LOW-REYNOLDS-NUMBER INSTABILITY OF THE LAMINAR FLOW BETWEEN WAVY WALLS
Instability of a viscous incompressible flow in a channel with wavy walls is investigated theoretically, numerically and experimentally. Linear stability analysis shows that appropriately chosen wall waviness leads to flow destabilization at surprisingly low Reynolds numbers. The unstable mode of disturbances forms a vortex array, which travels downstream. The remarkable feature is that the most destabilizing waviness does not introduce any additional flow resistance. The outcome of the stability analysis are consistent with the result of direct numerical simulation obtained using CFD finite volume package FLUENT (Ansys Inc.). Preliminary experimental data gained for a channel with appropriately corrugated wall seem to confirm these predictions.
TURBULENCE IN MICRO-CHANNELS
Fluid mechanics in small channels, i.e. channels of micrometer size, is dominated by surface effects and often exhibits striking differences of flow characteristics when compared with macro scale. One of important microfluidic problems is flow destabilization and occurrence laminar- turbulent transition. In this paper we describe our experimental and numerical attempts to understand growth of flow instabilities and development of turbulent structures in small channels. In the first configuration flow of water through 1mm long and 0.4mm high microchannel formed between two planes is investigated varying Reynolds number from 1000 to 6770. Fluorescent traces are used for flow visualization and microPIV acquisition of temporary velocity fields. The microPIV data are used to evaluate turbulent flow characteristics. Our experimental study shows that destabilization of flow in such a micro-channel does not necessarily occurs when it is usually expected. Nearly laminar flow structure is present within the channel even for the highest investigated flow Reynolds number. These findings are confirmed by numerical simulation performed using finite volume code. On the other hand it appears possible to achieve unstable flow pattern even for quite low Reynolds number flow regime by proper modification of the channel walls. In the second experimental and numerical study we demonstrate that appropriately chosen wall waviness of the micro-channel may lead to flow destabilization already at quite low flow Reynolds number (~100).
Fatigue effects on the coordinative pattern during cycling: kinetics and kinematics evaluation.
The aim of the present study was to analyze the net joint moment distribution, joint forces and kinematics during cycling to exhaustion. Right pedal forces and lower limb kinematics of ten cyclists were measured throughout a fatigue cycling test at 100% of PO(MAX). The absolute net joint moments, resultant force and kinematics were calculated for the hip, knee and ankle joint through inverse dynamics. The contribution of each joint to the total net joint moments was computed. Decreased pedaling cadence was observed followed by a decreased ankle moment contribution to the total joint moments in the end of the test. The total absolute joint moment, and the hip and knee moments has also increased with fatigue. Resultant force was increased, while kinematics has changed in the end of the test for hip, knee and ankle joints. Reduced ankle contribution to the total absolute joint moment combined with higher ankle force and changes in kinematics has indicated a different mechanical function for this joint. Kinetics and kinematics changes observed at hip and knee joint was expected due to their function as power sources. Kinematics changes would be explained as an attempt to overcome decreased contractile properties of muscles during fatigue.