Group Study for Mechanical & Automobile Engineering | Check Your Knowledge | Viva

Replies

• 

  Harshad Italiya

  Re: Group Study for Mechanical & Automobile Engineering | Check Your Knowledge | Viv
  
  

  ~Reserved for the Questions~​

  
  3rd Set of Questions:-
  
  11. What is the effect of surface roughness on the friction drag coefficient in laminar and turbulent flows?
  
  12.Is temperature a scalar/vector quantity? Explain your answer.
  
  13.Is heat transfer a scalar/vector quantity? Explain your answer.
  
  14. A person forgets a cold drink in a cylindrical bottle on a table. How will you analyze that problem? In other words, will you analyze it to be 1D, 2D or 3D analysis?
  
  15.What are the various examples of heat generation in solid which you can point out?
  
  16. In context of heat transfer, what is the difference between isotropic and unisotropic material?
  
  PS:- Questions are Updated !! Date:- 11/05/2011
• 

  Harshad Italiya

  Re: Group Study for Mechanical & Automobile Engineering | Check Your Knowledge | Viv
  
  

  Score Sheet
  
  Crazy_gaurav:- 8
  CivilPrincess:- 11
  ​
• 

  Harshad Italiya

  Re: Group Study for Mechanical & Automobile Engineering | Check Your Knowledge | Viv
  
  Hall of Fame
  
  1.A person weighs himself on a waterproof spring scale placed at the bottom of a 1m deep swimming pool. Will the person weigh less or more in water? Why?
  
  2.Consider a fluid whose volume does not change with temperature at constant pressure. What can you say about natural convection heat transfer in this medium?
  
  3.What does Grashof's number represent physically? How does it differ from Reynold's number?
  
  4.In which type is the heat transfer coefficient high? Natural or Forced? Why?
  
  5.Where will the hull of ship sink deeper? -> In fresh water or Salt water?
  
  6.If we have a hot egg kept in a plate. Where will it cool faster?-> In outer space
  (Universe) or Earth?
  
  7. What is the difference between upstream and free stream velocity? For what types of flow are these two velocities equal to each other?
  
  8. What is the difference between streamlined and blunt bodies? Is tennis ball streamlined or blunt body?
  
  9. What is ‘drag’? What causes it? Why do we usually try to minimize it?
  
  10. What is ‘Lift’? What causes it? Does wall shear contribute to lift?
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  crazy_gaurav

  1) less because weight as measured by the spring scale is gravitational force applied by person mass on it, but when one weights oneself in water there is force of buoyancy opposing this force of gravity.
  4) forced because in case of forced convection velocity is higher than that imparted by difference in density as in free convection and this high velocity ensures more amount of air coming in contact with object and hence more matter available to object to dump/gain heat.
  5) in fresh water as density of fresh water is less than that of salt water and hence more amount of water is needed to b displaced before buoyancy force (weight of liquid displaced) can equal the weight of ship and this calls for more height to be submerged under water.
  6)in earth as in earth heat transfer(HT) will be composed of conduction to plate, convection and radiation both whereas in outer space due to absence of air medium HT is reduced to only conduction and radiation
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  Harshad Italiya

  Good job Gaurav !
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  CIVILPRINCESS

  1. he'll weigh less. because the weight of an object under water is lesser due to the buoyant forces.
  2. transfer of heat is constant.
  3.it is a coefficient which represents the heat transfer by convention. the reynold's number is a coefficient which classifies the flow regimes like laminar or turbulent.
  4. forced
  5. in fresh water. since the density is lesser
  6.earth i guess. because it needs a medium to transfer it's heat.
  
  it was a nice test. but i've guessed some questions 😉
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  Harshad Italiya

  CP~
  Good job there now you can give away marks to Gaurav too. Please do that. 😀
• 

  ISHAN TOPRE

  Good going people, so here are my answers.
  
  1.Reason The weight always remains same anywhere we go. Whether we are in water or air or universe. But Measurement varies. There will be an opposite force in upward direction which is called the force of buoyancy. So the,
  
  Weight measured by spring scale = (Actual weight)-(Force of Buoyancy).
  
  My answer: Hence the spring balance will measure less weight.
  
  2. Reason Heat transfer rate changes with change in volume and shape of material. But as the volume remains constant, the assuming that there is no change in shape or any such parameter at constant pressure
  
  My answer:The heat transfer rate will remain constant in this medium
  
  3. Grashof’s number is physically ( Buoyant force/ viscous force)* Reynolds number.
  It is physically significant in case of Natural convection while, Reynolds number is used to determine whether the flow is turbulent or not.
  
  4. ReasonForced convection will be having high heat transfer coefficient. Because with air ‘moving’ over a hot surface, there will not be any stagnant air film which will avoid that heat transfer.
  
  5. Reason:The hull of the ship will experience a greater buoyant force in case of Salty (Sea water) which has a density of around 1035 Kg/m3 while fresh water has density of about 1000Kg/m3.
  My answer: So The hull will sink deeper in case of fresh water.
  
  6. Reason:The egg will get colder in case if it is kept on earth, because it requires a medium for heat transfer. However this answer is valid only if we neglect the temperature of Universe outside which will instantly burn that egg.
  My answer:So to make egg colder we need to place it on Earth.
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  ISHAN TOPRE

  Crazy_gaurav: 2+0+0+2+2+2=8 Nice reasons Gaurav, Hope to see you on Forum quite often. 😀
  
  CP: 2+2+2+1+2+2=11 You forgot to mention the reason but yes it was simple may be you knew it. Practically you can get 12 but for you forgot. 😀
  
  Me?? :razz:
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  Harshad Italiya

  #-Link-Snipped-# & CP:- You can rate Ishu's Answer. 😀 He's already answered so you can just judge it and offer your rating.
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  Harshad Italiya

  I have updated the Score Sheet.
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  Harshad Italiya

  Question List is Updated. 😀
• 

  Ramani Aswath

  Instructions to readers:
  Please do not rate answers from this poster. He is responding for the fun of it. Bioramani
  
  
  1.A person weighs himself on a waterproof spring scale placed at the bottom of a 1m deep swimming pool. Will the person weigh less or more in water? Why?
  
  Ans.: Weight in water will be less.
  The average density of human being (Most mammals) is about 1.01 kg/litre, just 1% more than water. In 1m deep water an adult will have the legs almost completely immersed with the torso out. Weight of the immersed portion, which is about 1/3 of the whole body weight, will be almost totally cancelled by buoyancy. The net weight shown should be near 2/3 weight on land.
  
  
  2. Consider a fluid whose volume does not change with temperature at constant pressure. What can you say about natural convection heat transfer in this medium?
  
  Ans.: Natural Convection will be zero. Heat transfer will be by conduction as in a solid. It is assumed that radiation is not present. The Grashof's number(See q.:3 below) controls natural convection It has the coefficient of thermal expansion in the numerator. Since this is given as zero in this case, thermal siphon effect will be absent.
  
  3.What does Grashof's number represent physically? How does it differ from Reynold's number?
  
  Ans.: Grashof's number represents convection in heat transfer. (See above) This takes into account the thermal siphoning set up by the difference in density created by temperature difference and also the Reynold’s number for the flow pattern.
  
  4.In which type is the heat transfer coefficient high? Natural or Forced? Why?
  
  Ans.: Forced convection coefficients are higher.
  
  In any solid-fluid interface there is a stationary layer of fluid through which heat transfer takes place essentially by conduction. The thickness of this layer controls the heat transfer coefficient. In forced convection this layer gets thinned quite a bit and is not constant over the whole surface because of turbulence. Generally the average thickness of this stationary layer in forced convection is much less than in natural convection. Hence, the heat transfer is also proportionately higher.
  
  5.Where will the hull of ship sink deeper? -> In fresh water or Salt water?
  
  Ans.: Sinks more in fresh water.
  Archimedes principle states that the weight of the displaced liquid equals the weight of the floating object. The volume of fresh water will be more than the volume of sea water of the same weight as its density is lower. This implies that the hull will sink to a greater depth in fresh water.
  
  6.If we have a hot egg kept in a plate. Where will it cool faster?-> In outer space
  (Universe) or Earth?
  
  Ans.: In space. The space temperature is taken as absolute zero. Radiant cooling will be maximum. An ordinary egg may explode instantly because of the (near) absolute vacuum before any cooling starts.
  
  
  Bioramani
  
  My scoring of the other answers will be in another post
• 

  sureshkumargce

  Re: Group Study for Mechanical & Automobile Engineering | Check Your Knowledge | Viv
  
  Q.NO. 9 & 10:
  ans: If we placed one object at the middle of stream flow that object will be attacked by two forces at the direction of stream flow. One is downward and another one is upward.
  
  downward force is called "Drag" & upward force is called "Lift"
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  ISHAN TOPRE

  Very good Suresh. Keep up. 😀
  
  Reserved for my answers. 😀
• 

  Ramani Aswath

  As before please do not grade these answers
  Second Set
  7. What is the difference between upstream and free stream velocity? For what types of flow are these two velocities equal to each other?
  Ans.: When a fluid flows, the velocity of the fluid that remains stable over a zone away from all restraining walls is the free stream velocity. Upstream velocity is the velocity of the fluid ahead of any obstruction (orifice plate, weir, venturi or valve). In any real fluid flow the velocity ahead of an obstruction gets modified and reduced. However, if the fluid is perfect (Zero viscosity and zero compressibility) and the obstruction is a thin symmetric air foil oriented parallel to the streamline (Drag is zero in such cases) and the velocity is at a low Reynolds number, the upstream velocity is likely to be not too different from the free stream velocity.
  (This is just an educated (?) guess)
  
  
  8. What is the difference between streamlined and blunt bodies? Is tennis ball streamlined or blunt body?
  Ans.:A stream lined body has a shape, which when oriented in a fluid stream parallel to the flow direction offers minimum drag and creates minimum flow disturbance. A tennis ball (or any other spherical shape) is blunt and not streamlined. When moving in air such objects create turbulence.
  
  9. What is ‘drag’? What causes it? Why do we usually try to minimize it?
  Ans.: Drag is the resistance offered by any object located in a flowing fluid stream because of the shear forces operating on the interface area. This is a force parallel to the direction of flow of the fluid. Drag wastes energy. In transportation vehicles this leads to excess fuel consumption. Streamlining the outer shell minimizes drag loss and improves performance.
  
  
  10. What is ‘Lift’? What causes it? Does wall shear contribute to lift?
  Ans.: Lift is a force arising out of a surface (such as an airplane wing) held at an angle (angle of attack) to the direction of flow of the fluid. This force is perpendicular to the direction of fluid flow. During cruising conditions, this is essentially a Bernoulli lift. During landing and takeoff this is probably more a Newtonian reaction lift. Both operate to greater or lesser extent all the time. Wall shear contributes to Bernoulli lift.
  
  11. What is the effect of surface roughness on the friction drag coefficient in laminar and turbulent flows?
  Ans.: To the extent the surface area increases, drag in laminar flow will increase. Depending on the sharpness of the asperities the effect could be more or less in turbulent flow. (I am not quite sure about this though)
  Bioramani
• 

  Harshad Italiya

  @Ramani Sir ~
  Nice Answers again.
• 

  hansmukh jain

  Re: Group Study for Mechanical & Automobile Engineering | Check Your Knowledge | Viv
  
  10. lift is also known as a elevator. it is a device which is used to transfer weight up to down ann vice versa.
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  Ramani Aswath

  Re: Group Study for Mechanical & Automobile Engineering | Check Your Knowledge | Viv
  
  

  hansmukh jain

  10. lift is also known as a elevator. it is a device which is used to transfer weight up to down ann vice versa.

  Likewise drag is what you do to your heavy baggage after a tiresome journey.
  Bioramani
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  ISHAN TOPRE

  hello Hasmukh,
  
  Please be serious this is a group study. you are technically correct but your answer does not seem to be correct to reference. 😀
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  Harshad Italiya

  3rd Set is Posted:- #-Link-Snipped-#
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  Harshad Italiya

  You can post your answers. 😁
• 

  Ramani Aswath

  Re: Group Study for Mechanical & Automobile Engineering | Check Your Knowledge | Viv
  
  As before please do not grade these answers.
  
  11. What is the effect of surface roughness on the friction drag coefficient in laminar and turbulent flows?
  
  Ans.: This issue is of great relevance in airfoils (Planes, wind turbines and such) and to some extent in propellers of ships and agitators as well. In air foils there is a zone where the flow separation starts. This is the point at which the streamline tends to break and moves away from attachment to the wall of the airfoil. This leads to turbulence. As a consequence there is a reduction in lift and increase in drag.
  However, this is not very simple to predict. Depending on the roughness value, its orientation and the flow conditions, the effect on drag and lift can vary. There was even a demo by an aerospace giant that fixing a designed pattern of roughness (sort of parallel grooves a few microns in size) reduced fuel consumption. (This was ages back and I am not able to locate the reference.)
  
  This paper reports a study of the effect of roughness on wind turbine performance:
  
  #-Link-Snipped-#
  
  This one is a study on the same subject on air foils:
  
  #-Link-Snipped-#
  
  12.Is temperature a scalar/vector quantity? Explain your answer.
  
  Ans.: Temperature is defined as a measure of the average kinetic energy of the molecules in a body of matter. There is no direction associated with such a quantity. So, temperature is scalar.
  
  13. Is heat transfer a scalar/vector quantity? Explain your answer.
  
  Ans.: The question may need some rewording. Heat transfer is a mechanism, a process. It has neither dimension nor direction, except that the Second Law of Thermodynamics stipulates that heat can flow spontaneously only from a higher temperature to a lower temperature.
  
  Heat Transfer Coefficient does have a value. It has no direction except that it requires a temperature gradient to happen. It is scalar.
  
  14. A person forgets a cold drink in a cylindrical bottle on a table. How will you analyze that problem? In other words, will you analyze it to be 1D, 2D or 3D analysis?
  Ans.: If the L/D ratio of the bottle is large, it may be possible to approximate it as a 1D problem.
  
  If the L/D is small, there are two possibilities. The ambient may be still air. The warming will vary from top to bottom of the glass. Since warm fluid is lighter than the cold, there will be no convection currents in the drink. However, there will be convection currents in the air surrounding the glass. A 2D model will fit this situation. If the glass is standing in a stream of air blowing across it, the air flow pattern around the glass will determine the local warming rates and a 3D model will be needed.
  
  (Tongue in cheek) If the air flow is unsteady in direction and speed, then a 4D model including time variation may be needed.
  
  15. What are the various examples of heat generation in solid which you can point out?
  
  Ans.: Ordinarily there is no self generation of heat in a solid. If the solid has an ohmic resistance and a voltage is applied across the material it will become hot due to the wattage delivered into it.
  
  If the material is combustible and is ignited the surface in contact with air will burn and produce heat at the surface.
  
  Elastic materials like rubber behave rather oddly. If a rubber band is stretched it heats up. This is because the act of stretching aligns the tangled long molecules. The resulting ordering releases heat as a consequence of the second law of thermodynamics. What is interesting is that if a rubber band is stretched at room temperature and held stretched till the heat dissipates and the band returns to room temperature, releasing the band to regain the original condition causes the aligned molecules to tangle again. The band will cool now. This is the reverse of what one expects. Expansion should cool and compression should heat.
  
  Why do rubber bands contract when heated instead of expanding?
  
  16. In context of heat transfer, what is the difference between isotropic and anisotropic material?
  
  Ans.: An isotropic material has the same behaviour in all directions. On the other hand, an anisotropic material will show variation in properties in different directions. Extruded polyethylene foam is a commonly used insulation and shock absorbing material, which is anisotropic. It has a different structure parallel and perpendicular to the direction of extrusion. In terms of heat transmission such lamellar materials can have variation in thermal conductivity parallel and perpendicular to the lamellae.
  
  Here is a paper on this material though it does not cover heat transfer:
  
  #-Link-Snipped-#
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  Harshad Italiya

  Superb show again by Ramani sir,
  
  Guys where are you ? Please share your knowledge too. 😀
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  ISHAN TOPRE

  Bioramani sir,
  
  Superb job 👍 . I just want to discuss a few minor queries.
  
  Q14: L/D ratio larger or greater that what? is the value compared to '1'? L/D<1 or>1?
  
  Q15: There are certain examples of heat generation in solids. Ripening of fruits, then radioactive materials etc.