An Experimental Study of Amplitude and Frequency Effects upon a Pulsating Jet

Abstract

Pulsating jets are very common and useful in various industrial aspects. However, they have some different basic characteristics from steady jets. In this research, the authors reveal both the frequency and the amplitude effects of pulsations on a jet, including large-amplitude cases. Experiments are conducted at a Reynolds number of 5,000, Strouhal numbers of 0.011 -0.27 and velocity-amplitude ratios of 0.1 -1.0. Using a hot-wire anemometry, the authors show centre velocities, radial profiles, half widths and total flow rates at several positions downward a nozzle exit. From centre velocities and radial profiles, we can see that the streamwise scale of the potential core is less than three times the diameter. From half widths and total flow rates, we can see that the pulsation enhances mixing, and confirm the frequency and the amplitude effects specifically. On the other hand, from centre velocities and radial profiles, we cannot see the both effects clearly. As a result, the authors show the combined frequency-and-amplitude effect upon total flow rate, which consists with small-amplitude results by Crow and Champagne (1971), and find the optimum frequencies.

Consecutive PIV Analyses on a Finite-Amplitude Pulsating Jet

“…We can expect some qualitative similarities between those results, considering ref. (4) On the other hand, the present study will show the difference between those results in the convection speeds of ring vortices, with which we will deal elsewhere. where t is time.…”

“…In general, the random high-frequency fluctuations become relatively negligible at larger pulsation amplitudes, while the distortions diminish at smaller pulsation amplitudes (see ref. (4) ).…”

“…Until now, there have been some studies on the pulsating jet (see ref. (4) ). Recently, the authors have systematically conducted a series of experiments concerning both the frequency effect (the Strouhal number St effect) and the amplitude effect (velocity-amplitude ratio α effect) for large-amplitude pulsations upon a circular jet, and found out the optimum St.…”

“…However, in small-amplitude conditions, turbulence intensity is suitable as an indicator rather than the total flow rate, because the total flow rate is not sensitive (see ref. (4) ). Of course, it is useful to discuss the consistency between small-amplitude and large-amplitude results.…”

Quantitative Measurement of Mass Transport Using Vortex Rings

“…(1), (2) ．工学的には，推力を生み出す最適な 形状の渦輪やその形成方法 (3)～(6) や噴流の制御 (7), (8) など，応用を目指した基礎的研究もすすめられている． 渦輪の特徴として，コンパクトで孤立波のように持続的に並進運動し，渦輪の内外の流体交換が尐なく，その 経路に渦輪を構成する流体を取り残すことが尐ないことが挙げられる．この性質を利用して，噴出口から離れた 自由空間中の特定の領域に流体を輸送する手段として期待できる．不必要な空間に輸送流体が拡散してしまうこ とを削減できれば，省エネルギーやコストダウンに貢献できる．また，渦輪自身の形成パラメータが豊富なだけ でなく，渦相互の干渉によって流れ場を大きく変えられるため，通常の噴流より制御が柔軟に工夫できる．しか しながら，渦輪による流体輸送について定量的に調査した例は多くない． 例えば，土門ら (9) は，直径 0.…”