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Farklı Delik Çaplarına Sahip Gözenekli Levhalar Boyunca Oluşan Serbest Salınımların İncelenmesi

Investigation of Self-Sustained Oscillations Along Perforated Surfaces which have Different Hole Sizes

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Abstract (2. Language): 
The shear flow of a fully turbulent boundary layer along a perforated plate, which is bounded by a closed cavity on its backside, can give rise to highly coherent, self-sustained oscillations. These oscillations are characterized in terms of velocity fluctuations and quantitative images of the instantaneous and averaged flow structure using a technique of highimage- density particle image velocimetry (PIV). Variations of the effective length L of the perforated plate show nearly invariant values of dimensionless frequency fL/U; in which f is the predominant frequency of oscillation and U is the freestream velocity. In fact, this relationship holds even when the diameter of the hole pattern is altered. Variation of the hole diameter D does, however, strongly influence the amplitude and degree of organization of the self-sustained oscillation. Four different plates were employed, with whole diameters of D = 6.4 mm, 12.7mm, 19.1mm and 25.4mm. The plate was maintained constant thickness at t =11mm for all experiments. The freestream velocity was maintained at a value U = 240 mm/s and the momentum thickness of the turbulent boundary layer was  =7.5 mm. [The Reynolds number based on was Re = 1800.] It is demonstrated that, as the hole diameter becomes larger relative to the inflow boundary layer thickness, the amplitude of the predominant spectral peak is substantially attenuated and, in a limiting case, undetectable.
Abstract (Original Language): 
Kapalı bir kavite üzerine yerleştirilmiş gözenekli bir levha boyunca oluşmuş tam türbülanslı sınır tabaka içerisinde kendiliğinden sürekli salınımlar oluşur. Bu çalışmada parçacık görüntülemeli (PIV) hız ölçme metodu kullanılarak anlık ve ortalama hız değerleri, girdap çizgileri, Reynolds gerilmeleri ve levha boyunca belli bir zaman aralığında ölçülen hız değerleri ile bu salınımlara ait üç boyutlu hız spektrumları verilmiştir. Gözenekli levha boyunca akış yönünde ölçülen basınç değerleri neticesinde hesaplanan boyutsuz frekansın fL/U hemen hemen sabit kaldığı görülmüştür. Burada f salınımın frekansını U da serbest akış hızını ifade etmektedir. Aynı şekilde boyutsuz frekansın gözenek çapının değişmesiyle de sabit olduğu gösterilmiştir. Gözenek çapı D nin değiştirilmesi periyodik salınımlara neden olmakta ve etkili bir şekilde salınımların genliğini etkilemektedir. Dört farklı gözenekli levha delik çapı D = 6.4 mm, 12.7mm, 19.1mm and 25.4mm deneylerde kullanılmıştır. Çalışmada serbest akış hızı U=240 mm olarak alınmış bu hıza karşılık gelen türbülanslı sınır tabaka momentum kalınlığı =7.5 mm olarak bulunmuştur. [Momentum kalınlığına bağlı Reynolds Sayısı = 1800 dir.] Gözenkli levhaların kalınlıkları sabit olarak t=11 mm olarak alınmıştır. Sınır tabaka kalınlığına göre gözenek çapı arttırıldığında salınımların genliğinin sönümlendiği ve frekansın belirsiz hale geldiği görülmüştür.

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