Ml Munjal

Theory of a two source-location method for direct experimental evaluation of the four-pole parameters of an aeroacoustic element

A new method, called the two source-location method, is described for measurement of the four-pole or the transfer matrix parameters for an acoustic element or a subsystem of elements by means of the four-microphone technique and use of the transfer function approach. Theoretical expressions are derived for the two source-location method, and these are shown to be identical to those for the two-load method. However, the two methods are conceptually different. The two-source method is shown to be functionally much more stable and entirely independent of the loading terminations. An uncertainty analysis is presented and used for estimation of possible nonequalization errors in the transfer matrix parameters. Finally, several examples of successful measurement use of the new method are included.

Aeroacoustic analysis of perforated muffler components

Sullivan and Crockers equations for two-duct perforated elements were solved recently by Jayaraman and Yam by means of a decoupling approach, which was valid only for equal mean-flow Mach numbers in the two interacting ducts. The present paper provides a generalized decoupling approach for the analysis of two-duct as well as three-duct muffler configurations consisting of perforated elements, taking into account the actual mean flow Mach numbers in the adjoining tubes. The theoretical results of transmission loss of typical muffler configurations are compared with the available measurements of Sullivan, and predictions of Jayarman, Yam and Thawani. A comparison has been made with the TL values calculated by means of Sullivans segmentation approach. The agreement between the two seems to be good.

Velocity ratio-cum-transfer matrix method for the evaluation of a muffler with mean flow

The paper deals with a method for the evaluation of exhaust muffers with mean flow. A new set of variables, convective pressure and convective mass velocity, have been defined to replace the acoustic variables. An expression for attenuation (insertion loss) of a muffler has been proposed in terms of convective terminal impedances and a velocity ratio, on the lines of the one existing for acoustic filters. In order to evaluate the velocity ratio in terms of convective variables, transfer matrices for various muffler elements have been derived from the basic relations of energy, mass and momentum. Finally, the velocity ratiocum-transfer matrix method is illustrated for a typical straight-through muffler.

Experimental evaluation of impedance of perforates with grazing flow

This paper is concerned with measurement of grazing flow impedance of a perforated plate. The measurements have been made for various grazing flow velocities with an impedance tube facility, at different frequencies. The effects of the thickness of the plate and the diameter of the holes on the impedance of the perforate have been included. It is found from the results that resistance increases with the grazing flow velocity, but is more or less independent of thickness and hole diameter, whereas reactance increases with thickness and diameter of the hole. Finally an empirical formula for computing the normal incidence impedance of an orifice or cluster of orifices (perforated plate) is given, and verified for use in prediction of noise reduction across concentric tube resonators.

Plane wave analysis of side inlet/outlet chamber mufflers with mean flow

Integration of side inlet and/or side outlet expansion chambers into the rest of the muffler system for prediction of overall transmission loss (TL) or insertion loss requires knowledge of the four-pole parameters of these elements for propagation of plane waves in a moving medium. In this paper, transfer matrices have been derived for side inlet and side outlet elements for typically small mean flow Mach numbers. These are validated against the 3-D model existing in the literature for a stationary medium and for acoustically long chambers. The effect of mean flow on TL of the side inlet/outlet chambers has been demonstrated. Incidentally, it has been shown that side inlets and side outlets behave more or less like extended inlets and extended outlets, respectively

A HYBRID APPROACH FOR AEROACOUSTIC ANALYSIS OF THE ENGINE EXHAUST SYSTEM

This paper presents a new hybrid approach for prediction of noise radiation from engine exhaust systems. It couples the time domain analysis of the engine and the frequency domain analysis of the muffler, and has the advantages of both. In this approach, cylinder/cavity is analyzed in the time domain to calculate the exhaust mass flux history at the exhaust valve by means of the method of characteristics, avoiding the tedious procedure of interpolation at every mesh point and solving a number of equations simultaneously at every junction. This is done by making use of an interrelationship between progressive wave variables of the linear acoustic theory and those of the method of characteristics. In this approach, nonlinear propagation in the exhaust pipe is neglected and free expansion is assumed at the radiation end of the exhaust pipe. In the case of a muffler proper, expansion from the exhaust pipe into the first chamber is assumed to be a free expansion. Various results of this approach are compared with those of the method of characteristics and the classical acoustic theory, and various peaks and troughs in insertion loss curves are analytically validated.

On plane‐wave propagation in a uniform pipe in the presence of a mean flow and a temperature gradient

A theoretical study on the propagation of plane waves in the presence of a hot mean flow in a uniform pipe is presented. The temperature variation in the pipe is taken to be a linear temperature gradient along the axis. The theoretical studies include the formulation of a wave equation based on continuity, momentum, and state equation, and derivation of a general four-pole matrix, which is shown to yield the well-known transfer matrices for several other simpler cases.

A simple numerical method for three-dimensional analysis of simple expansion chamber mufflers of rectangular as well as circular cross-section with a stationary medium

Three-dimensional effects are a primary source of discrepancy between the measured values of automotive muffler performance and those predicted by the plane wave theory at higher frequencies. The basically exact method of (truncated) eigenfunction expansions for simple expansion chambers involves very complicated algebra, and the numerical finite element method requires large computation time and core storage. A simple numerical method is presented in this paper. It makes use of compatibility conditions for acoustic pressure and particle velocity at a number of equally spaced points in the planes of the junctions (or area discontinuities) to generate the required number of algebraic equations for evaluation of the relative amplitudes of the various modes (eigenfunctions), the total number of which is proportional to the area ratio. The method is demonstrated for evaluation of the four-pole parameters of rigid-walled, simple expansion chambers of rectangular as well as circular cross-section for the case of a stationary medium. Computed values of transmission loss are compared with those computed by means of the plane wave theory, in order to highlight the onset (cutting-on) of various higher order modes and the effect thereof on transmission loss of the muffler. These are also compared with predictions of the finite element methods (FEM) and the exact methods involving eigenfunction expansions, in order to demonstrate the accuracy of the simple method presented here.

The two-microphone method incorporating the effects of mean flow and acoustic damping

Analytical expressions for the reflection coefficient and acoustic impedance of a passive termination, and intensity of the acoustic flux, have been derived incorporating the effects of mean flow as well as the acoustic damping. These expressions reduce to those available in the literature when one or both of these effects is dropped. Finally, the instability condition is derived and the stabilizing effect of the acoustic damping is demonstrated.

Experimental evaluation of the aeroacoustic characteristics of a source of pulsating gas flow

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