Fluid Mechanics interviewquestions and answers

1. Why the Centrifugal Pump is called High Discharge pump?
Centrifugal pump is a kinetic device. The centrifugal pump uses the centrifugal force to push out the fluid. So the liquid entering the pump receives kinetic energy from the rotating impeller. The centrifugal action of the impeller accelerates the liquid to a high velocity, transferring mechanical (rotational) energy to the liquid. So it discharges the liquid in high rate. It is given in the following
formulae:
Centrifugal force F= (M*V2)/R.
Where, M-Mass;  V-Velocity; R-Radius

2. How Cavitation can be eliminated by Pump?
Cavitation means bubbles are forming in the liquid. ·
To avoid Cavitation, we have to increase the Pump size to One or Two Inch;
To increase the pressure of the Suction Head, or  Decrease the Pump Speed.

3. Why Cavitation will occur in Centrifugal Pump and not in Displacement Pump?
The formation of cavities (or bubbles) is induced by flow separation, or non-uniform flow velocities, inside a pump casing. In centrifugal pumps the eye of the pump impeller is smaller than the flow area of pipe. This decrease in flow area of pump results in increase in flow rate. So pressure drop happened between pump suction and the vanes of the impeller. Here air bubbles or cavities are formed because of liquid vapour due to increase in temperature in impeller. This air bubbles are transmitted to pump which forms cavitation.

4. Which Pump is more Efficient Centrifugal Pump or Reciprocating Pump?

Centrifugal pump. Because flow rate is higher compared to reciprocating pump. Flow is smooth and it requires less space to install. Lower initial cost and lower maintenance cost.

5. Why Centrifugal Pump is not called as a Positive Displacement Type of Pump?

The centrifugal has varying flow depending on pressure or head, whereas the Positive Displacement pump has more or less constant flow regardless of pressure.
Likewise viscosity is constant for positive displacement pump where centrifugal pump have up and down value because the higher viscosity liquids fill the clearances of the pump causing a higher volumetric efficiency. When there is a viscosity change in supply there is also greater loss in the system. This means change in pump flow affected by the pressure change. One more example is, positive displacement pump has more or less constant efficiency, where centrifugal pump has varying efficiency rate.

6. How Cavitation can be eliminated in a Pump?
Cavitation means bubbles are forming in the liquid.
· To avoid Cavitation, we have to increase the Pump size to One or Two Inch;
· To increase the pressure of the Suction Head, or
· Decrease the Pump Speed.

7. Which pump is more efficient Centrifugal pump or Reciprocating pump?
Centrifugal pump. Because flow rate is higher compared to reciprocating pump. Flow is smooth and it requires less space to install. Lower initial cost and lower maintenance cost.

8. Why Centrifugal Pump is not called as a Positive Displacement Type of Pump?
The centrifugal has varying flow depending on pressure or head, whereas the Positive Displacement pump has more or less constant flow regardless of pressure.  Likewise viscosity is constant for positive displacement pump where centrifugal pump have up and down value because the higher viscosity liquids fill the clearances of the pump causing a higher volumetric efficiency. When there is a viscosity  change in supply there is also greater loss in the system. This means change in pump flow affected by the pressure change. One more example is, positive displacement pump has more or less constant efficiency, where centrifugal pump has varying efficiency rate.

9. What is a radial-flow turbine?
In a radial-flow turbine, steam flows outward from the shaft to the casing. The unit is usually a reaction unit, having both fixed and moving blades.

10. What are four types of turbine seals?
Carbon rings fitted in segments around the shaft and held together by garter or retainer springs. Labyrinth mated with shaft serration’s or shaft seal strips. Water seals where a shaft runner acts as a pump to create a ring of water around the shaft. Use only treated water to avoid shaft pitting. Stuffing box using woven or soft packing rings that are compressed with a gland to prevent leakage along the shaft.

11. What are two types of clearance in a turbine?
Radial – clearance at the tips of the rotor and casing.
Axial – the fore-and-aft clearance, at the sides of the rotor and the casing.

12. What is the function of a thrust bearing?
Thrust bearings keep the rotor in its correct axial position.

13. What is a stage in a steam turbine?  In an impulse turbine, the stage is a set of moving blades behind the nozzle. In a reaction turbine, each row of blades is called a "stage." A single Curtis stage may consist of two or more rows of moving blades.

14. What is a diaphragm?
Partitions between pressure stages in a turbine’s casing are called diaphragms. They hold the vane- shaped nozzles and seals between the stages. Usually labyrinth-type seals are used. One-half of the diaphragm is fitted into the top of the casing, the other half into the bottom.

15. What are the two basic types of steam turbines?
Impulse type. Reaction type.

16. What are topping and superposed turbines?Topping and superposed turbines arc high- pressure, non-condensing units that can be added to an older, moderate-pressure plant. Topping turbines receive high-pressure steam from new high-pressure boilers. The exhaust steam of the new turbine has the same pressure as the old boilers and is used to supply the old turbines.

17. What is a combination thrust and radial bearing?
This unit has the ends of the Babbitt bearing extended radically over the end of the shell. Collars on the rotor face these thrust pads, and the journal is supported in the bearing between the thrust collars.

How to study fluid mechanics and mathematics?

How to study fluid mechanics and mathematics?

From my side, I will add this.  Fluid mechanics is a subject that you will be using in your career long after you have passed AeSI, so I suggest that spend maximum time in understanding the concepts . Relate them to your life and make them spring out of the text book . Source sink, fluid dynamics, viscosity, Reynolds's number are the core pillars of further studies. So while learning understand them well, use your imagination to connect them to your life. Study with question answer style for maximum retention.

While reading the material, write down small quiz questions on the topic you are reading. They will
be used to revise the concepts latter. My tuition teacher used this approach. While going through  the topics he made us write down trivial questions like what is source? Give examples of sink? What is the most viscous materials you have seen? etc. That time we felt, the questions were waste of time, but latter I realized how useful they were for revising the concepts.

For every study session. Spend ten minutes revising what you have already studied. If you have formed those small questions then use them to jog your memory. This ten minutes jogging session not only reinforces the previous studied material but help you begin the next session with a  positive momentum.

This three concepts can be applied to mathematics and the other subjects. Just remember, that if you can setup a simple process for your study and follow it consistently, that is more than sufficient to help you tackle any subject that AeSI throws at you.

Good luck!!
If you have some specific tip, do chip in and comment!!

New method for propulsion in fluids

Researchers discover a way for temperature gradients in fluids to move objects.

David L. Chandler | MIT News Office

Researchers at MIT have discovered a new way of harnessing temperature gradients in fluids to propel objects. In the natural world, the mechanism may influence the motion of icebergs floating on the sea and rocks moving through subterranean magma chambers.
The discovery is reported this week in the journal Physical Review Letters by associate professor of mechanical engineering Thomas Peacock and four others. The finding was an unexpected outcome of research on other effects of temperature differences, such as the way winds form over glaciers in a valley, Peacock says.

These winds are generated by natural convection that arises from temperature differences between a fluid and a heated or cooled boundary. “People had only ever studied this phenomenon in relation to a fixed object,” Peacock says. But his group realized that “if you can induce these kinds of flows on the
boundaries of a floating object, you can generate forces.”
Peacock’s first study of the concept, about four years ago, focused on slow flows caused by diffusion — work that demonstrated that induced
boundary flows can generate small propulsive forces. But diffusion is a very gradual process, he says, and the resulting forces are perhaps too
small to be exploited.
“I always thought, and expected, that the equivalent flows you could generate by selective heating and
cooling of an object could be more significant,”
Peacock says.
But perfecting the experimental setup was challenging. Fully calming a floating object and tank of water before beginning a test and devising a way to heat the object without causing ripples or movement were particularly difficult tasks. The
team decided to use a metal wedge, about 5 inches long, containing a heating element that could be
activated by a remote control unit.
This experiment was the first to demonstrate that a temperature differential between the surface of an
object and the surrounding fluid can drive movement — an effect that might have widespread significance in the natural world, and potential for
future technologies.
The effect itself is surprisingly simple, Peacock explains: “By virtue of heating or cooling the surface
of an object, you change the density of any fluid next to that surface.” In the valley winds previously considered, the object was either a glacier or a valley wall heated by the sun, and the fluid was the air passing over it; in this case, it’s the solid wedge and its surrounding water.
The changed density of the fluid generates a flow over the surface, Peacock says, adding, “That flow
then creates unbalanced forces, with lower pressure on one side, and higher on the other” — an imbalance that propels the object from the higher pressure toward the lower.

The phenomenon applies to “any situation where an object is immersed in fluid, and its temperature is different” from that of the fluid, Peacock says.

The basic equations that govern convection are well known, Peacock says. “This type of flow has been
studied for over 100 years, but somehow, in all that time, no one had thought to do this.”
Colm-cille Caulfield, an applied mathematician and theoretical physicist at Cambridge University who was not involved in this research, says it is indeed surprising that this phenomenon has been
overlooked for so long. “That such a generic and naturally occurring process … has been identified,
demonstrated, and explained for the first time is a significant and surprising discovery,” he says. Coalfield adds that while the initial laboratory proof involved a small object, the effect presumably also
applies to larger systems. “The real prize is to demonstrate that this process is also significant on a larger scale,” he says. “If such a scale-up can be achieved, this work has the potential to be central to our understanding and modeling of many
environmentally and industrially relevant flows.”

Peacock is already working on such follow-up experiments, to figure out “whether the effect can be exploited, in an engineering sense, and also
whether nature might already be exploiting it.”
The method could prove useful in controlling how particles move through microfluidic devices, or in
understanding the motion of material floating in magma. It may, Peacock says, even turn out to be something that living organisms have learned to
harness: If a very small creature can propel itself by selectively heating or cooling itself, that could turn out to be a significant mechanism, he says.
“It’s very rare in fluid mechanics to discover a new phenomenon like this,” Peacock says. “There are
so many fields that this could potentially impact. ….
I hope other researchers will hear about the effect and investigate it in their particular fields and discover new things.”

In addition to Peacock, the work was carried out by former MIT postdoc Matthieu Mercier, now at the
Institut de Mécanique des Fluides de Toulouse in France; MIT affiliates Brian Doyle and Michael
Allshouse; and Arezoo Ardekani, now a faculty member at the University of Notre Dame.

VENTURIMETER

VENTURIMETER:
A venturimeter is essentially a short pipe consisting of two conical parts with a short portion of uniform cross-section in between. This short portion has the minimum area and is known as the throat. The two conical portions have the same base diameter, but one is having a shorter length with a larger cone angle while the other is having a larger length with a smaller cone angle.

FM

I'll admit when I first saw the words "Dimensional Analysis," I felt my skin flush; my heart started beating faster; my mind began racing; and I scanned the exits of the room I was in. Classic fight or flight, with an emphasis on the latter.

But as I read, I realized my first reaction was silly. That said, I couldn't stop thinking that a lot of this was hand-waving. Can we legitimately summarize this discussion (or least summarize the justification) by observing that "in physics, almost everything is continuous" so arguments like this just work?

More precisely, what exactly is "length scale" or "characteristic length" supposed to represent? Is this along the lines of the length of the box everything is contained in, or is this the length of the smallest phenomenon observable/significant? What about in problems with a large container and small phenomena of global significance?

Also, why do we put a bar on the velocity scale U?

Finally, how is Reynolds number in any way well-defined? Can't I just say the scales are approximately this or that and get entirely different values?

Onto the next section, when can we legitimately make the lubrication assumption and get realistic results? I want to say for "slippery" fluids, but what does that even mean?

When we get a time estimate for the length of time needed to remove an adhering object, what assumptions are we making about the way it's pulled off? I feel like this should be clear, but wasn't really for me.

Overall, really cool stuff. I'm amazed that despite the sophistication of the equations, we can get tangible and useful numerical results.

SUNDAY, APRIL 13, 2008

Reading 4/14/2008: Lecture 7

First derivation is very cool. Energy minimization leads to the fluid cylinder instability. Makes sense, and the derivation is simple enough.

Very cool to see Bessel functions popping up, though given the type of equations and the space on which we're solving them, this doesn't seem particularly surprising, comparing to experience with Math 180.

Overall, it all makes sense to me. It's very interesting to see that what is fundamentally a stability analysis can be performed by linearizing and solving the system and then looking at solutions for which the waves grow. I'm a little unclear as to where the "asymmetric modes" part of the final paragraph comes from, but the fact that wavelengths greater than a threshold value grow to infinity actually makes sense to me.

NB: Sorry I've missed so many blog entries. If I can find time, I'm going to go back and write them, but these past two weeks have been absolutely vicious.

WEDNESDAY, APRIL 2, 2008

Reading 4/2/2008: Lecture 3 Notes

I wish I'd had more time to blog recently, but life has been a little too crazy.

At any rate, this is cool stuff. It's nice to see how the free-surface boundary conditions play out in the mathematical PDEs framework, and it's even cooler to see a fairly rigorous proof of Bernoulli's theorem. Obviously the same concepts are there, but, well I'm a mathematician, so it's better now.

Definitely cool to see the Fourier transform appear in the end, too. I'd be curious to hear about the general applicability of the FT in fluids - it's certainly a big hammer and great for making this smooth. (No pun intended.)

The series expansion strangely reminded by of perturbation theory from big quantum; I'm guessing this is a fairly standard approach, I think it's more the notation. That said, I wonder how applicable the linearized equations are and/or what are their drawbacks?

WEDNESDAY, MARCH 12, 2008

Reading 3/10/2008: Section 3.6

Late, I know, but better than never.

This stuff honestly is straightforward. Having seen complex variable before, the approach is a little weird (partial derivatives of a complex function and chain rule usage are a little suspect, but it works).

Laplace's equation is solved pretty thoroughly in a bunch of classes, so that's pretty much par for the course. It is really cool to see the stuff on p. 197 about using the real vs. complex part as the potential. Didn't know you could look at it that way.

What exactly happens physically at the interface where, mathematically, the pressure becomes negative? That was really my only major question from the chapter.

WEDNESDAY, MARCH 5, 2008

Reading 3/5/2008: Sections 3.4-3.5

Section 3.4: Very cool; we have a number that can tell is whether we make the assumption of diffusion-dominance or vorticity-dominance. Makes sense to me, though where do we get the characteristic length scale from? And why does it shrink with turbulence? Otherwise, everything is clear.

Section 3.5: The derivation of small Bernoulli is very straightforward, though the result is quite cool. For big Bernoulli, we demand that a flow is irrotational - where are some examples where this really breaks down in a big way? Also, how might one measure &phi, the velocity potential function, much less its time rate of change?

The connection with the Laplacian here makes sense given our assumptions, but is a nice touch. The dipole/etc. thing is worth discussing in class. I've never understood physicists' fascinating with dipoles, but maybe I can with a little more detail.

MONDAY, MARCH 3, 2008

Reading 3/3/2008: Sections 3.2-3.3

Section 3.2: Okay, cool. Our equations reduce with the acoustic approximation to something much more tractable. Very nice. I'm a little curious why we can assume (3.58) can only be satisfied in the two ways mentioned in the book. If I had a little more time, I would sit down and just prove this, but I wonder if there's a quick answer?

What are the real-world implications of S-waves decaying so rapidly? If the waves are only significant very, very close to the source, where do they arise/where are they important in practice?

How is the scattering effect of particles in p. 163 accounted for in fluid models?

Overall, this section seemed pleasantly simple. We get some nasty dispersion relations, but they're easy enough to use, and reduce to forms that are fairly easy to work with. Cool stuff.

Section 3.3: A section with "Theorem" in the title. Yay, math. Honestly, everything here made good sense. I wish I could see a more rigorous proof of the theorem, but for our purposes, this seems pretty good to me.

The bit at the end about vortex tubes is awesome. So THAT'S what a tornado is...

Reading 2/27/2008: Section 3.1

Woo, Fluids.

So we can immediately dispense with &mu, simplifying things quite nicely. Very cool derivation, and seemingly quite rigorous. I'm not entirely clear why we can assume &xi_n,n is zero, but I'm assuming it's because S_mm is &Phi.

The derivation of the new equation of state is very cool. It's remarkable to see that dp can be characterized completely and uniquely in terms of &rho. I'm not quite sure where the book is going with the "exact differential" comment, but I'm assuming that means something to physicists that it lacks in meaning to me.

Why do we assume viscous stresses are linearly proportional to velocities? What is the origin of this postulate? That was one of the major aspects unclear in the section. Otherwise, the derivation of Newtonian viscosities was clear enough.

And holy cow, we have Navier-Stokes! If only we could solve them generally...

I'm a little unclear what is meant by a volume force in (3.27) - is this just to emphasize that this is a force separate from the external (e.g. gravitational) force?

On p. 151 I just want to point out that the word "magma" is bloody awesome. Everyone should incorporate it into their daily speech immediately. No, seriously. I mean it.

Overall a fantastic section.