Position Of Flow Separation Research Articles

Artificial intelligence control of flow separation from a curved ramp

This work aims to control flow separation from a two-dimensional curved ramp. The Reynolds number examined is Reθ = 5700 based on the momentum thickness of the turbulent boundary layer right before the ramp. Three steady jets, blowing tangentially along the ramp from three spanwise slits, are deployed at the most likely flow separation position, upstream and downstream of this position, respectively. Three different control modes are investigated, i.e., a single jet, multiple jets, and genetic algorithm-optimized blowing rates of three jets. The single jet placed at the time-averaged flow separation position is found to be most effective and efficient in eliminating flow separation among the first and second control modes. However, it is the third control mode that may not only eliminate the separation bubble completely but also cut down the energy consumption, by up to 30%, compared to the single jet blowing at the flow separation position. The flow physics underlying the control modes is also discussed.

Computational modeling of heat transfer and flow separation in supersonic cooled nozzles: Parametric study

Flow separation is necessary for the construction of a rocket engine nozzle. Regenerative cooling is one of the most significant criteria for the safety of wall nozzles because of the high temperature and pressure in the thrust chamber. A review of a comprehensive numerical investigation of the boundary layer separation and heat transfer in a 30??15? cooled nozzle is presented. The accuracy of the SST-V turbulence model in this study was numerically investigated. For this purpose and for a wide range of chamber conditions, the effects of various parameters, such as wall temperature, turbulent Prandtl number, and constant specific heat ratio vary from 1.31 to 1.4 for constant fluid properties for N2O, CH4, Cl2, and air, respectively. The variable specific heat ratio ranged from 1.39 to 1.66 for variable fluid properties for air, CH4, O2 and Helium, respectively, and we investigated how various parameters impact the position of flow separation and local wall heat transfer.

Mechanisms Contributing to the Dynamic Stability of a Flexible Subscale Rocket Nozzle

Forced motion simulations of an overexpanded subscale rocket nozzle were performed to investigate the transient mechanisms that lead to self-exciting fluid–structure interaction as observed in preceded studies. The pressure response to the deformation could be separated into two regions upstream and downstream the flow separation position. Within these regions the transient part of the pressure was analyzed using fast Fourier transform based on the method of generalized aerodynamic forces. The amplitude spectrum and phase shift distribution of the pressure response could be explained by superposition of three independently acting mechanisms: the inclination effect, the existence of a moving axial pressure wave, and intrinsic oscillations caused by the turbulence created by the strong shock system. Simplified simulation setups using a bent flat plate and a detailed unsteady simulation of the flow in the undeformed nozzle were analyzed to validate these assumptions.

Experimental investigation on the multiphase flow characteristics of oblique water entry of the hollow cylinders

The oblique water entry of the hollow cylinders with various inner diameters has been investigated experimentally. The multiphase flow characteristics of the cavity, through-hole jet and multiscale bubbles have been revealed and discussed. In the experimental setup, a high-speed photography technique is used to capture the process of water entry based on digital video images. The results show that the fluid flows to the inside and outside of the hollow cylinder along the flow separation position, the through-hole jet and cavity are formed. The deep closure mode of the cavity pinched off by line contact appears, the upper cavity moves upward and the secondary jet is formed, and the lower cavity presents the evolution process of stable transparent surface, fluctuating rippling surface, blurry broken surface and collapse. Three bubble clusters are formed due to the closure and pinching off of the cavity, the breaking up of the through-hole jet, and the blurry broken evolution of the lower cavity, and are shedding upward along the moving paths of the upper cavity, the tail and the head of the hollow cylinder, respectively. Moreover, a parametric study is conducted on the effects of inner diameters on the multiphase flow structure and dynamic characteristics.

Theoretical analysis on the ignition of a combustible mixture by a hot particle

Mechanical spark is an important ignition source in various industrial processes involving combustible mixtures and it may cause serious safety issues. In this work, we analysed the ignition induced by hot particles in a combustible mixture. In Semenov's transient ignition criterion, we introduced a hypothetical heat loss coefficient accounting for the temperature inhomogeneity and obtained a revised ignition criterion which allows us to calculate the ignition delay time. Explicit expressions for the critical ignition temperature were derived and used to demonstrate the primary impacts of temperature inhomogeneity on the ignition process. Consistent with experimental and numerical results, the temperature inhomogeneity is intensified by either reducing the particle size or convective heat transfer at the particle surface, resulting in an increase of the critical ignition temperature. For flow separation on the particle surface, the boundary layer problem was solved based on a Blasius series. A temperature gradient for ignition was defined at the location of flow separation to reproduce the experimentally observed phenomenon that ignition prefers to occur first near the flow separation position. It is shown that the unsteadiness of particle cooling makes negligible contribution to the ignition process because of the exceedingly large density ratio between the particle and the ambient gas. In addition, the finite residence time of ignition for a fluid parcel due to its elevation from the particle surface leads to additional growth in the critical ignition temperature. However, such a correction appears to be inconsequential because the ignition of the fluid parcel restricted to the Frank–Kamenetskii region is close to the particle surface.

Flow Separation Control over an Airfoil Using Plasma Co-Flow Jet

A NACA0025 coflow jet airfoil model, including the inner and outer airfoils, was designed and manufactured through 3D printing technology. Using the duct connecting the leading- and trailing-edge slots formed by the inner and outer airfoils, eight plasma actuators are arranged in the duct and synchronously driven by two high-voltage alternating-current sources to realize the plasma coflow jet (PCFJ) flow control technique. The test angle of attack range is 8–30°, and the Reynolds number is 68,000. The induced airflow characteristics were studied, and the flow control mechanism of the PCFJ airfoil was explored using the detailed two-component particle image velocimetry technique. The results in the quiescent air show that in addition to the injection at the leading-edge slot and the suction at the trailing-edge slot, the actuators on the upper surface of the inner airfoil provide local flow acceleration and momentum injection. The results in the wind tunnel show that the PCFJ airfoil has a better performance in delaying flow separation, and its time-averaged flow separation position is delayed by 46% compared with the baseline airfoil. The mixing shear layer that becomes turbulent in advance and the subsequent coherent vortex structures, the injection at the leading edge and suction at the trailing edge, and local flow acceleration and momentum injection of the plasma actuators are the key factors for the mechanism of separation delaying of the PCFJ flow control technique.

Investigation of Reverse Swing and Magnus Effect on a Cricket Ball Using Particle Image Velocimetry

Lateral movement from the principal trajectory, or “swing”, can be generated on a cricket ball when its seam, which sits proud of the surface, is angled to the flow. The boundary layer on the two hemispheres divided by the seam is governed by the Reynolds number and the surface roughness; the swing is fundamentally caused by the pressure differences associated with asymmetric flow separation. Skillful bowlers impart a small backspin to create gyroscopic inertia and stabilize the seam position in flight. Under certain flow conditions, the resultant pressure asymmetry can reverse across the hemispheres and “reverse swing” will occur. In this paper, particle image velocimetry measurements of a scaled cricket ball are presented to interrogate the flow field and the physical mechanism for reverse swing. The results show that a laminar separation bubble forms on the non-seam side (hemisphere), causing the separation angle for the boundary layer to be increased relative to that on the seam side. For the first time, it is shown that the separation bubble is present even under large rates of backspin, suggesting that this flow feature is present under match conditions. The Magnus effect on a rotating ball is also demonstrated, with the position of flow separation on the upper (retreating) side delayed due to the reduced relative speed between the surface and the freestream.

Large eddy simulation of dynamic stall flow control for wind turbine airfoil using plasma actuator

To solve aerodynamic performance deterioration caused by dynamic stall, a large eddy simulation numerical calculation based on dynamic grid and sliding grid technology was conducted and the dynamic flow control mechanism of unsteady pulsed plasma was explored. The results showed that plasma aerodynamic actuators can effectively control the airfoil’s dynamic stall, improve the mean and transient aerodynamic forces, and reduce the negative peak value of the pitch moment and hysteresis loop area. A negative pressure “bulge” appears in the plasma application areas, and the peak suction of the airfoil’s upper surface obviously increases. Two unsteady control parameters, the pulsed frequency and duty cycle, significantly influence the flow control. When the dimensionless pulsed frequency is 1.5, plasma control improves, and when the duty cycle is 0.8, it is close to the aerodynamic benefits under the continuous working mode. In the deep stall state, plasma impels the flow separation position to obviously move backward, resisting large-scale dynamic stall vortices. The structure of the separation vortices is broken, dissipated, and reattached to the airfoil by the plasma, and the influence area of the vortices is reduced. In the light stall state, the plasma actuator easily controls the shear layer, inducing the transition of the airfoil’s boundary layer and promoting the momentum mixing with the main flow. “Vortex clusters” near the airfoil’s leading edge induced by plasma actuation play a role in the virtual aerodynamic shape. The harmonic oscillation of aerodynamic force/moment is caused by the nonlinear and strong coupling effect between dynamic vortex structures with different scales and frequencies and plasma aerodynamic actuation. The amplitude of low-order mode energy concentration is relatively large, which is mainly caused by the pitching motion. The high-order fluctuation energy concentration is caused by the evolution process of starting vortices and derived secondary vortices with different frequencies induced by plasma.

Numerical Investigation of Flow Transition Behavior in Cold Flow Dual-Bell Rocket Nozzles

The dual-bell nozzle is an altitude-adaptive nozzle concept. It combines the advantages of a nozzle with small area ratio under sea-level conditions and a large area ratio nozzle under high-altitude conditions. Reynolds-averaged Navier–Stokes and unsteady Reynolds-averaged Navier–Stokes simulations on two-dimensional axisymmetric grids were conducted at DLR, German Aerospace Center in Lampoldshausen to investigate the transition from one mode to the other of a dual-bell nozzle model with positive pressure gradient extension. A cold flow test campaign conducted at DLR’s cold flow test facility P6.2 provided validation data for the numerical approach. The present study investigates the influence of different turbulence models and feeding pressure gradients on the dual-bell flow transition behavior. Better results were achieved for the Spalart–Allmaras and Reynolds stress turbulence model. A clear impact of the feeding pressure ramp on the dual-bell transition pressure ratio and the flow separation position velocity was shown. The transition nozzle pressure ratio was predicted with an accuracy of 1%. For the hysteresis between transition and retransition nozzle pressure ratio, an accuracy of approximately 10% was reached. The calculated values of the experimental and numerical transition duration were on the same order of magnitude.

Design and Performance Analysis of Compressor in Solid Propellant Air Turbo Rocket (SPATR)

The design and characteristic analysis of the single-stage centrifugal compressor were performed to meet the requirements of the integral performance of a certain solid propellant air turbo rocket (SPATR). Based on the limitation of the geometry and aerodynamic demands, the impeller and diffuser were designed and optimized by considering the disc friction loss and the flow quality. A theoretical approach was applied to evaluate feasibility and primary aerodynamic performance of the compressor. Focused on the operation performance of the compressor, the detailed flow structure in the compressor was investigated by using numerical simulation. The results show that the total pressure ratio and isentropic efficiency of the compressor in design point are 3.5 and 0.78 respectively, which meets the design demands. The compressor has excellent aerodynamic characteristics during the starting procedure or under the partial loading condition. The position of flow separation and the reason of flow loss are analyzed to conduct further optimization.

Experimental Research on Transition and Flow Separation of Laminar Airfoil

Experimental research on transition and flow separation of laminar airfoil and the influence of transition and flow separation on the performance of laminar airfoil are carried through. The experimental device and instruments used in the research are introduced. It is shown from the experimental results that at certain scope of attack angle, flow transition shifts from 50%c to 30%c on the upper surface of airfoil model along with 1 degree increase of airfoil attack angle; For flow speed of 35m/s and 50m/s with attack angle from 4 degree to 8 degree, flow separation position don’t vary but only separation scope increase; For flow speed of 15m/s laminar flow separation on the upper surface of airfoil take place at attack angle of 4 degree, flow become attaching at attack angle of 6 degree and turbulent flow separation take place at attack angle of 8 degree.

The Influence of the Total Pressure Profile on the Performance of Axial Gas Turbine Diffusers

Large industrial gas turbines for combined heat and power generation normally have axial diffusers leading to the heat recovery steam generator. The diffusers operate with high inlet axial Mach number (0.6) and with a nonuniform inlet total pressure profile from the turbine. Tests have been carried out on a generic highly loaded axial diffuser in a scaled axial diffuser test rig, with different inlet total pressure profiles including those that might be met in practice. The results show that the inlet total pressure profile has a strong effect on the position of flow separation, whereby a hub-strong profile tends to separate at the casing and the tip-strong profile on the hub. Steady computational fluid dynamics (CFD) simulations using the shear stress transport (SST) turbulence model have been carried out based on extensive studies of the best way to model the inlet boundary conditions. These simulations provide good agreement with the prediction of separation in the diffuser but the separated regions often persist too long so that, in this highly loaded case with flow separation, the calculated diffuser pressure recovery can be in error by up to 30%.

On the generation of a helicopter aerodynamic database

AbstractThe GOAHEAD (Generation of an Advanced Helicopter Experimental Aerodynamic Database for CFD code validation) consortium was created in the frame of an EU-project in order to create an experimental database for the validation of 3D-CFD and comprehensive aeromechanics methods for the prediction of unsteady viscous flows. This included the rotor dynamics for complete helicopter configurations, i.e. main rotor – fuselage – tail rotor configurations with emphasis on viscous phenomena like flow separation and transition from laminar to turbulent flow. The wind tunnel experiments have been performed during two weeks in the DNW-LLF on a Mach-scaled model of a modern transport helicopter consisting of the main rotor, the fuselage, control surfaces and the tail rotor. For the sake of controlled boundary conditions for later CFD validation, a closed test section has been used. The measurement comprised global forces of the main rotor and the fuselage, steady and unsteady pressures, transition positions, stream lines, position of flow separation, velocity profiles at the test section inlet, velocity fields in the model wake, vortex trajectories and elastic deformations of the main and tail rotor blades.

On quasi‐steady laminar flow separation in the upper airways

AbstractAccurate prediction of the position of flow separation along a constriction is important to model fluid–structure interaction phenomena in the upper airways such as phonation and obstructive sleep apnea. Flow assumptions underlying common flow descriptions along the upper airways are formulated. Flow separation positions obtained from theories with different degrees of complexity are qualitatively and quantitatively discussed. In particular, geometrical and flow features determining the influence of viscosity are varied. Increasing the constriction degree and the constriction length is shown to affect the position of flow separation. Boundary layer solutions and simulations with the two‐dimensional Navier Stokes equations result in an accurate quantitative prediction of flow separation. Furthermore, Jeffery–Hamel flow solutions qualitatively predict the effect of both constriction height and length on the position of flow separation. The ad hoc assumption applied in quasi‐one‐dimensional flow descriptions does not accurately predict flow separation. Copyright © 2008 John Wiley & Sons, Ltd.

Modified two-mass model for the vocal cords

The two-mass approximation is often presented as a simple but efficient model for the purpose of numerical synthesis of voiced sounds. However, the quality and the naturalness of the generated signal is still to be improved. A simple modification of the two-mass model is proposed in which the mechanical dynamics of that of two point masses. The movement of these point masses drives a two-parameter curve describing the smooth surface of the vocal folds. This representation allows description of unsteady flow separation within the glottis. A semiempirical boundary layer theory is used to calculate the position of flow separation. This theory is experimentally tested using stationary flow measurements and unsteady flow visualization. Furthermore, computed volume flux and Bernoulli force are presented and compared with classical models. The description also includes an inelastic collision which yields a prediction of an additional volume flux due to the deformation of the vocal folds after the through flux has been stopped by the closure of the glottis. This additional deformation appears to have a significant influence on the voiced sound production.

Axisymmetric Couette flow at large Taylor numbers

Measurements have been made in flow between concentric cylinders with only the inner one rotating, for Reynolds numbers (based on flow gap) from 17 000 to 120 000, corresponding to Taylor numbers from 8 × 107 to 4 × 109. At the lower speeds (Reynolds numbers less than 30 000), the large-scale motion consists of toroidal eddies, highly uniform in spacing and intensity and convected by a slow axial flow past fixed sensors. By synchronizing an external oscillator with the passage frequency, flow velocity and small-scale turbulent intensity may be sampled at defined stages of the passage cycle and averaged to provide maps of the velocity fields and the associated distributions of small-scale intensity and Reynolds stress.At higher speeds, the passage of toroidal eddies becomes too irregular to establish the passage cycle, but, by comparing the velocity fluctuations from four inclined hot wires placed near the outer cylinder, the current location of large-scale flow separation from the outer cylinder can be approximately determined. Statistics of the temporal variations of the location show that the large-scale motion still approximates to the toroidal form, but that there are azimuthal variations of separation position and velocity that indicate a change from toroidal to helical eddies. Conditional averages of flow velocity and small-scale turbulent intensity, based on relative distance from the position of flow separation, are very similar in form and magnitude to phase-selected averages obtained at lower speeds.The considerable changes in the large-scale motion that occur as the Reynolds number increases from 300 to 1200 times the critical value are believed to arise from increase in the ‘turbulent Taylor number’ of the central flow, based on variation of angular momentum and on the eddy diffusion coefficients for linear and angular momentum. Effects on the motion of the slow axial flow, always less than 1% of the peripheral flow velocity, are also discussed.

Dissolution of solid spheres by isothermal free convection

When a solid sphere dissolves isothermally in a liquid solvent, the consequent changes in density set up convection currents that govern the rate of mass transfer. In this paper the boundary layer flow itself as well as the local and overall mass transfer has been studied for a wide range of the characteristic numbers: 2·0.10 3 < Gr < 2·0.10 6 and 3·9.10 2 < Sc < 1·1.10 4, obtained by using different dimensions, solvents and solids. The experimental results are compared with those of other workers and with theory. The dependency of the position of flow separation and minimum mass transfer on Sc and Gr has been studied.

Some Properties of Laminar Boundary Layers on Curved Surfaces

Equations have been developed [1] which describe the flow in a steady, two-dimensional, incompressible, laminar boundary layer on a curved surface. The method of “similar solutions” yields a fourth-order, nonlinear, ordinary differential equation which may be solved on a digital computer. Account is taken of the effects of both surface curvature and displacement thickness, and in this paper attention is given to the influence of these effects on other boundary-layer properties up to and including the position of flow separation.