Y. Zhou

Turbulent intensity and Reynolds number effects on an airfoil at low Reynolds numbers

This work investigates the aerodynamics of a NACA 0012 airfoil at the chord-based Reynolds numbers (Rec) from 5.3 × 103 to 2.0 × 104. The lift and drag coefficients, CL and CD, of the airfoil, along with the flow structure, were measured as the turbulent intensity Tu of oncoming flow varies from 0.6% to 6.0%. The analysis of the present data and those in the literature unveils a total of eight distinct flow structures around the suction side of the airfoil. Four Rec regimes, i.e., the ultra-low ( 5.0 × 106), are proposed based on their characteristics of the CL-Rec relationship and the flow structure. It has been observed that Tu has a more pronounced effect at lower Rec than at higher Rec on the shear layer separation, reattachment, transition, and formation of the separation bubble. As a result, CL, CD, CL/CD and their dependence on the airfoil angle of attack all vary with Tu. So does the critical Reynolds number Rec,...

Flow around two non-parallel tandem cylinders

An experimental investigation at a Reynolds number (Re) of 5.6 × 104 is carried out to understand fluid dynamics associated with two nonparallel cylinders in a tandem configuration. The upstream and downstream cylinders of identical diameter D are oppositely rotated by 7.5° measured from the normal to free stream direction, which leads to an included angle of 15° between the cylinders. Measurements of Strouhal number (St) and time-mean and instantaneous flow fields are conducted to explore the flow structure variation in the spanwise direction at different spacing ratio L* (=L/D = 1–4.05, where L is the cylinder center-to-center spacing). Three distinct flows are identified, namely, alternating reattachment flow (regime I, 1 ≤ L* < 2.15), bi-stable flow (regime II, 2.15 ≤ L* ≤ 3.1), and coshedding flow (regime III, 3.1 < L* ≤ 4.05). Based on shear layer reattachment and its influence on quasi-steady vortex in the gap and wake regions, regime I is further divided into two subregimes, IA and IB. The three f...

Jet mixing optimization using machine learning control

We experimentally optimize mixing of a turbulent round jet using machine learning control (MLC) following Li et al. (Exp Fluids 58(article 103):1–20, 2017). The jet is manipulated with one unsteady minijet blowing in wall-normal direction close to the nozzle exit. The flow is monitored with two hotwire sensors. The first sensor is positioned on the centerline five jet diameters downstream of the nozzle exit, i.e. the end of the potential core, while the second is located three jet diameters downstream and displaced towards the shear-layer. The mixing performance is monitored with mean velocity at the first sensor. A reduction of this velocity correlates with increased entrainment near the potential core. MLC is employed to optimize sensor feedback, a general open-loop broadband frequency actuation and combinations of both. MLC has identified the optimal periodic forcing with small duty cycle as the best control policy employing only 400 actuation measurements, each lasting for 5 s. This learning rate is comparable if not faster than typical optimization of periodic forcing with two free parameters (frequency and duty cycle). In addition, MLC results indicate that neither new frequencies nor sensor feedback improves mixing further—contrary to many of other turbulence control experiments. The optimality of pure periodic actuation may be attributed to the simple jet flapping mechanism in the minijet plane. The performance of sensor feedback is shown to face a challenge for small duty cycles. The jet mixing results demonstrate the untapped potential of MLC in quickly learning optimal general control policies, even deciding between open- and closed-loop control.Graphical abstract

Vibration Initiation of a Cylinder in the Wake of Another

This paper presents the cross-flow induced vibration response of a both-end-spring-mounted circular cylinder (diameter D) placed in the wake of a rigid circular cylinder of a smaller diameter d. The cylinder diameter ratio d/D and the spacing ratio L/d are 0.4 and 2.0, respectively, where L is the distance between the center of the upstream cylinder to the forward stagnation point of the downstream cylinder. The focus is given to investigate how the initiation of the vibration occurs, gaining insight into physics in the transition period where the cylinder starts to vibrate. The transition period can be divided into pre-initial, initial and late transitions. The role of added mass, added damping and work done in the transition process is studied in detail.

K41 Versus K62: Recent Developments

For the past 50 years or so, Kolmogorov’s (1962) correction (K62) to his 1941 hypotheses (K41) has been embraced by an overwhelming majority of turbulence researchers. Our recent work suggests that there are no valid reasons for abandoning K41. In particular, analytical considerations, based on the NS equations, which take into account the finite Reynolds number (FRN) effect, together with the available experimental laboratory data, seem to confirm a tendency towards the simple and elegant predictions of K41 as the Reynolds number increases. This is especially true when the focus is on the length scales which lie in the dissipative range. Incorrectly accounting for the FRN effect and the inclusion of the atmospheric surface layer (ASL) data, likely to have been affected by the proximity to the surface, appear to be the major factors which have contributed to a nearly unchallenged acceptance of K62.

Boundedness of the mixed velocity-temperature derivative skewness in homogeneous isotropic turbulence

The transport equation for the mean scalar dissipation rate ϵ¯θ is derived by applying the limit at small separations to the generalized form of Yaglom’s equation in two types of flows, those dominated mainly by a decay of energy in the streamwise direction and those which are forced, through a continuous injection of energy at large scales. In grid turbulence, the imbalance between the production of ϵ¯θ due to stretching of the temperature field and the destruction of ϵ¯θ by the thermal diffusivity is governed by the streamwise advection of ϵ¯θ by the mean velocity. This imbalance is intrinsically different from that in stationary forced periodic box turbulence (or SFPBT), which is virtually negligible. In essence, the different types of imbalance represent different constraints imposed by the large-scale motion on the relation between the so-called mixed velocity-temperature derivative skewness ST and the scalar enstrophy destruction coefficient Gθ in different flows, thus resulting in non-universal app...

Wake and Vortex-Sheddings from Different Diameter Cylinders in Tandem

The paper associated with two tandem cylinders presents the upstream cylinder size (diameter d) effect on global parameters of the downstream cylinder (diameter D) including time-mean and fluctuating drag (C D , ( C^{prime}_{D} )), fluctuating lift (( C^{prime}_{L} )), Strouhal number (St), and flow structures at spacing ratio L/d = 1.0–8.0, and diameter ratio d/D = 0.2–1.0, where L is the distance between the center of the upstream cylinder and the forward stagnation point of the downstream cylinder. The Reynolds number is kept constant at 4.27 × 104 based on D. C D , ( C^{prime}_{D} ) and ( C^{prime}_{L} ) are measured using a sectional load cell, flow structures are obtained using a PIV technique. The critical L/d dividing the reattachment and coshedding flows is larger at smaller d/D. In the coshedding regime, the shedding frequency of the downstream cylinder locks-in with that of the upstream cylinder for d/D = 1.0, 0.8 and 0.6, in addition to a subharmonic lock-in for d/D = 0.6. The lock-in however does not occur at d/D = 0.4 and 0.2. C D in general increases with d/D. ( C^{prime}_{D} ) and ( C^{prime}_{L} ) both generally wane and grow as d/D decreases from 1.0 to 0.6 and 0.4 to 0.2, respectively.

Effect of Nose Shape on Separation Bubble and Surface Pressure on a Cylinder in Axial Flow

The flow characteristics is experimentally studied over the leading edge of a circular cylinder with blunt, conical, and hemispherical nose shapes, for a range of Reynolds number Re D = 2.5 × 103− 4.2 × 104. While mean and fluctuating pressure (C p , C p ′) around the leading edge are measured with a pressure transducer, PIV and flow-visualization experiments are performed to assimilate how an increase in Re D influences the shear layer reattachment length x R , shear layer transition length x Tr , and bubble width W. The results reveal that x R , x Tr , and W all shrinking significantly with Re D up to Re D = 104. Their shrinkage is however inconsequential for Re D > 104. At a given Re D , when the nose changes as blunt, conical and hemispherical, x R and W shorten, but x Tr enlarges. Following the separation bubble size, C p and C p ′ in the bubble are highly sensitive to Re D for Re D 104.

Turbulent Kinetic Energy Budget in the Far Field of a Square Cylinder Wake

In this paper, the transport equation for the turbulent kinetic energy (TKE) is tested in the far field of a square cylinder wake (hereafter denoted SC). It is found that the physical mechanism for transporting TKE along the axis is different from that in a circular cylinder (hereafter denoted CC) wake. For the SC wake, the (overline{{q^2}} ) diffusion term is negligible compared to advection term along the axis and the advection and energy dissipation terms dominate the budget. However, in the CC wake, aside from the advection and energy dissipation terms, the (overline{{q^2}} ) diffusion term also contributes significantly to the budget. At the region close to the centreline, the gain of the energy due to the contributions from the advection and diffusion terms is equal to the loss due to the isotropic dissipation, indicating that the isotropic dissipation rate ({overline{varepsilon }_{iso}}) is a good surrogate of the mean TKE dissipation rate ({bar{varepsilon }}).

A Cylinder Vibration Induced by Shear Layers of Another of Smaller Diameter

Flow-induced vibration of a circular cylinder (diameter D) submerged in the wake of a fixed circular cylinder of smaller diameter d is investigated. The cylinder was spring mounted, allowed to vibrate in the transverse direction only. While d is varied from 0.2D to 1.0D, cylinder spacing L systematically is changed from 1.0 to 5.5, where L is the distance between the forward stagnation point of the downstream cylinder to the center of the upstream cylinder. Measurements of vibration amplitude, lift force and flow field are carried out. A map of vibration regime is provided showing how the vibration of the cylinder is dependent on d and L. At a smaller d, vibration occurs for a longer range of L. The vibration is excited by the two shear layers reattaching alternately on the downstream cylinder. An increase in vibration amplitude is accompanied largely by an increase in lift amplitude.