M. Vezza

Alleviation of Airfoil Dynamic Stall Moments via Trailing-Edge-Flap Flow Control

Trailing-edge-flap flow control for the mitigation of large negative pitching moments and negative aerodynamic damping caused by helicopter rotor blade dynamic stall was studied by means of computational fluid dynamics. A discrete vortex method was used for the simulations. The model geometry was a NACA 0012 airfoil oscillating in an α(t) = 15 deg + 10 deg sin(ωt) motion at the reduced frequency of k = 0.173. The freestream flow conditions were of M = 0.117 and Re = 1.463296 x 10 6 . The flap actuation was a brief pulse signal of a sinusoidal shape

Prediction of unsteady flow around square and rectangular section cylinders using a discrete vortex method

Abstract A discrete vortex method has been developed at the Department of Aerospace Engineering, University of Glasgow to predict unsteady, incompressible, separated flows around closed bodies. The basis of the method is the discretisation of the vorticity field, rather than the velocity field, into a series of vortex particles which are free to move in the flow. The grid free nature of the method allows analysis of a wide range of problems for both stationary and moving bodies. This report presents a brief description of the numerical implementation, and presents the results of an extensive validation of the method on bluff body flow fields. Results are presented for the mean force coefficients, surface pressure coefficients and Strouhal numbers on a square section cylinder at varying angle of incidence. Also presented are the mean force coefficients and Strouhal numbers on rectangular cylinders. The results from the vortex method show good agreement, both qualitative and quantitative, with results taken from various experimental data.

Discrete Vortex Method for Simulating Unsteady Flow

Amodi® ed discrete vortexmethod to simulate the separated ow around an aerofoil undergoingpitchingmotion is described. The vorticity generated in the thin layer around the body is discretized into vortices in accordance with themultipanel surface representation. By convectionand diffusion the vortices are released from the body and advanced in the wake as determined by the Biot± Savart law and random-walkmodel, respectively. Both unsteady static and pitching cases are presented, and comparisonwith the test data illustrates that, without prior knowledge of the developing separation and reattachment points for the model, good agreement has been achieved.

Calculation of the flow field around a square section cylinder undergoing forced transverse oscillations using a discrete vortex method

Abstract A discrete vortex method has been developed at the Department of Aerospace Engineering, University of Glasgow, to predict unsteady, incompressible, separated flows around closed bodies. The basis of the method is the discretisation of the vorticity field, rather than the velocity field, into a series of vortex particles which are free to move in the flow. The grid-free nature of the method allows analysis of a wide range of problems for both stationary and moving bodies. This paper presents a brief description of the method and presents the results of a validation programme on bluff bodies undergoing forced transverse oscillations. The results demonstrate that the method successfully predicts the vortex lock-in phenomena around the resonance point, as well as capturing the various states of the flow field expected above and below vortex lock-in. Results in the form of fluctuating lift coefficients, surface pressure coefficients and phase angle, are presented for a square cylinder undergoing transverse oscillations over a range of frequencies and amplitudes. The results from the vortex method show good agreement, both qualitatively and quantitatively, with results from various experimental data.

Discrete Vortex Method for Simulating Unsteady Flow Around Pitching Aerofoils

Amodi® ed discretevortex method to simulatetheseparatedowaround an aerofoil undergoing pitching motion is described. The vorticity generated in the thin layer around the body is discretized into vortices in accordance withthemultipanel surfacerepresentation. Byconvectionand diffusionthevorticesarereleased fromthebody and advanced in the wake as determined by the Biot± Savart law and random-walk model, respectively. Both unsteady static and pitching casesare presented, and comparison with the test data illustratesthat, without priorknowledge of the developing separation and reattachment points for the model, good agreement has been achieved. Nomenclature A = area of body (section) B = volume within the body c = aerofoil chord Fb = volume within the control zone Fw = volume outside the control zone K = number of subpanels for each panel k = unit vector k = reduced pitch rate k = X c/2V l = panel length m = index number of subpanel within the panel N = number of panels for the body P = static pressure Re = Reynolds number r, r = position vector and its magnitude S = surface of the body s, n = unit vector along and normal to the surface t = time U = ¯ ow velocity V = velocity Z = position in the form of complex number z = vortex position in the form of complex number a = angle of attack C = circulation c = circulation density 4 t = time step d = distance of nascent vortex off the body m = kinematic viscosity q = ¯ uid density r = vortex core radius W ,W = vector potential and stream function X = rotational velocity x = vorticity Subscripts

ALLEVIATION OF ROTOR BLADE DYNAMIC STALL VIA TRAILING EDGE FLAP FLOW CONTROL

A trailing-edge flap flow control for the mitigation of large negative pitching moments and the associated negative damping during helicopter rotor blade dynamic stall was studied by means of CFD. A discrete vortex method was used for the simulations. The model geometry was a NACA 0012 airfoil, oscillating in a fi(t) = 15o + 10osin(!t) motion at the reduced frequency of k=0.173. The freestream flow conditions were M=0.117 and Re=1,463,296. The flap actuation was a brief pulse signal and it was shown that for optimum results upward flap deflection for the duration of about the 1/3 of the oscillation time period should be employed. The pulse signal should start in the 3rd quarter of the azimuth. Detailed analysis of the flowfield showed that the trailing-edge vortex (TEV), induced by the downstream convecting dynamic stall vortex (DSV), is responsible for the occurence of large negative pitching moments and negative damping. The flap flow control technique proved to be successful in mitigating these eects by displacing the TEV to a higher location where the DSV could only push it o the trailing edge, thus eliminating its eect. The method was shown to be promising for other cases from the helicopter flight envelope as well.

Simulating the Wake Downstream of a Horizontal Axis Tidal Turbine Using a Modified Vorticity Transport Model

To decrease the need for fossil fuels, the alternative energy resources must be not only economically viable but also sustainable in the long term. One of the most promising alternatives is the marine renewable energy resource. The relatively young marine energy industry is presented with two challenges: first, to deliver a continuous reliable power supply, and second, to minimize potentially harmful effects of the power extraction on the marine environment. The requirement to understand the interactions between a tidal turbine and the surrounding flow environment motivated this work. A tidal turbine mounted on the seabed induces a wake that extends far downstream of the device. As the direction of tidal flow changes, so does the position of the wake with respect to the device. The detailed study of the turbine wake has been conducted by means of computer simulations. An existing finite-volume computer model called the vorticity transport model has been modified to suit the purpose of simulating the wake of a horizontal axis tidal turbine subjected to a nonuniform flow typical of that close to the seabed. High-resolution computer simulations suggest that a progressive fragmentation of the vortical structure occurs during the development of the wake of a tidal turbine. The predicted fragmentation generates small-scale unsteady flow phenomena beyond five rotor diameters downstream of the device in the area previously thought unaffected by the presence of a tidal turbine. The effects of nonuniform flow on the vorticity structure downstream of a tidal turbine and the fragmentation process are analyzed in this work.

Simulation of parallel blade–vortex interaction using a discrete vortex method

Numerical results are presented for two-dimensional vortex-aerofoil interaction using a grid-free discrete vortex method. The effects of the passing vortex on the surface pressure distribution and hence the aerodynamic force and moment of the aerofoil are examined in detail for a variety of interaction geometries. For some head-on interaction cases, vortex-induced local flow separation is also predicted on the aft part of the aerofoil surfaces