Flow separation

About: Flow separation is a research topic. Over the lifetime, 16708 publications have been published within this topic receiving 386926 citations.

High frequency wall‐pressure fluctuations in turbulent boundary layers

Abstract: Directly comparable spectral measurements with piezoelectric and pinhole‐microphone transducers show pinhole measurements to be significantly in error. The increase in measured rms pressure with reduction of transducer size is not nearly as dramatic as previous work has suggested.

Numerical Simulation of Turbulent Mixing Layers via Vortex Dynamics

Abstract: The simple concept of effective transport coefficients for turbulent flows is starting to be replaced with the realization that turbulent flows consist of structures which are created and destroyed, and it is their interactions that produce the transport of mass, momentum, and energy. The bursting phenomena in boundary layers is one aspect of that structure, and vorticity (or eddy) pairing is the structure of the turbulent mixing layer. This paper describes numerical experiments using discrete vortices in order to determine to what detail they can simulate the planar turbulent mixing layer. Calculations involving several thousand vortices have duplicated the mixing layer growth, eddy pairing, turbulent shear-stress profile, and root mean square velocity fluctuations of moderate Reynolds number flows. With the exception of the normal rms velocity fluctuation, good agreement is also found at high Reynolds number.

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.

Investigation of the backflow phenomenon in falling liquid films

Abstract: The phenomenon of backflow in the capillary wave region of laminar falling liquid films is studied in detail. For the first time, the mechanism leading to the origination of the phenomenon is identified and explained. It is shown that backflow forms as the result of a separation eddy developing at the bounding wall similar to the case of classical flow separation. Results show that the adverse pressure distribution causing the separation of the flow in the capillary wave region is induced by the strong third-order deformation (i.e. change in curvature) of the liquid–gas free surface there. This deformation acts on the interfacial pressure jump, and thereby the wall pressure distribution, as a result of surface tension forces. It is shown that only the capillary waves, owing to their short wavelength and large curvature, impose a pressure distribution satisfying the conditions for flow separation. The effect of this capillary separation eddy on momentum and heat transfer is investigated from the perspective of modelling approaches for falling liquid films. The study is centred on a single case of inclined liquid film flow in the visco-capillary regime with surface waves externally excited at a single forcing frequency. Investigations are based on temporally and spatially highly resolved numerical data obtained by solving the Navier–Stokes equations for both phases. Computation of phase distribution is performed with the volume of fluid method and the effect of surface tension is modelled using the continuum surface force approach. Numerical data are compared with experimental data measured in the developed region of the flow. Laser-Doppler velocimetry is used to measure the temporal distribution of the local streamwise velocity component, and confocal chromatic imaging is employed to measure the temporal distribution of film thickness. Excellent agreement is obtained with respect to film thickness and reasonable agreement with respect to velocity.