Large-scale Separated Flow Research Articles

Dual separation control and drag mitigation in high speed flows using viscoelastic materials

Boundary layer separation and friction drag form key delimiting phenomena that subvert the aerial platforms from achieving greater efficiency and accessing wider operation envelope. Both these phenomena are significantly aggravated in supersonic platforms due to the interactions between shock waves with the boundary layer that develops over the vehicle surface and within the engines. The present work demonstrates a new paradigm that leverages the native or programmable material properties of the aerostructures to engender simultaneous reduction in the separation scales and mitigation of skin friction drag. As a first step toward realizing this paradigm, the present work demonstrates, for the first time, the simultaneous skin friction drag mitigation in a Mach 2.5 boundary layer and control of shock induced boundary layer separation, both using viscoelastic implants placed under the flow. It is experimentally demonstrated that the appropriately chosen viscoelastic materials can simultaneously reduce the skin friction coefficient at the measurement location by 11% and mitigate the size of a large-scale separated flow by up to 28%. The reported performance matches the current generation flow effectors in both separation scale and skin friction mitigation. The present study opens a new application space for soft/programmable materials in high speed aerial vehicles.

Advances in complex low speed flow around a prolate spheroid

Understanding and predicting the flow around the prolate spheroid is of great engineering significance to guide the design of vehicles such as aircraft and submarines. In recent years, a lot of experimental and numerical studies have been carried out on the flow around the prolate spheroid. The qualitative description and quantitative research of flow separation around prolate spheroid at attack angle are presented, promoting the identification and topology research of three-dimensional separation. The experimental results of oil flow, smoke, dye, hydrogen bubble, and LDV are given. The flow field characteristics are analyzed, and the existing problems are pointed out. Based on the introduction of the above phenomena, the effects of separation on aerodynamic force, noise, and wake are introduced. The effects of test conditions such as transition zone, protrusion, depression, and tail support on flow are also discussed. There are obvious differences between the above steady flow and the unsteady maneuvering process. The unsteady maneuvering process can not be treated as a steady or quasi-steady problem. During the maneuvering process, the separation will be delayed obviously, and the aerodynamic force will also change obviously. The greater the angle of attack, the higher the maneuvering rate, the more noticeable this effect will be. At present, RANS turbulence model is still the primary engineering method to solve the large-scale separated flow around the prolate spheroid. However, LES, DES, and other methods have gradually been widely used. Due to the limitation of computer capability, DNS can only be used in the case of lower Reynolds number but not in high Reynolds number flow. The difference between the numerical simulation and the unsteady simulation is more significant. Finally, the research progress of prolate spheroid transition is introduced. The mechanism and identification of TS transition and cross-flow transition are more accurate. The numerical simulation results are basically consistent with the experimental results, but the understanding of reattachment transition is not clear enough, especially on the windward side. Therefore, the research of prolate spheroid transition still needs to rely on experiments.

Numerical investigation of unsteady cavitating turbulent flows around a three-dimensional hydrofoil using stress-blended eddy simulation

The mechanism of flow instability, which involves complex gas–liquid interactions and multiscale vortical structures, is one of the hot research areas in cavitating flow. The role of turbulence modeling is crucial in the numerical investigation of unsteady flow characteristics. Although large-eddy simulation (LES) has been used as a reliable numerical method, it is computationally costly. In this work, we used a hybrid Reynolds-averaged Navier–Stokes (RANS) and LES model, that is, stress-blended eddy simulation (SBES), to improve the prediction capability for the cloud cavitating flow. Our hybrid approach introduces a shielding function to integrate the RANS model with the LES applied only regionally, such as to large-scale separated flow regions. The results showed that the periodic shedding of cavity growth, break off, and collapse around a three-dimensional Clark-Y hydrofoil was reproduced in accordance with experimental observations. The lift/drag coefficients, streamwise velocity profiles, and cavity patterns obtained by the SBES model were in better agreement with the experimental data than those obtained by the modified RANS model. The re-entrant jet dynamics responsible for the break off of the attached cavity were discussed. Further analysis of vorticity transportation indicated that the stretching and dilatation terms dominated the development of vorticity around the hydrofoil. In conclusion, the SBES model can be used to predict cavitating turbulent flows in practical engineering applications.

Part 2: Simulation methodology and numerical uncertainty for RANS-CFD for the hydrodynamics of wind-assisted ships operating at leeway angles

A Reynolds-averaged Navier Stokes computational fluid dynamics (RANS-CFD) package will be one of the primary tools used during the development of a performance prediction program for wind-assisted commercial ships. This paper describes the simulation verification exercise, performed in support of the experimental validation presented in Part 1 of this two-part series describing the RANS-CFD method employed in this research. The predominance of large-scale separated flow structures in the wake of the sailing ship, an artefact of sideforce production necessary for sailing, points to a careful verification exercise and estimate for the numerical uncertainty to support the systematic investigation of wind-assisted ship hydromechanics and meshing guidelines within the available computer resources. Methods for CFD uncertainty quantification are defined and implemented for verification cases at leeway angles equal to 0ᵒ, 6ᵒ, and 9ᵒ. Analysis for four sets of grids with different meshing strategies and for varying time steps results in a grid definition and time step for simulation validation. Numerical uncertainty as adopted in Part 1 for validation is defined. Finally, the meshing strategy for full-scale simulation is described, as used for the production runs of the Delft Wind Assist Series.

Investigation of corner separation and suction control in constant area duct

To control shock-wave/boundary-layer interaction with suction, numerical simulations in a constant-area rectangular isolator with asymmetry nozzle are conducted to understand the complex flow phenomena. Results show that three-dimensional effect significantly weakens isolator resistance to backpressure, which indirectly emphasizes enormous role of viscous effect and geometry to the dynamic behavior of shock train or pseudo-shock wave. The large-scale separated flow only locates at the corner of the upper wall. The mechanism analysis indicates that the flow is accelerated more rapidly along the bottom wall, thereby providing more momentum at the nozzle exit. The displacement and momentum thicknesses and their increasing rate on the upper wall are all greater than that of the bottom wall, which means that upper corner flow is subjected easily to separation. The vortex structure of different cross sections in the pseudo-shock region is formed in different ways and the large-scale vortex still exists after the airflow mixing, which contributes to the deceleration and pressurization. Through the control effect analysis of suction slot, the suction slot in spanwise direction on the side wall can shorten effectively the length of shock train, which indicates that removing the low-momentum fluid near the corners can control significantly the shock-wave/boundary-layer interactions.

Investigation of Unsteady Surface Pressures over a Two-Wheeled Landing-Gear Model

Unsteady flow around deployed landing gears is a significant contributor to aircraft noise on approach. The far-field sound is largely determined by the fluctuating pressures on the gear surface, quantities that have yet to be fully characterized even for simplified representations. The current work reports axle- and wheel-surface data from a comprehensive set of measurements on a generic, two-wheel, landing-gear model. The results show the expected high levels of unsteadiness on areas exposed to large-scale separated flow, of which some (wheel rear surfaces and forward edges) can be identified from steady data, but others (wheel faces adjacent to axle wakes, flow impingements on sidewalls) cannot. The forward-edge values for the current, untripped configuration are markedly greater than previously reported measurements with artificial transition fixing. Spectral analysis highlights a further category of potentially important contributions that are not easily detected in overall rms values: high-frequency unsteadiness associated with turbulent attached, or attaching, flows. A key implication of these results is the need for accurate representation of the true, full-scale, boundary-layer state in computations and model-scale testing.

Analysis of the dominant frequency in a subsonic compressor stator at near stall condition

IDDES method was applied to investigate the highly unsteady flow in a subsonic compressor stator with very large hub clearance and high incidence angle. The blade loading variation frequency was found close to the rotating instability (RI) frequency f RI ≈ 333 Hz observed in the experiment. Detailed analysis of the flow physics shows that the loading variation is caused by the periodic swing of the large scale separated flow on the blade suction side surface. The breakdown of the leakage vortex has no significant dominant frequencies, thus cannot be the cause of RI in this compressor stator as normally believed. Furthermore, the vortex shedding and vortex breakdown due to shear layer instability at the outer edge of the blade suction surface separation region excite high frequency unsteadiness that can form the sources of noise.

Analysis of Turbulent Separated Flows for the NREL Airfoil Using Anisotropic Two-Equation Models at Higher Angles of Attack

Numerical simulations of the turbulent flow fields at stall conditions are presented for the NREL (National Renewable Energy Laboratory) S809 airfoil. The flow is modeled as compressible, viscous, steady/unsteady and turbulent. Four two-equation turbulence models (isotropic k–ε and q–ω models, anisotropic k–ε and q–ω models), are applied to close the Reynolds-averaged Navier-Stokes equations, respectively. The governing equations are integrated in time by a new LU-type implicit scheme. To accurately model the convection terms in the mean-flow and turbulence model equations, a modified fourth-order high resolution MUSCL TVD scheme is incorporated. The large-scale separated flow fields and their losses at the stall and post-stall conditions are analyzed for the NREL S809 airfoil at various angles of attack (α) from 0 to 70 degrees. The numerical results show excellent to fairly good agreement with the experimental data. The feasibility of the present numerical method and the influence of the four turbulence models are also investigated.

114 側壁吸い込みによる風洞内はく離流れの可視化

In order to investigate large-scale separated flow in the wind tunnel, surface tuft method was applied to wind tunnel flow which was sucked from side wall. In the region of flow separation, many vortices were observed, which center lines were normal to either side wall or bottom wall.

Base Drag Reduction Caused by Riblets on a GAW(2) Airfoil

AMONG various methods explored for turbulent drag reduction on aerodynamic surfaces, riblets have beer the most promising.' As much as 4-8% of viscous drag reduction has been reported for simple two-dimensional configurations Plastic sheets with symmetric v-grooves (manufactured by the 3M Co.) have been employed widely in research- Assessment of viscous drag reduction on two-dimensional airfoils. both at low and transonic speeds, has been reported as well.- Excellent reviews on the subject covering aspects of drag reduction and flow structure are contained in Refs. 1 and 7. 13; There have been very few attempts exploring the fuse of giblets in separated flows, either from the point of view of drag reduction or separation control-r'xB0; Recently. Krishnan et al.' showed that riblets actually increase the base drag (about 8.7 on a long axisymmetric body with a blunt base at low speeds: the base diameter was about four times the boundary -layer thickness ahead of the base corner-They used 3M riblet sheers and systematically studied the effect of h+` on base pressure. They also speculated that, while riblets caused an increase in the base drag for a large-scale separated flow (like on the axisymmetric blunt base'), the effect could be favorable on an airfoil with a blunt trading edge, which is a case of a small-scale separated flow. 13; The present investigation was undertaken specifically to assess the effect of 3M riblets on the base pressure of an airfoil with a blunt trailing edge. Experiments were made at low speeds on a 13.6% thick GAW(2) airfoil model, which has a trailing-edge thickness ratio of 0.5%. The results show very clearly that the base drag reduction of an engineering value can be achieved for the optimized riblet geometry.

Concerning inviscid solutions for large-scale separated flows

The large-scale separated eddies set up behind a bluff body at high Reynolds number are considered, for steady laminar planar flow. The main eddies are massive and are controlled predominantly by inviscid mechanics, with uniform vorticity inside. Analytical and computational solutions of the massive-eddy (vortex-sheet) problem are then described. A further possibility studied is that, even with lateral symmetry assumed, there may still be an extra degree of nonsymmetry of skewing with respect to the streamwise direction. Small-scale separations, where a Benjamin-Ono equation also possibly yielding nonsymmetric solutions can come into play, are discussed briefly.