Modeling of the propagation and attenuation of noise in aeraulic networks with SILDIS^® software

Modeling the propagation and attenuation of noise in aeraulic networks is possible with the SILDIS^® software in different contexts :

• for air conditioning and ventilation systems (not only for applications in the building construction domain, but also in the transport sector: automobiles, trucks, trains, planes, rockets, boats and even: submarines)
• for circuits involving air and other industrial fluids (sometimes: under pressure) of various processes (e.g. in relation to cooling, heating, evacuation of combustion products, compressed air supply to equipment); in this field, turbomachines (e.g. compressors, pumps but also fans of all kinds: helical aka axial, centrifugal aka radial which are frequent components of all kinds of hardware - the smallest e.g. extractor of a Controlled Mechanical Ventilation (CMV) facility of a few centimeters of radius to the largest e.g. fan of a cooling tower whose diameter can exceed 10 meters) constitute a field of recurring applications, the production of energy based on fossil fuels (e.g. thermal engines, gas turbines) being another, implying - for this latter - operating conditions at ambient temperature (for the air intake) and at very high temperature (for the exhaust, with regard to the burnt gases, with or without a chimney)

This often involves limiting the sound impact of such aeraulic networks :

• at one or more ends (air inlet orifice, chimney outlet, vent of all kinds)
• with regard to the transmission of noise through the walls of ducts or pipes

It is therefore appropriate to assess, for the various components of such aeraulic networks, the performance in terms of noise transmission limitation, taking into account - for each element - on the one hand the attenuation - in dB - but also self-noise - in general, in dB ref. 1 pW - (which can, in certain cases, be prohibitive with regard to obtaining a significant dynamic insertion loss - in dB -, e.g. in the case of an excessive gas velocity, then at the origin of too much flow noise).

All this depends (like most performance indicators in the field of acoustics) on the frequency, if only because it influences the conditions of sound waves propagation (the fundamental mode i.e. a plane wave front, propagating alone below a cutoff frequency, which is a function, for each component, of its cross section, of the speed of sound and of the Mach number); even with the plane wave assumption, the attenuation of many aerodynamic circuit components is often highly variable with frequency.

Among the usual components for which the modeling of the axial propagation and attenuation of noise in aeraulic networks with the SILDIS^® software is possible, can be mentioned (for cross sections which may be rectangular, square or circular) :

• straight sections, with or without specific sound absorbing lining sounds (dissipative silencers i.e. whose effect - in general: broadband - is based on the interaction between the fluid which is transported and the pores of one or more material(s) more or less permeable, or resonant i.e. with perforations or plates, then effective in a more restricted frequency range); with or without taking into account the transmission through the walls for the calculation of the axial sound propagation
• elbows (with right angles or with a radius of curvature)
• sudden changes in section (sudden expansion or narrowing), not only at the entry or exit of silencers with splitters (baffles)
• divisions i.e. bifurcations
• nozzles e.g. ventilation orifice, air conditioning outlet, stack outlet

The SILDIS^® software is used to model the propagation and attenuation of noise from such components of aeraulic networks :

• separately
• mounted in series (for relatively simple aerodynamic circuits), then calculating the cumulative performance

The noise transmission through the duct walls (for cross sections which may be rectangular, square or circular - eventually gradually varying -) or through partitions of larger dimensions, possibly multilayered, plane or with corrugations is also possible with the SILDIS^® software.

In terms of modeling the propagation and attenuation of noise in ventilation systems, the SILDIS^® software has various advantages :

• its versatility, its various functionalities allowing simulations involving a wide variety of air network components for which acoustic and aerodynamic performance can be assessed
• its power, when it comes to performing calculations involving the resolution of transcendent equations (in particular: propagation of sound in porous media) involving trigonometric functions (e.g. Bessel, Neumann functions) with complex argument whose modulus is variable from infinitely small to infinitely large values, by the means of an analytical method (i.e. without recourse to finite element calculations, often complex to prepare and long to complete)
• its accessibility, allowing use by personnel without specific skills in computer programming
• its practicality :
  • the preparation (often: tedious) of a geometry model with mesh of the component for which a simulation is desired is not necessary; the simulations can be carried out with easily accessible calculation means (in terms of processors & memory) (compared to those required for FEM ^[1] or BEM ^[2] calculations, for which ITS is also equipped); input data entry times and calculation durations are very short (all parameter changes are quick and easy. For example, the calculation of the acoustic performance, over the entire frequency interval 20 Hz - 20 kHz, of a chimney with a diameter of 5 m and a height of 50 m with an (internal) peripheral sound-absorbing lining does not take more time with the SILDIS^® software than the dimensioning of a (much) smaller silencer e.g. of size 10, 20 or 50 times lower (time is counted in seconds or minutes with SILDIS^® software, whereas it would be quite different with a simulation tool based on a mesh if a usual criterion of cell size ratio to wavelength of 12, 10, 8 or even only 6 would be applied: simulation times with a PC, even equipped with a multi-core processor, would then be counted in hours or even in days)
  • the calculations of all the components of an aeraulic circuit (or of pressurized fluids networks) listed above can be carried out using a single computer file, with the exception of reactive silencers (rarely considered for applications linked to ventilation, often useful for internal combustion engines exhausts), for which there is a separate computer file
  • the properties of dry air (adiabatic exponent, density, sound speed, dynamic viscosity, specific heat, thermal conductivity) are calculated by the software for a wide range of thermodynamic conditions (the temperature can vary from 200 K i.e. -73.15 °C à 1200 K i.e. 926.85 °C); evaluations of such properties (and also: of the individual gas constant) for gas mixtures are facilitated (the mole fraction of water vapor is calculated for humid air depending on the hygrometry)
  • the properties of many filling materials e.g. fibrous wool (rock, glass, basalt, ceramic, polyester), foams and surfacings e.g. non-woven cloths, fabrics as well as perforated protections are available in libraries, based on laboratory measurements (e.g. for porous media: air flow resistance, porosity, tortuosity, thermal and viscous characteristic lengths); such menus (and others) are drop-down
  • model choices are possible for the key steps of the calculations, as soon as alternatives make sense (e.g. the calculation of dissipative silencers can - for most configurations - be carried out by considering a filling either locally reacting or isotropic, or anisotropic then considering 2 perpendicular directions)
  • for each of such elements of an aerodynamic network, a single document (1 page) synthesizes the data taken into account and the results of the calculations (insertion loss without flow noise, self noise i.e. sound power of the flow, sound power level downstream of the element considered - when the upstream sound power level has been entered by the user or has been the subject of a previous calculation for the serial component provided upstream -, and when this makes sense: total pressure loss); many calculations are done in fine frequency band (1/21 octave) and converted to 1/3 octave and 1/1 octave band - the overall performance in dB(A) against a reference spectrum chosen by the user is also possible -; results are available in the form of tables of numerical values and graphs
  • many performance indicators obtained with the SILDIS^® software are comparable with standardized measurements, when they exist e.g. for a silencer, the results of the calculations are comparable with standard NF EN ISO 7235 Acoustics - Laboratory measurement procedures for ducted silencers and air-terminal units - Insertion loss, flow noise and total pressure loss
• its precision and reliability
  
  • measurement (e.g. in laboratories) results were used for the evaluation of some performance indicator components (e.g. for dissipative silencers: bypass correction, reflection loss, self-noise i.e. sound power of the flow, total pressure loss)
  • hundreds of comparisons between simulation results with SILDIS^® software and bibliographic data (e.g. results of measurements, calculations by other approaches) have been carried out during the validation process

Proposed in ASP mode ^[3] or which can be carried out (within the framework of engineering missions) by the human resource of ITS, the modeling of the propagation and the attenuation of noise in ventilation networks with the SILDIS^® software constitutes a decision aid and a soundproofing device sizing tool being very valuable for many projects (in the construction sector as well as in industry).

^[1] Finite Element Method
^[2] Boundary Element Method
^[3] Application Service Provider