Mapping of the turbulent round jet developing region using a constant temperature anemometer (CTA)

Abstract

The fully developed round turbulent jet has been extensively studied, whereas the developing region is much less understood. The high shear and turbulence intensities in the most interesting parts of the developing region make them inaccessible to common measurement techniques such as Constant Temperature Anemometry (CTA) due to the high demands on the measurement techniques for accuracy of the measurements. Turbulence measurements are therefore planned using our in-house laser Doppler anemometer (LDA) system based on its capability to provide accurate measurements and with its inherent ability to properly distinguish velocity components. A rigorous measurement with the intended LDA system however demands impractical processing time, so knowing the critical points at which measurement are to be taken will save valuable time. This information is herein acquired significantly faster and more practically, however less accurately, with single-wire CTA. A high-resolution measurement was done using a computer-controlled single-wire CTA with the wire probe mounted perpendicular to the incoming flow from the jet orifice. The measurements covered several points in the radial (r-direction) along x/D=10, x/D=15, x/D=20 and x/D=30 downstream (where D is the jet exit diameter), with spatial resolutions ranging from 1 to 3 mm between the points, depending on how far the measurement was from the jet centerline. A proper alignment was also conducted prior to measurement so that the same points can be reached again for LDA measurement on the same jet afterwards. The radial profiles of mean velocity and turbulence intensity at each downstream position are presented to show the statistics of the air flow inside and outside the jet. As expected from theory, the mean profiles display a nearly Gaussian shape, spread out and tapered with the downstream direction. The highest velocities are located at the centerline.

Experimental investigation of the turbulent cascade development by injection of single large-scale Fourier modes

“…A jet generator, which is the one used in several projects of the Turbulence Research Laboratory of DTU e.g. [29][30][31], produces an axisymmetric fully developed turbulent flow 30 jet exit diameters downstream of the nozzle exit. The jet nozzle, shown in Figure 3, has an exit diameter of D=10mm, a contraction ratio of 3.2:1 and can be moved in two horizontal directions, manually or by computer control.…”

A novel laser Doppler anemometer (LDA) for high-accuracy turbulence measurements

“…The two beams are directed through a dual Bragg cell (BC in the following) in order to distinguish the moving direction of the particles along the measured component axis [25,26]. This issue was remarked to be critical, in particular within the flow region where fluctuations and turbulence intensities are high [27], since the velocity variations are more likely to exist in both directions. The frequency of one of the beams is shifted by 40 MHz, which value is known from the BC module used, while the frequency of the other beam is shifted by 37 MHz.…”

Mapping of Energy Cascade in the Developing Region of a Turbulent Round Jet

“…2. This measurement scheme was determined from the mapping obtained during the preliminary experiment 24) , which was performed earlier to capture the regions where the turbulence development was most marked. The high level of fluctuations observed from that experiment has also highlighted the need to use a suitable optical frequency shift in order to obtain unbiased velocity measurement especially in the shear layers and the outer region of the jet.…”