Fluid mechanics introduction, basic definitions

FM

1. Chapter 1 Introduction to Fluid Mechanics

2. Definition Mechanics is the oldest physical science that deals with  both stationary and moving bodies under the influence forces. The branch of mechanics that deals with bodies at rest called statics , while the branch that deals with bodies motion is called dynamics . of is in  The subcategory fluid mechanics is defined as the science that deals with the behavior of fluids at rest ( fluid statics)  or in motion ( fluid dynamics), and the interaction with solids or other fluids at the boundaries. The study of fluids at rest is called fluid statics . of fluids  2

3. Definition The study of f1uids in motion, where pressure forces are not considered, is called fluid kinematics and if the  pressure forces are also considered for the fluids in motion. that branch of science is called fluid dynamics . Fluid mechanics itself is also divided into several categories. The study of the motion of fluids that are practically incompressible (such as liquids, especially water, and   gases at low speeds) is usually referred to as hydrodynamics. A subcategory of hydrodynamics is hydraulics, which deals with liquid flows in pipes and open channels.  3

4. Definition  Gas dyn a m ics dea ls w i t h the f l ow o f fl u i d s t h a t unde r g o s ig n i f i c an t d e n s i t y c ha n g es, s u ch as the flow of ga s es through nozzles at high speed s .  The c a t e g o ry a e r odyn a m ics d e a ls with the f l o w of gases (es p ec i a l l y a i r ) ov er b o d ies s u ch as a i r c ra f t, ro c ke ts, a n d auto m ob i l e s at high or low sp e ed s .  Some other specialized categories such deal as meteorology, oceanography, and hydrology with naturally occurring flows. 4

5. What is a Fluid? A substance exists in three primary phases: solid, liquid, and gas. A substance in the liquid or gas phase is referred to as a fluid.  Distinction between a solid and a fluid is made on the basis of the substance’s ability to resist an applied shear (or tangential) stress that tends to change its shape. A solid can resist an applied shear stress by deforming, whereas a fluid deforms continuously under the influence of shear stress, no matter how small.   In solids stress is proportional to strain, is proportional to strain rate. but in fluids stress  5

6. What is a Fluid? When a constant shear f o r c e is applied, a sol i d even t ually stops defor m ing, at so m e fixed st r ain angl e , w h ere a s a f luid never stops defor m ing and app r oaches a ce r ta i n ra t e of st r ain.  Figure. Deformation of a rubber eraser placed between two parallel plates under the influence of a shear force. 6

7. What is a Fluid? In a liquid, m o l ecu l es can move  relative to each other, but the volume remains relatively constant because of the strong cohesive forces between the molecules. A s a r e s u lt, a l i q u i d t a k e s t he shape of t h e con t a i n e r i t i s i n, a nd i t f o r m s a f r ee s urfa c e i n a l a r g e r c on t ainer i n a gr a v i t a t i o n al fie l d . A g a s , o n t h e o t h e r h a nd, e xpa n ds u n t i l i t e n c o unt e rs t he wal l s o f the c o n t a i n e r a n d fil l s t h e e n t i re a v a i l a b l e sp a c e . This is because the gas molecules are widely spaced, and the cohesive forces between them are very small. Unlike liquids, gases cannot form a free surface     7

8. What is a Fluid?  Differences between liquid and gases 8 Liquid Gases Diff i c u l t to co m p r e s s and o f t e n re g ar d ed as inco m p r e s s i b l e Eas i ly to co m p r e s s – cha n ges of v o l u m e is l a r g e , can n ot n o r m a l l y be neglec t ed and are rela t ed to t e m per a t ur e Occupies a fixed volume and will take the shape of the container N o f i x e d v o l u m e, i t c hang e s v o l u m e t o e x p a nd t o fi l l t he co n t a ining vessels A free surface is formed if the volume of container is greater than the liquid. Completely fill the vessel so that no free surface is formed.

9. Application areas of Fluid Mechanics  Mechanics of fluids is extremely important of engineering and science. Examples are:  Biomechanics  Blood flow through arteries and veins  Airflow in the lungs  Flow of cerebral fluid  Households in many areas  Piping systems for cold water, natural gas, and sewage and  Piping and ducting systems vacuum network of heating air- conditioning  refrigerator, cleaner, dish washer, washing machine, water meter, radiator, etc. natural gas meter, air conditioner,  Meteorology and Ocean Engineering  Movements of air currents and water currents 9