Instability of the flow in the vicinity of trailing edge of a class of thin aerofoils

A numerical study has been undertaken to investigate the notion of absolute/convective instability in laminar incompressible trailing edge flows past wedge-like shapes with curved boundaries of the form y=α(−x)^m. The effects of various trailing edge shapes m and relative thickness α on the flow separation and the development of instabilities in the vicinity of trailing edge are investigated. The nonlinear viscous-inviscid interaction equations, which have been derived by means of the asymptotic theory of flow separation, are solved first numerically to construct genuine mean velocity profiles representing the correct flow in the vicinity of the trailing edge. The absolute/convective nature of the asymptotically formed velocity profiles via a composite expansion is then ascertained by using a spatio-temporal analysis based on the Briggs-Bers pinching criterion. Although no absolute growth is encountered upstream of the trailing edge of the airfoil shapes considered, in particular the wake region behind the trailing edge of Joukowski type profiles is found to be persistently susceptible to absolute instability. It is found that separation is enhanced as the relative thickness of the airfoil gets bigger. This, in turn, is shown to lead to an additional enhancement of the absolute instability character by both increasing the absolute growth rate as well as the extent of the unstable region. Shedding frequency of the Karman vortex street is also determined behind the trailing edge shapes considered.     

Shock-Vortex Interactions at High Mach Numbers

We perform a computational study of the interaction of a planar shock wave with a cylindrical vortex. We use a particularly robust High Resolution Shock Capturing scheme, Marquina's scheme, to obtain high quality, high resolution numerical simulations of the interaction. In the case of a very-strong shock/vortex encounter, we observe a severe reorganization of the flow field in the downstream region, which seems to be due mainly to the strength of the shock. The numerical data is analyzed to study the driving mechanisms for the production of vorticity in the interaction.     

Numerical prediction of unsteady vortex shedding for large leading-edge roughness

A full two-dimensional Navier-Stokes algorithm is used to investigate unsteady, incompressible viscous flow past an airfoil leading edge with surface roughness that is characteristic of ice accretion. The roughness is added to the surface through the use of a Prandtl transposition and can generate both small-scale and large-scale roughness. The focus of the study is a detailed flow analysis of the unsteady velocity fluctuations and vortex shedding induced by the surface roughness. The results of this study are compared to experimental data on roughness-induced transition for the same roughness geometry. A comparison is made between “fluctuation intensity” values from the current algorithm to experimentally determined turbulence intensity values. The effects of the roughness Reynolds number, Re_k, are investigated and compared to experimental values of the critical roughness Reynolds number. The authors speculate that there may be a possible correlation between unsteady roughness-induced vortex shedding and the onset of experimentally measured transitional flow downstream of large-scale roughness.     

A thermal-calorimetric gas flow meter with improved isolating feature

This paper presents design and characterization of a novel thermal-calorimetric flow-meter using suspended-cantilever-structure. There is an air gap between the heater and each individual thermistor providing a good thermal isolation. Due to the suspended-structure which consists of three cantilevers, the thermal convection effect is present on both sides of the active area. Also the velocity boundary layer thickness of the cantilever is much less than closed-membrane one. This characteristic enhances the sensitivity of sensor. The simulation results indicate that the average temperature difference between upstream and downstream thermistors are 36.5 and 1.04 K for flow rate of 1 m/s and the worst case of 0.1 m/s respectively. This solution significantly improves the sensitivity compared to the closed-membrane-structures. The maximum temperature difference causes 94 mV at the output of Wheatstone bridge with 3 V of voltage supply. The calculated and simulated results show that the maximum power consumption of sensor is 4.7 mW at the maximum flow velocity of 1 m/s. The operational range of the designed flow meter is from 0 to 1 m/s. The features of the device are analytically evaluated and simulated under various conditions.

     

Parallel DNS for vortex structure of late stages of flow transition

A new DNS using compact high order scheme and MPI parallel computation has been conducted with 1920 × 241 × 128 grid points for non-linear stages of flow transition. The coherent vortex structure of the late flow transition stages and the mechanism of formation of single vortex ring, multiple vortex rings, and small length scales are discussed. The ring-like vortex formation from the Λ-vortex is the result of the interaction of two pairs of counter-rotating primary and secondary streamwise vortices. The formation of the multiple ring structure follows the first Helmholtz vortex conservation law. A bridge must be formed to link two Λ-vortex legs. The bridge finally develops as a new ring. This process keeps going onto form a multiple ring structure. The U-shaped vortices are part of existing coherent large vortex structure. Actually, the U-shaped vortex, which is a third level vortex, serves as a second neck to supply vorticity to the multiple rings. The multiple ring-like vortex structure is found quite stable and can travel for a long distance. The “hairpin vortex breakdown” does not happen. The small vortices can be found on the bottom of the boundary layer near the wall surface. It is believed that the small vortices are generated by the interaction of higher level vortices with the solid wall, but not by the “vortex breakdown”.     

液体冲压发动机仿真模型研究

发展了一种弹用液体冲压发动机仿真模型.利用小偏离线性化理论和分段集总参数法,分别研究了超声速进气道、燃烧室和可调收敛喷管的建模方法.以正激波作为上边界,燃油流量的小变化作为扰动方程,喷管作为下边界,从而建立了液体冲压发动机仿真模型.根据所建立的仿真模型,对冲压发动机系统进行了仿真实验.仿真结果验证了仿真模型的有效性和合理性.该模型可用于液体冲压发动机控制系统的研究.     

Large-eddy simulations of film cooling flows

Large-eddy simulations (LES) of a jet in a cross-flow (JICF) problem are carried out to investigate the turbulent flow structure and the vortex dynamics in gas turbine blade film cooling. A turbulent flat plate boundary layer at a Reynolds number of Re_∞ = 400,000 interacts with a jet issued from a pipe. To study the effect of the jet inclination angle α on the flow field, two angles are chosen, the perpendicular injection at 90° and the streamwise inclined injection at 30°. For the normal injection case a small blowing ratio of the jet velocity to the cross-stream velocity R = 0.1 is examined. For the streamwise inclined injection case two blowing ratios R = 0.1 and R = 0.48 are investigated to check the impact of the jet velocity on the cooling performance. The time-dependent turbulent inflow information for the cross-flow is provided by a simultaneously performed LES of a spatially developing turbulent boundary layer. Whereas in the perpendicular injection case a rather large separation region is found at the leading edge of the jet hole, in the streamwise inclined injection cases no separation is observed. Compared with the normal injection case at the same blowing ratio, the streamwise inclination weakens the jet-cross-flow interaction significantly. Thus, the first appearance of the counter-rotating vortex pair (CVP) is shifted downstream and its strength is reduced. The increase of the blowing ratio leads to a stronger penetration of the jet into the cross-flow, resulting in a more upstream located and more pronounced CVP. Downstream of the jet exit the streamwise vortices are so large that besides the jet fluid also the cross-stream is partially entrained into this zone, which yields the worst cooling performance.     

Capabilities and limitations of 2-dimensional and 3-dimensional numerical methods in modeling the fluid flow in sudden expansion microchannels

This paper deals with computational and experimental investigations into pressure-driven flow in sudden expansion microfluidic channels. Improving the design and operation of microfluidic systems requires that the capabilities and limitations of 2-dimensional (2-D) and 3-dimensional (3-D) numerical methods in simulating the flow field in a sudden expansion microchannel be well understood. The present 2-D simulation results indicate that a flow separation vortex forms in the corner behind the sudden expansion microchannel when the Reynolds number is very low (Re∼0.1). However, the experimental results indicate that this prediction is valid only in the case of a sudden expansion microchannel with a high aspect ratio (aspect ratio >> 1). 3-D computational fluid dynamics simulations are performed to predict the critical value of Re at which the flow separation vortex phenomenon is induced in sudden expansion microchannels of different aspect ratios. The experimental flow visualization results are found to be in good agreement with the 3-D numerical predictions. The present results provide designers with a valuable guideline when choosing between 2-D or 3-D numerical simulations as a means of improving the design and operation of microfluidic devices.     

Localized flow control in microchannels using induced-charge electroosmosis near conductive obstacles

In this paper, we investigate the use of induced-charge electroosmosis (ICEO) as a means of providing localized flow control near conductive obstacles within bulk pressure-driven flow. In an experimental device, this ICEO flow was induced by an on/off switchable AC field applied across a section containing gold post(s). A simple numerical model, adapted from Levitan et al. (Colloids Surf A 267:122–132, 2005), was implemented and used to provide guidance for the design of the experimental devices. The induced flow was combined with an applied bulk pressure-driven flow to modify flow patterns. We have specifically observed single and multiple stream patterns downstream of the posts in the experimental devices, suggesting the presence of ICEO flow in the experimental system. The custom devices were obtained using a fabrication process that relies on relatively standard steps in the MEMS community, however, unlike other fabrication processes, it has been shown to create fully conductive posts with vertical sidewalls. Utilizing various combinations of number(s) of post(s), geometry and position, useful flow patterns can be created.     

Stagnation point viscous layers with mass transfer

Effects of injecting helium, argon and hydrogen into ionizing air boundary layers and thin viscous shock layers including the effects of shock slip (finite thickness bow shock waves) were studied. Finite rate chemistry and complete multicomponent diffusion were used in all models. Comparison with experimental data and predictions based upon equilibrium chemistry showed significant effects of the viscous flowfield models and wall catalyticity on heat transfer. Effects of theoretical models on deduced experimental freestream and stagnation conditions are discussed. At low Reynolds numbers boundary-layer theory is not applicable, and a model is described to include higher-order effects with hydrogen injection. Viscous-layer predictions were made up to the point where the sensible heat transfer was reduced to zero under which conditions the viscous layer was still attached to the surface.