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John Mcrain
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If I put funnel out of car when driving 100km/h, can speed in narrow section in funnel be higher than 100km/h(freestream) and does bernuli equation works for this case?
yes I know how to solve it using bernulli,total pressure must be same,but I don't know can I use it for this case..boneh3ad said:Have you tried to work on this at all on your own?
Lnewqban said:The speed of airflow within the narrow section should be higher than the speed of the freestream.
You are welcome.John Mcrain said:thanks for answer
Lnewqban said:You are welcome.
Consider that more mass of air will be able to flow through the narrow section as the slope of the funnel is smaller (more gradual transition between wide and narrow sections).
Lnewqban said:Also, watch the attached video to see how much disturbance of air is created by the the car next to the funnel.
That disturbance greatly affects the the stability of the direction of the flow entering the funnel, as well as the static pressures upstream and downstream, all of which move the situation away from the conservation of energy involved in Bernoulli's equation.
How would you improve the statement, if at all possible?boneh3ad said:That's not at all a generally true statement.
Lnewqban said:How would you improve the statement, if at all possible?
I just double check .So every constriction reduce speed compare to freestream..Arjan82 said:I don't see how any of my arguments would be different for this case. Why do you think this makes a difference?
My first funnel...Arjan82 said:The flow coming out of the funnel needs to have the same static pressure as the surroundings
John Mcrain said:Isnt flow higher speed then freestream when leave funnel outlet,becuase air has mass, so it has inertia,so it keep going for short time?
John Mcrain said:Or flow decelerate inside funnel narrow section to match freestrem speed when leave outlet?
John Mcrain said:So every constriction reduce speed compare to freestream..
So basicly you answer is opposite from other membersArjan82 said:Therefore you 'loose' pressure and the flow through the small end of the funnel is lower than the surrounding flow, always.
John Mcrain said:So basicly you answer is opposite from other members
I am really suprised that @boneh3ad has opposite answer from yours!Arjan82 said:Apparently, should I have conceded? ;)
John Mcrain said:I am really suprised that @boneh3ad has opposite answer from yours!
So bernulli will take me to wrong answer...
Leave now exit of outlet velocity,this was disccused between only you and me..Arjan82 said:First: I don't see @boneh3ad making any statement about the exit velocity of the funnel being higher or lower than free stream in any of his posts here.
Second: Even if he did, so what?!? Are we going to vote now as to whether the outlet velocity is higher or lower than the surroundings? Do you think the air would care? What are you trying to say here? My arguments don't count because someone else who you apparently think high of doesn't agree? Shall we please keep it to actual arguments on physical processes please? This is a physics forum, not an opinion forum!
Third: Even if he was correct and I was wrong. What are you trying to say here? You are still to point out why he is correct! He is not correct because he's called boneh3ad!
Fourth: I also used Bernoulli (that's how you write it) in my argument...
John Mcrain said:You said that velocity in narrow section in funnel will be allways slower then freestream.
John Mcrain said:bonh3ad wrote " There will almost always be some inviscid core flow through such a funnel in which Bernoulli is perfectly applicable" as respons to Lnewquban post. So he implies that using Bernulli I will get answer...
John Mcrain said:Lnewquban wrote that speed will be higher then freestream(opposite from yours),
John Mcrain said:bonh3ad didnt complain nothing about it,he just add it that I can use bernulli for core flow..
John Mcrain said:I am suprised becuase I think he is proffesor of fluid dynamics
Arjan82 said:Maybe he is, maybe he knows more about fluid dynamics than I do, but what does that mean? Status doesn't count, only arguments do. (status does make me take his arguments more serious though :) )
Similarly, the flow outside your funnel might be faster than freestream. But that doesn't imply, that the flow somewhere in the funnel will be faster than freestream.John Mcrain said:wing has higher speed on upper surface then freestream, without pump.
Maybe the more general question would be:boneh3ad said:First, if fluid is incompressible and the flow is steady and moving into the funnel at a rate of ##\dot{m}##, it is therefore also leaving the funnel at a rate of ##\dot{m}##. Obviously the exit velocity has to be faster than the inlet velocity. It would break conservation of mass otherwise.
Second, one of the issues that people seem to have here is that the flow through the funnel does not occur independently of the flow outside the funnel. The funnel presents some resistance to flow that doesn't exist in the free stream. This makes it very difficult to generalize anything. If the opening is small, consider that it will behave in some ways like a Pitot probe where the flow in the front slows down (though doesn't entirely stagnate). If the opening is very large, this effect would only be large near the edges.
Also consider that the flow outside the funnel would be important. The outer shape of the funnel is important in determining what the wake of the funnel looks like into which the flow through the center would be discharged. The external flow will accelerate around the funnel and the pressure will drop, so the pressure into which the funnel discharges is not the same as that of the free stream.
But all of this is heavily dependent on both the internal and external shapes of the funnel. Incompressible fluids are governed by elliptical equations. What happens in one location affects every other location simultaneously (obviously an approximation to what we observe in real life, where nothing is actually perfectly incompressible).
With the initial configuration, I would say no.A.T. said:Is it at all possible to increase the flow speed through some kind of "funnel" beyond the free-stream speed?
boneh3ad said:Also consider that the flow outside the funnel would be important. The outer shape of the funnel is important in determining what the wake of the funnel looks like into which the flow through the center would be discharged. The external flow will accelerate around the funnel and the pressure will drop, so the pressure into which the funnel discharges is not the same as that of the free stream.
I think perpetuum mobile is if we increase total pressure of flow,without adding energy.Arjan82 said:Increasing the flow speed to beyond the free stream velocity with an object of any shape without it adding energy to the flow, disregarding local flow accelerations (i.e. behind the object), is a perpetuum mobile...