Interview question on Reynold's Number
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Interview question on Reynold's Number
Hi everybody,
I always thought that the last time I would have to fathom Reynold's Number was for ATP written exams but it seems this is not the case.
A particular airline has been asking interview questions relating to Reynolds Number recently. I understand that the number determines whether the flow of a fluid is turbulent or laminar. Does this mean that the Reynold's Number is directly proportional to Total Drag? In the instance of an aircraft on the wrong side of the drag curve, accelerating, will its Re decrease progressively until Vimd and then begin increasing?
My next question is this: What about high speed flight? The standard Reynold's Number explanation ignores the effects of compressibility. What happens when you're cruising at .78M? What happens to Re at Mcrit? No matter how many thermodynamics websites I visit and how many ATP text books I read, I can't figure this one out. Can any of you geniuses out these shed some light on this mysterious phenomenon for me (in simple terms, if there is such a thing)?
Thanks in advance!
I always thought that the last time I would have to fathom Reynold's Number was for ATP written exams but it seems this is not the case.
A particular airline has been asking interview questions relating to Reynolds Number recently. I understand that the number determines whether the flow of a fluid is turbulent or laminar. Does this mean that the Reynold's Number is directly proportional to Total Drag? In the instance of an aircraft on the wrong side of the drag curve, accelerating, will its Re decrease progressively until Vimd and then begin increasing?
My next question is this: What about high speed flight? The standard Reynold's Number explanation ignores the effects of compressibility. What happens when you're cruising at .78M? What happens to Re at Mcrit? No matter how many thermodynamics websites I visit and how many ATP text books I read, I can't figure this one out. Can any of you geniuses out these shed some light on this mysterious phenomenon for me (in simple terms, if there is such a thing)?
Thanks in advance!
Last edited by skywalker001; 20th Aug 2012 at 14:52.
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Not easy to give specific comment without knowing what is being asked about Rn.
However I would not suggest you offer any interpretation of what you think Rn might mean or imply. That way you could open the door to invite other questions that will soon reveal any lack of depth in understanding.
Better in my view to stick to basics to show you are familiar with them. Depending on what you are asked the basics The basics would include such things as:
" Reynolds number is non-dimensional (meaning it is pure number and has no units) and is calculated in the case of an aircraft by mutiplying the density of the air it is in by the speed of the aircraft and multiplying again by the length of the aircraft and dividing the result by the viscosity of the air".
(Which just puts into words R = Density x Speed x Length / Viscosity)
"Reynolds umber is particularly important when comparing results between practical cases. Any large difference in the Reynolds number between cases may invalidate to some extent any comparison - for example in a wind tunnel test the length of the model will be very small compared to that of the real aircraft and it may be beneficial to increase the model Rn by pressurising the tiunnel. This will reduce the so called "Scale Effect" issues of very different Rns".
Wikipedia is helpful where it says
Reynolds numbers frequently arise when performing dimensional analysis of fluid dynamics problems, and as such can be used to determine dynamic similitude between different experimental cases.
They are also used to characterize different flow regimes, such as laminar or turbulent flow: laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion; turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend to produce chaotic eddies, vortices and other flow instabilities.
But like I said don't get suckered into conjecturing.
As a last result
" I am familiar with the basics of Rn but I am not sure I quite understand your question"
However I would not suggest you offer any interpretation of what you think Rn might mean or imply. That way you could open the door to invite other questions that will soon reveal any lack of depth in understanding.
Better in my view to stick to basics to show you are familiar with them. Depending on what you are asked the basics The basics would include such things as:
" Reynolds number is non-dimensional (meaning it is pure number and has no units) and is calculated in the case of an aircraft by mutiplying the density of the air it is in by the speed of the aircraft and multiplying again by the length of the aircraft and dividing the result by the viscosity of the air".
(Which just puts into words R = Density x Speed x Length / Viscosity)
"Reynolds umber is particularly important when comparing results between practical cases. Any large difference in the Reynolds number between cases may invalidate to some extent any comparison - for example in a wind tunnel test the length of the model will be very small compared to that of the real aircraft and it may be beneficial to increase the model Rn by pressurising the tiunnel. This will reduce the so called "Scale Effect" issues of very different Rns".
Wikipedia is helpful where it says
Reynolds numbers frequently arise when performing dimensional analysis of fluid dynamics problems, and as such can be used to determine dynamic similitude between different experimental cases.
They are also used to characterize different flow regimes, such as laminar or turbulent flow: laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion; turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend to produce chaotic eddies, vortices and other flow instabilities.
But like I said don't get suckered into conjecturing.
As a last result
" I am familiar with the basics of Rn but I am not sure I quite understand your question"
Last edited by John Farley; 21st Aug 2012 at 10:39.
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Hi,
As you say, Re can give an indication of laminar or turbulent flow given a number of assumptions which include incompressible flow. You could always work it in reverse: what happens to the airflow at Mcrit? How do you correct for the effects which are not accounted for in the calculation of Re? What Re do you end up with once those effects are accounted for?
I have never used Re in wing calculations but there may be limits to how far it will apply given the increasing dominance of factors such as shocks and their effect of the properties of the fluid.
It may be that your interviewers only wish to explore your knowledge of Re rather than achieve a specifically 'correct' answer.
Best wishes
AD
As you say, Re can give an indication of laminar or turbulent flow given a number of assumptions which include incompressible flow. You could always work it in reverse: what happens to the airflow at Mcrit? How do you correct for the effects which are not accounted for in the calculation of Re? What Re do you end up with once those effects are accounted for?
I have never used Re in wing calculations but there may be limits to how far it will apply given the increasing dominance of factors such as shocks and their effect of the properties of the fluid.
It may be that your interviewers only wish to explore your knowledge of Re rather than achieve a specifically 'correct' answer.
Best wishes
AD
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Thanks for the insightful info. They certainly helped quite a lot.
When I posed the same question to a friend he said: "Whatever you do, don't ask the examiner during the sim check the whereabouts of the Reynolds Number indicator in the cockpit."
Take care.
When I posed the same question to a friend he said: "Whatever you do, don't ask the examiner during the sim check the whereabouts of the Reynolds Number indicator in the cockpit."
Take care.
John Farley's answer is the best actually, it doesn't get more concise or simpler than that! otherwise that's an impossible question to answer exactly each type exhibits different characteristics, either it's a ridiculous question or they're trying to test your honesty...
If I were asked I'd say do you have four hours and do you like Navier-Stokes, differential operators and compressible flows
... if only but to tell them that I don't know...
If they're serious, well then that has to be the dumbest question for a pilot interview I've ever seen
If I were asked I'd say do you have four hours and do you like Navier-Stokes, differential operators and compressible flows
... if only but to tell them that I don't know...If they're serious, well then that has to be the dumbest question for a pilot interview I've ever seen
Last edited by Pugilistic Animus; 22nd Aug 2012 at 09:25.
