Many thanks for this explanation, very useful/helpful. Is there a way to calculate theoretically how far in front of an object a pressure sensor would need to be located for the object behind to not be influencing the measurement (due to the object slowing the airflow and increasing the dynamic pressure (if I have that correctly understood..)). If it helps having a particular example.. the case I have in mind is that I have started using an "aerosensor" to assess relative changes in CdA of a time trial bike set-up. The sensor is mounted at the front of the bike and a "calibration factor" is required to account for the bike and I slowing the air down at the sensor. I am finding this C.F. to be very variable and would like to just get the sensor into the "clean air". Is there a way to calculate how far in front of me+bike the sensor would have to be mounted?? Thanks in advance for any thoughts....
Good question. The answer, as you have suggested, depends on the object behind it. The more bluff the object, the greater the disturbance upstream usually is. Unfortunately, for this example, you need to run a simulation or experiments to see how far upstream the pressure changes. If you have calibration curves, do you have the point at which the the calibration factor drops to 0? If so, then that is the point at which the pressure doesn't change. Also, you could tryin mounting the sensor above the rider, from the front forks, for example. By putting it up above, you reduce the impact the rider has on it.
Some of the time, but I'm not sure that it holds true all of the time. One "kind of" exception would be a converging-diverging nozzle; the dynamic pressure goes up in the contraction because the velocity increases, but there really isn't much of a change in the dynamic pressure elsewhere. The only place it drops would be slightly before the nozzle because of the back pressure. But that drop is not nearly as great as the increase you get in the converging section and it's not due to a redistribution of the energy of the flow. In other cases, then it definitely makes sense that an increase must come from somewhere, so a drop elsewhere is likely. (and from an energy point of view)
No, it is valid for all flows, however, you need to make sure you are using the right dynamic pressure. For example, in supersonic flows, there are shocks and those shocks reduce the total energy of the flow, hence the total pressure drops. Much of that has to do with the flow being slower after the shock, hence the dynamic pressure has dropped post shock.
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Thank you for helping me out
You're welcome!
Thanks this was really helpful
You're welcome.
Perfect, Thanks a lot!
You're welcome!
Many thanks for this explanation, very useful/helpful. Is there a way to calculate theoretically how far in front of an object a pressure sensor would need to be located for the object behind to not be influencing the measurement (due to the object slowing the airflow and increasing the dynamic pressure (if I have that correctly understood..)). If it helps having a particular example.. the case I have in mind is that I have started using an "aerosensor" to assess relative changes in CdA of a time trial bike set-up. The sensor is mounted at the front of the bike and a "calibration factor" is required to account for the bike and I slowing the air down at the sensor. I am finding this C.F. to be very variable and would like to just get the sensor into the "clean air". Is there a way to calculate how far in front of me+bike the sensor would have to be mounted?? Thanks in advance for any thoughts....
Good question. The answer, as you have suggested, depends on the object behind it. The more bluff the object, the greater the disturbance upstream usually is.
Unfortunately, for this example, you need to run a simulation or experiments to see how far upstream the pressure changes. If you have calibration curves, do you have the point at which the the calibration factor drops to 0? If so, then that is the point at which the pressure doesn't change.
Also, you could tryin mounting the sensor above the rider, from the front forks, for example. By putting it up above, you reduce the impact the rider has on it.
Is there some 3rd law involved in pressure? For every dynamic pressure increase, there's a corresponding dynamic pressure decrease?
Some of the time, but I'm not sure that it holds true all of the time. One "kind of" exception would be a converging-diverging nozzle; the dynamic pressure goes up in the contraction because the velocity increases, but there really isn't much of a change in the dynamic pressure elsewhere. The only place it drops would be slightly before the nozzle because of the back pressure. But that drop is not nearly as great as the increase you get in the converging section and it's not due to a redistribution of the energy of the flow.
In other cases, then it definitely makes sense that an increase must come from somewhere, so a drop elsewhere is likely. (and from an energy point of view)
is the total pressure =static + dynamic valid only for incompressible flows?
No, it is valid for all flows, however, you need to make sure you are using the right dynamic pressure. For example, in supersonic flows, there are shocks and those shocks reduce the total energy of the flow, hence the total pressure drops. Much of that has to do with the flow being slower after the shock, hence the dynamic pressure has dropped post shock.
that the total pressure equal the lift force ?
No, the lift force is a little more to do with differences in pressures.
ok .. The total pressure in up of airfoile > total pressure in down of airfoile to make lift
It is more the pressure felt on the surface (so the static pressure).