EMD Adaptive Filtering and Time-Frquency Analysis for Low Turbulence Intensity
Turbulence intensity is an important flow field parameter in wind tunnel, but variation interference noise cause the measurement of low turbulence intensity much trouble, so that lots of methods were used to process turbulence intensity signal. For purpose of improving measurement precision, an EMD(Empirical Mode Decomposition) adaptive filter is designed and HHT(Hilbert-Huang Transform) time-frquency analysis is used initially for low turbulence intensity. Experiment is carried out in three kinds of wind tunnel, includes a low turbulence wind tunnel, a general low speed wind tunnel and an open jet wind tunnel. The turbulence intensity signal is acquired with hot wire anemometry, the EMD method is compared with traditional methods, includes BPF(Band Pass Filter), ENC(Electronic Noise Correction), HPIA(High Pass Inertia Attenuation Filter). Using the EMD filter, the flow field turbulence intensity in the low turbulence wind tunnel test section is measured and found to be less than 0.05% at speed of 30m/s~100 m/s. Using HHT, the low frequency flow fluctuation signal is found in the open jet wind tunnel. To conclude, method of EMD adaptive filter can be used to control the interference of noise in hot wire signal, which is a practical method for low turbulence intensity digital signal process.
The further research on the design of low (varying) turbulence wind tunnel
Based on intensive investigation and successfully constructed wind tunnels in the past twenty years,the paper reviews some new ideas on the low(varying) turbulence wind tunnel design and they can be reference for engineering design.The paper gives the integrative curve for the design of stable section which plays a crucial role in reducing turbulence intensity,and the universal method for the contraction section which plays an important role in reducing turbulence intensity.Also it is proposed some personal opinion on the design of the power section and turning deflector,which optimize the whole system.
Hot-wire anemometry in transonic flow
The use of hot-wire anemometry for obtaining fluctuating data in transonic flows has been evaluated. From hot-wire heat loss correlations based on previous transonic data, the sensitivity coefficients for velocity, density, and total temperature fluctuations have been calculated for a wide range of test conditions and sensor parameters. For sensor Reynolds numbers greater than 20 and high sensor overheat ratios, the velocity sensitivity remains independent of Mach number and equal to the density sensitivity. These conclusions were verified by comparisons of predicted sensitivities with those from recent direct calibrations in transonic flows. Based on these results, techniques are presented to obtain meaningful measurements of fluctuating velocity, density, and Reynolds shear stress using hot-wire and hot-film anemometers. Examples of these measurements are presented for two transonic boundary layers.