Introduction to CFD Questions & Answers
Question by Student 201327103
professor this is my counter plot for cylinder.
I can't get shock wave. I Think my boundary condition is wrong, but i don't know where is wrong. here is my code
 05.31.18
The problem may be that you are imposing free stream boundary conditions on all boundary nodes..? You should check your boundary conditions are well imposed using the -on flag.
Question by Student 201327103
Professor, i check the boundary node and it looks like good. but the result still has same problem
This looks fine: your boundary conditions are well imposed. The contours you showed previously seem to have not been iterated at all. After the iteration process, are you reading in the data file (obtained after 1000 iterations or so) before outputting the post file?
 Question by Student 201327132 Dear professor, I have a question about order of accuracy P. Assuming P>1, We obtained $GCI_f$. If P<1, Should we change to$\left\vert\frac{\triangle x_c}{\triangle x_f }-1\right\vert$? I obtained P that is smaller than 1 and minus value. Did I do it wrong way? Thank you.
 06.11.18
Very good question. We made this assumption only when deriving the GCI. So, within the $\rm GCI_{f}$ equation, you should change the term to $\left\vert\left(\frac{\triangle x_c}{\triangle x_f }\right)^p-1\right\vert$. That is, the GCI should always be positive. However, when determining order of accuracy $p$, we did not use the GCI and we did not assume that $p$ should be greater than 1. So you should not change any of the equations used to determine $p$.
 Question by Student 201427116 Professor, I have a question about WENO. We used below terms in WENO: $${\bar w_0} = \frac{\gamma_{0}}{{(\epsilon+\beta_0)}^2}, {\bar w_1} = \frac{\gamma_{1}}{{(\epsilon+\beta_1)}^2}$$ There is $\epsilon$ in denominator but what is it for? and how can I decide the value of $\epsilon$ ?  Another question is about $\gamma_0$ and $\gamma_1$. With Taylor series expansion, we found $\gamma_0=\frac{1}{3}$ and $\gamma_1=\frac{2}{3}$ for 3rd order accuracy. With 3rd order accuracy, that is, WENO3, does $\gamma_0$ and $\gamma_1$ have fixed value of $\frac{1}{3}$ and $\frac{2}{3}$, respectively? Or are they also variants?
 06.13.18
The user-defined constant $\epsilon$ is included to prevent a division by zero. Set it to a very small value. As for $\gamma_0$ and $\gamma_1$, they are fixed constants: don't change them.
 Question by Student 201327132 Professor I have a question about bdry condition. In my note, One property must come from out of the domain at subsonic outflow bdry condition. So We choose Pressure. And We use $P_1^{n+1}=P_\inf$. Other bdry condition case that subsonic inflow, Two properties must be obtained from outside domain. So we choose Temperature and Pressure. And we use Stagnation Temperature and Pressure for time level n+1. Why we use difference methods to obtain pressure?(Stagnation pressure and Freestream pressure) Thank you.
For the inflow BC, the stagnation pressure can be assumed equal to the one in the freestream because the flow along a streamline is isentropic. But such is not the case for the outflow BC. What if there is a shock somewhere within the domain? Then, the entropy rises and the stagnation pressure will go down and not be equal to the one in the freestream. However, for external flows around a body, the pressure will eventually become equal to the freestream pressure even if shocks are present (as long as the BC is far away from the body). Hence why it's better at the outflow BC to choose to fix pressure rather than stagnation pressure.
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