Skin friction probes for field applications

A technique has been developed which measures skin friction in marine, limnological, and flume boundary layers, with and without suspended particulate matter, by constant temperature anemometry. With sensor scales of ∼3 mm and frequency responses ≧20 Hz, flush‐mounted, epoxy‐coated hot films yield mean and fluctuating components of wall shearing stress at water depths ≦7000 m. Calibration and deployment techniques show that the probes can measure the magnitude of skin friction τ up to 2 N m −2 with sensitivities ≧0.1 V/0.1 N m −2 and accuracies of ±10% (20%) for mean (fluctuating) wall shearing stresses ≧0.016 N m −2 at the 95% uncertainty level. Two‐dimensional versions of the probes measure magnitude and direction of τ as well as the longitudinal vorticity ω x . Measurement errors of the skin friction probes are 2 to 6 times smaller than uncertainty levels for wall shearing stresses calculated by the logarithmic layer technique from velocity profiles under the same flow conditions, topography permitting. The seagoing skin friction sensors yield data without special requirements on shape or steadiness of the mean vertical flow profile, resolve turbulent wave boundary layers, and measure skin friction in sediment‐laden flows as long as (partial) viscous sublayers are present.