Georges Charnay

Measurements and image analysis of the turbulent field in an axisymmetric jet subject to a sudden velocity decrease

Abstract The unsteady turbulent far field of a transient axisymmetric turbulent jet is studied experimentally. The initial flow perturbation consists of a sudden and large decrease in the ejection velocity. Flow visualization provides a qualitative understanding of the response of the jet structure, and the temporal evolution of the mean aand fluctuating velocity field is measured by using X-hot-wire probes. The longitudinal adaptation of the perturbation time scale to the local jet time scale can be described by showing a local self-similarity property of the flow. Transient mean radial velocities are deduced, and a large radial inflow of external fluid is found to occur during the phase of velocity decrease. The anisotropic reaction of the turbulent second-order moments reflects the particular energy-transfer mechanisms encountered in free shear flows. A local negative energy production from the turbulent field to the mean field appears on the jet axis during the transient evolution. Moreover, we can expect that the kinetic energy budget measured in steady jets is perturbed.

PIV Measurements of Internal Aerodynamic of Diesel Combustion Chamber

Characteristics of the in-cylinder air motion in Diesel engine has been investigated by PIV from intake to exhaust phase. This paper presents the methodology used to perform in cylinder aerodynamics measurements (PIV) using a Renault tansparent single cylinder engine in motored conditions. Measurements have been achieved with exact engine geometry configuration (high compression ratio) even at top dead center (TDC). Two different configurations have been studied: flat and bowl-in-piston. First results, which are high quality, demonstrate that swirl is better centred with bowl-in-piston than flat piston. Nevertheless, swirl still remain dissymetrical. Its evolution during compression measured on experimental engine is close to theory. Finally, squish phenomena is visualized and its intensity appears to be weaker than theory.

Transient behavior of an axisymmetric turbulent jet

The prediction of the response of unsteady flows submitted to external excitation is a real challenge for the optimization of industrial processes. As the jet flow is a very basic turbulent flow related to mixing and entrainment phenomena via turbulent structure dynamic, we investigate the transient behavior of an axisymmetric jet submitted to a large and sudden decrease of its ejection velocity. The non stationary flow evolution is studied experimentally. Measurements along the jet axis based on pure ensemble averaging show clearly the convective motion of the perturbation and the adaptation of the local interaction to the local jet time scale. A transverse investigation in the non stationary region show that the mean flow and its turbulence is deeply affected during the local velocity decrease.

Experimental study of the behavior of confined variable density jets in a time varying crossflow

An experimental work and a physical analysis dedicated to the study of a low density jet subjected to a time varying crossflow with high acceleration/deceleration levels are presented in this paper. Relevant nondimensional numbers are derived and show that unsteady effects associated with the presence of the jet in the acceleration field have noticeable consequences on the flapping of the jet. The Schlieren technique is applied in the test section of a square duct to obtain time resolved images of the jet. Analysis of the results is focused on the influence of the unsteady effects on the global dynamic behaviour of the jet in the near field. The interaction between the jet and the crossflow is analysed in three contrasted situations corresponding to different values of the jet outlet velocity U0. We predict and observe an increase of the jet deflection during the acceleration phase and a competition between drag and acceleration during the deceleration. This competition is particularly clear for the two lowest ejection velocities of the jet and we have shown that the jet is initially deflected upstream the nozzle. The influence of exit jet injection angle is finally considered. We show that upstream or downstream injections induce a very strong modification of the mixing process of the jet fluid with the pulsed crossflow.

Round Turbulent Air Jet Submitted to a Pulsed Coflow

Mean and turbulent properties of an unsteady round air jet submitted to a pulsed coflowing airstream were studied from laser Doppler Velocimetry measurements. The ejection velocity of the jet is kept at a constant value, whereas the coflowing stream is pulsed. These measurements revealed that the unsteadiness leads to a longitudinal partition of the jet. Near the jet exit, the flow is a quasi-steady jet flow. Farther downstream, the flow is unsteady with the creation of a large and propagative structure in the jet flow. The objective of the study is comprehen- sive understanding of the main physical mechanisms induced by the coflow unsteadiness. Consequences of the entrainment process are also discussed.

Strong Baroclinic Effects in a Light Jet in a Pulsed Coflow

The role of strong baroclinic effects in a light jet injected into a pulsed coflow is analyzed. This case is believed to be a model situation because the induced baroclinic torque competes with the jet shear. Phase-averaged mean and turbulent properties are presented. A correct scaling is obtained from physical analysis. The striking observation is here a transition from a jet to a wake behavior during the deceleration phase. This has a dramatic influence on the evolution of the Reynolds stresses

Approaches to analytical solutions of laminar compressed free jets

WE study here the effect of a uniform mass density time variation on a subsonic gas jet by generalizing the classical similarity hypothesis. Both plane and circular laminar jet evolutions are precisely established and compared. I. Laminar Jet Self-Preserving Region We are first interested in characterizing the behavior of the plane or axisymmetric laminar jet self-preserving region. The jet flow is well into the subsonic flow so that the gas mass density p is uniform in space and governed by the displacement of the pistons (see Fig. 1). Two characteristic times rule the flow evolution - the mass density variation time scale