Unsteady Aerodynamic Performance of a Maglev Train: The Effect of the Ground Condition

The effect of the ground condition on unsteady aerodynamic performance of maglev train was numerically investigated with an IDDES (Improved Delayed Detached Eddy Simulation) method. The accuracy of the numerical method has been validated by wind tunnel experiments. The flow structure, slipstream and aerodynamic force around the train under stationary and moving ground conditions were compared. Compared with the stationary ground condition, the vortex structure under the condition of moving ground generated by the wake region is narrower and higher because of the track. Near the nose point of the head and tail vehicles, the peak value of slipstream under the condition of moving ground is slightly higher than that under stationary ground. In the wake area, the effect of the main vortex structure on both sides of the tail vehicle and the track makes the vortex structure in the wake area stronger than that under moving ground, the slipstream peak is larger and the locus thereof is further forward. Under the two ground conditions, the vortex structure is periodically shed from both sides of the train into the wake area, and the shedding frequency of the main vortex under the moving ground condition is lower than that under the stationary ground condition. Moving ground can increase the resistance of the maglev train, reduce the lift of the maglev train, and decrease the standard deviation of the maglev train’s aerodynamic force.

Application of a Projection Method for Simulating Flow of a Shear-Thinning Fluid

In this paper, a first-order projection method is used to solve the Navier–Stokes equations numerically for a time-dependent incompressible fluid inside a three-dimensional (3-D) lid-driven cavity. The flow structure in a cavity of aspect ratio δ = 1 and Reynolds numbers ( 100 , 400 , 1000 ) is compared with existing results to validate the code. We then apply the developed code to flow of a generalised Newtonian fluid with the well-known Ostwald–de Waele power-law model. Results show that, by decreasing n (further deviation from Newtonian behaviour) from 1 to 0.9, the peak values of the velocity decrease while the centre of the main vortex moves towards the upper right corner of the cavity. However, for n = 0.5 , the behaviour is reversed and the main vortex shifts back towards the centre of the cavity. We moreover demonstrate that, for the deeper cavities, δ = 2 , 4 , as the shear-thinning parameter n decreased the top-main vortex expands towards the bottom surface, and correspondingly the secondary flow becomes less pronounced in the plane perpendicular to the cavity lid.

Numerical analysis of the effect of the change in the ride height on the aerodynamic front wing–wheel interactions of a racing car

An investigation into the influence of the ground clearance on the aerodynamic interactions between the inverted front wing and the wheel of a racing car was conducted using computational fluid dynamics. Height-to-chord ratios h/ c from 0.075 to 0.27 were assessed for a single-element wing with a fixed angle of 4° and for two wing spans, one of which completely overlapped the wheel and the other which had its endplate coincident with the inside face of the wheel. With a narrower span, a lower peak downforce was achieved at a higher ground clearance owing to changes in the lower endplate vortex strength whereas, with a wider span, no downforce loss was observed, with decreasing clearance for those tested. This contrasted distinctly with the performance of the wing in isolation. The wheel lift was scarcely affected with decreasing wing ground clearance for the narrower span but decreased significantly for the wide-span wing at low ground clearances. The vortex paths changed considerably with the ground clearance, with a strong coupling to the wing span; a state in which the main vortex was destroyed in the contact patch of the wheel was identified.

Dynamics of a vortex ring moving perpendicularly to the axis of a rotating fluid

The dynamics of a vortex ring moving orthogonally to the rotation vector of a uniformly rotating fluid is analysed by laboratory experiments and numerical simulations. In the rotating system the vortex ring describes a curved trajectory, turning in the opposite sense to the system's anti-clockwise rotation. This behaviour has been explained by using the analogy with the motion of a sphere in a rotating fluid for which Proudman (1916) computed the forces acting on the body surface. Measurements have revealed that the angular velocity of the vortex ring in its curved trajectory is opposite to the background rotation rate, so that the vortex has a fixed orientation in an inertial frame of reference and that the curvature increases proportionally to the rotation rate.The dynamics of the vorticity of the vortex ring is affected by the background rotation in such a way that the part of the vortex core in clockwise rotation shrinks while the anti-clockwise-rotating core part widens. By this opposite forcing on either side of the vortex core Kelvin waves are excited, travelling along the toroidal axis of the vortex ring, with a net mass flow which is responsible for the accumulation of passive scalars on the anti-clockwise-rotating core part. In addition, the curved motion of the vortex ring modifies its self-induced strain field, resulting in stripping of vorticity filaments at the front of the vortex ring from the anti-clockwise-rotating core part and at the rear from the core part in clockwise rotation. Vortex lines, being deflected by the main vortex ring due to induction of relative vorticity, are stretched by the local straining field and form a horizontally extending vortex pair behind the vortex ring. This vortex pair propagates by its self-induced motion towards the clockwise-rotating side of the vortex ring and thus contributes to the deformation of the ring core. The deflection of vortex lines from the main vortex ring persists during the whole motion and is responsible for the gradual erosion of the coherent toroidal structure of the initial vortex ring.

The vortex-street structure of turbulent jets. Part 2

This paper is an extension of the work reported earlier as part 1 in a companion paper. It endeavours first to discover how the vortex-street structure in the round jet develops during its passage downstream, and secondly to resolve some of the questions concerning the flow structure in the outer part of the mixing region. It appears that the main vortex street that was reported in part 1 has a trajectory that converges on the jet axis at about two potential core lengths from the nozzle. This brings the vortex cores towards the axis and contributes to the erosion of the potential core. A branch vortex street apparently forms on the outer part of the jet mixing region downstream of x/D = 1.5, and as the main vortex street approaches the axis, the branch vortex street moves outwards. The vortices in the branch are jostled occasionally by those in the main vortex street, and various effects are caused, including the frequency modulation of the signals in the outer part of the mixing region. Measurements at higher Mach numbers confirm the appropriateness of Strouhal number scaling of the jet data for the full range of subsonic Mach numbers and suggest that the structure described in this paper is valid up to the sonic speed.

Numerical analysis of turbulent end-wall boundary layers of intense vortices

After a careful consideration of the laws of generation, advection, diffusion and dissipation of turbulent kinetic energy proposed by Prandtl (1945) and Emmons (1954), equations of motion and turbulent kinetic energy for the vortex flow near a solid end wall are established. These equations are then evaluated by a numerical procedure. Care is taken to specify boundary conditions such that satisfactory matching of the solution with the main vortex is assured. The agreement between the predicted mean velocity distribution and the experimental data is remarkably good. In addition, several interesting characteristics are predicted by the theory: (i) the vertical distribution of horizontal velocity is oscillatory in the inner region, whilst it is of the ordinary boundary-layer type in the outer region; (ii) the maximum velocity in the boundary layer can exceed that in the main vortex by a considerable amount and (iii) the minimum pressure of the vortex does not occur in the vortex-core root as has been generally believed.

An Analytical Model for the Incompressible Flow in Short Vortex Chambers

A momentum integral analysis is presented for the incompressible, steady, axisymmetric flow in a short vortex chamber of the type commonly used in vortex valves. The analysis is developed with the aid of flow visualization photographs and considers the interaction which occurs between the main vortex core flow and the viscous chamber end wall boundary layers. The radial pressure distributions predicted by the analysis compare favorably with measured end wall static pressure distributions.