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Fig.6 shows the completed open jet wind tunnel inside the ISVR’s anechoic chamber (control valve and primary silencer are in the roof space of the chamber and are not shown in the figure).圖6示出了在英國南安普頓大學(xué)聲與振動(dòng)研究所（ISVR）消聲室（控制閥和主消聲器是在消聲室的屋頂空間,在圖中沒有顯示）內(nèi)的完整的開放式風(fēng)洞.Also shown is the new coordinate system (x,y,z) employed for the cross section of the nozzle exit plane.示出的還有用于噴嘴出口平面橫截面的新坐標(biāo)系統(tǒng)（x,y,z）.We now present the overall facility background noise characteristics for the entire rig as a function of exit jet velocity.現(xiàn)在我們介紹整個(gè)裝置（rig）總的設(shè)施背景噪聲特性與出口射流速度的函數(shù)關(guān)系.The flow uniformity and turbulence intensity variation over the jet nozzle of the jet were also measured and
are also presented below.在射流噴嘴范圍的流動(dòng)均勻性和湍流強(qiáng)度變化也進(jìn)行了測量,并在下面加以介紹.Note that both the acoustic and aerodynamic measurement results are plotted using the new coordinate system (x,y,z) as defined in Fig.6.請注意,聲學(xué)和空氣動(dòng)力學(xué)的測量結(jié)果都用新的坐標(biāo)系統(tǒng)（x,y,z）繪制,入圖6所示.
4.1.Analysis of background noise levels
背景噪聲水平的分析
A microphone was placed at (x,y,z) = (0,0.5,0),i.e.0.5 m vertically above the centre of the cross-sectional nozzle exit plane to measure the background noise level inside the anechoic chamber
at different exit jet velocities.在（x,y,z）＝(0,0.5,0)處,即在橫截面的噴嘴出口平面中心正上方0.5m處放置一個(gè)麥克風(fēng),以測量不同出口射流速度下消聲室內(nèi)的背景噪聲水平.This corresponds to a polar angle,h = 90 ,where h is the angle from the jet axis,as shown in Fig.1.這相當(dāng)于h＝90度的極角,這里,h為離開射流軸線的角度,入圖1所示.In addition,another microphone was placed at h = 45 (0.35,0.35,0) to assess the noise directivity of the exit jet.此外,在h＝45度（0.35,0.35,0）處放置另一個(gè)麥克風(fēng),以評估出口射流的噪聲方向性.Fig.7a and b show the narrowband (spectral density) sound pressure level at h = 45 and 90 ,respectively pertaining to the open jet wind tunnel over a range of jet velocities between 33.1 and 99.6 ms 圖7a和b分別示出了在h＝45度和90度時(shí),關(guān)于射流速度在33.1和99.6m/s之間范圍時(shí)開放式風(fēng)洞的窄帶（譜密度）聲壓水平.1.These figures are plotted in the form of power spectral density with a 1 Hz bandwidth and a frequency resolution,Df of 6.25 Hz.這些圖是以1Hz帶寬的功率譜密度和Df為6.25Hz的頻率分辨率的形式繪制的.The spectra are smoothly varying and decay slowly with frequency.該譜平滑變化,并隨頻率緩慢衰落.It is also insightful to examine how the sound pressure level varies with jet velocity as the function of frequency.研究聲壓水平作為頻率的函數(shù)如何隨射流速度變化也是很有見識(shí)的.Fig.8 shows the dependence of sound pressure level on jet velocity,p2 / VN for h = 45 and 90 .圖8示出了,聲壓水平與射流速度的依存關(guān)系,在h＝45度和90度下p2
/VN.For h = 45 ,the sound pressure level is observed to scale as V7.5–V8 in the frequency range between 400 Hz and 10 kHz.在h＝45度時(shí),在400Hz和10kHz之間的頻率范圍,觀察到的聲壓水平標(biāo)度為V7.5－V8.This power law is classically associated with quadrupole jet mixing noise.For h = 90 ,a power law of V6.5 in the frequency range 100 Hz–2 kHz is observed.這一冪率經(jīng)典來說與四極射流混合噪聲相關(guān)聯(lián).在h＝90度時(shí),在100Hz－2kHz的頻率范圍觀察到了V6.5的冪率.This velocity dependence implies that dipole aerodynamic noise sources are dominant at this measurement angle.這一速度依存關(guān)系表明了,雙極空氣動(dòng)力學(xué)噪聲源在這一測量角度時(shí)是主導(dǎo)性的.One possible dipole noise source is due to the boundary layer being scattered at the nozzle lip.Another possible dipole noise contributor at this frequency range could be due to the noise breakout from inside of the rig.一個(gè)可能的雙極噪聲源是由于邊界層在噴嘴唇部被散射而引起的.在這一頻率范圍,另一個(gè)可能的雙極噪聲貢獻(xiàn)源可能是由于在裝置（rig）內(nèi)部的噪聲爆發(fā)而引起的.From 2 kHz and above,the從2kHz及以上,……