Titreşen diskten girdap halkası kopmalarının deneysel incelenmesi

Öz

Pah kenarlı titreşen bir diskten girdap kopmaları incelenmiştir. Deneyler yüksek hızlı bir Parçacık Görüntülemeli Hız Ölçümü (PGHÖ) sistemi kullanılarak 160 Hz görüntü toplama hızında yapılmıştır. Altı tam periyot, çevrim başına 333 hız ölçümü yapılarak ve toplamda 1999 görüntü alınarak kaydedilmiştir. Suda titreşen 175 mm çaplı diskin titreşim frekansı 0.5 Hz ve titreşim mesafesi 20 mm’dir. Rijid obje takibi ve görüntü stabilizasyon teknikleri kullanılarak disk görüntüsünü PGHÖ ham görüntülerinden çıkarabilmek için dinamik görüntü maskeleme uygulanmıştır. Burada bir koordinat değişimi söz konusudur ve bunun sayesinde disk yerinde çakılı duruyormuş gibi diskin etrafındaki akış alanı sonuçlarının incelenmesi mümkün olmuştur. Aynı zamanda istatiksel analizlerin ve faz kilitli ortalamanın kullanılması da mümkün olur. Sonuçlar her çevrimde girdap oluşumunun ve kopmasının çok düzenli ve öngörülür bir şekilde gerçekleştiğini göstermiştir. Pahlı disk kenarı geometrisinin girdap dinamiğine çok büyük etkisi vardır: Öteleme pahlı tarafa olduğunda diğer düz tarafta büyük ve yapışık bir girdap oluşurken, ötelemenin yönü değişip düz tarafa doğru olduğunda daha önce oluşan büyük ve yapışık girdap merkezden uzaklaşarak kopmuş ve yatay eksene göre 45° eğimi olan bir kayma tabakası gözlemlenmiştir. Bu da asimetrik disk kenarı geometrisinin tüm çevrimde net bir kuvvet oluşturduğu anlamına gelir.

Anahtar Kelimeler

• Girdap kopması
• Titreşen disk
• Parçacık görüntülemeli hız ölçümü
• Dinamik maskeleme
• Obje takibi
• Görüntü stabilizasyonu

Experimental investigation of vortex ring shedding from oscillating disc

Abstract

Vortex shedding from an oscillating disc with a chamfered tip is investigated. The experiments are performed using a high-speed Particle Image Velocimetry (PIV) system with 160 Hz frame acquisition frequency. Six full cycles are recorded with a total of 1999 recordings, i.e. 333 velocity measurements resolving a single cycle. The 175-mm-diameter disc oscillation frequency was 0.5 Hz within a 20 mm displacement in water. Dynamic image masking is performed to remove the disc shape from the PIV raw images, using rigid object tracking and image stabilization techniques. This implies a coordinate transformation and allows the investigation of flow field results with respect to the disc, as if it were stationary. This allows the use of statistical analysis and phase-locked averaging. The results indicate that the vortex formation and shedding from-cycle-to-cycle is very stable and predictable. The chamfered disc tip geometry has great influence in the vortex dynamics: When the motion is towards the chamfered side, a big, attached trailing vortex is present on the flat side; and when the motion is reversed towards the flat side the big trailing vortex is shed off outwards from the center, and a shear layer with a 45° orientation is formed. This means the asymmetric disc tip geometry generates a net force for the complete cycle.

Keywords

• Vortex shedding
• Oscillating disc
• Particle image velocimetry
• Dynamic masking
• Object tracking
• Image stabilization

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