Experimental and computational fluid dynamics analysis of turbulent jet in crossflow

As it is well known that, most of the flows in the nature are turbulent. One of the turbulent flows seen in the nature is jet in cross flow or transverse jet. Jet in cross flow expresses the flow at which its direction is completely different from the cross flow. Free stream forces the jet flow to change its direction due to its momentum. Jet in crossflow is encountered from the stack smoke to flow in combustion room and hypersonic misille and rocket control systems, low mass flux boundary layer control systems to V/STOL aircrafts.In this study, jet in cross flow is taken into account by using experimental and computational fluid dynamics (CFD). In computational fluid dynamics calculations a CFD code of FLUENT 6.3.26 which has been used commonly in industry by engineers and in institutions and universities by academicians and researchers are considered. In the numerical section, nozzle geometry, number of nozzles, velocity ratio and inclination angle between the crossflow and nozzle are investigated to explore the effect of these parameters on the flow structure. For this reason two different nozzle section geometries; two different nozzle arrangements; circular and square nozzles, one and three-side by side nozzles, four different velocity ratios; R=0.2, 0.5, 1.0 and 2.0, five different inclination angle; a=0°, 45°, 60°, 75° ve 90° are considered. It was realized that, the high velocity ratio and inclination angle between the channel and nozzle, the more penetration of jet and the enlarge the counter-rotating vortex pair (CRVP) in size. It was also reported that using three nozzles side by side lead to high CRVP in size than the use of one nozzle. Film cooling which is very important for cooling technique was also investigated for different jet inclination angle and blowing ratios. It can be concluded that, the high thermal film cooling efficiency was obtained for the lowest inclination angle of 30°.Experimental measurements are done in a wind tunnel which has 305 mm´305 mm´914 mm test section for different jet-to-crossflow velocity ratios at different stations. Jet in crossflow are measured by using constant temperature anemometer (CTA) which is traditional but most commonly used in the experiments. Nozzles used in the experiments are considered as circular and the inclination angle between the nozzle and crossflow are designed as 90°. All measurements are conducted for R=0.5, 1.0 and 1.5. The results are then decomposed by using Fourier, wavelet and proper orthogonal decomposition (POD) techniques. Besides power spectrums and histograms dealing with the flow, velocity distributions are showed. According to Fourier analysis, the power spectrum at the center of the nozzle are more similar to the laminar character that the streamwise stations. When jet exit velocity was increased, it was seen that the energy of the flow decreased ironically. Comparisons were made for three velocity ratios, (R=0.5, 1.0 and 1.5) and it was seen that, the highest velociy ratio has the lowest energy. At the center of the nozzle, jet acts as an obstacle to the coming crossflow and damps the effect of the crossflow. The most dominant frequency of the flow was detected as 45 Hz at the ratio of R=0.5 by using wavelet technique. At R=1.0, dominant frequencies were seen at 85 Hz to 105 Hz. Energy contribution of the flow was calculated by using proper orthogonal decomposition (POD) technique and it was seen that the lower velocity ratio leads to carry more energy of the flow than the higher velocity ratio.Flow visualization technique was also performed to reveal the flow structure. A fog generator and digital video camera were used for this purpose. The flow field was investigated for different jet to crossflow velocity ratio by changing the velocity of the jet and wind tunnel, respectively. At the highest velocity ratio, jet penetrate directly into the crossflow due to its high momentum and acts as obstacle on the coming crossflow. It was also showed that, when the jet velocity was increased, crossflow become more dominant and sweep the jet flow, bending it to the crossflow direction. Shear layer vortices were detected especially at lower velocity ratio while it could not be seen at high crossflow.Keywords: Computational fluid dynamics (CFD), turbulence models, transverse jet, constant temperature aneometer (CTA), wavelet transform, proper ortogonal decomposition (POD), Fourier decomposition, power sprectrum.