Alric P. Rothmayer

Dynamic Roughness as a Means of Leading-Edge Separation Flow Control

The aircraft industry, as a whole, has been deeply concerned with improving the aerodynamic efficiency of current and future flight vehicles, particularly in the commercial and military markets. However, of particular interest to the field of aerodynamics is the elusive concept of a workable flow control mechanism. Effective flow control is a concept which if properly applied can increase aerodynamic efficiency. Various concepts and ideas to obtain successful flow control have been studied in an attempt to reap these rewards. Some examples include boundary layer blowing (steady and periodic), suction, and synthetic jets. The overall goal of flow control is to increase performance. The specific objectives of flow control include: 1) delay or eliminate flow separation, 2) delay boundary layer transition or 3) reduce skin friction drag. The purpose of this research is to investigate dynamic surface roughness as a novel method of flow control technology for external boundary layer flows. As opposed to standard surface roughness, dynamic roughness incorporates small time dependent perturbations to the surface of the airfoil. These surface perturbations are actual humps and/or ridges that are on the scale of the laminar boundary layer, and oscillate with an unsteady motion. Research has shown that this can provide a means to modify the instantaneous and mean velocity profile near the wall and favorably control the existing state of the boundary layer. The results of this study have shown that dynamic roughness can be a viable alternative in delaying and/or eliminating the leading edge laminar separation bubble and hence reaping some of the rewards of an effective flow control system, while also maintaining some physical advantages over other techniques.

Effects of Surface Ice Roughness on Dynamic Stall

A two-dimensional Navier-Stokes algorithm is used to investigate unsteady, incompressible viscous flow past an airfoil leading edge with surface roughness that is characteristic of early-growth ice accretion. The roughness is added to the surface through the use of a Prandtl transposition and can generate both small-scale and large-scale roughness geometries. The algorithm is used to simulate steady or unsteady flow at constant angle of attack or pitch up corresponding to dynamic-stall conditions. Investigations of the dynamic stall show that some types of surface roughness can significantly alter the unsteady flow separation pattern and the formation of the dynamic-stall vortex. This includes both small-scale and large-scale roughness

Numerical prediction of unsteady vortex shedding for large leading-edge roughness

Abstract A full two-dimensional Navier–Stokes algorithm is used to investigate unsteady, incompressible viscous flow past an airfoil leading edge with surface roughness that is characteristic of ice accretion. The roughness is added to the surface through the use of a Prandtl transposition and can generate both small-scale and large-scale roughness. The focus of the study is a detailed flow analysis of the unsteady velocity fluctuations and vortex shedding induced by the surface roughness. The results of this study are compared to experimental data on roughness-induced transition for the same roughness geometry. A comparison is made between “fluctuation intensity” values from the current algorithm to experimentally determined turbulence intensity values. The effects of the roughness Reynolds number, Re k , are investigated and compared to experimental values of the critical roughness Reynolds number. The authors speculate that there may be a possible correlation between unsteady roughness-induced vortex shedding and the onset of experimentally measured transitional flow downstream of large-scale roughness.

Application of triple-deck theory to the prediction of glaze ice roughness formation on an airfoil leading edge

Abstract A viscous–inviscid interaction triple-deck structure is developed to describe the thermomechanical interaction of an air boundary layer with ice sheets and liquid films. Linear stability results are compared with nonlinear triple-deck computations, and a number of nonlinear simulations of air–water–ice interactions are presented. An icing instability is encountered in regimes with simultaneous wall and air cooling that is believed to admit small scale and highly irregular surface roughness. The stabilization of the smallest scale icing disturbances is obtained through the Gibbs–Thomson relation. This local thermodynamic condition relates the freezing temperature of a pure substance to the surface tension and the mean curvature of the interface and provides a short scale stabilizing mechanism for icing instability modes. Comparison with available experimental data on glaze ice roughness diameters, accreted on NACA 0012 airfoil leading edges under glaze icing conditions, is provided. It is also found in all cases computed in this study that water beads can be formed on a wetted ice surface once the water film is locally ruptured by ice roughness elements.

Separation and Instabilities in the Viscious Flow over Airfoil Leading edges

Abstract The two-dimensional, unsteady, leading-edge flow over stationary, pitching and oscillating airfoils is studied using the Navier–Stokes equations for flow past a parabola. Grid-converged solutions have been obtained for the initial stages of unsteady separation for impulsively-started unsteady flow and for pitch-up at constant rate. The results compare favorably with corresponding full-airfoil computations. It is shown that small perturbations in the flow can generate eddies in the boundary layer before flow reversal occurs in the base flow. Oscillations appear in the skin friction which grow in amplitude and convect downstream with time. Above a certain amplitude, the skin friction becomes negative locally, giving rise to small recirculating eddies. This type of eddy creation prior to reversal in the base flow is in general agreement with the theory of Rayleigh instabilities. The wavelength of these instabilities scales as Re −0.43 which is in reasonable agreement with the theoretical value of Re −1/2 for the Rayleigh instability.

An Experimental Investigation on Wind-Driven Rivulet/Film Flows over a NACA0012 Airfoil by Using Digital Image Projection Technique

Aircraft icing is a serious threat to aviation safety. Icing accretion process usually interacts with surface water run back flow under glaze icing condition. In the present study, an experimental investigation was conducted to characterize the surface wind-driven water film/rivulet flows over a NACA 0012 airfoil in order to elucidate the underlying physics of the transient surface water transport behavior pertinent to aircraft icing phenomena. The experimental study was conducted in an icing research wind tunnel available at Aerospace Engineering Depratment of Iowa State University. A novel digital image projection (DIP) measurement system was developed and applied to achieve quatitative measurements of the thickness distributions of the surface water film/rivulent flow at different test conditions. The measurement results reveal clearly that, after impinged on the leading edge of the NACA0012 airfoil, the micro-sized water droplets would coalece to form a thin water film in the region near the leading edge of the airfoil. Water rivulets were found to be generated as the water film flow runs backs. The width and the spacing of the water rivulets were found to decrease monotonically with the increasing wind speed.

Solutions for Two-Dimensional Instabilities of Ice Surfaces Uniformly Wetted by Thin Films

Solutions are presented for the interaction between a cold airstream and a growing ice surface. The ice is uniformly wetted by a thin water lm created by the impact of small water droplets. The cold airstream causes an instability of the ice surface, leading to roughness elements whose size and shape are controlled primarily by the level of supercooling. Properties of the roughness are examined in two dimensions.

Stall Suppression of a Low-Reynolds-Number Airfoil with a Dynamic Burst Control Plate

An experimental study was conducted to investigate the use of a dynamic burst control plate to suppress stall on a NACA 0012 airfoil by preventing the bursting of the low Reynolds number leading edge separation bubble. Pressure measurements, force measurements, and particle image velocimetry (PIV) data show the ability of the dynamic burst control plate to reattach the leading edge separation bubble to the surface at higher angles of attack than a stationary burst control plate and increase the lift performance of the airfoil.

Linearized Solutions of Three-Dimensional Condensed Layer Films

Numerical methods are developed for linearized condensed layer films. The condensed layer film is a simplified model that can be used to compute both shear and pressure driven films. Numerical methods are developed for two versions of the condensed layer film: i) a strong-lubrication approximation where a lubrication approximation in the film is coupled to a boundary-layer air flow, and ii) a weak lubrication approximation where inertia effects are retained in the film. Typical solutions are presented for both versions of the model.

Impact of Surface Roughness on Local Aerodynamics Using a Three-Dimensional Navier-Stokes Solver

Accurate simulation of glaze ice accretion requires detailed models which account for the impact of small scale surface roughness on boundary layer development and heat transfer. The current study investigates the feasibility of using a Direct Numerical Simulation (DNS) approach in order to study the flow over roughness and subsequently develop the desired roughness models. First, the limits of the DNS approach are investigated and the method is found to be limited by the value of the roughness Reynolds number. Second, a preliminary estimate of grid requirements is made for a particular case based on a series of two–dimensional calculations. The approach is validated for flow upstream of an isolated three–dimensional hemispherical roughness by making comparisons with experiment. Finally, qualitative results are shown in the roughness wake region which indicate the ability of the method to capture unsteady multi–scale phenomena. Based on this information and a performance study which shows good scalability to larger numbers of processors, the authors discuss the need to continue development of the approach in order to improve efficiency and make the calculations viable for the study of ice roughness.