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

THE velocity and vorticity fields around an oscillating airfoil in compressible dynamic stall are reported. Phase averaged, two component laser velocimetry data were obtained at a freestream Mach number of 0.3 and a reduced frequency of 0.05. This is the first set of velocity data available at a high Reynolds number (540,000) under compressible flow conditions and it serves as a good database for development and validation of computer codes. Of particular interest is the formation of a separation bubble, which bursts coincidentally with the formation of the dynamic stall vortex, adding an extra degree of physical complexity to the problem. Contents The performance of helicopters and the maneuverability of fixed wing aircraft is limited by dynamic stall and the gross separation of the unsteady flow that accompanies it. The dynamic stall flowfield is a complicated combination of a multitude of fluid dynamic effects that include tremendous fluid acceleration around the leading edge; formation of strong suction peaks; development of the local boundary layer under strongly adverse pressure gradients; transition of the leading-edge laminar boundary layer, its separation, and reattachment resulting in a separation bubble that grows and eventually bursts just when the dynamic stall vortex forms; under compressibility conditions (M>0.3), formation of shock(s) and the induced separation due to it; addition of large amounts of coherent vorticity into the flow and its coalescence into the dynamic stall vortex that later convects over the airfoil upper surface and interacts with the trailing edge separated flow that propagates toward the leading edge; and so on. The complex interactions governing the physical problem have hitherto made understanding the origin of dynamic stall and attempts at modeling the flow very challenging. The study being reported provides a comprehensive set of velocity data along with vorticity distributions derived from it, which can be used to model the flow. In addition, it also serves as a database to verify computational results and enables development of new codes.

https://apps.dtic.mil/sti/pdfs/ADA271217.pdf

Velocity and vorticity distributions over an oscillating airfoil under compressible dynamic stall

Two component, phase averaged mean velocity data have been obtained with an Laser Doppler Velocimeter (LDV) system, on the upper surface, near the leading edge of an oscillating airfoil undergoing compressible dynamic stall In particular, the effect of oscillation amplitude has been studied The results show that at an oscillation amplitude of 10 degrees, a separation bubble forms, that eventually bursts on the upstroke, well beyond the static stall angle At 2 degrees amplitude, the bubble forms on the upstroke, but dynamic stall occurs on the downstroke The results reveal new flow physics and the data sets serve as valuable quantitative information for validation of unsteady flow codes at transitional Reynolds numbers The maximum velocity seen in the flow is about 16 times the free stream value and it occurs slightly downstream of the suction peak location Some of the measurement difficulties are also discussed

LDV Measurements in Dynamically Separated Flows

Reynolds number (Re) effects on oscillating airfoil compressible dynamic stall flow have been investigated. The Re was doubled at constant Mach number by using two NACA 0012 airfoils, one with 3-inch chord and another with 6-inch chord. Pressure distributions documented using point diffraction interferometry (PDI) are compared with each other for Mach numbers of 0.3 and 0.45 at different reduced frequencies. Comparisons with a reference data set at Re = 4 x 10 exp 6 for a Mach number of 0.3 at reduced frequencies of 0.05 and 0.1 showed good agreement for the tripped 6-inch airfoil, thus establishing a minimum model Re for simulating full-scale results. Re influence manifests as an increase in the airfoil peak suction pressure coefficient, smaller laminar separation bubble, increase in dynamic stall onset angle, and an increased ability to withstand adverse pressure gradient in the flow. At lower Re, dynamic stall arises from the bursting of the laminar separation bubble at both M = 0.3 and 0.45. At higher effective Re, dynamic stall is induced by a large adverse pressure gradient in the flow for M = 0.3 and shock induced dynamic stall onset for M = 0.45. (Author)

Reynolds number influence on 2-D compressible dynamic stall

: Control of dynamic stall under compressible flow conditions appropriate for a helicopter rotor has been demonstrated using a variable droop leading edge airfoil. The airfoil leading is drooped in phase with its sinusoidal pitch oscillations to eliminate the dynamic stall vortex that induces many adverse effects on the blade. The approach results in dramatic reductions in drag and pitching moment, while also decreasing the maximum lift slightly. This loss was recovered using a trailing edge mounted vertical Gurney flap of 1% chord height, without undue drag penalty. Drooping the leading edge substantially modifies the airfoil pressure distribution such that the dynamic stall onset mechanism is changed from shock-induced to pressure gradient induced for certain flow conditions. The changes also manifest in significantly lowered peak vorticity fluxes preventing the flow vorticity from coalescing into a very tightly organized dynamic stall vortex. The behavior of transition and its role in the process are also addressed.

M. S. Chandrasekhara

Papers

Velocity and vorticity distributions over an oscillating airfoil under compressible dynamic stall

LDV Measurements in Dynamically Separated Flows

Reynolds number influence on 2-D compressible dynamic stall

Aerodynamic Flow Control using a Variable Droop Leading Edge Airfoil

Compressible dynamic stall control - A comparison of different approaches