Average Tip Velocity Research Articles

Distinctions of dendritic behavior influenced by constant pressure and periodic pressure

The distinctions of dendritic morphology and sidebranching behavior when solidified under atmosphere pressure, constant pressure which is higher than atmosphere pressure (hereinafter referred to as constant pressure) and periodic pressure were investigated using 3-D phase field method. When growing at atmosphere pressure, side branches (secondary dendritic arms) are irregular. When solidified under constant pressure with a relatively high value, side branches are much more luxuriant, with more developed high-order side branches. When applied with periodic pressure, resonant sidebranching happens, leading to many more regular side branches and the smallest secondary dendritic arm spacing (SDAS) in the three cases. The significant difference in dendritic morphology is associated with tip velocity modulated by total undercooling including pressure and temperature undercooling. In the case of constant pressure, tip velocity increases linearly with total undercooling, and it varies periodically in periodic pressure case. The different variation trend in tip velocity is the reason for the distinct dendrite growth behavior in different cases. Unlike the phenomenon in constant pressure case where the dendrite grows faster with higher pressure, the dendrite grows slower under periodic pressure with higher amplitude, resulting in less developed primary dendrite and side branches. This is influenced by tip remelting due to low undercooling or even negative undercooling. It is revealed that the accelerated velocity of tip remelting increases with the decline of undercooling. The greater the amplitude of periodic pressure, the faster the tip remelting velocity during one period. This is the reason why the average tip velocity decreases with the rise of amplitude of periodic pressure.

Experimental trajectory optimization of a flapping fin propulsor using an evolutionary strategy

The experimental optimization of bio-inspired flapping fin trajectories are demonstrated for potential applications as a side or a rear propulsor of an autonomous underwater vehicle. The trajectories are scored based upon their difference from a force set-point and upon their efficiency and are parameterized by 10 variables inspired by fish swimming. The flapping fin is a generic rectangular rigid flat plate with a tapered edge. Optimization occurs as follows. First, a generation of trajectories is created. Second, the trajectories are executed by a spherical parallel manipulator, during which the forces are acquired. Third, the trajectories are scored and a new generation of trajectories is created using the covariance matrix adaptive evolutionary strategy. This loop repeats ad-infinitum until the search converges. Within the first set of searches, two trajectories for optimal side-force generation are found, one is fully three-dimensional while the other is artificially constrained to a line, and one trajectory for optimal thrust generation is found. All searches demonstrate good convergence properties and match the desired force set-point almost immediately. Additional generations primarily improve the efficiency of the maneuver. The two trajectories for generating side-force have a similar efficiency, which shows potential in utilizing a simple trajectory limited to a line. Comparison between the trajectories for generating side-force and thrust suggests that side-force generation is more efficient around Re ~1000, based on the average tip velocity and length of the fin. The second set of searches explores the behavior of the optimal trajectories for generating side-force at a lower force set-point and the third set of searches explores the sensitivity and repeatability of the optimization.

An Experimental Study on the Macroscopic Spray Characteristics of Biodiesel and Diesel in a Constant Volume Chamber

The objective of this study was to investigate the macroscopic spray characteristics of different 0%–100% blends of biodiesel derived from drainage oil and diesel (BD0, BD20, BD50, BD80, BD100), such as spray tip penetration, average tip velocity at penetration, spray angle, average spray angle, spray evolution process, spray area and spray volume under different injection pressures (60, 70, 80, 90, 100 MPa) and ambient pressures (0.1, 0.3, 0.5, 0.7, 0.9 MPa) using a common rail system equipped with a constant volume chamber. The characteristic data was extracted from spray images grabbed by a high speed visualization system. The results showed that the ambient pressure and injection pressure had significant effects on the spray characteristics. As the ambient pressure increased, the spray angle increased, while the spray tip penetration and the peak of average tip velocity decreased. As the injection pressure increased, the spray tip penetration, spray angle, spray area and spray volume increased. The increasing blend ratio of biodiesel brought about a shorter spray tip penetration and a smaller spray angle compared with those of diesel. This is due to the comparatively higher viscosity and surface tension of biodiesel, which enhanced the friction effect between fuel and the injector nozzle surface and inhibited the breakup of the liquid jet.

Numerical Simulation of Linear Explosive Formed Penetrator by Endpoint Initiating Manner

The formation of linear explosion formed penetrator(LEFP) from linear shaped charge detonated from one end point was studied both theoretically and experimentally, and a numerical simulation was also carried out. By decomposing the detonation velocity vector into spherical and slide components, the theoretical model described the tip velocity change of LEFP. The numerical model illustrated the formation of LEFP and the tip velocity agreed with the theoretical model. The validation of the theoretical and numerical models was verified by experiment, in which steel targets were used to show the effect of LEFP. The results showed that average tip velocity was 2658.4 m/s at 100 mm to the charge mouth. The theoretical and numerical velocity values and distribution curve were very close to the experimental ones. This work may further develop the theory of LEFP and linear shaped charge.

A correlation image velocimetry-based study of high-pressure fuel spray tip evolution

A correlation image velocimetry (CIV) technique has been developed to study the evolution of the leading edge, or tip, of isothermal high-pressure fuel sprays. Adaptations of the analysis permit determination of both the average spray tip motion and the spatial distribution of velocity along the spray edge. From these measurements, three distinct regions of the tip’s evolution have been observed and scaling relations developed. Further investigation has revealed significant uniformity in the radial evolution of the spray tip, despite the apparent similarity to turbulent jet flow. Examination of pdfs of the average tip velocity reveals among the many repeatable injection events a significant amount of variability and that this variability extends to regions near the nozzle, implying that among the sources of shot-to-shot viability is the atomisation process itself.

Kinematic theory for scale-invariant patterns in acicular martensitas

We present a kinematic theory which explains the emergence of scale-invariant patterns in acicular martensites which occur, for example, in FeC and FeNi alloys. Scale-invariant structures emerge naturally as a consequence of competition between the average tip-velocity, ν, and the rate of nucleation, I, of the martensite grains. Martensite growth is analyzed in terms of fixed points of a well-defined renormalization group. It is shown that the stationary probability distribution of the martensite grain size P( l), is governed by two fixed points - an unstable (noncritical) fixed point at ν/ I = 0, characterized by a Gamma distribution and a stable (critical) fixed point at ν/ I = ∞, characterized by a Lévy distribution, P( l) ∼ l − α . A universal crossover function describes the SPD at intermediate values of ν/ I. The analysis may also be used to compute the tip-velocity from optical micrograph pictures of martensite grains.

Transition of viscous fingering patterns in polymer solutions

Viscous fingering patterns of aqueous hydroxypropyl methyl cellulose (HPMC) solutions pushed by air in the Hele–Shaw cell were observed as a function of isopropyl alcohol content under a constant pressure of 15 cm H2O. A morphological transition from side branching patterns to tip splitting ones with increasing isopropyl alcohol content, accompanied with a decrease in surface tension and an increase in viscosity is found. The observed morphology transition was correlated with the dimension of the fingering pattern, as well as the average tip velocity in the fingering.

The velocity of streamer tips in impulse point-to-plane corona in air, using Lichtenberg figure techniques

Positive and negative streamer lengths were measured in point-to-plane impulse corona in air at atmospheric pressure using Lichtenberg figure technique. Applying short time pulses from 9 to 40 nsec duration time-distance plots were obtained, which allowed one to determine the streamer tip velocity. For a 4 cm gap, a 1 mm diameter point, and a 30 kV pulse an average tip velocity of 1.8×108 cm/sec was found. Comparing these data withHudson's photomultiplier measurement, we may identify the Lichtenberg figure with the “primary” streamer ofHudson. The negative streamer tip velocity in the cathode part of the gap was approximately an order of magnitude smaller than the positive streamer velocity.