Spark Shadowgraphs Research Articles

Numerical study of acoustic radiation due to a supersonic turbulent boundary layer

AbstractDirect numerical simulations are used to examine the pressure fluctuations generated by fully developed turbulence in a Mach 2.5 turbulent boundary layer, with an emphasis on the acoustic fluctuations radiated into the free stream. Single- and multi-point statistics of computed surface pressure fluctuations show good agreement with measurements and numerical simulations at similar flow conditions. Consistent with spark shadowgraphs obtained in free flight, the quasi-homogeneous acoustic near field in the free-stream region consists of randomly spaced wavepackets with a finite spatial coherence. The free-stream pressure fluctuations exhibit important differences from the surface pressure fluctuations in amplitude, frequency content and convection speeds. Such information can be applied towards improved modelling of boundary layer receptivity in conventional supersonic facilities and, hence, enable a better utilization of transition data acquired in such wind tunnels. The predicted acoustic characteristics are compared with the limited available measurements. Finally, the numerical database is used to understand the acoustic source mechanisms, with the finding that the supersonically convecting eddies that can directly radiate to the free stream are confined to the buffer zone within the boundary layer.

Interaction of supersonic wing-tip vortices with a normal shock

An experimental study involving interaction of concentrated streamwise wing-tip vortices and normal shock fronts was carried out in a Mach 2.49 flow. The interaction scheme involved positioning a vortex-generator wing section upstream of a pitot-type normal shock inlet such that the wing-tip vortices interacted with the normal shock formed in front of the inlet. The vortex strength was varied by placing the vortex generator wing at different angles of attack while a normal shock was created by adjusting the mass flow rate passing through the inlet. Spark shadowgraphs, laser sheet planar visualizations, and pitot pressure measurements of the flowfield indicated a significant change in the structure of streamwise vortices generated by the vortex generator wing at 5.7- and 10.4-deg angle of attack upon encountering a normal shock discontinuity. Results of the Investigation showed that the interactions lead to the formation of an unsteady conical shock wave far upstream of the inlet as well as a highly turbulent flow downstream for both vortices. Pitot pressure measurements using a fast response pressure transducer in conjunction with the spark shadowgraphs revealed a bimodal feature of the flowfield. The frequency of oscillation of the generated structure was found to be higher for increased vortex strength. Measurements of pitot pressure in the vortex core in the absence of a shock wave revealed high-frequency oscillations that were attributed to the vortex meandering phenomenon.

Experimental investigation of the aerodynamic breakup of liquid drops

An experimental investigation was made of the deformation and the mechanism of stripping-type breakup of liquid drops. Experiments were conducted in a 60 X 150 mm cross-sectional shock tube equipped with pulsed laser holographic interferometry. Water drops having diameters of 1030 and 4300 fim were examined for shock wave Mach numbers from 1.3 to 1.5 in atmospheric air. The Weber and Reynolds numbers under these conditions were in the range of 600 to 7600 and 1.38 to 10.4 x 10, respectively. In previous works, the experimental data of the stripping-type breakup were obtained by spark shadowgraphs and streak schlieren methods and high-speed movies. However, in previous works, due to the effect of light scattering through the micromist, the structure of disintegrating drops could not be visualized. The purpose of the present work is, by using holographic interferometry, to re-examine the classical problem of the stripping-type breakup of liquid drops. As a result, a four-stage mechanism of the stripping-type breakup of liquid drops was established.

Experimental Study of the Flowfield of a Two-Dimensional Premixed Turbulent Flame

A turbulent reacting shear layer in a premixed propane/air flow has been studied in a two-dimensional combustor, with a flame stabilized behind a rearward facing streamlined step. Spark shadowgraphs show that in the range of velocities (7.5-22.5 m/s corresponding to Reynolds numbers of 0.5xl0 4-1.5xl04 cm1) and equivalence ratios (0.4-0.7) studied, the mixing layer is dominated by Brown-Roshko type large coherent structures in both reacting and nonreacting flows. High-speed schlieren movies show that these eddies are convected downstream and increase their size and spacing by combustion and coalescence with neighboring eddies. Tracing individual eddies shows that on average eddies accelerate in the reacting shear layer as they move downstream, with the highest acceleration close to the origin of the shear layer. Combustion is confined to those large structures which develop as a result of vortical action of the shear flow. On the average, the reacting eddies have a lower growth rate than nonreacting eddies. A turbulent boundary layer created by means of a tripping wire upstream of the edge of the step virtually eliminates the large coherent structures in the shear layer, while for the case in which the wire could not trigger the transition to turbulence, the large coherent structures dominated the reacting and nonreacting flows.

Tangential slot injection in supersonic flow.

The flowfield associated with tangential slot injection of a gas into a supersonic external stream is of current technological and academic interest. The present study can be divided into three areas. First, detailed profiles in the near flowfield were determined with an interferometer for air injection into a Mach 2.1 airstream past a flat plate. Second, an analytical treatment of the turbulent mixing zone including the effects of a splitter plate boundary layer is presented and shown to provide predictions in excellent agreement with the experiments. Lastly, the stability of the mixing region is considered and experiments with spark shadowgraphs are reported.

Noise Radiation from Impinging Jet Flows

Experimental investigations of the noise from an underexpanded axisymmetric (d=38 in.) main jet flow at 100 psig stagnation pressure interacting with a transversely impinging concentric annular (34‐in. diam by 364 in. wide) jet flow have been conducted in an anechoic chamber. The impingement location of the annular flow (within one diameter downstream of the main jet nozzle exit) and percent impingement (here equal to the annular jet stagnation gauge pressure) were varied. Between 0% and 8% impingement, the total acoustic power radiated increased from 142 dB re 10 13 W to 148 dB. Significantly, between 8% and 33% impingement, the power radiated decreased to a relative minimum that occurred between 15% and 25% impingement, depending on the impingement location. This relative minimum was within 2 dB of the power radiated by the main jet flow alone. Spark shadowgraphs of the underexpanded main jet flow and its interaction with annular jet flow revealed interesting changes in the shock pattern. Possible bearing of these changes on the observed behavior of the acoustic power is discussed. [Research sponsored by the National Aeronautics and Space Administration.]

On the mechanism of bluff body flame stabilization at low velocities

Previous workers on bluff body laminar flame stabilization have observed, under a wide variety of circumstances, flame sheets that were wrinkled into ‘nodal’ waves with well-defined wavelengths and velocities of propagation. In this paper, experiments are described that explain the observed wave forms. It is believed that they are forced, unstable, propagating, interfacial waves—forcing being caused by the oscillation of the point of attachment of the flame in the boundary layer and wake surrounding the flameholder. Spark shadowgraphs are presented to support this conclusion. Data are presented to show the dependence of the frequency of oscillation of flow velocity, air/fuel ratio, and sphere diameter. Flameholding phenomena and the formation of ‘envelope’ flames are also discussed.

Experimental study of a reversed jet flameholder

Experiments and analyses were performed to contribute to a basic understanding of the holding of flame in a stream of a combustible gas mixture by a small gas jet directed upstream. The main stream, a mixture of natural gas and air, entered a cylindrical transparent test section at uniform velocity. A flameholding jet of air was directed counter to the stream along the axis of the test section. Observations of the flow field and flame behaviour were made by means of time-exposure photographs, high-speed motion pictures, and spark shadowgraphs. A pilot flame was delineated, and its significant time-averaged dimensions were noted as a function of jet and main-stream velocities. New evidence was obtained indicating the occurrence of recirculation. Transmission of flame from the pilot to the main stream is accomplished through a mixture continuously shedding from the pilot flame. Rough burning near a stability limit occurs when significant portions of the shedding mixture are not inflammable. The influence of non-uniform fuel concentration patterns in the main stream on flame stability limits was also investigated. Interpretation of the data leads to conclusions which conflict with a currently held mechanism for reversed-jet flame stabilization. A mechanism is proposed which is plausible in the light of current observations.

LXXV.Hot wire and spark shadowgraphs of the airflow through an air-screw

Summary A method of delineating the flow-pattern round bodies mounted in an air-stream is described and applied to the case of the flow around an air-screw.