Thread Subject: Simscape - domains with multiple across variables

I am using Simscape to create a thermal system models. For proof of concept, I am trying to model a Brayton Cycle (compressor, combustor and turbine in series, where the compressor is driven by the work output of the turbine). Since both pressure and temperature of the air entering the turbine are needed to predict the work output, I am using my own domain adapted from the Simscape hydraulic foundation library that includes both pressure and temperature as across variables, while the only through variable is mass flow rate. All of the Demos and examples from Mathworks consist of domains that contain only one through and one across variable. How, if at all, does my domain differ and what sort of workarounds do you recommend?

"John Kreuder" <gedoboarder@hotmail.com> wrote in message <h0mbdm$5sv$1@fred.mathworks.com>...
> I am using Simscape to create a thermal system models. For proof of concept, I am trying to model a Brayton Cycle (compressor, combustor and turbine in series, where the compressor is driven by the work output of the turbine). Since both pressure and temperature of the air entering the turbine are needed to predict the work output, I am using my own domain adapted from the Simscape hydraulic foundation library that includes both pressure and temperature as across variables, while the only through variable is mass flow rate. All of the Demos and examples from Mathworks consist of domains that contain only one through and one across variable. How, if at all, does my domain differ and what sort of workarounds do you recommend?

Hi,

I would suggest adding heat flow as a through variable. The tricky part is defining temperature-dependent domain-wide parameters, such as fluid properties. If you manage to get it working, I would encourage you to post it to the FEX for sharing with the rest of the community. Good luck!

Arnaud

"Arnaud Miege" <arnaud.miege@nospam.mathworks.co.uk> wrote in message <h0meqe$m2b$1@fred.mathworks.com>...
> "John Kreuder" <gedoboarder@hotmail.com> wrote in message <h0mbdm$5sv$1@fred.mathworks.com>...
> > I am using Simscape to create a thermal system models. For proof of concept, I am trying to model a Brayton Cycle (compressor, combustor and turbine in series, where the compressor is driven by the work output of the turbine). Since both pressure and temperature of the air entering the turbine are needed to predict the work output, I am using my own domain adapted from the Simscape hydraulic foundation library that includes both pressure and temperature as across variables, while the only through variable is mass flow rate. All of the Demos and examples from Mathworks consist of domains that contain only one through and one across variable. How, if at all, does my domain differ and what sort of workarounds do you recommend?
>
> Hi,
>
> I would suggest adding heat flow as a through variable. The tricky part is defining temperature-dependent domain-wide parameters, such as fluid properties. If you manage to get it working, I would encourage you to post it to the FEX for sharing with the rest of the community. Good luck!
>
> Arnaud

Arnaud,
But can one still have two across variables with one variable? What problems might arise if such a domain definition is used?

Won't adding heat flow as a through variable actually cause problems since the heat flow of a component such as a heat exchanger, combustor, turbine, etc. will actually change "across" the component?
Rob

"Arnaud Miege" <arnaud.miege@nospam.mathworks.co.uk> wrote in message <h0meqe$m2b$1@fred.mathworks.com>...
> "John Kreuder" <gedoboarder@hotmail.com> wrote in message <h0mbdm$5sv$1@fred.mathworks.com>...
> > I am using Simscape to create a thermal system models. For proof of concept, I am trying to model a Brayton Cycle (compressor, combustor and turbine in series, where the compressor is driven by the work output of the turbine). Since both pressure and temperature of the air entering the turbine are needed to predict the work output, I am using my own domain adapted from the Simscape hydraulic foundation library that includes both pressure and temperature as across variables, while the only through variable is mass flow rate. All of the Demos and examples from Mathworks consist of domains that contain only one through and one across variable. How, if at all, does my domain differ and what sort of workarounds do you recommend?
>
> Hi,
>
> I would suggest adding heat flow as a through variable. The tricky part is defining temperature-dependent domain-wide parameters, such as fluid properties. If you manage to get it working, I would encourage you to post it to the FEX for sharing with the rest of the community. Good luck!
>
> Arnaud

Arnaud,
But can one still have two across variables with one variable? What problems might arise if such a domain definition is used?

Won't adding heat flow as a through variable actually cause problems since the heat flow of a component such as a heat exchanger, combustor, turbine, etc. will actually change "across" the component?
Rob

Hi,
It seems that the our questions have some common ground.
my question is:
http://www.mathworks.com/matlabcentral/newsreader/view_thread/248136#641479

"Robert " <rjseme@rit.edu> wrote in message
news:h0n62q$dme$1@fred.mathworks.com...
> "Arnaud Miege" <arnaud.miege@nospam.mathworks.co.uk> wrote in message
> <h0meqe$m2b$1@fred.mathworks.com>...
>> "John Kreuder" <gedoboarder@hotmail.com> wrote in message
>> <h0mbdm$5sv$1@fred.mathworks.com>...
>> > I am using Simscape to create a thermal system models. For proof of
>> > concept, I am trying to model a Brayton Cycle (compressor, combustor
>> > and turbine in series, where the compressor is driven by the work
>> > output of the turbine). Since both pressure and temperature of the air
>> > entering the turbine are needed to predict the work output, I am using
>> > my own domain adapted from the Simscape hydraulic foundation library
>> > that includes both pressure and temperature as across variables, while
>> > the only through variable is mass flow rate. All of the Demos and
>> > examples from Mathworks consist of domains that contain only one
>> > through and one across variable. How, if at all, does my domain differ
>> > and what sort of workarounds do you recommend?
>>
>> Hi,
>>
>> I would suggest adding heat flow as a through variable. The tricky part
>> is defining temperature-dependent domain-wide parameters, such as fluid
>> properties. If you manage to get it working, I would encourage you to
>> post it to the FEX for sharing with the rest of the community. Good luck!
>>
>> Arnaud
>
> Arnaud,
> But can one still have two across variables with one variable? What
> problems might arise if such a domain definition is used?
>
> Won't adding heat flow as a through variable actually cause problems since
> the heat flow of a component such as a heat exchanger, combustor, turbine,
> etc. will actually change "across" the component?
> Rob

Hi Rob,

You can have two across variable and one through variable, there are no
rules as to the number of across and through variables in a domain, but it's
generally good practice to have a pair of across and through variables, in
your case pressure/mass flow rate and temperature/heat flow. The effect of
heat flow (in J/s) on temperature is similar to that of mass flow rate on
pressure. The through variable is what flows through a port (and therefore
you measure it in series) whereas the across variable is something that you
measure relative to a reference, i.e. in parallel. Think of voltage and
current in the electrical domain. Additionally if you are using the latest
versuion (R2009a), the source code for most of the blocks in the Simscape
foundation library is provided.

HTH,

Arnaud

Arnaud,

I keep seeing that it is "best practice" to have a pair of one through and one across variable, but why? What is the technical reason for having pairs versus something else? Will some problems arise under certain situations that I should be aware of?

Thanks,
Rob

Arnaud,

I keep seeing that it is "best practice" to have a pair of one through and one across variable, but why? What is the technical reason for having pairs versus something else? Will some problems arise under certain situations that I should be aware of?

Thanks,
Rob

"Robert " <rjseme@rit.edu> wrote in message <h0rb50$79k$1@fred.mathworks.com>...
> Arnaud,
>
> I keep seeing that it is "best practice" to have a pair of one through and one across variable, but why? What is the technical reason for having pairs versus something else? Will some problems arise under certain situations that I should be aware of?
>
> Thanks,
> Rob

Hi Rob,

This is only a suggestion and reflects only my opinion, but if you think of the energy/power that is being exchanged in a physical network, it is generally the product of an across and a through variable, e.g:
- electrical power = voltage x current
- mechanical power = force x velocity or torque x rotational velocity
- hydraulic power = pressure x flow rate
- thermal power = heat flow x temperature

That's why I suggested having a pair of across and through variables, it just makes sense from a physical point of view