Fluid Mechanics Questions and Answers – Fluid Properties – 1

This set of Fluid Mechanics Multiple Choice Questions & Answers (MCQs) focuses on “Fluid Properties – 1”.

1. Which one of the following is the unit of mass density?
a) kg = m3
b) kg = m2
c) kg = m
d) kg = ms

Answer: a
Explanation: Mass Density(p) is defined as the mass(m) per unit volume(V ), i.e., p = m ⁄v
Thus, the unit of p is kg = m3.

2. The specific gravity of a liquid has
a) the same unit as that of mass density
b) the same unit as that of weight density
c) the same unit as that of specific volume
d) no unit

Answer: d
Explanation: The specific gravity of a liquid is the ratio of two similar quantities (densities) which makes it unitless.

3. The specific volume of a liquid is the reciprocal of
a) weight density
b) mass density
c) specific weight
d) specific volume

Answer: b
Explanation: Specific volume(v) is defined as the volume(V ) per unit mass(m).
v = v⁄m = 1 / m⁄v = 1⁄p
where p is the mass density.

4. Which one of the following is the unit of specific weight?
a) N = m3
b) N = m2
c) N = m
d) N = ms

Answer: a
Explanation: Specific weight(γ) is defined as the weight(w) per unit volume(V ), i.e.,
γ = w / v
Thus, unit of is N = m3.

5. Which one of the following is the dimension of mass density?
a) [M1 L-3 T0].
b) [M1 L3 T0].
c) [M0 L-3 T0].
d) [M0 L3 T0]

Answer: a
Explanation: Mass Density(p) is defined as the mass(m) per unit volume(V ), i.e.,
[p] = [m]/[v] = [m] /[L3] = [ML-3].

6. Which one of the following is the dimension of specific gravity of a liquid?
a) [M1 L-3 T0].
b) [M1 L0 T0].
c) [M0 L-3 T0].
d) [M0 L0 T0].

Answer: d
Explanation: The specific gravity of a liquid is the ratio of two similar quantities (densities) which makes it dimensionless.

7. Which one of the following is the dimension of specific volume of a liquid?
a) [M1 L-3 T0].
b) [M-1 L3 T0].
c) [M-1 L-3 T0].
d) [M0 L3 T0].

Answer: b
Explanation: Specific volume(v) is defined as the volume(V ) per unit mass(m). Thus,
[v] = [V]/[m] = [L3]/[M] = [M-1L3].

8. Which one of the following is the dimension of specific weight of a liquid?
a) [ML-3 T -2].
b) [ML3 T-2].
c) [ML-2 T-2].
d) [ML2 T-2].

Answer: c
Explanation: Specific weight(γ) is defined as the weight(w) per unit volume(V ), i.e.,

9. Two fluids 1 and 2 have mass densities of p1 and p2 respectively. If p1 > p2, which one of the following expressions will represent the relation between their specific volumes v1 and v2?
a) v1 > v2
b) v1 < v2
c) v1 = v2
d) Cannot be determined due to insufficient information.

Answer: b
Explanation: Specific volume(v) is defined as the volume(V ) per unit mass(m).
v = v⁄m = 1 / m⁄v = 1⁄p
where p is the mass density. Thus, if p1 > p2, the relation between the specific volumes v1 and v2
will be represented by v1 < v2.

10. A beaker is filled with a liquid up to the mark of one litre and weighed. The weight of the liquid is found to be 6.5 N. The specific weight of the liquid will be
a) 6:5 kN = m3
b) 6:6 kN = m3
c) 6:7 kN = m3
d) 6:8 kN = m3

Answer: a
Explanation: Specific weight(γ) is defined as the weight(w) per unit volume(V ), i.e.,
γ = w⁄V
Thus, γ = 6:5 ⁄10-3 N ⁄ m3 = 6:5 kN/m3.

11. A beaker is filled with a liquid up to the mark of one litre and weighed. The weight of the liquid is found to be 6.5 N. The specific gravity of the liquid will be
a) 0.65
b) 0.66
c) 0.67
d) 0.68

Answer: b
Explanation: Specific gravity(S) of a liquid is defined as the ratio of the density of the liquid(pl) to that of water(pw).

Thus, S = 0:66.

12. A beaker is filled with a liquid up to the mark of one litre and weighed. The weight of the liquid is found to be 6.5 N. The specific volume of the liquid will be
a) 1 l =kg
b) 1:5 l =kg
c) 2 l =kg
d) 2:5 l =kg

Answer: b
Explanation: Specific volume(v) is defined as the volume(V ) per unit mass(m). Thus,

WHY MECHANICAL ENGINEERING IS BETTER ????

Over the past few decades, mechanical engineering degree has raised its value and demand among aspirants and no decline has yet been seen in reputation and scope of mechanical engineering degree. That is why it is often said to be an ever green field.

Mechanical Engineering is a diverse field of engineering. This diversity allows mechanical engineers to pursue a career in any field of life. So it gives you a wide variety of choice after graduation. There are many students who wish to change their field of work after they graduate. So if you are a mechanical engineer, you will find it easy to plunge yourself into a completely new field. Either you want to be a project manager or a technical consultant, being a mechanical engineer you can survive in any field.
    I know many mechanical engineers who are doing awesome jobs as environmentalists because they did masters in environmental engineering after bachelors in mechanical. This is why I said mechanical engineering is diverse in its scope, Even if you don’t like to be a mechanical engineer at work, you can change your path easily. You can pursue higher education in civil engineering, environmental engineering, computer science, software engineering, quality control, project management or even supply chain management. Swapping field of work is not difficult for mechanical engineers.

A Different Kind of Heart Attack: New Research Finds a Broken Heart Permanently Weakens the Organ in a Similar Way

Researchers discover that severe emotional distress can damage the heart physically like a heart attack.

At least 3,000 adults in the U.K. suffer from Takotsubo cardiomyopathy, colloquially termed as the “broken heart syndrome,” each year. It is caused by bereavement and severe emotional distress, which stuns and weakens the heart. This causes the left ventricle – one of the main chambers of the heart – to change shape.

The deformed heart looks similar to an octopus pot, or “takotsubo” in Japanese, which gave the condition its name. Doctors have long presumed that the damage was temporary and would eventually heal over time. ( Broken hearts may actually be able to cause death.)

Interestingly, a major project by Swiss researchers last year found that the condition can also be triggered by extreme happiness as well as sorrow.

Researchers at the University of Aberdeen have discovered that the condition may cause permanent damage to the heart, similar to a heart attack. Following 37 patients with Takotsubo for two years, they found that the heart damage remained long after the event that first triggered the condition. This was seen through regular ultrasound and MRI scans.

Many of the patients reported becoming tired very easily and were thus unable to do much exercise. The researchers suggest that patients should be offered the same drugs as those whose hearts have been damaged by a heart attack.

Lead researcher Dr. Dana Dawson said that Takotsubo is more common than they initially thought, and that it may actually cause permanent damage to the hearts of the patients. Moreover, the condition is affecting the patients’ everyday lives with frequent exhaustion and lack of physical activity.

“Our research shows that Takotsubo needs to be treated with same urgency as any other heart problem, and that patients may need ongoing treatment for these long-term effects,” said Dr. Dawson.

Furthermore, the study suggests that women are more prone to developing this condition than men.

 Treat your heart to some of these healing herbs:

• 1.Hawthorn (Crataegus spp.) – This helps strengthen the cardiovascular system, lower your cholesterol levels, and maintain a healthy blood pressure. One way to prepare it is to crush its (dried) berries and add a teaspoon of it with some sweetener in a cup of hot water, to sip as tea.
• 2.Lemon balm (Melissa officinalis) – This herb is most helpful in relieving insomnia and restlessness, that may be caused by intense emotional stress like heartbreak. Add two teaspoons of dried lemon balm, a little bit of sweetener, and lemon juice to a cup of hot water for a flavorful tea.
• 3.Rose petals (Rosa rugosa) – The rose has long been a symbol of love, whether romantic (red rose) or platonic (yellow rose). Roses can calm the nerves and soothe physical fatigue. A tablespoon of rose petals and some light-flavored honey mixed in hot water is the perfect mood-lifter.

Have You Ever Visit Davis Falls in Monsoon Season ?

Davi's Falls or Devi's Falls  is a waterfall located at Pokhara n Kaski District, Nepal.

                                      

The water forms an underground tunnel after reaching the bottom. This tunnel is approximately 500 feet (150 m) long and runs 100 feet (30 m) below ground level. On 31 July 1961, a Swiss couple Davi went swimming but the woman drowned in a pit because of the overflow. Her body was recovered 3 days later in river Phusre with great effort. Her father wished to name it "Davi's falls" after her. Its Nepali name is Patale Chango, which means "underworld waterfall".

After exiting the tunnel, the water passes through a cave called Gupteshwor Mahadev or "cave beneath the ground". The Phewa Lake dam is the water source.

It is an attraction for tourists and locals. Thousands of Nepalis visit for recreation and enjoyment. Visitors can try their luck on the luck pond constructed there by throwing and placing the coin on the statue of God. Likewise, one can find a model of traditional typical Nepali house and a series of statue of Nepali people wearing traditional dresses where visitors can click photo. Once it was considered as a suicide point since the locals as well as the tourists jumped from the cliff down to the bottom of the fall. Safety measures were applied soon after increase in such activities by constructing metal fences around the cliff to prevent further suicides.

Physical properties of Fluids

Fluid mechanics is  the study of fluids either in motion or at rest,  and  the subsequent effects of the fluids upon the boundary conditions.  Both  gases and liquids are classified as fluids.  

The study of fluid properties involves the basic understanding of flow or static condition of fluids. The important properties are: 

1. Density (ρ)
2. Viscosity ()
3. Surface tension (σ) 
4. Bulk modulus (K)
5. Vapour pressure (Pv) 

From  the  point  of  view  of  fluid  mechanics,  all  matter  consists  of  only  two  states,  i.e.  fluid  and  solid. The main distinction between a solid and a  fluid  lies  in  their  respective  reaction  towards  applied shear and tangential stress. 

A  fluid  further  can  be  divided  as  liquid and  gas. The main distinction between a liquid and a gas is  the effect of cohesive forces on them.

A  liquid,  with  relatively  low  value  of  molecular  spacing  (molecules  are  closely  packed  as  compared  to  gas)  exhibit  strong  cohesive  forces,  and tends to retain its volume. On the other hand,  gases  have  large  molecular  spacing  and  exhibit  low cohesive forces and have a tendency to freely  expand until it is confined.  

Density

It is defined as the mass per unit volume.  

It is denoted by Greek letter ‘Rho’ symbolised as  ρ.  It  is  highly  variable  in  gases  and  it  increases  proportionately with the gas pressure. Density  of  liquids  is  almost  constant,  i.e.  the  density of water is 1000 kg/m³.

Points to remember:

• In general, at atmospheric pressure, liquids are almost three times denser than the gases.
• The densest common liquid is mercury (Hg) with a density value of 13,600 kg/m³ and the least dense gas is hydrogen with a density of 0.0838 kg/m³.

If  the  density  of  a  fluid  varies  significantly  with  moderate  changes  in  pressure  or  temperature,  then  the  fluid  is  termed  as  compressible fluid.  Generally  gases  and  vapours  can  be  regarded  as  compressible fluids. 

If  the  density  variation  of  a  fluid is  small  due  to  changes  in  temperature  or  pressure,  then  the  fluid  is  termed  as  incompressible fluid.  All  liquids are classified under this category. 

Specific Weight

It  is  defined  as  the  weight  per  unit  volume  (N/m³), and is denoted by the letter߱ . Mathematically, it can be written as, . 

Where g = acceleration due to gravity = 9.8 m/s² The  specific  weight  of  water  is  9800 N/m³ at  200^oC and atmospheric pressure. 

Specific Gravity (S.G.)

It  is  defined  as  the  ratio  of  fluid  density  to  the  density of a standard reference fluid.  

Specific gravity of mercury is 13600/1000 = 13.6.  It is also referred to as Relative density "R.D".

Viscosity

When a fluid is subjected to shearing, it results in  the  movement  of  the  fluid  creating  a  strain  rate  which  is  inversely  proportional  to  a  property  called  the  coefficient  of  viscosity (μ) .  The  resulting shear stress (τ) is expressed as, . 

The  figure  shown  below,  illustrates  the  velocity  profile  and  development  of  shearing  in  an  elemental  fluid  element.  It  shows  that  shear  stress  is  maximum  at  the  wall.  Further,  the  velocity ሺuሻ is zero at  the wall, otherwise known  as no‐slip condition.  

The units of μ are N‐s/m², and the term is known  as  the  coefficient  of  dynamic  viscosity  or  simply  viscosity. More popular unit  for expressing value  of viscosity is Poise (P). 

1 Poise = 0.1 N-s/m²

Equation shown above is known as Newton’s  Law. 

The fluids which follow equation (1) are  known  as Newtonian fluids.

The  value of  viscosity of Newtonian  fluids  varies  with temperature and pressure. The viscosity of a  fluid  varies  very  weakly  with  pressure.  Temperature,  on  the  other  hand  has  a  strong  effect  on  the  viscosity.  For  gases,  viscosity  increases  with  an  increase  in  temperature,  and  decreases with an increase in temperature. 

Point to remember:

• For common engineering applications, we tend to neglect the pressure effects.

The ratio of dynamic viscosity to the fluid density  is defined as kinematic viscosity (ν).  Mathematically, ν = μ/ρ. 

The units of kinematic viscosity are, Stokes. 

1 stoke = 1 cm²/sec = 10^-4 m²/s

Flow between plates:

In this problem, we have a fixed lower plate and a  moving  upper  plate,  moving  with  a  constant  velocity  V.  The  condition  is  illustrated  in  the  figure below, 

The clearance between  the plates is ‘h’,  fluid is a  Newtonian  fluid  with  no  slip  condition  at  the  bottom  plate.  It  is  assumed  that  there  is  zero  acceleration  and  no  pressure  variation  in  the  direction of flow. We can conclude, that the shear  stress is constant throughout the fluid, and, 

Upon integration of the above equation, we get, 

The  above  expression  validates  that  the  velocity  distribution  is  linear.  The  terms  ‘a’  and  ‘b’  are  constant  values  and  can  be  evaluated  from  the  no‐slip condition at the upper and lower walls as  follows, 

  

  

The boundary conditions give us,  

  

Therefore the velocity profile between the plates  is given by the relation,

From  the  above  matheatical  expression,  we  can  observe that the viscous stresses are negligible in magnitude, although viscosity, as a property, has  profound effects on the fluid motion.   

Non-Newtonian Fluids:

Fluids  which  do  not  obey  Newton’s  laws  are  called Non‐Newtonian fluids.  

• Dliatant fluid or a shear thicknening fluid  increases  flow  resistance  with  an  increasing applied stress  
• Pseudoplastic fluid  or  a  shear  thinning  fluid  decreases  flow  resistance  with  an  increasing  applied  stress.  If  the  thinning  effect  is  very  strong,  then  the  flow  is  termed as Plastic
• Bingham plastics show  that  the  flow  is  linear after  the yielding, but in actual  the  flow may be non linear too.

The transient effect in Non-Newtonian flow can be illustrated by the rheopectic and thixotropic fluids. 

• Rheopectic fluids show an increase in the shear stress with respect to time.
• Thixotropic fluids show a decrease in the shear stress with respect to time. 

Surface tension:

A liquid being unable to expand freely, will form an interface with a second liquid or gas. This causes a phenomenon to occur, surface tension. 

Surface tension is defined as the apparent tensile stress that acts whenever a liquid has a density interface, such as when the liquid comes in contact with a gas, vapour or another liquid. The effect of surface tension is that, the liquid surface at an interface appears to act as a stretched elastic membrane. The symbol to denote surface tension is ‘σ’ and its units are N/m.

There are two types of forces that act between molecules and matter.

1. The first kind of force is directly proportional to the product of the masses of two molecules and inversely proportional to the square of the distance between their centres of masses. This force is referred to as mass attraction.
2. The second kind of force is an electro-chemical attractive force between molecules, which results in cohesion and adhesion

• Adhesion is the attractive force between molecules of solid and liquid or between molecules of two different liquids which do not mix
• Cohesive forces are the resistive forces that exist between molecules of the same substance
• The relative magnitude of cohesive and adhesive forces will decide whether the liquid will or will not wet the given solid surface
• If the adhesive forces between a liquid and a solid surface are stronger, then liquid will wet the solid surface
• If the cohesive forces among the liquid itself are stronger, then the liquid molecules will resist such adhesion, and will not wet the surface causing the liquid to form spherical beads

It is due to the phenomena of surface tension, that the liquid drops appear to be spherical.

• The cohesive force between gas molecules or between liquid molecules and gas molecules is negligible, because of greater intermolecular distances
• Presence of gas in case of liquid-gas interface has negligible effect on curvature of the surface

The pressure inside a spherical bubble is given as,  or .

Where, p = pressure inside the droplet, in excess of the atmospheric pressure,  d = diameter of the droplet and σ = surface tension of the liquid. 

The above relation shows that, the pressure increases with the decrease in the size of the drop. 

The pressure inside a liquid jet is given as, 

, where p = pressure inside the jet, in excess of the atmospheric pressure, d = diameter of the jet, σ = surface tension of the liquid and L = length of the jet.

Another important surface effect is the contact angle (θ) which appears when a liquid interface intersects with a solid surface, as illustrated in the figure below,

• If the value of θ<90^o then the liquid is said to wet the solid
• If the value of θ>90^o then the liquid is non-wetting

Capillary Rise or Capillary Depression

This is one of the most important applications of cohesion and adhesion, concerning small diameter tubes and in interstices of porous materials.

If the adhesive forces predominate, the liquid will wet the glass surface and liquid will rise in a vertical capillary tube dipped in the liquid.

If the surface tension predominates over adhesion, then the liquid does not wet the surface and will tend to depress the point of contact causing capillary depression.

The value of capillary rise or fall (Δh) can be given as, . 

• Smaller the tube radius, greater will be the capillary rise
• The curved surface of liquid in the tube is known as meniscus
• For tubes of diameter 6 mm or more, the capillary rise is negligible for water

Bulk Modulus (K)

Fluids can be compressed by applying external pressure, and they expand back to their original volume once this external pressure is removed. This indicates the elastic property of a fluid.  Due to the compressible nature, the fluid density changes with pressure. But this change in density due to pressure variations is very small in case of liquids, and due to this fact liquids are considered to be incompressible.

But the mass density of gases changes appreciably with variation in pressure, which concludes that the gases are compressible.

The compressible fluids are characterized by the property of Bulk Modulus of Elasticity (K), which is defined as the ratio of volume change under a pressure increase of Δp.

K=-V dp/dV

Negative sign indicates a decrease in volume upon increase in pressure.

The value of bulk modulus of elasticity is not constant for a fluid but necessarily depends upon pressure.

• As for gases, there is a very fine relation between pressure and temperature, therefore, the K for a gas is dependent upon temperature also.
• The units for bulk modulus are N/m²
• In case of liquids, the effect of compressibility can be neglected, however in some cases like sudden closure of valves, it must be taken into account
• In cases like air flowing in a ventilating system the gas may be treated as incompressible because the pressure variation is so small and change in air density is negligible

Vapour Pressure

All liquids show a tendency to evaporate when exposed to the atmosphere. The rate of evaporation depends upon:

• Nature of the liquid
• Temperature of the liquid
• Condition of atmosphere just above the liquid surface

Consider a closed container partially filled with a liquid and is maintained at a constant temperature. As the liquid evaporates the number of molecules in the region above the liquid start to increase. As this is happening, simultaneously a small number of molecules re-enter the liquid.

With the passage of time, the concentration of vapour molecules above the liquid surface increase, such that an equilibrium conditions exists with the help of which air above the liquid surface is saturated with vapour molecules.

The pressure exerted by the vapour molecules (or saturated vapour molecules) on the liquid surface is called vapour pressure.

• A liquid with high vapour pressure will evaporate more readily as compared to the liquid with a low value of vapour pressure
• When the vapour pressure of a liquid is slightly greater than the pressure exerted on the surface, the liquid will boil
• If the pressure at any point in the liquid approaches the vapour pressure, the liquid starts vapourising and creates bubbles or pockets of gases and vapours. This phenomenon is called cavitation.

What is difference between Cupola furnace and blast furnace?

Answer:

In essence nothing, a Cupola furnace is a type of blast furnace in that it is charged at the top and tapped at the bottom and air is blasted into the furnace via a wind belt and tuyres.
A blast furnace in a steel works is a huge structure operated for long periods of time; it is charged with iron ore, coke and limestone and reduces the iron ore into pure iron.
Cupola furnaces, as used in the foundry re melts pig iron, from the blast furnace, along with foundry scrap, steel scrap and scrap iron engine blocks and produced iron alloys of various specifications. A Cupola furnace is usually operated on a daily basis but some types can be continuously operated for several weeks

·       Cupola varies in sizes from diameter from 1 to 2 meters with height from four to five times the diameter.

·       With 1m diameter and 4m height it would melt about 2 tons of iron per hour.

·       It is operated when desired and not continuously and only when pigs are to be melted to form various irons or steel.

·       Coke fire is first lit at the bottom.

·       Then charging starts at the bottom with alternate layers of coke, pig iron and little lime stone.

·       The pigs before feed are broken into lengths of 0.25 meter and are mi8xed with agreed proportion of iron and steel scrap,

·       Adding low carbon scrap will reduce percentage of carbon in the melt.

·       After an hour or two when the charge is burnt the hot air blast is gradually increased and cupola is closed.

·       The Iron melts and sinks to the bottom from where it is tapped off into ladle or directly into moulds.

·        The amount of coke consumed is not large as iron ha just to be melt without any chemical change.

·       The coke of 50 to 125kg / ton of iron to be melted may be needed. Lime stone adds with sand at the bottom and forms a liquid slag floating above melted iron.

·       About 3 to 5 Kg of lime stone / ton of iron may be needed.

IMPACT OF MOVIES AND TV PROGRAMS ON SOCIETY

                  IMPACT OF MOVIES AND TV PROGRAMS ON SOCIETY

"I think that although movies or TV programs have some good aspects, they affect people and society more negatively. Children who spend a lot of time watching television tend to get lower grades. In addition, violent movies or TV programs make our society more aggressive. Now, I will state these reasons to support my opinion.

First of all, I state the relationship between children and TV programs. According to some surveys, children who spend much time watching the TV at home tend to get lower grades at school. They prefer to watch their favorite programs to do their home works. In particular, today, many children have their own TV set in their bedrooms. Therefore, more and more, they tend to indulge in watching the TV. To bu sure, if there are many valuable programs, we will not drag their children away from the TV. However, because there are little educational programs, it is difficult for us to allow children to sit in front of the TV for a long time.

Secondly, I state about the bad aspect of influence that movies or TV programs give to society. Although there are a wider variety of programs today, some of them are a little more violent. These programs often make people more aggressive because many viewers tend to be influenced by media. In fact, they say that some crimes were committed by young men who were inspired due to watching violent movies in the past."