EXPERIMENTAL STUDY ON THE TURUBLENTCE STRUCTURE CHARACTERISES OF THE JET SHEAR LAYER IN A COUNTERFLOWING WALL JET

The experimental measurement of the flow field of counterflowing wall jet using the particle image velocimetry. The jet velocity ratio to the main flow velocity is 8.89, and the Reynolds number based on the jet pipe is 9127. This paper focuses on the statistical characteristics of turbulence at different streamwise positions in the jet shear layer, including scale characteristics and structural characteristics. Statistical analysis of the fluctuating velocity field at different streamwise direction positions on the jet centerline shows that: in the range of x/D = 30\sim43 , Q1 and Q4 events dominate due to the feedback mechanism. Q3 event is dominant in the region near the stagnation point ( x/D = 43\text \sim50 ). The spatial scale of the turbulent structure in the jet shear layer is analyzed. The total scale in the interval x/D = 0\sim37 shows an increasing trend downstream, and it is almost unchanged in the interval x/D = 37\sim46 . The total scale in the interval x/D = 46\sim 51 tends to decrease downstream. In the interval x/D = 0\sim35 , the upstream scale of the reference point is similar to the downstream scale. In the interval x/D = 35\sim41 , the downstream scale of the reference point is larger than the upstream scale. In the interval x/D = 41\sim51 , the downstream scale of the reference point is smaller than the upstream scale. The spectral proper orthogonal decomposition is used to quantitatively analyzes the turbulent structure. It shows that the energy of the mode is concentrated in low frequency. The most energetic mode in the flow field has a frequency offD/U_j = 0.000\;5 and appears in the recirculation region. The first mode with a frequency equal tofD/U_j = 0.002\;6 indicates that the turbulent structure is generated when the jet is deflected by the interaction with the main flow, and transported along the periphery of the recirculation region. The configurations of high-frequency structures are similar, all located in the jet shear layer, and the higher the frequency, the closer to the jet outlet, the smaller the scale.