Unsteady Detached Eddy Simulations Research Articles

Noise field properties of marine propellers in open water

Aim of the paper is to gain a physical understanding of the main noise generating mechanisms related to vorticity and turbulence downstream the disk of a marine propeller in open water. Through the application of the Ffowcs Williams and Hawkings acoustic analogy (FWH) for permeable surfaces, the analysis of the noise field generated by the INSEAN E779A propeller model at different advancing ratios allows to detect correlations between acoustic signatures in the near/mid field and wake evolution. Turbulence noise-induced effects are captured by combining the FWH formulation with a Detached Eddy Simulation (DES) of the flow past the propeller. Besides, a phase-locked post-processing of the unsteady DES data on the acoustic surface is used to yield FWH signatures governed by vorticity effects, which are then compared with the pressure fluctuations predicted by the Bernoulli equation, within a potential hydrodynamics platform solved by a Boundary Integral Element Method (BIEM). It is found that turbulence is the most annoying source of propeller noise, whereas vorticity-driven phenomena are important only in a narrow region surrounding the propeller. Moreover, numerical results highlight that acoustic pressures are correlated to the vortex pairing hydrodynamic phenomenon associated to a blade-to-shaft harmonic energy transfer, detected in literature by time-resolved visualizations and velocimetry measurements.

Numerical analysis of vortex and cavitation dynamics of an axial-flow pump

ABSTRACT This study focuses on the correlative mechanism of the ambient pressure and inflow uniformity on the vortex and cavitation dynamics of an axial flow pump. The shear stress transport k – ω turbulence model and Schnerr–Sauer cavitation model are applied in the unsteady detached eddy simulation. The results show that the vortex merging between the primary and secondary tip leakage vortices (TLV) happens earlier with the growth of the cavity at a lower ambient pressure. The contact position of the merged TLV to the adjacent blade moves upstream with the decrease in the cavitation number. As the uniformity of the axial inflow decreases, TLV merging and vortex shedding are also promoted. The nonlinear variation of the initial angle of attack of the impeller blade leads to the compression or expansion of sheet cavitation under non-uniform inflow conditions. The evolution process and energy transfer of the vortices are verified quantitatively using a power spectral density analysis of kinetic energy fluctuations, and the short-wave instability leads to the fast decline of spectrum peaks at a higher frequency in the cavitating flow. It is crucial to avoid severe changes of ambient pressure and inflow uniformity to ensure the design performance of pump in actual working environment.

Combining simultaneous density and velocity measurements of rotor blade tip vortices under cyclic pitch conditions

An investigation into blade tip vortices of a sub-scale rotor under cyclic pitch conditions is carried out. Background oriented schlieren (BOS), particle image velocimetry (PIV), and computational fluid dynamics (CFD) are applied to the same test cases. This approach allows to combine the velocity data from PIV in a measurement plane, the density related data from BOS in a measurement volume, and the comprehensive set of flow variables provided by unsteady detached eddy simulations. Vortices up to an age of varPsi _{{text {v}}}= {70}^circ in case of PIV and CFD, and up to varPsi _{{text {v}}}= {200}^circ in case of BOS are considered. The vortex locations are obtained through all three techniques. The unsteadiness of the vortices was obtained by the experimental results, whereas CFD provides an average solution. An increased position scatter was observed during the downstroke of the pitch cycle with both experimental methods and was found to be in good agreement. In the second part, the PIV velocity data are compared to common vortex models. An approach to link the density distribution and the swirl velocity is applied to the measured data. Based on the CFD results, it is shown that the assumption of isothermal flow yields better agreement between velocity and density than isentropic flow.Graphic abstract

Effect of hydrogen jets in supersonic mixing using strut injection schemes

The prevalence of complex phenomena associated with the fuel mixing of a supersonic stream in scramjet combustor is inherently occurred due to the short residence time. An efficient injection mechanism is required to enhance the mixing and improve combustion efficiency. This numerical simulation study aims to reveal the performance of modified strut injection strategies within a Mach 2.0 flow field. Two-dimensional steady and transient Navier-Stokes computations of the DLR scramjet experiment is performed for various strut injection locations. The Reynolds Averaged Navier Stokes equation with the SST k-ε turbulence model is utilized to solve the flow field under steady conditions. The critical parameters examined to investigate steady solutions are wall static pressure, flow Mach number, and total pressure loss across the combustor. The dual injection configuration in the flow considerably reduces the shock waves impact at the downstream of the strut and preserves the magnitude of internal forces acting on combustor walls and total pressure loss. Unsteady Detached Eddy Simulation (DES) results for hydrogen concentration and velocity field are analyzed by applying Dynamic Mode Decomposition (DMD). Multiple injections are observed to alter the frequency and the number of dominant modes.

Unsteady Detached-Eddy Simulation (DES) of the Jetstream 31 aircraft in One Engine Inoperative (OEI) condition with propeller modelling

Numerical results from a three-dimensional (3D) computational fluid dynamics (CFD) model of the Jetstream 31 aircraft in conditions of one engine inoperative are presented. The objective of this work is to analyse the performance of the Jetstream 31 aircraft and provide transient data using an unsteady Detached-Eddy Simulation (DES) CFD approach and a numerical propeller model to compare computational results with a single engine flight test experiment. The propeller modelling approach has been implemented through User-Defined Functions using C programming language to replicate the propeller effect. Different angles of attack and sideslip are studied, based on records from flight test data, both with unsteady (DES) and steady-state Reynolds-Averaged Navier-Stokes (RANS) models. An error analysis on the flight test data provides an error band from 2% to 16% among all cases, high values due to the lack of many data samples. Across the RANS approach, an average deviation of 6.9% and 3.8% for respectively lift and drag coefficients is achieved. By applying the DES turbulence modelling approach, a better lift prediction is achieved (5.4%) despite a slightly worse drag (4.5%). It has also been found that 80% of the numerical results are within the error band defined. A close agreement has been found within moment coefficients, with average percentage deviations from 3.3% to 7.0%. Overall, an analysis has been carried out in the present work, both on the flight test and computational sides to provide reliable numerical results of these aerodynamic properties.

Numerical study of a symmetric submerged spatial hydraulic jump

ABSTRACT Hydraulic jumps are often employed as energy dissipators below hydraulic structures. If a high-velocity conduit drains into an expanding open channel a spatial hydraulic jump can be formed. Even though this type of hydraulic jump is common in civil engineering practice, it has not received sufficient consideration by researchers. Three-dimensional, unsteady detached eddy simulation of a symmetric submerged spatial hydraulic jump was carried out to evaluate the turbulent flow features. The volume of fluid multiphase model was used to capture the free surface. The complete three-dimensional flow features of symmetric submerged hydraulic jump are reported herein for the first time. The three-dimensional jump roller was found to be stronger near the side walls than at the centre. The instantaneous vortex shedding from the conduit outlet was also captured by the simulations. To describe the internal structure of turbulence within a symmetric submerged spatial hydraulic jump, the coherent flow structures are qualitatively and quantitatively examined using a vortex identification technique and proper orthogonal decomposition.

Grid Convergence Study for Detached-Eddy Simulation of Flow over Rod-Airfoil Configuration Using OpenFOAM

A rod-airfoil is a benchmark configuration for simulating Airfoil-Turbulence Interaction Noise (ATIN). The numerical simulation is modelled using unsteady Detached Eddy Simulation (DES). The grid refinement involves two stages of assessments. The first compares the sensitivity of two reasonable estimated cells to the flow behaviours of the case. Based on the finding from the first stage, further grid refinement is proceed in stage two. In stage two, a minimum of three different grid resolutions are considered; the fine, medium and coarse grids in order to investigate the grid independency. Richardson extrapolation and Grid Convergence Index (GCI) are introduced to quantitatively evaluate the grid independency. Based on the results between those three different grids, a monotonic convergence criterion has been achieved. The reduction in GCI value indicates that the grid convergence error has been significantly reduced, in which the fine grid has a GCI value is around less than 0.5%.

Numerical Simulations of Imperfect Bifurcation of Jet in Crossflow

This paper investigates the flow structure nearby a rectangular cooling hole on a flat plate by numerical simulations and topological analysis. The velocity ratio ranges from 0.5 to 1.5. Steady Reynolds-averaged NavierStokes simulations and unsteady detached eddy simulations are performed based on the Spalart-Allmaras turbulence model. The time-averaged velocity profiles in the central plane are verified by grid convergence study and comparison with the experimental data. The discussions mainly focus on the effect of the velocity ratio on the vortex structure and the stability of the jet-in-crossflow (JICF) system. The simulation results show that a bifurcation of the vortex system exists with respect to the velocity ratio. The averaged flow goes from a symmetric structure to an asymmetric structure on this bifurcation. The detached eddy simulation results show that both the symmetric vortex system and the asymmetric vortex system with different velocity ratios are stable.

A fluid structure coupling of the Ariane-5 nozzle section during start phase by detached eddy simulation

Concerning the requirements of future rocket technologies, providing a cost-efficient access to orbit as well as an increase in system reliability, a deeper insight into the unsteady phenomena during ascent of modern launchers is essential. Unsteady interactions and resonances of the turbulent separated launcher wake and the nozzle structure play an important role for the design of future main stage propulsion systems. The so-called buffeting coupling phenomenon is one of the main challenges during ascent. In the present study, a coupled simulation of the afterbody of the Ariane-5 launcher with a realistic structural and aerodynamic representation of different nozzle configurations is carried out. On the computational fluid dynamics side, unsteady detached eddy simulations are coupled with structural computations for different nozzle configurations. The essential features of the interaction process are well captured. The coupling algorithm is validated by a nonlinear supersonic panel flutter simulation with highly transient mechanical interaction.

Detached Eddy Simulation of Film Cooling Performance on the Trailing Edge Cutback of Gas Turbine Airfoils

The present study deals with the unsteady flow simulation of trailing edge film cooling on the pressure side cut back of gas turbine airfoils. Before being ejected tangentially on the inclined cut-back surface, the coolant air passes a partly converging passage that is equipped with turbulators such as pin fins and ribs. The film mixing process on the cut back is complicated. In the near slot region, due to the turbulators and the blunt pressure side lip, turbulence is expected to be anisotropic. Furthermore, unsteady flow phenomena like vortex shedding from the pressure side lip might influence the mixing process (i.e., the film cooling effectiveness on the cut-back surface). In the current study, three different internal cooling designs are numerically investigated starting from the steady RaNS solution, and ending with unsteady detached eddy simulations (DES). Blowing ratios M=0.5; 0.8; 1.1 are considered. To obtain both, film cooling effectiveness as well as heat transfer coefficients on the cut-back surface, the simulations are performed using adiabatic and diabatic wall boundary conditions. The DES simulations give a detailed insight into the unsteady film mixing process on the trailing edge cut back, which is indeed influenced by vortex shedding from the pressure side lip. Furthermore, the time averaged DES results show very good agreement with the experimental data in terms of film cooling effectiveness and heat transfer coefficients.