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.
On the Convection of Heat from Small Cylinders in a Stream of Fluid: Determination of the Convection Constants of Small Platinum Wires with Applications to Hot-Wire Anemometry
The general problem of the convection of heat from bodies immersed in moving media has recently received considerable attention both from the theoretical and experimental point of view. The equation of the conduction of heat in a moving fluid was stated by Fourier as long ago as 1820,(1) and a few years later was expressed by Poisson(2) and Ostrogradsky(3) in the familiar form c D θ /D t = ∂/∂ x ( k ∂ θ /∂ x ) + ∂/∂ y ( k ∂ θ /∂ y ) + ∂/∂ z ( k ∂ θ /∂ z ), . . . . . (1) where θ is the temperature of the fluid at any point ( x, y, z ), c the heat capacity of the fluid per unit volume, k its thermal conductivity, and D/D t the “mobile operator” D/D t = ∂/∂ t + u ∂/∂ x + v ∂/∂ y + w ∂/∂ z of the hydrodynamical equations. In 1901 the problem was taken up by Boussinesq,(4) whose memoir on the subject in 1905 contains a great number of successful calculations of heat losses from bodies of various shapes immersed in a stream of fluid.