If you’re somebody studying fluid mechanics and you are contemplating not watching this video in full, take my word, this is worth every minute. I’ve seen a lot of videos on UA-cam regarding this topic, and no other video builds upon theoretical fundamentals and challenges adaptation of real life phenomena quite like this video. Just finished watching this video and I am very very pleased. I considered switching to a new video at about 4/5 mins in. I am very glad I stuck it through. Excellent work here. Thank you.
That was great. Am I correct in assuming that when we do a cut thru a solid even if there is no ATM pressure in that section we still imagine ATM pressure is there to use the trick and work with gauge pressure? For instance in the horizontal nozzles at the left end there is no air acting on the solid and liquid but we still assume its there and instead work with gauge.
I not 100% sure of your question. The absolute pressure inside a pipe will be affected by local atmospheric pressure even though "there is no air acting on the solid and liquid" inside the pipe. It will be the pressure provided by, say a pump, plus to local atmospheric pressure. So, yes, the atmospheric pressure "is there" and you need to work with gauge pressure. I hope that helps.
Hi Mr Fluid, how to factor different nozzle type in one flowing at time, let says 1 same direction as inlet another is 30degree off the axis, would that reate some vorttex force?
I'm not entirely sure what you are asking. Here is another solved example where the nozzle outlet is "off the axis", which may be what you are looking for: ua-cam.com/video/1tp7-e1fxhc/v-deo.html
A change in the outlet diameter will change the inlet pressure. You could work out p_1 using Bernoulli (plus some estimated head losses, if you want more accuracy). I haven't done the analysis. But my intuition is that the thrust due to the change in moment of the water might go up or down. Although the outlet velocity (V_2) will increase as D_2 decreases, but the the mass flow rate will decrease. Not sure if that answers your question.
is it correct to use guage pressure,because in real world ,it is the absoute pressure that contribute to the force.we just consider guage pressure since they cancel out,but here they do not cancel out right? according to F= P * area , since A1>A2,the force component due to atmospheric pressure does not cancel out,there is a net value there,but how could you take it as zero,can you please explain on this
I suspect you need to rewatch the explanation/theory at the at the beginning of this video. All is explained in the first 7 minutes of this video. You have to use gauge pressure in these problems. If you use absolute pressures, you have to integrate the non-zero atmo. pressure over the outer surface of the nozzle. That is part of the control surface. It can be done in theory, but it's not necessary. If you work in gauge, the pressure on the outer surface of the nozzle is zero (gauge). This does not result in an error because the integral of the atmospheric pressure over the ENTIRE control volume is zero (for reasons explained in the video -- nature takes care of that force balance.). So, you subtract off p_atm over the entire cv, and work in gauge.
If you’re somebody studying fluid mechanics and you are contemplating not watching this video in full, take my word, this is worth every minute. I’ve seen a lot of videos on UA-cam regarding this topic, and no other video builds upon theoretical fundamentals and challenges adaptation of real life phenomena quite like this video. Just finished watching this video and I am very very pleased. I considered switching to a new video at about 4/5 mins in. I am very glad I stuck it through. Excellent work here. Thank you.
Thanks for the very nice comment. Glad to hear the video was helpful.
All the videos for this introductory Fluid Mechanics course are now available at: www.drdavidnaylor.net/
That was great. Am I correct in assuming that when we do a cut thru a solid even if there is no ATM pressure in that section we still imagine ATM pressure is there to use the trick and work with gauge pressure? For instance in the horizontal nozzles at the left end there is no air acting on the solid and liquid but we still assume its there and instead work with gauge.
I not 100% sure of your question. The absolute pressure inside a pipe will be affected by local atmospheric pressure even though "there is no air acting on the solid and liquid" inside the pipe. It will be the pressure provided by, say a pump, plus to local atmospheric pressure. So, yes, the atmospheric pressure "is there" and you need to work with gauge pressure. I hope that helps.
Hi Mr Fluid, how to factor different nozzle type in one flowing at time, let says 1 same direction as inlet another is 30degree off the axis, would that reate some vorttex force?
I'm not entirely sure what you are asking. Here is another solved example where the nozzle outlet is "off the axis", which may be what you are looking for: ua-cam.com/video/1tp7-e1fxhc/v-deo.html
so can you say that if the diameter of the nozzle decreases (so that it is not a nozzle anymore, it is a basic circle), we obtain more thrust force ?
A change in the outlet diameter will change the inlet pressure. You could work out p_1 using Bernoulli (plus some estimated head losses, if you want more accuracy). I haven't done the analysis. But my intuition is that the thrust due to the change in moment of the water might go up or down. Although the outlet velocity (V_2) will increase as D_2 decreases, but the the mass flow rate will decrease. Not sure if that answers your question.
Great video!
i think there's a little mistake in the Area of the nozzle , you mean pi*r^2 instead of pi*D^2
No. Note the divided by 4, since r^2=(D/2)^2=D^2/4 (at about 22:16)
is it correct to use guage pressure,because in real world ,it is the absoute pressure that contribute to the force.we just consider guage pressure since they cancel out,but here they do not cancel out right? according to F= P * area , since A1>A2,the force component due to atmospheric pressure does not cancel out,there is a net value there,but how could you take it as zero,can you please explain on this
I suspect you need to rewatch the explanation/theory at the at the beginning of this video. All is explained in the first 7 minutes of this video. You have to use gauge pressure in these problems. If you use absolute pressures, you have to integrate the non-zero atmo. pressure over the outer surface of the nozzle. That is part of the control surface. It can be done in theory, but it's not necessary. If you work in gauge, the pressure on the outer surface of the nozzle is zero (gauge). This does not result in an error because the integral of the atmospheric pressure over the ENTIRE control volume is zero (for reasons explained in the video -- nature takes care of that force balance.). So, you subtract off p_atm over the entire cv, and work in gauge.
Whuh.just proved the pinned comment.thank you so much ❤