Round impinging jets with relatively large stand-off distance

Abstract

Large eddy simulation and particle image velocimetry measurements have been performed to evaluate the characteristics of a turbulent impinging jet with large nozzle height-to-diameter ratio (H/D = 20). The Reynolds number considered is approximately 28 000 based on the jet exit velocity and nozzle diameter. Mean normalized centerline velocity in both the free jet and impingement regions and pressure distribution over the plate obtained from simulations and experiments show good agreement. The ring-like vortices generated due to the Kelvin-Helmholtz instabilities at the exit of the nozzle merge, break down and transform into large scale structures while traveling towards the impingement plate. A Strouhal number of 0.63 was found for the vortices generated at the exit of the nozzle. However, this parameter is reduced along the centerline towards the impingement zone. A characteristic frequency was also determined for the large scale structures impinging on the plate. The expansion, growth, tilt, and three-dimensionality of the impinging structures cause dislocation of the impinging flow from the centerline, which is significantly larger when compared with flows having small H/D ratios. Contrary to the behavior of impinging jets with small stand-off distance, due to the loss of coherence, the large scale structures do not result in significant secondary vortices in the wall jet region and consequently less fluctuations were observed for wall shear stress.

Towards an absolute scale for adhesion strength of ship hull microfouling

“…Stopping criteria were set to ensure convergence of simulation results, corresponding to an asymptotic limit for the pressure at the stagnation point of the jet of 0.01 Pa maximum absolute variation within 10 iterations in the validation part, and within 100 iterations in the empirical study. These criteria were selected based on the expected stagnation pressure for the simulated cases, which was expected to be $440 Pa in the validation part (Shademan et al 2016) and $1.2 Â 10 3 -3.9 Â 10 4 Pa in the empirical study, estimated using Equation 1 with current flow parameters.…”

“…Round vertically impinging jets under immersed conditions, ie water-in-water jets or air-in-air jets, have been extensively studied in the past. These jets are of interest in many fields of engineering, from aerospace (Wu, Banyassady, and Piomelli 2016) to heating/cooling, metal cutting and industrial cleaning (Shademan et al 2016), as well as in soil erosion testing (Hanson and Cook 2004).…”

“…This means that the ratio between inertia and viscous forces (Reynolds number) is similar, and therefore previous work may be used for validation and calibration purposes. Poreh, Tsuei, and Cermak (1967) 8 -24 1.11 Â 10 6 -5.00 Â 10 6 Floating element (air) Orifice Kataoka & Mizushina (1974) 3.86-8.24 9.60 Â 10 3 -3.62 Â 10 4 Electro-chemical (water) Convergent Beltaos & Rajaratnam (1974) 21.2-65.7 3.89 Â 10 4 -8.04 Â 10 4 Preston tube (air) Convergent Giralt, Chia, and Trass (1977) 3.0-25.0 3 Â 10 4 -8 Â 10 4 Not determined (air) Convergent Hanson, Robinson, and Temple (1990) 16.5 2.3 Â 10 4 -8.4 Â 10 4 Hot-film sensor (water) Convergent Alekseenko & Markovich (1994) 2 -8 4.16 Â 10 4 Electrochemical (water) Convergent Phares, Smedley, and Flagan (2000) 3 -20 3.7 Â 10 3 -1.15 Â 10 4 Particle resuspension (air) Short pipe Shademan, Balachandar, and Barron (2013) 2 -18.5 1 Â 10 5 RANS Uniform inlet Ghaneeizad, Atkinson, and Bennett (2015) 24.7 2.3 Â 10 4 -3.4 Â 10 4 PIV (water) JET nozzle Shademan et al (2016) 20 2.8 Â 10 4 LES, PIV (water) Convergent RANS, Reynolds-averaged Navier-Stokes; LES, large eddy simulation; PIV, particle image velocimetry; JET, jet erosion test. Depending on the working fluid, usually air or water, different experimental techniques have been used in the past for determining the tangential component of wall surface forces, ie the wall shear stress (Table 1).…”

“…Simulated flow cases are defined in Table 2, where a stand-off ratio of H/D ¼ 20 was kept throughout. The first case, with nozzle diameter D ¼ 0.01 m, is used for validation against experimental and numerical results from Shademan et al (2016). The four remaining cases, with nozzle diameter D ¼ 1.6 mm, represent conditions specific to microfouling adhesion tests using calibrated waterjets (Swain and Schultz 1996;Finlay et al 2002), with the sole difference that here the waterjets are immersed, rather than in air.…”

“…In the validation part of this study, results were compared to experimental and numerical data on immersed jets (Shademan et al 2016, among other studies). Additionally, numerical uncertainties in terms of grid convergence were quantified using the grid convergence index (GCI) for the fine grid solution, as detailed in Roache (1994):…”