Forces Exerted on a Hovercraft by a Moving Pressure Distribution: Robustness of Mathematical Models

2006 ◽  
Vol 50 (01) ◽  
pp. 38-48 ◽  
Author(s):  
Gregory Zilman
Keyword(s):  
Free Surface ◽  
Mathematical Models ◽  
Pressure Distribution ◽  
Constant Pressure ◽  
Wave Resistance ◽  
Excess Pressure ◽  
Physical Parameters ◽  
Rectangular Region ◽  
Heavy Fluid ◽  
Side Force

The wave resistance, side force, and yawing moment acting on a hovercraft moving on the free surface of a heavy fluid is studied. The hovercraft is represented by a distributed excess pressure. Various types of pressure and bounding contours are considered. The sensitivity of the results to numerous uncertainties in the problem's physical parameters is investigated. It is found that constant pressure over a rectangular region moving with an angle of drift results in peculiar side force values. Several robust mathematical models of a moving hovercraft are proposed and analyzed.

Flow Due to a Moving Pressure Distribution on Free-Surface With Finite-Depth Bottom

2002 ◽  
Author(s):  
Iskender Sahin ◽  
Noriaki Okita
Keyword(s):  
Free Surface ◽  
Pressure Distribution ◽  
Current Method ◽  
Finite Depth ◽  
Steady State Solution ◽  
Wave Resistance ◽  
Surface Elevation ◽  
Bottom Pressure ◽  
Approximation Techniques ◽  
Pressure Profiles

Surface elevation and dynamic bottom pressure profiles caused by a moving pressure distribution over the free-surface are obtained. A direct numerical integration approach for the linear, two-dimensional, and steady-state solution has been developed. The behavior of the surface elevation and bottom pressure profiles along with wave resistance for increasing Froude number or depth are presented. The agreement of the wave resistance calculations using the profiles obtained by the current method and the expression given by Newman and Poole (1962) indicates that the current method can be used as a reliable tool for prediction as well as validation for other numerical approximation techniques.

scholarly journals Nonlinear stability of two-layer shallow water flows with a free surface

2020 ◽  
Vol 476 (2236) ◽  
pp. 20190594
Author(s):  
Francisco de Melo Viríssimo ◽  
Paul A. Milewski
Keyword(s):  
Free Surface ◽  
Initial Data ◽  
Nonlinear Stability ◽  
Mathematical Proof ◽  
Passive Layer ◽  
Physical Parameters ◽  
Immiscible Fluid ◽  
Dimensional System ◽  
The Cauchy Problem ◽  
The Difference

The problem of two layers of immiscible fluid, bordered above by an unbounded layer of passive fluid and below by a flat bed, is formulated and discussed. The resulting equations are given by a first-order, four-dimensional system of PDEs of mixed-type. The relevant physical parameters in the problem are presented and used to write the equations in a non-dimensional form. The conservation laws for the problem, which are known to be only six, are explicitly written and discussed in both non-Boussinesq and Boussinesq cases. Both dynamics and nonlinear stability of the Cauchy problem are discussed, with focus on the case where the upper unbounded passive layer has zero density, also called the free surface case. We prove that the stability of a solution depends only on two ‘baroclinic’ parameters (the shear and the difference of layer thickness, the former being the most important one) and give a precise criterion for the system to be well-posed. It is also numerically shown that the system is nonlinearly unstable, as hyperbolic initial data evolves into the elliptic region before the formation of shocks. We also discuss the use of simple waves as a tool to bound solutions and preventing a hyperbolic initial data to become elliptic and use this idea to give a mathematical proof for the nonlinear instability.

scholarly journals Numerical Investigation on the Influence of the Streamlined Structures of the High-Speed Train’s Nose on Aerodynamic Performances

2021 ◽  
Vol 11 (2) ◽  
pp. 784
Author(s):  
Zhenxu Sun ◽  
Shuanbao Yao ◽  
Lianyi Wei ◽  
Yongfang Yao ◽  
Guowei Yang
Keyword(s):  
Drag Reduction ◽  
Pressure Distribution ◽  
Flow Separation ◽  
High Speed ◽  
Leeward Side ◽  
Detached Eddy Simulation ◽  
Side Force ◽  
Eddy Simulation ◽  
Delayed Detached Eddy Simulation ◽  
Asymmetrical Design

The structural design of the streamlined shape is the basis for high-speed train aerodynamic design. With use of the delayed detached-eddy simulation (DDES) method, the influence of four different structural types of the streamlined shape on aerodynamic performance and flow mechanism was investigated. These four designs were chosen elaborately, including a double-arch ellipsoid shape, a single-arch ellipsoid shape, a spindle shape with a front cowcatcher and a double-arch wide-flat shape. Two different running scenes, trains running in the open air or in crosswind conditions, were considered. Results reveal that when dealing with drag reduction of the whole train running in the open air, it needs to take into account how air resistance is distributed on both noses and then deal with them both rather than adjust only the head or the tail. An asymmetrical design is feasible with the head being a single-arch ellipsoid and the tail being a spindle with a front cowcatcher to achieve the minimum drag reduction. The single-arch ellipsoid design on both noses could aid in moderating the transverse amplitude of the side force on the tail resulting from the asymmetrical vortex structures in the flow field behind the tail. When crosswind is considered, the pressure distribution on the train surface becomes more disturbed, resulting in the increase of the side force and lift. The current study reveals that the double-arch wide-flat streamlined design helps to alleviate the side force and lift on both noses. The magnitude of side force on the head is 10 times as large as that on the tail while the lift on the head is slightly above that on the tail. Change of positions where flow separation takes place on the streamlined part is the main cause that leads to the opposite behaviors of pressure distribution on the head and on the tail. Under the influence of the ambient wind, flow separation occurs about distinct positions on the train surface and intricate vortices are generated at the leeward side, which add to the aerodynamic loads on the train in crosswind conditions. These results could help gain insight on choosing a most suitable streamlined shape under specific running conditions and acquiring a universal optimum nose shape as well.

A note on the unsteady motion under gravity of a corner point on a free surface: a generalization of Stokes' theory

2008 ◽  
Vol 465 (2101) ◽  
pp. 165-173 ◽  
Author(s):  
D.J Needham ◽  
J Billingham
Keyword(s):  
Free Surface ◽  
Corner Point ◽  
Constant Pressure ◽  
Unsteady Motion ◽  
Constant Speed ◽  
Inviscid Fluid ◽  
Irrotational Motion

In this paper, we develop a theory based on local asymptotic coordinate expansions for the unsteady propagation of a corner point on the constant-pressure free surface bounding an incompressible inviscid fluid in irrotational motion under the action of gravity. This generalizes the result of Stokes and Michell relating to the horizontal propagation of a corner at constant speed.

On the Effects of Aero Boundary Layer Control on Pressure Drag Reduction in Supercavitating Bodies

2005 ◽  
Author(s):  
Yasmin Khakpour ◽  
Miad Yazdani
Keyword(s):  
Boundary Layer ◽  
Drag Reduction ◽  
Pressure Distribution ◽  
Constant Pressure ◽  
Viscous Drag ◽  
Cavity Surface ◽  
Gradient Flows ◽  
Supercavitating Vehicles ◽  
Pressure Drag ◽  
Layer Control

Supercavitation is known as the way of viscous drag reduction for the projectiles, moving in the liquid phase. In recent works, there is distinct investigation between cavitation flow and momentum transfer far away from the cavity surface. However, it seems that there is strong connection between overall flow and what takes place in the sheet cavity where a constant pressure distribution is assumed. Furthermore as we’ll see, pressure distribution on cavity surface caused due to overall conditions, induct nonaxisymetric forces and they may need to be investigated. Primarily we describe how pressure distribution into the cavity can cause separation of the aero boundary layer. Then we present some approaches by which this probable separation can be controlled. Comparisons of several conditions exhibits that at very low cavitation numbers, constant pressure assumption fails particularly for gradient shaped profiles and separation is probable if the flow is sufficiently turbulent. Air injection into the NATURALLY FORMED supercavity is found as an effective way to delay probable separation and so significant pressure drag reduction is achieved. In addition, the position of injection plays a major role to control the aero boundary layer and it has to be considered. Moreover, electromagnetic forces cause to delay or even prevent separation in high pressure gradient flows and interesting results obtained in this regard shows significant drag reduction in supercavitating vehicles.

Exact solutions for a class of nonlinear free surface flows

1991 ◽  
Vol 434 (1892) ◽  
pp. 677-682 ◽  
Keyword(s):  
Free Surface ◽  
Free Boundary ◽  
Infinite System ◽  
Free Stream ◽  
Constant Pressure ◽  
Velocity Ratio ◽  
Mapping Function ◽  
Free Surface Flows ◽  
Surface Flows ◽  
Nonlinear Free Surface

We consider a class of inviscid free surface flows where the free surface is of finite length and in which the pressure on the free boundary p b is different from the free stream pressure p ∞ . The aim of the paper is to determine the shape of the free surface as a function of the velocity ratio parameter λ . The free boundary problem is tackled by seeking a mapping z ═ f (ζ) such that the flow past a circle in the ζ-plane maps to a flow with constant pressure p b on the free surface in the z -plane. The formulation leads to an infinite system of coupled nonlinear equations for the coefficients in the mapping function. Remarkably, the system can be solved exactly to yield two families of free surface flows of the form z ═ ζ + λ 2 /ζ + a ( λ ) ln (ζ + b ( λ )/ζ ─ b ( λ )). The nature of the solutions, their limitations and possible extensions to them are discussed.

Suction Pile Splash Zone Crossing Modelling

2020 ◽  
Author(s):  
Jean-Luc Pelerin ◽  
David Terribile ◽  
Emmanuel Sergent ◽  
Gerard Fernandez
Keyword(s):  
Free Surface ◽  
Peak Load ◽  
Field Measurements ◽  
Model Tests ◽  
Water Dynamics ◽  
Physical Parameters ◽  
Splash Zone ◽  
Entrapped Air ◽  
Vessel Motion ◽  
The Impact

Abstract One of the critical phases that drive allowable seastates during suction pile installation is the splash zone crossing (SPZC). Offshore experience shows that anticipated loads and slack events are often over predicted, which directly affect installation vessel operability. If conservatism is required to prevent damaging installation assets, a better risk balance is required to avoid unnecessary asset stand-by. Despite the above, basin tests have shown that the peak load/slack criteria can also be under-estimated with the current methodology which may lead to a dangerous situation offshore. Because the applicable methodology is regardless of the installation crane capacity (i.e. slack) and because it does not account for the entrapped water dynamics, it cannot accurately predict the loads on the crane. We present here a physics based model of the free surface inside the suction pile that provides the loads applied on the crane while crossing the splash zone. This allows mitigation to be incorporated from day-1 of design phase and avoid late change from installation contractor while pile are fabricated and increase their vessel operability in the meantime. The model accounts for the entrapped air compressibility, the air/water flow through the pile openings, the vessel motion and the surrounding wave field. The numerical implementation has been performed in Python and packaged as an Orcaflex module. Some of the model physical parameters such as the opening pressure drop coefficients have been derived with the help of CFD. The impact of the free surface on the pile top cap is modelled as a polynomial function of the impact velocity and the coefficients values have been derived using CFD. The model has been validated against model tests and compared to field measurements and observations. The numerical results have shown good agreement with both model tests and offshore measurements at a qualitative level (the observed phenomenon are properly reproduced) and at a quantitative level. The application of the validated model to projects will allow broadening of the operating envelope and the optimization of the installation vessel planning by reducing the standby time. This new methodology shows some high potential and could be applied to projects on a more regular basis.

A simple example from flat-ship theory

1988 ◽  
Vol 189 ◽  
pp. 301-310 ◽  
Author(s):  
Susan Cole
Keyword(s):  
Exact Solution ◽  
Free Surface ◽  
Energy Balance ◽  
Aspect Ratio ◽  
Pressure Distribution ◽  
Wave Drag ◽  
Vortical Flow ◽  
Far Field ◽  
Low Aspect Ratio ◽  
Free Surface Waves

This paper describes the induced pressure distribution, free-surface waves, vortical flow and wave drag of an exact solution of low-aspect-ratio flat-ship theory. An energy balance is derived which relates the spray drag, the energy carried away by the far-field waves and the vortical flow to the total wave drag.

Sign in / Sign up

Export Citation Format

Share Document