Measurements of velocity and wall shear stress fluctuations were made in an external turbulent boundary layer developed over a towed surface-piercing flat plate. An array of eight flush-mounted wall shear stress sensors was used to compute the space-time correlation function. A methodology for in situ calibration of the sensors for ship hydrodynamic applications is presented. The intensity of the wall shear stress fluctuations, τ
 rms/τ
 avg was measured as 0.25 and 0.36 for R
 θ
 =3,150 and 2,160 respectively. The probability density is shown to exhibit positive skewness, and lack of flow reversals at the wall. Correlations between velocity and wall shear stress fluctuations are shown to collapse with outer boundary layer length and velocity scales, verifying the existence of large-scale coherent structures which convect and decay along the wall at an angle of inclination varying from 10 to 13° over the range of Reynolds numbers investigated. The wall shear stress convection velocity determined from narrow band correlation measurements is shown to scale with outer variables. The space-time correlation of the wall shear is shown to exhibit a well-defined convective ridge, and to decay 80% over approximately 3δ for R
 θ
 =3,150.

Measurements and scaling of wall shear stress fluctuations

A new gauge for skin‐friction measurements in turbulent flows involving a wire close to the surface and a second, flush‐mounted film which serves as a constant temperature guard heater is described and evaluated. It is shown that a laminar calibration of the gauge applies to measurements in turbulent flow and that the frequency response of the gauge is appropriate for time‐resolved data in such a flow. Initial results from a turbulent channel flow are presented.

Modified gauge for time‐resolved skin‐friction measurements

Measurements of velocity and wall shear stress Uo fluctuations were made in an external turbulent boundary layer developed over a towed surface-piercing flat plate. An array of eight flush-mounted wall shear stress sensors _ was used to compute the space-time correlation function. _ A methodology for in situ calibration of the sensors for ship hydrodynamic applications is presented. The inten- 6 sity of the wall shear stress fluctuations, Trras/Tavg, was 3" measured as 0.25 and 0.36 for Ro=3,150 and 2,160 re- 0 spectively. The probability density is shown to exhibit v positive skewness, and lack of flow reversals at the wall. g Correlations between velocity and wall shear stress 2c fluctuations are shown to collapse with outer boundary zw layer length and velocity scales, verifying the existence of Tavg large-scale coherent structures which convect and decay Zrm~ along the wall at an angle of inclination varying from 10 Offo) to 13 ~ over the range of Reynolds numbers investigated. The wall shear stress convection velocity determined from narrow band correlation measurements is shown to scale with outer variables. The space-time correlation of the wall shear is shown to exhibit a well-defined convective ridge, and to decay 80% over approximately 3(5 for R0=3,150.

shear stress fluctuations

: Fluid shear stress is one of the most important quantities characterizing the interaction between a flowing fluid and a solid body. A knowledge of surface shear stress is essential in determining the drag on submerged bodies, the energy losses in internal flows, local surface heat transfer rates in both internal and external flows, flow separation, cavitation, and unsteady flow behavior in rotating machinery. (Author)

C Forbes Dewey

Papers

Measurement Methods for Fluid Shear Stress.