Showing papers by "Jørgen Fredsøe published in 1998"

Turbulent combined oscillatory flow and current in a pipe

Abstract: This work concerns the combined oscillatory flow and current in a circular, smooth pipe. The study comprises wall shear stress measurements, and laser-Doppler-anemometer velocity and turbulence measurements. Three kinds of pipes were used, with diameters D=19 cm, 9 cm, and 1.1 cm, enabling the influence of the parameter R/δ to be studied in the investigation (R/δ ranging from about 3 to 53), where R is the radius of the pipe, and δ is the Stokes layer thickness. The ranges of the two other parameters of the combined flow processes, namely the current Reynolds number, Rec, and the oscillatory-flow boundary-layer (i.e. the wave–boundary layer) Reynolds number, Rew, are: Rec=0−1.6×105, and Rew=0−7×106. The transition to turbulence in the combined flow case occurs at a current Reynolds number larger than the conventional value, ca. 2×103, depending on Rew, and R/δ. A turbulent current can be laminarized by superimposing an oscillatory flow. The overall average value of the wall shear stress (the mean wall shear stress) may retain its steady-current value, it may decrease, or it may increase, depending on the flow regime. The increase (which 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 stress oscillating around the mean wall shear stress can also increase with respect to its oscillatory-flow-alone value. For this to occur, the originally laminar oscillatory boundary layer needs to become a fully developed turbulent boundary layer, when a turbulent current is superimposed. This increase can be as much as O(3–4). The velocity profiles across the cross-section of the pipe change near the wall when an oscillatory flow is superimposed on a current, in agreement with the results of the wall shear stress measurements. The period-averaged turbulence profiles across the cross-section of the pipe behave differently for different flow regimes. When the two components of the flow are equally significant, the turbulence profile appears to be different from those corresponding to the fundamental cases; the level of turbulence increases (only slightly) with respect to those experienced in the fundamental cases.

Wave scour around group of vertical piles

Abstract: The paper presents the results of an experimental investigation on scour around pile groups with different configurations exposed to waves. Two kinds of tests were carried out: the actual scour experiments, and bed shear stress measurements. Seven kinds of pile-group arrangements were tested, including the side-by-side arrangements of piles, tandem arrangements of piles, and triangular and square group arrangements with two-, three-, and four-pile groups. The variations with the pile spacing as well as the Keulegan-Carpenter number, \IKC\N, were investigated. The scour depth can increase with respect to its single-pile value by as much as a factor of 3 for moderate \IKC\N numbers [such as \iO(10)] for most of the pile-group configurations for small pile spacings. For small \IKC\N numbers, this increase can be even higher, by as much as a factor of 10 or more. For a given pile spacing, the scour depth is mainly governed by the \IKC\N number. In agreement with the single-pile case, the larger the \IKC\N number, the larger the scour depth. The bed was live in all the tests except a few cases.