1,721,032 research outputs found
Book review. Statistical theory and modeling for turbulent flows (2000) by P.A. Durbin and B.A. Pettersson Reif. Wiley, Chichester, ISBN 0471497444, £29.95
No full textNotes on meteorological balloon mission planning
No full textIn the 21st century the high altitude gas balloon remains an indispensable tool in atmospheric science, meteorology and other applications requiring stratospheric observations. A prerequisite of the effectiveness of many types of balloon operations is an accurate trajectory forecasting capability, complete with appropriate error estimates. This is particularly important in targeted flights, sample return missions or flights of expensive instruments, whose recovery is essential. The ASTRA (Atmospheric Science Through Robotic Aircraft) initiative led to the development of such a forecast model, which is at the centre of the present paper. A key source of error in such models is our incomplete understanding of the drag opposing the rise of balloons in the free atmosphere – here we propose a new, stochastic model based on empirical data derived from thousands of radiosonde flights. We also examine other sources of prediction error affecting the accuracy of the flight path forecast, such as uncertainties in the wind profile and balloon envelope manufacturing variability. A Monte Carlo framework is used to provide probabilistic touchdown point estimates taking these error sources into account. The above elements have been integrated into a web service, which can be used as a flight planning tool – here we review the key features of its architecture
Neutral flow over a series of rough hills: a laboratory experiment
No full textThis paper presents laboratory experiments of aerodynamically fully rough, neutral flow over a series of sinusoidal hills. Two sets of hills, with maximum gradients (slopes) of 0.2 (10°) and 0.4 (20°), were considered. The flow remained attached in the former case while separation occurred in the latter. Characteristics of the mean flow and turbulence statistics are discussed and compared with profiles over a flat surface covered with the same roughness as the hills. Comparisons are made with linear theory predictions for the flow in the inner region and aloft. Accurate measurements of the surface pressure were also made, enabling the comparison between the measured pressure drag and predictions from theoretical and computational work with different turbulent closure schemes. Organised secondary flow in the spanwise direction, observed previously in both experimental and computational studies, was also observed here over the small hills
Near-surface flows over random (urban) roughness
No full textComprehensive sets of vertical mean velocity and turbulence quantities have been obtained at 64 spatial locations within a unit (plan) area of random roughness. The roughness consisted of rectangular blocks arranged in a regular staggered pattern covering 25% of the wall surface. Each element had the same square cross-section in the horizontal but the heights were randomised. The dispersive stress arising in the roughness sublayer from the spatial inhomogeneity in the mean flow profiles was calculated and is shown to be negligible compared with the usual Reynolds stresses. The data are also compared with corresponding data for a uniform-height roughness of the same pattern and total volume. It is shown that the inertial sublayer, although having an upper limit which is almost identical to that of the uniform surface, is much thinner (i.e. the roughness sublayer was much thicker for the random surface than for the uniform surface). We conclude that the inertial sublayer may not exist at all in some urban areas in which the dominant features are high, irregular and heterogeneous roughness elements
Turbulence in rough-wall boundary layers: universality issues
No full textWind tunnel measurements of turbulent boundary layers over three-dimensional rough surfaces have been carried out to determine the critical roughness height beyond which the roughness affects the turbulence characteristics of the entire boundary layer. Experiments were performed on three types of surfaces, consisting of an urban type surface with square random height elements, a diamond-pattern wire mesh and a sand-paper type grit. The measurements were carried out over a momentum thickness Reynolds number (Reh) range of 1,300–28,000 using two-component Laser Doppler anemometry (LDA) and hot-wire anemometry (HWA). A wide range of the ratio of roughness element height h to boundary layer thickness d was covered (0:04!h=d!0:40). The results confirm that the mean profiles for all the surfaces collapse well in velocity defect form up to surprisingly large values of h/d, perhaps as large as 0.2, but with a somewhat larger outer layer wake strength than for smooth-wall flows, as previously found. At lower h/d, at least up to 0.15, the Reynolds stresses for all surfaces show good agreement throughout the boundary layer, collapsing with smooth-wall results outside the near-wall region. With increasing h/d, however, the turbulence above the near-wall region is gradually modified until the entire flow is affected. Quadrant analysis confirms that changes in the rough-wall boundary layers certainly exist but are confined to the near-wall region at low h/d; for h/d beyond about 0.2 the quadrant events show that the structural changes extend throughout much of the boundary layer. Taken together, the data suggest that above h/d & 0.15, the details of the roughness have a weak effect on how quickly (with rising h/d) the turbulence structure in the outer flow ceases to conform to the classical boundary layer behaviour. The present results provide support for Townsend’s wall similarity hypothesis at low h/d and also suggest that a single critical roughness height beyond which it fails does not exist. For fully rough flows, the data also confirm that mean flow and turbulence quantities are essentially independent of Reh; all the Reynolds stresses match those of smoothwall flows at very high Reh. Nonetheless, there is a noticeable increase in stress contributions from strong sweep events in the near-wall region, even at quite low h/
Near-wall flow development after a step change in surface roughness
No full textAn experimental study of the initial flow field downstream of a step change in surface roughness is presented. The roughness length of the downstream surface was approximately tenfold that of the upstream roughness and, unlike all previous studies, attention was concentrated on the roughness sublayer region beneath the inertial (log-law) region. The experiments were conducted at a boundary layer Reynolds number of about 6 × 104 (based on layer thickness and free-stream velocity) and around a longitudinal location where the (downstream) roughness length, zo2, was about 1% of the boundary-layer thickness at the roughness change point.
The thickness of the roughness sublayer was found for the two roughness. It was observed that the vertical profiles of mean velocity and turbulence characteristics started to show similarity after about 160z02 downstream of the roughness change. The presence of a shear stress overshoot is shown to depend strongly on the precise location (with respect to the roughness elements) at which the measurements are made and the thickness of the equilibrium layer is shown to be very sensitive to the way it is defined. It is demonstrated that the growing equilibrium layer has first to encompass the roughness sublayer before any thickness of inertial sublayer can be developed. It follows that, in some practical cases, like flows across some urban environments, the latter (log-law) region may never exist at all
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