Unsteady Reynolds-averaged Navier-Stokes Solutions Research Articles

Gust load alleviation on an aircraft wing by trailing edge Circulation Control

This paper reports an investigation of gust load alleviation using Circulation Control for a three-dimensional wing including aerodynamic and structure interaction. The Field Velocity Method is introduced to the unsteady Reynolds averaged Navier–Stokes solutions for gust response simulations. Structural dynamic equations of motion are coupled with the unsteady Reynolds averaged Navier–Stokes solutions to consider the aeroelastic effects due to the gust response. The effects of gust load alleviation on the BAH wing using Circulation Control are tested under typical ‘one-minus-cosine’ gust profiles from certification specification defined by European Aviation Safety Agency. The results show a promising capability of Circulation Control for gust load alleviation as significant gust load alleviation effects have been achieved for both the rigid and elastic BAH wings in the test cases. For the BAH wing considering aeroelasticity, the displacement oscillations induced by gusts have been effectively suppressed by Circulation Control. The results also show that Circulation Control has the fast frequency response characteristics, where more than 50% of the total change in lift coefficient can be obtained within the non-dimensional time s =U∞tc¯=1.

Two-dimensional simulations of vortex-induced vibration of a circular cylinder

In the present study, one of the cross-disciplinary problems known as vortex-induced vibration is numerically investigated. Effects of four different low mass-damping ratios; ζ = 0.013, 0.028, 0.074, and 0.124 of a smooth cylinder are taken into account for transition of shear layer 2 (TrSL2) type flow that falls between the Reynolds numbers from 2500 to 10,830 utilizing a two-dimensional cylinder that is free to move in normal-direction. Unsteady Reynolds-Averaged Navier–Stokes solutions indicate that the general trend is well captured with the adopted shear stress transport k-ω turbulence model, however, due to two-dimensional limitations some results are not consistent with experimental data. An inverse relation between the mass-damping ratio and the transition from the upper to the lower branch is detected. Change of drag and lift coefficients with the reduced velocities revealed that the maximum drag coefficient increases with reduced velocity until it reaches Ur = 5 and then decreases dramatically while the lift coefficients decrease consistently from the beginning.

Airfoil gust load alleviation by circulation control

This paper presents a numerical investigation on the gust load alleviation effects based on circulation control (CC) via blowing over airfoil trailing-edge Coanda surface. The NACA0012 airfoil was chosen for the study from subsonic to transonic speeds. The field velocity method is introduced to the unsteady Reynolds averaged Navier-Stokes solutions to simulate responses of the airfoil to arbitrarily shaped gust penetrations. For validation, gust responses to a step change in angle of attack, sharp-edged gusts and one-minus-cosine gusts from the numerical solutions are compared against available theories and other numerical references. Experimental data are used to validate the numerical results of CC. Thereafter circulation control via steady blowing with different momentum coefficients are tested for the gust load alleviation effects in terms of lift coefficients. The results show that circulation control can effectively suppress the lift disturbances caused by the gust for subsonic flows. Most importantly, CC has a fast frequency response characteristic as more than 50% of the total change in lift coefficient can be obtained within the non-dimensional time s=U∞t/b=1. Based on the rapid response characteristic, circulation control via unsteady blowing with dynamically adaptive momentum coefficients proportional to the vertical gust velocities is proposed and tested. The results demonstrate that a near constant lift coefficient can be achieved under gust condition for subsonic incoming flow indicating its potential for real-time adaptive load control. It has been demonstrated that CC is also able to reduce gust load at transonic speed, but it is less effective as compared with that at subsonic speed.

Numerical Study of Deterministic Fluxes in Compressor Passages

Computational fluid dynamics (CFD) has been widely adopted in the compressor design process, but it remains a challenge to predict the flow details, performance, and stage matching for multistage, high-speed machines accurately. The Reynolds Averaged Navier-Stokes (RANS) simulation with mixing plane for bladerow coupling is still the workhorse in the industry and the unsteady bladerow interaction is discarded. This paper examines these discarded unsteady effects via deterministic fluxes using semi-analytical and unsteady RANS (URANS) calculations. The study starts from a planar duct under periodic perturbations. The study shows that under large perturbations, the mixing plane produces dubious values of flow quantities (e.g., whirl angle). The performance of the mixing plane can be considerably improved by including deterministic fluxes into the mixing plane formulation. This demonstrates the effect of deterministic fluxes at the bladerow interface. Furthermore, the front stages of a 19-blade row compressor are investigated and URANS solutions are compared with RANS mixing plane solutions. The magnitudes of divergence of Reynolds stresses (RS) and deterministic stresses (DS) are compared. The effect of deterministic fluxes is demonstrated on whirl angle and radial profiles of total pressure and so on. The enhanced spanwise mixing due to deterministic fluxes is also observed. The effect of deterministic fluxes is confirmed via the nonlinear harmonic (NLH) method which includes the deterministic fluxes in the mean flow, and the study of multistage compressor shows that unsteady effects, which are quantified by deterministic fluxes, are indispensable to have credible predictions of the flow details and performance of compressor even at its design stage.

A Method for Approximate Prediction of Laminar–Turbulent Transition on Helicopter Rotors

An approximate method is presented to predict the laminar-turbulent transition onset within the unsteady Reynolds-averaged Navier-Stokes (URANS) simulations of helicopter rotors. Based on the presented method, the influence of laminar flow on the required rotor power and generated rotor thrust is investigated and compared to fully turbulent results. To compute the laminar boundary layer, an integral method according to Schlichting/Walz and a set of rotating velocity profiles according to Blaser and Velkoff are used. The laminar-turbulent transition onset is predicted by empirical criteria with respect to Tollmien-Schlichting and crossflow instabilities, bypass mechanisms, and attachment line contamination. The method presented accounts for the turbulence-level variation of the simulated rotor wake. A detection of the boundary layer edge within the URANS solution is not required. The approximate method is validated using the experimental onset positions of representative transition test cases. This includes a laminar flow airfoil and a swept wing. To cover a rotor in forward flight, the onset positions on the model rotor of a Mach-scaled helicopter configuration are computed and compared to available hot-film measurements. In general, the predicted laminar flow lengths on the rotor blades reduce the required rotor power by 4-5%. The rotor thrust remains practically unaffected.

Mixed convection heat transfer of turbulent flow in a three-dimensional lid-driven cavity with a rotating cylinder

A numerical study has been carried out to investigate the combined forced and natural convection heat transfer in a differentially heated 3D obstructed cavity with a thermally insulated rotating circular cylinder. The cavity has a hot stationary bottom wall and a cold top lid-driven wall, and all the other walls completing the domain are motionless and adiabatic. The simulations are performed for different Reynolds numbers, Re=5000, 10,000, 15,000 and 30,000, and for dimensionless rotational speeds of the cylinder, 0≤Ω≤10. The performance of two turbulence methods, Large Eddy Simulation (LES) and Unsteady Reynolds-Averaged Navier-Stokes (URANS), has been evaluated in this research. The flow and thermal fields are studied through flow vectors, isotherm contours and iso-surfaces temperature, as well as through the average Nusselt number (Nuav) and velocity components. The results demonstrate clearly that the flow patterns and the thermal fields are influenced strongly by increasing either the rotating cylinder speed or the Reynolds number. Furthermore, both LES and URANS solutions can capture the essential feature of the primary eddies in the cavity. But this study has shown convincing evidence that only the LES method can predict the structure details of the secondary eddies that have profound effects on the heat transfer behaviour within the enclosure.

Reynolds-averaged and wall-modelled large-eddy simulations of impinging jets with embedded azimuthal vortices

We have performed simulations of an impinging jet with embedded azimuthal vortices, a model of the wake of a helicopter hovering near the ground. This problem has considerable practical importance since, when the landing area is covered with sand or snow, the interaction between the helicopter wake, the rotor-tip vortices and the solid particles on the ground can result in the lift up of a cloud of sediment that limits the pilot’s vision, causing accidents and potentially loss of life. We compare the results of well-resolved large-eddy simulations (LES) at laboratory scale, with solutions of the Unsteady Reynolds-Averaged Navier–Stokes (URANS) equations with three turbulence models, and also with LES in which the wall layer is modelled (WMLES), using the Delayed Detached Eddy Simulation (DDES) approach, or employing approximate boundary conditions. The URANS solutions do not yield a reliable prediction of the development of the azimuthal vortices (a well-known shortcoming of most eddy-viscosity turbulence models in this type of configuration), while the WMLES and DDES predict vortex decay in good agreement with the resolved LES data. Simulations at a Reynolds number higher by a factor of 20 (intermediate between the laboratory scale and the real configuration) were also carried out. All the simulation methods predict very little effect of the Reynolds number: the flow appears to be driven by the evolution of the large-scale embedded vortices. The trends are the same observed at laboratory-scale: the RANS turbulence models predict a much faster decay of the embedded vortices compared to the WMLES and DDES. The need for reliable experimental data is highlighted.

Effects of blunt trailing edge flow discharge in supersonic regime

Flow discharge at the blunt trailing edge of bodies immersed into supersonic stream alters significantly the downstream aerodynamic characteristics. This paper studies the changes in the base region topology for a wide range of purge ejection, using Unsteady Reynolds Averaged Navier–Stokes solutions. At low purge rate the base pressure increases, resulting in a reduction of the trailing edge shock wave intensity. Supersonic purge flows diminishes the base pressure and thus strengthens the shock wave, a linear relationship between the base pressure and the shock strength was obtained for the investigated conditions. Furthermore, an auxiliary shock wave system was observed to form upstream of the main fish tail shock at certain discharge rates.

Behavior of Detached-Eddy Simulations for Mild Airfoil Trailing-Edge Separation

�is simulated using unsteady Reynolds-averaged Navier–Stokes solution, detached-eddy simulation, and delayeddetached eddy simulation. This case features a relatively thick boundary layer with a mild trailing-edge separation. Detailed comparisons of the modeled Reynolds stresses are carried out with the available experimental data at different locations on the suction side of the airfoil and in the wake region. The impact of the delayed switching on modeled Reynolds stresses from delayed-detached-eddy simulation as compared with the original detached-eddy simulation is studied. On the suction side of the airfoil, the modeled turbulent Reynolds stresses computed through the detached-eddy simulation are generally lower than the unsteady Reynolds-averaged Navier–Stokes solutions, whereas the delayed-detached-eddy simulation gives comparable values with the unsteady Reynolds-averaged Navier–Stokessolutionsresults.NoneofthemgiveconsistentcomparisonwiththeexperimentalReynoldsstresses.It is observed that for the cases with the mild separation, the original detached-eddy simulation may perform better than the delayed-detached-eddy simulation, due to its relatively lower turbulent dissipation levels. The detachededdy simulation Reynolds stresses in the wakeregion show better results, in comparisonwith the experimental data.