Types of Fluid Flow

IntroductionA fluid is a state of matter in which its molecules move freely and do not bear a constant relationship in space to other molecules.In physics, fluid flow has all kinds of aspects: steady or unsteady, compressible or incompressible, viscous or non-viscous, and rotational or irrotational to name a few. Some of these characteristics reflect properties of the liquid itself, and others focus on how the fluid is moving. Fluids are :- • Liquid : blood, i.v. infusions)• Gas : O2 , N2O)• Vapour (transition from liquid to gas) : N2O (under

compression in cylinder), volatile inhalational agents (halothane, isoflurane, etc)

• Sublimate (transition from solid to gas bypassing liquid state) : Dry ice (solid CO2), iodine

Definition of Flow

Flow is defined as the quantity of fluid (gas, liquid, vapour or sublimate) that passes a point per unit time.A simple equation to represent this is: Flow (F) = Quantity (Q) Time (t)Flow is sometimes written as ∆Q (rate of change of a quantity).

Types of Fluid Flow

Aerodynamic force CavitationCompressible flow Couette flow Free molecular flow Incompressible flowInviscid Flow Isothermal FlowLaminar Flow

Aerodynamic ForceAerodynamic force is exerted on a body by the air (or some other gas) in which the body is immersed, and is due to the relative motion between the body and the gas. Aerodynamic force arises from two causes: • The normal force due to the pressure on the

surface of the body.• The shear force due to the viscosity of the gas,

also known as skin friction.

Forces on an Aero foil

CavitationCavitation is the formation of vapour cavities in liquid i.e. small liquid free zones that are the consequence of forces acting upon the liquid.It usually occurs when a liquid is subjected to rapid changes of pressure that cause the formation of cavities where the pressure is relatively low.When subjected to higher pressure, the voids implode and can generate an intense shock wave.

Cavitating propeller model in a water tunnel experiment

Compressible Flow

Compressible flow is the branch of fluid mechanics that deals with flows having significant changes in fluid density. Gases, but not liquids, display such behaviour.To distinguish between compressible and incompressible flow in air, the Mach number (the ratio of the speed of the flow to the speed of sound) must be greater than about 0.3 (since the density change is greater than 5% in that case) before significant compressibility occurs. The study of compressible flow is relevant to high-speed aircraft, jet engines, rocket motors, hyper loops high-speed entry into a planetary atmosphere, gas pipelines, commercial applications such as abrasive blasting, and many other fields.

Couette FlowCouette flow is the laminar flow of a viscous fluid in the space between two parallel plates, one of which is moving relative to the other. The flow is driven by virtue of viscous drag force acting on the fluid and the applied pressure gradient parallel to the plates. This type of flow is named in honour of Maurice Marie Alfred Couette, a Professor of Physics at the French University of Angers in the late 19th century.

Free Molecular FlowFree molecular flow describes the fluid dynamics of gas where the mean free path of the molecules is larger than the size of the chamber or of the object under test.For tubes/objects of the size of several cm, this means pressures well below 10−3 Torr. This is also called the regime of high vacuum. This is opposed to viscous flow encountered at higher pressures.In free molecular flow, the pressure of the remaining gas can be considered as effectively zero. Thus, boiling points do not depend on the residual pressure.The flow can be considered to be individual particles moving in straight lines. Practically, the "vapour" cannot move around bends or into other spaces behind obstacles, as they simply hit the tube wall.

Incompressible FlowIn fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric flow) refers to a flow in which the material density is constant within a fluid parcel an infinitesimal volume that moves with the flow velocity. An equivalent statement that implies incompressibility is that the divergence of the flow velocity is zero.

Inviscid FlowAn inviscid flow is the flow of an ideal fluid that is assumed to have no viscosity. In fluid dynamics there are problems that are easily solved by using the simplifying assumption of an inviscid flow. The flow of fluids with low values of viscosity agree closely with inviscid flow everywhere except close to the fluid boundary where the boundary layer plays a significant role.

Isothermal Flow

Isothermal flow is a model of compressible fluid flow whereby the flow remains at the same temperature while flowing in a conduit.In the model, heat transferred through the walls of the conduit is offset by frictional heating back into the flow. Although the flow temperature remains constant, a change in stagnation temperature occurs because of a change in velocity. The interesting part of this flow is that the flow is choked at and not at Mach number equal to one as in the case of many other model such as Fanno flow. This fact applies to real gases as well as ideal gases.

Laminar Flowlaminar flow (or streamline flow) occurs when a fluid flows in parallel layers, with no disruption between the layers. At low velocities, the fluid tends to flow without lateral mixing, and adjacent layers slide past one another like playing cards. There are no cross-currents perpendicular to the direction of flow, nor eddies or swirls of fluids. In laminar flow, the motion of the particles of the fluid is very orderly with all particles moving in straight lines parallel to the pipe walls. Laminar flow is a flow regime characterized by high momentum diffusion and low momentum convection.