TURBULENT COMBINED OSCILLATORY FLOW AND CURRENT IN A PIPE

This work concerns the combined oscillatory flow and current in a circ ular, smooth pipe. The study comprises wall shear stress measurements, and laser-Doppler-anemometer velocity and turbulence measurements. Th ree kinds of pipes were used, with diameters D = 19 cm, 9 cm, and 1.1 cm, enabling the influence of the parameter R/delta to be studied in t he investigation (R/delta ranging from about 3 to 53), where R is the radius of the pipe, and delta is the Stokes layer thickness. The range s of the two other parameters of the combined flow processes, namely t he current Reynolds number, Re-c, and the oscillatory-flow boundary-la yer (i.e. the wave-boundary layer) Reynolds number, Re-w, are: Re-c = 0-1.6 x 10(5), and Re-w = 0-7 x 10(6). The transition to turbulence in the combined flow case occurs at a current Reynolds number larger tha n the conventional value, ca. 2 x 10(3), depending on Re-w, and R/delt a. A turbulent current can be laminarized by superimposing an oscillat ory flow. The overall average value of the wall shear stress (the mean wall shear stress) may retain its steady-current value, it may decrea se, or it may increase, depending on the flow regime. The increase (wh ich can be as much as a factor of 4) occurs when the combined flow is in the wave-dominated regime, while the oscillatory-flow component of the flow is in the turbulent regime. The component of the wall shear s tress oscillating around the mean wall shear stress can also increase with respect to its oscillatory-flow-alone value. For this to occur, t he originally laminar oscillatory boundary layer needs to become a ful ly developed turbulent boundary layer, when a turbulent current is sup erimposed. This increase can be as much as O(3-4). The velocity profil es across the cross-section of the pipe change near the wall when an o scillatory flow is superimposed on a current, in agreement with the re sults of the wall shear stress measurements. The period-averaged turbu lence profiles across the cross-section of the pipe behave differently for different flow regimes. When the two components of the flow are e qually significant, the turbulence profile appears to be different fro m those corresponding to the fundamental cases; the level of turbulenc e increases (only slightly) with respect to those experienced in the f undamental cases.