Flow separation

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

Analysis of the flow and mass transfer processes for the incompressible flow past an open cavity with a laminar and a fully turbulent incoming boundary layer

Abstract: The three-dimensional incompressible flow past a rectangular two-dimensional shallow cavity in a channel is investigated using large-eddy simulation (LES). The aspect ratio (length/depth) of the cavity is L/D = 2 and the Reynolds number defined with the cavity depth and the mean velocity in the upstream channel is 3360. The sensitivity of the flow around the cavity to the characteristics of the upstream flow is studied by considering two extreme cases: a developing laminar boundary layer upstream of the cavity and when the upstream flow is fully turbulent. The two simulations are compared in terms of the mean statistics and temporal physics of the flow, including the dynamics of the coherent structures in the region surrounding the cavity. For the laminar inflow case it is found that the flow becomes unstable but remains laminar as it is convected over the cavity. Due to the three-dimensional flow instabilities and the interaction of the jet-like flow inside the recirculation region with the separated shear layer, the spanwise vortices that are shed regularly from the leading cavity edge are disturbed in the spanwise direction and, as they approach the trailing-edge corner, break into an array of hairpin-like vortices that is convected downstream the cavity close to the channel bottom. In the fully turbulent inflow case in which the momentum thickness of the incoming boundary layer is much larger compared to the laminar inflow case, the jittering of the shear layer on top of the cavity by the incoming near-wall coherent structures strongly influences the formation and convection of the eddies inside the separated shear layer. The mass exchange between the cavity and the main channel is investigated by considering the ejection of a passive scalar that is introduced instantaneously inside the cavity. As expected, it is found that the ejection is faster when the incoming flow is turbulent due to the interaction between the turbulent eddies convected from upstream of the cavity with the separated shear layer and also to the increased diffusion induced by the broader range of scales that populate the cavity. In the turbulent case it is shown that the eddies convected from upstream of the cavity can play an important role in accelerating the extraction of high-concentration fluid from inside the cavity. For both laminar and turbulent inflow cases it is shown that the scalar ejection can be described using simple dead-zone theory models in which a single-valued global mass exchange coefficient can be used to describe the scalar mass decay inside cavity over the whole ejection process.

Incompressible Boundary-Layer Separation

Abstract: For high-Reynolds-number flow over bodies or in confined channels the effects of viscosity are generally limited to a thin layer, the boundary layer, adjacent to the bounding surface. When the imposed pressure gradient is adverse, however, the thickness of the viscous layer increases as momentum is consumed by both wall shear and pressure gradient, and at some point the viscous layer breaks away from the bounding surface. Downstream of this point (or line) of breakaway the original boundary-layer fluid passes over a region of recirculating flow. The point at which the thin boundary layer breaks away from the surface and which divides the region of downstream-directed flow, in which the viscous effects are quite limited in extent, from the region of recirculating flow is known as the separation point.! Two different types of post-separation behavior are known to exist. In some cases the original boundary layer passes over the region or ' recirculating fluid and reattaches to the body at some point downstream, trapping a bubble of recirculating fluid beneath it. The characteristic length of this separation bubble may be of the same order as the upstream boundary-layer thickness or ma�y times the boundary-layer thickness. In other cases, the original boundary-layer fluid never reattaches to the body but passes downstream, mixing with recirculating fluid, to form a wake. For this wake-type of separation the characteristic dimension of the recirculating region is generally of the same order as the characteristic body dimension. In either case, the recirculating flow alters the effective body shape and hence the inviscid flow about the body.

Velocity measurements in a plane turbulent air jet at moderate Reynolds numbers

Abstract: An experimental investigation of the moderate Reynolds number plane air jets was undertaken and the effect of the jet Reynolds number on the turbulent flow structure was determined. The Reynolds number, which was defined by the jet exit conditions, was varied between 1000 and 7000. Other initial conditions, such as the initial turbulence intensity, were kept constant throughout the experiments. Both hot-wire and laser Doppler anemometry were used for the velocity measurements. In the moderate Reynolds number regime, the turbulent flow structure is in transition. The average size and the number of the large scale of turbulence (per unit length of jet) was unaffected by the Reynolds number. A broadening of the turbulent spectra with increasing Reynolds number was observed. This indicated that there is a decrease in the strength of the large eddies resulting from a reduction of the relative energy available to them. This diminished the jet mixing with the ambient as the Reynolds number increased. Higher Reynolds numbers led to lower jet dilution and spread rates. On the other hand, at higher Reynolds numbers the dependence of jet mixing on Reynolds number became less significant as the turbulent flow structure developed into a self-preserving state.

Flow Separation and Side-Loads in Rocket Nozzles

Abstract: Flow separation in nozzles of rocket engines is undesired because it can lead to dangerous lateral forces, which might damage the nozzle The origin of side-loads is not fully clear, although different possible origins were identified in the past Meanwhile, it seems to be clear that in thrust-optimized or parabolic nozzles, a major side-load is due to the transition of separation pattern from free shock separation to restricted shock separation and vice versa After a literature review, the reasons for the transition between the separation patterns are discussed, and the cap shock pattern, which is identified to be the cause of this transition, is closely analyzed It turns out that this pattern can be interpreted as an inverse Mach reflection of the internal shock at the centerline The separation and side-load behavior of thrust-optimized and parabolic nozzles is described in detail In order to be able to predict the pressure ratio pc/pa at which the transition of separation patterns occurs, a model is developed which uses TDK-data as an input With the oblique shock relations and a momentum balance, both the ratio of chamber to ambient pressure and the value of the lateral force can be predicted with fair accuracy

Structure of laminar juncture flows

Abstract: A computational study of both steady and periodic laminar horseshoe vortex flows generated upstream of a cylinder/flat plate juncture is presented. The flowfields are simulated using the full three-dimensional unsteady Navier-Stokes equations and a time-accurate implicit algorithm. A new type of laminar horseshoe vortex topology is identified. For the case of a single primary vortex, this new topology is found to be independent of the computational grid and is also supported by recent experimental flow visualizations. The flat plate skin-friction portraits corresponding to the new and to the standard horseshoe vortex topologies are equivalent, pointing out the nonunique relation between the wall limiting streamline pattern and the three-dimensional flow above the plate. For the new topology, the foremost line of coalescense is an attachment rather than a separation line. This unusual feature illustrates the fact that convergence of skin-friction lines is a necessary but not sufficient condition for separation. As the Reynolds number increases, the flow topology evolves from a single to multiple primary horseshoe vortices, in agreement with experimental observations. At least two different types of triple horseshoe vortex systems are shown to be possible. Above a certain value of the Reynolds number, the juncture flow becomes unsteady and periodic at a frequency that increases with Reynolds number. The unsteady horseshoe vortex process upstream of the cylinder is found in qualitative agreement with experiment. Horseshoe vortices are periodically generated and convected toward the juncture. Vorticity intensification by vortex stretching, and the eruption of vorticity from the plate surface are observed.