Hello all,
I've got a 2D simulation case in which the flow separates from the sharp leading edge of rectangular bluff body and reattaches to the wall some distance downstream. The main goal is a accurate prediction of pressure distribution along the body's face parallel to the flow.
I'm doing a transient simulation using SST model in conjunction with gamma-Re transition model. The time- cord-averaged y+ is less than 2~3 and the inflation layer around the face of interest contains 10 prism layers. The Re number based on the body's width (perpendicular to the flow) is 1.7e+4.
The problem is that my model overpredicts the reattachment length, which in turn leads to delayed pressure recovery.
I have a suspicion that longitudinal decay of turbulence values specified at the inlet might be to blame. Consulting the Ansys CFX-solver Modeling Guide, I learnt that one solution is to prescribe appropriate turbulence values at the inlet based on the desired values at the body. An alternative approach also suggests some additional source terms for k and w transport equations in order to preserve the inlet values up to some distance upstream the body, from where decay is allowed.
Here are my questions:
1- Is my suspicion valid in the case of my problem?
2- Is the decay of turbulence of physical basis or a numerical artifact?
3- which of the two methods works better? Are there any attempts in the literature?
I appreciate your comments.
First of all, you stated an overprediction but compared to what, numerical or experimental results? And are you sure to prescribe the same inflow variables?
A further issue is in the grid resolution, are you able to resolve the BL along the bluff body (that is at least 3-4 cells within y+
Dear Filippo Maria Denaro
Dear
I appreciate your comments very much. I would like to share with you my latest progress with a bit more detail and then kindly ask for your helpful tips.
I have been trying to validate my CFD model through the experimental measurements of pressure along the body's face adjacent to the regions of separation from the leading edge and reattachment further downstream.
Figure 1 shows the results of some of my trials with different turbulence models and adjustments. I started with a SST model and then compared its performance by refining the mesh. However, not much improvement was observed; both predicted a delayed pressure recovery associated with a separation bubble larger than reality. Then I switched to the transitional Gamma-Re model coupled with SST, which yielded a very similar pressure behaviour (not shown here in the figure). The best result I got so far (see the blue curve) was by integrating SST+Gamma-Re with the so-called "Reattachment Modification (RM)" model offered by Ansys CFX-pre. According to CFX guide, this RM model promotes reattchment by enhancing the production term in the turbulence transport equations. For all these simulations, the time-averaged y+ along the body's face ranges between 5 or higher (at the very few cells next to the leading edge due to high values of shear stress) to below 2 (for cells further downstream of the leading edge), so let say 1.5~2 on average for the whole face.
With all these observations in mind, I am wondering if there still might be some room to further improve the results by preventing the decaying of turbulence intensity that enters the domain through the inlet, thereby forcing the free shear layer to reattach faster. As I said in my previous comment, CFX suggests adding some source terms to the turbulence transport equations in order to allow turbulence decay only from close proximity to the body such that a minimum turbulence intensity of 0.1% is ensured around the body.
This idea sounds weird to me, because if turbulence decay is a physical phenomenon actually occurring in the flow, on what basis can we tamper with that? Doesn't that introduce unphysical effects?
Dear Armin Hajighasem Kashani
trying to achieve a match with experimental measurements is not a simple task. In your formulation you should match the statistics of the experimental inflow and that is not straightforward also for a laminar flow problem as detailed in Sec.4 here
Article Analysis of 3-D backward-facing step incompressible flows vi...
A further issue could be also due to the fact your grid resolution is not sufficient to resolve the BL. You should be able to set at least 3-4 nodes at y+
Hi Armin and All,
In my research on turbulence modeling for blood flow simulations in hemodialysis cannulae (Salazar et al., 2008, found in ResearchGate at: Article Turbulence Modeling in the Numerical Estimation of Hemolysis...
), I explored the impact of turbulence modeling on predicting hemolysis (red blood cell damage). This work might be relevant to the query about turbulence values affecting reattachment length in bluff body simulations.The paper highlights the importance of accurate turbulence modeling since blood flow in cannulae is often turbulent, and turbulence significantly impacts hemolysis. I discuss the selection of an appropriate turbulence model for accurate flow predictions.
We validated our approach using a benchmark case of a coaxial jet array, which shares similarities with cannula flow. The findings suggest that the Shear Stress Transport (SST) model with Gamma-Theta transition yielded the best results compared to standard k-ε and k-ω models.
Here's how this research might be helpful to your situation:
- It emphasizes the critical role of turbulence modeling for accurate flow simulations, especially in complex geometries and turbulent regimes. This is likely applicable to bluff body simulations.
- It underscores the need for validation, particularly when selecting a turbulence model. While the benchmark case involved a coaxial jet array, the validation process provides valuable insights for selecting appropriate turbulence models for specific geometries, including bluff bodies.
- The SST model with Gamma-Theta transition might be a good candidate for your simulation. It could be worthwhile to explore how this model performs in your case compared to the model you're currently using.
While my paper focuses on hemolysis in cannulae, it offers valuable considerations for turbulence modeling in general. It highlights the importance of validation and suggests a potentially suitable turbulence model for your bluff body simulation.
I hope this information is helpful!
Best regards,
Luis
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