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• # Query regarding reference values in Ansys solver

Posted by on September 20, 2023 at 8:23 am

Hi everyone,

I had a query regarding how to get the reference values (especially area) for our geometries to simulate.

When I was simulating the flow over a car I used the project area tool to find the area which doesn’t change after a particular lower value as the reference area value. However, this technique didn’t work for me when I was simulating flow over a cylinder so for that case, I just used the curved surface area.

Right now I am simulating an axisymmetric flow over a venturi nozzle and wanted to know how to calculate the reference area from a 2D geometry. I did some searching and found one reply from the forums stating we need to assume a 3D geometry of the 3D case and take the frontal area from that case. I just wanted to know if that is correct or if there is a generalised easier or right way of approaching this.

Additionally, when we use pressure value as the inlet condition, do we need to put the reference value velocity as 0?

Finally, when we use the reference values do we always compute it from the inlet?

replied 9 months ago 2 Members · 5 Replies
• 5 Replies
• ### Sandeep

September 20, 2023 at 8:47 am

Reference values are only for post-processing. It will not be used for the actual solution. But once you get the solution of primitive variables then further post-processing like calculation of CD, HTC etc can be done using reference value. In 2D case the depth will be taken as 1 m so you have to input the length scale accordingly. But most of the time we don’t depend on the reference values for 2D calculations. We will directly use the results and do further calculations.
In the case of cylinder/car etc. the reference area is the frontal area. The projected area in the direction of flow.

Member
September 20, 2023 at 11:42 am

Thank you sir, I just had a few more doubts

I am trying to find the mass flow rate for a 2-D axisymmetric geometry using Fluent but I am getting negative values for the mass flow rate at the outlet. I am not sure why I am getting negative values. I am attaching the plot below for reference. i tried plotting the velocity vectors and they show the fluid going outwards from the outlet.

Also for calculating the coefficient of discharge, do we need to take the Poiseuilles equation to calculate the ideal flow rate and then divide by the mass flow rate from fluent to get the values?

• ### Sandeep

September 20, 2023 at 11:51 am

It will be negative as the flow is outside the domain. The inlet will be positive so that the net is zero for mass conservation.
For calculating ideal mass flow, you can use the Bernoulli equation. Do some literature survey about the Cd of an orifice.

Member
September 20, 2023 at 2:32 pm

Hi Sir,

I used Bernoulli’s equation to calculate the volume flow rate manually and got a Q_theoretical = 2 m^3/s

However, when I take the inlet volumetric flow rate from fluent I get a value of 0.008 m^3/s as shown in the screenshots. But this lies way outside the permissible Cd value of 0.6.

I gave the following BCs,

Outlet: gauge pressure = 0 Pa

Inlet: total gauge pressure = 50000 Pa

axis line

and 0-shear walls

I am not sure where I am making the mistake. It would be great if you could suggest some possible mistakes I could be making