Emmanuel Jondeau

Space-Time Correlations in Two Subsonic Jets Using Dual Particle Image Velocimetry Measurements

Dual particle image velocimetry (dual PIV) measurements have been performed to investigate the space-time correlations in two subsonic isothermal round jets at Mach numbers of 0.6 and 0.9. The correlation scales are analyzed along the centerline and in the shear-layer center over the first 11 jet diameters from the nozzle exit To provide robust results over a wide range of flow conditions, these correlation scales are given in terms of their appropriate quantities, namely, the mean or rms velocity in reference to velocity and the momentum thickness or the half-velocity diameter in reference to length in the shear layer and on the jet axis, respectively. From these results, a discussion on the modeling of turbulence in jets is addressed. The self-similarity of some space correlation functions in the shear layer and on the jet axis is shown. Furthermore, far enough downstream in the shear layer, some of the ratios between the space and time scales are relatively close to the values expected in homogeneous and isotropic turbulence. It is also found that the ratio between the integral length and the time scales in the fixed frame is of the order of the local mean flow velocity. In the convected frame, the appropriate scaling factor is the rms velocity.

Broadband Noise Reduction With Trailing Edge Brushes

Airfoil broadband trailing edge noise is reduced by modification of the trailing edge geometry. A brush made of a single row of flexible polypropylene fibers is integrated in the trailing edge of a cambered airfoil. Far field acoustic measurements show a noise reduction potential reaching 3 dB on a wide frequency range. Due to high curvature of the incident flow, a secondary acoustic source partly masks the trailing edge noise reduction. Hot wire correlation measurements in the very near wake of the airfoil show that longitudinal as well as transversal length scales are affected by the brush. Span wise coherence length of boundary layer eddies falls off by 25 % in the presence of a brush in the adequate frequency range, possibly explaining a 1.3 dB contribution to the noise reduction mechanism. Boundary layer turbulence exhibits a preferred coherence length l y v on a wide frequency range. l y v /d ≈ 2, is considered a proper brush design law, d being the diameter of the brush.

Space-time correlations in two subsonic jets using dual-PIV measurements

Dual Particule Image Velocimetry (dual-PIV) measurements have been performed to investigate the space-time correlations in two subsonic isothermal round jets at Mach numbers 0.6 and 0.9. Measurements are obtained along the centerline and the shear-layer region which are closely connected with the noise generation. Integral scales have been calculated with as much as 2000 samples which provides high-quality data. The correlation scales are given in function of appropriate references, namely the local momentum thickness (respectively the jet diameter) for the shear-layer (respectively the jet axis) and the mean or rms local velocity. Far enough downstream in the shear-layer, some of the ratios between the space scales and between the time scales are relatively close to the values expected in isotropic an homogeneous turbulence. Furthermore, the relation between the time and space scales follows well the Taylor’s assumption in the shear-layer.

An experimental characterisation of wall pressure wavevector-frequency spectra in the presence of pressure gradients

The goal of this experimental study is to investigate the wall pressure wavenumberfrequency spectra induced by a turbulent boundary layer in the presence of a mean pressure gradient. The mean pressure gradient is achieved by changing the ceiling angle of a rectangular channel flow. Wall pressure spectra are measured for zero-, adverse- and favorablepressure-gradient boundary layers by using a pinhole microphone in conjunction with a high-frequency-calibration procedure. A linear antenna based on a non-uniform distribution of remote microphones mounted on a rotating disk is also developed to obtain a direct measurement of both aerodynamic and acoustic components of wavenumber-frequency spectra. First results, comparisons and analyses are then discussed.

LES of noise induced by ow through a double diaphragm system

This paper aims at predicting the noise generated in internal flows using unstructured LES. A canonical geometry representative of complex air conditionning system parts is selected to perform extensive validation of the numerical approach. An experimental campaign carried out at LMFA provides both detailed aerodynamic description of the flow and the wall pressure spectra. The impact of important numerical parameters such as numerical scheme and boundary conditions is then investigated, before assessing the ability of the method to capture physical trends between two similar configurations.

Cold wire constant voltage anemometry to measure temperature fluctuations and its application in a thermoacoustic system

The knowledge of temperature fluctuations is essential for most thermoacoustic systems. In the present paper, cold wire constant-voltage anemometry (CVA) to measure temperature fluctuations is presented. Corrections for the thermal inertia and for the end losses of the wire are applied during the post-processing. The correction for the thermal inertia of the cold wire is achieved by applying a time dependent thermal lag as proposed originally for a constant-current anemometry (CCA) system. This thermal lag is measured in parallel by a hot wire. The thermal end losses of the wires to their supports are also considered and approximate corrections are proposed. The procedure for the cold wire CVA is validated in the acoustic field of an acoustic resonator with wires of different lengths. A comparison between a CVA and a CCA measurement also confirms the CVA measurement. Furthermore, the proposed measurement procedure is applied close to the stack of a thermoacoustic refrigerator. Supposing a two-dimensional flow, the simultaneous measurement of velocity and temperature fluctuations is possible. This allows a detailed examination of the acoustic field close to the stack, including the study of the correlation between temperature and velocity.

Tip Leakage Flow: Advanced Measurements and Analysis

Advanced measurements of a tip clearance flow are carried out in the large subsonic anechoic wind tunnel of the LMFA in Lyon. The experimental set-up is obtained by placing the airfoil between two plates into the potential core of a turbulent rectangular jet, the upper plate being the hub and the lower the casing. The jet that exits a converging nozzle has a 450 mm cross-stream width and a 200 mm height in the span-wise direction. An h=10 mm gap is maintained between the blade and the casing plate. The incoming free stream turbulence u’/U0 is 0.5%, and the flat plate boundary layer half a chord upstream of the airfoil leading edge is about 8 mm thick. Except for the thickness being halved, the configuration is the same as the one discussed by Jacob et al.1, I.J.A, 9(3), (2010) , and Camussi et al.2, J.F.M., 660, (2010): a M ~ 0.2 and Rec ~ 950000 jet flow past a NACA5510 airfoil with 15° angle of attack. Measurements are carried out in the region of the Tip Leakage Vortex (TLV) in order to characterise its unsteady velocity. Both 2D–2 Component and stereo Time Resolved PIV techniques are successfully applied in order to provide an insight into the low frequency content of the velocity field, that are compared to LDV measurements. This experiment corresponds to a novel type of configuration that has only been studied by Jacob et al. Moreover, the TR-PIV and specifically the stereo-TR PIV are quite new techniques that have not often been applied to such complex configurations at such high speeds. The benefits of the time resolution are promising for the understanding of such broadband noise sources. Among the findings, there is a low frequency oscillation of the TLV, whose mechanism is yet unclear but which does not seem to radiate into the far field. Additionally, a hump at medium and high frequencies (0.7 – 7 kHz) is found in the far field. It can also be related to time decay double space-time correlations.

Time-resolved PIV measurements of a tip leakage flow

Time-resolved PIV and time-resolved stereo PIV measurements are carried out in the tip leakage vortex of a single non-rotating airfoil placed in the potential core of a flanged rectangular jet in free field conditions. The experiment is based on the improvement of an existing rig: a cambered airfoil (NACA5510) now mounted with a 16 . 5 ∘ ± 0 . 5 ∘ angle of attack between two horizontal plates, a 10 mm gap being maintained between the airfoil tip and the lower (casing) plate. The mean flow velocity is 70 m / s , which corresponds to a 0 . 2 Mach number and a chord-based Reynolds number of 933 , 000 . Unlike in the former experiment carried out with this rig, the boundary layer thickness is now smaller than the gap, which significantly reduces the interaction between the upstream turbulence and the airfoil leading edge as well as the resulting interaction noise. The measurements described here include the far field. The upstream flow is characterised with hotwire anemometry. LDV profiles are also obtained in the tip leakage region and compared to the PIV measurements. The experiment is also designed to provide validation data for unsteady CFD computations of the same configuration as shown in a companion paper.

An experimental investigation of wall pressure fluctuations beneath pressure gradients

The understanding of the structural response under excitation is often the first motivation to study wallpressure fluctuations. Vibrations and noise induced by turbulent-boundary-layer pressure is of importance in hydroacoustics, 7, 26 but also in aeronautics 27 and more recently in automotive applications. 5, 18 The aerodynamic part of wall-pressure fluctuations is associated with the indirect contribution to cabin noise through panel vibration while the acoustic part represents a direct contribution to this noise. Direct measurements of the wall pressure excitation by a turbulent boundary layer, including both aerodynamic and acoustic components of loading, is thus desirable. The large dynamic range between these two components makes this experimental characterization quite tricky. 11, 12 Mainly the incompressible part of wall-pressure fluctuations has been reported over the past fifty years. 16 It must be mentioned that these difficulties are also encountered in numerical simulations. 15, 17 Moreover, zero-pressure-gradient turbulent boundary layers are often considered, but numerous engineering applications involves the presence of pressure gradients. Only a fragmented view is currently offered regarding pressure gradient effects, even for modelling the aerodynamic loading. 20, 25 In a previous study by Arguillat et al., a rotating microphone array was used to estimate both the aerodynamic and the acoustic part of the wall pressure wavevector-frequency spectrum through an original post-processing. Results have been reported for a turbulent boundary layer at a Reynolds number Reδθ = uτδθ/ν = 1716 and at a moderate velocity U∞ = 44 m.s −1 and the feasibility of obtaining pressure spectra by this original approach was demonstrated. In these expressions, uτ denotes the friction velocity, δθ the momentum thickness and U∞ the free stream velocity of the boundary layer, and ν is the kinematic viscosity of the fluid. It was also noticed that some improvements could be carried out in the future regarding the test channel as well as the antenna. Starting from this study, wall pressure fluctuations induced by a turbulent flow were investigated and the experimental approach was revisited and significantly improved. In the present work, two experimental set-ups were used to better describe wall-pressure features beneath a turbulent boundary layer.

Experimental Investigation of Trailing-Edge Noise from a Linear Cascade of Cambered Airfoils

Glegg’s cascade broadband trailing-edge noise model is successfully compared to measurements on a linear cascade of seven cambered airfoils immersed in a low Mach number (M=0.23)-low Reynolds (Rec = 5.3× 10 ) quiet flow. Cascade resonances are also found and interpreted in addition to the broadband noise. The cascade solidity is σ = c/s = 1.43 and the blade chord c = 0.1 m. Far field acoustic PSD follows a classical U power law and scales with a Helmholtz number rather than a Strouhal number. Suction side surface pressure spectra Φpp are analysed and fitting laws are proposed for the spanwise coherence length lz and the convection velocity Uc to apply three analytical models. Amiet’s isolated airfoil noise model is used to assess the extent of the blade-to-blade interactions in the experiment. It is able to reproduce the far field noise when the acoustic wavelength is smaller than the cascade pitch. However at low frequencies, the effect of the cascade becomes very strong and the level increase reaches 15 dB at 200 Hz. Howe’s cascade trailing-edge model is applied but in its present form this model seems not able to reproduce the measured noise spectra. Glegg’s cascade trailing-edge model shows a very good agreement in the mid to high frequency range f > 800 Hz. Additionally cascade resonance frequencies are well predicted by Koch’s model.