This is all about the flow of fluids (liquids and gases). There are several areas of speciality, including aerodynamics (the study of air and other gases in motion) and hydrodynamics (the study of liquids in motion). Hence the importance of this to the whole field of model engineering.

**Aerodynamics** – Resistance of a vehicle body to motion through the air. A smooth surface has less drag than a rough one.

It may be broken down into three main components:

**skin friction:**this is drag due to the surface texture and area.**profile drag:**this is drag from the three-dimensional shape of the aircraft/vehicle.**service drag:**this is drag from air ducted to cooling components.

**Aerodynamic Noise** – when we flow gas through a pipe or over a surface we generate noise and sometimes vibration. The noise that is produced can be extreme if there is a feedback system between the noise that is generated and the gas flow.

**Boundary Layer** – the portion of a fluid flowing past a body that is in the immediate vicinity of the body and that has a reduced flow due to the forces of adhesion and viscosity.

Friction between the body and the surrounding air holds back the flow nearest the surfaces, whilst the air further from the body in the mainstream flows past at unabated speed.

Example– the layer of air next to the Earth′s surface, typically a few hundred metres thick. Ordinarily, only the boundary layer is appreciably affected by the properties of the surface. The region above the boundary layer is called the ′free atmosphere′.

**Centre of Pressure** – The point at which the aerodynamic forces on a body appear to act, and at which there is no aerodynamic movement. It is similar to the centre of gravity in mechanical terms.

Compressible Fluids– Compressible flow requires the integration of the equations of conservation of mass and momentum with that of energy conservation. These three conservation requirements are closely coupled to each other in a way that increases the complexity of the problem. Also, the energy conservation equation explicitly incorporates thermodynamic properties of the fluid. A result of this is there is no general solution to such flows, and we resort instead to simple flows, such as unsteady one-dimensional and steady two-dimensional inviscid flows of perfect gases having constant specific heats.

**Differential Pressure Indicator** – An indicator which signals the difference in pressure between two points, typically between the upstream and downstream sides of a filter, valve or expansion element.

**Dynamic Pressure** – The pressure of a fluid resulting from its motion when brought to rest on a surface. Also known as impact pressure, stagnation pressure, and total pressure.

**Flow Noise** – A term generally used to describe aerodynamic noise produced when a gas flows within a duct or when the gas exits the duct. This is a typical problem observed in the exhaust systems of internal combustion engines and high engine rotational speeds and hence high gas flow rates.

Fluids– Substances in which the binding forces are weaker than in solids, so that the atoms or molecules do not occupy fixed positions and move at random. Liquids and gases are fluids.

**Hydrostatic Pressure** – The pressure at any point in a liquid at rest; equal to the depth of the liquid multiplied by its density and acceleration due to gravity.

**Kinematic Viscosity** – is the dynamic viscosity of a fluid divided by the fluid density.

**Pressure Coefficient** – A dimensionless value which acts as a means of indicating the local pressure at some point of interest around a body, and which is independent of velocity.

**Turbulence** – Describes fluid motion disturbed from its average behaviour by random fluctuations over a range of temporal and spatial scales.

These fluctuations often constitute major deformations of the flow and are capable of transporting:

- momentum
- energy
- suspended matter

The transportation rates can be far in excess of the rate of transport by the molecular processes of diffusion and conduction in a nonturbulent or laminar flow.

**Turbulent Boundary Layer** – when the Reynolds number is high, there is a turbulent layer adjacent to the laminar boundary layer. Within this layer the Reynolds stresses are much larger than the viscous stresses.

**Viscosity** – That molecular property of a fluid which enables it to support tangential stresses for a finite time and thus to resist deformation; the ratio of shear stress divided by shearing strain. A measure of the internal friction within a fluid. The time required for a liquid to drain out of a capillary tube is directly proportional to its viscosity.