cylinder. The Reynolds number investigated is 41300, which corresponds to the sub-critical flow state. All the measurements were conducted at the midspan of the cylinder and the measurement area according to the cylinder diameter was 2.6Dx0.9D, in X and Y directions, respectively. The free stream velocity was 0.588 m/s and the inlet turbulence intensity and turbulence length scale of the free stream at this velocity were 1.73% and 0.22D, respectively. The analysis of the flow fields extracted from the PIV images are processed with two different turbulent flow field decomposition techniques including Reynolds decomposition, and Proper Orthogonal Decomposition (POD). The flow properties such as vorticity, stress distribution and oscillation frequency are derived from the analysis. The results of the measurements conducted for the bare cylinder are compared with the data in the open literature. These indicate that the results are in good agreement with the results of the similar studies in literature. Experimental results show that the vortex generators highly affect the near wake and hence the boundary layer of the cylinder. First of all, Reynolds averaged flow fields indicate that the shear layers get closer to each other, bend towards the centerline, and the width of the near wake decreases. These events clearly indicate that the flow separation is delayed on the measurement plane. One can observe that the vortex formation length and the wake closure length are increased considerably with the downstream motion of the separation location as shown in the literature. The amount of the global mean circulation in the flow fields is also increased because of the vortex generators while the width of the shear layers is decreased. The more concentrated pattern of the shear layers provide the vortex formation process to occur at a longer distance from the cylinder. It is expected that the increase in the vortex formation length causes the low pressure area behind the cylinder to move with the vortices and hence decreases the suction at the rear side of the cylinder. With the displacement of the shear layers towards the wake centerline and the thinning of the shear layers, the Strouhal number is increased approximately 41% at VG1650, and then is decreased slowly probably due to the reduction of strength of the forming vortex. It is obvious that the strength of the stresses due to both turbulent and coherent motions decreases because of the vortex generators.

Özet

Dairesel silindir etrafındaki akım, son derece karmaşık yapısı ve mühendislik uygulamalarında sıkça karşılaşılan bir akım tipi olması nedeniyle, yaklaşık bir asırdır hidro-aerodinamik alanda araştırmacıların temel konularından birisi olmuştur. Öte yandan, sınır tabaka ayrılması özellikle dolgun cisimler etrafındaki akışı değiştirerek büyük miktarda enerji kaybına yol açan bir olaydır. Buna bağlı olarak, hidro-aerodinamik pek çok aracın performansı akım ayrılmasının yeri tarafından kontrol edilmektedir. Akım ayrılmasının engellenmesi ya da geciktirilmesi için kullanılan en etkili yöntemlerden biri pasif girdap yaratıcılardır. Girdap yaratıcılar, neden oldukları sınır tabaka içine gömülen girdaplardan dolayı oluşan makro ölçekteki hareketler ile duvar yakınındaki akışın yeniden düzenlenmesini sağlamaktadır. Çalışmadaki amaç, girdap yaratıcıların dairesel silindir etrafındaki akışa ve girdap oluşma mekanizmasına olan etkilerinin deneysel olarak ortaya konmasıdır. Çalışmada, dairesel silindirin yakın iz bölgesi DPIV ile görüntülenmiştir. Deneyler, Newcastle Üniversitesi Emerson Kavitasyon Tüneli’nde gerçekleştirilmiştir. Çalışmada, 1.6 mm yüksekliğinde, akış yönüne ±10 derece açı yapacak şekilde kullanılan girdap yaratıcıların silindir çevresine dört farklı açıda yerleştirildikleri vakalar ve girdap yaratıcı içermeyen yalın silindir vakası yer almaktadır. Çalışmadaki Reynolds sayısı 41300 ve buna karşılık gelen silindir akış rejimi ise kritik-altı rejimdir. Ölçümlenen akış alanlarının analizi, Reynolds Ayrıştırması ve POD olmak üzere, iki ayrı türbülanslı akış alanı ayrıştırma tekniği ile gerçekleştirilmiştir. İz bölgesindeki girdaplılık, gerilme dağılımı, salınım frekansı gibi pek çok akış özelliği ortaya konmuştur. Yalın silindir vakası için yapılmış olan ölçümler literatür ile karşılaştırılmıştır. Girdap yaratıcıların etkisi, vakaların kendi aralarında ve yalın silindir ile karşılaştırmaları ile belirlenmiş, sonuçlar tartışılmıştır.