The generation and growth of wind waves are re-examined using linear viscous shear flow instability theory by solving the coupled in-air and in-water Orr-Sommerfeld equations. To enable comparison with the available laboratory observations, model simulations are performed for a wide range of wavelengths spanning the gravity-capillary and gravity wave regimes typical of such experiments. The sensitivity of the results to key modelling assumptions is investigated, including the friction velocity, the surface drift velocity at the air-water interface as well as the shapes of velocity profiles in air and in water, which are modelled using the mixing-length approach. Airflows both over an initially smooth surface and over a surface modified by the emergence of fast-growing short ripples, and thus effectively rough, are considered. A detailed energy budget analysis, based on eigenfunctions of the coupled Orr-Sommerfeld equations across different wavelengths, provides further insight into the mechanisms governing energy transfer from wind to water waves under diverse flow conditions.

The spatial evolution of various statistical parameters of fetch-limited waves generated by steadily blowing wind over mean water flow in a wind-wave flume is investigated experimentally. Measurements are performed in both along- and against-wind current conditions, and compared with measurements in the absence of current. A rake of capacitance-type wave gauges is used to measure surface elevation for a wide range of wind and water current velocities; additionally, an optical wave gauge is used to measure the directional properties of the wind-wave field in the presence of a mean water current at multiple locations. The variation with fetch of essential wave parameters such as characteristic wave energy, dominant frequency, power spectra and temporal coherence, as well as higher-order statistical moments that characterize wave shape, is presented for co- and counter-wind water currents, and compared with the no-current condition. The findings in the presence of mean water flow are interpreted in the framework of the viscous shear flow instability model of Geva & Shemer (Phys. Rev. Lett., vol. 128, 2022, 124501).

The growth with fetch of young wind waves under steady wind forcing that is commonly attributed to shear flow instability results in a spatially inhomogeneous wave field with a spectrum evolving along the tank. The present laboratory study accounts for multiple co-existing statistically stationary random frequency harmonics. Single-point synchronous measurements of the instantaneous surface elevation and of its along-wind slope component are performed by optical methods at numerous locations. Assuming exponential spatial growth, the phase shift between the surface elevation and surface slope at each frequency is related to the spatial growth rate of each harmonic. The validity of the assumption that the wave energy varies exponentially with fetch is examined in a separate set of experiments; the instantaneous surface elevation at various wind-forcing conditions is measured at multiple locations along the tank. The spatial variation of the energy of individual frequency harmonics is determined. It is found that, below the local peak frequency, the energy of each harmonic grows exponentially, while the evolution of waves at frequencies approaching and exceeding the local peak is strongly affected by sheltering by the dominant wave, as well as by nonlinear bound waves. The outcomes of two independent methods of determination of spatial growth rates at a range of young wave frequencies are compared. The accumulated data also enable quantitative analysis of the sheltering phenomenon. The essential difference between the spatial and the temporal wind-wave evolution cases is discussed.

A diffuser-augmented wind turbine is used to enhance the overall performance characteristic of the wind turbine. Flanges near the trailing end of the diffuser create a pressure difference that accelerates the free-stream velocity into the diffuser, increasing the performance of the wind turbine. In the present analysis, a numerical method is used to analyze the effect of flange vibration at different amplitude and operating frequencies. The diffuser is operated at no-load (Ct = 0) condition for all cases. An optimum opening angle is determined in the absence of flange vibration at a constant airflow rate and is used for further analysis. Performance characteristics such as diffuser speed-up ratio and coefficient of power are estimated for all cases. It is found that the velocity at the centerline oscillates with the same frequency as that of the flange.

Understanding the air velocity profile over random and three-dimensional wind-waves is crucial for evaluating momentum and energy transfer between air and water. Unfortunately, determining accurate air velocity profiles in field conditions is nearly impossible. It is well accepted, though, that the airflow above water waves has a logarithmic profile usually expressed in terms of the effective roughness parameter. However, this parameter cannot be evaluated directly from the wave measurements at the water surface. For young wind waves, it was recently shown that the airflow over the waves maintains wall similarity. In this case, the airflow vertical velocity distribution can be described by the Nikuradse fully rough logarithmic profile, using the root mean square value of the water surface elevation as the equivalent sand grain roughness height. The existence of mean current in water in either co- or counter-wind direction may significantly modify the wave field, the vertical wind-velocity profile and thus the momentum and energy transfer from air to water. The effect of the water current on the spatially developing boundary layer over young wind waves and the wall similarity is examined. Combined laboratory measurements at several fetches of finely resolved mean air velocity profile above the water surface and of the characteristics of the wind-wave field are performed at multiple wind-forcing and mean water current conditions. The shear stress at the air–water interface estimated using two independent approaches is weakly dependent on current, while the resultant wave field differs significantly.

Synchronous single point measurements of the instantaneous surface elevation and of two components of the surface slope in a wind-wave tank are carried out using a high-speed camera and a laser slope gauge. The applied technique allows study of joint statistics between the temporal variation of surface elevation and the slope components at a point, as well as computation of directional wave energy spectra. It is demonstrated that while significant coherence exists between surface elevation and along-wind slope component, the coherence between crosswind slope and surface elevation is almost negligible. Evolution of the directional spectra along the test section is presented for different wind velocities and compared with results obtained in other relevant field and laboratory studies.

Different wavy regimes in stratified air–water pipe flow are determined for a wide range of gas and liquid flow rates in a 10 m long horizontal pipe with a diameter of 24 mm. Three sub-regions of wavy stratified flow are identified: ripples, roll waves, and pre-annular wavy flow. Statistical parameters, such as local mean film thickness and its higher moments (root-mean-square, skewness, excess kurtosis) as well as wave characteristics (mean heights and wave height distributions, lengths, propagation velocities, etc.), are measured and analyzed. It is demonstrated that ripples are essentially linear waves and their propagation velocities are described reasonably well by linear wave theory. High amplitude roll and pre-annular waves are substantially nonlinear, and their propagation velocities differ significantly from that of ripples. Transition to roll waves causes a sharp increase in higher statistical moments. Evolution of wave and statistical parameters characterizing each sub-region of stratified gas–liquid pipe flow is studied. Simplified models describing roll waves are presented; the model predictions are verified by experiments.

The effect of multiple airfoil-slat configurations for the airfoil NACA 2415 is numerically investigated in this study. Numerical simulations were carried out with the finite volume method using k-omega-SST (shear stress transport) and k-k(l)-omega turbulence models. The obtained numerical result for the airfoil NACA 2415 is validated with the experiment results of Genc et al. (2009). The k-omega-SST and k-k(l)-omega turbulence models were able to predict the coefficient of lift and drag accurately, but the value deviates after 20 degrees without slat configuration. The value predicted by the k-omega-SST turbulence model aligns well with the experimental data. The maximum coefficient of the lift (C-l) for a single airfoil slat was obtained at an angle of attack alpha = 22 degrees with a value of 2.22 and for a double airfoil slat at an angle of attack alpha = 26 degrees with a value of 2.33 at Re = 2 x 10(5). Further, it is observed that with a single-slat configuration, the coefficient of the lift is increased by 72% compared to the airfoil with no slats. For the given range of Re, the coefficient of lift (C-l) and drag (C-d) follow a similar trend for both the single- and double-slat configurations. For the post-stall condition, C-l is significantly reduced for the single-slat configuration as compared to the double-slat configuration. At alpha <= 10 degrees, the C-d for the double-slat airfoil is far less than for the single-slat airfoil, which results in a high C-l-to-C-d ratio.

In this work, the optimization for the optimum position of the secondary slat was investigated with an emphasis on enhancing the aerodynamic performance. The method adopted here merges the computational fluid dynamic (CFD) technique with the response surface method. The multi-objective genetic algorithm was used for the optimization of the positioning of the secondary slat, and the Pareto ranking was done using a non-dominated sorting method. In CFD analysis, the Reynolds average Nervier–Stokes equation is solved using the k-ω shear stress transport model which is very popular due to its accuracy. The obtained numerical result for the primary airfoil NACA 2415 and the airfoil with a single slat is validated with the experimental data. The NACA 22 airfoil profile is selected to serve as a slat to impediment the separation of boundary layer and enhance airfoil characteristics. The addition of slat at the leading edge of the primary slat increases the overall aerodynamic performance of the configuration and enhances the stall angle from 12º to 22º. Further, the addition of slat significantly reduces the boundary layer thickness as a result delays the separation to a higher angle of attack. The method used in this work can be employed as a valuable tool for positioning optimization of the secondary slat at the leading edge of the primary slat of the airfoil.

This study numerically examines the characterization of fluid flow and heat transfer in microchannels with grooves of different aspect ratios. Rectangular and circular grooves at different aspect ratios are used as flow turbulence promoters. A correlation is developed for predicting the hydrothermal characteristics of the microchannel using nonlinear regression analysis. Simulations were conducted for a moderate-to-high Reynolds number in the transition-turbulent flow regime. The influences of parameters like inlet velocity, groove depth, and groove shape on the hydrothermal properties of the microchannel are examined. Analysis of the result shows that, with an increase in the groove's aspect ratio, the Poiseuille and Nusselt numbers decrease. The pressure drops across the microchannel increase with an increase in Reynolds number, and it decreases with an increase in groove depth. The microchannel with a circular groove exhibits a higher friction factor and pressure drop and, in turn, higher heat transfer than the rectangular microchannel.

A three-dimensional numerical simulation is conducted to study the characteristic of fluid flow and heat transfer in the rectangular microchannel heat sink with arc grooves. The numerical model of the microchannel is validated with the experimental data at various Reynolds numbers. The groove configurations in the present analysis are grooves on the floor of the microchannel, grooves on both the sidewalls and grooves on the sidewall, and the floor of the microchannel. The groove depth is varied from h = 20 mu m to h = 80 mu m for all different configurations. The simulations are carried out for Reynolds number (Re) 200, 900, and 2000, with constant bottom wall temperature. Nusselt number, Poiseuille number, and performance factor are analyzed for the above cases to quantify the effect of groove configuration and groove depth on the hydrothermal characteristics of the microchannel. It is observed that the addition of grooves on the surface of the microchannel, results in the formation of pseudo secondary flow which enhances the heat transfer in the microchannel at the expense of increased Poiseuille number. The groove depth only affects the performance of the microchannel at lower Reynolds number, and at higher Reynolds number, the groove depth does not have any role in the performance enhancement. The groove configuration also plays a major role in the performance enhancement of the microchannel.

Centrifugal atomizer has been widely used to produce metal powders, slag granules through dry slag granulation process. In this process, liquid is poured at the center of the spinning disc, which disintegrates to form droplets, and becomes solid granules/powders on cooling. In this study, a numerical simulation has been carried out to estimate powder size considering phase change into the model, and the obtained results are validated with experimental measurements. Different mechanisms of powder production viz., direct powder formation, ligament to powder formation, and film to powder formation have been captured at various liquid flow rates and angular disc speed. The effect of composition of the model liquid slag and its properties like viscosity, density, surface tension, etc. has been studied using this mathematical model. Further, the effect of superheat on solidification was also considered. Finally, powder size distribution has been correlated with properties of the slag, superheat of the liquid, and process variables.