Starting vortex

About: Starting vortex is a research topic. Over the lifetime, 4785 publications have been published within this topic receiving 100419 citations.

Three-dimensional direct numerical simulation of wake transitions of a circular cylinder

Abstract: This paper presents three-dimensional (3D) direct numerical simulations (DNS) of flow past a circular cylinder over a range of Reynolds number ( ) up to 300. The gradual wake transition process from mode A* (i.e. mode A with large-scale vortex dislocations) to mode B is well captured over a range of from 230 to 260. The mode swapping process is investigated in detail with the aid of numerical flow visualization. It is found that the mode B structures in the transition process are developed based on the streamwise vortices of mode A or A* which destabilize the braid shear layer region. For each case within the transition range, the transient mode swapping process consists of dislocation and non-dislocation cycles. With the increase of , it becomes more difficult to trigger dislocations from the pure mode A structure and form a dislocation cycle, and each dislocation stage becomes shorter in duration, resulting in a continuous decrease in the probability of occurrence of mode A* and a continuous increase in the probability of occurrence of mode B. The occurrence of mode A* results in a relatively strong flow three-dimensionality. A critical condition is confirmed at approximately , where the weakest flow three-dimensionality is observed, marking a transition from the disappearance of mode A* to the emergence of increasingly disordered mode B structures.

Numerical Studies of Two-Dimensional Vortex Motion by a System of Point Vortices

Abstract: The motions of a vortex tube with an elliptic cross section and of a vortex sheet of finite length in an inviscid, incompressible fluid are simulated by a number of discrete vortex filaments, each vortex moving under the action of the velocity field of all the other vortices. By the use of this simulation, rotation of the vortex tube and rolling-up of the vortex sheet are investigated numerically as initial value problems. Comparison with exact solutions shows the validity of this method of approximation. In order to improve the results, an artificial viscosity is introduced to the equations of motion. This diminishes randomization of vortices inherent to the treatment without viscosity and thus leads to regular rolling-up of a vortex sheet.

Transient Separation Control Using Pulse-Combustion Actuation

Abstract: The transitory response of the flow over a stalled, two-dimensional (NACA 4415) airfoil to pulsed actuation on time scales that are an order of magnitude shorter than the characteristic convective time scale is investigated experimentally (Re = 570, 000). Actuation is effected by momentary [O(1 ms)] pulsed jets that are generated by a spanwise array of combustion-based actuators integrated into the center section of the airfoil. The flowfield in the cross-stream plane above the airfoil and in its near wake is computed from multiple high-resolution particle image velocity images that are obtained phase locked to the actuation waveform and allow for tracking of vorticity concentrations. The brief actuation pulse leads to a remarkably strong transitory change in the circulation about the entire airfoil that is manifested by a severing of the separated vorticity layer and the subsequent shedding of a large-scale clockwise vortex that forms the separated flow domain. The clockwise severed vorticity layer that follows behind this detached vortex has a distinct sharp streamwise edge that grows and rolls up as the layer is advected along the surface. It is shown that the shedding of the severed vortex and the accumulation of surface vorticity are accompanied by a transitory increase in the magnitude of the circulation about the airfoil that lasts 8—10 convective time scales. The attached vorticity layer ultimately lifts off the surface in a manner that is reminiscent of dynamic stall, and the flow separates again.

Flow in a Viscous Trailing Vortex

Abstract: The equations of motion for an isolated laminar viscous vortex at moderate to large Reynolds numbers are linearised, by assuming that both the rotational velocity and the deficit of longitudinal velocity are small compared with that in the free stream. The rotational motion and the longitudinal motion may then be superimposed and solutions are readily obtained for each. If the vortex is generated by a body with profile drag it is predicted that the deficit of longitudinal velocity will be positive, which is in agreement with experimental observation. Further details of the solution and its relation to the flow in real vortices are discussed; and the theory is compared with some measurements in a turbulent vortex.