Acoustic perturbation equations for reacting flows (APE-RF) are derived to investigate combustion noise in conjunction with a hybrid approach. The method uses large-eddy simulation (LES) data in the first step and the APE-RF system in the second step. The newly derived APE-RF system contains several source terms on the right-hand side (RHS). They are discussed in detail with respect to their relation to various sound mechanisms. The acoustic sources contain the impact of unsteady heat release, non-isomolar combustion, species diffusion, heat diffusion, viscous effects, non-uniform mean flow and non-constant combustion pressure effects, and the influence of acceleration of density inhomogeneities. It is shown that the unsteady heat release occurs in the total time derivative of the density that is immediately available from an LES. By computing via the hybrid method the sound field of an open turbulent non-premixed flame being generated just by the substantial derivative of the density, which contains besides the unsteady heat release, the effect of non-isomolar combustion, heat and species diffusion, and viscous effects, i.e., the remaining terms are neglected, and by comparing the numerical data with experimental findings the total temporal derivative of the density is shown to contain for a wide frequency range the major sound sources in reacting flows. However, it is also indicated that to simulate all the details in the complete frequency range the remaining source mechanisms occurring on the RHS of the APE-RF system are to be taken into account in the analysis.
In this study, combustion noise and sound source mechanisms of an unconfined turbulent non-premixed flame, i.e., the DLR-A flame is investigated. A hybrid LES/CAA approach is thereby employed in which a low Mach number variable density large-eddy simulation (LES) is combined with the acoustic perturbation equations for reacting flows (APE-RF). In the first step of the hybrid analysis the flamelet/progress variable (FPV) model is employed as combustion model followed by the acoustic simulation in the second step using the acoustic perturbation equations for reacting flows (APE-RF). In the acoustic analysis, special emphasis is placed on the impact of the thermoacoustic source contributions within the pressure-density relation of the APE-RF system on the radiated acoustic field and the applicability of these source formulations in terms of a hybrid CFD/CAA approach. The flamelet/progress variable database has been extended in terms of acoustic source terms. The unsteady heat release rate, the source describing the effect of non-isomolar combustion, and the species diffusion term are described by two independent parameters, i.e., the mixture fraction and the progress variable. From the findings in the present study, the analysis of the acoustic field of low Mach number reacting flows induced by the thermoacoustic sources such as the unsteady heat release leads to a very stiff problem formulation, since the related sources require highly resolved regions in the source area, which restricts the possible time step during temporal integration of the equations. The numerical bottleneck is not so restrictive when a source term formulation based on the density distribution is used. Spectra obtained from the simulated acoustic field, using two different source term formulations involving derivatives of the density are in good agreement with the experimental data even in the higher frequency range.
Porcine epidemic diarrhea (PED) virus (PEDV) is a globally emerging and re-emerging epizootic swine virus that causes massive economic losses in the swine industry, with high mortality in piglets. In Vietnam, PED first emerged in 2009 and has now developed to an endemic stage. This is the first cross-sectional survey performed to evaluate the proportion of PEDV-positive swine farms in Vietnam from January 2018 to February 2019. Fecal samples from 327 pig farms in northern Vietnam were collected and tested for PEDV infection by reverse transcription-loop-mediated isothermal amplification (RT-LAMP) method. The proportion of PEDV-positive farms was 30.9% and PEDV-positive farms were distributed throughout the study area. The highest proportion of PEDV-positive farms was 70% (7/10) among nucleus production type farms (P < 0.05). Higher proportions of PEDV-positive farms were found in the Northeast and Red River Delta areas, which are the major areas of pig production (P < 0.05). The proportion of PEDV-positive farms was higher among larger farms (P < 0.05). Our findings illustrate the high proportion of PEDV-positive farms in the Vietnamese pig population and will help to better understand the epidemiological dynamics of PED infection, to estimate impact, and establish and improve prevention and control measures.
Porcine epidemic diarrhea virus (PEDV) causes enteritis, vomiting, watery diarrhea, and high mortality in suckling pigs, threatening the swine industry. Porcine epidemic diarrhea (PED) re-emerged globally in 2013 in many important swine-producing countries in Asia and the Americas. Several studies have identified the risk factors for the spread of PEDV in acute outbreaks. However, limited information is available on the risk factors for the transmission of PEDV in endemic regions. We hypothesized that poor biosecurity, location, and some social or cultural practices are the main risk factors for PEDV transmission in the Vietnamese pig population. The aim of this study was to evaluate the potential risk factors for the transmission of PEDV in an endemic area in Vietnam. In this case-control study, questionnaires containing 51 questions were completed for 92 PEDV-positive and 95 PEDV-negative farms. A logistic regression analysis was performed to assess the risk factors associated with PEDV infection. Province and the total number of pigs were included as random effects to determine their influence on the risk of PEDV infection. Twenty-nine variables of interest that have been associated with PEDV status were analyzed in a univariate analysis (P < 0.20), with backward stepwise selection. Only three of these 29 variables in four models remained significant PEDV risk factors in the final model: farrow-to-wean production type, distance from the farm to the slaughterhouse (< 1,000 m), and the presence of chickens on site (P < 0.05). This is the first study to identify the main risk factors for PEDV infection in an endemic area. Our findings suggest that hygiene measures should be strictly implemented on farms for the effective control and prevention of PEDV infection.
Combustion noise analyses in a hybrid computational aeroacoustics (CAA) context are presented. Acoustic perturbation equations for reacting flows (APE-RF) describe the wave propagation, while unsteady results from a variable density incompressible reactive large-eddy simulation (LES) are used to evaluate the acoustic source terms. To simulate combustion generated noise via such a hybrid approach, an appropriate source description has to be taken, which preferably matches two requirements, i.e., on the one hand, to efficiently and accurately predict the generated acoustic field, and on the other hand, to easily determine the source term from the LES. In this study, three source formulations for the APE-RF system are examined on two acoustic meshes with different resolutions in the source region to predict the acoustic field of an open turbulent nonpremixed flame. Using the source term, which is expressed via the scaled partial time derivative of the density, the acoustic field can be reproduced best up to a maximum Strouhal number of StD = 2 on the fine mesh. However, for this source formulation spurious noise can be observed depending on the CAA resolution. It will be shown in this study that this observation can be related to the "artificial interpolation induced acceleration" effect. A compromise between efficiency and accuracy represents the source formulation expressed via the scaled material derivative of the density.
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