Luiz M. Lourenco

On the use of microjets to suppress turbulence in a Mach 0.9 axisymmetric jet

We have experimentally studied the effect of microjets on the flow field of a Mach 0.9 round jet. Planar and three-dimensional velocity field measurements using particle image velocimetry show a significant reduction in the near-field turbulent intensities with the activation of microjets. The axial and normal turbulence intensities are reduced by about 15% and 20%, respectively, and an even larger effect is found on the peak values of the turbulent shear stress with a reduction of up to 40%. The required mass flow rate of the microjets was about 1% of the primary jet mass flux. It appears that the microjets influence the mean velocity profiles such that the peak normalized vorticity in the shear layer is significantly reduced, thus inducing an overall stabilizing effect. Therefore, we seem to have exploited the fact that an alteration in the instability characteristics of the initial shear-layer can influence the whole jet exhaust including its noise field. We have found a reduction of about 2 dB in the near-field overall sound pressure level in the lateral direction with the use of microjets. This observation is qualitatively consistent with the measured reduced turbulence intensities.

Turbulence and noise suppression of a high-speed jet by water injection

An experimental investigation has been carried out on a supersonic jet of air issuing from an M =1.44 convergingx2013;diverging rectangular nozzle of aspect ratio 4. Particle13; image velocimetry measurements of the flow field along with near-field acoustic measurements were made. The effect of injection of a small amount of water (x223C;5% of the mass flow rate of the jet) into the shear layer of the jet, on the unsteady flow structure and sound generation were examined. The presence of water droplets in the jet modified the turbulence structure significantly, resulting in axial and normal r.m.s.velocity reductions of about 10% and 30%, respectively, as compared to that of a13; normal jet. An even larger effect is found on the peak values of the turbulent shear stress with a reduction of up to 40%. The near-field noise levels (OASPL) were found13; to reduce by about 2x2013;6 dB depending on the location of the injection and the water mass flow rate. Far-field acoustic measurements carried out on a heated M =0.9 (jet13; exit velocity=525msx2212;1) jet show significant (6 dB) reductions in the OASPL with moderate amounts of water injection (17% of the mass flow rate of the jet) suggesting13; that the technique is viable at realistic engine operating conditions.

Vortex ring formation at the open end of a shock tube: A particle image velocimetry study

The vortex ring generated subsequent to the diffraction of a shock wave from the open end of a shock tube is studied using particle image velocimetry. We examine the early evolution of the compressible vortex ring for three-exit shock Mach numbers, 1.1, 1.2, and 1.3. For the three cases studied, the ring formation is complete at about tUb/D=2, where t is time, Ub is fluid velocity behind shock as it exits the tube and D is tube diameter. Unlike in the case of piston generated incompressible vortex rings where the piston velocity variation with time is usually trapezoidal, in the shock-generated vortex ring case the exit fluid velocity doubles from its initial value Ub before it slowly decays to zero. At the end of the ring formation, its translation speed is observed to be about 0.7 Ub. During initial formation and propagation, a jet-like flow exists behind the vortex ring. The vortex ring detachment from the tailing jet, commonly referred to as pinch-off, is briefly discussed.

Structure of a screeching rectangular jet: a stereoscopic particle image velocimetry study

The unsteady velocity field generated by an underexpanded jet has been investigated using stereoscopic particle image velocimetry (PIV). A 4:1 aspect ratio converging–diverging rectangular nozzle designed to operate at a fully expanded condition of

Particle image velocimetry

M=1.44

Suppression of Self-Sustained Oscillations in a Supersonic Impinging Jet

was used. The nozzle was operated at off-design conditions to generate imperfectly expanded jets with intense screech tones. Phase-locked PIV measurements show the spatial and temporal evolution of the three-dimensional jet with high fidelity. In addition to the globally averaged mean and turbulence velocity field data, the phase-averaged data for the velocity and vorticity fields were also obtained. The turbulence quantities were resolved into contributions from the periodic and random motions. The deformation of the periodic spanwise structures results in the formation of strong streamwise vortices that appear to govern the mixing of the jet. It is shown that the presence of coherent vorticity of significant strength, in addition to the shock cell strength, is largely responsible for determining the screech intensity.

Investigation of flow at leading and trailing edges of pitching-up airfoil

One of the most challenging and time-consuming problems in experimental fluid mechanics is the measurement of the overall flow field properties, such as the velocity, vorticity, and pressure fields. Local measurements of the velocity field (i.e., at individual points) are now done routinely in many experiments using hot-wire (HW) or laser velocimetry (LV). However, many of the flow fields of current interest, such as coherent structures in shear flows or wake flows, are highly unsteady. HW or LV data of such flows are difficult to interpret, as both spatial and temporal information of the entire flow field are required and these methods are commonly limited to simultaneous measurements at only a few spatial locations.

Structure of Supersonic Twin Jets

A passive control technique was used to suppress the self-sustained oscillations in a supersonic impinging jet. A rigid plate, placed in the ambient region close to the jet exit, was used to interrupt the upstream propagating acoustic waves originating from the impingement region. The effectiveness of this technique was examined using the particle image velocimetry. The results clearly showed that the large-scale coherent vortical structures, which dominated the normal impinging jet, are completely suppressed with the passive control technique. As a result, a signie cant reduction in induced entrainment velocity was found. A recovery of about 16% of the lift loss on the attached lift plate was achieved when compared to an uncontrolled impinging jet. A reduction of about 11 dB in the near-e eld overall sound pressure level was observed.

A PIV Study of a Supersonic Impinging Jet

The dynamic stall process of an NACA 0012 airfoil undergoing a constant-rate pitching-up motion is studied experimentally in a water towing tank facility. This study focuses on the detailed measurement of the unsteady separated flow in the vicinity of the leading and trailing edges of the airfoil. The measurements are carried out using the particle image velocimetry technique. This technique provides the two-dimensional velocity and associated vorticity fields, at various instants in time, in the midspan of the airfoil. Near the leading edge, large vortical structures emerge as a consequence of van Dommelen and Shen type separation and a local vorticity accumulation. The interaction of these vortices with the reversing boundary-layer vorticity initiates a secondary flow separation and the formation of a secondary vortex. The mutual induction of this counter-rotating vortex pair eventually leads to the ejection process of the dynamic stall vortex from the leading-edge region. It is found that the trailing-edge flowfield only plays a secondary role on the dynamic stall process.

Stereoscopic PIV Measurements of a Screeching Supersonic Jet

The flow characteristics of coupled oscillatory motion of M j = 1.5 supersonic twin jets are described using the particle image velocimetry measurements. As a result of self-excitation caused by screech, sinuous oscillation of individual jets is observed. The coupling between the jets, characterized by the presence of coherent large-scale eddies, results in a symmetric mode with respect to the midplane separating the two jets. A control technique using microjets placed at the nozzle exit is used to suppress the unsteady flow