**Advection **

Advection is the transportation of substance or quantity by bulk motion.

During advection, fluid transports some conserved quantity or material via bulk motion.where fluid described by vector and transported material as scalar quantity. Advection requires currents in the fluids,and so cannot happen in rigid solids.It does not include transport of substance by molecular diffusion.

**Advection** is the reason of transport of some property of the atmosphere or ocean,such as heat,humidity or salinity.

**Advection **is important for the formation of clouds and the precipitation of water from clouds ,as part of the hydrological cycle

**Adverse Pressure**

In fluid dynamics,an adverse pressure gradient occurs when the static pressure increases in the flow direction.

**dp/dx >0**, for flow in positive x direction. This is important in boundary layers,increasing the fluid pressure of the fluid ,leading to a reduced kinetic energy & a deceleration of the fluid.As the fluid inside the boundary layer is slower, it is greatly affected by the increasing pressure gradient.

For a large enough pressure increase , this fluid may slow to zero velocity or even become reversed causing flow separation.

this has large consequences in aerodynamics since flow separation significantly modifies the pressure distribution along the surface and hence the **lift and drag characteristics** .

**Aeroacoustics **

It is a branch of acoustics that studies noise generation via either turbulent fluid motion or aerodynamic forces interacting with surfaces.

Noise generation can also be occur due to periodically varying flows. A notable example of this phenomenon is the aaerolian tones produced by wind blowing over fixed objects.

**Aerodynamics**

It is the study of motion of air particularly as interaction with a solid object ,such as an airplane wing.It is a subfield of fluid dynamics and gas dynamics . It often used as gas dynamics applies to the motion of all gases,and is not limited to air.

Recent work has focused on issues related to compressible flow ,turbulence and boundary layers and has become increasingly computational in nature.

**Aerosol **

An aerosol is a suspension of fine solid particles or liquid droplets ,in air or another gas. (typically solid particles diameters < 1×10^-6 m)

It can be natural or anthropogenic.

Examples of natural aerosol are : fog, dust , forest exudates and geyser

Examples of anthropogenic aerosols are : haze , particulate air pollutants and smoke

**Aerostatics **

A subfield of fluid statics , **aerostatics** is the study of gases that are not in motion with respect to the coordinate system in which they are considered.

Aerostatics studies **density allocation,especially in air.**

**Airfoil**

An airfoil is the cross sectional shape of a wing,blade etc.

The geometry of the airfoil is described with a variety of terms :

- The
*leading edge*is the point at the front of the airfoil that has maximum curvature (minimum radius) - The
*trailing edge*is defined similarly as the point of maximum curvature at the rear of the airfoil. - The
*chord line*is the straight line connecting leading and trailing edges. The*chord length*, or simply**chord**, , is the length of the chord line. That is the**reference dimension**of the airfoil section.

The shape of the airfoil is defined using the following geometrical parameters:

- The
*mean camber line*or*mean line*is the locus of points midway between the upper and lower surfaces. Its shape depends on the thickness distribution along the chord; - The
*thickness*of an airfoil varies along the chord. It may be measured in either of two ways:- Thickness measured perpendicular to the camber line, sometimes described as the “American convention”;
- Thickness measured perpendicular to the chord line, sometimes described as the “British convention”

**Annular Velocity**

Speed of a fluid’s movement in a column called an annulus in wells being drilled with circulating drilling fluid.**It is actually defined as actual flow rate divided by the annulus area **

Measured in ft/min or m/min . **Annular velocity** is described as used in drilling fluid applications in the oil exploration industry.

**Anti bubble **

**An antibubble is a droplet of liquid surrounded by a thin film of gas**, as opposed to a gas bubble, which is a sphere of gas surrounded by a liquid. *Antibubbles are formed when liquid drops or flows turbulently into the same or another liquid*.

**Apparent viscosity **

Apparent Viscosity is the * ratio of the shear stress to the shear strain* .

For Newtonian fluids , it is constant and same as viscosity ,but for the Non-Newtonian fluids it changed,and depends on the shear rate

SI unit : Pa.s

In many non-Newtonian fluids, the shear stress due to viscosity, , can be modeled by

where

*k*is the consistency index*n*is the flow behavior index*du/dy*is the shear rate, with velocity*u*and position*y*

These fluids are called power-law fluids.