Measurement of Shear Stress Acting on Flat Plate Using Oil Film Interferometry

Measurements of frictional resistance play an important role in engineering practice. There are several types of air resistance acting on an aircraft, for example. One of them, frictional resistance, accounts for half of the air resistance. Oil film interferometry is one of methods for measuring the frictional resistance. Oil dropped on an object is thinly stretched by the frictional resistance. The bright and dark fringe pattern is generated when monochromatic light is applied to the oil film. The gradient of the oil thickness decreases with the lapse of time, and thus the spacing between neighboring the dark lines increases. The rate at which the spacing increases is proportional to the frictional resistance. In this study, the frictional resistance acting on a small area on a plate was measured and compared with the theoretical value. As a result, these results qualitatively agree well with each other.

Structure and Screech Tone of Radially Underexpanded Jet Emitted by Facing Cylinders

An underexpanded jet has typical shock-cell structure and strongly oscillates, its behavior being known to cause many industrial problems. An underexpanded jet radially issues from intake and exhaust valves of an internal combustion engine, a pressure control valve and so on. When a supersonic jet exhausted from a circular nozzle impinges on a flat plate, the wall jet formed on the plate often becomes underexpanded and spreads out radially. Such underexpanded impinging jet is one of models of supersonic jets on laser cutting process and glass tempering process. In this study, an underexpanded jet radially discharged from a circular slit nozzle, which consists of two cylinders, was experimentally examined for different nozzle pressure ratios and for different diameters of cylinder. Jet structure was analyzed by means of a visualization e.g. Schlieren method. A noise emitted from the jet was measured and the frequency of screech tone was analyzed. The experimental results were compared with those of a two dimensional jet issuing from a rectangular nozzle. Furthermore, a comparison of visualized sound waves with the screech tone frequency reveals that the sound source measured was in the vicinity of the end of the second cell and that the length of the second or third cell was one of the most important parameters to determine the frequency of the emitted screech tone.

Structure and Oscillation of Underexpanded Jet

In this study, flow characteristics are experimentally and numerically discussed, especially concerning shock-cell structure and oscillatory phenomena of supersonic jet. The jet becomes underexpanded when the nozzle pressure ratio of convergent nozzle exceeds the critical value. The underexpanded jet is not uniform because of the presence of the expansion wave, the compression wave and the shock wave formed in it. Many vortices are induced in jet boundary region by shearing stress generated between supersonic jet flow and atmospheric air. They interact with shock waves composing cell node of jet, which is closely related to a noise radiating from the jet. In experiment, flow field is visualized and the shape of cell structure is examined. Furthermore, frequency of vortex generation is measured. The jet is simulated using TVD scheme and the results are compared with the experimental ones. Distortion of cell structure caused by three dimensional oscillation of jet and behavior of vortices are discussed.

A Study on Broad Band Noise Emitted from Underexpanded Radial Jet

This study focuses on a noise emitted from a radial underexpanded jet. The underexpanded jet is well known as one of supersonic jets and it is formed when the pressure ratio across a convergent nozzle is more than the critical value. The underexpanded jet has been used in many fields, such a propulsion of rocket, laser cutting and a glass tempering process. Such jet is exhausted from a circular nozzle. The expansion wave, the compression wave and the shock wave are periodically formed in the jet, which leads to quasi-periodical shock-cell structure. On the other hand, an underexpanded jet radially issues from intake and exhaust valves of an internal combustion engine and a pressure control valve. The radial underexpanded jet does not have the quasi- periodical shock-cell structure such as the jet from the circular nozzle, its cell length decreasing along jet axis. In this study, an underexpanded jet radially discharged from a circular slit nozzle, which consists of two circular tubes, is experimentally examined for different nozzle pressure ratios. The jet structure is visualized using Schlieren method and a noise emitted from the jet is measured. Typically, the shock associated broadband noise is analyzed and a relation between the jet structure and the radiation noise frequency is discussed. As a result, it is found that the broadband noise lies on lower frequency than a screech tone and that the 1st cell length plays an important role in broadband noise radiation.

A Numerical Study on Jet Structure and Noise Emission of Underexpanded Radial Jet

In this study, an underexpanded radial jet issuing from a small gap between two circular tubes facing each other is investigated numerically. Radial jet is formed, for example, downstream of high-pressure valves in piping system and of poppet valves in engines, and causes many industrial problems such as the noise generation and the fatigue failure of structure. In this study, the jet issuing from a small gap between two tubes with same diameter is numerically simulated. The flow field is assumed to be axisymmetric against the central axis of tubes and to be symmetric against the intermediate plane between the exits of two tubes. The axisymmetric Euler equations are solved using symmetric TVD (Total Variation Diminishing) scheme. The effects of nozzle pressure ratio and of diameter of circular tubes on the structure and the behavior of jets are examined. Typical cell structure of underexpanded jet appears in radial jet and the length of cell becomes smaller in downstream region because the jet spreads radially like a disc. The length and width of first cell are larger with higher nozzle pressure ratio. Many vortices are generated one after another near the jet boundary and move downstream, which cause the oscillation of jet. Outside of jet, two types of density waves are observed. One of them propagates toward the nozzle (toward the upstream region) and the other propagates in opposite direction. Focusing on the pressure change caused by the former waves, which is related to well-known screech, dominant frequency obtained by FFT analysis was found to become lower with higher pressure ratio and smaller diameter of tube.