Linear frequency signal(LFM) signal is first used in radar fields due to its remarkable properties,such as wide bandwidth,high energy and high signal-to noise *** excitation has recently been introduced in medical ult...
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Linear frequency signal(LFM) signal is first used in radar fields due to its remarkable properties,such as wide bandwidth,high energy and high signal-to noise *** excitation has recently been introduced in medical ultrasonic scanning,and much exciting results were obtained by research ***,there is Utile study aiming to inspect solid materials utilizing coded excitation,In the present study,LFM signal were distinguished with pulsed compression method even merged *** signal with large time-bandwidth product can be designed theoretically,but the limited bandwidth of a transducer will lead to energy *** coded signals were used to excite transducer,then the received signals' amplitude and spectrum were *** results show that the amplitude will decrease when the frequency exceed the transducer's *** order to investigate the transmitting law in solid materials,the waveform and spectrum were analyzed after LFM signals penetrating steel materials with different *** the analysis,it is found that wave at high frequency band attenuates much quickly than wave at low frequency *** results of investigations of the field structure in a steel plate are *** results of measured pressure field distributions before and after compression were compared with the recorded signals using short pulse *** characteristics of waveform and spectrum were analyzed with LFM coded *** SNR of LFM signal at output side will be improved with pulsed compression *** the same time,the range resolution increase after matched *** sound field on the surface of a steel plate was measured using several LFM coded signal *** paper shows the feasibility of inspecting steel material with LFM signals.
High-speed flows in cavities such as grooves,wheel wells,and weapon bays,are often encountered in aerospace and aeronautical *** most cases the flow field is complex and unsteady,featured by shear-layer instability,fl...
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High-speed flows in cavities such as grooves,wheel wells,and weapon bays,are often encountered in aerospace and aeronautical *** most cases the flow field is complex and unsteady,featured by shear-layer instability,flow separation and formation of vortex structures,shock wave/boundary-layer interactions at transonic and supersonic *** unsteady flow characteristics occur therein,such as fluctuating pressure and *** aerodynamic noise inside the cavity can reach 170 dB,which can damage some installed apparatus inside the cavity and structural components of the ***-stream boundary layer has significant influence on aero-acoustic characteristics inside *** paper presents that sound pressure level(SPL) distributions and sound pressure frequency spectrum(SPFS) characteristics at different measurement positions on cavity floor centerline utilizing two terms of experimental results under experimental *** of different free-stream boundary-layer thickness to cavity depth ratios on shallow cavities(transitional and closed cavity flow) were *** data presented herein was obtained about the cavities with length to depth ratios of 12 and 15 over a Mach number of 0.6 at a Reynolds number of 1.23×10～7 per *** results show that decrease in the boundary-layer thickness to the cavity depth ratios(δ/D)causes SPL amplification and noise enhancement at different measurement positions inside the cavities,especially the front and ***,decrease in δ/D)has a significant influence on SPFS characteristics and leads to increase in SPL in the frequency range of 0Hz
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