Flow separation

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

Boundary-layer pressures and the Corcos model: a development to incorporate low-wavenumber constraints

Abstract: This paper re-examines the theoretical arguments that indicate the structure of the pressure field induced on a flat surface by boundary-layer turbulence at low Mach number. The long-wave elements are shown to be dictated by the acoustics of the flow, and the limit of the acoustic range is the coincidence condition of grazing waves where the spectrum is singular and proportional to the logarithm of the flow scale. The surface spectrum is shown to be proportional to the square of frequency at low-enough frequency and to the square of wavenumber at those low wavenumbers with subsonic phase speed.The similarity model successfully used by Corcos for the main convective elements of the field is used in this paper to model the turbulent sources of pressure, not the pressure itself, so that a Corcos-like description of the pressure spectrum is derived that is consistent with constraints imposed by the governing equations. This results in a fairly compact specification of the pressure spectrum with yet-undetermined constants, which must be derived from experiment. Despite an extensive search of published data on the pressure field, it is concluded that existing information is an inadequate basis for setting those constants and that new free field experiments are needed. Boundary layers formed on gliders or buoyant underwater bodies offer the most promising source of such data.The paper concludes with a study of how large flush-mounted transducers discriminate against the local flow noise field and i t is shown that they do so at a rate of 9 decibels per doubling of transducer diameter. This different conclusion from Corcos’ correct 6 decibel rate for small transducers is entirely due to the low- wavenumber constraints on the spectrum, which are misrepresented in the simple similarity model. This result, which conforms with the constraints imposed by the weak compressibility of the fluid, is the same as that later suggested by Corcos for transducers that are large on the boundary-layer scale.

Separation Control on Low-Pressure Turbine Airfoils Using Synthetic Vortex Generator Jets

Abstract: Oscillating vortex generator jets have been used to control boundary layer separation from the suction side of a low-pressure turbine airfoil. A low Reynolds number (Re = 25,000) case with low free-stream turbulence has been investigated with detailed measurements including profiles of mean and fluctuating velocity and turbulent shear stress. Ensemble averaged profiles are computed for times within the jet pulsing cycle, and integral parameters and local skin friction coefficients are computed from these profiles. The jets are injected into the mainflow at a compound angle through a spanwise row of holes in the suction surface. Preliminary tests showed that the jets were effective over a wide range of frequencies and amplitudes. Detailed tests were conducted with a maximum blowing ratio of 4.7 and a dimensionless oscillation frequency of 0.65. The outward pulse from the jets in each oscillation cycle causes a disturbance to move down the airfoil surface. The leading and trailing edge celerities for the disturbance match those expected for a turbulent spot. The disturbance is followed by a calmed region. Following the calmed region, the boundary layer does separate, but the separation bubble remains very thin. Results are compared to an uncontrolled baseline case in which the boundary layer separated and did not reattach, and a case controlled passively with a rectangular bar on the suction surface. The comparison indicates that losses will be substantially lower with the jets than in the baseline or passively controlled cases.Copyright © 2003 by ASME

Numerical modelling of separated flow in river bends: model testing and experimental investigation of geometric controls on the extent of flow separation at the concave bank

Abstract: The occurrence of separated flow in sharply curving river bends, and the associated deposition of fine-grained sediment, is widely recognized, but the factors controlling the existence and extent of separated flow have not been clearly identified. A three-dimensional, fixed lid computational fluid dynamics program was used to simulate time-averaged flow in idealized channel bends in order to investigate geometric controls on flow separation. The code was applied first to a field site; it successfully modelled the qualitative features of the flow, including a separation zone, and was insensitive to choice of turbulence closure or exclusion of lateral variation in water surface topography. The simulations of idealized bends demonstrate that the existence and extent of concave bank flow separation can be significantly influenced by changes in bend planform, point bar topography and upstream planform. © 1998 John Wiley & Sons, Ltd.

Dynamics of bubble growth and detachment in a viscous shear flow

Abstract: The quasistatic injection of air bubbles at the wall in a viscous shear flow is experimentally investigated by high-speed video pictures and image processing. The bubble radius and center of gravity position are determined throughout the growth. Experimental results are then used to validate a force balance model during bubble growth and after detachment for small bubble Reynolds numbers. The contact angles at the bubble foot calculated from the force balance agree well with those experimentally obtained from image processing. Finally, the force balance model is used to predict the departure bubble radius versus the gas flow rate and liquid flow shear rate.