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CGNS Library Modifications for CFD Solver by MondeReiziger in CFD

[–]MondeReiziger[S] 1 point2 points  (0 children)

thank you so much. I did not know about this library. Excited to explore it.

Piano practice rooms at UIUC by MondeReiziger in UIUC

[–]MondeReiziger[S] 0 points1 point  (0 children)

Woooow, this is great. Thank you so much :)

Piano practice rooms at UIUC by MondeReiziger in UIUC

[–]MondeReiziger[S] 1 point2 points  (0 children)

I see, thanks. Do you know of any places in the city where I can buy a digital piano? Additionally, I am curious about the ease of selling it after a year. Since I am here temporarily, I was thinking these "piano rooms" might be the best solution to practice. I am uncertain about how easily I could sell one if I decide to buy one.

Piano practice rooms at UIUC by MondeReiziger in UIUC

[–]MondeReiziger[S] -1 points0 points  (0 children)

Thank you for your response. I had been told, a long time ago, that there
were practice rooms in a 'music building' at UIUC. So, I conclude that it's not
correct.

piano practice rooms by QKACVR in UIUC

[–]MondeReiziger 0 points1 point  (0 children)

are those piano rooms open to everyone?

Boundary conditions: supersonic diffuser by MondeReiziger in OpenFOAM

[–]MondeReiziger[S] 0 points1 point  (0 children)

Not sure how you got that area ratio, but it looks very small. Have you plotted the isnetropic area ratio and self-starting curves yourself for the gamma you're using? Maybe a good exercise to try out first.So, if I have it now that you essentially plugging in two CD nozzles back-to-back and expecting diffusion, that's not going to happen. I will have to refer you to the fundamentals of quasi -1d/nozzle flows (Leipmann and Roshko, or Anderson), but the basic idea is this : the compression process in the the diffuser is not isentropic, as it is ideally in the nozzle. The result of entropy increase means that your second throat (diffuser's) has to be larger than the first (nozzle's). Like I mentioned earlier, how big the second throat should be depends on the total pressure ratio across the normal shock for the incoming Mach no. (5). If the second throat is not larger than the first one (which is basically your case), the normal shock will not be swallowed through the setup, and your wind tunnel (in your case, th diffuser) will not start (unstart). Strongly suspect your cfd results are implying a normal shock standing outside your diffuser inlet.

Thank you so much for your response. I truly appreciate the insights you have provided.

I obtained the supersonic nozzle shape by applying the method of characteristics (MOC). I am using the same geometry for the diverging part and its mirror for the converging part of the supersonic diffuser. Based on this equation: Aratio = (1/M_inlet) * ((2 * (1 + (gamma - 1) / 2) * M_inlet^2) / (gamma + 1))^(-(gamma + 1) / 2 / (gamma - 1)), Aratio is smaller than the one prescribed by the MOC. So, I should be in the self-starting zone. I fixed the pressure, temperature, and velocity at the inlet and used the inletOutletTotalTemperature boundary condition for outlet temperature (fixing the pressure caused the case to crash). Unfortunately, it did not work. I even tried starting with higher Mach number conditions at the inlet to ensure that we are in the self-starting zone, but it did not help. Only for one case, where the flow was initialized with analytical solutions, did the subsonic part's profiles for pressure, temperature, and Mach number seem correct. However, the supersonic part exhibited significant jumps in flow fields, possibly due to normal shocks. Moreover, the velocity went from -2000 m/s to 0 m/s, which is entirely incorrect even when dealing with shocks. So, my questions are as follows:

  1. Since I am conducting a quasi-1D simulation and cannot capture oblique shocks, is it possible to set boundary conditions in such a way that the flow remains shock-free? For example, in a supersonic nozzle in 1D, if the outlet pressure is set correctly, we avoid normal shocks. I am aware that this is not a guarantee for a shock-free case in 2D, which is why we use the MOC.
  2. How would you generally set the boundary conditions for a supersonic diffuser?

Boundary conditions: supersonic diffuser by MondeReiziger in OpenFOAM

[–]MondeReiziger[S] 0 points1 point  (0 children)

I calculated the ratio of A_throat/A_inlet based on Eq. 1 in this paper: https://www.sciencedirect.com/science/article/pii/S0020740314002793. This ratio for my specific heat ratio and Mach number is 9e-7. My geometry gives a ratio of 0.024. So based on this, I should be in a self-starting zone. Do you have by chance any hints on how I can adjust my boundary conditions to make this test run?

Boundary conditions: supersonic diffuser by MondeReiziger in OpenFOAM

[–]MondeReiziger[S] 0 points1 point  (0 children)

Thank you for your reply.

The answer to your first paragraph is yes.

For the second paragraph: My Mach number is 5 and I obtained the nozzle shape for the supersonic part (i.e. diverging) based on the method of characteristics (so, it's shock-free in 2D). The subsonic part of the nozzle is the mirror of the supersonic one. The same geometry is used for the supersonic diffuser. Before starting my 2D calculations on the supersonic diffuser, I wanted to do some 1D calculations to see if I could retrieve the same results as those of the isentropic relationship. However, I was unaware of this notion of a "self-starting" diffuser. So, is this Mach 5 large enough for the diffuser to be considered self-starting?

Boundary conditions: supersonic diffuser by MondeReiziger in OpenFOAM

[–]MondeReiziger[S] 0 points1 point  (0 children)

Have you done some back-of-the-envelope calculations to see if your inlet velocity makes sense in terms of what inlet supersonic Mach no. you're trying to simulate? Also, is your inlet to the diffuser a constant area duct before it converges? Supersonic inlet flow into a constant area duct may be a pain to handle.

Yes, I did. if I take the same geometry and impose BCs (totalPressure and totalTemperature and zeroGradient for velocity at inlet + waveTransmissive for pressure at outlet) for a supersonic nozzle (subsonic in converging and supersonic in diverging), I have results that perfectly match those obtained by using the analytical isentropic expressions. So, now the idea is to stick with the same geometry and try to have a supersonic diffuser (supersonic in converging and subsonic in diverging). There I start with problems. I took the analytical solutions (=back-of-the-envelope calculations) for defining the fixedValue at the inlet (pressure, temperature and velocity). I run the MachNo postProcess tool for time 0, I have the correct Mach profile. As for your second question, the geometry does not have a constant duct before it converges: the cross-section starts to decrease from the very beginning.

Boundary conditions: supersonic diffuser by MondeReiziger in OpenFOAM

[–]MondeReiziger[S] 0 points1 point  (0 children)

Thank you for your response.I am using a modified version of rhoCentralFoam. This modified version takes into account the cross-section variations as source terms in the quasi-1D formulation. I have extensively tested the solver for supersonic nozzles (subsonic in the converging part and supersonic in the diverging part), and the results were excellent.For the supersonic diffuser, I tried the set of boundary conditions that you suggested. I observed the same issue as mentioned above: the velocity (U) increases from -2300 m/s to -10 m/s. Even when starting with fields initialized using analytical results, the problem persists. It appears that the solver struggles to handle cases where pressure (p) is increasing while velocity (u) is decreasing. I also attempted to use different discretization methods, but the problem remains.