Experimental Investigation on Flow Structures behind Tandem Bluff Bodies

Abstract

The vortex shedding phenomenon in the wake region behind bluff bodies is the most significant feature of flow around these bodies. One consideration regarding this phenomenon is the induced unsteady loading on the structure which can lead to possible structural damages. Therefore, to prevent any damages, the engineers are required to consider this phenomenon during design stages of industrial systems. In this study, vortex shedding in the wake region of single and dual bluff bodies have been investigated in detail. Firstly, the effects of entrainment of fluid through the hollow space in square and triangular cylinders have been studied experimentally. The flow structures have been investigated in terms of velocity components, turbulent kinetic energy and similarities and dissimilarities in coherent and incoherent structures have been addressed in detail. It was found that the wake region behind the U shape cylinder exhibits a delay in the wake recovery in comparison to the square cylinder. It was observed that the coherent turbulent kinetic energy (TKE) peak of U shape cylinder is about 25 % lower than the coherent TKE peak of square cylinder in the near wake region while on average it is about 15 % higher in the far wake region. Similarly, it was also observed that the wake region behind the L shape cylinder exhibits a delay in the wake recovery in comparison to the triangular cylinder. It was seen that the coherent TKE peak of L shape cylinder is about 22 % lower than the coherent TKE peak of the triangular cylinder in the near wake region while on average it is about 20 % higher in the far wake region. Moreover, interacting wakes of two inclined flat plates in tandem arrangement have been studied experimentally. The effects of gap ratio (g/D) between the plates and the angle of attack (α) on the flow structure in the wake region have been investigated. Different set of experiments have been conducted for gap ratios 0.5 and 1.0 between the plates. In addition, various angles of attack (70-90 degrees) have been considered to probe the effects of inclination on the wake region in terms of the shedding frequency and the Strouhal number variation. The results revealed that both the coherent TKE peak production and the Strouhal number decrease as the angle of attack increases. Moreover, it was observed that for a given angle of attack ࢻ, the Strouhal number increases as the gap ratio g/D increases. Furthermore, the coherent TKE peak production decreases as the gap ratio g/D increases in cases of the angle of attack of 70 and 80 degrees while in case of the angle of attack of 90 degree, the coherent TKE peak production increases as the gap ratio g/D increases. Keywords: Vortex shedding, Incoherent and coherent flow structure, Triangular cylinder, Inclined flat plate, Interacting wake, Tandem arrangement

ÖZ: Cisimlerin arkasındaki dalga alanındakı girdap oluşumu cisimleri etrafındakı akışın en belirgin özelliklerden biridir. Bu oluşum yapılar üzerinde buna bağlı oluşan karasız yüklemelere sebebiyet verir ki bunlar da yıkıma sebebiyet verebilirler. Bu yüzden mühendisler endüstriyel yapıları tasarlarken oluşabilecek zararları önlemek için bu konuyu dikkate almaktadırlar. Bu çalışmada tek ve çift gövdelerin dalga alanında oluşan girdapları detaylı bir şekilde incelenmiştir. Öncelikle akışkanın kare ve üçgen silindirler arasına nüfuz etmesinin etkileri deneysel olarak araştırılmıştır. Akış yapıları hız komponenti, türbülans kinetik enerjisi, koherent ve inkoherent yapılardaki benzerlikler ve farklılıklar araştırılmıştır. U kesitli silindir kare şeklindeki silindire göre dalga düzenlemelerinde gecikme göstermektedir. Ayrıca koherent türbülans kinetik enerjisinin U kesitli silindirde kare silindire göre dalga yakın bölgelerinde % 25 az olduğu ve uzak dalga bölgelerinde ise % 15 fazla olduğu gözlemlenmiştir. Ayni şekilde L kesitli silindirin dalga düzenlemesinin üçgen silindire göre daha geç oluştuğu gözlemlenmiştir. Koherent türbülans kinetik enerjisinin L kesitli silindirede üçgen silindire göre yakın dalga bölgesinde % 22 az olduğu ve uzak dalga bölgesinde ise % 20 yüksek olduğu gözlemlendi. Bunun yanında arka arkaya yerleştirilmiş iki eğimli düz plakanın dalga etkileşimleri deneysel olarak incelenmiştir. Anahtar Kelimeler: Vorteks oluşumu, Inkoherent ve koherent akış yapıları, Üçgen silindir, Eğimli dik plaka, Etkileşimli dalga, Tandem düzen