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The response of the wall shear stress in uniformly and nonuniformly accelerating pipe flows

L.R. Joel Sundstrom
Division of Fluid and Experimental Mechanics Lulea University of Technology SE-971 87, Lulea, Sweden

Michel J. Cervantes
Lulea University of Technology, Sweden Division of Fluid and Experimental Mechanics, Department of Engineering Sciences and Mathematics, Norrbotten 97187, Sweden; Water Power Laboratory, Norwegian University of Science and Technology, Trondheim, Norway

Sinopsis

Wall shear stress measurements in an accelerating turbulent pipe flow have been performed. Four different imposed accelerations have been studied including one uniform and three nonuniform ones. The initial-to-final Reynolds numbers as well as the acceleration time were approximately the same for each case, thereby isolating the effect of the type of acceleration. Data have been taken using hot-film anemometry, and it has been established that the time-development for each case is qualitatively similar, although there are significant quantitative differences between each case. The previously established view that the time-development of an accelerating flow resemble a laminar-to-turbulent bypass transition is confirmed. An explanation for the transitional behavior is sought through the Poisson equation describing the pressure fluctuations. It is postulated that the fast pressure induces asymmetries in the time-development of the wall-normal velocity fluctuations thereby leading to a route to transition. Due to lack of data, however, the proposed explanation cannot be confirmed or rejected, for that, further experimental as well as numerical studies have to be performed.