Sound pulse scattering from an edge of thick elastic plates:experimental and numerical investigations

I.IntroductionItisdifficulttoanaIysesoundscattcringfromanedgeofathickpIatetheoretica11y.Inmostprevious.ork['-'],thecdgcdifTractionsofidca1ha1fp1aneswereconsidered,i.e.theplanesarepcrfcct1yhardorpcrfcctlysoftoroncsurfacehardandonesurfacesoftandtheirthicknessisneg1cctcd,thccorrespondingthporicsarcquitecomp1ctc.Inthisstudy,thecdgcscattcringfromthickelasticplatesinwatcrisinvestigated.Theex'perimentalrcsultsarccxamincdphysica1lybythcp1atethcoryandcomparcdquantitative1ywithca1culationsfromanumeric…     

Study of extracting characteristic parameters from a blood vessel

I.IntroductionTheinvcrseprob1cmofsoundscattcringisinterestingtou1trasonicdiagnosis,identificationofanundcrwatcrtargct,speechsynthesisctc.UsingthismethodDucheneandTabbara[1.21.studiedtheu1trasonicdiagnosisofab1oodvessel.Inthispaperweusedasoft-tubetosimulateabloodvesselanddeaIswiththeu1trasonicdiagnosisofthesofc-tube.Invcrsescatteringprob1emdcterminingatubeparamctcrswasgivcn.Agcomctrica1acousticsmethodwasuscdindescribingthescattcrcdfield.Fivcparametcrsofthctubcwercrcconstructcd.Therelativcdc-…     

A postbeamformer adaptive noise cancelling system

I.IntroductionApostbeamformernoisecancel1erisasing1e-channeladaptiveprocessorinbeamspace['l.Ithasnotonlysimplerstructure,buta1sobetterpropertiesthane1ementspaceprocessorinimp1ementation.L.C.GodaraandA.Conto1ihavestudiedthesignaltonoiseratioofthebroadbandPIC[1].Theauthorshavesolvedtheprob1emofapplicationconditionsofnarrowbandPIC['],andproposedthatRMGSEFa1gorithmistheoptimuma1gorithminthenarrowbandPICadaptiveprocessing['1.Atpresent,thepfob1emofadaptivecance11ingofbroadbandtowing-shipnoi…     

Acoustics of Chinese woodwind reed instrument Sheng

I.IntroductionTheancientfolkwindinstrumentShengwasfounda1readyintheShangDynasty3000yearsago.AtraditionalShenghasl7bambooreed-pipes.IthasbeendevclopedintothepresentShengwithdifferentkindsofstructure.TherichsoundofShengisbri1liantandhar-monious.ThespecialityofShengisthatsevera1tonescanbep1ayedinharmonyandsomechords.Inthispaperitisana1yzedthattheprincip1esandacousticcharacteristicsofShengbasedonthe17lreed-pipeSheng.II.Acousticequiva1cntcircuitInFig.lisshowna17rced-pipeShengwithnames0fvari…     

Overall evaluation for concert hall acoustics

I.IntroductionTherearetwowaysofcvaluatingconcertha1lacoustics.Oneistoevaluatetheacousticalqualityofaconcertha11ateachlistener'spositionrespectively,andtheotheristoevaluatetheacousticalqua1ityofaha1lasawho1e.Bothwaysarebasica1lysimilar.Howcvcr,sincesomcob-icchveparametersmayhavedifTerentvalucsatdifferentpositions,onemustdcfinesuitab1ein-dicestodescribetheseparameters(e.g.meanva1ucsorothers),sothatacomparisonbetweenha1lsbecomcspossib1e,Thispaperdea1swiththelattereva1uationbyusingfuzZysettheory…     

The design and dynamic performance of a moment actuator

I.IntroductionMachinenoiseisanenvironmcnta1po11utantanditscontrolremainsamajorunresolvedprob1cm.Inadditiontoair-bornesound,machinesimpartvibrationalenergyintosupportingandconnectedstructuresbybothtrans1ationa1androtationa1motions.Translationa1compo-nentsarerelative1yeasytoestimatesinccnearpureforcescanbegeneratedandmeasured.ThisisnotthecaseforrotationalcomponentSsincemomcntsaremoredifficu1ttogenerateandmeasure.Suchmeasurementswould1eadtoafu11undcrstandingofthis1ittleunderstoodbutoftenimporta…     

Bulk acoustic wave Barker code sequence and the application in ultrasonic NDT

I.IntroductionUptonowtheultrasonicpu1seechotechniquehasbeenwide1yusedinthefieldofnon-destructivetesting.Toachievehightargetreso1ution,narrowpu1seshou1dbeadoptcd.Butthiswill1imitthetransmitpowerandthcdetectab1erange,whichisveryboportantcspecia1lyinthemediumwithstrongacousticattenuation.Itmeansthatthetargetresolutionanddetcctablerangehavccontradictorydcmands.Thcrcforethepulsccompressionandcorre1ationtechniqucdeve1opedinradarsystemhavebeenintroducedintotheultrasonicNDTandsomeprogresseshavebeenp…     

A study of multi-beam DICANNE system

I.IntroductionTheprob1cmofsuppressingstrongpointinterferencesourceisavcryimportantthesisinsonardesign.Fortheana1ogsonar,thesolutionofthisprob1emisquitedifficu1t.Untilthedevelopmentofdigitalsonar,thereissomerescarchreportsaboutsuppressingstrongpointin-terferencesource.Theear1ierworkin1969,dedicatcdbyV.C.Anderson,istheconccptofDICANNE(DigitalInterfcrcnceCance1lingAdaptiveNu1lNetworkEquipment)['l.Thisisadoublecompensatormulti-bcamsystcm.Oneisusedtoformintcrfercncebeamandanothcrisusedtofor…     

Free-field attenuation of earmuffs and earplugs both singly and in combination

I.IntroductionExposuretoenvironmcnta1noisc1cadstoannoyanceande1evationofhearingthrcsho1d.Itisnotpracticetoiso1atesuchthcsenoiscsbyengineeringmethods.Analternativecommonlypractica1andinexpcnsivemethodforhcaringprotectionisthcuseofhcaringprotcctivedcvicespePDs)suchasearplugsandearmuffe.AsdcfinedbyBcrgcr['lthatthenoisereductionpro-videdbyHPDispopu1ar1yrefcrredtoasitsattenuation.InfaCtoricsandindustrialplaceswhcretheworkersareexposedtohazardousnoisc,thcattcnuationofsing1ehearingprotectordevice…     

Ultrasonic study of nitric acid

I.IntroductionBymeansofu1trasonicwavesthedynamicspecificpropertiesofmolecu1esinmediacouldbeapprehended.Fromthe195ostol96osacousticiansstudiedmo1ecu1aracousticandthermodynamicspecificpropertiesof1iquidsandmixtureswithgreate.th.siasm['-'],andkeptonbringto1ighttheirreportsofrc1evantstudiesinthe1astten-oddyearsl'-'l.overrecentyearstheauthorhasbeenengagedinthemeasurementandstudyofsoundve-locityofsevera1chemica1so1utionsinassociationwithu1trasonicmeasurementstudyofthecon-centrationofsolutionsproduce…     

A robust numerical scheme for highly compressible magnetohydrodynamics: Nonlinear stability,implementation and tests

The ideal MHD equations are a central model in astrophysics, and their solution relies upon stable numerical schemes. We present an implementation of a new method, which possesses excellent stability properties. Numerical tests demonstrate that the theoretical stability properties are valid in practice with negligible compromises to accuracy. The result is a highly robust scheme with state-of-the-art efficiency. The scheme’s robustness is due to entropy stability, positivity and properly discretised Powell terms. The implementation takes the form of a modification of the MHD module in the FLASH code, an adaptive mesh refinement code. We compare the new scheme with the standard FLASH implementation for MHD. Results show comparable accuracy to standard FLASH with the Roe solver, but highly improved efficiency and stability, particularly for high Mach number flows and low plasma β. The tests include 1D shock tubes, 2D instabilities and highly supersonic, 3D turbulence. We consider turbulent flows with RMS sonic Mach numbers up to 10, typical of gas flows in the interstellar medium. We investigate both strong initial magnetic fields and magnetic field amplification by the turbulent dynamo from extremely high plasma β. The energy spectra show a reasonable decrease in dissipation with grid refinement, and at a resolution of 512³ grid cells we identify a narrow inertial range with the expected power law scaling. The turbulent dynamo exhibits exponential growth of magnetic pressure, with the growth rate higher from solenoidal forcing than from compressive forcing. Two versions of the new scheme are presented, using relaxation-based 3-wave and 5-wave approximate Riemann solvers, respectively. The 5-wave solver is more accurate in some cases, and its computational cost is close to the 3-wave solver.     

Compressible turbulent flame speeds of highly turbulent standing flames

In this paper we present the first measurement of turbulent burning velocities of a highly turbulent compressible standing flame induced by shock-driven turbulence in a Turbulent Shock Tube. High-speed schlieren, chemiluminescence, PIV, and dynamic pressure measurements are made to quantify flame–turbulence interaction for high levels of turbulence at elevated temperatures and pressure. Distributions of turbulent velocities, vorticity and turbulent strain are provided for regions ahead and behind the standing flame. The turbulent flame speed is directly measured for the high-Mach standing turbulent flame. From measurements of the flame turbulent speed and turbulent Mach number, transition into a non-linear compressibility regime at turbulent Mach numbers above 0.4 is confirmed, and a possible mechanism for flame generated turbulence and deflagration-to-detonation transition is established.     

Comparison of methods for calculating turbulent diffusion coefficients

Different methods for calculating the turbulent diffusion coefficient D _T of a passive scalar impurity in an infinite homogeneous isotropic stationary turbulent medium are examined. The values of D _T calculated by these methods are compared for two limiting types of turbulence, viz., turbulence with a δ-function spectrum and turbulence with a Kolmogorov-type spectrum. The temporal dependence of the velocity correlators is assumed to be exponential. It is shown that the most accurate method is based on the use of the solution of the nonlinear equation for the averaged Green’s function with consideration of the contribution from the four-point turbulent velocity correlators. A comparison with the results of other methods that are simpler from the mathematical standpoint shows that some of them also permit the calculation of D _T with relatively good accuracy. Zh. éksp. Teor. Fiz. 111, 871–881 (March 1997)     

Turbulent diffusion in a compressible medium

This paper examines the diffusion of impurity particles in a compressible turbulent medium and compares it to diffusion in an incompressible medium. The turbulent diffusion coefficients are calculated using exact formulas expressed in terms of the Green’s function describing impurity transport in an infinite homogeneous, isotropic, stationary turbulent medium. To obtain an approximate expression for the Green’s function, numerical solutions of the nonlinear DIA (direct interaction approximation) equation (which in this paper are obtained for the first time for the case of compressible turbulence) are employed. Two types of turbulence are examined, acoustic and a mixture of shock waves. These are described by different generalized spectra. Finally, it is shown that compressibility significantly enhances the diffusion coefficient in the case of acoustic turbulence and reduces it in the second case. Zh. éksp. Teor. Fiz. 114, 930–945 (September 1998)     

Investigation of temperature fields in supersonic flow behind a backward-facing step

The results of numerical modelling and experimental investigations of high-enthalpy turbulent flows in the neighborhood of 90-degree backward-facing steps at the Mach numbers M_∞ = 2–4 are presented. The experiments were conducted in the hot-shot wind tunnel IT-302M of ITAM SB RAS. The computations were carried out on the basis of the full Favres-averaged Navier — Stokes equations augmented by the Wilcox turbulence model. The temperature factor influence on the flow structure in the separated zone and temperature distributions was investigated numerically for different Mach numbers. The wall temperature is shown to affect significantly the quantity and sizes of recirculation vortices as well as the temperature distribution in the zone of flow separation and reattachment. The computational results are compared with experimental data on the pressure distribution on the model surface and the wave structure of the flow.     

可压缩湍流研究进展

高马赫数可压缩湍流的运动是一个多尺度多过程的物理现象。采用了多过程分解的方法,将可压缩湍流分解为剪切和胀压过程,分析其统计行为和动力学行为。发展了一种新的紧致差分和WENO格式相结合的混合型数值格式,准确模拟了可压缩湍流场;研究了其多尺度多过程行为和对粒子的输运影响;研究了激波结构对湍流场的影响;在高雷诺数可压缩湍流中,证明存在惯性区,其中流运动和压力做功引起的动能流通量都是常数;证明可压缩湍流中存在从大尺度到小尺度的动能级串过程;证明动能流通量的剪切部分和胀压部分在惯性区都为常数;分析亚格子应力项和亚格子质量流动项对动能级串的影响。     

Mixing of a passive scalar across a thin shearless layer: concentration of intermittency on the sides of the turbulent interface

The advection of a passive scalar through an initial flat interface separating two different isotropic decaying turbulent fields is investigated in two and three dimensions. Simulations have been performed for a range of Taylor’s microscale Reynolds numbers from 45 to 250 and for a Schmidt number equal to 1. Different to the case where the transport involves the momentum and kinetic energy only and one intermittency layer is formed in the low-turbulent energy side of the system, in the passive scalar concentration field two intermittent layers are observed to develop at the sides of the interface. The layers move normally to the interface in opposite directions. The dimensionality produces different time scaling of the passive scalar diffusion, which is much faster in the two-dimensional case. In two dimensions, the propagation of the intermittent layers exhibits a significant asymmetry with respect to the initial position of the interface and is deeper for the layer which moves towards the high kinetic energy side of the system. In three dimensions, the two intermittent layers propagate nearly symmetrically with respect the centre of the mixing region. During the temporal decay, inside the mixing, which is both inhomogeneous and anisotropic but devoid of a mean velocity shear, the passive scalar spectra are computed. In three dimensions, the exponent in the scaling range gets in time a value close to that of the kinetic energy spectrum of isotropic turbulence (?5/3). In two dimensions, instead the exponent settles down to a value that is about one-half of the corresponding isotropic case. By means of an analysis based on simple wavy perturbations of the interface we show that the formation of the double layer of intermittency is a dynamic general feature not specific to the turbulent transport. These results of our numerical study are discussed in the context of experimental results and numerical simulations.     

Structure function scaling in compressible super-Alfvénic MHD turbulence

Supersonic turbulent flows of magnetized gas are believed to play an important role in the dynamics of star-forming clouds in galaxies. Understanding statistical properties of such flows is crucial for developing a theory of star formation. In this Letter we propose a unified approach for obtaining the velocity scaling in compressible and super-Alfvénic turbulence, valid for the arbitrary sonic Mach number, M(S). We demonstrate with numerical simulations that the scaling can be described with the She-Lévêque formalism, where only one parameter, interpreted as the Hausdorff dimension of the most intense dissipative structures, needs to be varied as a function of M(S). Our results thus provide a method for obtaining the velocity scaling in interstellar clouds once their Mach numbers have been inferred from observations.     

Transition of the boundary layer on a flat plate at supersonic and hypersonic velocities

The transition of the boundary layer from the laminar to the turbulent state on a smooth flat plate at a zero angle of attack is studied in the range of Mach numbers M_∞ = 2–6. It is demonstrated that the results measured at the end of the transition region can be approximated by a simple dependence suitable for applications, which does not require additional measurements, is valid in the range of Mach numbers M_∞ = 2–10, and, with an error lower than 20 %, can be used to estimate the location of the transition region on a flat plate in geometrically similar wind tunnels.     

应用GAO-YONG可压缩湍流模式数值模拟RAE2822翼型绕流

应用Gao-Yong可压缩湍流模式,数值模拟RAE2822二维翼型在两种不同来流情况下的跨音速粘性绕流问题.湍流模式的对流项用ROE格式离散,扩散项用中心差分格式离散,空间离散后的控制方程用多步Runge-Kutta显式时间推进格式求解.计算结果预测了翼型表面的压力系数的分布、平均速度剖面、激波的位置、马赫数等值线等情况.同时,对翼型表面激波与边界层相互干扰以及转捩问题进行分析计算,结果表明,Gao-Yong可压缩湍流模式结合适当的数值方法能够成功地模拟翼型跨音速粘性流动.最后,基于Gao-Yong可压缩湍流模式各项异性湍流粘性的机理,初步提出一种预测转捩起始位置的方法.