Determination of Turbulent Scales in Subsonic and Supersonic Jets from LDV Measurements

The turbulence statistics of high speed jets provide the basis for the acoustic source mechanisms which are responsible for noise generation. The correlation functions and spectra together with the length and time scales of the turbulence associated with sub and supersonic jets are examined for two point Laser Doppler Velocimetry (LDV) measurements. The paper presents results for two point/single component measurements at Mach numbers of 0.75 and 1.2 for isothermal conditions (i.e. Tj/Ta = 1.0). Analysis has been performed to extract the turbulence statistical characteristics using both the slot correlation and sample-and–hold procedures. A technique by which the frequency dependence of the scales can be determined from the auto and cross spectra of the measure ments is derived. The results from the two analysis methods are compared and shown, in the main, to be equivalent. The figure below shows the length and time scales determined from the measurements in a subsonic isothermal jet flow at M=0.75