Simon C. M. Yu

Active control of vortex-induced vibrations of a circular cylinder using windward-suction- leeward-blowing actuation

This paper studies the control of two-dimensional vortex-induced vibrations (VIVs) of a single circular cylinder at a Reynolds number of 100 using a novel windward-suction-leeward-blowing (WSLB) concept. A lattice Boltzmann method based numerical framework is adopted for this study. Both open-loop and closed-loop controls are implemented. In the open-loop control, three types of actuation arrangements, including the pure suction on the windward side of the cylinder, the pure blowing on the leeward side, and the general WSLB on both sides, are implemented and compared. It is found that the general WSLB is the most effective, whereas the pure suction is the least effective. In the closed-loop control, the proportional (P), integral (I), and proportional-integral (PI) control schemes are applied to adjust the WSLB velocities according to the flow information obtained from a sensor. The effects of four key control parameters including the proportional gain constant, the integral gain constant, the length of data history used for the feedback, and the location of the sensor are investigated. It is found that the use of only P control fails to completely suppress the VIV, the use of only I control can achieve the complete suppression, and the PI control performs the best in terms of both the control effectiveness and efficiency. In the PI control, there exists an optimal length of data history for the feedback, at which the VIV control is the most efficient. There also exist the minimum required WSLB velocities for the VIV suppression, independent of the control schemes. Moreover, it is found that the VIV control is independent of the sensor location.

Numerical Investigation of a Trapped Vortex Miniature Ramjet Combustor

The design of a miniature ramjet combustor using gaseous methane fuel for Mach 2.5 has been conducted. The main challenges stem mainly from the insufficient space for mixing and burning, short residence time, and the flame stabilization. Impossible utilization of relatively large air-blast fuel injectors provides more difficulties for the design. The trapped vortex combustor, as a novel way of flameholding by trapping the pilot flame inside a cavity instead of exposing it to the mainstream, is selected. Three main parts are studied numerically, which include the cold flowfield characteristics, the fuel-injection schemes, and the overall combustion performance. The results show that the drag coefficient can help to determine the optimum cavity size for trapping a stable vortex. Injecting all the fuel in the cavity always leads to an overly fuel-rich condition, whereas injecting in front of the cavity with a momentum flux ratio q between 0.61 and 1.0 can successfully achieve stoichiometric mixing in the cav...

Aerodynamic characteristics of a wrap-around fin rocket

Purpose – The purpose of this paper is to investigate the aerodynamics characteristics (especially the side force/moment and rolling characteristics), to analyze the impacts generated by different parameters of wrap-around fins (WAFs) and to find the corresponding mechanism. Design/methodology/approach – The paper has adopted three different types of WAFs for the rocket configurations and the sub-regions divided technology to investigate the lateral and rolling characteristics of WAFs, including the fins with variations in span to chord ratio, thickness, leading-edge sweep, curvature radius, fin numbers, setting angles and rotated angles. Simulations have been performed at Mach numbers from 3 to 4 through an angle-of-attack range of about 0° to 10° and at model rolling angles of 45° to 90°. Findings – The paper shows that the WAF configurations can greatly improve the longitudinal stability and enhance the longitudinal aerodynamic characteristics for the whole rocket. The total drag of the whole rocket is...

Numerical Investigations of Small-Size High Throughflow Speed Combustors with Trapped-Vortex Concept

The main challenges in designing a small-size combustor operating at relatively high throughflow speeds are the insufficient space for mixing and burning due to the short residence time of mixtures and the difficulties in flame stabilization. In this study, the trapped vortex combustor (TVC) concept is investigated because it provides a more stable way for mixing and flame holding by holding the pilot flame inside a cavity instead of exposing it to the mainstream. The computations are performed using a well-validated flow solver with appropriate models for turbulence, species transport and combustion. Three main parts are studied including the flow field characteristics of the combustor without fuel injection; the effects of fuel injection, mixing and penetration without combustion; and the combustion process. The optimum cavity configuration in the cold flow, the mixing efficiency, the total pressure and the combustion efficiency are considered in these parts. The results show that TVC provides a good overall performance for the designed combustor and also has the potential to be compact. This paper represents a contribution to understand the capability and working processes of trapped vortex combustors for high speed flows.

Control of vortex-induced vibration using a pair of synthetic jets: Influence of active lock-on

While conventional vortex-induced vibration (VIV) of bluff bodies is suppressed through reducing the strength of asymmetric vortex shedding, it can also be mitigated by shifting the vortex shedding frequency away from the natural frequency of the body structures using active lock-on. Recently Du and Sun [“Suppression of vortex-induced vibration using the rotary oscillation of a cylinder,” Phys. Fluids 27, 023603 (2015)] utilized periodical rotation to induce the lock-on of the frequency of vortex shedding from a transversely vibrating cylinder to the rotation frequency and demonstrated successful VIV suppression. However, questions were raised from this investigation: Does the occurrence of active lock-on always suppress VIV? If not, how to ensure the appropriate usage of active lock-on for VIV suppression? To address these research questions, a numerical investigation is conducted on the active VIV control of a circular cylinder using a pair of synthetic jets (SJs) at a low Reynolds number of 100. The SJ...

Vortex ring formation in starting forced plumes with negative and positive buoyancy

The limiting process of vortex ring formation in starting forced plumes, with Richardson number in the range of −0.06 ≤ Ri ≤ 0.06, was studied numerically under the Boussinesq approximation. The examination of the dynamics of the starting flow evolution reveals that the plume-ambient density difference affects the vortex ring pinch-off mainly through three mechanisms, i.e., the baroclinic production of vorticity, the buoyancy acceleration (or deceleration) on the vortical structures, and its effect on the trailing shear layer instability. As Ri increases from negative to positive values, three regimes can be identified in terms of the vortex interaction patterns during the pinch-off process, i.e., the weak-interaction regime (−0.06 < Ri < − 0.02), the transition regime (−0.02 ≤ Ri < 0), and the strong-interaction regime (0 ≤ Ri < 0.06). By eliminating the influence of the baroclinic vorticity production, the circulation method proposed for the starting jets is revised to determine the buoyant formation number F in the starting forced plumes. Besides the formation number F, another dimensionless time scale (dubbed as the separation number S), which corresponds to the end of the pinch-off process, is identified by the time of vanishment of the vorticity flux feeding the leading vortex ring. The numerical results show that the variation trends of formation number and separation number against Ri change near the critical value of Ric ≈ − 0.02. In the weak-interaction regime, both formation number and separation number increase rapidly against Ri. While in the transition and strong-interaction regimes alike, the formation number increases at a much slower rate than in the weak-interaction regime, and the separation number declines dramatically as Ri increases. Finally, a qualitative explanation on the variation patterns of formation number and separation number is proposed based on the buoyancy effects on the dynamic properties of the leading vortex ring and the vortex interaction patterns.