Qing Xiao

A numerical study of transonic buffet on a supercritical airfoil

The flow of the BauerGarabedianKorn (BGK) No. 1 supercritical airfoil is investigated by the solution of the unsteady Reynolds-averagedNavierStokes equations with a two-equation lagged kωturbulent model.Two steady cases (M=0.71, α=1.396 deg and M=0.71, α=9.0 deg) and one unsteady case (M=0.71, α=6.97 deg), all with a far-stream Reynolds number of 20106, are computed. The results are compared with available experimental data. The computed shock motion and the evolution of the concomitant flow separation are examined. Space-time correlations of the unsteady pressure field are used to calculate the time for pressure waves to travel downstream within the separated region from the shock wave to the airfoil trailing edge and then back from the trailing edge to the shock outside the separated region. The reduced frequency so calculated agrees well with the computed buffet frequency, supporting the signal propagation mechanism for buffet proposed by Lee (Lee, B. H. K., Oscillation Shock Motion Caused by Transonic Shock Boundary-Layer Interaction, AIAA Journal, Vol. 28, No. 5, 1990, pp. 942944).

Numerical Investigation of Supersonic Nozzle Flow Separation

Separation of supersonic flow in a planar convergent–divergent nozzle with moderate expansion ratio is investigated by solving the Reynolds-averaged Navier–Stokes equations with a two-equation k-!turbulence model. The focus of the study is on the structure of the fluid and wave phenomena associated with the flow separation. Computations are conducted for an exit-to-throat area ratio of 1.5 and for a range of nozzle pressure ratios. The results are compared with available experimental data in a nozzle of the same geometry. The flow separates by the action of a lambda shock, followed by a succession of expansion and compression waves. For 1:5 < NPR < 2:4, the computation reveals the possibility of asymmetric flow structure. The computationally obtained asymmetric flow structuresareconsistentwithpreviousexperimental flowvisualizationsstudies.Inaddition,other flowfeaturessuch asshocklocationandwallpressuredistributionsarealsoingoodagreementwiththeexperimentaldata.Thepresent study provides new information that confirms earlier conjectures on the flow–wave structure relevant to the instability of the separated flow in convergent–divergent nozzles of moderate expansion ratio.

A bio-inspired study on tidal energy extraction with flexible flapping wings

Previous research on the flexible structure of flapping wings has shown an improved propulsion performance in comparison to rigid wings. However, not much is known about this function in terms of power efficiency modification for flapping wing energy devices. In order to study the role of the flexible wing deformation in the hydrodynamics of flapping wing energy devices, we computationally model the two-dimensional flexible single and twin flapping wings in operation under the energy extraction conditions with a large Reynolds number of 106. The flexible motion for the present study is predetermined based on a priori structural result which is different from a passive flexibility solution. Four different models are investigated with additional potential local distortions near the leading and trailing edges. Our simulation results show that the flexible structure of a wing is beneficial to enhance power efficiency by increasing the peaks of lift force over a flapping cycle, and tuning the phase shift between force and velocity to a favourable trend. Moreover, the impact of wing flexibility on efficiency is more profound at a low nominal effective angle of attack (AoA). At a typical flapping frequency f * = 0.15 and nominal effective AoA of 10°, a flexible integrated wing generates 7.68% higher efficiency than a rigid wing. An even higher increase, around six times that of a rigid wing, is achievable if the nominal effective AoA is reduced to zero degrees at feathering condition. This is very attractive for a semi-actuated flapping energy system, where energy input is needed to activate the pitching motion. The results from our dual-wing study found that a parallel twin-wing device can produce more power compared to a single wing due to the strong flow interaction between the two wings.

A new flow regime in a Taylor–Couette flow

In this Brief Communication, we report a new finding on a Taylor–Couette flow in which the outer cylinder is stationary and the inner cylinder is accelerated linearly from rest to a desired speed. The results show that when the acceleration (dRe/dt) is higher than a critical value of about 2.2 s−1, there exists a new flow regime in which the flow pattern shows remarkable resemblance to regular Taylor vortex flow but is of shorter wavelength. However, when the acceleration is lower than 2.2 s−1, a wavy flow is found to occur for the same Reynolds number range. To our knowledge, this is probably the first time that such a phenomenon has been observed. For completeness, the case of a decelerating cylinder is also investigated, and the results are found to be almost the same.

Computation of Transonic Diffuser Flows by a Lagged k-! Turbulence Model

The lag model proposed by Olsen and Coakley is applied in combination with the baseline k‐! two-equation turbulence model to simulate the steady and unsteady transonic e ows in a diffuser. A fully implicit time-accurate multigrid algorithm is used to solve the unsteady Navier ‐Stokes equations and the coupled k‐! turbulence model equations. Two test cases are investigated, one with a weak shock in the channel corresponding to an exit-static-toinlet-total pressure ratio Rp=0.82 and the other with a strong shock corresponding to Rp=0.72. Unsteady e ows are induced by imposing e uctuating backpressure. Computational results are compared with experimental data and demonstrate notable improvement by the lag model for e ows with strong shock ‐boundary-layer interactions.

NUMERICAL STUDY OF ASYMMETRIC EFFECT ON A PITCHING FOIL

This study investigates numerically the effect of asymmetric sinusoidal oscillating motion on the propulsion performance of a pitching foil and attempts to gain insight in whether the low thrust generated by pure pitching could be improved by asymmetric motion. The propulsion performance and flow structure are explored by solving the unsteady Navier–Stokes equations. Computations are conducted for a range of oscillation frequency, pitching amplitude, and asymmetry. The results show that the higher asymmetry can induce the stronger reverse Von Karman vortex in the wake, which in turn leads to the increased thrust. However, the propulsion efficiency reduces as increasing asymmetry. Computed results are further adopted to shed insight into the mechanism of thrust enhancement.

Second Taylor vortex flow: Effects of radius ratio and aspect ratio

This paper is motivated by our earlier investigation on the stability of Taylor–Couette flow in which we discovered a previously unidentified flow regime, which we refer to as “Second Taylor vortex flow” (STVF) when an inner cylinder is subjected to some critical acceleration [Lim, Chew, and Xiao, Phys. Fluids 10, 3233 (1998)]. The aim here is to explore how the STVF regime is affected by changes in radius ratio and aspect ratio. Results show that the STVF regime is sensitive to the gap size between the two cylinders, and does not exist for some radius ratios, whereas it increases with decreasing aspect ratio.

Experimental and Numerical Study of Jet Mixing from a Shock-Containing Nozzle

The compressible jet plume emerging from a planar convergent-divergent nozzle containing a separation shock is investigated experimentally and numerically. The investigation encompasses exit-to-throat area ratios (Ae=At) from 1.0 to 1.8 and nozzle pressure ratios from 1.2 to 1.8. Experiments were conducted in a variable-geometry nozzle facility, and computations solved the Reynolds-averaged Navier-Stokes equations with several turbulence models. The computed mean velocity field outside the nozzle compares reasonably well with the experimental data. Among the different turbulence models tested, the two-equation shear stress transport model is found to provide the best agreement with the experiments. Jet mixing is governed by Ae=At and, to a lesser extent, by nozzle pressure ratios. Increasing Ae=At results in an increased growth rate and faster axial decay of the peak velocity. The experimental trends of jet mixing versus Ae=At and nozzle pressure ratios are captured well by the computations. Computations of turbulent kinetic energy show that, with increasing Ae=At , the peak turbulent kinetic energy in the plume rises and moves toward the nozzle exit. The significant increase of turbulent kinetic energy inside the nozzle is associated with asymmetric flow separation.

Numerical study of propulsion mechanism for oscillating rigid and flexible tuna-tails

Numerical study on the unsteady hydrodynamic characteristics of oscillating rigid and flexible tuna-tails in viscous flow-field is performed. Investigations are conducted using Reynolds-Averaged Navier–Stokes (RANS) equations with a moving adaptive mesh. The effect of swimming speed, flapping amplitude, frequency and flexure amplitude on the propulsion performance of the rigid and flexible tuna-tails are investigated. Computational results reveal that a pair of leading edge vortices develop along the tail surface as it undergoes an oscillating motion. The propulsive efficiency has a strong correlation with various locomotive parameters. Peak propulsive efficiency can be obtained by adjusting these parameters. Particularly, when input power coefficient is less than 2.8, the rigid tail generates larger thrust force and higher propulsive efficiency than flexible tail. However, when input power coefficient is larger than 2.8, flexible tail is superior to rigid tail.

Numerical Study of Jet Plume Instability from an Overexpanded Nozzle

*† ‡ The compressible jet plume from a planar overexpanded nozzle is investigated by solving the Reynolds-Averaged Navier-Stokes equations with several turbulence models. Computations are conducted for a series of exit-to-throat area ratios (Ae/At) from 1.0 to 1.8 and a range of nozzle pressure ratios (NPR) from 1.2 to 1.8. The results are compared with available experimental data in a nozzle of the same geometry. The asymmetric jet plume is well predicted by the simulation and is consistent with the experiments. Among the different turbulence models tested, the two-equation Shear Stress Model (SST) is found to agree closest to the experiments. The simulations are able to predict the velocity profiles, total pressure decay, and axial jet thickness distribution in the jet plume reasonably well. Jet mixing is governed by Ae/At a n d t o a l e s s e r e x t e n t b y N P R . I n c r e a s i n g Ae/At results in a significant increase of mixing rate. Computations of turbulent kinetic energy (TKE) show that, with increasing Ae/At, the peak TKE in the plume rises and moves towards the nozzle exit. Significant increase of TKE inside the nozzle results from the asymmetric flow separation. Nomenclature