Takumi SATO
Numerical Computation of Jet which Recirculation Occurs at Outflow Boundary
Yang Hongxuan
Turbulent jets have been applied for discharging waste water into the whole river / seawater and other, and Numerical Simulation (CFD) of the jets in the stagnant environment is performed to know their characteristic. This is generally performed in limited space, but when a recirculation occurs at the outflow boundary, as the properties of the recirculation fluid are unknown, the computation become unreliability and difficult to converge.
To solve this problem, firstly, a uniform co-flow flow (U0) is given in the computation domain and the jet in discharged into this co-flow environment. Secondly, the velocity Us which is expected to be close to the result of the jet discharged into the stagnant fluid is calculated by subtracting U0 from the calculated result (U). Finally, we make visible its streamline and velocity vector, evaluate it by making mean velocity distribution and comparing with experimental data.
OpenFOAM is software to be able to easily solve simultaneous partially differential equations based on a finite volume method (FVM). This can simulate various phenomena such as fluid flow or a rule equation of the economy by transcribing an equation in an original programming language.
The standard k-ŠĆ turbulent model for incompressible fluid is used in this study. SIMPLE is introduced to solve the governing equations. OpenFOMA is used to solve the partially differential equations.
At the two-dimensional model, the recirculation occurs at the outflow boundary, from mean velocity distribution, compared with experimental data of Us. It is showed that Us is reliable as a calculation result of the velocity. In addition, similar results were obtained in other cases.
By the way, to evaluate the error, Us is substituted into the governing equations to check whether the equations are satisfied, a term including U0 is found to be added in the equations of momentum, k and ŠĆ expecting the continuity equations.
To reduce the errors induced U0, a straight forward approach is to use lower value of U0, but it is necessary to keep U0 big enough to present that no recirculation occurs in the outflow boundary. The limited small value of U0 were examined under the condition Uj-U0=0.4[m/s] in several cases of computation.
As the result, when width doubles U0 shrank below half of that in the two-dimensional model. U0 shrank more smaller in the three-dimensional case.
As conclusion, the introduced co-flow U0 is a simple useful approach in the numerical simulation for jet discharged into stagnant environment, in which recirculation flow occurs at the outflow boundary.
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