2.1 Pressure
The equations of ﬂuid dynamics in CFD treat the ﬂuid as a continuous medium, or continuum. It is continuous in the sense that we consider the ﬂuid as having no “empty space” by ignoring its molecular nature. We assume it has properties that vary from point to point and are continuous throughout the solution domain, and whose derivatives are also continuous.
Pressure is an important property of a ﬂuid, denoted by . It describes the amount of force per unit surface area which acts on a surface, in the direction perpendicular to the surface.
Pressure is a scalar that produces a force vector with direction normal to the surface. When pressure is applied to one side of a segment of surface with area , the force points away from that side by

(2.3) 
While pressure exerts a force on a surface, the ﬂuid experiences a force which is compressive in nature, assuming is positive.
Pressure is measured in SI units of pascal . From Sec. 2.13 onwards, however, we generally use to represent the kinematic pressure, in units , obtained by dividing the true pressure by a constant density .
Scalar ﬁelds
The majority of properties, e.g. pressure, temperature, energy, density, volume, etc., can be represented by a single number, or scalar. A scalar ﬁeld describes a scalar property, e.g. pressure, which varies from point to point across some spatial domain.
Point locations can be deﬁned in any coordinate system of axes, e.g. Cartesian (, , ), and in any orientation. A scalar ﬁeld is invariant, meaning the scalar values are the same irrespective of the coordinate system used.
In this book, space and ﬁelds will be described in a coordinate system with righthanded rectangular Cartesian axes. The axes are constructed by deﬁning an origin from which three lines are drawn at right angles to each other, termed the , , axes. A righthanded set of axes requires that looking down the axis with nearest, an arc from the axis to the axis is in a clockwise sense.