Jet-surface Interaction Research Articles

Real-Time Estimation of Jet-Surface Interaction Noise

Motivated by the growing concerns for the noise emissions generated by the interaction between the engine jet and the airframe surfaces consequent to the progressive increase of the engine by-pass ratio in commercial aircraft architectures, we study the estimation of the far-field noise radiated by a jet in an installed configuration based on sensor readings in the near field of the jet. We carry out an experimental investigation of the jet-surface interaction phenomena in a simplified set-up where a flat plate is installed parallel to the nozzle axis of a subsonic jet. Real-time estimation is performed using empirical linear transfer functions identified between near-field sensors and far-field observers. The transfer function and the accuracy of the noise estimation performed are characterised as a function of jet flow conditions, type and streamwise position of near-field sensors and radial distance between the jet and the edge. This analysis can guide the positioning of near-field sensors for the implementation of closed-loop control strategies for the jet-surface interaction noise.

An experimental investigation into the influence of installed passively controlled jet flows on wall-pressure fluctuations

Jet-surface interaction(JSI) represents a significant noise problem in the installation of modern ultra-high bypass ratio turbofan engines. The use of chevron nozzles is known to reduce low-frequency jet mixing noise(JM) by increasing the velocity gradient in the jet shear layer. This effect is expected to influence the jet flow in the vicinity of the wing and to modify the jet-surface interaction noise. To clarify the physics of these two(JSI and JM) noise source mechanisms, an extensive experimental investigation has been conducted in the anechoic chamber of the Doak Laboratory at the University of Southampton. Various measurements were carried out on an isolated subsonic jet and an installed beneath a 2D NACA4415 airfoil using different nozzle shapes and passive vortex generator geometries. The wall-pressure field on the airfoil surface was investigated using a set of wall-pressure transducers flush-mounted in the streamwise and spanwise directions on the pressure side of the airfoil. The unsteady wall-pressure data were analysed in both time and frequency domains to assess changes in: 1) surface trailing edge spanwise correlation length, 2) streamwise convection velocity of the hydrodynamic pressure field, and 3) modification of the hydrodynamic pressure field in the vicinity of the surface trailing edge.

Numerical Investigation of a Rectangular Jet Exhausting over a Flat Plate with Periodic Surface Deformations at the Trailing Edge

Multiple competing factors are forcing aircraft designers to reconsider the underwing engine pod configuration typically seen on most modern commercial aircraft. One notable concern is increasing environmental regulations on noise emitted by aircraft. In an attempt to satisfy these constraints while maintaining or improving vehicle performance, engineers have been experimenting with some innovative aircraft designs which place the engines above the wings or embedded in the fuselage. In one configuration, a blended wing concept vehicle utilizes rectangular jet exhaust ports exiting from above the wing ahead of the trailing edge. While intuitively one would think that this design would reduce the noise levels transmitted to the ground due to the shielding provided by the wing, experimental studies have shown that this design can actually increase noise levels due to interactions of the jet exhaust with the aft wing surface and flat trailing edge. In this work, we take another look at this rectangular exhaust port configuration with some notional modifications to the geometry of the trailing edge to determine if the emitted noise levels due to jet interactions can be reduced with respect to a baseline configuration. We consider various horizontal and vertical offsets of the jet exit with respect to a flat plate standing in for the aft wing surface. We then introduce a series of sinusoidal deformations to the trailing edge of the plate of varying amplitude and wave number. Our results show that the emitted sound levels due to the jet–surface interactions can be significantly altered by the proposed geometry modifications. While sound levels remained fairly consistent over many configurations, there were some that showed both increased and decreased sound levels in specific directions. We present results here for the simulated configurations which showed the greatest decrease in overall sound levels with respect to the baseline. These results provide strong indications that such geometry modifications can potentially be tailored to optimize for further reductions in sound levels.

Jet-installation noise and near-field characteristics of jet–surface interaction

The link between jet-installation noise and the near-field flow features of the corresponding isolated jet is studied by means of lattice-Boltzmann numerical simulations. The computational set-up consists of a flat plate placed in proximity to a jet, replicating the interaction benchmark study carried out at NASA Glenn. Installation effects cause low-frequency noise increase with respect to the isolated case, mainly occurring in the direction normal to the plate and upstream of the jet's exit plane. It is shown that the Helmholtz number, based on the wavelength of eddies in the mixing layer and their distance to the plate trailing edge, predicts the frequency range where installation noise occurs. Based on the isolated jet near field, scaling laws are also found for the far-field noise produced by different plate geometries. The linear hydrodynamic field of the isolated jet shows an exponential decay of pressure fluctuations in the radial direction; it is shown that the far-field spectrum follows the same trend when moving the plate in this direction. In the axial direction, spectral proper orthogonal decomposition is applied to filter out jet acoustic waves. The resultant hydrodynamic pressure fluctuations display a wavepacket behaviour, which can be fitted with a Gaussian envelope. It is found that installation noise for different plate lengths is proportional to the amplitude of the Gaussian curve at the position of the plate trailing edge. These analyses show that trends of jet-installation noise can be predicted by analysing the near field of the isolated case, reducing the need for extensive parametric investigations.

Validation of a Jet–Surface Interaction Noise Model in Flight

Several methods for calculating jet–surface interaction noise have been developed recently and validated against laboratory measurements. However, further validation cases are required that include the effect of a flight stream. This Paper assesses the ability of the method of Lyu and Dowling to predict the effect of forward flight on jet–surface interaction noise produced by a laboratory jet. The main inputs to the method are near-field pressure measurements of the isolated jet, recorded within the flight-stream flow using a microphone with a nose cone. The predicted jet–surface interaction noise is then compared with measured far-field installed jet noise spectra over polar angles from 70 to 130 deg on the unshielded side of the jet. The results show that the model is capable of accurately reproducing both the amplitude and spectral shape of the jet–surface interaction noise up to a flight-stream Mach number of 0.2.

Offset height effect on turbulent characteristics of twin surface jets

The characteristics of square twin surface jets at various offset heights from the free surface were studied using a particle image velocimetry technique. The offset heights were varied from 1 to 4 nozzle widths at a fixed Reynolds number of 3890. The entrainment and mixing characteristics were examined using the potential core length, merging point, combined point, maximum velocity decay, half-velocity width and were found be nearly independent of offset height in near field. The jet–surface interaction was examined by surface velocity, vorticity thickness and surface turbulence intensities. The growth rate of the vorticity thickness reduced in the interaction region; and was more severe for shorter offset height. Turbulent structures were examined using weighted joint probability density function and two-point cross-correlations between swirling strength and velocity fluctuations. Weaker turbulent events were observed for deeper jet close to a free surface at the streamwise location beyond the attachment point.

On the modelling of wavepacket scattering noise with coherence effects.

An investigation of a wavepacket model for free-jet and jet-surface interaction noise was conducted. The source term for the axisymmetric mode was extracted from a Mach 0.9 jet large eddy simulation and employed to adjust the parameters of a simple source model. Streamwise coherence decay, in particular, was considered. The source model was propagated with both the free-field and tailored Green's function for a semi-infinite flat plate positioned at a distance of r/D = 1 from the jet axis. Significant deviations were observed in the prediction of the low-angle directivity of the isolated jet as well as in the reproduction of the characteristics of the source field. However, the effects of trailing edge noise were well reproduced. The installed jet case, at the region dominated by trailing-edge scattering, showed very little sensitivity to the coherence decay, a crucial feature in the isolated jet case. In this sense, the modelling of the installed-jet case proved to be much simpler.

Effects of Nozzle Geometry on Turbulent Characteristics and Structure of Surface Attaching Jets

The effects of nozzle exit geometry on the characteristics of a surface attaching jet were investigated experimentally. Three different types of nozzle geometries were studied, including a circular, square and rectangular nozzle with the same exit area. The jets were discharged with an offset height of two nozzle width at a Reynolds number of 5500. A particle image velocimetry system was used to measure the flow. Instantaneous visualizations revealed that the largest enhancements in the near field mixing and entrainment occur in the minor plane of the rectangular nozzle compared to the square and circular nozzles resulting in more rapid expansion of the shear layer. This also caused a faster deflection of the jet towards the free surface, maximum velocity decay and spread rates of the rectangular jet with minor axis orientation. The jet-surface interaction was examined using surface velocity, vorticity thickness and surface turbulence intensities. It was found that the damping of the surface-normal velocity fluctuations at the free surface is more severe in the minor plane of the rectangular jet than the square and circular nozzles. The influence of the free surface was also felt in the profiles of mean velocity and Reynolds stresses. The attenuation of Reynolds shear stress due to confinement was more dramatic in the minor plane of the rectangular nozzle than the other geometries. To quantify the influence of nozzle geometry on the coherent structures in the interaction region, two-point correlations of the velocity fluctuations and swirling strength; and proper orthogonal decomposition were performed.

Plasma-surface interactions in atmospheric pressure plasmas: In situ measurements of electron heating in materials

The energy flux to a surface during plasma exposure and the associated surface heating are of long standing interest as they contribute to the physico-chemical changes that occur during plasma-based materials synthesis and processing. Indeed, the energy delivered to the surface, via a flux of particles and photons, in concert with a flux of reactive species serves to chemically modify, etch, and/or deposit materials, with an efficacy that depends on the plasma processing environment. A unique feature of plasma synthesis and processing is that most of the delivered energy is absorbed at or very near the surface over short (picosecond) time scales. The dissipation of thermal energy proceeds through electron-electron and/or electron-phonon interactions as they propagate through the material, with relaxation time scales that can be orders of magnitude slower. Typically then, the surface is not in thermal equilibrium with the bulk material. Fast, surface-sensitive techniques are thus required to fully appreciate the dynamics of the plasma-surface interaction. In this work, we employ pump-probe Time-Domain Thermoreflectance, a surface sensitive technique typically used to measure thermal properties of thin films, to determine electron heating of thin metal films during exposure to an atmospheric pressure plasma jet. The results, in conjunction with current measurements, are used to develop a first order understanding of plasma jet-surface interactions. The results show that the energy delivered by the plasma jet causes a localized increase in electron energy within the thin film over an area commensurate with the plasma jet radius.

Turbulence statistics and distribution of turbulent eddies for jet flow and rigid surface interaction

The behavior of turbulent eddies within the self-similar region of a rigid surface interacting round jet is experimentally investigated. Results show that the turbulent jet flow structure is significantly affected due to the rigid surface interaction; particularly within the lower portion of the jet shear layer. It is observed that the jet and rigid surface interactions rather enhance the scale of axial velocity fluctuations within the intermediate region of the jet. An additional mixing layer is observed in the lower shear layer region close to the rigid surface due to the production of eddies from the rigid surface. The depth of penetration of the fluctuating eddies decreases significantly at the mixing layer region and this mixing layer acts like a shield which restricts the downward propagation of fluctuating eddies from the plane of symmetry of the jet. The results suggest that the region below the mixing layer can be treated as the shear less mixing region. The interesting consequence of this is that the rate of production of vorticity is enhanced below the mixing layer close to the rigid surface. Also, the enstrophy destruction is favored over enstrophy production at the upper portion of the mixing layer, and exactly the opposite phenomenon is observed in the lower portion of the mixing layer.

Experimental investigation of the far-field noise due to jet-surface interaction combined with a chevron nozzle

High-speed jets exhausting from engines, especially during takeoff, remain one of the main sources of external aircraft noise. Regulations on aircraft noise levels in regions close to airports are very strict and are forcing the aerospace industry to develop quieter aircrafts. Mixing enhancement devices at the exit of nozzles, such as chevrons, have assisted in the reduction of noise levels for some time. However, installation effects are becoming more intense due to highly integrated aircraft-engine configurations, which are currently employed by the aviation industry as an attempt to increase the aerodynamic efficiency and to reduce noise. Consequently, these installation effects may interfere with the noise reducing performance of such devices. This paper reports experimental investigations involving the combined effect of a flat plate integrated with nozzles, both with and without chevrons, exhausting cold subsonic jets. Assessment of the far-field noise was conducted for different Mach numbers and several configurations formed by relative positions between the nozzle and the plate. In general, it was observed that the acoustic benefit of chevrons, as found in isolated jets, remains effective in most of installed configurations. Nevertheless, for highly integrated configurations the low-frequency noise reduction provided by the chevron nozzle becomes almost negligible, whereas the increase of high-frequency noise becomes substantial and the shielding effect is observed to decrease.

Effect of de-correlating turbulence on the low frequency decay of jet-surface interaction noise in sub-sonic unheated air jets using a CFD-based approach

In this paper we extend the Rapid-distortion theory (RDT)-based model derived by Goldstein, Afsar & Leib (J. Fluid Mech., vol. 736, pp. 532-569, 2013) for the sound generated by the interaction of a large-aspect-ratio rectangular jet with the trailing edge of a flat plate to include a more realistic upstream turbulence spectrum that possess a de-correlation (i.e. negative dip) in its space-time structure and use results from three-dimensional Reynolds-Averaged Navier-Stokes (RANS) solutions to determine the mean flow, turbulent kinetic energy and turbulence length & time scales. Since the interaction noise dominates the low-frequency portion of the spectrum, we use an appropriate asymptotic approximation for the Rayleigh equation Green’s function, which enters the analysis, based on a two-dimensional mean flow representation for the jet. We use the model to predict jet-surface interaction noise for a range of subsonic acoustic Mach number jets, nozzle aspect ratios, streamwise and transverse trailing-edge locations and compare them with experimental data. The RANS meanflow computations are also compared with flow data for selected cases to assess their validity. We find that finite de-correlation in the turbulence spectrum increases the low-frequency algebraic decay (the low-frequency “roll-off”) of the acoustic spectrum with angular frequency to give a model that has a pure dipole frequency scaling. This gives better agreement with noise data compared to Goldstein et al. (2013) for Strouhal numbers less than the peak jet-surface interaction noise. For example, through sensitivity analysis we find that there is a difference of 10 dB at the lowest frequency for which data exists (relative to a model without de-correlation effects included) for the highest acoustic Mach number case. Secondly, our results for the planar flow theory provide a first estimate of the low-frequency amplification due to the jet-surface interaction for moderate aspect ratio nozzles when RANS meanflow quantities are used appropriately. This work will hopefully add to noise prediction efforts for aircraft configurations in which the exhaust systems are tightly integrated with the airframe.

Jet Surface Interaction – Scrubbing Noise in a Transversely Sheared Mean Flow

Generation of sound due to scrubbing of a jet flow past a nearby solid surface is investigated within the framework of the generalized acoustic analogy theory. The analysis applies to the boundary layer noise generated at and near a wall, and excludes the scattered noise component that is produced at the leading or the trailing edge. While compressibility effects are relatively unimportant at very low Mach numbers, frictional heat generation and thermal gradient normal to the surface could play important roles in generation and propagation of sound in high speed jets of practical interest. A general expression is given for the spectral density of the far field sound as governed by the variable density Pridmore-Brown equation. The propagation Green's function is solved numerically starting with the boundary conditions on the surface and subject to specified mean velocity and temperature profiles between the surface and the observer. It is shown the magnitude of the Green's function decreases with increasing source frequency or jet temperature. The phase remains constant for a rigid surface, but varies with source location when subject to an impedance type boundary condition. The equivalent sources of aerodynamic sound are associated with non-linear momentum flux and enthalpy flux terms that appear in a linear set of equations that govern the fluctuating components of the motion. These multi-pole sources are usually modeled and evaluated with input from a Reynolds-Averaged Navier-Stokes (RANS) solver with an appropriate turbulence model.

A ‘tissue model’ to study the plasma delivery of reactive oxygen species

We demonstrate the utility of a ‘tissue model’ to monitor the delivery of plasma jet-generated reactive oxygen species (ROS). We report on helium plasma jet interactions both across the surface and into the subsurface (defined as 150 µm to 1.5 mm) of the tissue model. The model comprises a gelatin gel encapsulating a homogeneously dispersed chemical or biological reporter molecule. Jet–surface interactions result in (i) star shaped patterns that resemble those previously reported for surface-plasma streamers on insulators (as imaged by Pockels sensing) and (ii) ‘filled’ or hollow circular surface features, which resemble the ‘killing’ patterns seen in plasma jet treatments of bacterial lawns.The use of reporter molecules show that plasma can deliver ROS from 150 µm to 1.5 mm below the tissue surface. Subsurface delivery of ROS is consistent with the use of plasma to decontaminate wounds (covered by wound exudate and clotted blood), the deactivation of whole biofilms, plasma-enhanced drug delivery through skin and the destruction of solid tumours.From the data presented, we argue that in these four cases (and others) ROS may be capable of directly accessing a tissue's subsurface, as opposed to other proposed mechanisms, which involve stimulating surface reactions that trigger a cascade of biomolecular signalling events (into the tissue).

Jet-Surface Interaction Test: Far-Field Noise Results

Many configurations proposed for the next generation of aircraft rely on the wing or other aircraft surfaces to shield the engine noise from the observers on the ground. However, the ability to predict the shielding effect and any new noise sources that arise from the high-speed jet flow interacting with a hard surface is currently limited. Furthermore, quality experimental data from jets with surfaces nearby suitable for developing and validating noise prediction methods are usually tied to a particular vehicle concept and, therefore, very complicated. The Jet-Surface Interaction Tests are intended to supply a high quality set of data covering a wide range of surface geometries and positions and jet flows to researchers developing aircraft noise prediction tools. The initial goal is to measure the noise of a jet near a simple planar surface while varying the surface length and location in order to: (1) validate noise prediction schemes when the surface is acting only as a jet noise shield and when the jet-surface interaction is creating additional noise, and (2) determine regions of interest for future, more detailed, tests. To meet these objectives, a flat plate was mounted on a two-axis traverse in two distinct configurations: (1) as a shield between the jet and the observer and (2) as a reflecting surface on the opposite side of the jet from the observer. The surface length was varied between 2 and 20 jet diameters downstream of the nozzle exit. Similarly, the radial distance from the jet centerline to the surface face was varied between 1 and 16 jet diameters. Far-field and phased array noise data were acquired at each combination of surface length and radial location using two nozzles operating at jet exit conditions across several flow regimes: subsonic cold, subsonic hot, underexpanded, ideally expanded, and overexpanded supersonic. The far-field noise results, discussed here, show where the jet noise is partially shielded by the surface and where jet-surface interaction noise dominates the low frequency spectrum as a surface extends downstream and approaches the jet plume.

The thermal signature of a low Reynolds number submerged turbulent jet impacting a free surface

The thermal signature of a low Reynolds number turbulent jet impacting a free surface was investigated experimentally. Three Reynolds numbers (1000, 3000, and 4800) were investigated for a configuration in which the jet nozzle diameter and the depth of the jet beneath the free surface were fixed. A high resolution infrared detector was used to collect thermal imagery of the surface temperature field. These data were then used to examine the detailed statistical nature of the resulting coupled thermal-hydrodynamic field. The analysis included an examination of the instantaneous, mean, and fluctuating surface thermal fields. Examination of the instantaneous fields strongly suggested the existence of a turbulent core region and a weaker outer region. The existence of this inner-outer structure associated with the surface flow was confirmed by a detailed examination of the mean surface temperature fields. In addition, the outer structure of the mean surface temperature appeared to correspond well with the existence of a surface current first observed by Anthony and Willmarth [“Turbulence measurements in a round jet beneath a free surface,” J. Fluid Mech. 243, 699 (1992)]. A self-similar region of the temperature field associated with the turbulent core was also clearly identified. Finally, the statistics of the surface thermal fluctuation fields were examined. These statistics revealed high thermal fluctuations near the edge of the flow field associated with flow intermittency there, as well as evidence of surface capillary waves which were generated as a result of the jet-surface interaction. In addition, the thermal fluctuation field was further examined using a Karhunen–Loeve analysis.

P1901 Factors associated with the level of HIV-1 proviral DNA in HIV-infected persons

In this paper we extend the Rapid-distortion theory (RDT)-based model derived by Goldstein, Afsar & Leib (J. Fluid Mech., vol. 736, pp. 532-569, 2013) for the sound generated by the interaction of a large-aspect-ratio rectangular jet with the trailing edge of a flat plate to include a more realistic upstream turbulence spectrum that possess a de-correlation (i.e. negative dip) in its space-time structure and use results from three-dimensional Reynolds-Averaged Navier-Stokes (RANS) solutions to determine the mean flow, turbulent kinetic energy and turbulence length & time scales. Since the interaction noise dominates the low-frequency portion of the spectrum, we use an appropriate asymptotic approximation for the Rayleigh equation Green’s function, which enters the analysis, based on a two-dimensional mean flow representation for the jet. We use the model to predict jet-surface interaction noise for a range of subsonic acoustic Mach number jets, nozzle aspect ratios, streamwise and transverse trailing-edge locations and compare them with experimental data. The RANS meanflow computations are also compared with flow data for selected cases to assess their validity. We find that finite de-correlation in the turbulence spectrum increases the low-frequency algebraic decay (the low-frequency “roll-off”) of the acoustic spectrum with angular frequency to give a model that has a pure dipole frequency scaling. This gives better agreement with noise data compared to Goldstein et al. (2013) for Strouhal numbers less than the peak jet-surface interaction noise. For example, through sensitivity analysis we find that there is a difference of 10 dB at the lowest frequency for which data exists (relative to a model without de-correlation effects included) for the highest acoustic Mach number case. Secondly, our results for the planar flow theory provide a first estimate of the low-frequency amplification due to the jet-surface interaction for moderate aspect ratio nozzles when RANS meanflow quantities are used appropriately. This work will hopefully add to noise prediction efforts for aircraft configurations in which the exhaust systems are tightly integrated with the airframe.

Growth Mechanism of Self-Induced Sloshing Caused by Jet in Rectangular Tank. 2nd Report. Multimode Sloshing Caused by Horizontal Rectangular Jet.

A jet under a free surface can induce sloshing in a tank without an oscillating external force. In the previous study, the authors proposed a new growth model of self-induced sloshing caused by a vertical plane jet. The model was based on a feedback loop which simply simulated the interaction between the jet and the free surface. The dependence of sloshing on the tank geometry, water depth and inlet velocity was qualitatively evaluated. In this study, the model was applied to self-induced sloshing caused by a horizontal rectangular jet. Under a certain geometric condition of a rectangular tank, 1st and 2nd mode sloshings were observed The growth of multimode sloshing was qualitatively explained by the model. The self-induced sloshing was considered to be excited by a jet-surface interaction regardless of the incident jet angle to the surface.

Influence of the Coanda effect on color Doppler jet area and color encoding. In vitro studies using color Doppler flow mapping.

We studied surface adherence and its effects on color Doppler jet areas and color encoding in an in vitro model with a noncompliant receiving chamber into which a steady flow jet was directed parallel to either a straight or a curved surface adjacent to and 4 mm away from the inflow orifice (1.50 mm2) with the control condition being a free jet matched for flow rates and driving pressures. Jets were imaged perpendicular to the plane of the surface, the plane in which most clinical images of jet-surface interactions are obtained. Ten different flow rates ranging from 0.13 to 0.30 l/min were used. Surface-adherent jet areas were smaller than control jets for every driving pressure-volume combination (paired t test, p less than 0.01). Computer analysis of color Doppler images showed more green and blue (reverse flow) pixels on the surface side of the adherent jets than the control jets (p less than 0.05), suggesting that viscous energy loss and flow deceleration and reversal play a role in the jet-surface interaction. Analysis of variance demonstrated that linear regression slopes of flow rate versus jet area for surface jets were lower (slopes, 11-21 cm2/l/min; r = 0.95-0.97) than those for the control (slope, 33 cm2/l/min; r = 0.97) (p less than 0.0001). Surface adherence (Coanda effect) influences jet size and color encoding, causing smaller color Doppler jet areas and greater variance and reverse velocity encoding.

Quantitative factors affecting parallel jet and jet surface interactions: An in-vitro study by color doppler and optical visualization

Quantitative factors affecting parallel jet and jet surface interactions: An in-vitro study by color doppler and optical visualization