Using a pressure controlled vortex design method to control secondary flow losses in a turbine stage

A turbine design method based on pressure controlled vortex design （PCVD） is presented to design a small-size turbine stage. Contrary to the conventional controlled vortex design （CVD） method, the main objective of PCVD is to control the axial velocity and radial pressure in the sta- tor rotor gap. Through controlling axial velocity, the PCVD establishes a direct tie to meridional stream surface. Thus stream surface variation is induced, resulting in a large secondary flow vortex covering the full blade passage in the respective stator and rotor. This secondary flow vortex could be dedicated to control the secondary flow mitigation and migration. Through radial pressure, the PCVD is also associated with the macroscopic driving force of fluid motion. So the better benefit of CVD can be achieved. The core concept behind PCVD is to mainly control the spanwise pressure gradient by altering profile loading at various spanwise locations. Therefore not only the local pro- file lift is affected, but also the resulting throat widths, stage reaction degree, and massflow rate are altered or redistributed respectively. With the PCVD method, the global stage efficiency is increased successfully while the mass flow rate keeps constant. Additionally there is no endwall shape optimization, stacking optimization, or pitch/chord variations, concentrating solely on varying blade profile deflections and stagger.

Experimental Study on Effect of A New Vortex Control Baffler and Its Influencing Factor

The horseshoe vortex generated around the sail-body junction of submarine has an important influence on the non-uniformity of submarine wake at propeller disc. The flow characteristics in the horseshoe vortex generated area are analyzed, and a new method of vortex control baffler is presented. The influence of vortex control baffler on the flow field around submarine main body with sail is numerically simulated. The wind tunnel experiment on submarine model is carried out, and it is proved that the vortex control baffler can weaken the hoi-seshoe vortex and decrease the non-uniformity of the wake at propeller disc. It is shown from the experiment results that the effect of vortex control baffler depends on its installation position; with a proper installation position, the non-uniform coefficient of submarine wake would be declined by about 50%; the Reynolds number of submarine model has an influence on the effect of vortex control baffler too, and the higher the Reynolds number is, the better the effect of the vortex control baffler is.

VORTEX CONTROL BY THE SPANWISE SUCTION FLOW ON THE UPPER SURFACE OF DELTA WING

The numerical investigation has been performed to explore the feasibility of vortex control by leading edge sucking excitation on a delta wing. The results reveal that the flow on the upper surface of the delta wing changes significantly in a wide range of the angle of attack. For the vortical flow at moderate angle of attack, the secondary and tertiary vortices are weakened or suppressed, and the total lift is almost unchanged. For the stalled flow at high angle of attack, the leading edge concentrated vortex is recovered, and the lift is enhanced with increasing suction rate. For the bluff-body flow at even high angles of attack, the lift can still be improved. The concentrated vortex disappears on the upper surface, and the load increment is nearly unchanged along the chordwise direction.

Tomographic PIV investigation on coherent vortex structures over shark-skin-inspired drag-reducing riblets

Nature has shown us that the microstructure of the skin of fast-swimming sharks in the ocean can reduce the skin friction drag due to the well-known shark-skin effect.In the present study,the effect of shark-skin-inspired riblets on coherent vortex structures in a turbulent boundary layer（TBL） is investigated.This is done by means of tomographic particle image velocimetry（TPIV） measurements in channel fl ws over an acrylic plate of drag-reducing riblets at a friction Reynolds number of 190.The turbulent fl ws over drag-reducing riblets are verifie by a planar time-resolved particle image velocimetry（TRPIV） system initially,and then the TPIV measurements are performed.Two-dimensional（2D） experimental results with a dragreduction rate of around 4.81% are clearly visible over triangle riblets with a peak-to-peak spacing s＋of 14,indicating from the drag-reducing performance that the buffer layer within the TBL has thickened;the logarithmic law region has shifted upward and the Reynolds shear stress decreased.A comparison of the spatial topological distributions of the spanwise vorticity of coherent vortex structures extracted at different wall-normal heights through the improved quadrant splitting method shows that riblets weaken the amplitudesof the spanwise vorticity when ejection（Q2） and sweep（Q4） events occur at the near wall,having the greatest effect on Q4 events in particular.The so-called quadrupole statistical model for coherent structures in the whole TBL is verified Meanwhile,their spatial conditional-averaged topological shapes and the spatial scales of quadrupole coherent vortex structures as a whole in the overlying turbulent fl w over riblets are changed,suggesting that the riblets dampen the momentum and energy exchange between the regions of near-wall and outer portion of the TBL by depressing the bursting events（Q2 and Q4）,thereby reducing the skin friction drag.

NUMERICAL STUDY OF FLOW IN CONICAL DIFFUSER WITH VORTEX GENERATOR JETS

To develop vortex generator jet （VGJ） method for flow control, the turbulence flow in a 14° conical diffuser with and without vortex generator jets are simulated by solving Navier-Stokes equations with k-ε turbulence model. The diffuser performance, based on different velocity ratio （ratio of the jet speed to the mainstream velocity）, is investigated and compared with the experimental study. On the basis of the flow characteristics using computation fluid dynamics （CFD） method observed in the conical diffuser and the downstream development of the longitudinal vortices, attempt is made to correlate the pressure recovery coefficient with the behavior of vortices produced by vortex generator jets.

Steady vortex force theory and slender-wing flow diagnosis

The concept vortex force in aerodynamics is sys- tematically examined based on a new steady vortex-force theory （Wu et al., Vorticity and vortex dynamics, Springer, 2006） which expresses the aerodynamic force （and moment） by the volume and boundary integrals of the Lamb vector. In this paper, the underlying physics of this theory is explored, including the general role of the Lamb vector in non- linear aerodynamics, its initial formation, and its relevance to the total-pressure non-uniformity. As a typical example, the theory is applied to the flow over a slender delta wing at a large angle of attack. The highly localized flow structures with high Lamb-vector peaks are identified in terms of their net contribution to various constituents of the total aerody-namic force. This vortex-force diagnosis sheds new light on the flow control and configuration optimization.

Control of Vortex Shedding at Moderate Reynolds Numbers

The suppression method of vortex shedding from a circular cylinder has been studied experimentally in the Reynolds number range from 300 to 1600. The test is performed in a water channel. The model cylinder is 1 cm in diameter and 38 cm in length. A row of small rods of 0.18 cm in diameter and 1.5 cm in length are perpendicularly connected to the surface of the model cylinder and distributed along the meridian, The distance between the neighboring rods and the angle of attack of the rods can be changed so that the suppression effect on vortex shedding can be adjusted. The results show that vortex shedding can be suppressed effectively if the distance between the neighboring rods is smaller than 3 times and the cylinder diameter and the angle of attack is in the range of 30degreesless than or equal tobeta<90&DEG;.

CONTROL OF UNSTEADY VORTICAL LIFT ON AN AIRFOIL BY LEADING-EDGE BLOWING-SUCTION

The effects of leading-edge blowing-suction on the vortex how past an airfoil at high incidence are investigated numerically by solving the Navier-Stokes equations. The results indicate that the frequency of the flowfield excited by the periodic blowing-suction locks into the forcing frequency, which is half of the dominant frequency for the flow past a fixed airfoil without injection. In that case, a well-developed primary leading-edge vortex occupies the upper surface of the airfoil and the largest lift augmentation is obtained.

Control of mean and fluctuating forces on a circular cylinder at high Reynolds numbers

A narrow strip is used to control mean and fluctuating forces on a circular cylinder at Reynolds numbers from 2.0 ×10^4 to 1.0 ×^ 10^5. The axes of the strip and cylinder are parallel. The control parameters are strip width ratio and strip position characterized by angle of attack and distance from the cylinder. Wind tunnel tests show that the vortex shedding from both sides of the cylinder can be suppressed, and mean drag and fluctuating lift on the cylinder can be reduced if the strip is installed in an effective zone downstream of the cylinder. A phenomenon of mono-side vortex shedding is found. The strip-induced local changes of velocity profiles in the near wake of the cylinder are measured, and the relation between base suction and peak value in the power spectrum of fluctuating lift is studied. The control mechanism is then discussed from different points of view.

NUMERICAL STUDY ON THE FLOW AROUND A CIRCULAR CYLINDER WITH SURFACE SUCTION OR BLOWING USING VORTICITY-VELOCITY METHOD

A vorticity-velocity method was used to study the incompressible viscous fluid flow around a circular cylinder with surface suction or blowing. The resulted high order implicit difference equations were effeciently solved by the modified incomplete LU decomposition conjugate gradient scheme ( MILU-CG). The effects of surface suction or blowing' s position and strength on the vortex structures in the cylinder wake, as well as on the drag and lift forces at Reynoldes number Re = 100 were investigated numerically. The results show that the suction on the shoulder of the cylinder or the blowing on the rear of the cylinder can effeciently suppress the asymmetry of the vortex wake in the transverse direction and greatly reduce the lift force; the suction on the shoulder of the cylinder, when its strength is properly chosen, can reduce the drag force significantly, too.

NUMERICAL SIMULATION AND EXPERIMENTAL STUDY OF THE NEW METHOD OF HORSESHOE VORTEX CONTROL

The horseshoe vortex generated around the sail-body junction of a submarine has an important influence on the uniformity of the submarine wake at the propeller disc.In this article,the horseshoe vortex is simulated by the Detached Eddy Simulation（DES） method,and a new method to control the horseshoe vortex by vortex control baffler is proposed.The numerical simulation shows that a kind of attached vortex,with the rotation direction opposite to that of horseshoe vortex,is generated by the vortex control baffler.With the attached vortex,the strength of the horseshoe vortex is significantly reduced.The wind tunnel experiment on a submarine model is carried out,and the axial velocities at the propeller disc of the submarine with and without vortex control baffler are measured by a hot wire anemometer system.It is shown from the experimental results that the vortex control baffler can enhance the uniformity of the wake at the propeller disc,which helps to improve the propeller performance.The engineering applicability of the vortex control baffler is discussed.

Effects of Vortex Generator on Cylindrical Protrusion Aerodynamics

Experimental and numerical studies were carried out to evaluate the effect of vortex generator on a small cylindrical protrusion at Mach number 2.0. The experiments were performed using the supersonic blow down wind tunnel on different heights of cylindrical protrusion with vortex generator placed ahead of them. The upstream and downstream flow around the cylindrical protrusion is influenced by vortex generator as is observed using both visualization and pressure measurement techniques. Numerical studies using three dimensional steady implicit formulations with standard k-ω turbulence model was performed. Results obtained through the present computation are compared with the experimental results at Mach 2.0. Good agreements between computation and experimental results have been achieved. The results indicate that the aerodynamic drag acting on cylindrical protrusion can be reduced by adopting vortex generator.

Parametric study of Liutex-based force field models

The motion of fluid consists of different scales of coherent vortical structures.These vortical structures determine the characteristics of fluid motion and are key to understand fluid dynamics.In this paper,we study the fine control method of vortical structures based on the Liutex force field model.This is achieved by constructing a source term using Liutex and directly add it to the Navier-Stokes equations.To investigate the influence of the constructed Liutex force model on vortical structures,a flow past a cylinder at Reynolds number of 100 is numerically studied with different source term magnitude and region.The drag and lift forces on the cylinder,as well as the flow field near and behind the cylinder are compared and analyzed.Results show that Liutex force model can effectively strengthen or weaken the vortical structures based on different purpose.

Applications of Liutex-based force field models for cavitation simulation

When studying the flow dynamics and the characteristics of fluid motion,vortex structure with their interactions is the key issue.In the present work,a vortex control method is investigated based on the vortex identification system of Liutex.The numerical study is carried out in OpenFOAM by directly adding a source term to the Navier-Stokes equations,which is called the centripetal force model in Liutex method.A 2-D test case is examined to justify the proposed method in cavitating flow around Clark-Y hydrofoil,the simulation results show that the improved Liutex solver is feasible.Methodologies of controlling the rotation strength of vortices are able to change the flow field and suppress the cavitation.The applicability of vortex-based control method in 3-D flow field is also studied.The results show that cavitation surrounded by particular vortex can be effectively influenced.

Unsteady Behavior and Control of Vortices in Centrifugal Compressor

Two examples of the use of vortex control to reduce noise and enhance the stable operating range of a centrifugal compressor are presented in this paper.In the case of high-flow operation of a centrifugal compressor with a vaned diffuser,a discrete frequency noise induced by interaction between the impeller-discharge flow and the diffuser vane,which appears most notably in the power spectra of the radiated noise,can be reduced using a tapered diffuser vane(TDV) without affecting the performance of the compressor.Twin longitudinal vortices produced by leakage flow passing through the tapered portion of the diffuser vane induce secondary flow in the direction of the blade surface and prevent flow separation from the leading edge of the diffuser.The use of a TDV can effectively reduce both the discrete frequency noise generated by the interaction between the impeller-discharge flow and the diffuser surface and the broadband turbulent noise component.In the case of low-flow operation,a leading-edge vortex(LEV) that forms on the shroud side of the suction surface near the leading edge of the diffuser increases significantly in size and blocks flow in the diffuser passage.The formation of an LEV may adversely affect the performance of the compressor and may cause the diffuser to stall.Using a one-side tapered diffuser vane to suppress the evolution of an LEV,the stable operating range of the compressor can be increased by more than 12 percent,and the pressure-rise characteristics of the compressor can be improved.The results of a supplementary examination of the structure and unsteady behavior of LEVs,conducted by means of detailed numerical simulations,are also presented.