Yuji Ohya

Experiments on vortex shedding from flat plates with square leading and trailing edges

Vortex shedding from flat plates with square leading and trailing edges having chord-to-thickness ratios 3–16 at Reynolds numbers (1–3) × 10 3 is investigated experimentally in low-speed wind tunnels. It is shown that vortex shedding from flat plates with square leading and trailing edges is characterized by the impinging-shear-layer instability where the separated shear layer becomes unstable in the presence of a sharp trailing edge corner. The Strouhal number which is based on the plates chord is approximately constant and equal to 0.6 for chord-to-thickness ratios 3–5. With further increase in the ratio it increases stepwise to values that are approximately equal to integral multiples of 0.6.

WIND-TUNNEL STUDY OF ATMOSPHERIC STABLE BOUNDARY LAYERS OVER A ROUGH SURFACE

Wind-tunnel simulations of theatmospheric stable boundary layer (SBL) developedover a rough surface were conducted by using athermally stratified wind tunnel at the Research Institutefor Applied Mechanics (RIAM), Kyushu University. Thepresent experiment is a continuation of the workcarried out in a wind tunnel at Colorado StateUniversity (CSU), where the SBL flows were developed over asmooth surface. Stably stratified flows were createdby heating the wind-tunnel airflow to a temperature ofabout 40–50°and by cooling the test-section floor toa temperature of about 10°. To simulate therough surface, a chain roughness was placed over thetest-section floor. We have investigated the buoyancyeffect on the turbulent boundary layer developed overthis rough surface for a wide range of stability,particularly focusing on the turbulence structure andtransport process in the very stable boundary layer.The present experimental results broadly confirm theresults obtained in the CSU experiment with the smoothsurface, and emphasizes the following features: thevertical profiles of turbulence statistics exhibitdifferent behaviour in two distinct stability regimes with weak and strong stability,corresponding to the difference in the verticalprofiles of the local Richardson number. The tworegimes are separated by the critical Richardsonnumber. The magnitudes in turbulence intensities andturbulent fluxes for the weak stability regime aremuch greater than those of the CSU experiments becauseof the greater surface roughness. For the very stableboundary layer, the turbulent fluxes of momentum andheat tend to vanish and wave-like motions due to theKelvin–Helmholtz instability and the rolling up andbreaking of those waves can be observed. Furthermore,the appearance of internal gravity waves is suggestedfrom cross-spectrum analyses.

TURBULENCE STRUCTURE IN A STRATIFIED BOUNDARY LAYER UNDER STABLE CONDITIONS

Turbulence structure in stably stratified boundary layers isexperimentally investigated by using a thermally stratified wind tunnel. Astably stratified flow is created by heating the wind tunnel airflow to atemperature of about 50 °C and by cooling the test-section floor to asurface temperature of about 3 °C. In order to study the effect ofbuoyancy on turbulent boundary layers for a wide range of stability, thevelocity and temperature fluctuations are measured simultaneously at adownwind position of 23.5 m from the tunnel entrance, where the boundarylayer is fully developed. The Reynolds number, Reδ, ranges from 3.14× 104 to 1.27 × 105, and the bulk Richardson number, Riδ,ranges from 0 to 1.33. Stable stratification rapidly suppresses thefluctuations of streamwise velocity and temperature as well as the verticalvelocity fluctuation. Momentum and heat fluxes are also significantlydecreased with increasing stability and become nearly zero in the lowest partof the boundary layer with strong stability. The vertical profiles ofturbulence quantities exhibit different behaviour in three distinct stabilityregimes, the neutral flows, the stratified flows with weak stability(Riδ = 0.12, 0.20) and those with strong stability (Riδ= 0.39,0.47, 1.33). Of these, the two regimes of stratified flows clearly showdifferent vertical profiles of the local gradient Richardson number Ri,separated by the critical Richardson number Ri cr of about 0.25. Moreover,turbulence quantities in stable conditions are well correlated with Ri.

The effects of turbulence on the mean flow past two-dimensional rectangular cylinders

There are two main effects of turbulence on the mean flow past rectangular cylinders, just as found earlier for square rods. Small-scale turbulence increases the growth rate of the separated shear layers through increased mixing. Large-scale turbulence weakens regular vortex shedding by reducing spanwise correlation. The consequences of increased mixing and weakened regular vortex shedding depend on the depth-to-height ratio of a rectangular cylinder. In particular, the mean base pressure of a rectangular cylinder can be varied significantly by both the scale and intensity of turbulence.

WAKE INTERFERENCE OF A ROW OF NORMAL FLAT PLATES ARRANGED SIDE BY SIDE IN A UNIFORM FLOW

The wake characteristic of groups of normal flat plates, consisting of two, three, or four plates placed side by side with slits in between, have been investigated experimentally. When the ratio of the slit width to the plate width (slit ratio) was small, the gap flows were observed to be biased either upward or downward in a stable way, leading to multiple, stable flow patterns for a single slit-ratio value. Some regularities were recognized in the gap-flow directions and the appearance of the flow patterns. The plates on the biased side showed high drag and regular vortex shedding, while those on the unbiased side showed the opposite. The origin of the biased flow has also been investigated with water-tank experiments, numerical calculations and wind-tunnel experiments. The-results showed that the origin of biasing is strongly related to the vortex shedding of each plate of a row.

Numerical simulation of atmospheric flow over complex terrain

Abstract In order to develop an overall efficient and accurate method of simulating an unsteady three-dimensional atmospheric flow over topography, we examined two grid systems and corresponding variable arrangements: one is a body-fitted coordinate (BFC) grid system based on a collocated variable arrangement; the other is an orthogonal grid system based on a staggered variable arrangement. Using these codes, we calculated the wind system over topography such as an isolated hill and real complex terrain. Both codes remarkably removed the numerical difficulties such as the convergence of the SOR method in solving the pressure Poisson equation, resulting in numerical results with much higher accuracy. Despite the differences in the grid system and in the variable arrangement, no significant differences in the flow pattern between the both numerical results were found. Compared with the previous studies, the numerical results obtained are very satisfactory in the sense that overall characteristic flows are successfully simulated irrespective of the simulation codes.

Experimental and numerical analysis of vortex shedding from elongated rectangular cylinders at low Reynolds numbers 200-103

Abstract Experimental and numerical investigation on vortex shedding from elongated rectangular cylinders at low Reynolds numbers 200-10 3 was performed in this paper. The experiment was made in a low speed wind tunnel, and the numerical analysis was based on the finite difference method applicable to the 2D Navier-Stokes equations. The side ratio of rectangular cylinders tested ranged from 3 to 16 in the experiment, and 3 to 10 in the numerical computation. Good agreement was obtained between experiment and numerical analysis of the Strouhal number. With increasing Reynolds number, transition was observed in the type of vortex shedding, i.e., from the Karman type shedding to the impinging-shear-layer instability, where a single separated shear layer can be unstable in the presence of a sharp trailing edge corner. The transitional Reynolds number was roughly equal to 300.

Turbulence structure of stable boundary layers with a near-linear temperature profile

By using a thermally stratified wind tunnel, we have successfullysimulated stably stratified boundary layers (SBL), in which the meantemperature increases upward almost linearly. We have investigated the flow structure and the effects of near-linearstable stratification on the transfer of momentum and heat. Thevertical profiles of turbulence quantities exhibit different behaviour in two distinct stability regimes of the SBLflows with weak and strong stability. For weak stability cases, theturbulent transfer of momentum and heat is basically similar to that for neutral turbulent boundary layers, although it is weakenedwith increasing stability. For strong stability cases, on the other hand,the time-mean transfer is almost zero over the whole boundary-layer depth.However, the instantaneous turbulent transfer frequently occurs in bothgradient and counter-gradient directions in the lower part of the boundary layer. This is due to the Kelvin–Helmholtz (K–H) shear instability and therolling up and breaking of K–H waves. Moreover, the internal gravity wavesare observed in the middle and upper parts of all stable boundary layers.

The effects of turbulence on the mean flow past square rods

There are two main effects of turbulence on the mean flow past rods of square cross-section aligned with the approaching flow. Small-scale turbulence increases the growth rate of the shear layer, while large-scale turbulence enhances the roll-up of the shear layer. The consequences of these depend on the length of a square rod. The mean base pressure of a square rod varies considerably with turbulence intensity and scale as well as with its length.

A thermally stratified wind tunnel for environmental flow studies

Abstract A wind tunnel was constructed to study the effects of thermal stratification on flow and diffusion in the atmospheric boundary layer. The wind tunnel is of a suction type and has a 1.5 m wide, 1.2 m high, 13.5 m long, rectangular test section. Designed to produce thermally stratified flows, the tunnel is equipped with two independent temperature systems, an air-flow heating unit (AHU) and a floor temperature controlling unit (FTCU). Using the AHU and FTCU, a wide range of thermal stratification can be generated with a wind speed in the range of (U = ) 0.2−2.0 m s−1. Both in neutral and stably stratified flows, the tunnel can provide a uniform smooth flow with a low turbulence intensity of about 0.4% at a wind speed less than 1 ms−1. The design features and performance of the wind tunnel are described. The characteristic of thermally stratified flows created in the test section is also discussed.