Interaction of the vortex structures formed in parallel pulsed jets

The experimental investigation of the lateral interaction of the heads of pulsed jets and primary shock waves at various nozzle spacings and pressure ratio numbers is described. The various stages of formation of a composite pulsed jet issuing from a multinozzle block are classified and the flow development mechanisms are explored. It is shown for both a block and a single nozzle the shock wave travels with almost the same velocity, whereas the jet front formed at the exit from a single nozzle moves much more slowly than the jet front formed beyond a nozzle block. Long-lived lateral bursts of gas, whose dimensions are an order greater than those of the jet bursts, are detected. Their long period of existence considerably increases the stabilization time of the steady-state structure and parameters as compared with a single pulsed jet with the same flow rate.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 153–159, November–December, 1987.     

Correction of wandering smoothing effects on static measurements of a wing-tip vortex

Wandering is a typical feature of wing-tip vortices and it consists in random fluctuations of the vortex core. Consequently, vortices measured by static measuring techniques appear to be more diffuse than in reality, so that a correction method is needed. In the present paper statistical simulations of the wandering of a Lamb-Oseen vortex are first performed by representing the vortex core locations through bi-variate normal probability density functions. It is found that wandering amplitudes smaller than 60% of the core radius are well predicted by using the ratio between the RMS value of the mean cross-velocity and its slope measured at the mean vortex center. Furthermore, the principal axes of wandering can be accurately evaluated from the opposite of the cross-correlation coefficient between the spanwise and the normal velocities measured at the mean vortex center. The correction of the wandering smoothing effects is then carried out through four different algorithms that perform the deconvolution of the mean velocity field with the probability density function that represents the wandering. The corrections performed are very accurate for the simulations with wandering amplitudes smaller than 60% of the core radius, whereas errors become larger with increasing wandering amplitudes. Subsequently, the whole procedure to evaluate wandering and to correct the mean velocity field is applied to static measurements, carried out with a fast-response five-hole pressure probe, of a tip vortex generated from a NACA 0012 half-wing model. It is found that the wandering is predominantly in the upward-outboard to downward-inboard direction. Furthermore, the wandering amplitude grows with increasing streamwise distance from the wing, whereas it decreases with increasing angle of attack and free-stream velocity.     

Separation control using synthetic vortex generator jets in axial compressor cascade

An experimental investigation conducted in a high-speed plane cascade wind tunnel demonstrates that unsteady flow control by using synthetic (zero mass flux) vortex generator jets can effectively improve the aerodynamic performances and reduce (or eliminate) flow separation in axial compressor cascade. The Mach number of the incoming flow is up to 0.7 and most tested cases are at Ma = 0.3. The incidence is 10° at which the boundary layer is separated from 70% of the chord length. The roles of excitation frequency, amplitude, location and pitch angle are investigated. Preliminary results show that the excitation amplitude plays a very important role, the optimal excitation location is just upstream of the separation point, and the optimal pitch angle is 35°. The maximum relative reduction of loss coefficient is 22.8%.The project supported by the National Natural Science Foundation of China (10477002 and 50476003) and the Ph.D. Innovative Foundation of Beihang University. The English text was polished by Yunming Chen.     

Experimental study of underwater rock drilling using a pulsed Ho:YAG laser-induced jets

This paper is primarily an assessment of laser-induced water jets for boring rock surfaces. It also reports the result of preliminary experiments of pulsed Ho:YAG laser-induced jets applied to drill a submerged rock specimen. The irradiation of pulsed Ho:YAG laser beams at 3 Hz inside a thin metal tube produces intermittent water vapor bubbles which result in liquid jet discharge from the exit of the metal tube. The laser-induced water jets are visualized by shadowgraphs and images are recorded by a high-speed digital video camera. High stagnation pressures were eventually generated by the jet impingements. Simultaneously shock waves of about 22.7 MPa were generated at bubble collapse, which effectively cracked the surface of the rock specimens. Repeated exposures of these laser-induced jets against submerged rock specimens have a potential to practically bore holes on rock surfaces.     

Density distribution in pulsed gas jets effusing into a rarefied space

Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 6, pp. 123–127, November–December, 1990.     

Identification of vortex motions in turbulent mixing of choked jets

A specially adapted schlieren system is used to generate fluctuating signals which respond strongly to large scale coherent components of a turbulent mixing jet flow and which have a relatively reduced response to random disturbances. The schlieren signals also provide a direct indication of the presence of vortex-like structures in the turbulent mixing layers by virtue of the phase relationship of the schlieren signals to the pressure field. This system gives a clear resolution of the fluctuating periodic effects associated with vortex structures in the flow from a choked convergent nozzle. It has thus been possible to determine that vortex-like eddies are associated with the feedback screech mechanism, and also generate periodic disturbances due to their passage through the diamond shaped wave structure in the flow. The regular disturbances in the flow move at 0.77 of the fully expanded flow velocity. Phase spectral observations demonstrate clearly the vortex like structure of coherent disturbances in the flow by virtue of the quadrature phase relation between the schlieren and microphone signals. Movement of the sensing microphone in the pressure field external to the flow shows disturbance propagation at the acoustic velocity, and also shows that disturbances at Strouhal numbers above 0.7 emanating from the inner mixing zone can be identified by an additional time delay to reach the microphone and only influence the microphone when it is located downstream of the flow sensing schlieren system due to confinement of pressure disturbances within Mach cones of the flow.     

Development of shock wave and vortex structures in unsteady jets

The paper deals with the formation of a gaseous jet behind a sonic nozzle. The nozzle was located at the end wall of a shock tube. A specially made two-direction shadow system, which ensured a simultaneous recording of side-on and head-on images of the jet, as well as a diffraction interferometer were used. On the basis of the data obtained, an analysis of the vortex structure of the jet was carried out, the amplitude of the azimuthal instability was measured and the spatial distribution of the density in the gaseous flow was obtained. Comparison between the experimental density distribution, numerical results and a nonself-similar point source blast wave model was conducted.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.     

Spatial correlation measurement in turbulent flows using pulsed wire anemometry

Details of a new technique in pulsed wire anemometry, developed to allow measurement of two-point spatial velocity correlations in highly turbulent flows, are described. An outline of the interface devices necessary for linking two anemometers to a microcomputer is given and examples of the use of the technique are presented. Firstly, measurements of spatial correlations with and without time delay in the near wake of a nominally two-dimensional cylinder normal to a uniform stream confirmed the viability of the technique. Secondly, measurements in the highly turbulent, separated region behind a normal flat plate fitted with a downstream splitter plate are presented as a demonstration of the effectiveness of the technique in such difficult flows. We believe that these are the first direct measurement of spatial correlation functions within a separated flow.     

Flow control on a 3D backward facing ramp by pulsed jets

This paper presents an experimental study of flow separation control over a 3D backward facing ramp by means of pulsed jets. Such geometry has been selected to reproduce flow phenomena of interest for the automotive industry. The base flow has been characterised using PIV and pressure measurements. The results show that the classical notchback topology is correctly reproduced. A control system based on magnetic valves has been used to produce the pulsed jets whose properties have been characterised by hot wire anemometry. In order to shed some light on the role of the different parameters affecting the suppression of the slant recirculation area, a parametric study has been carried out by varying the frequency and the momentum coefficient of the jets for several Reynolds numbers.     

Behavior of metallic shaped-charge jets with passage of a pulsed electric current through them

The paper reports results of experimental and numerical studies of the disruption of metallic shaped-charge jets by passage of a pulsed electric current through them. Experimental results are presented in the form of x-ray photographs of shaped-charge jets with and without a current and tables of penetration depths in targets. Numerical simulation of the disruption of shaped-charge jets with a current is performed for three possible mechanisms of disruption (development ofMHD instability of shaped-charge jets, volume fracture, and simultaneous development ofMHD instability and volume fracture). Numerical and experimental results are compared. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 41, No. 3, pp. 19–25, May–June, 2000.     

Flow analysis of two-dimensional pulsed jets by particle image velocimetry

A purely alternating jet without mean mass flux and a mixed pulsed jet containing an additional blowing component were investigated by particle image velocimetry (PIV). The jets issued from a two-dimensional slit connected to a converging nozzle, opening normally from a flat wall. The pulsation was driven by a loudspeaker. The mean velocity fields were characterized by the combination of downstream directional blowing and omni-directional suction. The velocity fluctuations were dominated by contra-rotating eddy pairs synchronized with the pulsation and formed at the jet edges during blowing. Phase-synchronized measurements permit the investigation of the averaged patterns and the cycle-to-cycle fluctuations of these vortices. The mean trajectories of vortex centers during a whole injection cycle show how large lateral jet expansions are achieved. For a purely alternating jet, the expansion takes place close to the slit. For a mixed pulsed jet, the vortices develop farther from the orifice. In addition, proper orthogonal mode decomposition demonstrates that only a few modes are required to represent the main events of the flow dynamics. Received: 10 August 1999 / Accepted: 10 January 2001     

Reynolds stress field in a turbulent wall jet induced by a streamwise vortex with periodic perturbation

An experimental study on the Reynolds stress tensor was conducted in the three-dimensional flow in the plane turbulent wall jet induced by an isolated streamwise vortex generated by the half-delta wing mounted on the wall. Oscillation of the angle of attack of the wing induced a periodic perturbation in the strength of the streamwise vortex. Analysis by triple velocity decomposition and phase averaging shows that the oscillation induces periodic variations in the strength, radius, and position of the streamwise vortex center. The effect of periodic perturbation manifests itself in the magnitude of the Reynolds stress components and Simulations prove that the periodic variations in the strength, radius, and position of the vortex center can generate an apparent shear stress, denoted herein as