small vortex generators in tube?

[allblowdup]

How would one calculate what size and how far apart to machine some small votex generators(basically slits) in a 8" dia tueb with water moving through it at 120 mph. The thinking here is that there is some drag that can be decreadsed by creating a boundry layer on the edges. Would this not make the tube act as a smaller diameter thus decreasing volume?

 

[allblowdup]

How about this instead. Fighter aircraft or high speed aircraft hav very small slits positioned very close together apparantly. What i want to do it the same thing for water that is running through a tube. A intake of a jet pump that is force fed with 120 mph water.

 

[btrueblood]

Are you talking about supersonic jet engine intakes on fighters? The spill slots on high-Mach no. intakes are there to vent the boundary layer overboard, to allow better "starting" characteristics when the intake is below design speed. There is no equivalance in Mach number from a supersonic jet fighter to 120 mph water (speed of sound for water is something like 10,000 m/s), thus no reason for a water intake to need to spill. Keep the intake pipe as short as possible, to reduce boundary layer effects.

 

[allblowdup]

Nope I am talking about on the fuselage. apparantly there are very fine v grooves running perpendicular to the airflow that help with drag. Much like a shark has for less drag in water.

http://www.rexresearch.com/lang/lang.htm

 

[rb1957]

sure, you can try just about anything, and some of them might work.

but it's not clear just waht you hope to achieve by this ...

a) limit the flow through the tube ? (sounds like you think a thicker BL is what you want)

b) maximise flow in the tube ?

is this s single phase problem ? ie how much dissolved gas in the water ? will it separate out (cavitate) ?

is the tube is completely immersed in water (on both sides of the wall) ?

what do you think the slits on the SS fighter do for it ? how can you apply this to your problem ?

Quando Omni Flunkus Moritati

 

[MikeHalloran]

You need a much bigger pipe.


Mike Halloran
Pembroke Pines, FL, USA

 

[rb1957]

i'm confused ... not for the first (nor last) time.

it sounds like you're trying to increase either the volume flow rate or the pressure recovery of your tube. trying to reduce surface drag, playing with the boundary layer Might Just improve things. The Only point to doing this is that you've calc'd that this tweek will get your where you want to be.

so, what's the problem ? is it flow rtae or pressure recovery (those could be different descriptions of the same problem) ?
how far out of bed are you ?
how long is the pipe ?
slits ?? is the pipe immersed in water ? is this like the intake on a personal watercraft ? if you had slits, which way would the water flow (into or out of the tube) ?

Quando Omni Flunkus Moritati

 

[allblowdup]

It actually is a a jet pump intake so ya . 3M made a rblet tape for that purpose but is not available to the public and besides who knows what the depth and spacing needs to be. The 3M riblet tape had very small V grooves(.003") and was longitunal not perpendicular to the flow.

 

[rb1957]

and it would be very hard to install on the inside of a tube ... though doable for an 8" tube.

are you not getting the required flow ?

Quando Omni Flunkus Moritati

 

[allblowdup]

For us it is about cutting down the friction. We usually have 60-80 psi presssure in the intake of the pump. Just trying to cut down the amount of energy requiered to make that pressure.

 

[rb1957]

if the grooves in the 3M tape are longitudinal ... what about machining them into the ID of the tube? maybe a spiral groove is even "better" ??

Quando Omni Flunkus Moritati

 

[allblowdup]

Machining them was my plan but from the studies that I have read all mention that if the spacing and depthy is not correct you will actually increase the friction. It may be one of those things that really can't tested or claculated properly.

 

[rb1957]

drag can be a very fnickety thing ... we had a mod (on a plane) where we were deflecting the airflow around an open door; in development it worked well, then they "productionised" the deflectors, changed some edge radius a little ... damned things didn't work, had to redo the development work with the productionised pieces.

if you can't access the tape (and "copy" it), can you do CFD ? i wouldn't trust the results without a test but they should get you some way along the path. if you can't do CFD then you're stuck with testing things. if you don't want to do this, see the previous post about using a bigger tube. i doubt that there is a canned solution for your problem.

again, how much are you trying to gain ? is the power required 2* what you want ? 1.1* ??

Quando Omni Flunkus Moritati

 

[allblowdup]

Basically these are high powered jet boats. Roughly 1300 hp. the main drag on the boat ends up being the pump inlt and impeller section. The amount of the boat that is actually touching the water is around 6sqft. a drag reduction of 1-2% for the intake would be noticable. It is very difficult to measure though other than outright speed tests.

 

[btrueblood]

These are straight-line "drag boats", or is there manuevering involved? In other words, if the intake has to operate at high angles of incidence, then you need a fatter, rounder-lipped intake like the bell of a trumpet (or something with blow-in doors which are complex and bulky in their own right). If the intake only needs to operate within a narrow range of angles, it can be something closer to a stovepipe. The main source of drag in either case is likely less from the internal flow, but from the external flow past/around the bulge formed by the intake (unless you could exhaust the water jet into the cavitation void right behind the inlet?). Some CFD and/or water tunnel studies of those flowfields (like rb suggests) would likely give you some pretty good payback.