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.
Dynamic flow quality measurements in the Langley Low Turbulence Pressure Tunnel
The results of tests to measure the disturbance levels across the cooler, screens, and nozzle in the test section of the rehabilitated Langley Research Center Low Turbulence Pressure Tunnel are reported and discussed. The velocity fluctuations measured upstream and downstream of the cooler were approximately equal and ranged from 5 to 15 percent. The attenuation of the measured velocity fluctuations across the screen agreed with predictions from Dryden's (1949) theory. Velocity fluctuations in the test section ranged from 0.025 percent at Mach 0.05 to 0.3 percent at Mach 0.2. Pressure fluctuations at the test section walls agreed with previous measurements made under a turbulent boundary. The autocorrelation functions indicated that there were large reductions in the integral scale of turbulence across the cooler and screens. The spectra measured upstream and downstream of the cooler were mostly broadband.
Hot-wire measurements in compressible flows
Conditions for hot-wire fluctuation measurements at high velocities are considered. Particular attention is given to the determination of sensitivity coefficients, the conditions for the application of constant-resistance and constant-current anemometers, and measurements at supersonic velocities. It is concluded that high-velocity measurements will be successful only if the following conditions are met: the availability of proper equipment, the development of the probe calibration method, and the substantiated interpretation of the measurement results. The method used at supersonic velocities not only can but must be used at high subsonic velocities.<<ETX>>