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The effect of the canopy fabric damage on parachute performance has been investigated numerically and experimentally in the present paper. For this aim, first, flow structure around the parachute canopy has been studied for both damaged and undamaged parachutes. Then, the drag coefficient as the main characteristic of a parachute performance has been examined and compared experimentally and numerically in four Reynolds numbers. Experimental tests for undamaged and damaged canopies have been carried out in an open-circuit wind tunnel laboratory at the velocities 15, 20, 25, and 30 m/s. In order to measure the drag force, a valid tensile load-cell has been used. Also, the smoke flow visualization has been utilized to find the flow behavior at different regions of the parachute. For the numerical simulation, an incompressible pressure-based CFD code using the finite volume method has been applied to solve the complex turbulent flow field around the inflated parachute canopy. To increase accuracy of the numerical simulation, the permeability boundary condition on the parachute canopy has been implemented and its effects have been considered on the flow field. The numerical results indicate that the permeability assumption on the canopy makes desirable results compatible with the experimental ones. Moreover, comparison of the results between damaged and undamaged canopies demonstrates significant differences in the streamlines, pressure distributions, and drag coefficients. As the reliability is the main criterion for the parachute system, investigation on the damage effects would be useful and it has been considered in the present work.

Present study concerns the development of a new approach for chemical reaction modelling in DSMC technique. The conventional quantum-kinetic (QK) method based on the VHS collision model is usually applied by the DSMC algorithm to simulate chemical reactions in rarefied gas flowfield. In this paper, a modified quantum-kinetic model (MQK) is presented to remove some limitation of the QK chemical model. The MQK method uses the GSS collision model based on the Stockmayer potential function which can capture both the attractive and repulsive intermolecular forces and polarization effect of the molecules in the reactive flow. To investigate the accuracy and capability of the proposed model, firstly, dissociation of the nitrogen (non-polar gas) and water vapour (polar gas) are simulated in a single cell DSMC for equilibrium and non-equilibrium conditions. The results of the MQK are assessed with those of the QK methods, analytical models and experimental data. Secondly, dissociation of these two types of gases is studies in the rarefied gas flowfield along the stagnation line of a typical hypersonic atmospheric blunt body problem. The flowfield properties including the density, velocity, transient, vibrational and total temperature along the stagnation line and surface properties including the surface pressure and heatflux at the stagnation point obtained by the MQK model are compared with results of the QK method and available analytical and experimental data. The results show that the MQK model provides more accurate results when the polarization effects and both the attractive-repulsive intermolecular forces are dominant especially at high temperature. Also, comparison of the computational cost between two models indicates higher efficiency using the proposed method rather than the QK model due to reduction of number of the collision pairs.

A new chemical model in the direct simulation Monte Carlo (DSMC) algorithm, entitled modified collision energy (MCE), has been developed for simulation of reactive rarefied flows without some limitations of the conventional macroscopic models. Determination of correct values of the experimental parameters for computing the Arrhenius reaction rate is a serious challenge in some macroscopic chemical reaction models such as total collision energy (TCE) and general collision energy (GCE). A slight variation of these constant parameters in the Arrhenius relation could lead to significant change in the results. On the other hand, these experimental parameters have been extracted empirically only for limit number of gases and so they cannot be used for simulation of chemical reactions in various types of gases. Since some of these constants have been determined experimentally by several studies, they have been reported by different values in different references. The proposed MCE model in the present study is a reliable method to properly determine values of these parameters for all types of gases with the intrinsic properties of the particles and without need of any experimental data. Extraction of the constant parameters has been carried out using the analytical method or numerical quantum kinetics (QK) or modified quantum kinetics (MQK) models. The proposed MCE method has been evaluated in four test cases, including assessment of the reaction rate in equilibrium and nonequilibrium conditions, studying of the rarefied flow along the stagnation line, and investigation of the hypersonic gas flow over the axial symmetry blunt nose. The results show that the proposed method has desirable accuracy without use of any experimental parameters. The MCE method can also be used to calibrate the macroscopic reactive models such as the TCE and GCE.

Three-dimensional unsteady flow field around a finite circular cylinder standing in a flat-plate boundary layer is studied. For this purpose, two different numerical turbulence approaches as wall adapted local eddyviscosity LES (LES-WALE) and the zonal hybrid RANS-LES approach of Detached-Eddy Simulation (Zonal-DES) are used. Analysis is carried out for a finite circular cylinder with diameter of D = 3 mm and length-to-diameter ratio of L/D=6 which leads to the Reynolds number 2×104. Numerical simulation has been performed based on the LES-WALE and Zonal-DES turbulence models using coarse and fine grids. Ability and accuracy of two models in capturing the complex physics of present phenomenon are investigated by comparing their results with each other and validated experimental results. Also, effect of several important parameters such as time-averaged pressure coefficient, velocity, vortex shedding frequency and performance of the LES-WALE and Zonal-DES turbulence models are studied.

Drop motion on different types of new proposed micro-structure surfaces has been numerically investigated to find the optimum structure in view point of ice formation delaying. The droplet automatically moves on the inclined surfaces due to gravity forces. To validate the numerical algorithm, three different bench mark problems have been considered. The results indicate that the present algorithm is trustable for the presented numerical simulations. Then the validated numerical approach has been used to simulate droplet motion on nine proposed superhydrophobic surfaces in the same conditions. Comparison the drop motion on different micro-structure surfaces at different time indicate that creation of most micro-structure coating on smooth surfaces causes larger equilibrium contact angles and delays ice formation. For better understanding the results, the air flow structure trapped in the grooves have been plotted and investigated for some cases and the extracted results have been discussed in view of ice formation. It is found that the micro-structure in which the grooves’ upper surface is greater than the lower one, may cause worse hydrophobicity conditions than the smooth one. Also, adding hierarchical structures to the micro grooves creates better ice repellency conditions. Structures which prevent formation of secondary vortices, like circular grooves, are more preferable to other micro-structures in view point of ice formation.

Longitudinal dynamic derivatives of an airfoil oscillating in pitching and plunging motions were calculated using variation of pitching moment coefficients with angle of attack in various conditions, based on wind tunnel data. The effect of reduced frequency on variation of longitudinal oscillatory derivatives was investigated, in three different regions of oscillation: before, over and post stall conditions. The results showed that reduced frequency has significant effects on longitudinal oscillatory coefficients in different conditions for both types of oscillations.

Two efficient computational procedures based on the boundary layer equations and approximate relations areassessedin prediction of the laminar hypersonic flowfield for both the perfect gas and equilibrium air around the axisymmetric blunt body configurations. For the boundary layer procedure, the boundary layer equationsutilize the integral matrix solution algorithm for the blunt nose and after body region by using a space marching technique. The integral matrix procedure able us to create accurate and smooth results using the minimum grid in the boundary layer and minimize the computational costs. Applying the approximate method creates a robust and efficient code for heating calculations over the blunt bodies which flies in hypersonic regimes. These algorithms are highly appropriate to design of hypersonic reentry vehicles. The effects of real gas on the flowfield characteristics are also studied in two procedures.

An appropriate combination of the thin-layer Navier–Stokes (TLNS) and parabolized Navier–Stokes (PNS) solvers is used to accurately and efficiently compute hypersonic transitional/turbulent flowfields of perfect gas and equilibrium air around blunt-body configurations. The TLNS equations are solved in the nose region to provide the initial data plane needed for the solution of the PNS equations. Then the PNS equations are employed to efficiently compute the flowfield for the afterbody region by using a space marching technique. Both the TLNS and the PNS equations are numerically solved by using the implicit non-iterative finite-difference algorithm of Beam and Warming. A shock fitting procedure is used in both the TLNS and PNS codes to obtain accurate solution in the vicinity of the shock. For turbulent flow simulations, both the Cebeci–Smith (CS) and the Baldwin–Lomax (BL) turbulence models are analyzed with the present technique for the case of long slender blunt bodies. The Baldwin–Lomax turbulence model, which does not need the determination of the edge of the boundary layer, is modified in the present work to include the effects of variable properties and damping factor to accurately calculate flowfield characteristics in turbulent regions. Detailed comparisons are made with other numerical and experimental results to assess the accuracy and efficiency of the present solution procedure for computing hypersonic transitional/turbulent flows over long slender blunt bodies. The results of these computations are found to be in good agreement with available data. The effects of real gas on the flowfield characteristics are also studied.

In the present study, two preconditioners proposed by Eriksson, and Choi and Merkel are implemented on a 3D upwind Euler flow solver on unstructured meshes. The mathematical formulations of these preconditioning schemes for the set of primitive variables are drawn and their eigenvalues and eigenvectors are compared with each others. A cell-centered finite volume Roe's method is used for discretization of the 3D preconditioned Euler equations. The accuracy and performance of these preconditioning schemes are examined by computing low Mach number flows over the ONERA M6 wing for different conditions.

SUMMARY Three preconditioners proposed by Eriksson, Choi and Merkel, and Turkel are implemented in a 2D upwind Euler flow solver on unstructured meshes. The mathematical formulations of these preconditioning schemes for different sets of primitive variables are drawn, and their eigenvalues and eigenvectors are compared with each other. For this purpose, these preconditioning schemes are expressed in a unified formulation. A cell-centered finite volume Roe's method is used for the discretization of the preconditioned Euler equations. The accuracy and performance of these preconditioning schemes are examined by computing steady low Mach number flows over a NACA0012 airfoil and a two-element NACA4412–4415 airfoil for different conditions. The study shows that these preconditioning schemes greatly enhance the accuracy and convergence rate of the solution of low Mach number flows. The study indicates that the preconditioning methods implemented provide nearly the same results in accuracy; however, they give different performances in convergence rate. It is demonstrated that although the convergence rate of steady solutions is almost independent of the choice of primitive variables and the structure of eigenvectors and their orthogonality, the condition number of the system of equations plays an important role, and it determines the convergence characteristics of solutions. Copyright © 2011 John Wiley & Sons, Ltd.

An appropriate combination of the thin-layer Navier–Stokes (TLNS) and parabolized Navier–Stokes (PNS) solvers is used to accurately and efficiently compute hypersonic flowfields of equilibrium air around blunt-body configurations. The TLNS equations are solved in the nose region to provide the initial data plane needed for the solution of the PNS equations. Then the PNS equations are employed to efficiently compute the flowfield for the afterbody region by using a space marching procedure. Both the TLNS and PNS equations are numerically solved by using the efficient implicit non-iterative finite-difference algorithm of Beam and Warming. A shock fitting technique is used in both the TLNS and PNS codes to obtain accurate solution in the vicinity of the shock. In the regions of the flowfield with upstream influences, the globally iterated PNS (IPNS) equations, which are computationally more efficient than the TLNS equations, can be used to improve the accuracy of the results. To validate the results of the developed TLNS code and study real gas effects on the flowfield, hypersonic laminar flow over a sphere at Mach number of 11.26 is computed. The computations are performed for hypersonic laminar flow over 5° long slender blunt cone at Mach number of 19.25 to demonstrate the accuracy and efficiency of using the present TLNS-PNS methodology. To show the capability and robustness of the present solution procedure, the comparisons are also made with the wall heat-transfer data on a 12.84°/7° bicone at Mach number of 6.89 and freestream temperature of 1604 K. The results of these computations are found to be in good agreement with available numerical and experimental data. The results of the perfect gas for these cases are also presented and the effects of real gas on the flowfield characteristics are studied in both the TLNS and PNS solutions.

The modified quantum kinetic (MQK) chemical reaction model introduced by Zakeri et al. is developed for applicable cases in axisymmetric reactive rarefied gas flows using the direct simulation Monte Carlo (DSMC) method. Although, the MQK chemical model uses some modifications in the quantum kinetic (QK) method, it also employs the general soft sphere collision model and Stockmayer potential function to properly select the collision pairs in the DSMC algorithm and capture both the attraction and repulsion intermolecular forces in rarefied gas flows. For assessment of the presented model in the simulation of more complex and applicable reacting flows, first, the air dissociation is studied in a single cell for equilibrium and non-equilibrium conditions. The MQK results agree well with the analytical and experimental data and they accurately predict the characteristics of the rarefied flowfield with chemical reaction. To investigate accuracy of the MQK chemical model in the simulation of the axisymmetric flo...