Turbulent Flow Behind an Open-Slit V

In this application, AcuSolve is used to simulate turbulent flow of air through and behind a two dimensional open-slit V. AcuSolve results are compared with experimental results adapted from Yang and Tsai (1993). The close agreement of AcuSolve results with experimental results validates the ability of AcuSolve to model the Coandă effect.

Problem Description

The problem consists of air flowing through and past an open-slit V positioned within a flow domain, as shown in the following image, which is not drawn to scale. The flow domain is 0.3 m long and 0.1 m high. The narrow end of the V is positioned 0.05 m from the domain inlet. The slit in the V is 0.002 m high. The V widens to 0.022 m at 0.075 m from the domain inlet. Air enters the domain with a constant velocity of 20 m/s. The upper and lower limits of the domain are defined as no-slip boundary conditions.

Figure 1. Critical Dimensions and Parameters for Simulating Turbulent Flow Behind an Open-Slit V

The simulation was performed as a two dimensional problem by restricting flow in the out-of-plane direction through the use of a mesh that is one element thick. A coarse mesh is used throughout much of the domain, with mesh refinement closer to the V.

Figure 2. Mesh used for Simulating Turbulent Flow Behind an Open-Slit V (the V Lies Within the Dense Area of the Mesh)

Figure 3. Mesh Surrounding the Open-Slit V

AcuSolve Results

The AcuSolve solution converged to a steady state and the results reflect the mean flow conditions. Air flows through the open slit in the V and accelerates while being forced to one side due to the Coandă effect. As air encounters the exterior of the widening gutter, velocity increases and the wake induces recirculation near the exit of the V. The recirculation zone extends downstream of the V gutter.

Figure 5. Contours of X-Velocity of Flow Through and Around an Open-Slit V (the Vertical Line Represents the Location, 0.022 m Downstream from the V exit, at Which Experimental Data is Compared to AcuSolve Results)

AcuSolve results were compared to experimental results at different heights 0.022 m behind the V exit. X-velocity is near zero at the bottom of the domain (height = -0.05 m), rising steeply with distance from the wall. The velocity gradient decreases until approximately 0.018 m below the center line of the domain, which is slightly below the bottom of the V exit. The velocity behind the V drops sharply with an increase in height, due to the blockage caused by the presence of the slanted walls. The lowest velocity is just above the center line of the domain.

The slight asymmetry in the velocity profile in the wake of the V is expected due to the Coandă effect.

Figure 6. X-Velocity Plotted Against Vertical Location 0.0022 m Downstream of the V Exit

Summary

The AcuSolve results compare well with the experimental results for flow through and around an open-slit V. In this application, the V-shaped geometry diverts flow towards the side walls of the channel and also creates a high-velocity jet through the slit. The low pressure associated with the jet causes a deflection towards the walls of the V, resulting in an asymmetric flow field within the V shape. This application illustrates the ability of AcuSolve to accurately predict the Coandă effect. The acceleration of the flow due to the deflection along the V-shaped walls and the momentum deficit in the wake of the V is captured accurately.

Simulation Settings for Turbulent Flow Behind an Open-Slit V

HyperMesh CFD database file: <your working directory>\v_gutter_turbulent\v_gutter_turbulent.hm

Global

Problem Description
- Analysis type - Steady State
- Turbulence equation - Spalart Allmaras
Auto Solution Strategy
- Relaxation factor - 0.5
Material Model
- Air
  - Density - 1.0 kg/m3
  - Viscosity - 1.8333e-05 kg/m-sec
Model
Volumes
- Fluid
  - Element Set
    - Material model - Air
Surfaces
- Inlet
  - Simple Boundary Condition
    - Type - Inflow
    - Inflow type - Velocity
    - X velocity - 20.0 m/sec
    - Eddy viscosity - 0.000800426 m2/sec
- LE_TE
  - Simple Boundary Condition
    - Type - Wall
- Outlet
  - Simple Boundary Condition
    - Type - Outflow
- Symm_MaxZ
  - Simple Boundary Condition
    - Type - Slip
- Symm_MinZ
  - Simple Boundary Condition
    - Type - Slip
- V_Walls
  - Simple Boundary Condition
    - Type - Wall
- Walls
  - Simple Boundary Condition
    - Type - Wall

References

J.-T. Yang and G.-L. Tsai. "Near-wake flow of a v-gutter with slit bleed". Journal of Fluid Engineering. 115:13-20. March, 1993.