X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from mail11.syd.optusnet.com.au ([211.29.132.192] verified) by logan.com (CommuniGate Pro SMTP 5.2c1) with ESMTPS id 2464511 for flyrotary@lancaironline.net; Sat, 10 Nov 2007 00:14:05 -0500 Received-SPF: none receiver=logan.com; client-ip=211.29.132.192; envelope-from=lendich@optusnet.com.au Received: from george (d211-31-228-51.dsl.nsw.optusnet.com.au [211.31.228.51]) by mail11.syd.optusnet.com.au (8.13.1/8.13.1) with SMTP id lAA5DILR003010 for ; Sat, 10 Nov 2007 16:13:19 +1100 Message-ID: <003f01c82358$6c6dab80$33e41fd3@george> From: "George Lendich" To: "Rotary motors in aircraft" References: Subject: Re: [FlyRotary] Re: Total,duct, Ambient or Velocity???? Date: Sat, 10 Nov 2007 15:13:24 +1000 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_003C_01C823AC.3D15E010" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.2180 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2180 X-Antivirus: avast! (VPS 0657-0, 12/12/2006), Outbound message X-Antivirus-Status: Clean This is a multi-part message in MIME format. ------=_NextPart_000_003C_01C823AC.3D15E010 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable To Both Ed and Al. Thanks! I guessed it was a gradual change subject to length and shape of the = inlet duct/ diffuser. I had hoped that with a decent design (we are all aiming for) that you = might expect to achieve max static pressure well prior to the Radiator. =20 However it may be that the optimum static pressure might be design = dependent and happen just before the rad. If figure if it was within the = rad it would be restrictive. Thinking on it further, the more further forward (of the Rad face) the = optimum static air pressure is, it may suggest that the rad is too = restrictive. I'm not sorry I asked, a little less confused and more things to think = about. George ( down under) =20 George, here you are getting into something we have not discussed in = depth. Two equations/laws of fluid dynamics are involved. Bernoulli's = equation and an equation called the law of continuity. This equation = relates to the fact that you don't create or lose mass in the duct, so = the mass flow is a constant everywhere in the duct. The mass flow is = frequently shown as the product of air density*cross section area*air = velocity =3D mass flow or simply p*A*V The equation goes something like this, the p1A1V1 (mass flow at point = 1) =3D p2A2V2 (mass flow at point 2). Since the air is normally = considered to act like it is incompressible at the lower speeds we are = talking about, that means the density p1=3D p2, so we can drop them = from the equation for this explanation. That leaves us with A1V1 =3D A2V2 or the product of the area and = velocity at point 1 is equal to the area and velocity at point 2 in the = duct. Now if A1 =3D A2 then V1 has to equal V2 for the two sides of = the equation to be equal. But, what if A2 =3D 2* A1 or the cross = section area of point 2 is made twice the cross section area of point 1. = Then if A2 =3D 2*A1, we can substitute 2*A1 for A2 in the equation and = we have the following. Taking A1V1 =3D A2V2 and substituting we have A1*V1 =3D (2*A1)*V2. = So what does that tell us about the air velocity at point 2 now that we = have doubled the cross section area there? =20 Well solving the equation for the new V2, We can call the new velocity = at point 2 V2n (for V2 new) with V2o being the old velocity at point 2. = So we have V2n =3D A1V2o/(2*A1) Now we can cancelled the A1 in the = numerator and denominator on right side of the equation leaving V2n =3D V2o/2 This shows us that the new velocity at point 2, V2n = is 1/2 the old velocity (V2o) at point 2 or V2n =3D 0.5V2o So what this says is the velocity starts changing (slowing in this = case and the pressure increasing ) as soon as the cross section area A2 = starts to increase from A1. The process continues until the area stops = expanding (or the kinetic energy of the moving air has all been = converted to a static pressure increase) and that is where the process = is finished as the duct/diffuser has expanded to its maximum area. = Actually, this process happens with both nozzles and diffusers just the = opposite way. Its derived from the Bernoulli equation and the = continuity law. So if you had a duct whose cross section area continued to expand for = a distance of 2" or 20" or 200" then theoretically the pressure would = continue to build and the velocity to decrease until all of the kinetic = energy of the moving air has been converted to pressure increase. This = is all theoretical, there are losses and turbulence and etc, that makes = a difference, but you get the ideal. It depends on your specific = diffuser dimensions. Think of it this way, George, some wind tunnels have diffuser which = expand over 10's of feet while some microscopic cooling systems have = diffusers measured in 10th's of an inch. Now aren't you sorry you asked {:>)? Ed ----- Original Message -----=20 From: George Lendich=20 To: Rotary motors in aircraft=20 Sent: Friday, November 09, 2007 4:52 PM Subject: [FlyRotary] Re: Total,duct, Ambient or Velocity???? Ed and Al,=20 This is all good info me, it either confirms, clarifies or informs. The straw concept is a timely reminder of pressure differentials, a = good example IMHO. One thing I would really love to know is - at what point in the = inlet duct does the dynamic flow change to static pressure. I would = assume this would vary with different shaped ducts and different dynamic = flow ( airflow speed).=20 Your opinions on this or guesstimates ie 1", 2" or 3" from the face = of the rad, would be of great interest to me. George (down under) Hi Al, Not picky - some good points as always . Yes, I agree, = generalization does have its pit falls, but on the other hand I think = they can help promote a conceptual understanding which can be refined = (through study and experiments) to meet a particular situation. As we = know, cooling airflow is attempting to balance conflicting aerodynamic = and thermodynamic principles. =20 I also agree that much of this stuff addresses the "Perfect = theoretical duct" out of necessity as there is only one perfect duct but = many, many implementations that fall short of perfect. So its more of = a conceptual goal to be aimed for - it may never be achieved, but = provides at least guidelines. But,this is just my opinion of course. Actually, I disagree, you can not "suck" air though anything. You = may create a partial pressure difference with the fan, but it is the = higher pressure air on the other end of the duct that pushes or "blows" = air through the duct into the area of lower pressure {:>) . =20 But, semantics aside, yes, I agree, lower exit pressure is what = you are after and that does not always equate to larger exit duct area. = In fact, if the air heated by the core flows through a nozzle it might = even produce thrust and lower exit pressure using a smaller exit. But, = in general, I still believe that in most of our cases, we are short of = the level of duct design that would reliably permit that. What we need = is someone to invest in one of those $$$$ Computer Fluid Flow software = programs and see what they would reveal. Ed ----- Original Message -----=20 From: Al Gietzen=20 To: Rotary motors in aircraft=20 Sent: Friday, November 09, 2007 1:09 AM Subject: [FlyRotary] Re: Total,duct, Ambient or Velocity???? It would seem "reasonable" that a low pressure area at the exit = will help flow through a duct - no argument on that point. What the = report appeared to say is that the after a certain point opening the = exit area wider does not appear to have any additional benefit. (Exit = "area" and exit "pressure" are not interchangeable terms) That if the = duct is capable of "using up" all of the kinetic energy in your air flow = by obstructions, pressure drops and friction losses then enlarging the = exit does not necessarily add to the flow. Remember you can not suck air through a duct, you can only blow = it through. (Of course you can suck air through a duct - I do it after = (and sometimes before) every flight with the fan I have on the back side = of the radiator) So in effect if the straw is pinched you can "suck" on = it all you want but it won't increase flow {:>). =20 If I understood the report, it appears that enlarging the exit = area beyond the frontal area of your core provides little if any = additional benefit. That does not mean cowl flaps never work or provide = benefit. In fact it appears that the better your duct, the more = benefit the cowl flaps appear to have, the worst your duct, the lesser = benefit - just the opposite of what you might think. Ed; Don't mean to be picky, but some of these generalities are = making me nervousJ. These things are applicable only when the = duct/diffuser is operating at max efficiency - which is rarely the case. Lot's of good info. Thanks. You're right; it's some kind of = magic, and you don't know for sure until you built it and try it. Al -------------------------------------------------------------------------= - No virus found in this incoming message. Checked by AVG Free Edition.=20 Version: 7.5.503 / Virus Database: 269.15.24/1115 - Release Date: = 7/11/2007 9:21 AM -------------------------------------------------------------------------= ----- No virus found in this incoming message. Checked by AVG Free Edition.=20 Version: 7.5.503 / Virus Database: 269.15.27/1121 - Release Date: = 9/11/2007 7:29 PM ------=_NextPart_000_003C_01C823AC.3D15E010 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
 
To Both Ed and Al.
Thanks!
I guessed it was a gradual change = subject to=20 length and shape of the  inlet duct/ diffuser.
I had hoped that with a decent design = (we are=20 all aiming for) that you might expect to achieve max static pressure = well=20 prior to the Radiator.
 
However it may be that the optimum = static=20 pressure might be design dependent and happen just before the rad. If = figure=20 if it was within the rad it would be restrictive.
 
Thinking on it further, the more = further forward=20 (of the Rad face) the optimum static air pressure is, it may suggest = that the=20 rad is too restrictive.
 
I'm not sorry I asked, a little less = confused=20 and more things to think about.
George ( down under)
 
George, here you are getting into something we = have not=20 discussed in depth.
 
Two equations/laws of fluid dynamics are = involved. =20 Bernoulli's equation and an equation called the law of =  continuity. =20 This equation relates to the fact that you don't create or lose = mass in=20 the duct, so the mass flow is a constant everywhere in the duct.  = The=20 mass flow is frequently shown as the product of air density*cross = section=20 area*air velocity =3D mass flow or simply p*A*V
 
The equation goes something like this, the=20  p1A1V1 (mass flow at point 1) =3D = p2A2V2=20 (mass flow at point 2).  Since the air is normally considered to = act like=20 it is incompressible at the lower speeds we are talking about, =  that=20 means the density  p1=3D p2, so we can drop them = from the=20 equation for this explanation.
 
That leaves us with A1V1 =3D = A2V2 or the=20 product of the area and velocity at point 1 is equal to the area and = velocity=20 at point 2 in the duct.  Now if A1 =3D A2 then = V1=20 has to equal V2 for the two sides  of = the=20 equation to be equal.   But, what if A2  =3D 2* = A1 or the cross section area of point 2 is made twice = the cross=20 section area of point 1.  Then if A2 =3D 2*A1, = we can=20 substitute 2*A1 for A2 in the = equation and=20 we have the following.
 
Taking A1V1 =3D A2V2 and = substituting we=20 have A1*V1 =3D (2*A1)*V2.   So what does = that tell us=20 about the air velocity at point 2 now that we have doubled the cross = section=20 area there? 
 
Well solving the equation for the new V2, We = can call=20 the new velocity at point 2 V2n (for = V2 new)=20 with V2o being the old velocity at point 2. =20
 
So  we have V2n = =3D=20 A1V2o/(2*A1)  Now we can cancelled the=20 A1 in the numerator and denominator on  right = side of=20 the equation leaving
 
V2n  =3D =20 V2o/2    This shows us that = the new=20 velocity at point 2, V2n is 1/2 the old velocity=20 (V2o) at point 2 or   V2n =3D=20 0.5V2o
 
So what this says is the velocity = starts changing=20 (slowing in this case and the pressure increasing ) as=20 soon as the cross section area A2 starts to increase from = A1. =20 The process continues until the area stops expanding (or the kinetic = energy of=20 the moving air has all been converted to a static pressure increase) =  and=20 that is where the process is finished as the duct/diffuser has = expanded to its=20 maximum area.  Actually, this process happens with both nozzles = and=20 diffusers just the opposite way.  Its derived from the Bernoulli = equation=20 and the continuity law.
 
So if you had a duct whose cross section area = continued=20 to expand for a distance of  2" or 20" or  200" then = theoretically=20 the pressure would continue to build and the velocity to decrease = until all of=20 the kinetic energy of the moving air has been converted to pressure=20 increase.  This is all theoretical, there are losses and = turbulence and=20 etc, that makes a difference, but you get the ideal.  It depends = on your=20 specific diffuser dimensions.
 
Think of it this way, George, some wind = tunnels have=20 diffuser which expand over 10's of feet while some microscopic cooling = systems=20 have diffusers measured in 10th's of an inch.
Now aren't you sorry you asked=20 {:>)?
 
Ed
 
----- Original Message -----
From:=20 George Lendich
To: Rotary motors in = aircraft=20
Sent: Friday, November 09, = 2007 4:52=20 PM
Subject: [FlyRotary] Re: = Total,duct,=20 Ambient or Velocity????

 Ed and Al,
This is all good info me, it either = confirms,=20 clarifies or informs.
The straw concept is a timely = reminder of=20 pressure differentials, a good example IMHO.
 
One thing I would really love to = know is - at=20 what point in the inlet duct does the dynamic flow change to static=20 pressure. I would assume this would vary with different shaped ducts = and=20 different dynamic flow ( airflow speed). 
Your opinions on this = or guesstimates ie=20 1", 2" or 3" from the face of the rad, would be of great = interest=20 to me.
 
George (down under)
Hi Al,
 
Not picky - some good points as always = .  Yes,=20 I agree, generalization does have its  pit falls, = but on=20 the other hand I think they can  help promote a conceptual=20 understanding which can be refined (through study and experiments) = to meet=20 a particular situation.  As we know, cooling airflow is = attempting to=20 balance conflicting aerodynamic and thermodynamic  = principles. =20
 
I also agree that   much of this = stuff=20 addresses the "Perfect theoretical duct" out of necessity as there = is=20 only one perfect duct but many, = many implementations=20  that fall short of perfect.  So its more of a = conceptual=20 goal to be aimed for  - it may never be achieved,=20 but provides at least guidelines.   But,this = is=20  just my opinion of course.
 
Actually, I disagree, you can not "suck" = air though=20 anything.  You may create a partial pressure difference with = the fan,=20 but it is the higher pressure air on the other end of the duct = that pushes=20 or "blows" air through the duct into the area of lower = pressure =20 {:>)  .  
 
 But, semantics aside, yes, I agree, = lower exit=20 pressure is what you are after and that does not always equate to = larger=20 exit duct area.  In fact, if the air heated by the core flows = through=20 a nozzle it might even produce thrust and lower exit = pressure using a=20 smaller exit.  But, in general, I still believe that in most = of our=20 cases, we are short of the level of duct design that would = reliably permit=20 that.  What we need is someone to invest in one of those $$$$ = Computer Fluid Flow software programs and see what they would=20 reveal.
 
Ed
----- Original Message ----- =
From:=20 Al=20 Gietzen
To: Rotary motors in = aircraft=20
Sent: Friday, November = 09, 2007=20 1:09 AM
Subject: [FlyRotary] Re:=20 Total,duct, Ambient or Velocity????

 

It =  would=20 seem "reasonable" that a low pressure area at the exit =  will help=20 flow through a duct - no argument on that point.  What the = report=20 appeared to say is that the after a certain point opening the = exit area=20 wider does not appear to have any = additional benefit. (Exit =93area=94 and exit = =93pressure=94 are=20 not interchangeable terms) That if the duct is capable = of "using=20 up" all of the kinetic energy in your air flow by obstructions, = pressure=20 drops  and friction losses then enlarging the exit does not = necessarily  add to the flow.

 

Remember you=20 can not suck air through a duct, you can only blow it=20 through. (Of = course you=20 can suck air through a duct =96 I do it after (and sometimes = before) every=20 flight with the fan I have on the back side of the=20 radiator) So in effect if the straw is pinched you = can=20 "suck" on it all you want but it won't increase flow=20 {:>).  

 

If = I understood=20 the report,  it appears that enlarging the exit area beyond = the=20 frontal area of your core provides little if any additional=20 benefit.  That does not mean cowl flaps never work or = provide=20 benefit.  In fact it appears that the better your duct, =  the=20 more benefit the cowl flaps appear to have, the worst your duct, = the=20 lesser benefit - just the opposite of what you might think.

Ed;

 

Don=92t mean to=20 be picky, but some of these generalities are making me=20 nervousJ.  These=20 things are applicable only when the duct/diffuser is operating = at max=20 efficiency =96 which is rarely the case.

Lot=92s of good=20 info. =20 Thanks.  You=92re right; it=92s some kind of magic, and you = don=92t know=20 for sure until you built it and try it.

Al

No virus found in this incoming message.
Checked by AVG = Free=20 Edition.
Version: 7.5.503 / Virus Database: 269.15.24/1115 - = Release=20 Date: 7/11/2007 9:21 AM

No virus found in this incoming message.
Checked by AVG Free = Edition.
Version: 7.5.503 / Virus Database: 269.15.27/1121 - = Release Date:=20 9/11/2007 7:29 PM
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