A numerical study of fluid injection and mixing under near-critical conditions

Nitrogen injection under conditions close vicinity of the liquid-gas critical point is studied numerically. The fluid thermodynamic and transport properties vary drasti- cally and exhibit anomalies in the near-critical regime. These anomalies can cause distinctive effects on heat-transfer and fluid-flow characteristics. To focus on the influence of ther- modynamics on the flow field, a relatively low injection Reynolds number of 1 750 is adopted. For comparisons, a reference case with the same configuration and Reynolds number is simulated in the ideal gas regime. The model accommodates full conservation laws, real-fluid thermody- namic and transport phenomena. Results reveal that the flow features of the near-critical fluid jet are significantly differ- ent from their counterpart. The near-critical fluid jet spreads faster and mixes more efficiently with the ambient fluid along with a more rapidly development of the vortex pairing pro- cess. Detailed analysis at different streamwise locations in- cluding both the flat shear-layer region and fully developed vortex region reveals the important effect of volume dilata- tion and baroclinic torque in the near-critical fluid case. The former disturbs the shear layer and makes it more unstable. The volume dilatation and baroclinic effects strengthen the vorticity and stimulate the vortex rolling up and pairing pro- cess

Dynamic performance and wake structure of flapping plates with different shapes

The dynamic performance and wake structure of flapping plates with different shapes were studied using multi-block lattice Boltzman and immersed boundary method.Two typical regimes relevant to thrust behavior are identified.One is nonlinear relation between the thrust and the area moment of plate for lower area moment region and the other is linear relation for larger area moment region.The tendency of the power variation with the area moment is reasonably similar to the thrust behavior and the efficiency decreases gradually as the area moment increases.As the mechanism of the dynamic properties is associated with the evolution of vortical structures around the plate,the formation and evolution of vortical structures are investigated and the effects of the plate shape,plate area,Strouhal number and Reynolds number on the vortical structures are analyzed.The results obtained in this study provide physical insight into the understanding of the mechanisms relevant to flapping locomotion.

Large Eddy Simulation of Stratified Turbulent Channel Flow with a Dynamic Subgrid Scale Model

In this paper, large eddy simulation coupled with a dynamic subgrid scale (SGS) model is used to study turbulent channel flows with heat transfer. Some fundamental flow behaviors are discussed. Based on our calculated results, the dynamic SGS model can reasonably predict some main characteristics of stratified turbulent channel flows. Our results are also in good agreement with theoretical analyses and previous calculated results.

A Consistent Characteristic Boundary Condition for General Fluid Mixture and Its Implementation in a Preconditioning Scheme

Characteristic boundary conditions that are capable of handling general fluid mixtures flow at all flow speeds are developed.The formulation is based on fundamental thermodynamics theories incorporated into an efficient preconditioning scheme in a unified manner.Local one-dimensional inviscid(LODI)relations compatible to the preconditioning system are proposed to obtain information carried by incoming characteristic waves at boundaries accurately.The approach has been validated against a variety of sample problems at a broad range of fluid states and flow speeds.Both acoustic waves and hydrodynamic flow features can pass through the boundaries of computational domain transparently without any unphysical reflection or spurious distortion.The approach can be reliably applied to fluid flows at extensive thermodynamic states and flow speeds in numerical simulations.Moreover,the use of the boundary condition shows to improve the computational efficiency.

Scaling laws for the intermittent swimming performance of a flexible plate at low Reynolds number

Many species of fish and birds travel in intermittent style,yet the combined influence of intermittency and other body kinematics on the hydrodynamics of a self-propelled swimmer is not fully understood.By formulating a reduced-order dynamical model for intermittent swimming,we uncover scaling laws that link the propulsive performance(cursing Reynolds number Rec,thrust T̄,input power P̄and cost of transport COT to body kinematics(duty cycle DC,flapping Reynolds number Ref).By comparing the derived scaling laws with the data from several previous studies and our numerical simulation,we demonstrate the validity of the theory.In addition,we found that Re_(c),T̄,P̄and COT all increase with the increase of DC,Ref.The model also reveals that the intermittent swimming may not be inherently more energy efficient than continuous swimming,depending on the ratio of drag coefficients between active bursting and coasting.

Large Eddy Simulation of a Vortex Ring Impacting a Bump

A vortex ring impacting a three-dimensional bump is studied using large eddy simulation for a Reynolds number Re=4×104 based on the initial diameter and translational speed of the vortex ring.The effects of bump height and vortex core thickness for thin and thick vortex rings on the vortical flow phenomena and the underlying physical mechanisms are investigated.Based on the analysis of the evolution of vortical structures,two typical kinds of vortical structures,i.e.,the wrapping vortices and the hair-pin vortices,are identified and play an important role in the flow state evolution.The boundary vorticity flux is analyzed to reveal the mechanism of the vorticity generation on the bump surface.The circulation of the primary vortex ring reasonably elucidates some typical phases of flow evolution.Further,the analysis of turbulent kinetic energy reveals the transition from laminar to turbulent state.The results obtained in this study provide physical insight into the understanding of the mechanisms relevant to the flow evolution and the flow transition to turbulent state.

Direct Numerical Simulation of Vertical Rotating Turbulent Open-Channel Flow with Heat Transfer

Direct numerical simulation of vertical rotating open-channel flow with heat transfer has been carried out for the rotation number Nτfrom 0 to 0.1,the Prandtl number 1,and the Reynolds number 180 based on the friction velocity of non-rotating flow and the height of the channel.The ob jective of this study is to reveal the effect of rotation on the characteristics of turbulent flow and heat transfer,in particular near the free surface and the wall of the open-channel.Statistical quantities,e.g.,the mean velocity,temperature and their fluctuations,turbulent heat fluxes,and turbulence structures,are analyzed.The depth of surface-influenced layer decreases with the increase of the rotation rate.In the free surface-influenced layer,the turbulence and thermal statistics are suppressed due to the effect of rotation.In the wall-influenced region,two typical rotation regimes are identified.In the weak rotation regime with 0<Nτ<0.06 approximately,the turbulence and thermal statistics correlated with the spanwise velocity fluctuation are enhanced since the shear rate of spanwise mean flow induced by Coriolis force increases;however,the other statistics are suppressed.In the strong rotation regime with Nτ>0.06,the turbulence and thermal statistics are suppressed significantly because the Coriolis force effect plays a dominant role in the rotating flow.To elucidate the effect of rotation on turbulent flow and heat transfer,the budget terms in the transport equations of Reynolds stresses and turbulent heat fluxes are investigated.Remarkable change of the direction of streak structures based on the velocity and temperature fluctuations is discussed.

Numerical Investigation of the Dynamics of a Flexible Filament in the Wake of Cylinder

Fluid-structure-interaction problems are ubiquitous,complicated,and not yet well understood.In this paper we investigate the interaction of a leading rigid circular cylinder and a trailing compliant filament and analyze the dynamic responses of the filament in the wake of the cylinder.It is revealed that there exist two flapping states of the filament depending on the cylinder-filament separation distance and the relevant critical distance distinguishing the two states is associated with the Reynolds number and the filament length.It is also found that the drag coefficient of the cylinder is reduced but that of the filament may be increased or decreased depending on its length.Compared with a single filament in a uniform flow,the filament of the same mechanical properties flapping in the wake of the cylinder has a lower frequency and a greater amplitude.