CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Main CFD Forum

What on earth is the criteria for the compressible

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   June 20, 2007, 09:41
Default What on earth is the criteria for the compressible
  #1
xumustc
Guest
 
Posts: n/a
What on earth is the criteria for the compressible fluid flow , Ma>0.3 or Ma>1 ?

When Ma>0.3, the fluid flow is considered as compressible. However, for the flow over a two-dimensional flat plate at Ma=0.5, the calculation of boundary layer thickness is about the same as the theory value for uncompressible case. Why?

After looking through some books ,such as F.M. White's Viscous Fluid Flow, H.Sclichting's Boundary-Layer Theory , what they mentioned the compressible fluid flow seems refer to the cases of Ma>1.

So, it comes to the conclusion that when Ma>1, we ,in practice, use the theory for the compressible case . Is this right, Sir?

  Reply With Quote

Old   June 20, 2007, 12:09
Default Re: What on earth is the criteria for the compress
  #2
Harish
Guest
 
Posts: n/a
Another rule of thumb of the measure of incompressibility is the change of density.If it lies within a certain range then flow can be assumed incompressible.Further as long as the incompressible code is able to capture the physics even if it slightly into the compressible regime it would be fine.
  Reply With Quote

Old   June 21, 2007, 10:14
Default Re: What on earth is the criteria for the compress
  #3
Mani
Guest
 
Posts: n/a
>Another rule of thumb of the measure of incompressibility is the change of density.

Not to increase the present confusion: Let's be more precise.

a) Compressibility is the ability of density to change due to pressure changes. Mathematically, the derivative of density with respect to pressure is a measure for compressibility. (Density can also change because of temperature changes, which has nothing to do with compressibility.)

b) The above not a "rule of thumb", it's the definition of compressibility.

Here's the rule of thumb. The density on the surface of an airfoil (or any other object) will vary between the absolute minimum (the density at the point of highest local Mach number) and the absolute maxmimum (stagnation density). The ratio of stagnation density to local static density is a function of local Mach number, for ideal gases. For air as an ideal gas (ratio of specific heats = 1.4), the stagnation density is about 5% higher than the static density at a Mach number of 0.3. Hence, if the maximum Mach number on the surface of an airfoil is 0.3, the density in the entire flow field will vary by no more than 5%, which is relatively small. This is the reason why Mach 0.3 is generally considered as upper limit for "incompressible" flow. For any Mach number above that, you will expect larger variations of density.

You'll notice that this rule of thumb is based on the variation of density within a flow field. This isn't exactly the definition of compressibility as stated above (hence, "rule of thumb"). Notice also, that it is the *local* maximum Mach number that matters, not the free-stream value! At the same free-stream Mach number, compressibility will be much more apparent on a thick airfoil than on a flat plate. With a flat plate in subsonic flow, the highest Mach number occurs very locally at the leading edge and that is a singular point, anyway!

I guess you'll see the complications of "rules of thumb". You may use them as you wish, but understand and keep in mind where they come from.
  Reply With Quote

Old   June 22, 2007, 04:12
Default Re: What on earth is the criteria for the compress
  #4
Ahmed
Guest
 
Posts: n/a
1- Incompressibility hypothesis is not a physical one, IT IS an engineering design hypothesis 2- As posted before, Engineers in their designs neglect changes in density as a result of pressure changes when they are less than 3% (Which corresponds to Mach =0.3 for air flows)(Check The derivation in Anderson Book) 3- It is up to you to decide whether this hypothesis is valid in your design/research or not
  Reply With Quote

Old   June 22, 2007, 07:16
Default Re: What on earth is the criteria for the compress
  #5
Anton Lyaskin
Guest
 
Posts: n/a
To introduce more confusion - sometimes (e.g. high lift multielement airfoils) you can have a significant change of density even at Ma<0.3. And for some applications like aeroacustics even very small density changes have to be accounted for, which can be observed even at nearly zero Mach numbers.
  Reply With Quote

Old   June 22, 2007, 08:44
Default Re: What on earth is the criteria for the compress
  #6
xumustc
Guest
 
Posts: n/a
Thanks Harish for---

"as long as the incompressible code is able to capture the physics "
  Reply With Quote

Old   June 22, 2007, 08:47
Default Re: What on earth is the criteria for the compress
  #7
xumustc
Guest
 
Posts: n/a
Thanks Mani for "I guess you'll see the complications of "rules of thumb". You may use them as you wish, but understand and keep in mind where they come from. "

  Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Compressible -> incompressible. Jinwon Main CFD Forum 6 November 23, 2007 22:07
How can I set the converge criteria properly? Alina FLUENT 3 September 7, 2007 17:55
compressible gas problem hx CFX 1 January 5, 2004 03:00
PHI file structure Eugene Phoenics 9 November 2, 2001 23:00
Compressible vs. Incompressible formulations Fernando Velasco Hurtado Main CFD Forum 3 January 7, 2000 17:51


All times are GMT -4. The time now is 08:23.