Search found 80 matches

by sam
Sat Apr 15, 2006 1:29 pm
Forum: Valved pulsejet forum
Topic: Sir Lord Rayleigh's criterion
Replies: 15
Views: 9087

re: Sir Lord Rayleigh's criterion

The First Law of Thermodynamics simply states that energy can be neither created nor destroyed (conservation of energy). Thus power generation processes and energy sources actually involve conversion of energy from one form to another, rather than creation of energy from nothing Well obviously what...
by sam
Fri Apr 14, 2006 10:32 pm
Forum: Valved pulsejet forum
Topic: Sir Lord Rayleigh's criterion
Replies: 15
Views: 9087

re: Sir Lord Rayleigh's criterion

If Heat is given to the air at the moment of greatest condensation, or be taken from it at the moment of greatest rarefaction, the vibration is encouraged You don't actually have to remove heat from the cycle for this effect to occur - you just have to have a less than average amount of heat additi...
by sam
Mon Apr 03, 2006 7:15 pm
Forum: Valved pulsejet forum
Topic: a Hydraulic Model for a P Combustor
Replies: 14
Views: 7885

re: a Hydraulic Model for a P Combustor

I believe this to be true for some cases ... but not all. I disagree with you but never mind that for now. Please can you give some more details about your model. For example: - How are you modelling the ingestion of reactants (sink flow perhaps?) - What are the two additional parameters which you ...
by sam
Sat Apr 01, 2006 8:44 am
Forum: Valved pulsejet forum
Topic: a Hydraulic Model for a P Combustor
Replies: 14
Views: 7885

re: a Hydraulic Model for a P Combustor

Forrest, I don't understand what you've done here. How can you possibly model a pulse combustor using incompressible theory? Pulse combustors work because heat is added periodically to an acoustic wave - this is known as a thermoacoustic instability. Any attempt to simulate acoustic phenomena requir...
by sam
Fri Feb 17, 2006 9:52 am
Forum: Valveless pulsejet forum
Topic: A Study In Pressure Vs Mass Flow
Replies: 31
Views: 14868

re: A Study In Pressure Vs Mass Flow

Larry, A compression wave will not necessarily induce a velocity in the direction of propagation. Instead it induces a *change* in velocity in that direction. This change might not be enough to switch the direction of mass flow - it depends on the strength of the wave. For a rarefaction wave, the op...
by sam
Mon Feb 13, 2006 2:12 pm
Forum: Tools and Construction
Topic: Connecting pressure transducer to computer
Replies: 12
Views: 6585

re: Connecting pressure transducer to computer

OK that's fine then - I misunderstood your application. Sorry for the confusion
by sam
Mon Feb 13, 2006 1:02 pm
Forum: Tools and Construction
Topic: Connecting pressure transducer to computer
Replies: 12
Views: 6585

re: Connecting pressure transducer to computer

That pressure transducer is no good for measuring unsteady pressures at frequencies greater than about 1kHz. Is that high enough? This limitation is because the diaphragm on which the strain gauge is mounted is remote from the measurement location. Ideally you want a pressure transducer with the dia...
by sam
Fri Dec 09, 2005 9:39 am
Forum: Valved pulsejet forum
Topic: Dimensionless Space and Time
Replies: 82
Views: 2841030

re: Dimensionless Space and Time

forrest, Care to reveal a little more about your code. What numerical scheme are you using? What boundary conditions are you including? (open and closed ends by the sounds of it). I assume its 1D - Does it allow for non-uniform cross-sections? I've got a fair bit of experience in CFD codes and I thi...
by sam
Wed Dec 07, 2005 7:31 pm
Forum: Valved pulsejet forum
Topic: Dimensionless Space and Time
Replies: 82
Views: 2841030

re: Dimensionless Space and Time

Hi Pezman, How's that shock tube simulation coming on? You can compare it directly with an exact solution, at least until the point where you start getting reflections from the boundaries. It will help to give some idea of the robustness of your code anyway. Can I ask what numerical scheme you are u...
by sam
Mon Dec 05, 2005 10:16 am
Forum: Valved pulsejet forum
Topic: Dimensionless Space and Time
Replies: 82
Views: 2841030

re: Dimensionless Space and Time

Forrest, [quote] I looked at the T(emperature) data and there's a 'jump' here from 0.22296745E+03 to 0.29997519E+03, so the acoustic velocity is pretty much 347.2 m/sec. [/quote] The 'jump' in temperature is physically correct and occurs when the code switches from outflow to inflow. During outflow ...
by sam
Sun Dec 04, 2005 3:55 pm
Forum: Valved pulsejet forum
Topic: Dimensionless Space and Time
Replies: 82
Views: 2841030

re: Dimensionless Space and Time

[quote]Well, I might have used the wrong speed of sound for non-dimensionalizing the flow velocity. I used 347.2 m/sec (the speed of sound in air at 300°K). [/quote] Well thats probably the problem. For the true Mach Number you have to non-dimensionalise by the true speed of sound, which varies wit...
by sam
Sat Dec 03, 2005 11:53 pm
Forum: Valved pulsejet forum
Topic: Dimensionless Space and Time
Replies: 82
Views: 2841030

re: Dimensionless Space and Time

fde,
I've just checked my solution to the problem and I too get an initial outflow Mach Number of 0.8, as your Method of Characteristic solution suggests. I'm not sure where you went wrong... maybe you weren't normalising the velocity by the correct speed of sound?
sam
by sam
Sat Dec 03, 2005 11:26 pm
Forum: Valved pulsejet forum
Topic: Dimensionless Space and Time
Replies: 82
Views: 2841030

re: Dimensionless Space and Time

[quote]Forgive my ignorance, but how do you 'compute' the time step during the data input stage?[/quote] The timestep is calculated by a stability crtierion (or CFL criterion as its usually called). You can't really work it out before hand, and the program calculates it as it goes along. Its given b...
by sam
Sat Dec 03, 2005 12:12 pm
Forum: Valved pulsejet forum
Topic: Dimensionless Space and Time
Replies: 82
Views: 2841030

re: Dimensionless Space and Time

Forrest, I suspect that the discrepancy in the velocity plot is because my open-end boundary condition is different to yours. I think I'm right in saying that you have been assuming that the static pressure stays constant at one atmosphere at all times at the open boundary. This is the usual "open-e...
by sam
Fri Dec 02, 2005 4:27 pm
Forum: Valved pulsejet forum
Topic: Dimensionless Space and Time
Replies: 82
Views: 2841030

re: Dimensionless Space and Time

Just to prove that my code (NUDiS) can be used to solve this problem too...
The code is different (better?) than UFLOW because it can model a proper closed end boundary condition.
Sam