Descent Speed Research Articles

Passive flight in density-stratified fluids

Leaves falling in air and marine larvae settling in water are examples of unsteady descents due to complex interactions between gravitational and aerodynamic forces. Understanding passive flight is relevant to many branches of engineering and science, ranging from estimating the behaviour of re-entry space vehicles to analysing the biomechanics of seed dispersion. The motion of regularly shaped objects falling freely in homogenous fluids is relatively well understood. However, less is known about how density stratification of the fluid medium affects passive flight. In this paper, we experimentally investigate the descent of heavy discs in stably stratified fluids for Froude numbers of order 1 and Reynolds numbers of order 1000. We specifically consider fluttering descents, where the disc oscillates as it falls. In comparison with pure water and homogeneous saltwater fluid, we find that density stratification significantly enhances the radial dispersion of the disc, while simultaneously decreasing the vertical descent speed, fluttering amplitude and inclination angle of the disc during descent. We explain the physical mechanisms underlying these observations in the context of a quasi-steady force and torque model. These findings could have significant impact on the design of unpowered vehicles and on the understanding of geological and biological transport where density and temperature variations may occur.

Drop attacks of the elderly

Falls are frequent in the elderly, often leading to significant injury, transfer to residential care or death. Well-recognised risk factors include medications (particularly polypharmacy and the use of psychotropic agents),...

Scale modeling experiments of fire-induced smoke and extraction via mechanical ventilation in an underground hydropower plant

This study explores experimental research that investigates the characteristics of fire-induced smoke and mechanical smoke extraction systems in the main powerhouse of the Ludila underground hydropower plant. Experiments adopting the similarity theory were conducted in a 1/12 scale model, which in turn was verified through empirical formulas. Experimental results reveal that the cooling effect was so striking that the maximum temperature in the occupied zone was 27.35 °C, and the egress time was over 18 min under conditions of fire intensity of 3 kW. Moreover, the influence of fire locations on CO concentration and smoke layer height are discussed in detail. Furthermore, the effects on temperature variations, the smoke ventilation system layout and exhaust rates caused by mechanical smoke extraction are analyzed. A turning point of smoke layer descent speed was discovered at 0.6 dimensionless height, and this study also recommends the optimal layout of smoke exhaust ventilators that should be used to reduce underground excavation loads. This will help to retrench the initial investment and thereby render a guideline in the design of sustainable underground engineering.

Dynamic guidance of orbiter gliders: alignment, final approach, and landing

A new algorithm capable of guiding an orbiter glider to a target point with a prescribed alignment and descent path angle is presented. This algorithm can initiate Terminal Area Energy Management (TAEM) before reaching steady state and perform the Final Approach and Landing (FA&L). During TAEM, runway alignment is done through a moving virtual target derived from steady state, while during FA&L, a transient (or flare) is used to reach the extremely shallow descent path angles. All decisions are made dynamically relying solely on local information (position, speed, attitude, and atmospheric parameters), and all structural limits of the glider are respected at all times. As a proof of concept, a Space Shuttle return flight is simulated. For a large multitude of initial conditions and targets, the algorithm is able to consistently deliver distance errors below 19 m (transverse errors below 4 m), alignment errors below $$1^{\circ }$$ , descent path angles at the intended $$-\,2^{\circ }$$ , and vertical descent speeds below 8.5 m/s with control time intervals of 0.1 s.

Morphological and functional seed traits of the wild medicinal plant Dioscorea strydomiana, the most threatened yam in the world.

Morphological and functional seed traits have important roles in characterising the species regeneration niche and help to understand the reproductive biology of rare and threatened plants, which can thus support appropriate plant conservation measures. Seed morphometric and dispersal kinetics of the critically endangered Dioscorea strydomiana were measured and compared with those of four other Dioscorea species, and seed germination response under constant temperatures (5-35°C) was compared with that of the congeneric and widespread D.sylvatica. Seed mass of D.strydomiana (ca. 14mg) was twice that of D.sylvatica, but similar to or smaller than the other species examined. Seeds of D.strydomiana have the lowest speed of descent and lowest variability in most of the morphological traits considered, suggesting lower phenotypic plasticity but higher variance in the wing-loading value. Seeds of D.strydomiana reached maximum germination at 15°C (ca. 47%), which decreased slightly to ca. 37% at 25°C and was completely inhibited at 35°C. D.sylvatica seeds started to germinate at 10°C (ca. 3%), reached 75-80% germination at 15-20°C and maximum (ca. 90%) at 25-30°C. Base temperatures for germination (Tb ) were 9.3 and 5.7°C, for D.strydomiana and D.sylvatica, respectively. Due to the higher germination percentages of D.sylvatica, ceiling and optimum temperatures could also be modelled for this species, suggesting higher sensitivity to high temperature for seeds of D.strydomiana. The detected poor seed lot quality of D.strydomiana suggests difficulties in reproduction from seed, highlighting the need for further investigation and conservation actions for this threatened yam species.

Effect of Stair Tread Markings on Foot Placement During Stair Descent

Falls during stair descent are common and can result in serious injury or even death. Many falls during stair descent are attributed to interactions with imperfections or defects in the stair construction, particularly, defects in the stair treads or nosing. While prior research has examined the placement of the foot related to the stair nosing during normal ambulation, limited research has examined the force distribution underneath the foot as it makes contact with the stair tread throughout the duration of the stride. An understanding of the location of the foot relative to the nosing and the force distribution underneath the foot is essential to evaluate the effect of stair imperfections during stair descent. As part of a larger laboratory study, the aim of this research was to determine the effect of markings placed on stair treads that represent pressure sensors, on descent speed, and on foot placement relative to the stair nosing. Ten healthy individuals descended a wooden staircase with and without markings placed on two stair treads while motion data of the feet were collected. Results showed that the presence of markings on the stair treads did not alter descent speed or foot placement relative to the tread nosing. Findings suggest that the presence of pressure sensors and other similar markings on stair treads do not alter descent speed or foot placement relative to the stair nosing.

A Numerical Study of the Flow Around a Model Winged Seed in Auto-Rotation

In this study the flow around a winged-seed in auto-rotation is characterized using direct numerical simulations (DNS) at Reynolds number in the range 80–240, based on the descent speed and a characteristic chord length. In this range, the flow is approximately steady when observed from a reference frame fixed to the seed. For all cases, the flow structure consists of a wing tip vortex which describes a helical path, a vortex shed behind the nut of the seed and a stable leading edge vortex above the wing surface which merges with the tip vortex. With increasing Reynolds number, the leading edge vortex becomes more intense and gets closer to the wing surface. The simulation results also show the formation of a spanwise flow on the upper surface of the wing, moving fluid towards the wing tip in a region downstream and beneath the leading edge vortex. This spanwise flow is rather weak inside the core of the leading edge vortex, and the analysis of the streamlines show a very weak transport of vorticity along the vortex for the cases under consideration. The analysis of the flow suggests that the stabilization of the leading edge vortex is mainly due to non-inertial accelerations, although viscous effects may contribute, specially at lower Re. Furthermore, the leading edge vortex has been characterized by analysing the flow variables averaged along cross-sections of the vortex. While some quantities, like the spanwise velocity or the pressure inside the vortex, are rather insensitive to the threshold used to define the leading edge vortex, the same is not true for the circulation of the vortex or its averaged spanwise vorticity, due to the viscous nature of the vortex. Finally, it is observed that the spanwise vorticity scales with the angular rotation of the seed for the different Re.

Vertical flight path segments sets for aircraft flight plan prediction and optimisation

ABSTRACTThe paper presents a method for constructing a set of vertical flight path segments, that would compose an aircraft's vertical flight envelope, by using an aircraft performance model. This method is intended to be used for aircraft flight plan prediction and optimisation algorithms. The goal is to reduce the volume of recurring segment performance computations currently required for flight plan prediction or optimisation. The method presented in this paper applies to a free-flight scenario. The flight-path segments composing the vertical flight envelope belong to one of the unrestricted climb, constant-speed level flight, step-climb and continuous descent segments, performed at the consigned climb, cruise and descent speed schedules and at the consigned air temperature values. The method employs an aircraft model using linear interpolation tables. Nine test scenarios were utilised to assess the performances of the resulting flight envelopes as a function of the number of cruise altitudes and descent flight paths. The set of evaluated performance parameters includes the range of total flight times and still-air flight distances, and the vertical profiles describing the minimum and maximum flight times, and still-air flight distances. The advantages of the proposed method are multiple. First, it eliminates the need for repetitive aircraft performance computations of identical vertical flight plan segments, and provides the means for quick retrieval of the corresponding performance data for use in the construction of a full flight plan. Second, the vertical flight path look-up structure and the vertical flight-path graph describe a set of vertical flight paths that consider an aircraft's and flight plan's configuration parameters and cover its maximum flight envelope. Third, the look-up structure and the graph provide the means for rapid and clear identification of the available options for constructing a flight-plan segment, as well as for detecting the points associated with changes in the flight phases, including climb, cruise, step-climb and descent.

Newtonian Analysis of a Folded Chain Drop

Consider a chain of length L that hangs in a U shape with end A fixed to a rigid support and free end E released from rest starting from the same initial height (call it y = 0) as A. Figure 1 sketches the chain after end E has fallen a distance y. Points O and A are assumed to be close enough to each other and the chain flexible enough that the radius of curvature r at the bottom point C can be taken to be negligibly small (compared to the length of the chain). The problem is to compare the speed of descent v(y) = dy/dt of the free end E of the chain to the speed vfree(y)=2gy of a free-falling point mass that has descended the same distance y. If v(y) > vfree (y) for all y > 0, then, in a race to fall any arbitrary distance Y (where 0 < Y < L), the chain end E will always beat a simultaneously released point mass, because the fall time t for E will be shorter than tfree for the point mass, t=∫0Ydyv(y)<tfree=∫0Ydyvfree(y).(1)Experimentally, this outcome of the race is observed in a “Veritasium” video.

Kinematics and dynamics of the auto-rotation of a model winged seed

Numerical simulations of the auto-rotation of a model winged seed are presented. The calculations are performed by solving simultaneously the Navier–Stokes equations for the flow surrounding the seed and the rigid-body equations for the motion of the seed. The Reynolds number based on the descent speed and a characteristic chord length is varied in the range 80–240. Within this range, the seed attains an asymptotic state with finite amplitude auto-rotation, while for smaller values of the Reynolds number no auto-rotation is observed. The motion of the seed is characterized by the coning and pitch angles, the angular velocity and the horizontal translation of the seed. The values obtained for these quantities are qualitatively similar to those reported in the literature in experiments with real winged seeds. When increasing the Reynolds number, the seed tends to rotate at higher speeds, with less inclination with respect to the horizontal plane, and with a larger translation velocity. With respect to the aerodynamic forces, it is observed that, with increasing Reynolds number, the horizontal components decrease in magnitude while the vertical component increases. The force distribution along the wing span is characterized using both global and local characteristic speeds and chord lengths for the non-dimensionalisation of the force coefficients. It is found that the vertical component does not depend on the Reynolds number when using local scaling, while the chordwise component of the force does.

Experimental Study on Descent Speed on Stairs of Individuals and Small Groups Under Different Visibility Conditions

Staircases play an essential role in multi-story buildings during emergencies, such as fire. Fires and the resulting smoke cause low visibility, which could affect the movement speed of the occupants. This must be taken into consideration during evacuation experimental design. This paper aims to study the stairwell evacuation performance of individuals and small groups under normal and impaired visibility conditions. The evacuation experiment was conducted in a five-story office building, with 75 subjects. The descent speed, the evacuation behavior, and the characteristic of small groups were analyzed and discussed. The descent speed of individuals decreased sharply when the visibility was reduced. The descent speed of small groups showed a relatively small downward trend. Group behavior had a negative effect on the evacuation performance during normal visibility. The group speed converged to the speed of the slowest group member. The group behavior contributed to a quicker movement speed in the low visibility condition. This was due to the stress of occupants being relieved. The group speed tended to be as fast as the fastest member of the group.

Atlantic sturgeon Acipenser oxyrinchus surfacing behaviour.

Atlantic sturgeon Acipenser oxyrinchus surfacing behaviour was investigated in Minas Basin (45° 20' N; 64° 00' W) and the Bay of Fundy with pop-up satellite archival tags (MiniPAT) measuring physical variables (pressure, temperature, light). Of six tags deployed during June and July, five provided pop-up locations and two were recovered after c. 4 months. Analysis of recovered archival data revealed that the frequency of surfacing events was highest (78·9%) when A. oxyrinchus were in Minas Basin at depths <10 m. Surfacing frequency decreased substantially when fish migrated into greater depths of the Bay of Fundy (>40 m). The tidal cycle in Minas Basin had a significant relationship to surfacing frequency, with the most surfacing events (49·5%) occurring on the flood tide, from mid- to high-tide. Surfacing events ranged from 0-12 a day and the maximum number occurred between 2300 and 0300 hours. Maximum surfacing ascent speeds ranged from 0·50 to 4·17 ms-1 and maximum descent speeds ranged from 0·17 to 3·17 ms-1 . Buoyancy control, by gulping air to inflate the gas bladder, is proposed as the main reason for surfacing behaviour in A. oxyrinchus.

Aircraft Vertical Route Optimization by Beam Search and Initial Search Space Reduction

This paper describes an optimization algorithm that provides an economical vertical navigation profile by finding the combinations of climb, cruise, and descent speeds, as well as altitudes, for an aircraft to minimize flight costs. The computational algorithm takes advantage of a space search reduction methodology to reduce the initial number of available speed and altitude combinations. The optimal solution was found by implementing the beam search algorithm. A bounding function that correctly estimates the flight cost by considering step climbs was developed to reduce the number of calculations required by the beam search algorithm. The full-flight fuel burn cost was obtained using a performance database-based method. The algorithm uses a numerical performance model instead of equations of motion to compute fuel burn. The database was developed by using flight experimental data. To validate the algorithm, its results were compared to those of three other algorithms: an exhaustive search, beam search, and search space reduction. The solution provided by the algorithm was also compared to the solution provided by a flight management system. Following this comparison, the algorithm systematically found the optimal solutions, which were better in terms of flight cost than those provided by the flight management system.

Diving activity of migrating silver eel with and withoutAnguillicola crassusinfection

Infection with the swim bladder nematode Anguillicola crassus has been hypothesised to threaten the spawning migration success of the endangered European eel (Anguilla anguilla). To examine this assumption, we compared the swimming behaviour of one Anguillicola crassus infested eel in the North Sea and three parasite-free eels in the Baltic using data recovered from data storage tags attached to migrating silver eels. In both areas, eel activity was characterized by frequent diving behaviour throughout the water column during the night, with reduced activity during the day. Despite substantial damage of the swim bladder, the behaviour of the infested eel from the North Sea was within the same range of migrating and diving activity parameters as the three parasite-free eels from the Baltic Sea. All eels had a similar frequency distribution of descent or ascent speeds and a similar average horizontal migration speed. The diving speeds and dive ranges exclude the possibility that the eels were in continuous hydrostatic equilibrium during their migrations and suggests therefore that the role of the swim bladder in vertical migration is likely to be more complex than currently thought. Our results suggest that eels infested by Anguillicola crassus are capable of diving in a similar manner to uninfested eels during the first stretch of their spawning migration.

Development of an eddy current disk brake capable of controlling descent speed

Development of an eddy current disk brake capable of controlling descent speed

Ultrafast eclogite formation via melting-induced overpressure

The conventional wisdom holds that metamorphic reactions take place at pressures near-lithostatic so that the thermodynamic pressure, reflected by the mineral assemblage, is directly correlated with depth. On the other hand, recent field-based observations and geodynamic simulations suggest that heterogeneous stress and significant pressure deviations above lithostatic (overpressure) can occur in Earth's crust. Here we show that eclogite, normally interpreted to form at great depths in subduction zones and Earth's mantle, may form at much shallower depths via local overpressure generated in crustal shear zones. The eclogites studied crop out as lenses hosted by felsic paragneiss in a sheared thrust slice and represent a local pressure and temperature anomaly in the Taconic orogenic belt, southern New England. Sharply-defined chemical zones in garnet, which record ∼5 kbar pressure rise and fall accompanied by a temperature increase of 150–200 °C, demonstrate extremely short timescales of diffusion. This requires anomalously fast compression (∼500 yrs) and decompression. We use coupled phase equilibria and garnet diffusion forward modeling to fit the observed garnet profiles and test the likely P–T–t paths using a Monte Carlo-type approach, accounting for off-center sectioning of garnet. The simulation shows that a ∼5 kbar pressure increase after the temperature peak is necessary to reproduce the garnet zoning. Remarkably, this post-peak-T compression (from 9 kbar to 14 kbar) lasted only ∼500 yrs. If the compression was due to burial along a lithostatic pressure gradient, the descent speed would exceed 30 m yr−1, defying any observed or modeled subduction rates. Local overpressure in response to partial melting in a confined volume (Vrijmoed et al., 2009) caused by transient shear heating can explain the ultra-fast compression without necessitating burial to great depth.

Aerodynamics and right-left symmetry in wind dispersal of maple, dipterocarps, conifers and some genera of apocyanaceae and magnoliaceae

The wind dispersing seeds have evolved remarkable aerodynamic optimisation to minimise the speed of descent on detachment from the tree, so that they could be carried away by the wind and deposited a distance away from the parent. An efficient physical mechanism that enables slowing down the descent is rotation, which reduces the translational kinetic energy. Rotation and translation always define a right or left-handedness. The present work examines the aerodynamics of the dispersal of seeds of many species in relation to right-left asymmetry, giving examples to show that in nature this symmetry is broken either spontaneously or intrinsically. In the former situation, seeds have no geometrical right-left asymmetry, but an initial instability chooses one sense of rotation (right or left). Whereas in the latter, geometrical asymmetry dictates the sense of rotation. Seeds of the maple familiy belongs to the first category and a theortical model is presented to explain the motion. Seeds of dipterocarps, conifers and some genera of apocyanaceae and magnoliaceae are found to possess a handedness, which determines the rotation during seed fall. In dipterocarps, conifers and magnoliaceae, both right-handed and left-handed seeds are produced in the same tree but correlated to the handedness of the sprial phyllotaxy of the branch that bears the fruit. Apocyanaceae is found to be unique and seeds of all genera have the same handedness. The details of extensive field observations, experiments and theoretical interpretations are presented to illustrate the relationship of right-left asymmetry to the aerodynamics of seed wind dispersal, pointing out the implications of evolutinary optimisation in practical aerodynamics.

The Effect of Electromagnetic Stirring on the Speed of the Solidification Front

This study follows and develops the work we have already done on electromagnetic stirring. This time, we highlight the effect of this stirring during solidification on the structure of a copper-tin alloy. We have used an experimental device to make an appropriate simulation of the continuous casting. The cooling of our crucible is carried out laterally by a stainless steel crown and at the bottom by a nozzle. In our experiments we have varied the parameters of which the most important are: the power of the stirring and the flow of cooling gas. We measured the cooling gases flow rate and its temperature, which allowed us to calculate the heat flow drawn off by this gas. Other measurements made it possible to determine the solidification front average speed and the values obtained made it possible to calculate theoretically this front position. In order to characterize the structure of the solidified ingots, an image analysis was carried out. The various experiments carried out have highlighted the importance of the ratio of the speed of descent of the crucible to the speed of the solidification front. Indeed, the choice of this ratio is very important for the optimization of the solidification process.

Squatting Biomechanics in Individuals with Symptomatic Femoroacetabular Impingement.

Identification of the biomechanical alterations in individuals with symptomatic femoroacetabular impingement (FAI) is critical to understand the pathology and inform treatments. Yet hip function in this condition is poorly defined. Squatting requires large hip flexion range and involves motion toward the position of hip impingement; thus, it is likely to expose modified biomechanics in these individuals. This study aimed to determine whether hip and pelvis biomechanics differ between individuals with and without symptomatic FAI during an unconstrained deep squat and a constrained squat designed to limit compensation by the pelvis and trunk. Fifteen participants with symptomatic cam-type or combined (cam plus pincer) FAI who were scheduled for arthroscopic surgery and 14 age- and sex-matched controls without FAI underwent three-dimensional motion analysis during the two squatting tasks. Trunk, pelvis, and hip kinematics and hip kinetics were compared between groups. There were no between-group differences in normalized squat depth for either task. Descent speed was slower for the FAI group during the unconstrained squat (P < 0.05). During the constrained squat, individuals with FAI demonstrated greater pelvic rise (P = 0.01) and hip adduction (P = 0.04) on the symptomatic side than controls. The hip external rotation moment was less in FAI patients during descent (P = 0.04), as was transverse plane hip angle variability (P = 0.04). Individuals with symptomatic FAI can squat to a depth comparable with controls, regardless of task design. When the task is constrained, FAI patients demonstrate greater ipsilateral pelvic rise and maintain a more adducted hip position, which may coincide with a compensatory strategy to avoid end range flexion as the hip approaches impingement. These biomechanical alterations may put additional stress on adjacent regions and have relevance for rehabilitation.

Exploring the Relationship between the Arid Valley Boundary’s Displacement and Climate Change during 1999–2013 in the Upper Reaches of the Min River, China

The arid valley is a unique type of ecological fragile landscape in the Hengduan Mountain Area, China. The boundary of the arid valley is one of the response indicators to mountainous climate change. Based on the meteorological data from 1999 to 2013 and the SPOT remote sensing images in 1999 and 2013 this study explored the response characteristics of the arid valley boundary to regional climate change in the upper reaches of the Min River in the Hengduan Mountains. The results are as follows: (1) During 1999–2013, the temperature, precipitation, and evaporation increased, and the sunshine duration and relative humidity showed decreasing trends at the rates of 0.008 °C/a, 2.25 mm/a, 5.51 mm/a, −8.72 h/a, and −0.19%/a, respectively. Meanwhile, the climate showed the warm-dry tendency in the southern region and the warm-humid tendency in the central and northern areas. (2) On the whole, the arid valley boundary mainly distributed between 1601–3200 m and moved downward to 2428 m at the speed of −0.76 ± 0.26 m/a along with global warming. The descent speeds in different regions showed the same decreasing order as the regional distributions of precipitation and sunshine duration. (3) The arid valley boundary’s displacement in the whole basin had significant negative correlations with current climate change (p &lt; 0.05), as well as with variations of moisture factors. Additionally, with the enhancements of the drought degree and humidity tendency, the variations of temperature, evaporation, and relative humidity, respectively, became the main factors that had significant correlations with the arid valley boundary’s displacement. Therefore, climate change during 1999–2013 shows beneficial effects on the improvement of the arid valley habitat in the upper reaches of the Min River. The study provides a new method and gives basic data for research on climate change.