Types of Fluid Flow

Introduction

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. Note that fluid flow can get very complex when it becomes

turbulent. Physicists haven’t developed any elegant equations to describe turbulence

because how turbulence works depends on the individual system whether you have

water cascading through a pipe or air streaming out of a jet engine. Usually, you have to

resort to computers to handle problems that involve fluid turbulence.

Types of fluid flow

Aerodynamic force

Cavitation

Compressible flow

Couette flow

Free molecular flow

Incompressible flow

Aerodynamic Force

Aerodynamic 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.

Pressure acts locally, normal to the surface, and shear force acts locally, parallel to the

surface. The net aerodynamic force over the body is due to the pressure and shear

forces integrated over the total exposed area of the body.

When an air foil (or a wing) is moving relative to the air it generates an aerodynamic

force, in a rearward direction at an angle with the direction of relative motion. This

aerodynamic force is commonly resolved into two components:

Drag is the force component parallel to the direction of relative motion.

Lift is the force component perpendicular to direction of relative motion.

In addition to these two forces, the body may experience an aerodynamic moment also,

the value of which depends on the point chosen for calculation.

The force created by a propeller or a jet engine is called thrust and it is also an

aerodynamic force (since it also acts on the surrounding air). The aerodynamic force on

a powered airplane is commonly represented by three vectors: thrust, lift and drag.

Forces on an Aero foil

Cavitation

Cavitation 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.

Cavitation is a significant cause of wear in some engineering contexts. Collapsing voids

that implode near to a metal surface cause cyclic stress through repeated implosion.

This result in surface fatigue of the metal causing a type of wear also called "cavitation".

The most common examples of this kind of wear are to pump impellers and bends

where a sudden change in the direction of liquid occurs. Cavitation is usually divided

into two classes of behaviour: inertial cavitation and non-inertial cavitation.

Inertial cavitation is the process where a void or bubble in a liquid rapidly collapses,

producing a shock wave. Inertial cavitation occurs in nature in the strikes of mantis

shrimps and pistol shrimps, as well as in the vascular tissues of plants. In man-made

objects, it can occur in control valves, pumps, propellers and impellers.

Non-inertial cavitation is the process in which a bubble in a fluid is forced to oscillate

in size or shape due to some form of energy input, such as an acoustic field. Such

cavitation is often employed in ultrasonic cleaning baths and can also be observed in

pumps, propellers, etc.

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 behavior .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, hyperloops high-speed entry into a planetary atmosphere, gas pipelines, commercial applications such as abrasive blasting, and many other fields.

Couette Flow

In fluid dynamics, Couette 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 honor of Maurice Marie Alfred Couette, a Professor of Physics at the French University of Angers in the late 19th century.

Mathematical description

Couette flow is frequently used in undergraduate physics and engineering courses to

illustrate shear-driven fluid motion. The simplest conceptual configuration finds two

infinite, parallel plates separated by a distance h. One plate, say the top one, translates

with a constant velocity u0 in its own plane. Neglecting pressure gradients

Free Molecular Flow

Free 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.

The definition of a free molecular flow depends on the distance scale under

consideration. For example, in the interplanetary medium, the plasma is in a free

molecular flow regime in scales less than 1 AU; thus, planets and moons are effectively

under particle bombardment. However, on larger scales, fluid-like behaviour is

observed, because the probability of collisions between particles becomes significant.

Incompressible Flow

In 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.

Incompressible flow does not imply that the fluid itself is incompressible. It is shown in

the derivation below that (under the right conditions) even compressible fluids can to

good approximation be modelled as an incompressible flow. Incompressible flow implies

that the density remains constant within a parcel of fluid that moves with the flow

velocity.

Inviscid flow

An 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.[1]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 flow

In fluid dynamics, laminar 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.

When a fluid is flowing through a closed channel such as a pipe or between two flat

plates, either of two types of flow may occur depending on the velocity and viscosity of

the fluid: laminar flow or turbulent flow. Laminar flow tends to occur at lower velocities,

below a threshold at which it becomes turbulent. Turbulent flow is a less orderly flow

regime that is characterised by eddies or small packets of fluid particles which result in

lateral mixing. In non-scientific terms, laminar flow is smooth while turbulent flow

is rough.

A sphere in Stokes flow, at very low Reynolds number.

An object moving through a fluid experiences a force in

the direction opposite to its motion

Open-channel flow

Open-channel flow, a branch of hydraulics and fluid mechanics, is a type of liquid flow within a conduit with a free surface, known as a channel. The other type of flow within a conduit is pipe flow. These two types of flow are similar in many ways, but differ in one important respect: the free surface. Open-channel flow has a free surface, whereas pipe flow does not.

Secondary flow

In fluid dynamics, a secondary flow is a relatively minor flow superimposed on the

primary flow, where the primary flow usually matches very closely the flow pattern

predicted using simple analytical techniques and assuming the fluid is inviscid. (An

inviscid fluid is a theoretical fluid having zero viscosity.)

The primary flow of a fluid, particularly in the majority of the flow field remote from solid

surfaces immersed in the fluid, is usually very similar to what would be predicted using

the basic principles of physics, and assuming the fluid is inviscid. However, in real flow

situations, there are regions in the flow field where the flow is significantly different in

both speed and direction to what is predicted for an inviscid fluid using simple analytical

techniques. The flow in these regions is the secondary flow. These regions are usually

in the vicinity of the boundary of the fluid adjacent to solid surfaces where viscous

forces are at work, such as in the boundary layer.

Superfluidity Superfluidity is a state of matter in which the matter behaves like a fluid with zero viscosity; where it appears to exhibit the ability to self-propel and travel in a way that defies the forces of gravity and surface tension. Superfluidity is found in astrophysics, high-energy physics, and theories of quantum gravity. The phenomenon is related to Bose–Einstein condensation, but neither is a specific type of the other: not all Bose-Einstein condensates can be regarded as superfluids, and not all superfluids are Bose–Einstein condensates.

The liquid helium is in the superfluid phase. As long as it remains superfluid, it creeps up the wall of the cup as a thin film. It comes down on the outside, forming a drop which will fall into the liquid below. Another drop will form and so on until the cup is empty.

Transonic In aeronautics, transonic refers to the condition of flight in which a range of velocities of airflow exist surrounding and flowing past an air vehicle or an airfoil that are concurrently below, at, and above the speed of sound in the range of Mach 0.8 to 1.0, i.e. 965–1,236 km/h (600–768 mph) at sea level. This condition depends not only on the travel speed of the craft, but also on the temperature of the airflow in the vehicle's local environment. It is formally defined as the range of speeds between the critical Mach number, when some parts of the airflow over an air vehicle or airfoil are supersonic, and a higher speed, typically near Mach 1.2, when most of the airflow is supersonic. Between these speeds some of the airflow is supersonic, but a significant fraction is not.

Aerodynamic condensation evidences of supersonic expansion fans around a transonic F/A-18

Turbulence

In fluid dynamics, turbulence or turbulent flow is a flow regime characterized

by chaotic property changes. This includes low momentum diffusion, high

momentum convection, and rapid variation of pressure and flow velocity in space and

time. Flow in which the kinetic energy dies out due to the action of fluid

molecular viscosity is called laminar flow. While there is no theorem relating the non-

dimensional Reynolds number (Re) to turbulence, flows at Reynolds numbers larger

than 5000 are typically (but not necessarily) turbulent, while those at low Reynolds

numbers usually remain laminar.

Flow visualization of a turbulent jet, made by laser-induced fluorescence.

Two-phase flow Two-phase flow is a flow of gas and liquid usually in a pipe. Two-phase flow is a particular example of multiphase flow. Two-phase flow can occur in various forms. For example, there are transient flows with a transition from pure liquid to a vapor flow as a result of external heating, separated flows and dispersed two-phase flows where one phase is present in the form of particles, droplets, or bubbles in a continuous carrier phase (i.e. gas or liquid).

References www.wikipedia.org (Fluid Dynamics)

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