Downwash Velocity Research Articles

A new method for predicting rotor wake geometries and downwash velocity field

A new method for predicting rotor wake in low speed and hovering flight is described to investigate the motion of the helical tip vortex. Beginning with the generalized wake model, a semi‐empirical correction for the vortex core effect on rotor wake is made and free wake calculation is carried out. As an example of its engineering application, the calculated downwash velocity field along the rocket launch line is presented and simply analysed. In terms of theory, the method developed here may provide of a referable basis for further study the formation mode of the tip vortex and vortex core interior structure.

A vortex pair model for plume downwash into stack wakes

A model is proposed that uses a streamwise vortex pair embedded in the plume to produce a downwash velocity that varies with downwind distance and competes with the trajectory centerline plume rise velocity. This streamwise vortex pair is an idealization of the connection back to the stack tip of the vertical von Karman vortices shed from the stack. The model is compared with the measurements of trajectories from non-buoyant momentum jets injected from a circular tube with subcritical external Reynolds number into a uniform cross flow in a water channel. Fluorescent dye and laser sheet-lighting were used to define the trajectories over a range of effluent to crosswind velocity ratios from 0.25 to 8.0. With an empirical function to account for the efficiency with which the jet entrains vorticity from the stack-tip vortex pair, the model accurately predicts the plume downwash effects, and accounts for the continuous downward deflection of the jet trajectory.

Examination of the flow around a hot-wire probe using particle image velocimetry

Previous work has shown that the k 2 term in the effective cooling velocity equation for inclined hot-wires can become negative under certain probe configurations and wire length-to-diameter ratios. It was hypothesised that this was due to a downwash component of velocity along the wire when prong interference effects were expected to be minimal. Direct measurements of the flow around a typical hot-wire probe using digital particle image velocimetry have shown that this downwash velocity component does exist, leading to negative values of k 2 as calculated from the angle of deviation from the free stream.

Selected design issues of some high-speed rotorcraft concepts

A study of vehicle concepts for High Speed Rotorcraft applications has been undertaken at Ames Research Center in cooperation with NASA Lewis and Langley Centers, with the objective of defining their technology needs. The design guidelines include a low downwash velocity in hover, good low speed maneuver capabilities and cruise speeds up to 450 knots. Four contractors and a systems analysis effort within NASA have defined promising configurations which may be capable of meeting these goals. This paper addresses challenging problems associated with some of the configurations in the areas of aerodynamics, propulsion, weights and aeroelastic stability.

On the use of the method of matched asymptotic expansions in propeller aerodynamics and acoustics

The applicability of the method of matched asymptotic expansions to both propeller aerodynamics and acoustics is investigated. The method is applied to a propeller with blades of high aspect ratio, in a uniform axial flow. The first two terms of the inner expansion and the first three terms of the outer expansion are considered. The matching yields an expression for the spanwise distribution of the downwash velocity. A numerical application shows that the first two terms of the inner solution do not yield an acceptable approximation for the downwash velocity. However, recasting the analytical expressions into an integral equation, similar to Prandtl's lifting line equation for wings, yields results for both aerodynamic and acoustic quantities, which agree well with experimental results. The method thus constitutes a practical analysis method for conventional propellers.

Aerodynamics optimum design procedure and program for the rotor of a horizontal-axis wind turbine

A design procedure and its computer program using aerodynamic optimization for the rotor of a horizontal-axis wind turbine is presented in this paper. It is based on the method of prediction of aerodynamic characteristics for the air-screw by the so-called “blade theory” and according to the feature of the flow around the rotor. Design methods now being widely employed are based on so-called “disk theory”, by which a vortex system with the assumption of an infinite number of blades is the aerodynamic model of the rotor. The influence of the number of blades has to be considered and evaluated according to some consideration. This will introduce a certain inaccuracy in the aerodynamic calculation of the rotor. In this paper, abandoning the assumption of an infinite number of blades, a vortex system adopted in this design method is able to give the induced downwash velocity at blade sections so that more accurate aerodynamic forces can be obtained. The power coefficient of the rotor is taken as the objective function for aerodynamic optimization to give an optimal geometry of the rotor. The design rotors which are of high efficiency and good performance are given. Comparison of their calculated and experimental performances shows good agreement.

移動波状壁面上を飛ぶ２次元薄翼の空力特性

This paper investigates the aerodynamic response of a thin airfoil flying over and in proximity to a wavy wall surface which moves in the same direction as free stream but with a different velocity. Integral equation method relating the pressure on boundaries to downwash velocity is adopted for formulation, based on inviscid and small perturbation flow. Supposing the wall to be sinusoidal and the airfoil to be a flat plate at zero incidence, numerical calculations are made with a set of important parameters as airfoil height from the wall, wave length of wall surface, and the wall velocity. The influence of such parameters on the aerodynamic coefficients is divided into two parts, “First-order Ground Effect” and “Second-order Ground Effect” The latter is discussed in detail through the solution of Kemp-type gust. Numerical results show that the wave length and the traveling velocity of wall within a certain range have a great influence on the “Second-order Ground Effect”, with proximity to the ground.

Spatial resolution and downwash velocity corrections for multiple-hole pressure probes in complex flows

This work was sponsored by the US Army Office of Aviation Research and Development, NASA-Defense Purchase Request No. C-80019-F. Dr. K. Civinskas was the program monitor.

Analytical Prediction of Vortex Lift

General integral expressions are derived for the nonlinear lift and pitching moment of arbitrary wing planforms in subsonic flow. The analysis uses the suction analogy and an assumed pressure distribution based on classical linear theory results. The potential flow lift constant and certain wing geometric parameters are the only unknowns in the integral expressions. Results of the analysis are compared with experimental data and other numerical methods for several representative wings, including ogee and double-delta planforms. The present method is shown to be as accurate as other numerical schemes for predicting total lift, induced drag, and pitching moment. b c c CL CD Cm CT Cs cc, ccd E2 Nomenclature = aspect ratio = wing span =chord = reference length = lift coefficient = drag coefficient = pitching moment coefficient = thrust coefficient = suction coefficient = section lift coefficient = section induced drag coefficient = section suction coefficient = pressure loading coefficient = drag = proportionality constant, Eq. (32) = proportionality constant, Eq. (53) = chordwise function, Eq. (44) ff(rj) = span wise f unction, Eq. (28) K = potential constant L =lift loading constant, Eq. (5) S = suction force SR = reference area s = suction force per unit length T = leading edge thrust, Eq. (7) T' = leading edge thrust per unit length V = freestream speed Wj = downwash velocity component, Eq. (11) a. = angle of attack F = vorticity p = freestream density £ = nondimensional chordwise coordinate 77 = nondimensional spanwise coordinate A = leading edge sweep angle Subscripts P = potential flow E =edge / = induced VLE = leading edge vortex VSE = side edge vortex

Effects of Modifying a Rotor Tip Vortex by Injection on Downwash Velocities, Noise, and Airloads

Effects of Modifying a Rotor Tip Vortex by Injection on Downwash Velocities, Noise, and Airloads

A Numerical Method for Calculating the Trailing Vortex System behind a Swept Wing at Low Speed

A wing of finite span in flight generates trailing vorticity which “rolls up” into two “discrete” vortices. During the rolling up process the trailing vorticity is displaced vertically downward (assuming that the aircraft is in horizontal flight) under the influence of the wing circulation and the self-induced downwash velocities. In this note a method is presented for the prediction of the spatial distribution of the trailing vorticity during the rolling up process, thus giving the overall downwash field behind the wing.

Cloud Modification by Helicopter Wakes

Abstract Helicopter wake effects on stratus and stratocumulus clouds are described. Holes and troughs were created in such clouds with dimensions several hundred feet across. The velocities and penetration distances of the downwash flow beneath the HH-53B helicopter employed in the experiments were investigated by the release of a “tracing agent” from the helicopter and by hovering at various altitudes above the water surface of a Florida bayou. The downwash velocities were as large as 100 ft sec−1, near the rotor, and the downwash extended some 1300 ft beneath the helicopter (under near dry adiabatic conditions). A suggestion is made that helicopters might be employed operationally to clear radiation-type ground fog from airfield runways. The cloud clearing capabilities of the HH-53B helicopter are discussed in support of this suggestion, by reference to observational data and to the theory of Hohler.

The Application of Lifting Line Theory to an Upright Bermudan Mainsail

SummaryUsing Glauert's analysis and taking into account the restraint due to the surface of the sea, lifting line theory is applied to calculate the distribution of loading, downwash velocity and vortex drag of a “Finn” dinghy's sail when upright. It appears that the loading can probably be represented with sufficient accuracy byI=I0(sin θ+sin 2θ)where k is largely dependent on the apparent twist in the sail and has a value generally of the order of 0-5, that the apparent twist may not be twist at all in the aerodynamic sense, and that the vortex drag is considerably greater than it would be if it were possible to make a sail to give elliptic loading.

Rotor Dynamics and Helicopter Stability

SUMMARY A detailed account is given of the aerodynamic and inertia forces acting on a rotor blade when the helicopter is undergoing arbitrary disturbance in velocity and angular velocity. A fairly generalized form is used for the downwash velocity. A principle is enunciated enabling the several second order terms of significance in the angle variables to be recognized and retained.

Experimental Downwash Velocity, Static Pressure, and Temperature Distributions in Ground Effect for a 75‐Foot Jet Driven Rotor

Experimental Downwash Velocity, Static Pressure, and Temperature Distributions in Ground Effect for a 75‐Foot Jet Driven Rotor