Process Save Reduction by Macro Joint Approach: The Key to Real Time and Efficient Vehicle Simulation

Applying a non-linear model reduction method to the tire suspension system of road vehicles enables an automatic transfer of complex offline simulation vehicle models to a mathematical model, which fits the real time simulation requirements. The basic assumption, that high frequent inner suspension dynamics are not relevant to handling manoeuvres, converts the differential algebraic equation system (DAE) of suspensions with kinematical closed loops into pure elasto-kinematical linkage equations. The equations of motions can be represented as an ordinary differential equation system (ODE) and considerable simulation time reductions are obtained for the off-line simulation and real time simulation is enabled. This so-called macro joint approach is an alternative modelling method to the well-known look-up table representation of suspension kinematics but it keeps the parameterisation of the original suspension model and is suitable to parameterised real time MBS models. With a second step the dynamics, caused by compliance in the suspension bushings, are reduced to their quasi-static behaviour. The consideration of these quasi-elasticity has nearly no influence on the necessary simulation time. This contribution shows the theoretical background and demonstrates the advantage of the macro joint model reduction approach on a typical vehicle example.     

Spatial Dynamics of Multibody Tracked Vehicles Part II: Contact Forces and Simulation Results

In this part of the paper, three dimensional computational capabilities, that includes significant details, are developed for the nonlinear dynamic analysis of large scale spatial tracked vehicles. Three dimensional nonlinear contact force models that describe the interaction between the track links and the vehicle components such as the rollers, sprockets, and idlers as well as the interaction between the track links and the ground are developed and used to define the generalized contact forces associated with the vehicle generalized coordinates. Tangential friction and contact forces are developed in order to maintain the stability of the track motion and avoid the slippage of the track or its rotation as a rigid body. Body and surface coordinate systems are introduced in order to define the spatial contact conditions. The nonlinear equations of motion of the tracked vehicle are solved using the velocity transformation procedure developed in the first part of this paper. This procedure is used in order to obtain a minimum set of differential equations, and avoid the use of the iterative Newton-Raphson algorithm. A computer simulation of a tracked vehicle that consists of one hundred and six bodies and has one hundred and sixteen degrees of freedom is presented in order to demonstrate the use of the formulations presented in this study.     

Spatial Dynamics of Multibody Tracked Vehicles Part I: Spatial Equations of Motion

In this paper, the nonlinear dynamic equations of motion of the three dimensional multibody tracked vehicle systems are developed, taking into consideration the degrees of freedom of the track chains. To avoid the solution of a system of differential and algebraic equations, the recursive kinematic equations of the vehicle are expressed in terms of the independent joint coordinates. In order to take advantage of sparse matrix algorithms, the independent differential equations of the three dimensional tracked vehicles are obtained using the velocity transformation method. The Newton-Euler equations of the vehicle components are defined and used to obtain a sparse matrix structure for the system dynamic equations which are represented in terms of a set of redundant coordinates and the joint forces. The acceleration solution obtained by solving this system of equations is used to define the independent joint accelerations. The use of the recursive equations eliminates the need of using the iterative Newton-Raphson algorithm currently used in the augmented multibody formulations. The numerical difficulties that result from the use of such augmented formulations in the dynamic simulations of complex tracked vehicles are demonstrated. In this investigation, the tracked vehicle system is assumed to consist of three kinematically decoupled subsystems. The first subsystem consists of the chassis, the rollers, the sprockets, and the idlers, while the second and third subsystems consist of the tracks which are modeled as closed kinematic chains that consist of rigid links connected by revolute joints. The singular configurations of the closed kinematic chains of the tracks are also avoided by using a penalty function approach that defines the constraint forces at selected secondary joints of the tracks. The kinematic relationships of the rollers, idlers, and sprockets are expressed in terms of the coordinates of the chassis and the independent joint degrees of freedom, while the kinematic equations of the track links of a track chain are expressed in terms of the coordinates of a selected base link on the chain as well as the independent joint degrees of freedom. Singularities of the transformations of the base bodies are avoided by using Euler parameters. The nonlinear three dimensional contact forces that describe the interaction between the vehicle components as well as the results of the numerical simulations are presented in the second part of this paper.     

Dynamics of inorganic nutrient species in the Bohai seawaters

Within the frame of a Sino-German Joint Research Program, two cruises of “R/V Dong Fang Hong 2” were carried out in September–October 1998 and April–May 1999, respectively, to understand the dynamics of nutrients in the Bohai. Nutrient species (NO₃⁻, NO₂⁻, NH₄⁺, PO₄³⁻ and SiO₃²⁻) are determined colorimetrically on board for five anchor and 30 grid stations. In situ incubation experiments are performed to determine planktonic nutrient uptake and benthic exchange flux. Nutrient concentrations display short-term variability and seasonal change in the Bohai, with higher levels in shallow coastal waters than in the Central Bohai. The influence of riverine discharge on nutrient levels can be seen from salinity isopleths, nutrient distribution and species ratios. Near-bottom (nb) waters have similar nutrient concentrations as to the surface waters in the Central Bohai, whereas stratification takes place in the Bohai Strait and North Yellow Sea. In situ incubation experiments provide evidence that the uptake ratio (i.e. N, P) by phytoplankton is proportional to the ratios among nutrient species in ambient waters. Based on the data of this study and previously publications, a preliminary estimate of nutrient budgets via riverine input and atmospheric deposition is established. The results indicate that atmospheric deposition gains importance over rivers in delivering nutrients into the Bohai and sustain the new production, following recent decrease in riverine inflow caused by drought periods in North China and damming practices. A historical review of nutrient data indicates that concentrations of nitrogen increase and phosphorus and silica decrease in the Central Bohai over last 40 years. This potentially has an important influence on the health of ecosystem in Bohai (e.g. food web and community structure), though further study is needed to examine the scenario in more detail.     

CFD Analysis of Ship-to-Ship Hydrodynamic Interaction

A numerical study of ship-to-ship interaction forces is performed using a commercial CFD code, and the results are compared with experimental data and with the results of a panel method analysis. Two ship models have been used in the interaction forces analysis: a tug and a tanker, advancing parallel to each other with different lateral distances and two different values of the fluid depth. Computations are carried out with four different flow models: inviscid and viscous flow with the free surface modeled as a rigid wall and inviscid and viscous flow with the deformable free surface. A fair agreement was obtained with available experimental data and results obtained by panel method. The influence of viscosity in the computations is found to be comparatively weak, while the wavemaking effects may be important, at small magnitude of the horizontal clearance.     

The structure of dissipation in the western Irish Sea front

We report on an intensive campaign in the summer of 2006 to observe turbulent energy dissipation in the vicinity of a tidal mixing front which separates well mixed and seasonally stratified regimes in the western Irish Sea. The rate of turbulent dissipation ε was observed on a section across the front by a combination of vertical profiles with the FLY dissipation profiler and horizontal profiles by shear sensors mounted on an AUV (Autosub). Mean flow conditions and stratification were obtained from a bed mounted ADCP and a vertical chain of thermistors on a mooring. During an Autosub mission of 60 h, the vehicle, moving at a speed of ~ 1.2 m s^− 1, completed 10 useable frontal crossings between end points which were allowed to move with the mean flow. The results were combined with parallel measurements of the vertical profile of ε which were made using FLY for periods of up to 13 h at positions along the Autosub track. The two data sets, which show a satisfactory degree of consistency, were combined to elucidate the space–time variation of dissipation in the frontal zone. Using harmonic analysis, the spatial structure of dissipation was separated from the strong time dependent signal at the M₄ tidal frequency to yield a picture of the cross-frontal distribution of energy dissipation. A complementary picture of the frontal velocity field was obtained from a moored ADCP and estimates of the mean velocity derived from the thermal wind using the observed density distribution. which indicated the presence of a strong (0.2 m s^− 1) jet-like flow in the high gradient region of the front. Under neap tidal conditions, mean dissipation varied across the section by 3 orders of magnitude exceeding 10^− 2 W m^− 3 near the seabed in the mixed regime and decreasing to 10^− 5 W m^− 3. in the strongly stratified interior regime. The spatial pattern of dissipation is consistent in general form with the predictions of models of tidal mixing and does not reflect any strong influence by the frontal jet.     

Quasi-two-dimensional flow on the polar β-plane: Laboratory experiments

Geophysical turbulence is strongly affected by the variation of the Coriolis parameter with latitude. This variation results in the so-called β-effect, which forces energy from small-scales to be transferred preferentially into zonal motions. This effect results in the formation of narrow jet-like zonal flows that dominate the dynamics and act as transport barriers. Here, laboratory experiments are used to reproduce this effect in decaying turbulent flows. An electromagnetic cell is used to generate an initial field of vorticity in a rotating tank. Under conditions of quasi-geostrophic flow, the β-effect is produced by depth variation of the flow instead of variation of the Coriolis parameter. The effects of changing the container geometry and the overall fluid depth on the production of jets are investigated. The results suggest that this laboratory configuration can be used to model jet formation in the oceans and that increasing fluid depth is a practical way to decrease viscous effects.     

The importance of oceanographic fronts to marine birds and mammals of the southern oceans

During the last 30 years, at-sea studies of seabirds and marine mammals in the oceans south of the Subtropical Front have described an association with major frontal areas. More recently, the advancement in microtechnology has allowed the tracking of individuals and investigations into how these marine predators actually use the frontal zones. In this review, we examine 1) the relative importance to apex predators of the different frontal zones in terms of spatial distribution and carbon flux; 2) the processes that determine their preferential use; and 3) how the mesoscale dynamics of frontal structures drive at-sea foraging strategies of these predators. We review published results from southern waters and place them in a broader context with respect to what has been learned about the importance of fronts in oceans farther north.Some fronts constitute important boundaries for seabird communities in southern waters. At a mesoscale the maximum values of seabird diversity and abundance correspond to the location of the main fronts. At-sea surveys show a strong curvilinear correlation between seabird abundance and sea surface temperatures. High mean species richness and diversity for whales and seabirds are consistently associated with the southern water mass boundary of the Antarctic Circumpolar Current, the Subtropical Front and the Subantarctic Front; in the case of the Polar Front mean seabird densities are more variable. At small-scales, variation in seabird occurrence has been directly related to the processes at fronts in a limited number of cases. A significant positive relation was found between some plankton feeding species and frontal temperature gradient–phytoplankton variables.Telemetric studies have revealed that several apex predators (penguins, albatrosses, seals) perform long, directed foraging trips either to the Subtropical front or Polar Front, depending on locality. Seabirds with low flight costs, such as albatrosses, are able to reach fronts at long distances from colonies, showing variable foraging strategies as a function of the distances involved. Diving birds such as King penguins, that travel at a higher cost and lower speed, rely on the predictable spatial distribution of mesopelagic fish found close to the Polar Front. They may use the currents associated with eddies as oceanographic cues in the active search for frontal zones. Once in these areas they dive preferentially in and below the depth of the thermocline where catches per unit effort are high. Elephant seals concentrate foraging activity principally inside or at the boundary of cyclonic eddies. These mesoscale features appear to offer exceptional productivity favourable for foraging by various diving top predators.The connection between biophysical parameters at fronts and predators is likely to be made through biological enhancement. Top predators appear to forage at locations where prey are advected by physical processes and others where prey are produced locally. Long-term research on at-sea distributions and demographic parameters of top predators are essential to assess the consequences of potential shift in front distributions in relation to global warming. Such environmental changes would add to the impact of fish extraction by the industrial fisheries on the southern food webs.     

Development of railway station choice models to improve the representation of station catchments in rail demand models

ABSTRACT

This paper describes the development of railway station choice models suitable for defining probabilistic station catchments. These catchments can then be incorporated into the aggregate demand models typically used to forecast demand for new rail stations. Revealed preference passenger survey data obtained from the Welsh and Scottish Governments was used for model calibration. Techniques were developed to identify trip origins and destinations from incomplete address information and to automatically validate reported trips. A bespoke trip planner was used to derive mode-specific station access variables and train leg measures. The results from a number of multinomial logit and random parameter (mixed) logit models are presented and their predictive performance assessed. The models were found to have substantially superior predictive accuracy compared to the base model (which assumes the nearest station has a probability of one), indicating that their incorporation into passenger demand forecasting methods has the potential to significantly improve model predictive performance.