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ABSTRACTIt is well known that drag reduction occurs when the flow is passing by a grooved circular cylinder at certain Reynolds numbers, which has been used as a powerful energy saving method in a broad range of circumstances. However, a challenge here is how to evaluate the combined effects of depth, width and location of a given triangular groove set covering half of the cylindrical surface area. A useful approach to quantitatively analyze the influence of these different factors on drag reduction using the response surface methodology is described here. The flow characteristics, including drag coefficient, flow velocity, turbulent kinetic energy and vorticity, were calculated by numerical simulation. The results showed a great drag reduction effect under the legitimate set of groove structure parameters at a super-critical Reynolds number providing a base for optimization process in various engineering applications. The drag reduction mechanism found from this research could extend to other cases and should provide insights into engineering applications like car grilles.

With further increasing in running speed of newer high-speed EMUs (electric multiple units), higher demand for wheelset dynamic balance is required. In order to study the influence of the dynamic unbalance of wheelset on the stability, ride quality, and curving performance for a high-speed car, a detailed dynamic model of a high-speed EMU car is established using the software SIMPACK. The analysis results indicate that the dynamic unbalance of the wheelset significantly influences the dynamic performance of the high-speed car. The increase in dynamic unbalance of the wheelset will decrease the hunting critical speed, worsen the vertical ride quality, and increase the wheelset lateral force, derailment coefficient, and wheel unloading ratio. Therefore, in order to improve the stability, ride quality, and running safety of high-speed car, the values of dynamic unbalance of wheelset should be strictly controlled in manufacturing, and periodically monitored in operation.

To achieve the purpose of backward-facing step flow control, a passive control approach, which consists in introducing nonsmooth structures in solid walls, is applied to the upstream of the backward-facing step. Based on STAR CCM+ software, the standard k-e turbulence model was established to simulate flow characteristics of the right angle step, fillet step and nonsmooth fillet step. The introduction of the nonsmooth surface leads to a significant reduction in recirculation region length (17.5%) and a decrease in downstream wall pressure coefficient. According to the analysis of separation point position and turbulent kinetic energy, the delay of separation point and the enhancement of momentum exchange are the main reasons for the success of flow control. Compared with the right angle step, the delay of flow separation of the fillet step leads to a reduction in step expansion ratio (ER), and the existence of the nonsmooth structure enhances the turbulent kinetic energy at the fluid separation points and the momentum mixing at the downstream of the step, thus reducing the reattachment length and the downstream wall pressure coefficient.