Accurate turbulence level estimations using PIV/PTV

The estimation of the turbulence level of flow facilities is very important for the comprehensive description of experimental results. While for low flow velocities various measurement techniques can be used (for example hot-wire, LDV, PIV) the task becomes difficult in the case of compressible flows as temperature and density fluctuations bias the measurement of the velocity fluctuations. In this work, we analyze the free-stream flow of the trisonic wind tunnel Munich (TWM) by means of particle image velocimetry (PIV) and particle tracking velocimetry (PTV). The goal is to determine the flow quality, i.e. the turbulence level, over the operating range of the facility without bias due to temperature and density variations. The capability of PIV/PTV for the estimation of small velocity fluctuations is investigated in detail. It is shown that a small particle shift on the measurement plane in combination with a large particle image displacement on the image plane allows for precise velocity measurements. Furthermore, a variation of the time separation between the PIV double images, $\Delta t$, enables the measurement uncertainty to be determined, which was estimated to be as low as $0.04\%$ of the mean displacement for a mean displacement of $\Delta x=100$ pixel and an interrogation window size of $32\times 32$ pixel. Regarding the wind tunnel turbulence, it was found that the turbulence level generally decreases with increasing Mach number for the TWM facility, starting with $1.9\%$ at $Ma=0.3$ and reaching $0.45\%$ at $Ma=3.0$. With this analysis, a methodology exists to perform accurate turbulence measurements in incompressible and compressible flows. Graphical abstract:

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