Alla förståelsefrågor (ish) från boken + tentor (ENG + SV) Flashcards
Definiera interna, externa och öppna kanalflöden
Externt - Ett ej avgränsad flöde, alltså fritt flöde över en platta eller dylikt
Internt - Ett avgränsat flöde, t.ex. genom ett rör eller dylikt
Öppet - Både internt och externt. Ett flöde från ett rör som rinner ut till en flod exempelvis
Definiera kompressibel och inkompressibel vätska
Kompressibel - Densiteten varierar
Inkompressibel - Densiteten förblir ungefär konstant.
Dock kan t.ex. luft approximeras som inkompressibel (trots att den är kompressibel) beroende på tryck- och temperaturförändringar etc.
Vad innebär no-slip?
Ett antagande man kan använda sig av som innebär att en fluid förblir stilla relativt en fast yta för de fluidpaket som har kontakt med väggen. Man kan säga att fluidpaketen fastnar mot ytan pga. av vätskans viskositet.
Skillnad på klassisk och statistisk approach?
Klassisk - Makroskopisk, baseras på experiment och analyser av vätskans uppförande
Statistisk - Mikroskopisk, baseras på det genomsnittliga uppförandet av en större grupp molekyler
Definiera stationär process?
En process som förblir oförändrad över tid inom systemet. Ex. Ett jämt flöde genom ett rör
Ostationär: En ballong som blåses upp, eller att vi förbränner något
Definiera system, omgivning, avgränsning (boundary).
System - Det område vi valt att studera
Omgivning - Området utanför systemet
Avgränsning - Den verkliga/imaginära avgränsningen mellan system och omgivning
Hur definieras “mach-numret” för ett flöde?
Mach nummer: Ma = V /c = speed of flow/speed of sound, där c = 346 m/s
Ma=1: sonisk (“ljudhastighet”), Ma<1: subsonisk, Ma>1: supersonisk (“överljudshastighet”),
Ma»1 hypersonisk
Hur förändras den dynamiska viskositeten för vätskor och gaser vid varierande temperatur?
Vätskor: Minskar vid ökande temperatur
Gaser: Ökar vid ökande temperatur
Påminnelse: Tänk bilmotor på vintern. Svår att starta pga. att oljan har högre viskositet vid låg temperatur.
Hur förändras den kinematiska viskositeten för vätskor och gaser vid varierande temperatur?
Vätskor: Minskar vid ökande temperatur
Gaser: Ökar vid ökande temperatur
Skillnad mellan mätar-tryck och absolut-tryck?
Mätar-tryck: Trycket relativt atmosfärstrycket
Absolut-trycket: Trycket relativt vacuumtryck
Två identiska fläktar, en vid havsnivå och en på toppen av ett berg. Jämför volymflöde och massflöde.
Luftens densitet är högre vid havsnivå, vilket inte kommer påverka volymflödet, men massflödet kommer vara högre vid havsnivån.
Definiera resulterande hydrostatiskt kraft som utövas på en nedsänkt yta, samt tryckets centrum.
Den resulterande hydrostatiska kraften som utövas på en nedsänkt yta är resultanten av alla tryckkrafter som utövas på ytan. Den resulterande kraftens angreppspunkt kallas tryckets centrum och den ligger generellt inte i mitten på kroppen pga. det hydrostatiska tryckets variation.
Varför är dammar tjockare i botten?
Pga. att tryckkrafterna ökar med djupet.
Vad innebär flytkraft?
Den uppåtriktade kraft som en fluid utövar på en nedsänkt kropp kallas flytkraft. Den beror på kommer från tryckökningen som bildas vid större djup.
F B= p fg V
Om F B> W, så flyter kroppen.
Påverkas flytkraften av djupet och kroppens densitet?
Oberoende av djupet och densitet (material spelar ingen roll), endast volymen påverkar.
Anta stationärt adiabatiskt flöde av en inkompressibel vätska. Om temperaturen är konstant under flödet, är det rätt att säga att friktionen är försumbar?
Ja, eftersom irreversibiliteter, som friktion, skulle öka entropin och temperaturen.
Vad är kinetisk energi korrektionsfaktor?
Den kompenserar för felet som uppstår då vi räknar med medelhastighet. Är ofta försumbar vid turbulenta flöden, men kan ha påverkan vid laminära flöden.
Rek. anv. α = 1, 05 vid fullt turbulent och α = 2.0 .
Kan göra stor skillnad om hastigheterna är
höga, annars kan den ofta försummas vid turbulenta flöden åtminstone.
Why are liquids usually transported in circular pipes?
Liquids are usually transported in circular pipes because pipes with a circular cross section can withstand large pressure differences between the inside and the outside without undergoing any significant distortion.
What is the physical significance of the Reynolds number?
Reynolds number is the ratio of the inertial forces to viscous forces, and it serves as a criterion for determining the flow regime. At large Reynolds numbers, for example, the flow is turbulent since the inertia forces are large relative to the viscous forces, and thus the viscous forces cannot prevent the random and rapid fluctuations of the
fluid.
How is the hydrodynamic entry length defined for flow in a pipe? Is the entry length longer in laminar or turbulent flow?
The region from the tube inlet to the point at which the boundary layer merges at the centerline is called the hydrodynamic entrance region, and the length of this region is called hydrodynamic entry length. The entry length is much longer in laminar flow than it is in turbulent flow. But at very low Reynolds numbers, L h is very small (e.g., L h = 1.2D at Re = 20).
Consider laminar flow in a circular pipe. Will the wall shear stress t w be higher near the inlet of the pipe or near the exit? Why? What would your response be if the flow were turbulent?
The wall shear stress τ w is highest at the tube inlet where the thickness of the boundary layer is nearly zero, and decreases gradually to the fully developed value. The same is true for turbulent flow.
What is the physical mechanism that causes the friction factor to be higher in turbulent flow?
In turbulent flow, it is the turbulent eddies due to enhanced mixing that cause the friction factor to be larger. This turbulent mixing leads to a much larger wall shear stress, which translates into larger friction factor.
Consider laminar flow of air in a circular pipe with perfectly smooth surfaces. Do you think the friction factor for this flow will be zero? Explain.
During laminar flow of air in a circular pipe with perfectly smooth surfaces, the friction factor is not zero because of the no-slip boundary condition, which must hold even for perfectly smooth surfaces.
Explain why the friction factor is independent of the Reynolds number at very large Reynolds numbers.
At very large Reynolds numbers, the flow is fully rough and the friction factor is independent of the Reynolds number. This is because the thickness of viscous sublayer decreases with increasing Reynolds number, and it be comes so thin that the surface roughness protrudes into the flow. The viscous effects in this case are produced in the main flow primarily by the protruding roughness elements, and the contribution of the viscous sublayer is negligible.
Define equivalent length for minor loss in pipe flow. How is it related to the minor loss coefficient?
Equivalent length is the length of a straight pipe which would give the same head loss as the minor loss component.
Which has a greater minor loss coefficient during pipe flow: gradual expansion or gradual contraction? Why?
A gradual expansion, in general, has a greater minor loss coefficient than a gradual contraction in pipe flow. This is due to the adverse pressure gradient in the boundary layer, which may lead to flow separation.
A piping system involves sharp turns, and thus large minor head losses. One way of reducing the head loss is to replace the sharp turns by circular elbows. What is another way?
Another way of reducing the head loss associated with turns is to install turning vanes inside the elbows. There are many other possible answers, such as: reduce the inside wall roughness of the pipe, use a larger diameter pipe, shorten the length of pipe as much as possible, etc.
Explain when an external flow is two-dimensional, three-dimensional, and axisymmetric. What type of flow is the flow of air over a car?
The flow over a body is said to be two-dimensional when the body is very long and of constant cross-section, and the flow is normal to the body (such as the wind blowing over a long pipe perpendicular to its axis). There is no significant flow along the axis of the body. The flow along a body that possesses symmetry along an axis in the flow direction is said to be axisymmetric (such as a bullet piercing through air). Flow over a body that cannot be modeled as two-dimensional or axisymmetric is three-dimensional. The flow over a car is three-dimensional.
What is the difference between streamlined and blunt bodies? Is a tennis ball a streamlined or blunt body?
A body is said to be streamlined if a conscious effort is made to align its shape with the anticipated streamlines in the flow. Otherwise, a body tends to block the flow, and is said to be blunt. A tennis ball is a blunt body (unless the velocity is very low and we have “creeping flow”).
What is drag? What causes it? Why do we usually try to minimize it?
The force a flowing fluid exerts on a body in the flow direction is called drag. Drag is caused by friction between the fluid and the solid surface, and the pressure difference between the front and back of the body. We try to minimize drag in order to reduce fuel consumption in vehicles, improve safety and durability of structures subjected to high winds, and to reduce noise and vibration.
What is lift? What causes it? Does wall shear contribute to the lift?
The force a flowing fluid exerts on a body in the normal direction to flow that tends to move the body in that direction is called lift. It is caused by the components of the pressure and wall shear forces in the direction normal to the flow. The wall shear contributes to lift (unless the body is very slim), but its contribution is usually small.
What is terminal velocity? How is it determined?
The maximum velocity a free falling body can attain is called the terminal velocity. It is determined by setting the weight of the body equal to the drag and buoyancy forces, W = F D + F B.
What is the difference between skin friction drag and pressure drag? Which is usually more significant for slender bodies such as airfoils?
The part of drag that is due directly to wall shear stress τw is called the skin friction drag F D, friction since it is caused by frictional effects, and the part that is due directly to pressure P and depends strongly on the shape of the body is called the pressure drag F D, pressure. For slender bodies such as airfoils, the friction drag is usually more significant.
What is flow separation? What causes it? What is the effect of flow separation on the drag coefficient?
At sufficiently high velocities, the fluid stream detaches itself from the surface of the body. This is called separation. It is caused by a fluid flowing over a curved surface at a high velocity (or technically, by adverse pressure gradient). Separation increases the drag coefficient drastically.
What is drafting? How does it affect the drag coefficient of the drafted body?
Drafting is when a moving body follows another moving body by staying close behind in order to reduce drag. It reduces the pressure drag and thus the drag coefficient for the drafted body by taking advantage of the low pressure wake region of the moving body in front.
What is the more common term for an energyproducing turbomachine? How about an energy-absorbing turbomachine? Explain this terminology. In particular, from which frame of reference are these terms defined—that of the fluid or that of the surroundings?
A more common term for an energy producing turbomachine is a turbine. Turbines extract energy from the moving fluid, and convert that energy into useful mechanical energy in the surroundings, usually in the form of a rotating shaft. Thus, the phrase “energy producing” is from a frame of reference of the fluid – the fluid loses energy as it drives the turbine, producing energy to the surroundings. On the other hand, a more common term for an energy absorbing turbomachine is a pump. Pumps absorb mechanical energy from the surroundings, usually in the form of a rotating shaft, and increase the energy of the moving fluid. Thus, the phrase “energy absorbing” is from a frame of reference of the fluid – the fluid gains or absorbs energy as it flows through the pump.
What are the primary differences between fans,
blowers, and compressors? Discuss in terms of pressure rise and volume flow rate.
A fan is a gas pump with relatively low pressure rise and high flow rate. A blower is a gas pump with relatively moderate to high pressure rise and moderate to high flow rate. A compressor is a gas pump designed to deliver a very high pressure rise, typically at low to moderate flow rates.
Explain why it is usually not wise to arrange two (or more) dissimilar pumps in series or in parallel.
Arranging dissimilar pumps in series can create problems because the volume flow rate through each pump must be the same, but the overall pressure rise is equal to the pressure rise of one pump plus that of the other. If the pumps have widely different performance curves, the smaller pump may be forced to operate beyond its free delivery flow rate, whereupon it acts like a head loss, reducing the total volume flow rate. Arranging dissimilar pumps in parallel can create
problems because the overall pressure rise must be the same, but the net volume flow rate is the sum of that through each branch. If the pumps are not sized properly, the smaller pump may not be able to handle the large head imposed on it, and the flow in its branch could actually be reversed; this would inadvertently reduce the overall pressure rise. In either case, the power supplied to the smaller pump would be wasted.
Consider a typical centrifugal liquid pump. For each statement, choose whether the statement is true or false, and discuss your answer briefly.
(a) V flow at the pump’s free delivery is greater than V flow
at its best efficiency point.
(b) At the pump’s shutoff head, the pump efficiency is zero.
(c) At the pump’s best efficiency point, its net head is at its
maximum value.
(d) At the pump’s free delivery, the pump efficiency is zero.
(a) True: The maximum volume flow rate occurs when the net head is zero, and this “free delivery” flow rate is typically
much higher than that at the BEP.
(b) True: By definition, there is no flow rate at the shutoff head. Thus the pump is not doing any useful work, and the
efficiency must be zero.
(c) False: Actually, the net head is typically greatest near the shutoff head, at zero volume flow rate, not near the BEP.
(d) True: By definition, there is no head at the pump’s free delivery. Thus, the pump is working against no “resistance”,
and is therefore not doing any useful work, and the efficiency must be zero.
What are the mechanisms of heat transfer? How are they distinguished from each other?
The mechanisms of heat transfer are conduction, convection and radiation. Conduction is the transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles. Convection is the mode of energy transfer between a solid surface and the adjacent liquid or gas which is in motion, and it involves combined effects of conduction and fluid motion. Radiation is energy emitted by matter in the form of electromagnetic waves (or photons) as a result of the changes in the electronic configurations of the atoms or molecules.
What is the physical mechanism of heat conduction in a solid, a liquid, and a gas?
In solids, conduction is due to the combination of the vibrations of the molecules in a lattice and the energy transport by free electrons. In gases and liquids, it is due to the collisions of the molecules during their random motion.
Consider heat transfer through a windowless wall of a house in a winter day. Discuss the parameters that affect the rate of heat conduction through the wall.
The parameters that effect the rate of heat conduction through a windowless wall are the geometry and surface area of wall, its thickness, the material of the wall, and the temperature difference across the wall.
Define emissivity and absorptivity. What is Kirch- hoff’s law of radiation?
Emissivity is the ratio of the radiation emitted by a surface to the radiation emitted by a blackbody at the same temperature. Absorptivity is the fraction of radiation incident on a surface that is absorbed by the surface. The Kirchhoff’s law of radiation states that the emissivity and the absorptivity of a surface are equal at the same temperature and wavelength.
What is a blackbody? How do real bodies differ from blackbodies?
A blackbody is an idealized body which emits the maximum amount of radiation at a given temperature and which absorbs all the radiation incident on it. Real bodies emit and absorb less radiation than a blackbody at the same temperature.
How do the thermal conductivity of gases and liquids vary with temperature?
The thermal conductivity of gases is proportional to the square root of absolute temperature. The thermal conductivity of most liquids, however, decreases with increasing temperature, with water being a notable exception.
Consider heat conduction through a plane wall. Does the energy content of the wall change during steady heat conduction? How about during transient conduction? Explain.
In steady heat conduction, the rate of heat transfer into the wall is equal to the rate of heat transfer out of it. Also, the temperature at any point in the wall remains constant. Therefore, the energy content of the wall does not change during steady heat conduction. However, the temperature along the wall and thus the energy content of the wall will change during transient conduction.
Consider heat conduction through a wall of thickness L and area A. Under what conditions will the temperature distri- butions in the wall be a straight line?
The temperature distribution in a plane wall will be a straight line during steady and one dimensional heat transfer with constant wall thermal conductivity.
What does the thermal resistance of a medium represent?
The thermal resistance of a medium represents the resistance of that medium against heat transfer.
Consider a window glass consisting of two 4-mmthick glass sheets pressed tightly against each other. Compare the heat transfer rate through this window with that of one con- sisting of a single 8-mm-thick glass sheet under identical conditions.
The window glass which consists of two 4 mm thick glass sheets pressed tightly against each other will probably have thermal contact resistance which serves as an additional thermal resistance to heat transfer through window, and thus the heat transfer rate will be smaller relative to the one which consists of a single 8 mm thick glass sheet.
Consider steady heat transfer through the wall of a room in winter. The convection heat transfer coefficient at the outer surface of the wall is three times that of the inner surface as a result of the winds. On which surface of the wall do you think the temperature will be closer to the surrounding air tem- perature? Explain.
Convection heat transfer through the wall is expressed as Q = hAs ( Ts − T∞ ) . In steady heat transfer, heat transfer rate to the wall and from the wall are equal. Therefore at the outer surface which has convection heat transfer coefficient three times that of the inner surface will experience three times smaller temperature drop compared to the inner surface. Therefore, at the outer surface, the temperature will be closer to the surrounding air temperature.
Consider two cold canned drinks, one wrapped in a blanket and the other placed on a table in the same room. Which drink will warm up faster?
The blanket will introduce additional resistance to heat transfer and slow down the heat gain of the drink wrapped in a blanket. Therefore, the drink left on a table will warm up faster.
What is thermal contact resistance? How is it related to thermal contact conductance?
The resistance that an interface offers to heat transfer per unit interface area is called thermal contact resistance, Rc . The inverse of thermal contact resistance is called the thermal contact conductance.
