X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from cdptpa-omtalb.mail.rr.com ([75.180.132.121] verified) by logan.com (CommuniGate Pro SMTP 5.2c1) with ESMTP id 2464866 for flyrotary@lancaironline.net; Sat, 10 Nov 2007 08:22:57 -0500 Received-SPF: pass receiver=logan.com; client-ip=75.180.132.121; envelope-from=eanderson@carolina.rr.com Received: from edward2 ([24.74.103.61]) by cdptpa-omta06.mail.rr.com with SMTP id <20071110132216.CSNC507.cdptpa-omta06.mail.rr.com@edward2> for ; Sat, 10 Nov 2007 13:22:16 +0000 Message-ID: <000b01c8239c$f4273660$2402a8c0@edward2> From: "Ed Anderson" To: "Rotary motors in aircraft" References: Subject: Re: [FlyRotary] Re: Total,duct, Ambient or Velocity???? Date: Sat, 10 Nov 2007 08:23:59 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0008_01C82373.0B072D10" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.3138 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.3138 This is a multi-part message in MIME format. ------=_NextPart_000_0008_01C82373.0B072D10 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable George, I'm not certain how valid this next statement is - but, I = think you want the minimum volume you can get away with after the = diffuser - meaning you want your radiator core as close to your diffuser = exit as possible. My reasoning is that apparently this "resistance" is = in large part responsible for the ability to have efficient diffusers = which have a much larger divergence angle than the 11 deg (which is = about the maximum for a diffuser without this resistance) before flow = separation occurs.=20 So does moving the radiator further way from you diffuser exit = adversely affect the effect of this resistance? I don't really know , = but I suspect it does. Ed ----- Original Message -----=20 From: George Lendich=20 To: Rotary motors in aircraft=20 Sent: Saturday, November 10, 2007 12:13 AM Subject: [FlyRotary] Re: Total,duct, Ambient or Velocity???? 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. 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) 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. =20 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_0008_01C82373.0B072D10 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
George, I'm not certain how valid =  this next=20 statement is  - but, I think you want the minimum volume you can = get away=20 with after the diffuser - meaning you want your radiator core as close = to your=20 diffuser exit as possible.  My reasoning  is that = apparently this=20 "resistance" is in large part responsible for the ability to have = efficient=20 diffusers which have a much larger divergence angle than the 11 deg = (which is=20 about the maximum for a diffuser without this resistance)  before = flow=20 separation occurs. 
 
 So does moving the radiator further way = from you=20 diffuser exit adversely affect the effect of this resistance?  I = don't=20 really know , but I suspect it does.
 
Ed
----- Original Message -----
From:=20 George=20 Lendich
Sent: Saturday, November 10, = 2007 12:13=20 AM
Subject: [FlyRotary] Re: = Total,duct,=20 Ambient or Velocity????

 
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=20 forward (of the Rad face) the optimum static air pressure is, it may = suggest=20 that the 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=20 not discussed in depth.
 
Two equations/laws of fluid dynamics are=20 involved.  Bernoulli's equation and an equation called the law = of=20  continuity.  This equation relates to the fact that = you=20 don't create or lose mass in the duct, so the mass flow is a = constant=20 everywhere in the duct.  The mass flow is frequently shown as = the=20 product of air density*cross section area*air velocity =3D mass flow = or simply=20 p*A*V
 
The equation goes something like this, the=20  p1A1V1 (mass flow at point 1) =3D=20 p2A2V2 (mass flow at point 2).  Since the air = is=20 normally considered to act like it is incompressible at the lower = speeds we=20 are talking about,  that means the density  p1=3D=20 p2, so we can drop them from the equation for this=20 explanation.
 
That leaves us with A1V1 =3D = A2V2 or=20 the product of the area and velocity at point 1 is equal to the area = and=20 velocity at point 2 in the duct.  Now if A1 =3D = A2 then=20 V1 has to equal V2 for the two = sides =20 of the equation to be equal.   But, what if = A2  =3D 2*=20 A1 or the cross section area of point 2 is made twice = the=20 cross section area of point 1.  Then if A2 =3D = 2*A1, we=20 can substitute 2*A1 for A2 in the = equation=20 and we have the following.
 
Taking A1V1 =3D A2V2 and = substituting=20 we have A1*V1 =3D (2*A1)*V2.   So what = does that=20 tell us about the air velocity at point 2 now that we have doubled = the cross=20 section area there? 
 
Well solving the equation for the new V2, We = can call=20 the new velocity at point 2 V2n = (for V2=20 new) with V2o being the old velocity at point = 2. =20
 
So  we have = V2n=20 =3D 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=20 of the moving air has all been converted to a static pressure = increase)=20  and that is where the process is finished as the duct/diffuser = has=20 expanded to its maximum area.  Actually, this process happens = with both=20 nozzles and diffusers just the opposite way.  Its derived from = the=20 Bernoulli equation and the continuity law.
 
So if you had a duct whose cross section = area=20 continued to expand for a distance of  2" or 20" or  200" = then=20 theoretically the pressure would continue to build and the velocity = to=20 decrease until all of the kinetic energy of the moving air has been=20 converted to pressure increase.  This is all theoretical, there = are=20 losses and turbulence and etc, that makes a difference, but you get = the=20 ideal.  It depends on your 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=20 systems 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 to me.
 
George (down under)
Hi Al,
 
Not picky - some good points as always = . =20 Yes, I agree, generalization does have its  pit = falls,=20 but on the other hand I think they can  help promote a = conceptual=20 understanding which can be refined (through study and = experiments) to=20 meet a particular situation.  As we know, cooling airflow = is=20 attempting to balance conflicting aerodynamic and = thermodynamic =20 principles. 
 
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=20 though anything.  You may create a partial pressure = difference with=20 the fan, but it is the higher pressure air on the other end of = the duct=20 that pushes or "blows" air through the duct into the area of = lower=20 pressure  {:>)  .  
 
 But, semantics aside, yes, I = agree, lower=20 exit pressure is what you are after and that does not always = equate to=20 larger exit duct area.  In fact, if the air heated by the = core=20 flows through a nozzle it might even produce thrust and lower = exit=20 pressure using a smaller exit.  But, in general, I = still=20 believe that in most of our cases, we are short of the level of = duct=20 design that would reliably permit that.  What we need is = someone to=20 invest in one of those $$$$ Computer Fluid Flow software = programs and=20 see what they would reveal.
 
Ed
----- Original Message ----- =
From:=20 Al=20 Gietzen
To: Rotary motors in=20 aircraft
Sent: Friday, November = 09, 2007=20 1:09 AM
Subject: [FlyRotary] = Re:=20 Total,duct, Ambient or Velocity????

 

It=20  would seem "reasonable" that a low pressure area at the = exit=20  will help flow through a duct - no argument on that = point. =20 What the report appeared to say is that the after a certain = point=20 opening the exit area wider does not appear to have any=20 additional benefit. (Exit =93area=94 and exit =93pressure=94 = are not=20 interchangeable terms) That if the duct is = capable of=20 "using up" all of the kinetic energy in your air flow by = obstructions,=20 pressure drops  and friction losses then enlarging the = exit does=20 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)=20 every 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=20 understood the report,  it appears that enlarging the = exit area=20 beyond the 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=20 to be picky, but some of these generalities are making me=20 nervousJ. =20 These things are applicable only when the duct/diffuser is = operating=20 at max 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=20 know 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=20 Edition.
Version: 7.5.503 / Virus Database: 269.15.27/1121 - = Release=20 Date: 9/11/2007 7:29 PM
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