Chelakara Subramanian

Effect of Reynolds number on a slightly heated turbulent boundary layer

Abstract Measurements in a slightly heated turbulent boundary layer, with essentially identical origins for momentum and thermal fields, indicate that the constants in the velocity and temperature logarithmic regions do not vary with Reynolds number. The extent of these regions, as a proportion of the boundary layer thickness, is approximately constant, independent of the momentum thickness Reynolds number Rm, when R m ⪞ 3100 . The deviation from the temperature logarithmic law in the outer layer is reasonably well described by expressions analogous to those which describe the velocity “ wake ”. The maximum value of this deviation increases with Rm over the range 990–4750 but is approx. constant for Rm > 4750, and equal to about half the maximum velocity deviation. Distributions of r.m.s. velocity and temperature scale moderately well with wall variables in the inner part of the sublayer at all Rm. Scaling on outer flow variables is only approximately achieved when R m ⪞ 3100 . The only noticeable effect of Rm on the turbulent Prandtl number and turbulence structure parameters is observed at the smallest Reynolds numbers investigated.

Temperature distribution in different materials due to short pulse laser irradiation

The purpose of this study is to analyze the heat-affected zone in materials such as meat samples, araldite resin-simulating tissue phantoms, and fiber composites irradiated using a mode-locked short pulse laser with a pulse width of 200 ps. The radial surface temperature profiles are compared with that of a continuous wave (CW) laser of the same average power. The short pulse laser results in a more localized heating than a continuous laser with a corresponding high peak temperature. A parametric study addressing the effect of pulse train frequency, material thickness, and amount of scatterers and absorbing agent in the medium and different initial sample temperatures is performed, and the measured temperature profiles are compared with the theoretical non-Fourier hyperbolic formulations and Fourier parabolic heat conduction formulations for both CW and pulsed laser cases.

BURSTS AND THE LAW OF THE WALL IN TURBULENT BOUNDARY LAYERS

The bursting mechanism in two different high-Reynolds-number boundary layers has been analysed by means of conditional sampling. One boundary layer develops on a smooth, flat plate in zero pressure gradient; the other, also in zero pressure gradient, is perturbed by a rough-to-smooth change in surface roughness and the new internal layer has not yet recovered to the local equilibrium condition at the measurement station. Sampling on the instantaneous uv signal in the logarithmic region confirms the presence of two related structures, ‘ejections’ and ‘sweeps’ which, in the smooth-wall layer, appear to be responsible for most of the turbulent energy production, and to effect virtually all that part of the spectral energy transfer that is universal. Ejections show features similar to those of Falcos ‘typical eddies’ while sweeps appear to be inverted ejections moving down towards the wall. The inertial structures associated with ejections show attributes of the true universal motion (Townsends ‘attached’ eddies) of the inner layer and these are therefore identified as ‘bursts’. In the outer layer, these become ‘detached’ from the wall. The large-scale structures associated with sweeps also appear to be ‘detached’ eddies (‘splats’), but these induce low-wave-number inactive motion near the wall and this is not universal even though the sweep itself is. Neither ejections nor sweeps detected in the rough-to-smooth layer are near a condition of energy equilibrium. The relation of ejections and sweeps to the law of the wall and other accepted laws is discussed.

Validation of a probabilistic model for hurricane insurance loss projections in Florida

The Florida Public Hurricane Loss Model is one of the first public models accessible for scrutiny to the scientific community, incorporating state of the art techniques in hurricane and vulnerability modeling. The model was developed for Florida, and is applicable to other hurricane-prone regions where construction practice is similar. The 2004 hurricane season produced substantial losses in Florida, and provided the means to validate and calibrate this model against actual claim data. This paper presents the predicted losses for several insurance portfolios corresponding to hurricanes Andrew, Charley, and Frances. The predictions are validated against the actual claim data. Physical damage predictions for external building components are also compared to observed damage. The analyses show that the predictive capabilities of the model were substantially improved after the calibration against the 2004 data. The methodology also shows that the predictive capabilities of the model could be enhanced if insurance companies report more detailed information about the structures they insure and the types of damage they suffer. This model can be a powerful tool for the study of risk reduction strategies.

Features of wavy vortices in a curved channel from experimental and numerical studies

Evidence of time‐dependent, wavy vortex motions associated with undulating and twisting Dean vortices is obtained experimentally in a curved channel with 40 to 1 aspect ratio, and mild curvature (radius ratio=0.979). These results are compared with direct numerical simulations of the time‐dependent, three‐dimensional Navier–Stokes equations using periodic boundary conditions in the spanwise and streamwise directions. When viewed in cross section, experimental visualizations of undulating and twisting vortex flows show rocking motion and changes in the direction of the flow between vortices that are like those observed in the simulations. Experimental spectra show that undulating vortices are replaced by the higher‐frequency, shorter streamwise wavelength twisting vortices at higher Reynolds numbers. When undulating vortices are present, experimental power spectra and visualizations give frequencies that are somewhat lower than the most unstable frequencies predicted by linear stability analysis. When twis...

Damage Characterization: Application to Florida Public Hurricane Loss Model

Florida’s population growth in recent decades has produced a steady increase in the concentration of population and infrastructure along its coastline. During this growth period, the vulnerability of the Florida coastline to hurricane impact has been amply demonstrated. The aftermaths unveiled the underlying potential for dramatic economic loss, which is disruptive for the state economy and has created a crisis in the property insurance market. To aid in preparedness efforts, the state of Florida commissioned a group of experts to develop the Florida Public Hurricane Loss Model (FPHLM). This paper describes how hurricane damage prediction techniques were developed and implemented in the vulnerability module of the FPHLM. The module consists of an external damage simulation engine coupled with internal damage and contents damage simulation engines. These components are described in this paper.

Reynolds-number dependence of the structure of a turbulent boundary layer

Conditional averages of longitudinal, normal velocity and temperature fluctuations and of their products have been obtained in a slightly heated boundary layer with zero pressure gradient over a momentum-thickness Reynolds-number range 990 ≤ R m ≤ 7100. These averages are based on the identification of coherent temperature fronts that extend across most of the layer. The average period between fronts is approximately independent of R m when R m is greater than about 1500. The streamwise length scale of the fronts and the magnitude of velocity and temperature derivatives associated with the fronts scale on the thickness of the layer except for R m , less than about 3000. This scaling is consistent with the Reynolds-number independence, for R m greater than about 3000, of both mean and turbulent velocity and temperature fields. Conditional averages are discussed in the context of Head & Bandyopadhyays (1978) suggestion, based on smoke-flow visualization, that the boundary layer consists almost exclusively of hairpin eddies.

Flow Around an Object Projected from a Cavity into a Supersonic Freestream

Abstract : The pressure and flow field of a supersonic flow over a cavity, with and without a store, was the focus of this experiment. One cavity geometry (length to depth ratio 3.6) was studied; the freestream Mach number and the placement of the store relative to the cavity floor were varied. The pressure spectra on the cavity floor were markedly different between Mach numbers of 1.8 and 2.9. The Mach 1.8 case exhibited clear spectral peaks consistent with predictions by Rossiter, whereas the Mach 2.9 flow did not. With the store placed within the free shear layer, the pressure fluctuations on the cavity floor decreased for Mach 1.8 and increased for Mach 2.9. High-speed Schlieren photography was used to visualize the interaction of the free shear layer and the modeled store. Images revealed that flow structures in the free shear layer of the Mach 2.9 flow exhibited less spanwise coherence then their Mach 1.8 flow counterparts. Images also revealed vertical displacement of the free shear layer as the store traversed through it. Pressure-sensitive paint (PSP) was utilized to quantify the full-field mean pressure on the cavity floor and store. A pressure rise near the trailing edge was noted for both freestream Mach numbers. The mean pressure contour of the floor for the Mach 1.8 exhibited considerable three-dimensionality, despite the generally spanwise coherent structures in the free shear layer. (4 tables, 33 figures, 37 refs.)

A wireless multipoint pressure sensing system: design and operation

This paper describes the development of a remote-sensing and wireless data acquisition system. The system was developed as part of a research effort currently underway to instrument coastal homes in Florida to monitor roof wind pressures during hurricanes. However, the design of the wireless data acquisition is flexible enough to adapt to other multisensor, wind tunnel, or field-testing applications. This paper describes the details of the design, theory of operation, and the results of field performance tests of the system. The new system offers the advantages of ease of installation, capability for more than 48 h of continuous data acquisition from up to 60 sensors at variable sampling rate, and relatively easy maintenance.

Response of a turbulent boundary layer to a sudden decrease in wall heat flux

Abstract Measurements are presented of mean and fluctuating temperature fields downstream of a sudden decrease in wall heat flux in a zero pressure gradient turbulent boundary layer. The growth rate of the thickness of the internal layer, estimated from r.m.s. temperature profiles, is found to be in good agreement with that obtained for a sudden increase in wall heat flux. When the internal thermal layer thickness and maximum temperature difference across the layer are used as the normalising length scale and temperature scale respectively, mean, r.m.s. and higher order moments of the temperature are approximately self-preserving, at least over the outer region of the internal layer. The sharp temperature jump at the back of the large structure is observed over a significant part of the boundary layer both upstream and downstream of the sudden change in heat flux.