Forward-backward elliptic anisotropy correlation in parton cascade

A potential experimental probe, forward-backward elliptic anisotropy correlation ($C_{FB}$), has been proposed by Liao and Koch to distinguish the jet and true elliptic flow contribution to the measured elliptic flow ($v_2$) in relativistic heavy-ion collisions. Jet and flow fluctuation contribution to elliptic flow is investigated within the framework of a multi-phase transport model using the $C_{FB}$ probe. We found that the $C_{FB}$ correlation is remarkably different and is about two times of that proposed by Liao and Koch. It originates from the correlation between fluctuation of forward and backward elliptic flow at low transverse momentum, which is mainly due to the initial correlation between fluctuation of forward and backward eccentricity. This results in an amendment of the $C_{FB}$ by a term related to the correlation between fluctuation of forward and backward elliptic flow. Our results suggest that a suitable rapidity gap for $C_{FB}$ correlation studies should be around $\pm$ 3.5.Comment: 4 pages, 3 figure

Forward-Looking Echoic Flow for Guidance of an Unmanned Aerial System

Echoic flow is a formula derived from natural phenomena that has the potential to control vehicles with great efficiency using range information. Initially studied in bats, echoic flow allows animals to use sonar as a navigation tool. Downward-facing echoic flow used in the vertical landing of an Unmanned Aerial System (UAS) has been studied in past research. Forward-looking echoic flow on a UAS could allow for new approaches to braking and following techniques in the horizontal plane of motion towards both fixed and moving targets. The goal of this project was to demonstrate forward-looking echoic flow guidance towards a fixed target using a quadcopter and to gather data showing the accuracy and precision of the process. In initial forward-looking tests, a modified Parrot AR Drone with an added ultrasonic sensor and Raspberry Pi were used as the UAS. Preliminary findings showed erratic and often inaccurate range finding measurements. These measurements were attributed in part to the inability of the UAS to aim directly at the small target. A software filter was designed to minimize the impact of erroneous measurements. Further tests conducted using a flat wall as the approach target still yielded trials that did not follow the ideal echoic flow approach accurately. In an attempt to improve the performance of trials, the equation used to convert velocities to motor thrust values was recalibrated. Though trial results did improve due to this modification, imprecise quadcopter movement control prevented the achievement of a smooth echoic flow approach. Finally, simulations of forward-looking trials were performed to test the impact of measurement and velocity error on the performance of echoic flow approaches. The values of measurement error that resulted in acceptable echoic flow performance were found to be lower than the expected values for the UAS in this study. Further forward-looking echoic flow research is recommended using a more accurate and robust rangefinder. A UAS capable of more precise horizontal plane movement is also recommended.No embargoAcademic Major: Electrical and Computer Engineerin

Directed flow in ultrarelativistic heavy-ion collisions

We study the generation of directed flow in the hydrodynamic expansion of the hot matter formed in ultrarelativistic heavy-ion collisions at 200GeV. The experimentally observed negative directed flow in a wide range of central pseudorapidities isreproduced assuming that the fireball is tilted away from the collision axis. The tilt of the source is consistent with a preferential emission in the forward/backward hemisphere from forward/backward participating nucleons. The model reproduces the experimentally observed scaling of the directed flow when going from Au-Au to Cu-Cu systems.Comment: figure adde

Measurements of a rotor flow in ground effect and visualization of the brown-out phenomenon

Quantitative and qualitative results of a series of experiments conducted on a rotor in ground effect at low forward speeds are presented. The velocity over a wide area of the ground effect wake was measured using Particle Image Velocimetry (PIV), and the evolution of the flow is described as the forward speed increases. Helicopter brown-out was simulated through a series of flow visualisation experiments. The technique involved sprinkling a fine powder on the ground below and ahead of the rotor. This helps to validate the experimental simulation of the brown-out phenomenon. Larger dust clouds were observed at lower advance ratio, and the dust cloud penetrated into the areas of the flow including those where vorticity levels were of low or negligible magnitude

A backward λ\lambda-Lemma for the forward heat flow

The inclination or $\lambda$-Lemma is a fundamental tool in finite dimensional hyperbolic dynamics. In contrast to finite dimension, we consider the forward semi-flow on the loop space of a closed Riemannian manifold $M$ provided by the heat flow. The main result is a backward $\lambda$-Lemma for the heat flow near a hyperbolic fixed point $x$. There are the following novelties. Firstly, infinite versus finite dimension. Secondly, semi-flow versus flow. Thirdly, suitable adaption provides a new proof in the finite dimensional case. Fourthly and a priori most surprisingly, our $\lambda$-Lemma moves the given disk transversal to the unstable manifold backward in time, although there is no backward flow. As a first application we propose a new method to calculate the Conley homotopy index of $x$.Comment: 31 pages, 6 figures. Comments most welcome. v2: Theorem 1.2 and Lemma 2.1 slightly improved, corrected typos. v3: minor modifications. To appear in {\it Math. Ann.

The effect of helicopter configuration on the fluid dynamics of brownout

Brown’s Vorticity Transport Model, coupled to an additional particle transport model, is used to simulate the development of the dust cloud that can form around a helicopter when operating in dusty or desert conditions. The flow field around a tandem rotor configuration is simulated during the final stages of landing. The time-averaged flow field around the helicopter is characterised by the existence of two stationary points immediately adjacent to the ground plane. Almost all entrainment of dust into the flow takes place forward of the rearmost stationary point; the dust initially remains in a thin, sheet-like layer above the ground. As the dust sheet approaches the forward stationary point, the layer thickens and forms a characteristic wedge-shaped ‘separation zone’. The amount of sand that is subsequently drawn up away from the ground then appears to be critically dependent on the strength and position relative to the separation zone of strong regions of recirculation. VTM simulations suggest that, for a tandem rotor helicopter at least, the sudden growth of the dust cloud that is responsible for the onset of brownout may be due to a change in mode within the flow field surrounding the aircraft. At higher advance ratios the flow is dominated by a strong ground vortex that is created by the rear rotor. The forward extent of the resultant dust cloud is limited though by the absence of any strong recirculation within the flow below the front rotor of the system. At lower forward speed the ground vortex of the rear rotor is replaced by a strong vortex that lies just below the leading edge of the front rotor. This vortex is responsible for drawing a significant amount of dust out of the surface layer of entrained particulates to form a dense wall of dust some distance upstream of the helicopter. A study of the effect of blade twist on the strength and shape of the dust cloud formed in the flow surrounding helicopters with tandem rotors suggests that systems with smaller blade twist but the same disc loading might produce denser dust clouds than those with high blade twist

Analysis of dynamic stall using unsteady boundary-layer theory

The unsteady turbulent boundary layer and potential flow about a pitching airfoil are analyzed using numerical methods to determine the effect of pitch rate on the delay in forward movement of the rear flow reversal point. An explicit finite difference scheme is used to integrate the unsteady boundary layer equations, which are coupled at each instant of time to a fully unsteady and nonlinear potential flow analysis. A substantial delay in forward movement of the reversal point is demonstrated with increasing pitch rate, and it is shown that the delay results partly from the alleviation of the gradients in the potential flow, and partly from the effects of unsteadiness in the boundary layer itself. The predicted delay in flow-reversal onset, and its variation with pitch rate, are shown to be in reasonable agreement with experimental data relating to the delay in dynamic stall. From the comparisons it can be concluded (a) that the effects of time-dependence are sufficient to explain the failure of the boundary layer to separate during the dynamic overshoot, and (b) that there may be some link between forward movement of the reversal point and dynamic stall

Lifting surface theory for a helicopter rotor in forward flight

A lifting surface theory has been developed for a helicopter rotor in forward flight for incompressible flow. The method utilized the concept of the linearized acceleration potential and make use of the vortex lattice procedures. Results in terms of lift coefficient slope for several forward flight conditions are given