Kewei Xu

The control of a ship’s airflow is especially tricky under yaw effects. This paper numerically studies the streamwise suction (SS) active flow control (AFC) on the Chalmers ship model (CSM) for suppressing force fluctuations on the ship body and turbulent fluctuations in the wake region. There are two yaw angles of interest, which are 15 and 45 deg, typically representing mild and severe yaw effects. The SS AFC is implemented immediately downstream of the sharp-edged front surface as inspired by prior studies in suction control. Beyond achieving effective mitigation of fluctuations, the goal here is to understand the flow control mechanism by analyzing flow characteristics and vortical activities. A large-eddy simulation with a wall-adapting local-eddy viscosity model is conducted and is complemented by experimental tests for validation. Two cases are studied at each yaw condition: the baseline CSM without control and the AFC case with SS activated along the leading edges (LEs) of the roof and of the leeward superstructure surfaces. Results show that full-flow attachments are achieved downstream of the suction slots, regardless of the sharp superstructure LEs. The airwake vortex structure and vortex dynamics are altered substantially, leading to the reduction of drag and side-force fluctuations. Turbulent fluctuation is also effectively suppressed at both yaw conditions, which can benefit maritime helicopter operations. As indicated by the proper orthogonal decomposition analysis, the suppression mechanism differs between the two cases, where the 15 deg case links to the weakening of shedding activities and the 45 deg case results from limiting vortex–vortex interactions.

The bogie-induced flow characteristics of two referenced variations of a streamlined high-speed train model, with and without bogie shields, are numerically studied by performing improved delayed detached eddy simulation. The results show that covering the bogies reduces the overall aerodynamic drag by 45%, primarily due to a significant reduction in pressure drag caused by the smoother underbody. While a localized increase in slipstream velocity is observed near the train's rear, the wake activity is diminished, resulting in a substantial drop in the nondimensional slipstream velocity to approximately 0.01 in the wake region, with decreases of 93% and 80% at a height of z* = 0.11 and 0.44, respectively. Additionally, the maximum turbulence intensity downstream of the bogie region decreased by 97% and the lateral motion of wake vortices was found to stabilize, with dominant frequencies occurring at StW = 0.139 and 0.425. These findings provide good references for the aerodynamic assessment of future high-speed trains.

The wake dynamics of two referenced variations of the squareback Windsor model with and without wheels is numerically studied by performing improved delayed detached eddy simulation. Numerical assessments are validated against publicly available experimental data. The focus of this study is on the wake states influenced by the wheels and the thick oncoming floor boundary layer. Results show that the addition of the wheels significantly changes the aerodynamic forces, the underbody flow, and the wake topology. The wake bi-stability is also enhanced with wheels in place due to the increased curvature of lateral shear layers in the near wake. However, the bi-stable behavior is largely suppressed when immersed in a thick boundary layer. These alterations depend on the degree of interaction between the wake recirculation and the bottom flow, and such degree is strongly affected by the underbody flow momentum. The evolution of low-order flow organizations and complementary spectral analysis highlight the differences in the coherent dynamics of the wake. The finding of this present work suggests that the wake bi-stability behind the squareback body can exist not only for a simplified geometry but also for a more realistic car with wheels in real-world upstream conditions.

View Video Presentation: https://doi.org/10.2514/6.2023-1430.vid This paper conducts a 2D numerical study of flows with extreme adverse pressure gradients (EAPG) enabled by Co-Flow Jet (CFJ) active flow control (AFC). An EAPG is defined to be an adverse pressure gradient of at least one order of magnitude greater than that which could be sustained by the non-controlled flow of a baseline case. The flow field is solved using the Unsteady Reynolds Averaged Navier-Stokes (URANS) equations with the one-equation Spalart-Allmaras turbulence model. A CFJ cylinder and CFJ airfoil with EAPG and attached flow are studied, showing lift coefficients up to 19.68 and 15.96 respectively. The CFJ airfoil maintains attached flow in a maximum streamwise adverse pressure gradient 1091 times greater than the baseline airfoil, and a maximum radial adverse pressure gradient 121 times greater than the baseline airfoil. Similarly, the CFJ cylinder shows attached flow with a streamwise pressure gradient ratio up to 17 and radial pressure gradient ratio up to 132. Such an extraordinary adverse pressure gradient ratios motivate this effort to investigate how the implementation of the CFJ can achieve this effect. Due to its ability to maintain attached flow in the presence of an EAPG, the CFJ offers a highly effective method of active flow control with the potential to be far more effective than other active flow control devices.

View Video Presentation: https://doi.org/10.2514/6.2023-0658.vid This paper numerically studies the control effectiveness and energy expenditure of pulsed Co-Flow Jet (PCFJ) applied on the widely used NASA hump. The 2D unsteady Reynolds averaged Navier-Stokes equations are solved for the simulations. An unsteady velocity boundary condition is developed for the in-house FASIP (Flow-Acoustics-Structure Interaction Package) CFD solver to simulate the pulsed CFJ. The pulsed jet simulation is validated with the NASA tested case actuated by a synthetic jet for the NASA hump flows. The predicted synthetic jet results are in good agreement with the experiment in terms of pressure coefficient distributions. For the pulsed CFJ, a novel actuation model with continuous suction and pulsed injection is proposed to mimic the zero-net-mass-flux pulsing process and calculate the power consumption. The pulsed CFJ and steady CFJ (SCFJ) are compared for their control effectiveness and power consumption at their minimum momentum coefficient to fully attach the flow. The pulsed CFJ is simulated with two reduced frequencies (F+) of 0.74 and 1.67. All the cases have identical CFJ injection and suction slot locations and sizes. Results show that the steady and pulsed CFJ cases have the similar control effectiveness as their pressure coefficient distributions are virtually overlapped. The PCFJ requires about 28% lower mass flow rate than that of the SCFJ. However, the PCFJ requires substantially higher power consumption due to significantly increased total pressure loss for pulsed jet actuation.