Flow Noise Sources Research Articles

A note on the Strouhal number dependence of the relative importance of internal and external flow noise sources in IC engine exhaust systems

This note discusses issues related to the estimation of flow noise emission from rear mufflers in IC engine exhaust systems, through the analysis of measurements performed in a steady cold flow bench. First, the net acoustic power transmitted along the outlet pipe is obtained from in-duct pressure measurements. The in-duct power is then compared with noise measurements carried out in a semianechoic chamber, in order to distinguish between flow noise contributions associated with internal generation (muffler and tailpipe) and flow noise produced in the discharge process (interaction between the outgoing flow and the outside atmosphere). The ratio between the tailpipe in-duct acoustic power and the radiated acoustic power is analysed as a function of the Strouhal number. The results provide some information about the relative importance of internal and external sources.

Sources of flow noise due to time‐varying tip relief

This paper begins with a brief review of the conventional cause of shed vorticity in two‐dimensional unsteady lifting flows around airfoils. It then poses the following three‐dimensional theoretical problem in support of experimental work recently conducted at Penn State/ARL: Take a wing tip of arbitrary three‐dimensional shape and place a wall near it such that its plane is normal to that of flight. Let the wall be sinusoidally corrugated in the direction of the flow and be convected by it, so that an observer stationed on the wing’s tip experiences time‐varying influences determined by the wall’s passing serpentine pattern. Our presentation will describe a linearizing perturbation analysis whereby the ratio of the wall’s sinusoidal amplitude to its wavelength becomes a small parameter. The zeroth‐order problem becomes then one of steady three‐dimensional aerodynamics barring the presence of external fluctuations. The first‐order problem is fundamentally unsteady and driven, as usual, by the zeroth‐order one. An unconventional source of flow noise emerges: the ultimately unacceptable axial changes in the strength of trailing vortices, as brought about by their irregular images behind the corrugated wall, lead to new neutralizing shed vortices and harmonic lift.

Numerical Study on Observed Decay Ratio of Coupled Neutronic-Thermal Hydraulic Instability in Ringhals Unit 1 under Random Noise Excitation

The core stability in the Ringhals Unit 1 was estimated under the numerical random noise that simulates indefinable two-phase flow noise sources in actual cores. This noise model is expressed as a product of band white amplitude and arbitrary shape functions. In evaluating decay ratios, the conventional free-decaying method based on a clean modal disturbance was replaced with the response analysis method based on a numerical moderator density noise. The stability monitoring procedure was reproduced numerically by giving the spatially random shaped noise disturbance and by linearly varying the moderator density reactivity multiplier. It was confirmed that the observed regional decay ratio based on differential LPRM signals proposed by Hagen sometimes shows a discontinuous jump from the stable to the unstable region as predicted by Pazsit. Nevertheless, the regional decay ratio based on the extracted modal response will show a continuous change under the same condition. It was clarified that this jumping is mainly induced by the local fluctuation of moderator density at the frequency range which overlaps with a predominance range of the fundamental mode. It was demonstrated that this kind of numerical noise analysis is useful in verifying the monitoring algorithm before applying it in actual plants.

Flow noise in automotive heating, ventilation, and air conditioning systems.

Acoustic noise generated by a typical automotive heating, ventilation, and air conditioning (HVAC) system can be dominated by unsteady flow and turbulence. A comprehensive flow field and acoustic field study has been conducted experimentally on a stand‐alone HVAC system operating in its maximum air conditioning mode. Acoustic intensity field mapping was conducted in an anechoic environment. Detailed flow visualizations were conducted underwater on an optically clear system operating at prototypical values of the Reynolds number. The blower is the dominant low‐frequency source of noise, while at higher frequencies, additional flow noise sources exist. These include high shear regions within the ducting, separated flows off of flow obstructions, and the exit flow from register vents. Flow visualizations of the blower internal flow field were also conducted on a three‐times‐larger optically clear model of the blower using underwater particle tracking techniques. The results identify significant regions of inefficient flow, such as blade passage flow separation, reentrant flow, and rotating stall. Collectively, these lead to poor aerodynamic performance as well as noise. [Work supported by Ford Motor Co. and is drawn from the thesis research of former students Timothy Brungart, George Denger, Lori Perry, and Mike Sullivan.]

Prediction of the far-field jet noise using Kirchhoff's formulation

The Kirchhoff method consists of the calculation of the nonlinear near- and midfields, usually numerically, with the far-field solutions found from a linear Kirchhoff formulation evaluated on a surface D surrounding the nonlinear field. The surface S is assumed to include all the nonlinear flow effects and noise sources

Flow-induced noise on a bluff body

Flow noise measurements were made on inertial type pressure gradient hydrophones, configured as three-dimensional cylinders in cross flow, over a diameter-based Reynolds number range of 4×103 to 1.8×104. The measurements were made at frequencies from 4.2 to 50 Hz as the bodies were towed in a quiescent body of water. Systematic changes were made in the cylinder geometry as to affect the flow noise level and to aid in the identification of dominant flow noise sources. The cylinder aspect ratio was varied from 0.5 to 2.5, and the endcap geometry was altered by relieving the 90° edge with radii that ranged from 0.0315D to 0.5D, where D is the diameter of the cylinder. The data from these (and other flow visualization) experiments shows that the presence of a radius at the corner between the endcap and the cylinder results in a significant reduction of the separated flow over the endcap, and that the flow noise levels decrease accordingly. The flow noise levels are also observed to decrease as the body aspect ratio increases which suggests that when three-dimensional effects (endcap flow) are suppressed, unsteady forces will be reduced and the cylinder self-noise will decrease. Thus the dominant source of flow noise on short cylinders in cross flow is attributed to the unsteady separation and turbulent flow over the cylinder endcaps.

Review: The Use of Kirchhoff’s Method in Computational Aeroacoustics

A comprehensive review of the use of Kirchhoff’s method in computational aeroacoustics is given. Kirchhoff’s integral formulation allows radiating sound to be evaluated based on quantities on an arbitrary control surface S if the wave equation is assumed outside. The control surface S is assumed to include all the nonlinear flow effects and noise sources. Thus only surface integrals are needed for the calculation of the far-field sound. A numerical CFD method can be used for the evaluation of the flow-field solution in the near-field and thus on surface S. Kirchhoff’s integral formulation has been extended to an arbitrary, moving, deformable piecewise-continuous surface. The available Kirchhoff formulations are reviewed and various aeroacoustic applications are given. The relative merits of Kirchhoff’s method are also discussed.

Studies on scaling of flow noise received at the stagnation point of an axisymmetric body

A description of the studies related to the problem of scaling of flow noise received at the stagnation point of axisymmetric bodies is provided. The source of flow noise under consideration is the transitional/turbulent regions of the boundary layer flow on the axisymmetric body. Lauchle has recently shown that the noise measured in the laminar region (including the stagnation point) corresponds closely to the noise measured in the transition region, provided that the acoustic losses due to diffraction are accounted for. The present study includes experimental measurement of flow noise at the stagnation point of three different shaped axisymmetric headforms. One of the body shapes chosen is that used by Lauchle in similar studies. This was done to establish the effect of body size on flow noise. The results of the experimental investigations clearly show that the flow noise received at the stagnation point is a strong function of free stream velocity, a moderately strong function of body scale but a weak function of boundary layer thickness. In addition, there is evidence that when body scale change is involved, flow noise amplitude scales but no frequency shift is involved. A scaling procedure is proposed based on the present observations along with those of Lauchle. At a given frequency, the amplitude of noise level obtained under model testing conditions is first scaled to account for differences in the velocity and size corresponding to the prototype conditions; then a correction to this is applied to account for losses due to diffraction, which are estimated on the basis of the geometric theory of diffraction (GTD) with the source being located at the predicted position of turbulent transition. Use of the proposed scaling law to extrapolate presently obtained noise levels to two other conditions involving larger-scale bodies show good agreement with actually measured levels, in particular at higher frequencies. Since model scale results have been used successfully to predict levels on larger-sized bodies tested in a totally different environment, the present data along with the proposed scaling procedure can be used to predict the expected flow noise levels at prototype scales during preliminary design studies.

Application of autoregressive modeling techniques in BWR noise analysis

Experience on development and application of an autoregressive (AR) modeling technique in BWR noise analysis is described. Based on the improvement of the AR model using an associate matrix, which specifies signal transfer paths in measured signals, BWR noise in reactor core and control systems is analyzed. In the reactor core, void and recirculation flow noise sources, which are most dominant in BWR plants, are quantitatively evaluated by the improved AR model. Also, ordinary and partial coherence functions between radially located neutron detector signals are estimated to clarify spatially distributed noise sources. In the control system, the open-loop and closed-loop responses are identified from steady-state noise data. Furthermore, a failure detection and isolation method for the control system is developed on the basis of the improved AR model.

Noise Source and Reactor Stability Estimation in a Boiling Water Reactor Using a Multivariate Autoregressive Model

A method for evaluating reactor stability in boiling water reactors has been developed. The method is based on multivariate autoregressive (M-AR) modeling of steady-state neutron and process noise signals. In this method, two kinds of power spectral densities (PSDs) for the measured neutron signal and the corresponding noise source signal are separately identified by the M-AR modeling. The closed- and open-loop stability parameters are evaluated from these PSDs. The method is applied to actual plant noise data that were measured together with artificial perturbation test data. Stability parameters identified from noise data are compared to those from perturbation test data, and it is shown that both results are in good agreement. In addition to these stability estimations, driving noise sources for the neutron signal are evaluated by the M-AR modeling. Contributions from void, core flow, and pressure noise sources are quantitatively evaluated, and the void noise source is shown to be the most dominant.

Effects of Surface Irregularity on Turbulent Boundary Layer Wall Pressure Fluctuations

Measurements were made of the mean velocity profiles and wall pressure field upstream and downstream of the flow over both a backward-facing and forward-facing step. For each configuration the velocity profiles show that the effects of the separation-reattachment process persist more than 24 step heights downstream of the step. Extremely high values of the RMS wall pressure are measured near reattachment. These values are 5 and 10 times larger than on a smooth flat plate for the backward-facing step and the forward-facing step, respectively. The spectral density of the wall pressure fluctuations in the recirculation region is dominated by low frequency components. Downstream of reattachment there is a reduction in the low frequency content of the wall pressures and an increase in the high frequency components. At the farthest measured position downstream, the spectral density is still higher than that found on a smooth flat plate. These results show that the complex turbulent flow generated by a surface irregularity can significantly increase the localized wall pressure field and these increases persist far downstream of the irregularity. Consequently, a surface irregularity can be a major source of turbulence-induced vibrations and flow noise, as well as a cause of the inception of cavitation in marine applications.

Multivariate analyses for neutron noise source identification in BWR plant.

A quantitative evaluation of neutron noise sources in a BWR plant is performed by using a multivariate autoregressive (AR) model. The mechanisms of noise propagation between the principal process signals are first examined, leading to the observations that the conventional AR model requires to be modified by means of an associate matrix in order to identify the source of flow noise. Using such a modified AR model, quantitative evaluation is made of the contribution of flow noise which is distinguished from that of void noise. Local neutron noise sources are also discussed, based on observations covering local power range monitor signals. These results indicate that flow noise, as well as void noise, contributes significantly to neutron fluctuation. The modified AR model is thus shown to be most effective for estimating the noise sources of neutron fluctuations.

Multivariate Analyses for Neutron Noise Source Identification in BWR Plant

A quantitative evaluation of neutron noise sources in a BWR plant is performed by using a multivariate autoregressive (AR) model. The mechanisms of noise propagation between the principal process signals are first examined, leading to the observations that the conventional AR model requires to be modified by means of an associate matrix in order to identify the source of flow noise. Using such a modified AR model, quantitative evaluation is made of the contribution of flow noise which is distinguished from that of void noise. Local neutron noise sources are also discussed, based on observations covering local power range monitor signals. These results indicate that flow noise, as well as void noise, contributes significantly to neutron fluctuation. The modified AR model is thus shown to be most effective for estimating the noise sources of neutron fluctuations.

On the successive stages of flow noise generation

The physics of flow noise generation is discussed and a qualitative survey of the successive stages of noise formation is presented in terms of the energy transport equations of acoustic waves. Initially, an acoustic wave, whose energy transport characteristics are assumed known, is subject to dissipative thermodynamic processes. The resultant frictional heat input into the fluid causes a continuous steepening of the entropy gradients and an eventual collapse of the disturbance, thus inducing mixing of the fluid strata. From the thermodynamic aspect of these phenomena, the mixing processes may be viewed as consisting of two parts. First, an expansion of the fluid takes place, due to the elevated pressure at the end of the frictional heat input. This expansion process, which brings the pressure of the fluid back to its ambient surroundings, is itself a source of an ideal, acoustic secondary wave, since its thermodynamic nature is purely nondissipative. Subsequently, the fluid attains thermodynamic equilibrium by means of an isobaric contraction process. By an analysis of the wave energy terms, it is shown that it is the frictional collapse and its resultant sudden expansion, which constitute the main acoustic flow noise sources.

A Study of Community Background Noise

A field measurement program was performed to determine the background noise levels as a function of time and location for a number of selected community areas in one city. Measurements of background noise were obtained according to a randomized sampling schedule. Concurrent observations were obtained of street traffic flow and other significant noise sources. This paper discusses variations in mean background noise spectra with time and location and short term fluctuations in noise levels. Inferences are drawn for the number and disposition of necessary field measurements to establish background noise levels with desired precision. The degree of correlation between background noise levels and street traffic flow is discussed and related to a previously reported procedure for estimating the noise from vehicular traffic [J. Acoust. Soc. Am. 29, 779(A) 1957]. Comparisons are drawn between observed aircraft noise and background noise from all other sources. [This work was supported in part by the U. S. Air Force under Contract Nos. AF 33(616)-3335 and AF 33 (616)-3938.]