Tomoya Houra

Structure of turbulent boundary layer subjected to adverse pressure gradient

Abstract A turbulent boundary layer subjected to a sustained adverse pressure gradient is experimentally investigated. Waveforms of fluctuating velocity components in the boundary layer, especially in the near-wall region, are remarkably elongated in time in comparison with those in zero-pressure-gradient flows, and thus time scales increase with an increasing pressure gradient parameter

Effects of adverse pressure gradient on quasi-coherent structures in turbulent boundary layer

P^+

Response Compensation Scheme for Constant-Current Hot-Wire Anemometry Based on Frequency Response Analysis

. The increase in time scales is not in proportion to the corresponding increase in the conventional viscous time scale

Turbulent Thermal Diffusion Over a Locally-Heated Two-Dimensional Hill

\nu/u_\tau^2

Turbulence Measurements Over a Heated Two-Dimensional Hill

. It is found that the Taylor time scale is most appropriate to describe the essential characteristics of non-equilibrium adverse pressure gradient flows. Even the near wall-limiting behavior of streamwise velocity fluctuations for different

Effects of Adverse Pressure Gradient on Heat Transfer Mechanism in Thermal Boundary Layer

P^+

DNS of velocity and thermal fields in turbulent channel flow with transverse-rib roughness

is well correlated in the coordinates based on the Taylor time scale. Moreover, in the boundary layer with an adverse pressure gradient, the contribution of sweep motions becomes equivalent to that of ejections, and outward and wallward interactions relatively increase near the wall, which evidently indicates a change in coherent structures.

Direct numerical simulation of stable and unstable turbulent thermal boundary layers

Abstract Statistical and kinematic characteristics of turbulent boundary layer flows subjected to adverse pressure gradients are found to differ significantly from those of zero-pressure-gradient ordinary boundary layers. The turbulent energy and shear stress transport vu 2 and vuv occurs in the direction toward the wall from the regions away from the wall, as the pressure gradient parameter P + increases. This event is completely opposite to that in zero-pressure-gradient cases. The instantaneous waveforms of third-order moments are markedly altered in the near-wall region. The quadrant splitting and trajectory analyses reveal that obvious changes do occur in large-amplitude sweep motions (Q4) and outward interactions (Q1). On the other hand, the contributions from other coherent motions, especially the ejection motions (Q2), significantly decrease and their durations become longer, i.e., these motions are dull and less active. Moreover, multi-point simultaneous measurements with five X-probes are made to depict the kinematic pictures of the effects of the adverse pressure gradient on the eddy structures.

Effects of adverse pressure gradient on heat transfer mechanism in thermal boundary layer

Hot-wire anemometry (HWA) is used for measuring velocity fluctuations such as turbulent flows. It is generally operated in three modes; constant-temperature (CT), constant-current (CC) and constant-voltage (CV) types. The constant-temperature anemometer (CTA) is the mainstream anemometer, while others are rarely used in measuring normal turbulent flows because of their insufficient response speed. However, since the constant-current anemometer (CCA) can be composed of simple circuits, the HWA can be realized at quite a low cost. In this study, the response characteristics of the CCA are theoretically analyzed, and a compensation scheme is proposed. The scheme is experimentally tested in a turbulent wake flow formed behind a cylinder. As a result, it has been confirmed that the root-mean-square (rms) velocities and the power-spectrum distributions compensated by the present scheme agree well with those measured with CTA. Hence, the CCA provides reliable measurement of turbulent velocity fluctuations.Copyright

Spatio-temporal turbulent structures of thermal boundary layer subjected to non-equilibrium adverse pressure gradient

We measure flow and thermal fields over a locally heated two-dimensional hill. The heated sections on the wall are divided into upstream and downstream portions of the hill model. These sections are heated independently, yielding various thermal boundary conditions in contrast to the uniformly heated case. In the separated region formed behind the hill, it is found that the mean temperature profiles in the uniformly heated case are well decomposed into the separately heated cases. This is because the velocity fluctuation produced by the shear layer formed behind the hill is large, so the superposition of a passive scalar in the thermal field can be successfully realized. The rapid increase in the mean temperature near the uniformly heated wall should be due to the heat transfer near the leeward slope of the hill. On the other hand, the mean temperature distributions away from the wall are strongly affected by the turbulent thermal diffusion on the windward side of the hill.© 2010 ASME