[mumtaz ersan]

Hello Everyone,

I am very new about cfd issue,i am very confused about calculating DeltaP(Pressure drop) and actually i need a help,

My first question is;
Is the inlet Pressure important for pressure-drop calculation in compressible-fluids (AIR). I mean that ; If air pass through from valve or nozzle with the same volume-flow or mass-flow but at different inlet pressures(For example; in my first test 2,5 bar inlet pressure at 120lt./min and in my second test 1,1 bar inlet pressure at 120lt./min )At the end, what can we say about their DeltaP(pressure-drop),For these two test ; Will their DeltaP(Pressure-Drop) be same or not ??????

My Second question is;
The formula for the ideal-gas, P=d*R*T ,
in this formula P symbolize which pressure ??? Static-pressure or Total-Pressure????

My third and last question is;

How do cfd programs calculate total-pressure and static pressure and density???? They calculate by the help of ideal-gas equation(P=d*R*T) or different way????? For example i define air at 120 lt./min at 298 Kelvin for inlet in a nozzle for calculating pressure-drop, and i define enviroment-pressure for outlet in solid-works flow simulation by the way my inlet dia is 18.95 mm.

Program gives me for a result;
Density : 1,18
Total Pressure :101407,47 pascal
Static Pressure :101427,47
Dynamic pressure : 30,11

If i calculate total-pressure by manually i find:

P=1.18*0,287*298 100.92 kpa==100920 Pascal and it isn't equal to total-pressure which is taken from program.So i think i miss sth....

P.S i know this forum topic isn't for solidworks-flow simulation but i watch few tutorials on the internet and the other programs use the same method with solidworks flowsimulation for calculating pressure-drop so i decide to write here for the purpose of reaching more people.....

I hope i didn't do wrong thing to write here,

And i hope explaine my question...

Can anyone explain this questions at least one of them????

Thanks for everyone...

[fluid23]

My first question is;
Is the inlet Pressure important for pressure-drop calculation in compressible-fluids (AIR). I mean that ; If air pass through from valve or nozzle with the same volume-flow or mass-flow but at different inlet pressures(For example; in my first test 2,5 bar inlet pressure at 120lt./min and in my second test 1,1 bar inlet pressure at 120lt./min )At the end, what can we say about their DeltaP(pressure-drop),For these two test ; Will their DeltaP(Pressure-Drop) be same or not ??????
This is a little confusing how you have worded it. Be careful about specifying mass flow rate and pressure at a boundary, they often contradict each other. Consider a simple pipe flow analysis, you can't specify both parameters. One will dictate the other. In general pressure drop would be inlet to outlet, but that also depends on your geometry, domain, and more. Please provide more info (but be concise).

My Second question is;
The formula for the ideal-gas, P=d*R*T ,
in this formula P symbolize which pressure ??? Static-pressure or Total-Pressure????
-Static.

My third and last question is;

How do cfd programs calculate total-pressure and static pressure and density???? They calculate by the help of ideal-gas equation(P=d*R*T) or different way????? For example i define air at 120 lt./min at 298 Kelvin for inlet in a nozzle for calculating pressure-drop, and i define enviroment-pressure for outlet in solid-works flow simulation by the way my inlet dia is 18.95 mm.
-Based on your last two questions, I would highly suggest you do some research. Basically; pressure, temperature and velocity are all results of the Navier-Stokes Equations. Total temperature and total pressure are calculated from pressure, temperature, and velocity. (They are post processed results and not directly calculated via CFD equations).

Program gives me for a result;
Density : 1,18
Total Pressure :101407,47 pascal
Static Pressure :101427,47
Dynamic pressure : 30,11

If i calculate total-pressure by manually i find:

P=1.18*0,287*298 100.92 kpa==100920 Pascal and it isn't equal to total-pressure which is taken from program.So i think i miss sth....

Again, kind of confusing based on how you present your info. Not very concise. However, it could be that the pressures you are specifying in your software are supposed to be gauge and not absolute (or vice versa).
For example, if I were to calculate total pressure (for low speed gas using Bernoulli) I would get Pt= Ps + 1/2 * rho * V^2. I have no idea what equation this is: P=1.18*0,287*298 100.92 kpa

[agd]

Just a note for the original poster - the total pressure should never be less than the static pressure. For compressible flow the isentropic relation should be used:

p_tot/p_static = (1 + 0.5*(gam-1)*M^2)^n

where gam is the ratio of specific heats, M is the Mach number, and n is equal to
n = gam/(gam-1)

This formula, along with a good definition of total conditions, can be found in most decent undergrad textbooks on compressible flow, or try googling for isentropic Mach number relations.

[mumtaz ersan]

First of all, thanks for both of you for your answers,

Actually , My first and third ques. are independent from each other,

About for my first question=

I asked it for general information it isnt for CFD analyze.

For example, Assume that i have a valve and i will do 2 different experiments for pressure-drop;

In my first test: I regulate supply pressure at 2 bar and i regulate volume-flow at 120 lt/min..Then let the air pass through from this valve and i will measure outlet pressure with a help of Pressure-sensor.And last, i calculate pressure-drop at this condition.(Supply pressure-outlet pressure).

In my second test:i regulate supply pressure at 4 bar and i regulate volume-flow at 120 lt./min..Then let the air pass through from this valve and i will measure outlet pressure with a help of Pressure-sensor.And last i calculate pressure-drop.

Now , my question is ; Pressure-drop which comes from my first test is equal (or not equal) to Pressure-drop which comes from my second test.

If they are equal, why???
If they are not equal , why ???


About my thrid question;

I will examine the navier-stokes equation , it will be very helpfull for me , thanks for suggestion...

Inlet Density : 1,18
Inlet Total Pressure :101407,47 pascal
Inlet Static Pressure :101427,47
Inlet Dynamic pressure : 30,11
Inlet Temperature : 298 Kelvin

these are results which i got from software after defining volume-flow at 120lt/min air for inlet-boundary and defining environment-pressure for outlet boundary...

In here, i want to calculate these pressure values manually by myself.

So can i use ideal-gas equations for static-pressure (P=d*R*T) in here?????

P==1,18*0,287*298==100920 Pascal isn't equal to 101427,47 Pascal which is come from software...

I can calculate dynamic pressure Pdynamic=0,5*d*V^2=0,5*1,18*7.19^2=30,11

But while calculating static pressure , i have a trouble actually....


I hope, at this time , i manage to explain my problem....