Abstract
Implicit large-eddy simulation is employed to simulate the flow in an asymmetric plane diffuser at Re=9000. Flow separation exists near the throat and evolves to large-scale, unsteady separation in the expansion section and the downstream region. An unconventional flow control method, namely, a cylindrical Karman-vortex generator (KVG) with different sizes and locations that induces periodic spanwise vortex shedding, is set upstream of the throat to suppress the flow separation. An appropriately designed KVG can enhance the mixing of the outer flow and the low energy fluid near the wall region by the periodic shedding Karman-vortices, and effectively reduce the separation bubble size. For the present optimal case, the length and height of the separation bubble are decreased 50.4% and 90.9%, respectively. The static pressure recovery coefficient is also increased by about 50%. Moreover, the velocity and total pressure distributions at the end of the expansion section are more uniform with lower fluctuation in the case with KVG installed. An optimal KVG diameter DK is suggested to be 3–4% of the expansion section length LE. The gap ratio to the lower wall G/DK and the length ratio to the throat Lt/DK are suggested to be 2.0–3.0 and 5.0–10.0, respectively.
Highlights
A diffuser is a device that decelerates fluid and converts the kinetic energy to static pressure.1 Diffusers are widely used in wind tunnels, turbomachinery, and automobiles
Based on the analysis of the averaged and instantaneous flow details and the characteristics of the flow around a circular cylinder, the following conclusions are reached: (1) The implicit large-eddy simulation (ILES) conducted by the combination of the simple low-dissipation advection upstream (SLAU) and minimum dispersion and controllable dissipation (MDCD) schemes is validated by both test cases with experimental data and a grid convergence test
Compared with the uncontrolled diffuser, the length and height of the flow separation region can be reduced by 54.9% and 90.9%, respectively, and the static pressure recovery coefficient can be increased about 50%. (3) The spanwise vortices generated by the Karman-vortex generator (KVG) in its wake can effectively enhance the mixing of the outer flow and the downstream near wall region
Summary
A diffuser is a device that decelerates fluid and converts the kinetic energy to static pressure. Diffusers are widely used in wind tunnels, turbomachinery, and automobiles. Zhang et al. utilized a ring-shaped Karman-vortex generator (KVG) to control the severe flow separation in a conical diffuser with 2θ = 29.14○ and AR = 3.43. The numerical results indicated that the periodic shedding Karman vortex street enhanced the flow mixing of the core and near wall region, and the pressure recovery performance of the diffuser achieved remarkable promotion. Investigations utilizing KVGs to improve the performance of diffusers are far less common than those utilizing van-shaped VGs and MVGs. The flow control mechanisms and the relevant design principles are ambiguous and restrict the application of KVGs in diffusers and similar problems to some extent. Considering that the diffuser was used to inhibit the flow and increase pressure and that the upstream KVG was supposed to enhance the mixing of the flow by the periodic shedding Karman vortex street, G/D > 1.5 is reasonable. The flow control mechanisms and design principles of KVGs are discussed based on the comprehensive analysis of the flow physics of the diffuser and flow characteristics around the circular cylinder
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