Chong Pan

Proper orthogonal decomposition analysis of vortex dynamics of a circular cylinder under synthetic jet control

Vortex dynamics of a circular cylinder controlled by a synthetic jet positioned at the back stagnation point is experimentally investigated using particle image velocimetry (PIV) technique. The proper orthogonal decomposition (POD) method is adopted to present the variations of the POD energy, mode, coefficient, corresponding dominant frequency, and the reconstructed spanwise vorticity. It is found that the dominant dimensionless control parameters should be the synthetic jet stroke length L0/D, where D is the diameter of the experimental circular cylinder, and the equivalent momentum coefficient Cμ. For the same stroke length L0/D=3.3, the states of the wake vortex shedding are determined by the momentum coefficient. They can be categorized into three groups summarizing all the parameters tested: antisymmetric Karman vortex shedding mode (Cμ≤0.027), vortex synchronization with shedding modes varying between the symmetric and antisymmetric ones (0.061≤Cμ≤0.109), and vortex synchronization with symmetric s...

Nature of sweep and ejection events in transitional and turbulent boundary layers

This paper represents an experimental investigation of the physical nature of the turbulence production mechanisms in boundary layers associated with three fundamental phenomena known from previous studies: (i) sweep events, (ii) ejection events, and (iii) coherent vortical structures. The main goal of this study is to clarify connections between all these phenomena in transitional and turbulent flows. The experimental approach is based on a modified version of the hydrogen-bubble (HB) visualization technique called the synchronous visualization method, which is combined with quantitative processing of video images in a PIV-like manner. The results of investigation of sweep/ejection events and their connection with vortical structures are obtained at late and super-late stages of boundary-layer transition, as well as in fully turbulent boundary layers. The developed approach gives us the possibility to obtain detailed synchronous information about instantaneous flow characteristics in a great range of spa...

Dynamical mode decomposition of Gurney flap wake flow

The present work uses dynamic mode decomposition (DMD) to analyze wake flow of NACA0015 airfoil with Gurney flap. The physics of DMD is first introduced. Then the PIV-measured wake flow velocity field is decomposed into dynamical modes. The vortex shedding pattern behind the trailing edge and its high-order harmonics have been captured with abundant information such as frequency, wavelength and convection speed. It is observed that high-order dynamic modes convect faster than low-order modes; moreover the wavelength of the dynamic modes scales with the corresponding frequency in power law.

Coherent structures in bypass transition induced by a cylinder wake

Flat-plate boundary layer transition induced by the wake vortex of a two-dimensional circular cylinder is experimentally investigated. Combined visualization and velocity measurements show a different transition route from the Klebanoff mode in free-stream turbulence-induced transition. This transition scenario is mainly characterized as: (i) generation of secondary transverse vortical structures near the flat plate surface in response to the von Karman vortex street of the cylinder; (ii) formation of hairpin vortices due to the secondary instability of secondary vortical structures; (iii) growth of hairpins which is accelerated by wake-vortex induction; (iv) formation of hairpin packets and the associated streaky structures. Detailed investigation shows that during transition the evolution dynamics and self-sustaining mechanisms of hairpins, hairpin packets and streaks are consistent with those in a turbulent boundary layer. The wake vortex mainly plays the role of generating and destabilizing secondary transverse vortices. After that, the internal mechanisms become dominant and lead to the setting up of a self-sustained turbulent boundary layer.

Phase identification of quasi-periodic flow measured by particle image velocimetry with a low sampling rate

This work mainly deals with the proper orthogonal decomposition (POD) time coefficient method used for extracting phase information from quasi-periodic flow. The mathematical equivalence between this method and the traditional cross-correlation method is firstly proved. A two-dimensional circular cylinder wake flow measured by time-resolved particle image velocimetry within a range of Reynolds numbers is then used to evaluate the reliability of this method. The effect of both the sampling rate and Reynolds number on the identification accuracy is finally discussed. It is found that the POD time coefficient method provides a convenient alternative for phase identification, whose feasibility in low-sampling-rate measurement has additional advantages for experimentalists.

On the instability and reproduction mechanism of a laminar streak

The instability and spanwise reproduction process of one single low-speed streak evolved in a laminar flat-plate boundary layer is experimentally investigated. The primary goal of this experiment is to bridge the gap between the understandings on streak characteristics and the dynamic mechanisms of free stream turbulence-induced bypass transition. The wake of an upstream wall-normal positioned interference wire is used to generate a low-speed streak through the leading edge receptivity process. Moreover, the wire wake introduces irregular perturbations with quasi-antisymmetry into the boundary layer, thus triggers a sinuous-typed secondary instability of the streak. The downstream evolution of the streak presents a re-growth trend in streak strength statistics. The first growth stage is dominated by the streak instability, while the secondary growth stage might be associated with the unsteady breakdown of the streak. Streamwise vortices generated through breakdown process are frequently observed on either flank of the streak in the secondary growth stage. They always organize into streamwise-elongated train with alternating vorticity sign, thus introducing antisymmetric perturbations onto the streak with significant strength to trigger additional breakdown. In the far downstream, two new-born low-speed streaks appear on either side of the original one, indicating the completion of the spanwise reproduction of the streak. On considering the consistent lift-up inducement from the streamwise vortex trains onto the underlying inner fluid, the streak breakdown process provides a dynamical precondition for the streak reproduction process.

Effects of roughness elements on bypass transition induced by a circular cylinder wake

The bypass transition of flat-plate boundary layer induced by a circular cylinder wake under the influence of roughness elements is experimentally investigated. The hydrogen-bubble visualization results show that the boundary layer separation occurs upstream of the roughness, and the separated shear layer is incised by roughness to different extent, resulting in different kinds of secondary vortices formed immediately downstream of the roughness. During the evolution of the secondary vortex, two types of instabilities are observed, which are denoted as large- and small-scale instabilities, respectively, according to different spatial scale of the hairpin vortices formed afterward. A merging process of hairpin vortices is also observed when the secondary vortices undergo the small-scale instability, and a potential new transition control strategy based on the present observation is proposed.Graphical Abstract

Reynolds-Number Dependency of Boundary-Layer Transition Location on Stratospheric Airship Model

Cd = drag coefficient of the airship model D = hull diameter of the airship model, m L = hull length of the airship model, m ReV = Reynolds number based on the cubic root of thevolume of the model U∞ = freestream velocity, m∕s urms = streamwise velocity fluctuation intensity, m∕s Xfr∕L = nondimensional axial length coordinate of sublimation front position Xtr∕L = nondimensional axial length coordinate at transition x = coordinate in freestream direction from hull nose, m y = coordinate in wall normal direction, mm z = coordinate in circumferential direction, mm α = angle of attack of the model, deg δ = boundary-layer thickness, mm