Quantification of the steady state turbulent pipe flow in a facility for detailed flow field measurements of water hammer using planar PIV

Abstract

Modelling of the initial pressure pulse amplitudes due to water hammer is well known, but including the modelling of the damping due to the wall friction and dissipation in the fluid in subsequent pulses is less straightforward. Some analytical solutions and detailed measurements exist for impulsively accelerating flow, while measurements and highly detailed simulations exist for impulsively decelerating turbulent flow in straight pipes. The authors are not aware of any detailed measurements of instantaneous velocity fields in a straight pipe for typical water hammer events. Measurements of this kind will enable validation of detailed flow simulations that might become attainable on short term and comparison with detailed analytical models. The authors recently presented a new facility for detailed flow field measurements of water hammer in a pipe (2). In this paper we present the quantification of the steady state flow and turbulence in the mentioned facility using planar Particle Image Velocimetry (PIV). We apply a calibration based on optical ray tracing analysis to the PIV data and present measurements up to a shear Reynolds number of Reτ = 827 and a spatial resolution in wall normal direction of at least Δy⁺ = 7.1, as found via a PIV analysis using non-square interrogation areas of 8 × 32 pixel. The presented PIV method and calibration lead to a match with literature data for steady flow and turbulence of up to 1.8% in turbulent normal stress (u^′+) peak height. The difference can likely be explained by the slightly lower value of Re_t than that for the reference case. This will enable the detailed study of the dissipative near-wall behavior and also of turbulent vortical interactions further from the wall in the logarithmic region to aid the understanding of the dissipation in impulsive water hammer flows in pipes.