Wall Shear Stress Generated by a Bernoulli Pad: Experiments and Numerical Simulations

TL;DR

This study addresses measuring wall shear stress generated by a Bernoulli pad to inform hull-cleaning applications. A flush-mounted hot-film sensor was calibrated in a water channel under laminar and turbulent conditions, enabling direct wall-shear measurements as the pad traverses a smooth workpiece. CFD simulations using Spalart-Allmaras and Transition-SST models were run in an axisymmetric domain to benchmark the experiments, accurately predicting the peak shear near the pad neck but overestimating shear at large radii. The work demonstrates the feasibility of validated, rapid predictions for peak shear while highlighting areas where turbulence modeling or relaminarization effects require further refinement for full-domain accuracy. Overall, the results support using simplified models to estimate maximum cleaning efficacy, with potential enhancements from LES/DNS for comprehensive flow-field predictions.

Abstract

Bernoulli pads generate locally large wall shear stresses on workpieces, which can be used for cleaning, but may also damage delicate surfaces. This work presents direct measurements of the wall shear stress using constant temperature anemometry for the first time. A hot-film sensor was calibrated in the laminar and turbulent flow regimes using a purpose-built water flow channel. The calibrated sensor was then flush-mounted onto a smooth surface and a Bernoulli pad was traversed over the sensor and wall shear stress data were acquired. Numerical simulations of the flow field were also performed; they accurately predicted the maximum shear stress near the jet corner but over-predicted at large radii.

Figures (12)

• Figure 1: Channel setup used for calibration of the hot-film sensor; figures are not drawn to scale: (a) top-view of the channel showing the eight pressure ports, (b) exploded view of the inlet flow conditioner, (c) exploded view of section c-c of channel with gasket, polycarbonate sheet, aluminum plate, and clamps (d) outlet flow conditioner showing internal vanes.
• Figure 2: A schematic of the hot-film sensor 55R46 by Dantec Dynamics (bib30)
• Figure 3: Assembled view of the channel setup in Fig.\ref{['Fig1']}: (a) top-view of the channel showing sensor mount (without sensor) and eight pressure ports (see Fig.\ref{['Fig1']}) connected via three-way valves, (b) inlet flow conditioner, (c) hot-film sensor, (d) magnified view of sensor mount with sensor, (e) sectional view of channel setup through the sensor mount and sensor.
• Figure 4: Pressure at the eight different ports of the channel (see Fig.\ref{['Fig1']}), computed based on pressure differential measurements relative to port and assignment of an arbitrary pressure to port . Note that the straight line fit was obtained by using the data from ports though .
• Figure 5: Calibration data showing the variation of $E_{\rm a}^2$ with $\tau_{\rm w}$