Chun Zhang

Bio: Chun Zhang is an academic researcher. The author has contributed to research in topics: Reynolds number & Cylinder. The author has an hindex of 1, co-authored 1 publications receiving 24 citations.

Vortex-induced vibrations of a confined circular cylinder for efficient flow power extraction

Abstract: The effects of confinement on vortex-induced vibration (VIV) of a circular cylinder (diameter D) and its flow power extraction capability are presented. A two-dimensional numerical study is performed on VIV of a circular cylinder inside a parallel plate channel of height H at Reynolds numbers 100 and 150. The blockage ratio (b = D/H) is varied from 1/4 to 1/2. The cylinder is elastically mounted with a spring such that it can only vibrate transversely to the flow. It has a fixed non-dimensional mass (m*) of 10. The energy extraction process is modeled as a damper (damping ratio ζ) attached to the cylinder. With increasing blockage, the vibration amplitude of the cylinder decreases, the lock-in happens at a larger non-dimensional natural frequency of the cylinder, and the initial branch of the vibration response of the cylinder shrinks. There is a minimum value of the channel height and Reynolds number for the existence of the initial branch. The extracted power is found to increase rapidly with the blockage. For the maximum blockage (b = 1/2), the flow power extracted by the cylinder is an order of magnitude larger as compared to what it would extract in an open domain with free stream velocity equal to the channel mean velocity. The optimal mass-damping (α = m*ζ) for extracting the maximum power is found to lie between 0.2 and 0.3. An expression is derived to predict the maximum extracted power from the undamped response of a confined/unconfined cylinder.

Wake events during early three-dimensional transition of a circular cylinder placed in shear flow

Abstract: Three-dimensional (3D) direct numerical simulations are carried out for shear flows past a circular cylinder in the early 2D (two-dimensional)–3D transition regime. The effect of incoming shear is buried in a source term employing a velocity transformation. Wake transition events are inspected for both planar and span-wise shear flows. Parallel, time-mean-symmetric shedding for planar shear and oblique vortex shedding for span-wise shear are observed with a near wake roll-up vortical structure. Vortex splits and dislocations are found without any order in time for moderate shear, while they give way to visibly higher levels of instabilities at higher shear rates. Mode “B” instabilities are noted for planar shear, while opposite streamwise vortices align in parallel horizontal layers for span-wise shear. Local Strouhal frequency (Stz) drops inside a span-wise cell for span-wise shear with finite jumps across cell boundaries. Wavelet multiresolution analysis indicates a strong flushing effect, triggered by vortex dislocations, which gives rise to a new frequency event. The dominant span-wise mode indicates periodic forcing of mode “A” instabilities at a rate close to the inverse of local Strouhal number. In contrast, the streamwise velocity modes result in a global span-wise similarity. Intrinsic secondary instabilities play a vital role in span-wise shear cases. The addition of planar shear makes the downstream defect layer nearly span-wise-invariant. However, the velocity defect is entirely controlled by the span-wise shear. The momentum thickness exhibits streamwise growth, similar to the Blasius profile. The shape factor of such profiles indicates a delay in laminar–turbulent transition for span-wise shear.