Force and particle image velocimetry measurements were conducted on a NACA 0012 airfoil undergoing small-amplitude sinusoidal plunge oscillations at a post-stall angle of attack and Reynolds number of 10,000. With increasing frequency of oscillation, lift increases and drag decreases due to the leading-edge vortices shed and convected over the suction surface of the airfoil. Within this regime the lift coefficient increases approximately linearly with the normalized plunge velocity. Local maxima occur in the lift coefficient due to the resonance with the most unstable wake frequency, its subharmonic and first harmonic, producing the most efficient conditions for high-lift generation. At higher frequencies a second mode of flowfield occurs. The leading edge vortex remains nearer the leading-edge of the airfoil and loses its coherency through impingement with the upward moving airfoil. To capture this impingement process high-fidelity computational simulations were performed which showed the highly transitional nature of the flow and a strong interaction between the upper and lower surface vortices. A sudden loss of lift may also occur at high frequencies for larger amplitudes in this mode.

Force and particle image velocimetry measurements were conducted on a NACA 0012 aerofoil undergoing small-amplitude high-frequency plunging oscillation at low Reynolds numbers and angles of attack in the range 0-20 • . For angles of attack less than or equal to the stall angle, at high Strouhal numbers, significant bifurcations are observed in the time-averaged lift coefficient resulting in two lift-coefficient branches. The upper branch is associated with an upwards deflected jet, and the lower branch is associated with a downwards deflected jet. These branches are stable and highly repeatable, and are achieved by increasing or decreasing the frequency in the experiments. Increasing frequency refers to starting from stationary and increasing the frequency very slowly (while waiting for the flow to reach an asymptotic state after each change in frequency); decreasing frequency refers to impulsively starting at the maximum frequency and decreasing the frequency very slowly. For the latter case, angle of attack, starting position and initial acceleration rate are also parameters in determining which branch is selected. The bifurcation behaviour is closely related to the properties of the trailing-edge vortices. The bifurcation was therefore not observed for very small plunge amplitudes or frequencies due to insufficient trailing-edge vortex strength, nor at larger angles of attack due to greater asymmetry in the strength of the trailing-edge vortices, which creates a preference for a downward deflected jet. Vortex strength and asymmetry parameters are derived from the circulation measurements. It is shown that the most appropriate strength parameter in determining the onset of deflected jets is the circulation normalized by the plunge velocity.

To program pluripotent cells into blood, a knowledge of the locations of precursors during their journey through the embryo and the signals they experience would be informative. The anterior (a) and posterior (p) ventral blood islands (VBIs) in Xenopus are derived from opposite sides of the pregastrula embryo. The aVBI goes through a "hemangioblast" state, characterized by coexpression of blood and endothelial genes at neurula stages, whereas the pVBI expresses these genes in a nonoverlapping fashion several hours later, after commitment to either a blood or an endothelial fate. We describe a novel role for fibroblast growth factor (FGF) in controlling the timing of Scl, Lmo2, and Runx1 expression in the 2 VBI compartments. Blocking FGF signaling during gastrulation expands expression at neurula stages into posterior regions. We show, by lineage labeling, explant analysis, and targeted blocking of FGF signaling, that this is due to the pVBI prematurely expressing these genes with the timing of the aVBI. In contrast, overex- IntroductionDuring early vertebrate embryogenesis, the first tissues to develop are blood and endothelium located in the blood islands of yolk sac mesoderm. 1 The blood islands are the source of primitive erythropoiesis and vasculogenesis, which supply the early embryo with a circulatory system carrying primarily erythroid and some myeloid cells. 2 As a result of early observations in chick, a common precursor to blood and endothelium, called the hemangioblast, was proposed. 3,4 Evidence for the existence of the hemangioblast comes from the observations that many genes are coexpressed and/or required for the very early stages of blood and endothelial development. 1,5 In addition, transient mouse embryonic stem cell (ESC)-derived blast colony forming cells (BL-CFCs) and more recently equivalent human colonies, give rise to both lineages and are considered to be the in vitro equivalent of hemangioblasts. 6,7 Comparable colonies have been detected in cultures from mouse embryos, suggesting that such bipotential precursors may also exist in vivo. 8,9 Very recently, clonal analysis of progenitor contributions to yolk sac populations using mouse ESCs, 10 and single cell resolution fate mapping in zebrafish late blastula/gastrula embryos 11 have arrived at contradictory conclusions on the existence of the hemangioblast. The fish study verifies a common progenitor for blood and endothelium, whereas the mouse study claims that few, if any, hemangioblasts exist. Further fate mapping may be required to resolve the issue.In Xenopus, blood and endothelium of the primitive lineage are located in the ventral blood island (VBI) of the tailbud stage embryo (approximately 26-36 hours after fertilization [hpf]). Fate mapping has demonstrated that the VBI is composed of 2 compartments, the anterior (a) and posterior (p) VBI, which are derived from opposite sides of the 32-cell stage embryo. 12 The precursors of the anterior VBI (aVBI) and posterior VBI (pVBI), therefore, take very different routes through the e...