The control of flow separation by periodic excitation

Nomenclature c = chord CD =drag coefficient, D/qA CL =lift coefficient, L/qa CP = pressure coefficient, P-PW /q CM = pitching-moment coefficient, M/qA D =drag, Ib k = reduced frequency, wc/2U L =lift, Ib M =Mach number; pitching moment, ft-lb P = pressure, lb/in q = dynamic pressure, 1 /2p U R, Re, Re, Rn = Reynolds number t time U,V = freestream velocity, ft/s x = distance a = angle of attack, deg f =nondimensional chord length, x/c $tr transition location co = rotational frequency, rad/s p = density, lb/ft A = sweep angle, deg

Progress in analysis and prediction of dynamic stall

The axial and radial velocity components w and u, and the concentration c of a Rhodamine 6G dye were measured simultaneously in a turbulent buoyant jet, using laser-Doppler anemometry combined with a recently developed laser-induced-fluorescence concentration measurement technique. These non-intrusive techniques enable measurements in a region of plume motion where conventional probe-based techniques have had difficulties. The results of the study show that the asymptotic decay laws for velocity and concentration of a tracer transported by the flow are verified experimentally in both jets and plumes. The momentum and volume fluxes and the mean dilution factor are determined in dimensionless form as a function of the normalized distance from the flow source. Contradictory results from earlier experimental plume investigations concerning the decay laws of w and c and the plume width ratio b_c/b_w are discussed. The turbulence properties and the transition from momentum-driven jets to buoyancy-driven plumes are presented. The turbulence is found to scale with the mean flow as predicted by dimensional analysis and self-similarity. Buoyancy-produced turbulence is found to transport twice as much tracer as jet turbulence. Although velocity statistics in jets and plumes are found to be highly self-similar there is a strong disparity in the distribution of tracer concentration in the two flows. This occurs in the time-average mean flows as well as the r.m.s. turbulent quantities. Instantaneous concentration fluctuations are found to exceed time averages by as much as a factor of 3. The experimental results should provide a reasonable basis for validation of computer models of axisymmetric plumes.

/pdf/investigations-of-round-vertical-turbulent-buoyant-jets-fts3lg6qh5.pdf

Investigations of round vertical turbulent buoyant jets

The characteristics of the unsteady boundary layer and stall events occurring on an oscillating NACA 0012 airfoil were investigated by using closely spaced multiple hot-film sensor arrays at . Aerodynamic forces and pitching moments, integrated from surface pressure measurements, and smoke-flow visualizations were also obtained to supplement the hot-film measurements. Special attention was focused on the behaviour of the spatial-temporal progression of the locations of the boundary-layer transition and separation, and reattachment and relaminarization points, compared to the static values, for a range of oscillation frequency and amplitude both prior to, during and after the stall. The initiation, growth and rearward convection of a leading-edge vortex, and the role of the laminar separation bubble leading to the dynamic stall, as well as the mechanisms responsible for the stall events observed at different test conditions were also characterized. The hot-film measurements were also correlated with the aerodynamic load and pitching moment results to quantify the values of lift increment and stall angle delay as a result of the observed boundary layer and stall events. The results reported here provide an insight into the detailed nature of the unsteady boundary-layer events as well as the stalling mechanisms at work at different stages in the dynamic-stall process.

Investigation of flow over an oscillating airfoil

The increasing need for thermal comfort has led to a rapid increase in the use of cooling systems and, consequently, electricity demand for air-conditioning systems in buildings. Heat-driven ejector refrigeration systems appear to be a promising alternative to the traditional compressor-based refrigeration technologies for energy consumption reduction. This paper presents a comprehensive literature review on ejector refrigeration systems and working fluids. It deeply analyzes ejector technology and behavior, refrigerant properties and their influence over ejector performance and all of the ejector refrigeration technologies, with a focus on past, present and future trends. The review is structured in four parts. In the first part, ejector technology is described. In the second part, a detailed description of the refrigerant properties and their influence over ejector performance is presented. In the third part, a review focused on the main jet refrigeration cycles is proposed, and the ejector refrigeration systems are reported and categorized. Finally, an overview over all ejector technologies, the relationship among the working fluids and the ejector performance, with a focus on past, present and future trends, is presented.

https://re.public.polimi.it/bitstream/11311/984261/1/2016_RSER_BesagniMereuInzoli.pdf

Ejector refrigeration: A comprehensive review

Dynamic stall calculations were carried out for an airfoil with a dynamically deformed leading-edge (DDLE) shape at a freestream Mach number of 0.3. The surface deformations were done about a baseline NACA 0012 airfoil, effectively increasing the airfoil leading-edge radius and thickness at high angles of attack. It was found that the DDLE airfoil had a slightly dynamic stall behavior compared to the baseline NACA 0012 airfoil. In particular, the lift, drag, and pitching-moment hysteresis loops were milder for the DDLE airfoil compared to the baseline airfoil. It was also found that a static shape that corresponds to the thickest deformed shape performed just as well as the DDLE shape, indicating that the shape itself, and not its time rate of change, was the reason for the improved performance. At higher Mach numbers around 0.4, the DDLE shape exhibited a strong dynamic stall triggered by a shock-induced separation, offsetting any benee t from the change in the shape of the airfoil. Additional work is needed on the development of DDLE shapes that will perform well at higher speeds.

Dynamic Stall Alleviation Using a Deformable Leading Edge Concept— A Numerical Study

Results of a study using a passive approach to recover the loss of li ft that occurred when a variable droop leading edge (VDLE) airfoil was used to successfully control compres ible dynamic stall by attach ing a small Gurney flap to it trailing edge are reported. Gurney flaps of' different heights were tested. The airfoil performance was evaluated by measuring the unsteady pressures while it executed a sinusoidaJ pitch -Up maneuver over a range of Mach numbers from 0.2 to 0.4 at different reduced frequencies, with both static and dynamic leading edge droops. Not only was the " lost" lift recovered completely with a 1 % chord-height Gurney flap, the drag and moment coefficients were also dramatically reduced and a lift-to-drag ratio greater than 10 was achieved, making it an acceptable choice for this purpose. The improved performance is explained througb the basic fl uid mechanics of the problem by discussing the various pressure distributions and the surface vorticity fl uxes derived from these.

Compressible Dynamic Stall Performance of a Variable Droop Leading Edge Airfoil with a Gurney Flap

Compressible steady and unsteady flowfields over a NACA 0012 airfoil at transitional Reynolds numbers are investigated. Comparisons with recently obtained experimental data are used to evaluate the ability of a numerical solution based on the compressible thin layer Navier-Stokes approximation, augmented with a transition model, to simulate transitional flow features. The discretization is obtained with an upwind-biased, factorized, iterative scheme. Transition onset is estimated using an empirical criterion based on the computed mean flow boundary- layer quantities. The transition length is computed from an empirical formula. The incorporation of transition modeling enables the prediction of the experimentally observed leading-edge separation bubbles. Results for steady airfoil flows at fixed angles of attack and for oscillating airfoils are presented. HE prediction of steady, inviscid flows over aerodynamic configurations is performed routinely nowadays. The computation of flows with separation bubbles or of fully sep- arated flows, on the other hand, is still a very challenging problem. For many practical applications, the assumption of fully developed turbulent flow yields good predictions of the flowfield. In other circumstances, such as the leading-edge dynamic stall flow, this assumption is not valid and compu- tations of such flows need improved methods. A characteristic feature of the dynamic stall flow is the onset of compressibility effects at a very low freestream Mach num- ber of O.2. 1-2 In addition, at transitional Reynolds numbers, it has been shown3-4 that the dynamic stall events are closely governed by the formation of a laminar separation bubble and its subsequent bursting. In fact, the above-cited studies demonstrate that the failure of the separated shear layer to reattach initiates dynamic stall, leading to the formation of the dynamic stall vortex. Further complications arise when the locally supersonic flow forms shocks that interact with the local boundary layer. It is important to recognize that the scales of the flow are very small here and the flow physics is not very clear. It is obvious that an accurate computational study of the problem demands a proper modeling of the phys- ics of the local compressible flow. In an effort to reach this eventual goal, it is necessary to include the transition physics that plays a key role in the dynamic stall process. The study to be reported represents a step in this direction. It is well known that prediction of the transition point and the transition length in such strongly adverse pressure gradient driven flows with current methods is difficult and involves uncertainties. Although several methods are available, the engineering prediction of transition relies on empirical for- mulation for boundary-layer flows. Whereas these methods have been moderately successful in steady and subsonic flows,

Analysis of low Reynolds number airfoil flows

Numerical and experimental results of steady and light dynamic stall flow over an oscillating NACA 0012 airfoil at a freestream Mach number of 0.3 and Reynolds number of 0.54 x 10 6 are compared. The experimental observation that dynamic stall is induced from the bursting of a laminar separation bubble points to the role of transition in influencing the flow development. Its modeling, including the changes in transition onset location and transition length with increase in airfoil angle of attack, is critical for computing the dynamic stall flow properly. In this study, the transition onset point is specified suitably and a simple transition length model is incorporated to determine the extent of the laminar separation bubble. The thin-layer approximations of compressible, Reynolds-averaged, Navier-Stokes equations are used for the numerical solution, with an implicit, upwind-biased, third-order-accurate scheme for the numerical integration.

Analysis of compressible light dynamic stall flow at transitional Reynolds numbers

Compressible dynamic stall control using a dynamically deforming leading edge airfoil is reported. The technique uses real-time leading edge shape adaptation to manipulate the local adverse pressure gradient over an oscillating airfoil. This produces attached leading edge flow at all angles of attack in the cycle. The shape change schedule is tailored to achieve the “right” airfoil shape through the dynamic stall regime by rounding the nose on the upstroke and sharpening it on the downstroke. The method exploits the favorable effects of shape adaptation on the potential flow to modify the pressure field and maintain the peak vorticity level below the critical level at which the dynamic stall vortex develops. The sensitivity of the flow to rate of nose curvature change and the angle of attack at which the initiation occurs has been investigated.

M. S. Chandrasekhara

Papers

Dynamic Stall Alleviation Using a Deformable Leading Edge Concept— A Numerical Study

Compressible Dynamic Stall Performance of a Variable Droop Leading Edge Airfoil with a Gurney Flap

Analysis of low Reynolds number airfoil flows

Analysis of compressible light dynamic stall flow at transitional Reynolds numbers

Compressible dynamic stall control using a shape adaptive airfoil