Flow over an obstacle emerging from the wall of a channel

Direct simulation of laminar flow over a rising obstacle (an actuator) reveals the presence of vortical structures identical to those found in flow over a stationary obstacle, intensified and stretched by the upward velocity of the boundary. Following deceleration of the actuator to a stationary position, this amplification leads to a vortex shedding event in the wake region as the flow evolves toward its steady state. Three obstacle shapes are analyzed: one is streamwise symmetric, and two are skew symmetric. The symmetric actuator is also raised into a higher Reynolds number flow and in a final test is raised at half-speed into the low Reynolds number flow. Results indicate that the time scale of the transient is independent of Reynolds number, depending primarily upon the rising time of the actuator and to a lesser degree its shape. A CTIVE control in the turbulent boundary layer of a wallbounded flow is an area of research that has significant implications for the air transport industry: substantial reductions in fuel costs may be achieved through small reductions in skin-friction drag; higher operating temperatures (and better efficiency) may be obtained in gas turbines through control of the heat transfer between exhaust gases from the combustor and the turbine blades. Different strategies for control include mass transfer through porous walls1 and so-called smart skins: an actuator on the wall responds to a flow by adjusting its height to steer the wall region dynamics (the bursting processes that are signatures of turbulent'flows and primary sources of momentum and heat transport).2 Critical to the success of active control is not only an understanding of near-wall turbulence (a great deal of research has been directed toward this over the past 30 years) but also an understanding of the phenomena that are to be used to effect control—the transient behavior of flow over a moving obstacle (concerning which little data are available). Flows over three-dimensional, stationary obstacles have been studied in the laboratory, in the physical domain, and through numerical simulation.3"6 In a comprehensive survey consisting of both experimental and computational tests, Mason and Morton7 have analyzed laminar, steady flows past a variety of stationary obstacles. As an extension of this body of work, focus is redirected toward transients associated with obstacles that emerge in time, utilizing an algorithm that simulates flow in a channel with three-dimensional, time-dependent wall geometries. The code has been verified using wall geometries for which there exist known steady and unsteady solutions8; this includes comparisons with data from Mason and Morton.7 Between these benchmark tests involving simpler flows and the control work involving turbulent flows fall intermediate analyses of transients associated with low and moderate Reynolds number laminar flows over rapidly emerging obstacles. To distinguish between the stationary and the rising obstacle, the latter shall be referred to as an actuator. Three actuator shapes have been selected, one streamwise symmetric and two skew symmetric. All are smooth functions, symmetric in the spanwise (crossflow) direction. As in the cases analyzed by Mason and Morton,7 Reynolds numbers have been chosen that lie below the value at which flow (over a stationary obstacle) becomes unsteady. Using a local Reynolds number (Ret), based on obstacle height and the mean velocity that would exist through the height interval