Concrete form of tunnel lining is generally demolded earlier than other concrete structures. In particular, the demolding of ceiling form of tunnel lining is approximately 12-20 hours of concrete age. Note is that such early strength of concrete is hardly examined at site. Hence, a proper strength-estimation of tunnel lining concrete at early age is needed to demold the concrete form. Strength development of concrete at early age must be strongly related to cement hydration process. To develop a non-destructive testing (NDT) technique evaluating concrete strength, a hyperspectral camera which can quantify cement hydration was used in this study. Hyperspectral images consist of wide-range wavelength, it can obtain invisible variations of hydrated cement compared to general red-green-blue (RGB) images. A fundamental study was conducted to examine cement hydration by using the hyperspectral camera. The study proposed a cumulative reflectance for quantitative evaluation of the hyperspectral image. The study examined the relation between the cumulative reflectance and compressive strength of hydrated cement paste. In addition, chemical reaction of alite and calcium hydroxide in the cement paste were examined to quantify the cement hydration. The fundamental test revealed that the cumulative reflectance is applicable to estimate cement hydration degree. The NDT technique using hyperspectral image has a possibility for estimating the strength of tunnel lining concrete.
In this study, the effects of the computational spanwise domain length on the flowfield with massive separation and on the flowfield with dynamic stall are investigated by largeeddy simulation. The objective airfoil is NACA0012 and the chord-based Reynolds number is of 2.56× 10. The objective flowfields are that around a fixed angle of attack of 10 and 25 degrees, and that around a pitching airfoil between AoA of 5 degrees and 25 degrees. The spanwise length effect become significant after the stall, as observed as the attenuation of the large vortices. Observations of the flowfield clarified that the undulation of two large vortices from the leading edge and the trailing edge is one of the mechanisms for the spanwise length effects. The qualitative analysis for this mechanism is performed to address the sufficient spanwise length, and the spanwise length have to be at least 1.0c for the flowfield with large vortex structures so as to resolve its spanwise distribution.
Mechanisms behind the pressure distribution and skin friction within a laminar separation bubble (LSB) are investigated by large-eddy simulations around a 5% thickness blunt flat plate at the chord length based Reynolds number 5.0 × 103, 6.1 × 103, 1.1 × 104, and 2.0 × 104. The characteristics inside the LSB change with the Reynolds number; a steady laminar separation bubble (LSB_S) at the Reynolds number 5.0 × 103 and 6.1 × 103, and a steady-fluctuating laminar separation bubble (LSB_SF) at the Reynolds number 1.1 × 104, and 2.0 × 104. Different characteristics of pressure and skin friction distributions are observed by increasing the Reynolds number, such that a gradual monotonous pressure recovery in the LSB_S and a plateau pressure distribution followed by a rapid pressure recovery region in the LSB_SF. The reasons behind the different characteristics of pressure distributions at different Reynolds numbers are discussed by deriving the Reynolds averaged pressure gradient equation. It is confirmed that the viscous stress distributions near the surface play an important role in determining the formation of different pressure distributions. Depending on the Reynolds numbers, the viscous stress distributions near the surface are affected by the development of a separated laminar shear layer or the Reynolds shear stress. In addition, we show that the same analyses can be applied to the flows around a NACA0012 airfoil.
A multi-block-and overset grid-based computational fluid dynamic (CFD) study was implemented for the unsteady flows surrounding a hovering hawkmoth with the realistic body-wing geometrical and kinematic models. The computed results show that downwash produced by a flapping hawkmoth has time dependency fairly. In particular, weak downwash is detected during downstroke, in contrast; strong downwash is predicted during upstroke. The reason the strong downwash is produced is that the combination of leading-edge vortex, shedding trailing-edge vortex and wing tip vortex forms a ring-shaped vortex characterized by strong downward-flows in the center of it during downstroke. The remarkable downward-flows travel to bottom area of a flapping insect during upstroke when the ring-shaped vortex is shed by wing rotation and flapping wings push it downward. Moreover downwash distribution does not exhibit a circle shape that has been explained by the Rankin-Fluid momentum theory.
Fluid physics associated with a pitching and plunging airfoil, while critical to the development of flapping wing air vehicles, is not adequately understood. To help assess the state-of-the-art of engineering predictive tools, we utilize recently obtained experimental information based on particle image velocimetry (PIV) in a water tunnel from two different facilities to examine the effects of chord Reynolds number, and the airfoil shape on the associated flow structures. Two rigid airfoils, SD7003 and flat plate, undergoing pitching and plunging motion in nominally two-dimensional conditions are investigated with the aid of the original Menter’s Shear Stress Transport (SST) turbulence model and a modified version which limits the production of turbulence kinetic energy to reduce the build-up of turbulence in stagnation regions. We consider two kinematic schemes, a pitching and plunging, and a pure plunging motion. For the SD7003 airfoil under pitching and plunging motion, the original SST model offers consistently favorable agreement with both PIV measurements. For the pure plunging SD7003 airfoil case, depending on the turbulence characteristics including those caused the motion of the wing, and the implied eddy viscosity level, qualitatively different flow structures are observed experimentally and computationally. The flat plate creates flow fields insensitive to the Reynolds number, and quite different from those around the SD7003 airfoil, due to the leading edge effect.
