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EXPERIMENT 3: PRESSURE/VELOCITY MEASUREMENTS IN A TURBULENT JET 1. Jets A jet is formed by flow issuing from a nozzle into ambient fluid, which can be either moving or at rest. A jet is a fundamental flow configuration with many practical applications, e.g . propulsion, combustion, various chemically-reacting flows, mixing of temperature and pollutants, and chemical lasers. The velocity at the exit of the nozzle of a typical laboratory jet has a smooth profile and a low turbulence level, about 0.1 0.5% of the mean centerline velocity. In practical cases, the turbulence can be as much as 20%, but the basic flow properties observed in a clean laboratory jet can still be generalized and applied to real world problems. Due to the velocity difference between the jet and the ambient fluid, a thin shear layer is created. Fluid from the jet and from the ambient mix inside the shear layer. In the central portion of the flow, there is a region with almost uniform mean speed, called the potential core. Due to entrainment of the outside fluid, the jet spreads in the streamwise direction (Fig. 1). Eventually, the potential core disappears at a distance of about five diameters downstream of the nozzle. JET Potential Core Shear Layer D ~5D Self-similar region Figure 1: Regions of a jet The entrainment process continues further downstream, but the rate of spreading is different from that observed upstream of the potential core. The velocity distributions in the region downstream of the potential core have a bell-shaped profile (Fig. 2). U U U y y y virtual origin X jet exit x Y ½ Y ½ Figure 2: Velocity profiles of a jet
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Y l/2 is defined as the distance between the jet axis and the location where the velocity equals half of the maximum velocity, U o . The cross-stream ( Y ) location of this characteristic point can be used to measure the thickness of the jet profile. The bell-shaped velocity profiles can be thought of as if they originate from a single point, called the virtual origin. The virtual origin is determined by joining the Y 1/2 points and extrapolating to the X axis. At some distance from the jet exit, the turbulent jet profiles collapse onto a single curve when the Y axis is normalized with X (Fig. 3).
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