Abstract
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, varDelta 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 varDelta x=100, pixel and an interrogation window size of 32times 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
Highlights
The quantification of turbulence levels in wind tunnels is very important as many flow phenomena are very sensitive to this quantity (Ol et al 2005)
The aim of this work is to investigate the capability of double-pulse particle image velocimetry (PIV)/particle tracking velocimetry (PTV) to estimate the wind tunnel turbulence level for free-stream velocities between u∞ = 100 m/s and 600 m/s
Depending on the mass flow, which is a function of the Mach number and the Reynolds number, the usable run time is between 50 s and 3 min for the analyzed configurations, refer to Fig. 2
Summary
The quantification of turbulence levels in wind tunnels is very important as many flow phenomena are very sensitive to this quantity (Ol et al 2005). Knowing the turbulence level allows for the comparison of results measured in different facilities and to set the inflow conditions for numerical flow simulations. The turbulence level Tu is usually described by the standard deviation of the fluctuations of the streamwise velocity component normalized by the mean streamwise velocity Tu = u 2 ∕⟨u⟩ (1). Where the standard deviation is computed from the ensemble of velocity measurements u 2 = N 1 − n un − 2 ⟨u⟩
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