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NAFEMS Glossary of Terms W-Z

WALL DAMPING FUNCTIONSFunctions used to modify the k- turbulence model for low Reynolds number flows.
WALL FUNCTIONSFunctions used to describe the effects of turbulent boundary layers in the region adjacent to a wall, without resolving details of the near wall flow and eliminating the need for high grid resolution in the viscous sub-layer.
WAVE PROPAGATIONThe dynamic calculation involving the prediction of the history of stress and pressure waves in solids and fluids.
WAVEFRONT (FRONT)The wavefront of a symmetric matrix is the maximum number of active nodes at any time during a frontal solution process. It is a measure of the time required to factorise the equations in a frontal solution. It is minimised be element renumbering.
WEIGHTED RESIDUAL FORMULATIONA form of the method of weighted residuals. The most general technique in finite element methods for defining an integral formulation of the physical problem, and which seeks to reduce errors through an appropriate choice of element weighting and interpolation functions.
WEIGHTED RESIDUALSA technique for transforming a set of partial differential equations to a set of simultaneous equations so that the solution to the simultaneous equations satisfy the partial differential equations in a mean sense.The form used in the finite element method is the Galerkin process. This leads to identical equations to those from virtual work arguments.
WEIGHTING FUNCTIONSAn averaging technique in which the discrete computed values are weighted according to the level of some property (such as mass, area, density etc.) in the cell relating to each value for which the average is required.
WELL-POSED PROBLEMA problem for which the solution depends, in a continuous way, on the initial and boundary conditions.
WHIRLING STABILITYThe stability of rotating systems where centrifugal and Coriolis are also present.
WHITE NOISEWhite noise has a constant spectral density for all frequencies.
WIGGLESPhysically unrealistic numerical oscillations in variable values resolved on the computational grid.
WILSON THETA METHODAn implicit solution method for integrating second order equations of motion. It can be made unconditionally stable.
WORD LENGTHWithin a digital computer a number is only held to a finite number of significant figures. A 32bit (single precision) word has about 7 significant figures. A 64bit (double precision) word has about 13 significant figures. All finite element calculations should be conducted in double precision.
Y+A non-dimensional description of distance from a wall in relation to local flow and wall shear stress parameters. The expression of distances from the wall in y+ ‘units’ is important in defining velocity and turbulence distributions in a universal form suited to wall functions. A very important requirement in the application of wall functions in CFD is that the computational cells adjacent to the wall have a height, usually expressed in y+ units, compatible with the wall  functions being employed.
YIELD CRITERIONIn the theory of plasticity, a law defining the limit of elastic behaviour under any possible combination of the stress components at any point: the criteria of Tresca and von Mises (q.v.) are common for metals.
YIELD STRESSThe stress level at which yielding commences in a uniaxial stress-strain state.
YIELDINGThe transition of material behaviour from elastic to plastic.
YOUNGS MODULUSThe material property relating a uniaxial stress to the corresponding strain.
ZERO ENERGY MODESSpurious element deformations (q.v. hourglass effects) that occur with zero strain energy, due to particular numerical integration schemes.
ZERO GRADIENT BOUNDARY CONDITIONA boundary condition where a variable is defined as constant across the boundary.
ZERO-EQUATION MODELSThe simplest type of turbulence  models, also known as mixing length turbulence models, in which the turbulent kinetic energy is determined from the mean velocity field.
ZONAL METHOD– a method in which different mathematical models are applied to different regions in the geometrical domain.
ZONE OF DEPENDENCESee region of dependence.
ZONE OF INFLUENCESee region of influence.