Modeling of a Reduced Hybrid H2–Air Kinetic Scheme Integrating the Effect of Hypersonic Reactive Air with Supersonic Combustion

A hybrid H2–air kinetic scheme of 11 species and 15 reactions is developed, which is capable of simulating the high-temperature air reaction flows and H2–O2 combustion flows respectively or simultaneously. Based on the Gupta scheme, the mole fraction varying with a Mach number at specific conditions is analyzed, and the weakly-ionized 7-species 7-reaction scheme is selected. The effect of nitrogenous species on the H2–O2 combustion is analyzed by a zero-dimensional simulation of steady-state and unsteady-state combustion under specified conditions, and the selected dominant nitrogenous reaction N + OH = NO + H is distinguished by the production rate of the nitrogenous species. The thermodynamic properties are verified by comparison using the NIST–JANAF database. The reaction rate coefficients of the dominant reaction of the hybrid kinetic scheme distinguished by a sensitivity analysis are corrected. The proposed kinetic scheme is validated by a zero-dimensional calculation of the ignition delay time and two-dimensional computational fluid dynamics (CFD) simulation with finite-rate chemistry on the shock-induced sub-detonative and super-detonative combustion. The ignition delay time of the hybrid kinetic scheme is almost in the middle between the Shang scheme and Jachimowski scheme, and all the calculated ignition delay times are acceptably greater than the experiments due to the errors of the experiments and numerical models. The clearly captured bow shock wave and combustion front using the hybrid kinetic scheme and Shang scheme are almost the same, which is strongly consistent with the schlieren image. In addition, a good agreement of the flow characteristics and mass fraction of the species along the stagnation line is also obtained, which indicates the accuracy and reasonableness of the hybrid kinetic scheme to simulate hybrid H2–air reactive flows.

On symmetric intrusions in a linearly stratified ambient: a revisit of Benjamin's steady-state propagation results

Previous studies have extended Benjamin's theory for an inertial steady-state gravity current of density $\rho _{c}$ in a homogeneous ambient fluid of density $\rho _{o} < \rho _{c}$ to the counterpart propagation in a linearly stratified (Boussinesq) ambient (density decreases from $\rho _b$ to $\rho _{o}$ ). The extension is typified by the parameter $S = (\rho _{b}-\rho _{o})/(\rho _{c}-\rho _{o}) \in (0,1]$ , uses Long's solution for the flow over a topography to model the flow of the ambient over the gravity current, and reduces well to the classical theory for small and moderate values of $S$ . However, for $S=1$ , i.e. $\rho _b = \rho _c$ , which corresponds to a symmetric intrusion, various idiosyncrasies appear. Here attention is focused on this case. The control-volume analysis (balance of volume, mass, momentum and vorticity) produces a fairly compact analytical formulation, pending a closure for the head loss, and subject to stability criteria (no inverse stratification downstream). However, we show that plausible closures that work well for the non-stratified current (like zero head loss on the stagnation line, or zero vorticity diffusion) do not produce satisfactory results for the intrusion (except for some small ranges of the height ratio of current to channel, $a = h/H$ ). The reasons and insights are discussed. Accurate data needed for comparison with the theoretical model are scarce, and a message of this paper is that dedicated experiments and simulations are needed for the clarification and improvement of the theory.

Numerical Analysis of the Take-Off Performance of a Seaplane in Calm Water

Nowadays, with the escalating tensions in maritime dispute and the development of marine economy, there has been renewed interest in seaplanes for their special capacity of taking off and landing on water. Prediction of take-off performance, involving aerodynamic analysis and hydrodynamic analysis, is a main challenge in seaplane design, while the prediction methods have been little improved since the 1960s. This paper aims to investigate the attitude and resistance characteristics of a seaplane at different speeds during the take-off by numerically modeling the air-water flow field using RANS equations with VOF method. The trim and heave motion of seaplane in response to aerodynamic forces, hydrodynamic forces, hydrostatic forces, and propeller thrust was realized by solving rigid body dynamics equations and adopting dynamic overset mesh technique. The variations in heave, trim angle, and resistance characteristics during the takeoff were investigated, and their inherent relationships with the aerodynamic, hydrodynamic, and hydrostatic performance were revealed. Particular investigation on the hydrodynamic resistance indicates that the stagnation line located at the convex bow would contribute a considerable increase of pressure resistance at the first hump, and the trim angel of a seaplane should be operated in an optimum trim range, typical between 4–6 deg, to minimize the hydrodynamic resistance at the second hump. Additionally, the dynamic motion convergence study proves that the utilization of damping terms was an effective way to accelerate the convergence of the dynamic motion ending with a quasi-static state.

Performance Prediction of a Hard-Chine Planing Hull by Employing Different CFD Models

This paper presents CFD (Computational Fluid Dynamics) simulations of the performance of a planing hull in a calm-water condition, aiming to evaluate similarities and differences between results of different CFD models. The key differences between these models are the ways they use to compute the turbulent flow and simulate the motion of the vessel. The planing motion of a vessel on water leads to a strong turbulent fluid flow motion, and the movement of the vessel from its initial position can be relatively significant, which makes the simulation of the problem challenging. Two different frameworks including k-ε and DES (Detached Eddy Simulation) methods are employed to model the turbulence behavior of the fluid motion of the air–water flow around the boat. Vertical motions of the rigid solid body in the fluid domain, which eventually converge to steady linear and angular displacements, are numerically modeled by using two approaches, including morphing and overset techniques. All simulations are performed with a similar mesh structure which allows us to evaluate the differences between results of the applied mesh motions in terms of computation of turbulent air–water flow around the vessel. Through quantitative comparisons, the morphing technique has been seen to result in smaller errors in the prediction of the running trim angle at high speeds. Numerical observations suggest that a DES model can modify the accuracy of the morphing mesh simulations in the prediction of the trim angle, especially at high-speeds. The DES model has been seen to increase the accuracy of the model in the computation of the resistance of the vessel in a high-speed operation, as well. This better level of accuracy in the prediction of resistance is a result of the calculation of the turbulent eddies emerging in the water flow in the downstream zone, which are not captured when a k-ε framework is employed. The morphing approach itself can also increase the accuracy of the resistance prediction. The overset method, however, overpredicts the resistance force. This overprediction is caused by the larger vorticity, computed in the direction of the waves, generated under the bow of the vessel. Furthermore, the overset technique is observed to result in larger hydrodynamic pressure on the stagnation line, which is linked to the greater trim angle, predicted by this approach. The DES model is seen to result in extra-damping of the second and third crests of transom waves as it calculates the stronger eddies in the wake of the boat. Overall, a combination of the morphing and DES models is recommended to be used for CFD modeling of a planing hull at high-speeds. This combined CFD model might be relatively slower in terms of computational time, but it provides a greater level of accuracy in the performance prediction, and can predict the energy damping, developed in the surrounding water. Finally, the results of the present paper demonstrate that a better level of accuracy in the performance prediction of the vessel might also be achieved when an overset mesh motion is used. This can be attained in future by modifying the mesh structure in such a way that vorticity is not overpredicted and the generated eddies, emerging when a DES model is employed, are captured properly.

Numerical Modelling of a Planing Craft with a V-Shaped Spray Interceptor Arrangement in Calm Water

V-shaped spray interceptors are a novel concept of spray deflection on planing craft. Conventional spray rails are positioned longitudinally on the bottom of the hull and detach the spray from hull deflecting it towards the sides or slightly down and aftward. The V-shaped spray interceptors, on the other hand, are located in the spray area forward of the stagnation line such that they would deflect the oncoming spray down and aftward, thereby producing a reaction force that reduces the total resistance. An experimental study reported that the V-shaped spray interceptors to reduce the total resistance at low planing speed by up to 4%. This paper features a numerical comparison of two planing craft, one equipped with a conventional setup of longitudinal spray rails and the other with a V-shaped spray interceptor. Both configurations were simulated in calm water conditions and were free to pitch and heave in a speed range of Fr∇ = 1.776 to 3.108. The numerical model was analyzed for grid sensitivity and numerical results were compared with experimental results. The two concepts were compared in terms of total resistance, lift, running position and wetted surface area. Conventional spray rails were shown to account for up to 5.6% of total lift and up to 6.5% of total resistance. The V-shaped spray interceptor was shown to reduce the total resistance by up to 8%. Since the V-shaped spray interceptor was located in the spray area forward of the stagnation line, it deflected the oncoming spray thereby producing a horizontal reaction force (-1.5% of RTM) in the direction of the craft’s motion. The rest of differences in the total resistance of the hulls equipped with the conventional spray rails and the V-shaped spray rails was due to absence of the resistance of the absent spray rails.

The Combined Influences of Film Cooling and Thermal Barrier Coatings on the Cooling Performances of a Film and Internal Cooled Vane

This paper presents a numerical investigation on the combined influences of film cooling and thermal barrier coatings (TBCs) on the cooling performances of a NASA C3X guide vane. The results show that: (1) film cooling on the pressure side is more effective than suction side, especially on the trailing edge where multiple cooling and thermal protection techniques include internal cooling and TBCs are necessary. (2) TBCs show positive and negative roles in improving cooling performance at the same time for the coated vane with or without film cooling. Without film cooling, TBCs show negative roles on the regions with lower temperature external hot gas, which is caused by flow acceleration from the stagnation line of the suction side. (3) Internal cooling improvement caused by coolant introduction leads to a larger cooling effectiveness inclement due to TBCs near coolant plenums and film cooling holes. However, the influence of TBCs on cooling effectiveness increment goes down and even shows negative roles on the regions away from coolant plenums and under the effective coverage of the film cooling. (4) Improving the convective heat transfer of coolant with the wall of coolant plenums and film cooling holes is the guarantee of improving the cooling performance of a coated vane.

The Discrete Green's Function for Convective Heat Transfer—Part 2: Semi-Analytical Estimates of Boundary Layer Discrete Green's Function

This is the second paper in a set that defines the discrete Green's function (DGF). This paper focuses first on the turbulent boundary layer and presents two different methods to estimate the DGF. The long-element formulation defines the DGF with just two simple algebraic equations, but it is not quantitatively accurate for short element lengths. A short element correction is derived, but must be recalculated for each selection of flow parameters and element lengths. A similarity solution is derived that allows accurate estimates of the DGF diagonal elements for laminar boundary layers and for turbulent boundary layers discretized with short element lengths. To illustrate other methods to derive DGFs in more complex flows, a low-resolution DGF for laminar stagnation line boundary layers is determined using the skin-friction formulation combined with similarity solutions for two different thermal boundary conditions. Stagnation line flow is shown to be highly sensitive to the thermal boundary condition, and this can be analyzed effectively using the DGF.

Bio-Inspired Icephobic Coatings for Aircraft Icing Mitigation: A Critical Review

A critical review is provided to summarize our recent efforts to utilize the state-of-the-art bio-inspired icephobic coatings/surfaces, i.e., 1). Lotus-leaf-inspired superhydrophobic surfaces (SHS) and 2). Pitcher-plant-inspired slippery liquid-infused porous surfaces (SLIPS) for aircraft icing mitigation. By leveraging the unique Icing Research Tunnel of Iowa State University (i.e., ISU-IRT), an experimental campaign was performed to evaluate the effectiveness of using SHS and SLIPS coatings to suppress impact ice accretion over the surfaces of typical airfoil/wing models. While both SHS and SLIPS were found to be able to suppress ice accretion over the airframe surfaces where strong aerodynamic forces are exerted, ice was still found to accrete in the vicinity of the airfoil stagnation line where the aerodynamic forces are at their minimum. A novel hybrid anti-/de-icing strategy concept to combine icephobic coatings with minimized surface heating near airfoil leading edge was demonstrated to effectively remove impact ice accretion over entire airfoil/wing surfaces. An experimental investigation was also conducted to examine the durability of the icephobic coatings/surfaces to resist "rain erosion" effects (i.e., the damage to the surface coatings due to continuous impingement of water droplets at high speeds) in considering the practical usage for aircraft icing mitigation. The rain erosion effects were characterized based on the variations of the ice adhesion strengths and surface morphology of the eroded test surfaces coated with SHS and SLIPS. The research findings are very helpful to elucidate the underlying physics for the development of novel and robust anti-/de-icing strategies for aircraft icing mitigation.

Coolant Jets Blowing Across the Airfoil Stagnation Line to Enhance Film Effectiveness

A novel airfoil leading edge film cooling design has been investigated, and its performance over conventional alternatives quantified. In conventional designs, the region near the geometric stagnation line is typically between the two most upstream film hole rows, which each emit coolant in the downstream direction on their respective sides of the airfoil. This region is thus relatively starved of coolant flow and adequate cooling is achieved inefficiently with a high density of holes expelling a large amount of coolant in order to dilute the nearby mainstream flow. Drawing inspiration from recent literature on reverse-blowing film cooling holes, several film cooling geometries have been designed and tested with a view to improving upon this situation by blowing coolant from each side of the airfoil geometric stagnation line to the other in a criss-cross pattern. This is found to be capable of producing much higher film effectiveness near the stagnation line than a series of more conventional designs which were also tested, without decreasing downstream film effectiveness. A method is also described for using experimental film effectiveness data to estimate two novel measures of the efficiency of leading edge film coolant usage: the proportion of the mainstream which interacts with leading edge film coolant and the proportion of coolant from the two most upstream film hole rows which reaches the stagnation line.