Acoustic Analogy Approach Research Articles

Computational optimization of trailing-edge designs to reduce airfoil self-noise

This paper presents an investigation and optimization of Trailing Edge (TE) design to reduce airfoil self-noise using Computational Fluid Dynamics (CFD). The case for study is a NACA0012 airfoil with a chord length (C) of 0.2286 m, a varying Angle of Attack (AoA) between 0° and 15°, and free stream velocity between 35 and 70 m/s. The flow domain consists of a c-type domain with a length and height of 18C and 9C, respectively. The parametric mesh maintains a structured mesh on the entire domain for different designs and TE shapes. Simulations employ a hybrid Stress-Blended Embedded Large-Eddy Simulations (SB-ELES) model to calculate the flow properties. Different turbulence models are tested to address their performance in determining pressure fluctuations. A correlation length also accounts for spanwise effects in the Ffowcs-Williams and Hawkings (FW-H) acoustic analogy approach to forecasting the far-field noise. Furthermore, multi-objective optimization is employed to determine the optimum airfoil TE configuration for different flow velocities and AoAs. The optimum designs generate the lowest Sound Pressure Level (SPL) without significantly sacrificing the aerodynamic performance of the airfoil within specified parameters.

Investigation on the behavior of flow and aerodynamic noise generated around the tandem seal-vibrissa-shaped cylinder

Through comparing with the tandem circular and elliptic cylinders with the same characteristic dimensions, the behavior of flow and flow-induced noise generated around the tandem seal-vibrissa-shaped cylinder is studied based on delayed detached-eddy simulation model and acoustic analogy approach. The co-shedding pattern of flow developed around the tandem cylindrical-like bars is investigated. The spatial modes, mode energy, and mode coefficients of turbulent flow around the geometries are analyzed through spectral proper orthogonal decomposition. Results show that the lift fluctuations of downstream bar are stronger than those of upstream bar, and more aerodynamic noise is radiated from the downstream bar than from the upstream bar. The alternative arrangement of nodal and saddle planes of seal-vibrissa-shaped cylinder introduces three-dimensional flow separations and suppresses the shear layer interactions, inhibiting the regular vortex shedding of Kármán vortex street occurring in the tandem cylinder wake. The reversed vortex shedding generated by two adjacent saddle surfaces in the wake of seal-vibrissa-shaped cylinder balances the lateral force and mitigates the lift fluctuations greatly, thereafter reduces the aerodynamic noise generated by wall pressure fluctuations introduced by unsteady fluctuating forces exerting on the surfaces of geometries. Compared to the tandem circular and elliptic cylinders, the good noise reduction effect with sound pressure level reduced at main frequency range has been achieved from the tandem seal-vibrissa-shaped cylinder. The calculated spectra and amplitude levels of aerodynamic noise agree well with the experimental measurements from the anechoic wind tunnel, verifying the accuracy of the numerical simulations.

Flow-induced noise from a seal-vibrissa-shaped cylinder

Inspired by the special structure of seal vibrissae and their ability to effectively suppress vortex-induced vibrations, this paper studies the characteristics of flow and aerodynamic noise generated around a seal-vibrissa-shaped cylinder at a Reynolds number (based on the hydrodynamic diameter and freestream velocity) of 6 × 104 using the delayed detached-eddy simulation model combined with the acoustic analogy approach. The far-field aerodynamic noise has also been measured at Reynolds numbers of 6 × 104∼1.2 × 105 in an anechoic wind tunnel to compare the seal-vibrissa-shaped cylinder with cylindrical and elliptical bars of the same characteristic dimensions. The calculated spectra of aerodynamic noise agree well with the wind tunnel measurements, verifying the accuracy of the numerical simulations. It is found that the three-dimensional flow separations introduced by the alternative saddle and nodal planes of the seal-vibrissa-shaped cylinder inhibit shear layer interactions and reduce the spanwise coherence, resulting in a wake with no dominant coherent structures and thus suppressing pressure fluctuations on the solid surfaces. The configuration of seal-vibrissa-shaped cylinder eliminates the regular Kármán vortex street which generally occurs in a cylinder wake, and consequently inhibits the tonal peak in the aerodynamic noise. The sound pressure level is also reduced in most frequencies. The consistency of the normalized spectra of the far-field aerodynamic noise and the vortex shedding frequency at different inflow speeds demonstrates the high accuracy of the anechoic wind tunnel measurements. Compared with cylinders with circular or elliptical cross sections, the seal-vibrissa-shaped cylinder effectively reduces the flow-induced noise by, respectively, 13 dB or 11 dB for a far-field receiver of 90° at Reynolds number of 6 × 104, showing great potential in reducing aerodynamic noise.

Effect of bogie fairing on aerodynamic and aeroacoustic behaviour of a high-speed train nose car

ABSTRACT The influence of bogie fairing on the flow feature and aerodynamic noise performance of a high-speed train nose car model is studied based on large-eddy simulation combined with acoustic analogy approach. For the case having no fairing, the strong flow interactions and impingements occur around the bogie cavity region due to the mass addition and expulsion induced from the cavity leading edges. When a streamlined fairing is mounted outside the bogie cavity, the fluid entrainment and vortices evolvement within the bogie cavity are weakened and the shear layer impingement and flow separation outside the fairing are mitigated greatly. Thus the wall pressure fluctuations on the solid surfaces of geometries located within bogie cavity are decreased and the flow-induced noise is reduced noticeably. Based on the semi-anechoic wind tunnel measurements on a high-speed train nose car model without and with bogie fairing, it is found that the dominant sound source around bogie cavity moves from its central area to rear wall region and the aerodynamic noise is reduced effectively with a streamlined fairing mounted around the bogie cavity. The numerical predictions of aerodynamic noise are verified by the experimental measurements. The investigation would be beneficial for guiding the low-noise design and configuration optimization of a high-speed train bogie fairing.

Experimental and numerical investigation of the sound field produced by a shrouded UAV propeller

This paper investigates the effect of an acoustically rigid shroud on the tonal noise produced by an unmanned aerial vehicle (UAV) propeller. Enclosing the propeller with a shroud alters the steady loading on the propeller blades, thereby changing the steady loading tones radiated from the sources on the propeller surface. In this paper it is shown that the time-average rotating pressure field on the inner surface of the shroud also makes a significant contribution to the tonal noise. The aerodynamics of a shrouded and an isolated propeller was investigated using computational fluid dynamics (CFD) simulations. The tones produced by the steady loading and thickness sources on the propeller, and the time-average rotating pressure field on the surface of the shroud were calculated using an acoustic analogy approach and data from the CFD simulations. These calculations show that the tonal noise radiated from the loading sources on the inner surface of the shroud make a significant contribution to the tonal noise levels and that the contribution from the loading sources on the other surfaces of the shroud are negligible. A probe microphone was used to measure the time-average rotating pressure field on the inner surface of the shroud and these measurements showed good agreement with the CFD simulation. These measurements were then used to calculate the contribution of the loading sources on the inner surface of the shroud to the total noise measured experimentally. High resolution acoustic measurements in the near-field of a shrouded and an isolated propeller were also taken. The near-field measurements were then projected to the acoustic far-field and these levels are compared with numerical predictions. The results show that the effect of the shroud is to decrease the tonal noise for observers below the rotational plane of the propeller.

Study on flow-induced noise propagation mechanism of cylinder–airfoil interference model by using large eddy simulation combined with vortex–acoustic equation

An innovative numerical prediction method of flow-induced noise is implemented to overcome the defect that the traditional acoustic analogy method cannot reflect the interaction between turbulence vortex and sound. The classical cylindrical–airfoil interference model is used to perform the simulation and compared with the experimental results. To start with the derivation of Powell’s vortex sound equation, an implicit three-dimensional model of the fluid–acoustic coupling field is established to process the unsteady iterative calculation. The large eddy simulation method is adopted to solve the unsteady flow, and the acoustic information is then calculated using the vortex acoustic equation at each iteration step. The vortex structures around the cylinder airfoil are identified and captured by the Q-criterion for further analysis of vortex–noise correlation mechanism. The flow-induced noise prediction results are finally compared with Ffowcs Williams–Hawkings (FW–H) acoustic analogy approach. The results show that the vortex shedding from the cylinder and the interaction between vortex shedding and airfoil have the greatest influence on the acoustic, and the far-field noise of the cylinder airfoil shows a partial “eight” dipole distribution. The calculated results of the vortex sound theory are closer to the experimental ones than the FW–H method. The research helps understand the vortex acoustic coupling mechanism of the cylinder–airfoil model and provides a more accurate numerical prediction of flow-induced noise.

Numerical study on cavitation–vortex–noise correlation mechanism and dynamic mode decomposition of a hydrofoil

The large eddy simulation model coupled with the modified Schnerr–Sauer cavitation model has been used to numerically simulate the unsteady cavitation and noncavitation flow of the three-dimensional NACA66 (National Advisory Committee for Aeronautics) hydrofoil under different operating conditions. The results show that the magnitude of the cavitation number plays a decisive role in the hydrofoil cavitation quasiperiodic phenomenon. The cavitation number of 1.25 is used as a typical working condition for analysis. Using the Ffowcs Williams–Hawkings acoustic analogy approach accompanied by the vorticity transport equation splitting, the growth and shedding of cavitation also lead to the growth and shedding of the vortex structure. The cavitation–vortex interaction is mainly influenced by the vortex stretching term and vortex dilatation term and amplitude of them are larger than 500. The baroclinic torque term may be responsible for generating vorticity during the cloud cavitation collapse and has a lower amplitude about 200. The cavity volume acceleration is the main influencing factor of the low-frequency pressure fluctuation around the cavitating hydrofoil. Moreover, the NACA66 hydrofoil surface-pressure data are collected for dynamic mode decomposition to locate the hydrofoil surface noise sources. The alternate high and low amplitude regions in the mode results overlap highly with the cavitation transformation regions. The cavity transformation and pressure fluctuations are the main reason for the generation of periodic low-frequency noise source regions on the hydrofoil surface. Moreover, the corresponding frequencies of each order mode are linearly correlated with the cavitation shedding frequency of 5.70 Hz. Combined with the results of the multiple mode comparisons, it can be inferred that the hydrofoil suction surface under the cavitation effect will generate quasiperiodic waves starting from upstream and moving downstream.

Investigation on aerodynamic noise generated from the simplified high-speed train leading cars

The aerodynamic noise behavior of flow passing the simplified leading car and nose car scale models of a high-speed train is investigated through the vortex sound theory and acoustic analogy approach. The unsteady flow developed around the geometries is solved numerically and the data are applied to study the near-field quadrupole sound source and calculate the far-field noise radiated. It is found that the turbulent flow developed around the leading car is characterized by multi-scale vortices separated from the geometries. The intensity of volume dipole source is much larger than that of volume quadrupole source and the volume dipole source becomes the predominate source of the near-field quadrupole noise. The flow is separated noticeably in the regions of the nose, bogies, bogie cavities, and train tail of the leading car where the pressure fluctuations are generated largely upon the solid surfaces and correspondingly a dipole noise of high level is produced. By comparison, the noise contribution from the leading bogie and bogie cavity is larger than that from the other components. Moreover, the numerical and experimental results of train nose car model demonstrate that the flow around the bogie region is the dominant aerodynamic sound source. Therefore, the flow-induced noise generated from the leading cars may be reduced efficiently within a certain frequency range and specific direction by mitigating the flow interactions around the areas of leading bogie and bogie cavity.

Investigation of unsteady cryogenic cavitating flow and induced noise around a three-dimensional hydrofoil

In this paper, the Large Eddy Simulation (LES) combined with the Schnerr–Sauer cavitation model and the permeable Ffowcs Williams–Hawkings (FW-Hpds) acoustic analogy approach are introduced to study the unsteady cavitation behaviors and the radiated noise characteristics of the transient liquid nitrogen (LN2) cavitating flow around a NACA66 (National Advisory Committee for Aeronautics) hydrofoil. Satisfactory agreement is obtained between the numerical predictions and experimental measurements. The cavitation noise is predicted based on the sound radiation theory for spherical bubbles and compared with the sound pressure levels of non-cavitating flow from the FW-Hpds equation. It is found that the cavity volume acceleration is directly responsible for driving the generation of cavitation noise, and the sound pressure caused by the development of LN2 cavitation is shown to vary with the periodic pulsing cavity volume evolution, indicating a strong link between cavity evolutions and radiated noises. The transient cavitation structures of the sheet and cloud cavitation are well captured, and the evolution features of the cavities and vortex structures are analyzed in detail. The collapse of the detached small cloud cavity downstream is the main mechanism for generating intense acoustic impulses for both sheet and cloud cavitation. While the strong interaction between the re-entrant jet and the main flow results in violent pressure fluctuations, and thus produces instantaneous extreme dipole noise, which accounts for another distinctive mechanism to induce intense acoustic impulses for cloud cavitation, the presented study provides a deep understanding of the nature of cavitation-dominated noise for cryogenic cavitating flow.

Numerical study of flow and noise predictions for tandem cylinders using incompressible improved delayed detached eddy simulation combined with acoustic perturbation equations

The method of combining incompressible hybrid Reynolds average Navier–Stokes/large eddy simulation (RANS/LES) with an acoustic perturbation equation was studied to accurately predict the flow and noise performances in NASA tandem cylinder experiments. In this approach, acoustic waves simultaneously propagate in the time domain with an unsteady turbulent flow. The experiment was performed under the condition of a Reynolds number of 1.66 × 105, based on the cylinder diameter, which provided detailed surface pressure and flow field measurements. The improved delayed detached eddy simulation (IDDES) model established in this study was consistent with the experimental results, except for the flow field behind the rear cylinder, which was caused by an early separation relative to the experiment. The acoustic results showed that the rear cylinder dominated the noise radiation, which indicated that the pressure fluctuation caused by wake interference was greater than that caused by vortex shedding. Acoustic predictions were also obtained using an acoustic analogy approach based on the FH-W method. Three kinds of integral surfaces (PS-open, PS-close, and IPS) were made for the acoustic predictions using the Ffowcs Williams–Hawkings method. This study also showed that wavenumber decomposition could be utilised to quantify the mechanisms related to turbulent convection and sound propagation. An IDDES hybrid model for predicting near-field and far-field noise was established, which could be applied to practical engineering applications.

Aeroacoustic topology optimization of noise barrier based on Lighthill's acoustic analogy

Topology optimization for the shape of noise barrier taking into account the aeroacoustic sound source is presented based on Lighthill's acoustic analogy approach. In the previous acoustic topology optimization based on the Helmholtz equation, only simple acoustic sound sources were taken into account. To overcome this limitation, the present aeroacoustic topology optimization is performed using Lighthill's equation that represents the sound propagation by an aeroacoustic sound source. In this study, aeroacoustic sound sources are randomly generated through the stochastic noise generation and radiation method. As an interpolation method of material properties by continuous design variables, the linear filter and the sigmoid penalty function are used. The top shape of noise barrier is designed by the optimization examples for various conditions, such as random aeroacoustic sound sources, interpolation methods, and target frequencies.

Transient cavitating flow structure and acoustic analysis of a hydrofoil with whalelike wavy leading edge

In this paper, the open source software OpenFOAM is used to perform a numerical investigation of the cavitating flow around a modified NACA634-021 hydrofoil with bioinspired, wavy leading edge, with particular emphasis on study of the interactions between the cavitation and the streamwise vortices and the far-field radiation noise. A modified k-ω shear-stress transport (SST) model coupled with the Schnerr-Sauer cavitation model and the Ffowcs Williams-Hawkings (FW-H) acoustic analogy approach are introduced to the simulation. The transient cavitation structure and the streamwise vortices are captured well and the results show significant interactions between the cavitation and the streamwise vortices. Cavitation can promote fragmentation of the streamwise vortices, while the streamwise vortices cause cavitation inception to occur earlier and bind the cavitation within the trough region by changing the pressure distribution on the hydrofoil. The transformation of the FW-H equation's solution indicates that the monopole noise is directly related to the cavitation volume acceleration and the dipole noise is related to the mechanical force of the hydrofoil on fluids and the rate at which this force changes. The collapse of cavitation cloud and the collision of the re-entrant jet and main flow will cause violent fluctuation of the mechanical force thus produce instantaneous extreme dipole noise values, while the monopole noise is relatively strong in the cavitation collapse stage due to significant cavitation volume acceleration. The time domain hydroacoustic characteristics are similar for the modified hydrofoil and the baseline hydrofoil.

Influence of Free Stream Effects on Jet Noise Generation and Propagation within the Goldstein Acoustic Analogy Approach for Fully Turbulent Jet Inflow Boundary Conditions

Sound generation and propagation by low-Reynolds number turbulent jets from a DNS database is modelled using the Goldstein acoustic analogy method. In comparison with previous work, the DNS data feature fully resolved turbulent boundary layers at the pipe nozzle exit and include the free-stream flow to study the influence of coflow on effective noise sources. To investigate the statistics of turbulent sources, fourth-order correlation analysis is applied to the DNS data. The results are in reasonable agreement with the conventional Gaussian turbulence statistics source model. For meanflow sound propagation modelling, adjoint linearised Euler equations are solved in the frequency domain. Far-field acoustic predictions are obtained for 40° and 90° observer angle to the jet flow and compared with the reference far-field noise spectra obtained from the same DNS database. To investigate the validity of some commonly made approximations, the results for the locally parallel meanflow propagation model and those for the compact eddy scale approximation are also presented and compared with the reference solution.

Three-stream jet noise measurements and predictions

An experimental and numerical investigation of the noise produced by high-subsonic three-stream jets was conducted. The exhaust system consisted of externally mixed-convergent nozzles and an external plug. Bypass- and tertiary-to-core area ratios between 1 and 1.75, and 0.4 and 1.0, respectively, were studied. Axisymmetric and offset tertiary nozzles were investigated for heated and unheated conditions. For axisymmetric configurations, the addition of the third stream was found to reduce mid- and high-frequency acoustic levels in the peak-jet-noise direction, with greater reductions at the lower bypass-to-core area ratios. The addition of the third stream also decreased peak acoustic levels in the peak-jet-noise direction for intermediate bypass-to-core area ratios. For the offset configurations, an s-duct was found to increase acoustic levels relative to those of the equivalent axisymmetric-three-stream jet while half-duct configurations produced acoustic levels similar to those for the axisymmetric jet for azimuthal observation locations of interest. Comparisons of noise predictions with acoustic data are presented for selected unheated configurations. The predictions are based on an acoustic analogy approach with mean flow interaction effects accounted for using a Green’s function, computed in terms of its coupled azimuthal modes, and a source model previously used for round and rectangular jets.

Tandem cylinder flow and noise predictions using a hybrid RANS/LES approach

The performance of a novel hybrid RANS/LES methodology for accurate flow and noise predictions of the NASA Tandem Cylinder Experiment is investigated. The proposed approach, the modified Flow Simulation Methodology (FSM), is based on scaling the turbulence viscosity and the turbulence kinetic energy dissipation rate with a damping function. This damping function consists of three individual components, a function based on the Kolmogorov length-scale ensuring correct behaviour in the direct numerical simulation (DNS) limit, a function ensuring that FSM provides the correct damping in large-eddy simulation (LES) mode, and a shielding function that forces the switch from Reynolds-Averaged Navier–Stokes (RANS) to LES to occur outside the boundary layer. The FSM is proposed for the kω-SST two-equation model (FSM-SST) and for an Explicit-Algebraic-Stress-Model (FSM-EASM), which is better suited to resolve anisotropy and non-equilibrium of the unresolved scales and the strain and rotation-rate dependent coefficients introduce a dynamic response of the model to the resolved flow field. Simulations are performed on a relatively coarse grid and the FSM data are compared with results obtained from the Scale-Adaptive-Simulation (SAS) and IDDES approaches. Acoustic predictions are obtained using an acoustic analogy approach based on Curle’s theory. The FSM-SST approach was found to predict the hydrodynamic field in very good agreement with reference data, whereas the FSM-EASM did not improve the predictions. The acoustic spectra predicted show good agreement with experimental results at various microphone positions, with some deficiencies in capturing the broadband noise levels at high Strouhal numbers.

Noise Predictions for Rectangular Jets Using a Conformal Mapping Method

An acoustic analogy approach is used to predict the turbulent mixing noise emitted by cold, subsonic, rectangular jets of aspect ratios 2, 4, and 8. Mean flow interaction effects are accounted for in terms of a Green’s function under the locally parallel mean flow approximation. The problem for the Green’s function is reduced to a set of coupled ordinary differential equations by modeling the mean flow as having level surfaces in crossflow planes coinciding with one coordinate in a cylindrical elliptical coordinate system and using a conformal mapping method. The Green’s function is combined with a previously developed hybrid source model to complete the prediction method. Source model coefficients are chosen based on the results of low-speed flow surveys in the nozzles considered. Results are compared with acoustic data, and it is found that the method is generally capable of predicting the measured azimuthal directivity characteristics of the sound field.

On Acoustic Radiation by Rotating Body Sources in Frequency Domain

For the complexity of the sound generated mechanism of rotating body source, this paper describes a frequency-domain numerical method for predicting sound radiation of rotating body sources based on the Kirchhoff integral equation with analytical Green's function of rotating monopole and dipole source in free space. The sound radiation model is established in free space and the relationship between characteristics of sound field and acoustic nature frequency of source, angular frequency and its harmonics can be revealed by the mathematical solution. Numerical simulation shows that sound field has a strong directivity, fundamental frequency transmitting in the rotary shaft direction and harmonics spreading along radial direction and frequency shift phenomena appearing clearly in higher rotating speed of source. The method has a theoretical significance for exploring the low-noise rotating machinery. Research about sound radiation of moving source originates from Lighthill. The first integral approach for acoustic propagation is the acoustic analogy. In the acoustic analogy, the governing Navier-Stokes equations are rearranged to be in wave-type form. There is some question as to which terms should be identified as part of the sound source and retained in the right-hand side of the equation and which terms should be in the left-hand side as part of the operator. The far-field sound pressure is then given in terms of a volume integral over the domain containing the sound source. However the major difficulty with the acoustic analogy is that the sound source is not compact in supersonic flows. Several modifications to Lighthill's original theory have been proposed to account for the moving sound source or other effects. The Ffowcs Williams and Hawkings (FW-H) equation was introduced to extend acoustic analogy in the case of solid surfaces. However, when acoustic sources are present in the flowfield a volume integration is needed. This volume integration of the quadrupole source term is difficult to compute and is usually neglected in most acoustic analogy codes. Errors could be encountered in calculating the sound field. Another alternative is the Kirchhoff method which assumes that the sound transmission is governed by the simple wave equation. Kirchhoff's method consists of the calculation of the nonlinear near- and mid-field, usually numerically, with the far-field solutions found from a linear Kirchhoff formulation evaluated on a control surface surrounding the nonlinear-field. The control surface is assumed to enclose all the nonlinear flow effects and noise sources. The sound pressure can be obtained in terms of a surface integral of the surface pressure and its normal and time derivatives. This approach has the potential to overcome some of the difficulties associated with the traditional acoustic analogy approach. The method is simple and accurate and accounts for the nonlinear quadrupole noise in the far-field. Full diffraction and focusing effects are included while eliminating the propagation of the reactive near-field. Kirchhoff's formula, published in 1882, is used in the theory of diffraction of light and in other electromaqnetic problems. It also has many applications to problems of wave in acoustics. One of the novel uses of this formula was proposed by Hawkinqs for predicting

Hybrid Source Model for Predicting High-Speed Jet Noise

This paper introduces a novel hybrid source model into an existing acoustic analogy approach to obtain improved predictions of the turbulent mixing noise from cold, round, subsonic, and supersonic jets. The model incorporates new features of the Reynolds stress autocovariance tensor components found in recent experiments. The model parameters are determined from a Reynolds-averaged Navier―Stokes flow solution and experimental data. It is shown that this model significantly improves the predictions relative to previous results, particularly at observer polar angles between 90 degrees to the jet axis and the peak noise direction, indicating the importance of properly modeling relatively subtle characteristics of the autocovariance functions. The results are used to infer the relative importance of individual terms that make up the formula for the acoustic spectrum as a function of jet Mach number, frequency, and observer location.

Measurement and analysis of the turbulent length scales in jet flows

The characteristic length and time scales of turbulence are reported in some detail for jet flows. The objective of the work is to determine the frequency dependence of these two-point turbulent properties, which are used to model the sources necessary for noise prediction using the acoustic analogy approach. A range of jet flow conditions for single and co-axial configurations are considered so that the effect of Mach number, temperature ratio and nozzle geometry is examined. The frequency dependence of both the fixed and moving frame length scales and the convection velocities for both the turbulence and the Reynolds stress are derived using a two-point complex coherence function. At higher frequencies, the integral scales are found to be strongly isotropic and inversely proportional to the Strouhal number. A frequency-dependent Taylor scale is derived and shown to agree well with the experimental results at the higher frequencies.

Determination of two-dimensional space–time correlations in jet flows using simultaneous PIV and LDV measurements

The two-dimensional space–time turbulence statistics of free shear jet flows in the form of the two-point velocity correlation tensor are important for aeroacoustic noise source modelling based on the acoustic analogy approach. This paper presents a direct application of the Point-Referenced Global Correlation (PRGC) technique to measure the components of this correlation tensor for a sector of the flow field in two jet configurations. The PRGC approach combines single point and global measurement techniques and enables two-point space–time correlations over a region of the flow to be obtained. The technique is applied to a single stream jet and a co-axial coplanar jet at a Mach number of 0.24 using commercial Laser Doppler velocimetry (LDV) and low-speed particle image velocimetry (PIV) systems. Results for the one-dimensional correlations are shown to compare well with two-point measurements. The results for the two-dimensional space–time correlations are presented and the characteristics for both configurations discussed.